i

HOMOEPITAXIAL DEPOSITION OF BORON-DOPED SINGLE CRYSTAL

DIAMOND

by

SUNIL KUMAR LAL KARNA

Y. K. VOHRA, COMMITTEE CHAIR

E. KHARLAMPIEVA

J. G. HARRISON

S. A. CATLEDGE

S. PILLAY

A DISSERTATION

Submitted to the graduate faculty of the University of Alabama at Birmingham,

in partial fulfillment of the requirements of the degree of

Doctor of Philosophy

BIRMINGHAM, ALABAMA

2013

ii

HOMOEPITAXIAL DEPOSITION OF BORON-DOPED SINGLE CRYSTAL DIAMOND

SUNIL KUMAR LAL KARNA

DEPARTMENT OF PHYSICS

ABSTRACT

The boron-doped single crystal diamond films were grown homoepitaxially on synthetic

(100) Type Ib diamond substrates using a microwave plasma assisted chemical vapor deposition.

The optical transmittance of the films was observed to change with the increasing boron content

in the film. The effect of boron and nitrogen on the surface morphology of the film has been

studied using atomic force microscopy. Use of nitrogen in process gas during boron doping

improves the surface topography as well as gives rise to an increase in growth rate of diamond

film. However, presence of nitrogen in the process gas significantly lowers the electrical

conductivity of the film. Raman spectra showed a few additional bands at the lower wavenumber

regions along with the zone center optical phonon mode for doped diamond. The change in the

peak profile of the zone center optical phonon mode and its downshift were observed with the

increasing boron content in the film. The sharpening and increase in intensity of the Raman line

has been also observed in boron doped diamond film when grown in the presence of nitrogen.

Temperature dependent electrical measurement between 90 to 680 K indicates two different

conduction mechanisms were responsible for the semiconducting behavior of the film. The

observed growth rate for homoepitaxial boron-doped diamond films were in the range of

5-16 µm / hour. Various level of boron doping (1018

to 1020

cm-3

) was achieved during this study.

The lowest resistivity of one of the boron doped samples at room temperature was calculated to

be 0.12 Ωcm. The potential of boron-doped single crystal diamond in electronic devices is

discussed.

Keywords: Epitaxial, Semiconductor, Diamond, Spectroscopy, Thin-film, Resistivity

iii

To my wife

iv

ACKNOWLEDGEMENTS

The Department of Physics, University of Alabama at Birmingham is a very

pleasant place to work in, and it is my pleasure to thank the department for providing me

an opportunity to pursue my career in physics. I am also very grateful to the graduate

assistantship fellowship program (GAFP), without its support this work would have been

very difficult to me. I am also very thankful to the department administrators and their

friendly assistants. Thank you Mark J. Case, Amanda J. Holt and Jerry Sewell.

Now, my sincere gratitude goes to my advisor, Prof. Y. K. Vohra for his

invaluable guidance, editorial and technical suggestions during the study. Along with my

advisor, I would like to thank Drs. S. Pillay, E. Kharlampieva, J. G. Harrison, and S. A.

Catledge for serving on my research committee. Special thanks go to Dr. S. A. Catledge,

who taught me to operate and maintain many project related instruments. I am thankful to

Drs. A. Stanishevsky and J. Dashdorje, D. Martyshkin, G. Tsoi and M. E. Zvanut for their

help rendered during this study. I would like to thank Dr. Samuel. T. Weir at Lawrence

Livermore National Lab, with whom I have had a good discussion on the project results. I

also thank him for reading and commenting on the manuscripts. I am also very grateful to

Dr. Patric Kung, University of Alabama, Tuscaloosa, for his help on electrical

characterization of samples. I would like to thank all the people who have helped and

inspired me during this study. I am very grateful for all the support of my colleagues and

friends who assisted me directly or indirectly in this work.

I would like to express my deepest thanks to my parents for their help and

support. Last but not least, my thanks go to my beloved wife Pushpa Kantha and lovely

daughters Priya and Roma for their love and patience during this study. They have also

been sources of strength and inspiration to work even during difficult times.

v

TABLE OF CONTENTS

Page

ABSTRACT ........................................................................................................................ ii

DEDICATION ................................................................................................................... iii

ACKNOWLEDGEMENTS ............................................................................................... iv

LIST OF TABLES ........................................................................................................... viii

LIST OF FIGURES ........................................................................................................... ix

LIST OF ABBREVIATIONS ........................................................................................... xii

CHAPTER 1 ........................................................................................................................1

1.0 INTRODUCTION ..................................................................................................1

1.1 Crystal Structure .....................................................................................................2

1.2 Synthesis of Diamond ............................................................................................3

1.2 (a) HPHT Method ..................................................................................................4

1.2 (b) CVD Method ....................................................................................................5

1.3 Microwave Plasma Chemical Vapor Deposition (MPCVD) .................................6

1.4 Homoepitaxial Growth...........................................................................................7

1.5 Twins......................................................................................................................9

