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SILICON CARBIDE SCHOTTKY AND P-I-N RECTIFIERS
By
SAURAV NIGAM
A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2003
Copyright 2003
by
Saurav Nigam
To my parents, my fiancée, my brother and my friends for their endless love and support.
ACKNOWLEDGMENTS
The past year has been one of the most crucial years of my life because for the first
time in my professional career, I was introduced to an exciting realm of microelectronics.
I would like to thank my advisor, Prof. Fan Ren, for giving me this golden opportunity to
join his research group. His commitment to provide world-class facilities in his
laboratory, engage students in the cutting-edge research and above all to provide
excellent guidance in all the spheres of life, both professional and personal, is truly
exceptional. Needless to mention, his extreme hard work and determination will always
act as a source of inspiration to me and instill in me the confidence to face every
adversity in life. He has always been able to find some time out of his extremely busy
schedule to help me to steer my efforts in the right direction. I will always look forward
to him as my true mentor.
I am also indebted to the other members of my advisory committee: Steve Pearton
and Tim Anderson. Their significant contributions to this work are greatly appreciated.
During the collaboration with Prof. Steve Pearton’s and Prof. Cammy Abernathy’s
research groups, I had an opportunity to develop interpersonal skills, emphasizing team
efforts and it was here that I came across some of the most remarkable people, like Brent
Gila and Mark, that I have ever known.
I would like to thank Brent for always lending me a helping hand whenever I
needed him and for his perspicacious views especially during the Wednesday meetings. I
iv
will also always cherish nice little discussions with Mark, who has provided me his
considerable time and energy to help me to set up the dicing saw.
I would also like to thank Ben Luo not just for providing the guidance in research
but also for providing his invaluable advice and guidance. I will always cherish those
moments when he would come up with his nice little humorous comments that would
take away all the stress after the day’s work. I would always count on him as a truly
sincere friend to seek for the right guidance. I would also like to thank Rishabh for his
support and care. He has been patient in providing me his valuable time to gain hands-on
experience with various equipment together with some delicious food and recipes. I have
also learned a lot from Jihyun, who has collaborated most closely with my research. I
would also like to thank Yoshi for providing me help and support and always showing a
great deal of concern. His friendship is one of the greatest gifts to me. Kelly, Kwang and
Kyu-pil have always been kind enough to lend their helping hand with the research.
I would like to thank Dennis Vince and Jim Hinnat of the Chemical Engineering
Department’s machine shop for all their help. I would also like to thank the staff in the
Chemical Engineering Department for all their administrative help. They are Peggy-Jo
Daugherty, Sonja Pealer, Santiago, Nancy Krell, Janice Harris and Debbie Sadoval. Their
assistance allowed me to focus on research.
I would like to thank my parents, my brother and my fiancée for their endless love,
care and support. They have provided me the strength and inspiration to continue my
studies at the University of Florida. This acknowledgement cannot be complete without
thanking my roommates Gopal, Nori, Sudeep and Archit, who have been very
understanding and caring throughout my stay at Gainesville.
v
TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ................................................................................................. iv
LIST OF TABLES........................................................................................................... viii
LIST OF FIGURES ........................................................................................................... ix
ABSTRACT...................................................................................................................... xii
CHAPTER
1 INTRODUCTION .........................................................................................................1
1.1 Background............................................................................................................. 1 1.2 Current Status of SiC Power Technology............................................................... 4 1.3 SiC Semiconductor Devices for Power Applications ............................................. 7
1.3.1 Material Availability and Quality ................................................................... 8 1.3.2 SiC Processing Technology ............................................................................ 9
1.3.2.1 Ion implantation ...................................................................................10 1.3.2.2 Oxides on SiC ......................................................................................11 1.3.2.3 Contacts................................................................................................12 1.3.2.4 Edge termination and passivation ........................................................13
2 SILICON CARBIDE SCHOTTKY RECTIFIERS .....................................................14
2.1 Experimental Characteristics of 4H-SiC Schottky Rectifiers............................... 14 2.1.1 Introduction................................................................................................... 14 2.1.2 Experimental Methods .................................................................................. 15 2.1.3 Results and Discussion.................................................................................. 17
2.2 Ni Contacts to N-Type 4H-SiC............................................................................. 26 2.3 Stability of SiC Schottky Rectifiers to Rapid Thermal Annealing....................... 27
2.3.1. Introduction.................................................................................................. 27 2.3.2 Experimental Methods .................................................................................. 29 2.3.3 Results and Discussion.................................................................................. 29 2.3.4 Summary and Conclusions............................................................................ 32
3 IRRADIATION AND PASSIVATION EFFECTS ON 4H-SiC RECTIFERS...........38
3.1 High Energy Proton Irradiation Effects on SiC Schottky Rectifiers .................... 38 3.1.1 Introduction................................................................................................... 38
vi
3.1.2 Experimental Methods .................................................................................. 39 3.1.3 Results and Discussion.................................................................................. 39
3.2 Influence of PECVD of SiO2 Passivation Layers on 4H-SiC Schottky Rectifiers41 3.2.1 Introduction................................................................................................... 41 3.2.2 Experimental Methods .................................................................................. 45 3.2.3 Results and Discussion.................................................................................. 45
3.3 Effect on 4H-SiC Schottky Rectifier of Ar Discharges Generated in a Planar Inductively Coupled Plasma Source ..................................................................... 47
3.3.1 Introduction................................................................................................... 47 3.3.2 Experimental Methods .................................................................................. 51 3.3.3 Results and Discussion.................................................................................. 51 3.3.4 Summary and Conclusions............................................................................ 54
4 JUNCTION TERMINATION EXTENSION GEOMETRY OF SiC RECTIFIERS..61
4.1 Influence of Edge Termination Geometry on Performance of 4H-SiC P-i-N Rectifiers ............................................................................................................... 62
4.1.1 Introduction................................................................................................... 62 4.1.2 Experimental Methods .................................................................................. 63 4.1.3 Results and Discussion.................................................................................. 63
4.2 Effect of Contact Geometry on 4H-SiC Schottky Rectifiers with Junction Termination Extension.......................................................................................... 65
4.2.1 Introduction................................................................................................... 65 4.2.2 Experimental Methods .................................................................................. 65 4.2.3 Results and Discussion.................................................................................. 72
4.3 Role of Device Area, Mesa Length and Metal Overlap Distance on Breakdown Voltage of 4H-SiC P-i-N Rectifiers...................................................................... 74
4.3.1 Introduction................................................................................................... 74 4.3.2 Experimental Methods .................................................................................. 74 4.3.3 Results and Discussion.................................................................................. 82 4.3.4 Summary and Conclusions............................................................................ 84
LIST OF REFERENCES...................................................................................................93
BIOGRAPHICAL SKETCH ...........................................................................................102
vii
LIST OF TABLES
Table page 1-1 Physical properties of important semiconductors for high voltage power devices. .5
1-2 Normalized unipolar figures of merit of important semiconductors for high voltage power devices..............................................................................................5
viii
LIST OF FIGURES
Figure page
1-1 Comparison of the ideal breakdown voltage of Si and SiC devices for different doping levels. ...........................................................................................................2
1-2 Comparison of the blocking layer thickness as a function of the ideal breakdown voltage for SiC and Si. .............................................................................................3
2-1 Cross-section of SiC Schottky rectifiers (top) and photograph of completed 2 inch diameter wafer (bottom).................................................................................16
2-2 Reverse I-V characteristics at 25 0C from Schottky rectifiers with different contact diameters. ..................................................................................................18
2-3 Reverse I-V characteristic from 154 µm diameter rectifier (top) and map of reverse breakdown voltage from a quarter of a 2 inch diameter wafer (bottom). .19
2-4 Comparison of breakdown voltage obtained from simulations and the fabricated devices....................................................................................................................21
2-5 Calculated reverse breakdown voltage as a function of epilayer doping. ..............23
2-6 Forward I-V characteristics in linear (top) and log (bottom) scales.......................24
2-7 Specific on-resistance as a function of breakdown voltage....................................25
2-8 Contact resistivity of Ni ohmic contact on n+ 4H-SiC as a function of annealing temperature (top), annealing time (center) and annealing ambient (bottom). .......28
2-9 Forward I-V characteristics as a function of anneal temperature for 60 sec anneals....................................................................................................................33
2-10 Change in RON as a function of anneal temperature for 60 sec anneals. ................34
2-11 Forward I-V characteristics (top) and change in VF (center) and RON (bottom) as a function of anneal time at 900˚C. ...................................................................35
2-12 Optical micrographs of Ni contacts before (top) and after annealing at 1000˚C for 60 sec (center) or 120 sec (bottom). .................................................................36
ix
2-13 Change in VF (top) and RON (bottom) as a function of measurement temperature after 900˚C, 60 sec anneals. ...................................................................................37
3-1 Reverse I-V characteristics from 4H-SiC Schottky rectifiers before and after proton irradiation at a dose of 5 x 109 cm-2 (top) and percentage increase in reverse leakage at –250V (bottom). .......................................................................42
3-2 Forward I-V characteristics from 4H-SiC Schottky rectifiers before and after proton irradiation at a dose of 5 x 109 cm-2 (top) and percentage decrease in forward current at 2V (bottom)..............................................................................43
3-3 Numerical change in values of n (top), RON (center) and VF (bottom) as a function of proton fluence......................................................................................44
3-4 Percentage change in VB as a function of plasma power (top), process pressure (center) and N2O content (bottom) during SiO2 deposition...................................48
3-5 Percentage change in VF as a function of plasma power (top) and process pressure (bottom) during SiO2 deposition..............................................................49
3-6 Percentage change in RON as a function of plasma power (top), N2O content (center) and process pressure (bottom) during SiO2 deposition. ...........................50
3-7 Planar coil ICP reactor............................................................................................56
3-8 Percentage change in VB (top) and RON (bottom) of 4H-SiC rectifiers as a function of ICP source power. ...............................................................................57
3-9 Percentage change in VF (top) and n (bottom) of 4H-SiC rectifiers as a function of ICP source power...............................................................................................58
3-10 Percentage change in VB (top) and RON (bottom) of 4H-SiC rectifiers as a function of process pressure...................................................................................59
3-11 Percentage change in VF (top) and n (bottom) of 4H-SiC rectifiers as a function of process pressure.................................................................................................60
4-1 P-i-N rectifier..........................................................................................................66
4-2 Forward I-V characteristics of p-i-n rectifier. ........................................................67
4-3 Reverse I-V characteristics from p-i-n rectifiers as a function of metal overlap distance onto mesa. ................................................................................................68
4-4 Variation of VB with metal overlap distance for p-i-n rectifiers. ...........................69
4-5 Reverse I-V characteristics from p-i-n rectifiers as a function of mesa length. .....70
x
4-6 Variation of VB with mesa length for p-i-n rectifiers with fixed metal overlap distance of 4 µm.....................................................................................................71
4-7 Schottky rectifier. ...................................................................................................75
4-8 Forward I-V characteristics of 0.64 mm2 rectifier..................................................76
4-9 Reverse I-V characteristics of 0.04 mm2 rectifiers with different contact shape. ..77
4-10 Reverse I-V characteristics of rectifiers with oval shaped contacts of different area.........................................................................................................................78
4-11 Reverse I-V characteristics of rectifiers with oval shaped contacts (0.04 mm2), as a function of extent of metal overlap.................................................................79
4-12 Reverse I-V characteristics of rectifiers with oval-shaped contacts (0.04 mm2), as a function of extent of metal overlap.................................................................80
4-13 Variation of VB with extent of metal overlap for different JTE lengths.................81
4-14 Reverse I-V characteristics of p-i-n rectifiers as a function of active area.............86
4-15 VB as a function of diode area for p-i-n rectifiers with mesa length 20 µm and metal overlap 4 µm. ...............................................................................................87
4-16 Reverse I-V characteristics of p-i-n rectifiers as a function of metal overlap distance. .................................................................................................................88
4-17 VB as a function of metal overlap length for p-i-n rectifiers with area 0.04 mm2 and zero mesa length..............................................................................................89
4-18 Reverse I-V characteristics of p-i-n rectifiers as a function of mesa length...........90
4-19 VB as a function of mesa length for p-i-n rectifiers with area 0.04 mm2 and zero metal overlap length...............................................................................................91
4-20 Forward I-V characteristics. ...................................................................................92
xi
Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science
SILICON CARBIDE SCHOTTKY AND P-I-N RECTIFIERS
By
Saurav Nigam
May 2003
Chair: Fan Ren Major Department: Chemical Engineering
There has been a significant research interest in silicon carbide (SiC) over the past
few years as a base material system for high frequency and high power semiconductor
devices. SiC Schottky diodes have shown an excellent performance for the realization of
power diodes for fast switching applications with nearly negligible power loss. In spite of
extremely high switching speeds, Schottky diodes suffer from high leakage current. P-i-N
diodes offer low leakage currents but show reverse recovery charge during switching and
have a large junction forward voltage drop due to the wide bandgap of 4H-SiC. However,
both the diodes have shown a considerable improvement in maximum breakdown voltage
values compared to that of planar junctions, with the employment of proper edge
termination methods.
The dc performance of SiO2 dielectric overlap-terminated 4H-SiC Schottky
rectifiers was simulated for different drift layer doping levels and dielectric thickness.
The devices were then fabricated with different Schottky contact diameters and the dc
xii
results compared to the simulations. There was a strong dependence of reverse
breakdown voltage (VB) on contact diameter (φ) ranging from –750 V for 100 µm φ to –
440 V for 1000 µm. The reverse leakage current scaled with diode diameter, indicating
that the surface contributions are dominant. The specific on-state resistance was ~0.36
mΩcm2, close to the theoretical minimum, with a forward turn-on voltage of 1.82 V at
100 A.cm-2. The contact resistivity of back-side Ni contacts annealed at 970 0C was ~1.2
x 10-6 Ωcm2.
The effect of deposition conditions of plasma enhanced chemical vapor deposition
(PECVD) SiO2 layers and inductively coupled Ar plasmas on the electrical properties of
4H-SiC Schottky rectifiers was also studied. For the former, the changes in reverse
breakdown voltage (VB), forward turn-on voltage (VF) and on-state resistance (RON) were
≤ 20% over a broad range of plasma conditions and show that 4H-SiC is relatively
resistant to changes induced by the ion bombardment and hydrogen flux present during
PECVD of dielectrics to surface passivation. In the latter, the VB increased by increase in
both incident ion energy and ion flux.
4H-SiC P-i-N rectifiers with SiO2 passivated mesa edge termination showed
forward current characteristics dominated by recombination at low bias (n~1.97) and
diffusion at high voltages (n~1.1). The forward turn-on voltage was ~4V, with a specific
on-state resistance of 15 mΩcm2, on/off current ratio of 1.5x105 at 3V/-450V and figure-
of-merit, VB2/RON, of 13.5 MWcm-2. The mesa extension distance did not have a strong
impact on reverse breakdown voltage.
xiii
CHAPTER 1 INTRODUCTION
1.1 Background
Silicon carbide (SiC) has shown outstanding potential for high power, high
temperature, high frequency and high voltage devices used in advanced communications
and radar system sensor and control systems, utility power switching and traction motor
control. The material has excellent physical and electrical properties such as extremely
high thermal conductivity (up to 4.9 W/cmK), high breakdown electric field (2-4 x 106
V/cm), a wide bandgap (3.26 eV for 4H-polytype) and high electron saturation velocity
(2 x 107 cm/s) as shown in Table 1-1 [1]. These characteristics result in significantly
higher current densities, operating temperatures and breakdown voltage than comparable
Si devices and in lower switching and on-state losses.
A comparison of ideal breakdown voltages versus blocking layer doping
concentration (Figure 1-1) suggests that the more highly doped blocking layer (more than
10 times higher) provides lower resistance for SiC because more majority carriers are
present than for comparably rated Si devices. A comparison of voltage blocking layer
thickness for a given breakdown voltage (Figure 1-2) shows a thinner blocking layer (n-
~5 x 1015 cm-3) of SiC devices (1/10th that of Si devices) also contribute to the lowering of
specific on-resistance by a factor of 10. The combination of 1/10th the blocking layer
thickness with 10 times the doping concentration can yield a SiC device with a factor of
100 advantage in resistance compared to that of Si devices.
1
2
1014 1015 1016
101
102
103
104
SiC
Si
Idea
l bre
akdo
wn
volta
ge (V
)
Doping of the blocking layer (cm-3)
Figure 1-1 Comparison of the ideal breakdown voltage of Si and SiC devices for different doping levels.
3
103 104100
101
102
103
SiC
Si
Wid
th o
f vol
tage
blo
ckin
g la
yer
(µm
)
Ideal breakdown voltage (V)
Figure 1-2 Comparison of the blocking layer thickness as a function of the ideal breakdown voltage for SiC and Si.
4
Several figures of merit, specifically to quantify the intrinsic performance
potentials of unipolar and bipolar devices, have been proposed [2-5]. The most important
material parameter is the avalanche electric field, followed by thermal conductivity and
carrier mobility. The unipolar figures of merit are shown in Table 1-2 [1] where SiC and
AlN unipolar devices have been projected to have intrinsic device improvement of 80 to
30000 times over conventional Si [4]. Also, bipolar devices, such as pin junction rectifier
and GTO thyristors, are expected to have lower total power loss when the switching
frequencies exceed 1 and 30 kHz respectively [5]. Here, it is worth pointing out that BN
is the best among the Group III-nitrides for power devices because of its high thermal
conductivity and indirect bandgap. Furthermore, due to the higher carrier mobility and
more isotropic nature of its properties, 4H-SiC has become the polytype of choice [6].
1.2 Current Status of SiC Power Technology
The overall goal for power electronic circuits is to reduce power losses, volume,
weight, and, costs of the system [7]. These factors become significant for applications in
future military and commercial ships especially electric propulsion. The first SiC based
device was a light emitting diode fabricated in 1907 by H. J. Round. However, a more
intensive silicon carbide semiconductor material and device development has been
underway for nearly 35 years in United States, the former Soviet Union, Europe and
Asia. Only in the past 15 years with the input of substantial funding from the US DOD
community (BMDO, DARPA, and ONR) have we seen the development of SiC wafers
industry in the US able to produce single crystal substrates with lower defect density
capable of sustaining device development programs [8]. Today, there are three suppliers
of SiC wafers in US (CREE, Sterling and Litton Airtron), one in Europe and one in Japan
with additional companies evaluating the market potential [8]. Within the last five years,
5
Table 1-1 Physical properties of important semiconductors for high voltage power devices.
Table 1-2 Normalized unipolar figures of merit of important semiconductors for high voltage power devices.
6
commercial devices and applications have emerged. Specifically, SiC is used for blue and
green LEDs, Northrop-Grumman makes Static Induction Transistors for internal
applications, and CREE has announced their recent SiC Zero Recovery™ Rectifiers.
Due to its exceptional switching speed, a high operational temperature and a high
breakdown electric field, SiC is having a profound impact on the design, topology and
circuits used in the power electronics especially where space weight, power density and
thermal management issues dominate as in mobile platforms; on aircraft, on vehicles and
especially on ships.
There have been many funded programs carried out at universities, industry and
government laboratories to commercialize SiC power switching devices. These programs
are designed to address device processing and fabrication technology, device physics and
design, packaging and applications.
The most recent results in Schottky and PiN diodes demonstrated exceptional
switching speeds with little or no ringing. When SiC diodes were packaged with Si
switches to form a simple hybrid inverter circuit, nearly all the stored energy was
eliminated due to the fast turn-off/turn-on SiC diodes [8].
At this time, US has a large but distribute program in SiC power device technology,
sustained with DOD programs at universities and larger companies such as GE and
Northrop Grumman. Traditional semiconductor solid-state switch manufacturers are now
actively pursuing this technology. On the other hand, the more integrated European
power industry, ABB and Siemens are actively pursuing research in SiC technology at all
levels in an internally coordinated manner. In Asia, Japan has an active research program
in SiC power technology that is now becoming focused under a new initiative [8].
7
1.3 SiC Semiconductor Devices for Power Applications
Silicon carbide offers significant advantages for power-switching devices because
the critical field for avalanche breakdown is about ten times higher than in silicon. SiC
power devices have made remarkable progress in the past five years, demonstrating
currents in excess of 100 A and blocking voltages in excess of 19000 V [9]. Over the past
decade a number of new SiC-based power switching devices have been demonstrated
such as GTOs, MOSFETs, pn diodes and schottky diodes with results approaching or
exceeding the theoretical limits of Si. However, there is significant difference between
demonstrating prototype devices and establishing an economically viable commercial
product. Silicon has proven to be an extremely difficult material to displace, largely
because of economic factors, and it remains to be seen whether SiC can accomplish what
other semiconductor materials have failed to do.
There are significant new challenges emerging, both in material science and device
design as the SiC technology is developed. One of the unique features of SiC is its high
thermal stability compared to silicon. This has both advantages as well as disadvantages-
an advantage because of improved reliability and higher temperature operability, and a
disadvantage because many of the typical fabrication processes either do not occur at all
in SiC or require extremely high temperatures. The most challenging aspect of the
thermal stability is the fact that due to the phase equilibrium in the Si-C system, SiC does
not melt, but instead gradually sublimes at temperatures above 2000 K. This makes it
impossible to form large single-crystal ingots by pulling a seed crystal from a melt, as in
Czochralski process that produces 200-300 mm diameter silicon ingots. Instead a
modified sublimation process that is presently limited to 100 mm diameter ingots forms
SiC crystals. The process is expensive and difficult and SiC wafer costs are astronomical
8
compared to silicon. However, the advantages of SiC power devices are so great that the
higher cost of the starting material can still be offset in certain cases, allowing SiC to
compete directly with silicon in specialized device markets.
The evolution of SiC power-switching technology can be divided into three phases:
prototype device demonstration, device scale-up, and commercial production. Many
types of SiC power devices are well along in the prototype demonstration phase and
several have demonstrated performance measures far superior to silicon. These include
rectifiers with blocking voltages above 19 kV [10], power metal-oxide-semiconductor
field-effect transistors (MOSFETs) [11], and bipolar junction transistors (BJTs) [12] with
performance figures 100 times higher than in silicon, and gate turn-off (GTO) thyristors
blocking over 3 kV and switching over 60 kW. Several of these devices are in the scale-
up phase, with terminal currents in excess of 100 A already demonstrated in single-device
packaged rectifiers [13].
1.3.1 Material Availability and Quality
SiC has a large number of different crystallographic forms or poly-types. Of these,
6H-SiC has the most developed growth technology on account of its relatively large
volume usage as a substrate for GaN blue LEDs. However, 4H-SiC is the preferred
material for many power applications owing to its nearly ten times higher on-axis
mobility compared to 6H-SiC. At present commercial 4H-SiC wafers are available in
sizes up to 50 mm diameter while 6H-SiC wafers are available in sizes up to 75mm
diameter, with 100 mm at the research stage. Substrates are available in both low
resistivity (n- and p-type) and semi-insulating forms [14]. However, the moderate to high
resistivity substrates desirable for high voltage devices is not available. Devices are
therefore, fabricated on homoepitaxial layers, which are routinely grown to thicknesses of
9
over 100 µm with doping densities as low as 1014 cm-3 using hot wall chemical vapor
deposition (CVD) [15]. High-level carrier lifetimes for this material are typically of the
order of several hundred nanoseconds [16] making it suitable for the fabrication of
devices with voltage blocking capabilities approaching 10 kV.
Historically, the major difficulty with SiC has been the presence of micropipes in
the substrates and epilayers. It is generally conceded that micropipes are the major
obstacles in the production of high-performance SiC devices. Micropipes are defects
unique to the growth of SiC and they are physical holes that penetrate through the entire
crystal. They are replicated into the device epitaxial layers and become “killer” defects if
they intersect the active region of SiC devices. However, the reports on material growth
with micropipe densities as low as 0.1/cm2 at research level (reduced from over 1000/cm2
in just a few years) [14], indications are that SiC is now adequate to fabricate devices
several millimeters square with reasonable yield [17, 18]. Indications are that a
dislocation density of less than 103 cm-2 is desirable for fabrication of power devices [18]
(current values are typically around 104 cm-2). A further difficulty concerns the
uniformity of epilayer doping and thickness across the wafer (typical results show 4 %
standard deviation in doping) and the uniformity of doping between runs (typically 40%)
[14].
1.3.2 SiC Processing Technology
SiC has a relatively mature processing technology. All the basic process steps, such
as doping (by implantation and during epitaxy), etching (plasma techniques), oxide
growth, Schottky and ohmic contacts have been successfully demonstrated [19]. There
10
are, however, several key processing issues that currently limit the performance of
fabricated devices.
1.3.2.1 Ion implantation
Ion implantation is employed widely for local p-type and n-type doping of SiC. The
most frequently used implanted ions are aluminum (Al) and boron (B) for p-type,
nitrogen (N) and phosphorous (P) for n-type. Key issues in SiC implantation technology
include the reduction of lattice damage occurring during implantation, successful
electrical activation of the dopants and preservation of good surface morphology.
Performing the implantation at elevated temperature is a common way to reduce the
lattice damage and reduce the requirements for subsequent annealing [20]. Typical
implantation temperatures are: 400 oC for Al [21], 500 oC for N [22], 800 oC for P [23].
These temperatures are sufficiently high to obtain good electrical conductivity after
annealing but not so high as to cause surface dissociation and/or large vacancy clusters
[24].
The annealing condition must be precisely determined, both for recording the
crystal and diffusing the dopants into substantial sites. In addition, evaporation of both
silicon and dopants must be avoided if the surface stoichiometry is to be preserved. This
is particularly important in cases where the surface layer forms an active part of the
device, for example in MOSFET and MESFET channel regions. There are two main
approaches to preserve the surface: (i) material encapsulation with AlN, graphite [25]; (ii)
increased Si partial pressure in the furnace reactor, either with silane gas [26] or with SiC
coated pieces [27]. Several authors have given results for optimum electrical activation
[28-30]. In all cases, it appears that good electrical activation is only possible if the
annealing temperature exceeds 1550 oC.
11
1.3.2.2 Oxides on SiC
One of the principal benefits of silicon carbide is that it oxidizes to form a stable
surface layer of silicon dioxide releasing carbon dioxide in the process [31]. However,
the detailed properties of the oxide and in particular the interface between the SiC and the
SiO2 are significantly different from Si [31].
The oxidation rate is crystal-orientation dependent and is far slower on the Si face
than the C face, though, in general, much better properties are found for the oxide on the
Si face. Oxidation temperature are normally around 1100 oC and unlike Si, a post
oxidation anneal in a hydrogen ambient is usually reported to have little effect at reducing
interface state density [32]. Interface state density on p-type material can be reduced to
levels below 1011 cm-2eV-1 in the lower half of the gap by the use of re-oxidation anneal
below 1000 oC in a wet ambient [33]. However, this treatment does not seem to improve
the density of states in the upper half of the gap [34]. Interface state densities on the 4H
polytype rise very rapidly towards the conduction band edge, typically exceeding 1013
cm-2eV-1, whereas on 6H the density is an order of magnitude lower. This has been
ascribed to a carbon related acceptor that is located just below the conduction band edge
for 4H but within the conduction band for 6H [35]. These defects have not been
successfully removed by any standard surface treatment or post oxidation anneal.
The best N channel inversion mode MOSFETs fabricated on 6H show electron
mobilities of ~100 cm2/Vs with a negative temperature coefficient, whereas 4H shows
mobilities of at best 25 cm2/Vs or lower [36, 37]. The fluctuation in potential resulting
from the charge in the interface states near the conduction band edge appear to be
responsible for these low values of mobility, particularly for 4H [38].
12
The breakdown and reliability properties of SiO2 are crucial for all MOS devices,
but unfortunately the situation is not as favorable as it is for Si. The dielectric constant of
SiC is ~10 whereas it is ~3.9 for SiO2, so any normal surface field will be ~2.5 times
higher in the oxide than the SiC. Hence to gain the full benefit of the 2.5 MV/cm
breakdown field of SiC, the oxide must withstand 6.25 MV/cm, which is higher than is
reliably usable, even on silicon [39]. High field stressing of oxides has shown that oxides
grown in a wet ambient, breakdown at lower fields than dry grown oxides [40], and that
extrapolated time-dependent dielectric-breakdown lifetimes of 10 years can only be
obtained on n-type at fields less than 5 MV/cm at room temperature. Studies have shown
that the lifetime for oxides drops rapidly at elevated temperatures, with a vulnerability to
negative bias-stress instability [41]. Electron injection into the oxide is more efficient
than for Si due to the lower barrier [32]. In addition, the barrier for injection of holes is
much lower than in Si, which is unfortunate since holes are particularly damaging to
oxides [42].
1.3.2.3 Contacts
Ohmic contacts to both n and p-type material have been demonstrated for 4H-SiC
with specific contact resistivities of the order of 10-5 Ωcm2 [43, 44]. To achieve this, it
has proved necessary to use rapid thermal annealing (RTA) of the wafer at temperatures
as high as 1400 oC. The majority of contacts are based around Ni and Al (for n- and p-
type, respectively) although these are generally not stable at temperatures above 400 oC.
To take advantage of the full potential for high temperature operation of 4H-SiC devices,
contacts that are thermally stable up to 600 oC over long periods are required. Recent
work based on Al/Ni/W/Au contact structures on p-type 4H-SiC have shown longevity of
over 100 h at 600 oC with a contact resistance of 10-3 Ωcm2 [45]. Other works have
13
shown similar results using a variety of materials including TaC [46] and AlSi [47]
alloys.
Several groups have reported excellent quality room temperature Schottky diodes,
with the barrier height dependent on the metal chosen. This demonstrates that the fermi
level is not pinned at the surface of the 4H-SiC. Values of ΦB vary from 1.10 eV (Ti) to
1.73 eV (Au), which are markedly larger than those observed on Si or GaAs. This
increase in ΦB makes SiC an ideal material for high-voltage, high-temperature, low –
leakage current Schottky diodes. Surface preparation, prior to deposition of the Schottky
contact, has been shown to be a key factor in achieving good performance with sacrificial
thermal oxidation being the optimum technique [48].
1.3.2.4 Edge termination and passivation
Many techniques applied to Si devices are also applicable to SiC. For example,
field plates [49, 50], guard rings and junction termination extensions [51, 52] have all
been used to good effect. Another simple technique involves the implantation of a high
dose of inert ions (either Ar [53] or B [54]). In this case the damage caused by the
implant causes a high resistivity region to be formed close to the surface, which acts in a
manner similar to semi-insulating poly crystalline silicon (SIPOS). The reverse leakage
performance of Ar and B implanted edge terminations may be improved by low
temperature (600 oC) annealing [55]. Passivation of SiC surface is not trivial and has yet
to be fully understood. The effects of inadequate passivation have been observed in
microwave MESFETs, where this leads to degradation of gain under CW operation [56].
Power switching devices, on the other hand, appear to perform well with conventional Si
passivation treatments such as polyamide.
CHAPTER 2 SILICON CARBIDE SCHOTTKY RECTIFIERS
Silicon carbide device technology has seen a tremendous development over last
five years. The technology once envisioned as a substitute to silicon technology has been
realized to practice. The feasibility of SiC devices has been shown for many different
types of devices, the development of a working production technology has started, yield,
reliability and costs now being the key issues. The high cost of SiC substrates at present
poses a major challenge for the technology to enter the device market. However, the
exceptional properties of the material promote a tremendous interest in some of the prime
applications like power electronics and high temperature sensor technology. The other
examples are in transportation and manufacturing sector where high efficiency needs
entails strong research efforts towards development of advanced power management and
control electronics. Central to this effort is the development of the solid-state devices
capable of delivering large currents and withstanding high voltages, without the need of
sophisticated cooling systems [56].
2.1 Experimental Characteristics of 4H-SiC Schottky Rectifiers
2.1.1 Introduction
SiC has shown outstanding potential for high power, high temperature electronics
in advanced communications and radar systems, sensor and control systems, utility power
switching and traction motor control [57-68]. The wide bandgap (3.25 eV for the 4H-poly
type), availability of doped substrates of either electrical conductivity type and high
thermal conductivity (up to 4.9 W/cmK) of SiC make it a preferred material for these
14
15
applications [69-77]. These characteristics result in significantly higher current densities,
operating temperatures and breakdown voltages than comparable Si devices and in lower
switching and on-state losses. There have been major advances in the bulk and epitaxial
growth of SiC in the past 5 years or so, along with improvements in device design and
fabrication. Those have resulted in very impressive performance from SiC power metal-
oxide semiconductor field effect transistors (MOSFETs), bipolar transistors and p-i-n and
Schottky rectifiers [78-93].
2.1.2 Experimental Methods
The starting substrates were n+ (1019 cm-3) 4H-SiC. Approximately 10µm of lightly
n-type (n ~ 5 x 1015 cm-3) 4H-SiC was grown on these substrate by vapor phase epitaxy,
followed by ~ 500 Å of thermal SiO2. A full-area back contact of e-beam deposited Ni
annealed at 970 0C was used for contact to the substrate. SiO2 layers deposited by plasma
enhanced chemical vapor deposition (PECVD) were employed as a part of the dielectric
overlap edge termination [94]. Holes were opened in the SiO2 stack by a combination of
dry and wet etching and a Schottky contact of e-beam evaporated (1000 Å thick) Ni
patterned by lift-off. The contact diameter ranged from 100-1000 µm. A schematic of the
completed structure and an optical micrograph of a processed 2 inch diameter wafer are
shown in Figure 2-1.
A key feature of achieving good Schottky performance is the quality of the ohmic
contacts. The contact resistivity ~1.2 x 10-6 Ωcm2 was obtained from transmission line
measurements of the Ni backside ohmic for 970 0C, 3 min anneals under a flowing N2
ambient in a Heatpulse 610 furnace. This is slightly higher than reported for annealed Ni
16
PECVD SiNx
Thermal SiO2Epi n- SiC (tepi=10 µm) Substrate n+ 4H/SiC (n+= 1e19)
Ni
Ni
Figure 2-1 Cross-section of SiC Schottky rectifiers (top) and photograph of completed 2 inch diameter wafer (bottom).
17
on 6H-SiC substrates (7 x 10-7 Ωcm2) [95], which have a slightly smaller bandgap (3.02
eV) than the 4H-polytype used here.
2.1.3 Results and Discussion
Figure 2-2 shows some reverse current-voltage (I-V) characteristics measured at
25oC schottky contact diameter. Note that the reverse breakdown voltage decreases
significantly with increasing contact size. This is due to the increased probability of
having crystal defects in the active region of the device as the area is increased. The
production of SiC substrates with large, defect free areas still represents the biggest
challenge to widespread use of power rectifiers in this materials system. Note also the
very low reverse leakage currents in the rectifiers at biases below breakdown. In this bias
regime, the reverse current was proportional to the length of the rectifying contact
perimeter, suggesting surface contributions were the most important in this voltage range.
At biases closer to breakdown, the current was proportional to the area of the rectifying
contact. Under these conditions, the main contribution to the reverse current is from
rectifiers with a 7000 Å thick SiO2 dielectric overlap layer, as a function of the under this
contact, i.e. from the bulk of the material.
A more detailed view of the reverse I-V characteristic from a 154 µm diameter
rectifier is shown in Figure 2-3 (top), along with a map of VB values measured over a
quarter of a 2-inch diameter wafer (bottom). The overall shape of the I-V curve is similar
to past reports for Ni/4H-SiC rectifiers [90], in which it was concluded that the origin of
the reverse current was due to a combination of both thermionic field emission and field
emission. Note that yield of rectifiers with VB > 750 V is over 50 % and with VB > 600 V
is over 75 %.
18
-800 -600 -400 -200 010-12
10-10
10-8
10-6
10-4
100 µm diode 500 µm diode 1000 µm diode
Cur
rent
(A)
Bias Voltage (V)
Figure 2-2 Reverse I-V characteristics at 25 0C from Schottky rectifiers with different contact diameters.
19
-1000 -800 -600 -400 -200 010-11
10-10
10-9
10-8
10-7
10-6
10-5
SiC Diode 5000 Å SiO2Dia: 154 µm
Cur
rent
(A)
Bias Voltage (V)
520 V 600 V
860 V 980 V800 V 900 V
520V940 V860 V820 V>1000 V
830 V660 V640 V750 V810 V
810V>1000 V930 V580 V400 V
560 V 820 V>1000 V 720 V 740 V
300 V>1000 V820 V750 V600 V
290 V980 V730 Vdamaged500 V
720 V
Figure 2-3 Reverse I-V characteristic from 154 µm diameter rectifier (top) and map of reverse breakdown voltage from a quarter of a 2 inch diameter wafer (bottom).
20
Simulations of the rectifier performance were carried out using the MEDICITM
code. The performance of the structure of Figure 2-1 (top) was simulated for different
oxide thicknesses and a metal overlap of 10 µm onto this oxide, as used experimentally.
Figure 2-4 shows a comparison of the experimental and theoretical results. Note that the
VB values obtained on the real devices are roughly half of that expected from the
simulations. The later do not take into account the presence of crystalline defects,
including micropipes in the SiC. This difference emphasizes the importance of crystal
quality in determining the performance of the rectifiers.
The reverse breakdown voltage is related to the epilayer doping (ND) through the
relationship [96]:
NEV
D
CB e2
2∈=
where ε is the dielectric constant of SiC, EC the critical field for breakdown and e the
electronic charge. Figure 2-5 shows our experimental data point for VB at a epilayer
doping concentration of 5 x 1015 cm-3 for 10 µm thick layers, along with the expected
results from the simulations with different carrier concentrations. Note that there is not
much improvement expected for decreasing the carrier concentration below 5 x 1015 cm-3
since the layer will be depleted under those conditions.
Our experimental devices still have performance limited by material defect density
rather than purity. The VB values obtained in the simulations were found to increase
linearly with SiO2 thickness from 2500 Å to 17500 Å and saturated thereafter.
21
2000 3000 4000 5000 6000 7000 8000 9000 10000 11000
-1500
-1400
-1300
-1200
-1100
-1000
-900
-800
-700
SiC Schottky DiodesDoping n:5e15 cm-3
tepi:10 µm
Experimental results Simulated results
Bre
akdo
wn
Vol
tage
(V)
Dielectric thickness (Å)
Figure 2-4 Comparison of breakdown voltage obtained from simulations and the fabricated devices.
22
However, thicknesses above ~5000 Å are difficult to achieve in practice because of
the long deposition times. The forward current characteristics of a Schottky rectifier are
dominated by the barrier height of the metal on the semiconductor and by series
resistance concentrations. The barrier height (φB) for Ni on 4H-SiC is ~1.3eV [90]. The
forward voltage drop, VF, is related to the barrier height and on-state resistance, RON,
through the relation:
JRTAJV FONB
FF n
enkT .)ln( 2* ++= Φ
where n is the diode ideality factor, k is Boltzmann’s constant, T the absolute
temperature, e the electric charge, JF the forward current density and A* is Richardson’s
constant. The value of RON is a function of epilayer thickness and doping. Figure 2-6
shows forward I-V characteristics in both linear (top) and log (bottom) form. A maximum
forward current of 2 A was obtained from these devices, and was limited by our
experimental system. The turn-on voltage was ~1.82 V at a forward current density of
100 Acm-2 and the on-state resistance was ~0.36 mΩcm2. The diode ideality was 1.13 at
25 0C, which is comparable to past results for Ni metallization [90].
To place our results in context, Figure 2-7 shows a theoretical plot of specific on-
resistance as a function of VB for Si, 4H-SiC and 6H-SiC, along with some experimental
data. The lines are derived from the relation [84-96]:
EVR
C
BON 3
24
∈=
µ
23
1014 1015 1016 1017102
103
Simulation Results Experimental Result
SiC Schottky Diode tepi=10 µmtdielectric=1.0 µm
Bre
akdo
wn
Vol
tage
(V)
Carrier Concentration (cm-3)
Figure 2-5 Calculated reverse breakdown voltage as a function of epilayer doping.
24
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.0
0.5
1.0
1.5
2.0
2.5
SiC Schottky DiodesSiO2: 7000 ÅDiode Dia.: 1000 µm
Cur
rent
(A)
Bias Voltage (V)
0.0 0.5 1.0 1.5 2.0 2.5 3.0100
101
102
SiC Schottky DiodesSiO2: 7000 ÅDiode Dia.: 1000 µm
Cur
rent
Den
sity
(A/c
m2 )
Bias Voltage (V)
Figure 2-6 Forward I-V characteristics in linear (top) and log (bottom) scales.
25
Figure 2-7 Specific on-resistance as a function of breakdown voltage.
26
where µ is the electron mobility in the epilayer. Note that the SiC rectifiers are expected
to have either much lower on-resistance at a given breakdown voltage than Si, or
equivalently a much larger breakdown voltage for a given on-resistance. Our results are
close to the theoretical minimum on-resistance for the VB’s obtained and indicate that the
device processing produces no major degradation in the electrical properties of the SiC.
2.2 Ni Contacts to N-Type 4H-SiC
Superior electrical and thermal properties of SiC make it promising material for
high temperature and high frequency electronics. However, it is widely appreciated that
the optimal performance of microwave devices is related to the quality of ohmic contacts.
Low contact resistance and high temperature stability are required [97]. Nickel is one of
the most popular materials used in advanced aerospace systems [98] and good nickel
ohmic contacts on n-SiC have been already demonstrated [99]. The popularity of Ni
contacts to SiC is due to its reproducibly low contact resistance and good temperature
stability [100]. In our study, we could see no appreciable degradation of the contacts up
to a temperature of 225 oC.
Annealing the contacts produces low ohmic contact resistivities on highly doped
substrates. For n-type SiC, low contact resistivities (<10-5 Ω cm-2) have been reported in
the past [101]. We obtained a contact resistivity of 1.2 x 10-6 Ω cm2, in our studies. A
study on the interfacial reactions between Ni and n-type SiC suggests that silicides such
as Ni31Si12 and Ni2Si are produced at an intermediate temperature range of 500-600 oC
[102]. X-ray diffraction (XRD) and Rutherford backscattering spectroscopy (RBS)
showed that the interfacial reaction forming Ni31Si12 and Ni2Si began at 500 oC [103], and
the Ni2Si phase remained up to 900 oC. The ohmic contact formation was thought to be
27
due to the formation of Ni2Si phase, even the Ni2Si formed at a temperature as low as 600
oC [101]. It is essential to optimize the annealing conditions to obtain contacts with least
contact resistivity. In our studies we have studied the effect of temperature, ambience and
time on the contact resistivity of Ni-SiC ohmic contacts. Figure 2-8 shows the effect of
annealing temperature (top-left), time (top-right) and annealing ambient (bottom) on the
contact resistivity obtained from transmission line measurements of the Ni backside
ohmic. A minimum value of ~1.2 x 10-6 Ωcm2 was achieved for 970 0C, 3 min anneals
under a flowing N2 ambient in a Heatpulse 610 furnace. This is slightly higher than
reported for annealed Ni on 6H-SiC substrates (7 x 10-7 Ωcm2) [95], which have a
slightly smaller bandgap (3.02 eV) than the 4H-polytype used here.
2.3 Stability of SiC Schottky Rectifiers to Rapid Thermal Annealing
2.3.1. Introduction
There is currently a lot of interest in the application of SiC power rectifiers to high
current switching in the >30A range because of the high thermal conductively
(~4.9W/cm·K) and large bandgap relative to Si [104-112]. The advantage in materials
properties mean that on-state resistance, RON, for a 4H-SiC rectifier can be over 100
times smaller than that of a Si rectifier at the same breakdown voltage, VB, since RON is
given by 4VB/µεEC3 (where µ is the electron mobility, ε the permittivity, and EC the
critical field for breakdown) [87].
There are numerous situations in which high temperature annealing might be
employed in the fabrication of SiC rectifiers, including activation of implanted dopants or
reduction of reverse leakage current in self-aligned Ar+-implanted edge terminations
28
950 960 970 980 990 10001.1x10-6
1.5x10-6
1.9x10-6
2.3x10-6
Anneal time: 3 minAnneal gas: N2
Rc (O
hm-c
m2 )
Temperature (°C)
2.0 2.5 3.0 3.5 4.01.0x10-6
1.4x10-6
1.8x10-6
2.2x10-6 Anneal temperature: 970 °CAnneal gas: N2
Rc (O
hm-c
m2 )
Time (min)
N2 Vacuum Ar1.1x10-6
1.3x10-6
1.5x10-6
1.7x10-6
Anneal time:3 minAnneal temp:970 °C
Rc (O
hm-c
m2 )
Anneal Gas
Figure 2-8 Contact resistivity of Ni ohmic contact on n+ 4H-SiC as a function of annealing temperature (top), annealing time (center) and annealing ambient (bottom).
29
[113-115]. In the latter case, Ar+ implantation is used to create a high-resistance region
around the contact periphery in order to spread the electric field distribution and avoid
breakdown due to field crowding. While this process does increase reverse breakdown
voltage, it also increases leakage current. Several studies have examined the effect of
anneals up to 700ºC for reducing this leakage current [114].
In this study we report the stability of 4H-SiC Schottky power rectifiers to rapid
thermal annealing treatments in the temperature range 700 - 1100ºC with the contacts
already in place. This is of interest for defining the stability of devices in which the Ni
rectifying contact is used as a self-aligned mask for implant edge termination. The
performance of the rectifiers is found to show a general improvement for anneals
≤1000ºC.
2.3.2 Experimental Methods
The Schottky contacts were fabricated on the wafer as described in section 2.1.2.
The annealing for these contacts was performed in a Heatpulse 610T System under
flowing N2 ambient at temperatures of 700 – 1100 ºC for 60 - 240 sec. The current-
voltage (I-V) characteristics were measured before and after this procedure. Prior to
annealing, the typical on-state resistance was 0.8 mΩ·cm2, the reverse breakdown voltage
(VB) ~450V and the forward turn-on voltage was 2.5V at 100Acm-2.
2.3.3 Results and Discussion
The reverse bias characteristics showed a general increase in current for anneals ≤
1000˚C, while the forward I-V characteristics also shoed significant changes under these
conditions, as shown in Figure 2-9. Note the decrease in turn-on voltage as the anneal
temperature increase. The forward current density, JF, can be expressed as:
30
−
−= 1exp2**
nkTeV
kTe
TAJ BF
φ
where A** is the Richardson’s constant for SiC, k is Boltzmann’s constant, e is the
electric charge, φ B is the barrier height for Ni on 4H-SiC, V is the applied voltage, n is
the ideality factor, and T is the measurement temperature. The data in Figure 2-9 are
consistent with a reduction in φ B. On 6H-SiC, Ni was observed to form Ni2Si at
temperature beginning at ~600˚C [116], eventually leading to ohmic behavior. In lower
temperature anneals, Ni/6H-SiC diodes showed a reduction of leakage current through
removal of low-barrier secondary diodes in parallel to the primary diode [113]. Our
measure φ B was ~1.4eV prior to annealing, which is consistent with past reports [116-
118]. It is likely that the reduced turn-on voltage observed in Figure 2-9 is the result of
silicide formation, as suggested previously [114]. The on/off current ratio was 1.5 ×105 at
3V/-450V in control samples and increased by a maximum of ~20% after optimum
annealing at 700˚C. The forward current characteristics of a SiC Schottky rectifier are
dominated by the Ni barrier height and by series resistance contributions and is related to
φ B and RON through the relationship:
FONBF
F JRnTA
Je
nkTV ++
= φ2**ln
With annealing, the value of φ B decreased as described above, but RON showed
improvement (Figure 2-10). Once again the annealing durations had to be kept<120 sec at
the highest temperature in order to retain some acceptable rectifying behavior of the top
Ni contact.
31
Figure 2-11 shows the effect of annealing time at 900˚C on the forward I-V
characteristics (top), VF (center), and RON (bottom). Note that the largest anneal (240 sec)
produced essentially ohmic behavior. Examples of the effect of the annealing duration on
the top Ni contact morphology are shown in the optical micrograph of Figure 2-12. It is
clear that there is already a strong metallurgical reaction between the Ni and the SiC at
1000˚C for 60 sec. To obtain the lowest ohmic contact resistivity for annealed Ni contacts
requires ~3 min at this temperature, and after 60 sec the contact still retains some
rectifying behavior. Note also the dark appearance of the contact after annealing. Auger
Electron Spectroscopy showed high levels of carbon present in these reacted contacts,
suggesting the reaction can be written as:
2Ni + SiC → Ni2Si + C
Figure 2-13 shows the measurement temperature dependence of the change in VF
(top) and RON (bottom) for samples annealed at 900˚C. There is little significant change
in VF while the improvement in RON decrease with increasing temperature as current
transport over the barrier. Arrhenius plots of the forward current indicated a barrier height
of ~1.14 eV after the 900˚C anneal, consistent with the observed increase in current after
annealing. The intersection of the lines with the temperature axis indicated a constant
electrically active contact area equivalent to ~9% of the physical contact area. This is
consistent with the expected behavior for thermionic emission still being the dominant
transport mechanism after 900˚C annealing.
32
2.3.4 Summary and Conclusions
The main points of our study may be summarized as follows:
• High voltage Ni/4H-SiC Schottky rectifiers using a simple SiO2/metal edge termination method show near-ideal forward characteristics (ideality factor, on-state resistance).
• The reverse breakdown voltage is approximately half that expected from a simple model simulation and decreases as the device area is increased.
• The reverse current appears to be dominated by surface combinations at low bias and by bulk combinations near breakdown.
• Both thermionic field emission and field emission mechanisms appear to be present. The yield of 750 V, 2 A devices was above 50 %.
• For high temperature anneals, the Ni rectifying contact becomes ohmic, with severe degradation of the contact morphology. The forward current characteristics are still dominated by thermionic emission until the onset of ohmic behavior.
• The benefits of annealing at moderate temperature include a decrease in forward turn-on voltage and an increase in the on/off current ratio.
• The reasons for those improvements may include annealing of defects or reduction of interfacial oxides between the Ni rectifying contact and the SiC.
• The use of more thermally stable rectifying contact metallization would allow higher annealing temperatures and possibly more improvement in device performance.
33
0.0 0.5 1.0 1.5 2.0 2.5 3.00.000
0.005
0.010
0.015
0.020
0.025
0.030
60 secanneals 1000 °C 900 °C
Control
Cur
rent
(A)
Bias Voltage (V)
Figure 2-9 Forward I-V characteristics as a function of anneal temperature for 60 sec anneals.
34
700 750 800 850 900 950 1000 1050 11000.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
60 secanneals
∆RO
N (10
-3)
Temperature (°C)
Figure 2-10 Change in RON as a function of anneal temperature for 60 sec anneals.
35
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
240 secs 60 secs Control
Cur
rent
(A)
Bias Voltage (V)
40 60 80 100 120 140 160 180 200 220 240 260-2.0
-1.9
-1.8
-1.7
-1.6
-1.5
-1.4
-1.3
-1.2
-1.1
∆VF
Anneal Time (Seconds)
40 60 80 100 120 140 160 180 200 220 240 2600.00
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
∆RO
N(10
-3)
Anneal Time (Seconds)
Figure 2-11 Forward I-V characteristics (top) and change in VF (center) and RON (bottom) as a function of anneal time at 900˚C.
36
Figure 2-12 Optical micrographs of Ni contacts before (top) and after annealing at 1000˚C for 60 sec (center) or 120 sec (bottom).
37
20 40 60 80 100 120 140 160-0.55
-0.50
-0.45
-0.40
-0.35
-0.30
-0.25
-0.20
∆VF
Measurement Temperature (°C)
20 40 60 80 100 120 140 1600.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
∆RO
N(10
-3)
Measurement Temperature (°C)
Figure 2-13 Change in VF (top) and RON (bottom) as a function of measurement temperature after 900˚C, 60 sec anneals.
CHAPTER 3 IRRADIATION AND PASSIVATION EFFECTS ON 4H-SIC RECTIFERS
3.1 High Energy Proton Irradiation Effects on SiC Schottky Rectifiers
3.1.1 Introduction
SiC power rectifiers with reverse breakdown voltages in the 300-1200 V range are
of current interest for a number of applications, including traction motor control, sensors
and advanced communications (including broad-band satellite transmission) and radar
systems [105-108]. In particular, 4H-SiC is attractive because of its larger bandgap and
higher and more anisotropic mobility relative to other SiC polytypes [119]. The 4H-SiC
has numerous advantages over Si for power device applications, including much larger
critical breakdown field, lower on-state resistance at a given voltage and higher thermal
conductivity. While a significant amount of work has been reported on ion-beam induced
damage and annealing in SiC [120-123], much less is known about radiation damage in
SiC-based devices. Recent results have shown that properly passivated SiC metal-oxide
semiconductor field effect transistors have good tolerance to MRad doses of 60Co γ-rays
[124]. One would expect SiC to be relatively resistant to radiation damage compared to
materials such as GaAs based on the empirical observation that the displacement
threshold energy for atoms in semiconductor is inversely proportional to lattice constant
[125] (a = 0.31 nm, c = 1.03 nm for 4H-SiC compared to a = 0.5653 nm for the zinc
blende GaAs).
There is particular interest in the response of SiC rectifiers to high-energy proton
irradiation because of the potential applications in satellites. In this work we reported on
38
39
the effects of 40 MeV proton irradiation of 4H-SiC rectifiers, at doses corresponding to
more than 10 years in low-earth orbit. The reverse leakage current increases by
approximately a factor of two under these conditions, while ideality factor, on-state
resistance and forward turn-on voltage all increase by ≤ 75%. These results are consistent
with creation of electron traps by protons that traverse the active region of the rectifiers.
3.1.2 Experimental Methods
The Schottky rectifiers with diameter of 154 µm were fabricated in the same way
as described in section 2.1.2. The devices were irradiated at the Texas A&M cyclotron
with 40 MeV protons at fluences from 5 × 107-5 × 109 cm-2, with the highest fluence
being comparable to at least 10 years exposure in low earth orbit. The projected range of
the protons is >50 µm in SiC. The irradiated devices were measured approximately 2
days after exposure.
3.1.3 Results and Discussion
Figure 3-1 (top) shows reverse current voltage (I-V) characteristics before and
after a proton fluence of 5 × 109 cm-2. The basic shape of the characteristic is unchanged
and shows only an increase in the magnitude of the reverse leakage current. Figure 3-1
(bottom) shows the percentage increase in reverse current measured at –250V, as a
function of the proton fluence. We suggest the creation of generation-recombination
centers related to atomic displacements created by the incoming protons is the main
mechanism for the increased leakage current, based on both this data and what follows
later in this section.
The forward I-V characteristics before and after proton irradiation at fluence of 5
x 109 cm-2 are shown at the top of Figure 3-2. There is an increase in low-bias (< 1V)
40
current consistent with the introduction of recombination centers associated with proton-
induced displacement damage. At higher forward voltages, the current is decreased.
Figure 3-2 (bottom) shows the magnitude of the decrease (measured at a forward voltage
of 2 V) as a function of the proton fluence. Note the percentage decrease in current is ~
6% after a fluence corresponding to more than a year in low earth orbit and 42% after
more than 10 years. The Schottky barrier height was extracted from the relationship:
]1))[(exp(2** −Φ
−=nkTeV
kTe B
F TAJ
where JF is the forward current density at voltage V, A** is the Richardson’s constant for
4H-SiC, T the absolute measurement temperature, e the electronic charge, ΦB the barrier
height for Ni on 4H-SiC, n the ideality factor and k the Boltzmann’s constant. The
extracted values were 1.32 ± 0.06 eV for both control and irradiated rectifiers, which is
consistent with past reports for Ni on 4H-SiC [54].
The forward voltage drop for a Schottky rectifier, VF, is related to the barrier
height and on-state resistance, RON, through:
JRTAJV FON
FF n
enkT
+Φ+= )ln( 2**
VF is usually defined as the voltage at which JF is 100 A.cm-2. Figure 3-3 shows
the increases in VF, n and RON as a function of proton fluence. The values in the
unirradiated control rectifiers were VF = 2.4V, n = 1.15 and RON = 3.7 × 10-4 Ωcm2. In
each case, the values of these parameters increase with increasing proton fluence. The
results are consistent with a net reduction in carrier density in the depletion region of the
rectifiers through the introduction of traps and recombination centers associated with
proton damage.
41
3.2 Influence of PECVD of SiO2 Passivation Layers on 4H-SiC Schottky Rectifiers
3.2.1 Introduction
There is great current interest in the development of manufacturable SiC power
rectifiers in the 300-1000 V, 25-50 A range for use in traction motor control, sensor and
control systems and the drive train of hybrid electrical vehicles [119-126]. Numerous
reports have shown the potential of both SiC Schottky and p-i-n rectifiers for these
applications [84, 87]. Record breakdown voltage (12.3 kV) and forward current (130 A)
results have been achieved for these devices [87]. The main focus now is to understand
the effect of crystal growth and device processing steps in the performance of SiC
rectifiers, in an attempt to push the devices into a robust manufacturing mode. One of the
important fabrication steps is the deposition of the thick dielectric layers for use as
surface passivation and as a part of the metal overlap edge termination. These layers are
generally deposited by plasma enhanced chemical vapor deposition (PECVD) using
SiH4-based chemistries and therefore the surface of the SiC is subject to both energetic
ion bombardment and a high flux of atomic hydrogen. The former can create surface and
bulk traps that degrade the electrical properties of semiconductors while the later can etch
the surface through the reaction:
SiC + 8H SiH4 + CH4
as well as passivate dopants in the near-surface region. Both donors (D) and acceptors
(A) can be passivated through the formation of neutral complexes with hydrogen,
according to following mechanisms:
D+ + H- (DH)0
A- + H+ (DH)0
42
-500 -400 -300 -200 -100 010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Post-irradiation Pre-irradiation
dose: 5x109 p/cm2
SiC Schottky diodeSiO2:7000 ÅDia.: 1000 µm
Cur
rent
(A)
Bias Voltage (V)
107 108 109 1010
100
101
102
103
SiC Schottky diodeSiO2:7000 ÅDia.: 1000 µm
% in
crea
se in
I L (-25
0V)
Proton Dose (cm-2)
Figure 3-1 Reverse I-V characteristics from 4H-SiC Schottky rectifiers before and after
proton irradiation at a dose of 5 x 109 cm-2 (top) and percentage increase in reverse leakage at –250V (bottom).
43
0.0 0.5 1.0 1.5 2.0 2.5 3.0
10-2
10-1
100
dose: 5x109 p/cm2
pre-irradiated post-irradiated
SiC Schottky diodeSiO2:7000 ÅDia.: 1000 µm
Cur
rent
(A)
Bias Voltage (V)
107 108 109 1010
0
10
20
30
40
50
SiC Schottky diodeSiO2:7000 ÅDia.: 1000 µm
% d
ecre
ase
in I F (2
V)
Proton Dose (cm-2)
Figure 3-2 Forward I-V characteristics from 4H-SiC Schottky rectifiers before and after
proton irradiation at a dose of 5 x 109 cm-2 (top) and percentage decrease in forward current at 2V (bottom).
44
107 108 109 10100.0
0.5
1.0
∆n
Proton Dose (cm-2)
107 108 109 10100
1
2
3
4
∆ R
ONx
10-4
Ω cm
2
Proton Dose (cm-2)
107 108 109 10100.0
0.5
1.0
∆V
F (V)
Proton Dose (cm-2)
Figure 3-3 Numerical change in values of n (top), RON (center) and VF (bottom) as a function of proton fluence.
45
In this study we report on the effects of PECVD SiO2 layers on the performance
of Ni/4H-SiC Schottky rectifiers. We also identify optimum deposition conditions under
which there is minimal degradation of the device performance.
3.2.2 Experimental Methods
The Schottky rectifiers with a diameter of 154 µm were fabricated in the same
way as described in section 2.1.2. PECVD of thin (~200 Å) SiO2 films were performed at
a substrate temperature of 250 0C using 2% SiH4 and N2O at a total flow rate of 600
sccm. The process pressure was raised from 600-900 mTorr, the N2O percentage of the
total flow rate from 5 to 50% and the rf power (13.56 MHz) varied from 25-400W. The
SiO2 layers were thin enough that we could probe through them to the underlying
contacts, allowing us to measure the current voltage (I-V) characteristics without having
to remove the SiO2 layers. Prior to SiO2 deposition, the rectifiers displayed reverse
breakdown voltage (VB) of 770 V, on-state resistance (RON) of 4.73 mΩ.cm2 and forward
turn-on voltage (VF) of 2.06 V at 100 A.cm-2.
3.2.3 Results and Discussion
Figure 3-4 shows the effect on VB of plasma power (top), pressure (center) and
N2O percentage (bottom) during the SiO2 deposition. There is no significant change in VB
at low powers, while at higher powers (>250 W), it increases. Since VB is inversely
dependent on the doping in the depletion region (ND), according to the relation:
NEV
D
CB e2
2∈=
where ∈ is the permittivity of SiC and e the electronic charge, this suggests a decrease in
doping in the unprotected region around the rectifying contact (and possibly under, since
atomic hydrogen ion diffuse in an isotropic fashion). This effect was largest at the lowest
46
process pressure, as seen in the center part of Figure 3-4. The bottom part of the figure
shows that VB actually decreased under SiH4-rich deposition conditions. Therefore, we
suggest that the main effect of the hydrogen flux is to decrease VB through creation of
surface states, whereas the effects of plasma power and pressure are mainly due to
creation of deep traps by energetic ion bombardment. In this later case, the effective
doping around the contact is decreased and this leads to an increase in breakdown
voltage.
The forward turn-on voltage was less sensitive to changes in the SiO2 deposition
parameters than was VB. This is expected, since VF is controlled by the barrier height of
Ni contact (1.3 eV) and by series resistance contributions, especially from contact
resistance. Since the area under the Schottky contact is protected by the overlying metal
and the rear ohmic contact is not exposed to the plasma, we would expect VF to be less
sensitive to the process conditions. Figure 3-5 shows the effect on VF of both plasma
power (top) and process pressure (bottom), with the changes in each case being less than
~ 10%.
The on-state resistance is related to the critical field for breakdown EC and the
reverse breakdown voltage through the relation [96]:
EVR
C
BON 3
24
∈=
µ
where µ is the electron mobility in the 4H-SiC epi-layer. Figure 3-6 shows the effect of
plasma power (top), N2O percentage (center) and process pressure (bottom) on RON. The
trends mirror those seen for VB under the same conditions, with the increases being
greatest at the highest powers, highest SiH4 content in the plasma and the lowest process
pressure.
47
3.3 Effect on 4H-SiC Schottky Rectifier of Ar Discharges Generated in a Planar Inductively Coupled Plasma Source
3.3.1 Introduction
SiC-based rectifiers are generating tremendous current interest for a broad range
of applications, including broad band satellite transmission systems, advanced radar, high
temperature sensors and traction motor control [119-127]. Most attention has been
focused on the 4H-SiC polytype because of its larger bandgap (3.25 eV) and higher
mobility relative to the other polytypes [70]. Within the two basic classes of rectifiers,
Schottky devices have the lowest on-state voltages and highest switching speeds; while p-
i-n diodes have the higher reverse breakdown voltage and lower reverse leakage current
[87]. Compared to Si rectifiers, SiC devices have on-state resistances approximately a
hundred times lower or equivalently much larger breakdown voltage at the same on-state
resistance [96]. While very high forward currents (up to 130 A) and breakdown voltages
(4.9 kV) have been achieved for 4H-SiC Schottky rectifiers [87], a lot of recent attention
has been focused on fabrication and materials technology for diodes in the 300-1000 V
range [105]. Plasma processing is required both for dry etching of mesas and deposition
of dielectrics for surface passivation and metal overlap edge termination. However little
has been reported on the effects of plasma exposure on the electrical performance of 4H-
SiC rectifiers.
In this study, we describe the effect of Inductively Coupled Plasma (ICP) Ar
discharges generated in a novel plasma source configuration, on the electrical properties
48
0 100 200 300 400-30
-20
-10
0
10
20
30
40
50SiC Schottky diodes
% c
hang
e in
Vb
Power (W)
600 700 800 900-20
0
20
40SiC Schottky diodes
% c
hang
e in
Vb
Pressure (mTorr)
0 10 20 30 40 50-100
-80
-60
-40
-20
0SiC Schottky diodes
% c
hang
e in
Vb
% N2O
Figure 3-4 Percentage change in VB as a function of plasma power (top), process pressure
(center) and N2O content (bottom) during SiO2 deposition.
49
0 100 200 300 4002
4
6
8
10
12 SiC Schottky diodes
% in
crea
se in
Vf
Power (W)
600 700 800 9002
4
6
8
10 SiC Schottky diodes
% in
crea
se in
Vf
Pressure (mTorr)
Figure 3-5 Percentage change in VF as a function of plasma power (top) and process pressure (bottom) during SiO2 deposition.
50
0 100 200 300 4000
5
10
15
20
25 SiC Schottky diodes
% in
crea
se in
Ron
Power (W)
0 10 20 30 40 500
2
4
6
8
10 SiC Schottky diodes
% in
crea
se in
Ron
% N2O
600 700 800 9000
5
10
15
20
25 SiC Schottky diodes
% in
crea
se in
Ron
Pressure (mTorr)
Figure 3-6 Percentage change in RON as a function of plasma power (top), N2O content
(center) and process pressure (bottom) during SiO2 deposition.
51
of Ni/4H-SiC Schottky rectifiers. Both ion flux and ion energy are found to play a role in
the extent of the observed changes in reverse breakdown voltage (VB), on-state resistance
(RON), ideality factor (n) and forward turn-on voltage (VF).
3.3.2 Experimental Methods
The Schottky rectifiers with a contact diameter of 154 µm were fabricated in the
same way as described in section 2.1.2. The as-fabricated rectifiers show RON of 4.4 mΩ-
cm2, VF = 1.96 V and VB = -330 V.
The devices were exposed to pure Ar discharges in a planar coil geometry ICP
reactor, shown schematically in Figure 3-7. A key feature of this system is the ability to
operate at very low source power (20 W) as well as chuck power (5 W), which allows
one to design etch or deposition processes that end with very low ion energy and flux
conditions to minimize damage. The ICP source power (2 MHz) was varied from 100-
700 W, the rf chuck power (13.56 MHz) from 25-200 W and the process pressure from
10-40 mTorr. All exposures were 30 sec in duration and removed < 150 Å of the surface
by sputtering. These plasma exposure simulate the ion bombardment received by the SiC
surface during processes such as dry etching of plasma enhanced chemical vapor
deposition and represent a worst-case scenario because there is no chemical etch
component that would remove some of the damaged surface region on a deposition of
dielectric to protect the surface from further ion bombardment.
3.3.3 Results and Discussion
Figure 3-8 shows the percentage changes in VB (top) and RON (bottom) as a
function of ICP source power (100 W) and process pressure (10 mTorr). The reverse
breakdown voltage decreases with increasing ion flux. Since the dc self-bias on the
52
sample electrode also decreases with increasing source power, this indicates that
increasing ion flux is a factor in degrading the breakdown voltage. Note however that the
changes are ≤ 25% even at the highest source power employed. For SiC there is an
empirical relationship between VB and the doping in the epilayer, ND, which is given by
[96]:
NV DB
75.0151075.1 −×=
One of the expected effects of Ar plasma exposure on the exposed SiC around the
contact periphery is a decrease in ND through creation of deep trap states. However this
would lead to an increase in VB, which is the opposite of what is observed
experimentally. Therefore, we suggest that the main effect of the plasma exposure under
our conditions is an increase in surface defects that initiate breakdown at much lower
values than expected from the bulk doping of the SiC epilayer.
The on-state resistance can be written as [106]:
EVR
C
BON 3
24
∈=
µ
where µ is the electron mobility, ∈ the permittivity of 4H-SiC and EC the critical field for
breakdown. Since VB decreases with increasing source power, we would also expect a
decrease in RON unless EC is also decreasing. Figure 3-8 (bottom) shows large increases
in RON as the source power is increased, which is consistent with the creation of surface
states that promote early breakdown of the damaged rectifier.
Figure 3-9 shows the percentage changes in VF (top, measured at 100 A.cm-2) and
ideality factor (bottom) as a function of source power at fixed rf power (100 W) and
pressure (100 mTorr). Note that the changes in both parameters are ≤ 20% provided the
53
source power is left below 300 W, corresponding to a dc self-bias ≤ -185 V. At higher
source powers both VF and n are severely degraded even though the self-bias under these
conditions is low (-41 V at 700 W source power). The average energy of the incident Ar+
ions is roughly given by the sum of the dc self-bias and the plasma potential
(approximately –25V in this system). Therefore even though the ion energy is low at high
source powers, the ion flux is sufficient to create significant degradation in the rectifier
characteristics. The forward voltage drop for a Schottky rectifiers is related to the barrier
height (φB) and RON through the relation [56]:
JRJV FONBF
Fn
TAenkT
++= φ)ln( 2**
where k is Boltzmann’s constant, T the absolute temperature, e the electronic charge, JF
the forward current density and A** is Richardson’s constant for 4H-SiC. Therefore VF
can be degraded by increases in ideality factor and on-state resistance along with a
reduction in forward current due to introduction of trap states.
The changes in rectifier performance were also dependent on the applied rf chuck
power, or equivalently on ion energy. The changes in VB, VF, RON and n were ≤ 20% for
VB and VF, ≤ 30% for RON and ≤ 40% for n over the range of these parameters that we
investigated.
Figure 3-10 shows the percentage changes in VB (top) and RON (bottom) as a
function of process pressure at fixed source power (300 W) and rf power (100 W). Note
that the decreases in both parameters are largest at the lowest pressure. This is consistent
with the fact that at low pressures the incident ions have a lower probability of collisions
with gas molecules or they traverse the sheath region and therefore they impact with their
full energy. A major advantage of the plasma ICP source is its ability to operate at higher
54
pressures (≥ 40 mTorr) relative to the more normal 10mTorr range of conventional
cylindrical coil sources. The ideality factor of the rectifiers was still more degraded at
higher pressures, while VF showed very small changes with pressure (Figure 3-11).
3.3.4 Summary and Conclusions
In conclusion 4H-SiC Schottky rectifiers show only modest changes in the
electrical characteristics after high-energy proton irradiation at fluences corresponding to
more than 10 years in low-earth orbit. These devices appear promising for both aerospace
and terrestrial applications where irradiation hardness is a prerequisite. The main
degradation mechanism appears to be creation of recombination centers and traps that
cause an increase in reverse leakage current, ideality factor and on-state resistance.
In another study, PECVD SiO2 layers were deposited on Ni/4H-SiC Schottky
rectifiers as a function of various plasma parameters. Both reverse breakdown voltage
and on-state resistance increase with increasing plasma power and SiH4 content through
creation of deep trap states and degradation of the SiC surface by reaction with atomic
hydrogen, respectively. The changes in forward turn-on voltage are less dramatic in each
case. Plasma conditions that utilize low-to-moderate powers, high process pressure and
low SiH4 percentages produce minimal changes in the rectifier performance.
4H-SiC power rectifiers were also exposed to Ar discharges in a plasma coil ICP
reactor as a function of various plasma parameters. The reverse breakdown voltage
decreases with increasing ion flux, which is controlled by the source power, whereas VF,
RON and n all increase under the same conditions. These results are consistent with
creation of surface states that promote reverse breakdown at lower applied voltages. The
same trends were observed with increasing of chuck power, which controls the incident
ion energy. The increases in VB and RON are minimized at high operating process
55
pressure, as expected since these conditions reduce the effectiveness of the ion
bombardment experienced during the plasma exposure. Damage during actual etch or
deposition process would be lower than produced during the Ar plasma exposure, but
clearly process conditions that utilize low ion energies and fluxes are desirable at the end
of an etch cycle or the beginning of a deposition cycle in order to minimize ion-induced
damage.
56
ICP (Inductively Coupled Plasma) Etching System
Reactor
13.56 MHZ
Frame Structure
Faraday Shield
Load Lock
He Cooling System
Plasma
Gas Inlet
Quartz
Sample
Powered Electrode
Figure 3-7 Planar coil ICP reactor.
57
100 200 300 400 500 600 700-25
-20
-15
-10
-5SiC Schottky RIE Power: 100 WPressure: 10 mTorr
ICP Power (W)
% c
hang
e in
VB
0
100
200
300
400
500
dc se
lf-bi
as (-
V)
100 200 300 400 500 600 700-200
0
200
400
600
800
1000
1200
1400
RIE Power: 100 WPressure: 10 mTorr
SiC Schottky
% c
hang
e in
Ron
ICP Power (W)
Figure 3-8 Percentage change in VB (top) and RON (bottom) of 4H-SiC rectifiers as a
function of ICP source power.
58
100 200 300 400 500 600 700
0
50
100
150
200
250
RIE Power: 100 WPressure: 10 mTorr
SiC Schottky
% c
hang
e in
VF
ICP Power (W)
100 200 300 400 500 600 700
10
20
30
40
50
60
70
80
RIE Power: 100 WPressure: 10 mTorr
SiC Schottky
% c
hang
e in
idea
lity
fact
or
ICP Power (W)
Figure 3-9 Percentage change in VF (top) and n (bottom) of 4H-SiC rectifiers as a
function of ICP source power.
59
10 15 20 25 30 35 40-20
-15
-10
-5
0
5
Pressure (mTorr)
% c
hang
e in
VB
0
50
100
150
200
250
SiC SchottkyRIE Power: 100 WICP Power: 300 W
dc se
lf-bi
as (-
V)
10 15 20 25 30 35 4014
16
18
20
22
24
26
28
30
32
RIE Power: 100 WICP Pwer: 300 W
SiC Schottky
% c
hang
e in
Ron
Pressure (mTorr)
Figure 3-10 Percentage change in VB (top) and RON (bottom) of 4H-SiC rectifiers as a
function of process pressure.
60
10 15 20 25 30 35 4017
18
19
20
21
22RIE Power: 100 WICP Pwer: 300 W
SiC Schottky
% c
hang
e in
VF
Pressure (mTorr)
10 15 20 25 30 35 40
20
25
30
35
40
45
50
55
60
RIE Power: 100 WICP Pwer: 300 W
SiC Schottky
% c
hang
e in
idea
lity
fact
or
Pressure (mTorr)
Figure 3-11 Percentage change in VF (top) and n (bottom) of 4H-SiC rectifiers as a
function of process pressure.
CHAPTER 4 JUNCTION TERMINATION EXTENSION GEOMETRY OF SIC RECTIFIERS
It is possible to make etching of SiC (0001) facet only by plasma methods because
of its high chemical stability [128]. After such etching the surface of the semiconductor is
strongly damaged and is not electrically neutral. It is a reason for arising breakdown at
considerable lower voltages than the values implied by the impurity concentration in the
base. In most of the cases the breakdown characterized by surface leakage current (in the
order of 10-6–10-8 A) and was irreversible. For decreasing probability of surface like
breakdown it is necessary to make surface electrical field strength less than the strength
in bulk.
This can be achieved by profiling of mesa structure during etching [129] or by
diffusion o acceptor impurities into the surface region of the n-base [130]. Also electrical
activity of the surface can be decreased by special treatment like oxidation.
Another problem encountered for high reverse voltage is when the breakdown
occurs through the air along the surface of the mesa-structure. It has been shown that the
breakdown voltage increases when the measurements are done by placing the samples in
a dielectric liquid with a high value of electric breakdown field strength like fluorientTM
FC-77 [131]. For high temperature and high power devices if is necessary to cover mesa-
structure by the thermostable dielectric with critical electrical field strength more than
that of air.
61
62
4.1 Influence of Edge Termination Geometry on Performance of 4H-SiC P-i-N Rectifiers
4.1.1 Introduction
4H-SiC has a larger bandgap (3.26 eV) compared to 6H-SiC (2.9 eV) and is
preferred for device applications because of its higher and more isotropic electron
mobility [70]. There is particular interest in the development of 4H-SiC power rectifiers
for high power electronics applications. The on-state resistance of SiC rectifiers can be
less than 1% that of Si rectifiers with the same breakdown voltage [106-108]. There are
two basic classes of rectifiers, namely Schottky and p-n junction. The former have lower
on-state voltages and higher switching speeds, while the latter have higher breakdown
voltage and lower reverse leakage current. Advanced designs such as junction barrier
Schottky (JBS) [109], dual metal trench [90] and merged p-i-n Schottky rectifiers (MPS)
[110] attempt to achieve the advantages of both types of rectifiers [84].
An empirically derived relationship of 4H-SiC shows that the critical electric field
for breakdown, EC, is related to the doping level in the drift layer, ND, through [112]:
)10
log(25.01
105.2
16
6
DC NE
−
×=
or in terms of breakdown voltage, VB [104]:
NV DB
75.0151075.1 ×=
Excellent high current (230 A) and high voltage (5.5 kV) performance have been
reported for 4H-SiC p-i-n rectifiers. The performance of manufacturable devices in the
300-1000V range is of particular interest, especially the development of robust designs
and fabrication sequences. In this paper we report on the influence of edge termination
design on the performance of 4H-SiC p-i-n rectifiers with a simple epi layer structure.
63
4.1.2 Experimental Methods
Epi layers of 12 µm of n-type (n~2x1015 cm-3) and 1 µm of p-type (p~1017 cm-3)
4H-SiC were grown on n+ 4H-SiC substrates. A schematic of the completed rectifiers is
shown in Figure 4-1. A full-area back metallization of Ni annealed at 970 0C for 3 min
provided ohmic contact to the substrate. Mesas were formed by dry etching and thermal
SiO2 was used as the part of the dielectric overlap edge termination. The e-beam
evaporated Ni Schottky rectifiers had areas between 0.005-0.04 mm2. Passivation layers
of SiO2 (3000 Å thick) were deposited by plasma enhanced chemical vapor deposition.
The adjustable parameters were the extent of metal overlap onto the mesa defining the
active device area and the length of this mesa. The mesa edge termination has been
demonstrated to provide excellent results on SiC p-n junctions provided the mesa surface
is properly passivated [90,94].
4.1.3 Results and Discussion
Figure 4-2 shows the forward I-V characteristics from one of the p-i-n rectifiers.
The I-V curve can be fitted to the standard relationship for p-n diodes, namely [107]:
)exp()exp(21 kTneVJ
kTneVJ
AIJ DORO +==
where A is the diode area, JRO and JDO are the saturation current densities for
recombination and diffusion current mechanisms, respectively, e is the electronic charge,
k is Boltzmann’s constant, T the absolute temperature, V the applied bias and n1 and n2
are the ideality factors for recombination and diffusion currents, respectively. A fit to the
data yields n1=1.97 and n2=1.10. Therefore, at lower current densities the transport is
dominated by recombination, whereas at higher forward voltages, the current is
64
dominated by the diffusion contribution. The forward turn-on voltage, VF, can be
represented as [84]:
VV Mi
F nnn
ekT
+= +− )ln( 2
where n- and n+ are the electron concentrations at the two end regions (p+/n and n+/n), ni
the intrinsic electron concentration and VM the voltage drop across the drift region. The
measured VF is ~4V, or roughly 0.74 V higher than the theoretical minimum. The best
on-state resistance was 15 mΩcm2, with an on/off current ratio of 1.5x105 at 3/-450 V.
Figure 4-3 shows reverse I-V characteristics as a function of metal overlap distance
for fixed mesa extension distance of 60 µm. The reverse breakdown voltage shows a
trend of decreasing as the metal overlap distance increases. The mechanism is not clear at
the moment, but could be due to several factors, including breakdown in the thermal
oxide or extension of the depletion region to include defects under the mesa. The figure-
of-merit VB2/RON was a maximum of 13.5 MWcm-2 for these rectifiers.
Figure 4-4 shows the variation of VB with metal overlap distance for rectifiers with
different active areas. There is no significant effect of area over the range we employed,
but we would expect to observe a decrease in VB in very large rectifiers because of the
increased probability of having a defect in the active region [127].
Figure 4-5 shows reverse I-V characteristics from rectifiers with fixed area and
extent of metal overlap, as a function of mesa extension length. The VB values extracted
from this data are shown in Figure 4-6. Within experimental error and the uniformity of
VB on a given wafer (~ ±10%), there is no significant change in breakdown voltage as a
function of mesa extension length.
65
4.2 Effect of Contact Geometry on 4H-SiC Schottky Rectifiers with Junction Termination Extension
4.2.1 Introduction
There is great current interest in SiC power Schottky rectifiers with reverse
breakdown voltages in the 500-1000V range for applications in traction motor control,
sensors and next generation communications and radar systems [125-132]. These devices
have significant advantages over Si diodes for these purposes, including lower switching
losses, higher operating temperatures and faster switching speeds [93, 96]. In addition the
high thermal conductivity of SiC (approximately three times that of Si) makes it ideal for
very high current applications. There is still a need to optimize the edge termination and
contact geometry for SiC rectifiers in order to find the most robust design for
manufacturing.
In this study we report on the effect of Schottky metal overlap and contact shape on
the reverse breakdown voltage of 4H-SiC rectifiers employing junction termination
extension (JTE). The JTE is used to reduce the electric field at the edges of the rectifiers.
It increases the breakdown voltage of the diode by reducing the electric field density
within the SiC near the edges of the rectifier. The p-doping of the JTE region counteracts
the bending of the depletion region around the edges of the anode. This effect spreads out
the electric field at the corners and edges of the depletion region.
4.2.2 Experimental Methods
The starting samples consisted of 1 µm p-type 4H-SiC (p~1017 cm-3) on 12 µm of
n-type 4H-SiC (n~2x1015 cm-3) on a n+ 4H-SiC substrate. A full area back contact of e-
beam deposited Ni annealed at 970 0C was used for contact to the substrate. Thermally
66
Figure 4-1 P-i-N rectifier.
67
0 1 2 3 4 5 6 7
0.0
2.0x10-3
4.0x10-3
6.0x10-3
8.0x10-3
1.0x10-2
1.2x10-2
1.4x10-2
SiC P-i-N diode
Cur
rent
(A)
Bias Voltage (V)
Figure 4-2 Forward I-V characteristics of p-i-n rectifier.
68
-700 -600 -500 -400 -300 -200 -100 010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4 metal overlap 0 µm metal overlap 2 µm metal overlap 4 µm metal overlap 6 µm
SiC P-i-N diodeSiO2: 3000 Å p-SiC:1 µmDiode area: 0.01 mm2
mesa length:60 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-3 Reverse I-V characteristics from p-i-n rectifiers as a function of metal overlap distance onto mesa.
69
0 1 2 3 4 5 60
200
400
600
area: 0.04 mm2
area: 0.01 mm2
area: 0.005 mm2
Bre
akdo
wn
Vol
tage
(V)
Metal Overlap (µm)
Figure 4-4 Variation of VB with metal overlap distance for p-i-n rectifiers.
70
-500 -450 -400 -350 -300 -250 -200 -150 -100 -50 010-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
mesa length: 60 µm mesa length: 40 µm mesa length: 20 µm mesa length: 0 µm
SiC P-i-N diodeSiO2: 3000 Å p-SiC:1 µmDiode Area: 0.04 mm2
metal overlap: 4 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-5 Reverse I-V characteristics from p-i-n rectifiers as a function of mesa length.
71
0 10 20 30 40 50 60100
150
200
250
300
350
400
450
SiC P-i-N diodeSiO2: 3000 Å p-SiC: 1 µmArea: 0.04 mm2 Metal Overlap: 4 µm
Bre
akdo
wn
Vol
tage
(V)
Mesa Length (µm)
Figure 4-6 Variation of VB with mesa length for p-i-n rectifiers with fixed metal overlap
distance of 4 µm.
72
grown SiO2 was on part of the dielectric overlap edge termination. Schottky contact holes
were etched into the SiO2 and through the p-SiC to expose the underlying n-SiC using
dry etching. Schottky contacts of e-beam evaporated Ni (area 0.04 – 0.64 mm2) were
patterned by lift-off. Three different contact shapes were employed, namely oval, circular
and square. Finally, SiO2 layer deposited by plasma enhanced chemical vapor deposition
(PECVD) was used for passivation. A schematic of the completed device structure is
shown in Figure 4-7.
4.2.3 Results and Discussion
The forward turn-on voltage VF, defined as the forward bias at which the current
density is 100 A-cm2, was < 2V (Figure 4-8). The on-state resistance, RON, was
4.2mΩcm2, which is close to the theoretical minimum for this structure of 4H-SiC. The
VF is ~25% higher than the theoretical minimum for Ni Schottky diodes on 4H-SiC and
may reflect the presence of some residual dry etch damage or surface contamination.
Figure 4-9 shows the reverse current-voltage (I-V) characteristics from rectifiers
with 0.04 mm2 area, junction termination extension of 40 µm and Ni metal overlap of 4
µm in extent, as a function of contact shape. Note that the oval diodes have the highest
leakage at low bias (<250 V) and the lowest leakage at higher voltages. The reverse
breakdown increases in the sequence square<circular<oval, where we define the
breakdown as the voltage at which the current reaches 1 µm. It is not immediately clear
why the oval shape rectifiers have higher breakdown and more work is needed to
establish any relationship to defect density in the SiC samples or the role of surface states
around the contact periphery. The breakdown voltage of all rectifiers is still
73
approximately a factor of two lower than expected from the depletion layer doping and
thickness employed.
Figure 4-10 shows reverse I-V characteristics from rectifiers of different contact
area and fixed JTE (40 µm) and extent of metal overlap (2 µm). The reverse breakdown
voltage decreases with increasing contact area. This is most likely due to the increased
probability of having crystal defects in the active region of the rectifier as the area is
increased. The shape of the I-V curves is similar to those reported earlier for Ni/4H-SiC
rectifiers [90], where it was suggested that both thermionic field emission and field
emission mechanisms are present.
There was little effect of metal overlap extent on the reverse I-V characteristics, as
shown in Figure 4-11 for oval-shaped rectifiers with small area (0.04 mm2) and a JTE of
20 µm. This is expected, since the field distribution in the depletion region is controlled
by the p-n junctions formed by the JTE regions on the drift region and not by the metal
overlying the p-regions. This gives considerable latitude in the patterning step for the
metal deposition. Similar results are shown in Figure 4-12 for rectifiers with a JTE length
of 40 µm.
A summary of VB data as a function of metal overlap distance for different JTE
length is shown in Figure 4-13. While the number of samples is too small to draw
definitive conclusions, VB is maximized at shorter JTE lengths. We believe that
breakdown may actually be initiated in the p-layer under some conditions because of its
low doping level and therefore longer JTE regions have a higher probability of having
defects that promote breakdown. The diodes show an excellent on/off current ratio (1.5/-
500V) of ≥ 4x105 at 25 0C.
74
4.3 Role of Device Area, Mesa Length and Metal Overlap Distance on Breakdown Voltage of 4H-SiC P-i-N Rectifiers
4.3.1 Introduction
SiC p-i-n rectifiers have several advantages over their simpler Schottky
counterparts, most importantly, a higher reverse breakdown voltage (VB) and lower
reverse leakage current [104-110]. SiC p-i-n rectifiers are expected to offer significant
performance advantages over Si rectifiers for high voltage applications, rf base station
infrastructure, advanced avionics, combustion control electronics in automobiles and
solid-state power conditioning [133-135]. While very high VB and forward conduction
currents have been reported (8.6 kV [87]), the need is to design robust device structures
and processes for the 300-1200 V range. In particular, to realize a manufacturable SiC
rectifier process, it is necessary to understand the effect of various edge termination
geometries on the breakdown voltage characteristics so that VB most closely approaches
the expected parallel-plate value for the epi layer doping and thicknesses employed.
Moreover, it is necessary to carry out these investigations on 4H-SiC, which has larger
bandgap and higher and more isotropic mobility than the 6H-polytype [70].
In this study we report the results of an investigation into the role of device area,
mesa length and metal overlap distance on the breakdown voltage of 4H-SiC p-i-n
rectifiers fabricated on thick (~50 µm) epi-layers under non-punch through drift region
conditions.
4.3.2 Experimental Methods
The starting substrates were n+ (1019 cm-3) 4H-SiC. Approximately 50 µm of n-type
(~2 x 1015 cm-3) and 1 µm of p-type (p ~ 1017 cm-3) 4H-SiC was grown on these
substrates by vapor phase epitaxy, followed by ~500 Å of thermal SiO2. A full area-back
75
Figure 4-7 Schottky rectifier.
76
0.0 0.5 1.0 1.5 2.0
0
20
40
60
80 SiC Schottky Diode(p-type edge termination)SiO2: 3000 ÅDiode Area: 0.64 mm2
Cur
rent
Den
sity
(A/c
m2 )
Bias Voltage (V)
Figure 4-8 Forward I-V characteristics of 0.64 mm2 rectifier.
77
-800 -700 -600 -500 -400 -300 -200 -100 0
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
oval diode circular diode square diode
SiC Schottky Diodes area:0.04 mm2
JTE:40 µmmetal overlap: 4 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-9 Reverse I-V characteristics of 0.04 mm2 rectifiers with different contact shape.
78
-700 -600 -500 -400 -300 -200 -100 010-11
10-10
10-9
10-8
10-7
10-6
10-5
area: 0.04 mm2
area: 0.16 mm2
area: 0.64 mm2
SiC Schottky Diodes ovalJTE:40 µmmetal overlap: 2 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-10 Reverse I-V characteristics of rectifiers with oval shaped contacts of different
area.
79
-800 -700 -600 -500 -400 -300 -200 -100 0
10-11
10-10
10-9
10-8
10-7
10-6
10-5
metal overlap: 6 µm metal overlap: 4 µm metal overlap: 2 µm
SiC Schottky Diodes (Oval)area:0.04 mm2
JTE:20 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-11 Reverse I-V characteristics of rectifiers with oval shaped contacts (0.04 mm2), as a function of extent of metal overlap.
80
-900 -800 -700 -600 -500 -400 -300 -200 -100 0
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
SiC Schottky Diodes (Oval)area:0.04 mm2
JTE:40 µm
metal overlap: 2 µm metal overlap: 4 µm metal overlap: 6 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-12 Reverse I-V characteristics of rectifiers with oval-shaped contacts (0.04 mm2), as a function of extent of metal overlap.
81
2 3 4 5 6
620
640
660
680
700
720
740
760
780
800
820
SiC Schottky Diodes area:0.04 mm2
JTE: 60 µm JTE: 40 µm JTE: 20 µm
Brea
kdow
n Vo
ltage
(V)
Metal Overlap (µm)
Figure 4-13 Variation of VB with extent of metal overlap for different JTE lengths.
82
contact of e-beam deposited Ni annealed at 970 0C for 3 min (contact resistivity of 1.2 x
10-6 Ωcm2), was used as the ohmic contact to the substrate. SiO2 layers ~ 3000 Å were
deposited by plasma enhanced chemical vapor deposition (PECVD) and were employed
as part of dielectric overlap edge termination. Mesas were formed by dry etching. Holes
were also opened in the SiO2 stack by a combination of wet and dry etching and Schottky
contacts of e-beam evaporated (1000 Å thick) Ni patterned by lift-off. The contact area
was varied from 0.01-0.36 mm2, while the mesa length was varied from 0-60 µm and the
extent of metal overlap from 0-20 µm. All current-voltage (I-V) measurements were
carried out at 25 0C in air.
4.3.3 Results and Discussion
Figure 4-14 shows reverse I-V characteristics as a function of rectifier active area
for a fixed mesa length of 20 µm and a metal overlap distance of 4 µm. The reverse
breakdown voltage shows a trend of decreasing as the area increases. This is most likely a
result of the greater probability for having defects such as micropipes within the active
region of the rectifier as the area increases. Therefore, our devices still have breakdown
characteristics dominated by defect density rather than bulk breakdown mechanisms. For
the smallest rectifiers the best on-state resistance (RON) obtained was ~ 20 mΩcm2,
leading to a maximum power figure-of-merit VB2/RON of 84.5 MW.cm-2 This is roughly a
factor of 6 higher than we reported previously for 4H-SiC p-i-n rectifiers fabricated on
thinner (~ 12 µm) n-layers [136].
The dependence of VB on diode area is shown in Figure 4-15 for fixed mesa length
of 20 µm and metal overlap distance of 4 µm. The fall-off in VB with diode area is most
pronounced at small areas (0.01-0.04 mm2) and most likely defines the region where
83
there is a probability of having either zero or one “killer-defect” within the device.
Compared to the theoretical best performance for 4H-SiC rectifiers of this structure, there
is still considerable improvement possible in both VB and RON [84].
The reverse I-V characteristics as a function of metal overlap distance for fixed
mesa extension length of 0 µm and fixed diode area of 0.04 mm2 are shown in Figure 4-
16. VB goes through a maximum with increasing metal overlap distance, which is a
somewhat different trend than reported previously where it continuously decreased as the
extension distance increased [136]. However, a difference in that case was the mesa
length was fixed at 60 µm [136]. In the present case, the trend may result from onset of
breakdown in the oxide or extension of the depletion region to include defects that initiate
breakdown.
Figure 4-17 shows the variation of VB with metal overlap distance for the same
diodes whose I-V characteristics are shown in Figure 4-16. It is clear that this parameter
has a strong influence on the breakdown voltage, with VB ranging from a low of ~ -800 V
to a maximum of ~ -1200 V depending on metal overlap distance.
The reverse I-V characteristics from rectifiers with fixed area and extent of metal
overlap are shown in Figure 4-18 as a function of mesa extension length. Over a wide
range of voltage, the reverse current density was found to vary as the square root of the
reverse voltage, which is characteristic of space-charge generation currents. The relation
between generation current Jo and effective generation lifetime ( τG) is given by:
5.05.0 )()2( Vn
eNeJ
G
i
BG τ
ε=
84
where e is the electronic charge, ε the permittivity of 4H-SiC, NB the drift layer doping
concentration, ni the extrinsic carrier concentration at the measurement temperature and
V is the applied reverse bias [87]. Given a typical value of τG =100 ns [87], this relation
yields a calculated generation current much lower than the measured value, suggesting
that other mechanisms such as surface leakage is playing a role. The larger area devices
should lower reverse current densities, indicating that leakage around the contact
periphery is the main contributor, as reported previously [87]. Figure 4-19 shows the
variation of VB with mesa length, extracted from the data of Figure 4-18. There is no
clear dependence of reverse breakdown on this parameter under our conditions.
Finally, Figure 4-20 shows a typical forward I-V characteristic from a small area
rectifier. The I-V characteristic is well represented by the relation:
)()(21 kTneV
DkTn
eV
R eIeII +=
where IR and ID are the saturation currents for recombination and diffusion current
mechanisms, respectively, e is the electronic charge, k is the Boltzmann’s constant, V the
applied voltage, T the absolute temperature and n1 and n2 are the ideality factors for
recombination and diffusion currents, respectively. At low currents, n ~ 2, indicating that
the dominant transport mechanism is recombination. At higher currents, n ~ 1-1.5,
indicating that the dominant mechanism is diffusion. A typical on/off current ratio was ~
104 at +3.5V/-1000V.
4.3.4 Summary and Conclusions
In conclusion mesa edge termination of p-n 4H-SiC rectifiers passivated with SiO2
is simple alternative to use of implanted junction termination extension (JTE) methods.
The mesa extension length did not have a significant impact on the reverse breakdown
85
voltage of our devices, whereas increasing metal overlap distance tended to decrease VB.
On/off current ratios >105 were achieved, with VF values ~25% higher than the
theoretical minimum.
In another study, 4H-SiC rectifiers were fabricated with epi JTE regions and
dielectric overlap for edge termination. Contact areas from 0.04-0.64 mm2 were
examined, with the reverse breakdown voltage being inversely dependent on contact area.
Oval and circular diodes showed higher VB than square contacts of the same area. The
metal overlap extent did not have a strong influence on VB for JTE lengths up to 40 µm.
4H-SiC p-i-n rectifiers with VB ~ -1000 V were also demonstrated. The role of
device area, mesa length and metal overlap distance on VB were examined. The
breakdown voltage was inversely dependent on device area but was not a function of
mesa length. On/off current ratios of ~ 104 were achieved at 3.5/-1000V. VB was a
maximum at ~ 5 µm metal overlap distance.
86
-1000 -800 -600 -400 -20010-11
10-9
10-7
10-5
area: 0.36 mm2
area: 0.16 mm2
area: 0.04 mm2
area: 0.01 mm2
SiC p-i-n diodesMesa length: 20 µmMetal overlap: 4 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-14 Reverse I-V characteristics of p-i-n rectifiers as a function of active area.
87
0.0 0.1 0.2 0.3 0.40
200
400
600
800
1000
1200
1400
SiC p-i-n diodesMesa length: 20 µmMetal Overlap: 4 µm
VB (-
V)
Diode Area (mm2)
Figure 4-15 VB as a function of diode area for p-i-n rectifiers with mesa length 20 µm and
metal overlap 4 µm.
88
-1400 -1200 -1000 -800 -600 -400 -20010-11
10-9
10-7
10-5
Metal overlap: 0µm Metal overlap: 2µm Metal overlap: 5µm Metal overlap: 10µm Metal overlap: 20µm
SiC p-i-n diodesDiode area: 0.04 mm2
Mesa length: 0 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-16 Reverse I-V characteristics of p-i-n rectifiers as a function of metal overlap
distance.
89
0 5 10 15 200
200
400
600
800
1000
1200
1400
SiC p-i-n diodesDiode area: 0.04 mm2 Mesa length: 0 µm
VB (-
V)
Metal Overlap (µm)
Figure 4-17 VB as a function of metal overlap length for p-i-n rectifiers with area 0.04
mm2 and zero mesa length.
90
-1200 -1000 -800 -600 -400 -20010-11
10-9
10-7
10-5
Mesa length: 0µm Mesa length: 20µm Mesa length: 40µm Mesa length: 60µm
SiC p-i-n diodeDiode area: 0.04 mm2
Metal overlap: 0 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-18 Reverse I-V characteristics of p-i-n rectifiers as a function of mesa length.
91
0 10 20 30 40 50 600
200
400
600
800
1000
1200
1400
SiC p-i-n diodesDiode area: 0.04 mm2
Metal overlap: 0 µm
VB (-
V)
Mesa Length (µm)
Figure 4-19 VB as a function of mesa length for p-i-n rectifiers with area 0.04 mm2 and
zero metal overlap length.
92
0 1 2 3 4 5
0.0
6.0x10-4
1.2x10-3
1.8x10-3
2.4x10-3
3.0x10-3
3.6x10-3
SiC p-i-n diodeDiode area: 0.04 mm2
Mesa length: 40 µmMetal overlap: 5 µm
Cur
rent
(A)
Bias Voltage (V)
Figure 4-20 Forward I-V characteristics.
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BIOGRAPHICAL SKETCH
Saurav Nigam was born April 13th, 1978, in the small village of Rewa in central
India. His family moved to Bhopal in 1981 and he started his schooling at Campion
School, Bhopal, where he spent his fourteen years before graduating from high school in
1996.
In 1997, he began his college at the Indian Institute of Technology (IIT), Roorkee,
one of the premier institutions in Asia. He graduated from IIT in May 2001.During the
undergraduate studies he conducted research in the area of biochemical engineering in
collaboration with Dr. Subhash Chand, professor, IIT-Delhi. He spent the summer of
1999 at the Department of Biotechnology and Biochemical Engineering at IIT-Delhi
conducting experiments on “Lipase Catalyzed Esterification Reactions in Supercritical
CO2.”
It was his strong interest in the courses on reaction engineering, material science
and its application and electrical engineering that led to his ever-increasing interest in
microelectronics. He enrolled at the University of Florida in the Department of Chemical
Engineering in August 2001. In the spring of 2002 he joined Prof. Fan Ren’s research
group to work on state-of-art compound semiconductor devices. He has worked on
silicon carbide based devices focusing on Schottky and P-i-N rectifiers for high power
electronics. He has also published more than ten papers in reputable refereed journals and
symposium proceedings during his graduate and undergraduate research.
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