1.6 Defects, Doping and Compensation ....................................................................10

1.7 Conduction in Semiconductors ............................................................................14

1.7.1 Hopping Conduction ....................................................................................16

vi

CHAPTER 2 ......................................................................................................................21

2.0 RESEARCH OBJECTIVES ................................................................................21

CHAPTER 3 ......................................................................................................................24

3.0 EXPERIMENTAL DETAILS .............................................................................24

3.1 Synthesis of Homoepitaxial Single Crystal Diamond ............................................24

3.2 Synthesis of Boron-Doped Homoepitaxial Single Crystal Diamond .....................25

3.3 Calculation of Boron to Carbon Ratio in Gas Phase (B/C)gas .................................25

3.4 Cleaning of Films after Deposition .........................................................................26

3.5 Characterizations.....................................................................................................28

3.5.1 Optical Transmission Spectroscopy .................................................................28

3.5.2 Optical Emission Spectroscopy (OES) ............................................................28

3.5.3 Optical Microscopy (OM)..............................................................................30

3.5.4 Atomic Force Microscopy (AFM) .................................................................30

3.5.5 Raman Spectroscopy ......................................................................................31

3.5.6 Single Crystal X-ray Diffraction (XRD) and Rocking Curve Experiment ....31

3.5.7 Fourier Transform Infra-Red spectroscopy ...................................................32

3.5.8 Diamond Surface and Metallic Contact .........................................................33

3.5.9 Four Point Probe and Hall Measurements .....................................................35

CHAPTER 4 ......................................................................................................................36

4.0 RESULTS AND DISCUSSION .........................................................................36

4.1 Optical Transmission Spectra ................................................................................36

4.2 Optical Emission Spectra .......................................................................................37

4.3 Optical Microscopy Images ...................................................................................40

vii

4.4 Atomic Force Microscopy Images .........................................................................44

4.5 X-ray Diffraction Pattern and Rocking Curve .......................................................48

4.6 Raman Spectra .......................................................................................................49

4.7 Fourier Transform Infrared Spectra .......................................................................52

4.8 Electrical Conductivity Measurements ..................................................................54

CHAPTER 5 ......................................................................................................................59

5.0 CONCLUSIONS ................................................................................................59

CHAPTER 6 ......................................................................................................................61

6.0 FUTURE WORK .................................................................................................61

REFERENCES .................................................................................................................62

viii

LIST OF TABLES

Table Page

3.1. Overview of experimental growth conditions (Set-I), growth rate (r)

and boron concentrations (calculated from four point probe

measurement data above transition temperature) in the grown

diamond films. Total feed gas flow rate was 400 sccm. .........................................27

3.2. Overview of experimental growth conditions (Set-II), growth rate (r)

and boron concentrations (calculated from four point probe

measurement data above transition temperature) in the grown

diamond films. Total feed gas flow rate was 400 sccm. .........................................27

3.3 Observed species in OES of H2/CH4/B2H6/N2/O2 plasma. ....................................29

3.4 Electrical nature of ideal MS contacts. ...................................................................34

4.1 Activation energies of doped films measured in four point probe experiment. 56

ix

LIST OF FIGURES

Figure Page

1.1 Unit cell of diamond, where ao is the cubic lattice constant and d is the

C-C bond length of 0.154 nm. The four thick lines represent

tetrahedral geometry. .................................................................................................3

1.2 (a) Schematic of AsTex type MPCVD system. The deposition chamber is

enclosed in a cylindrical Faraday cage. ...................................................................8

1.2 (b) Microwave power reactor used in this study to deposit boron doped

semiconductor diamond films. Three main units of the system are circled

as vacuum chamber, control unit, and microwave generator. ..................................9

1.3 (a) Penetration twins, and (b) Contact twins. .........................................................10

1.4 Diamond crystals with (a) Boron and (b) presence of defect in the form

of nitrogen–vacancy center in its lattice. The spheres green, red, and pink

represent boron atom, nitrogen atom, and vacancy in carbon lattice .....................11

1.5 Energy level diagram of a substitutional boron in diamond.

EA is an activation energy of boron atoms in diamond lattice and EG is an

energy band gap of diamond. ..................................................................................13

1.6 Schematics of excitations of carriers in a compensated p -type semiconductor. ....16

1.7 Schematics of carrier-hopping mechanism in localized states of a

semiconductor. Here Ef, Ej and Ei are the energy of a Fermi level,

an empty and a filled state respectively. .................................................................18

2.1 Design of water cooling stages (a) stainless steel stage, (b) copper stage,

and (c) copper cap over cooling drum. ...................................................................22

2.2 Design of substrate holders with shallow cavity (0.7mm deep and

4.5mm square shape) (a) narrow holder (diameter 12mm, length 25mm),

and (b) wide holder (diameter 22 mm, length 25mm). ...........................................22

3.1 Energy band in metal semiconductor systems before contact (a) sm ,

and (b) sm ; metal-semiconductor junction after contact

(c) sm , and (d) sm . .................................................................................34

x

4.1 Optical transmission spectra of seed crystal (Seed) and doped diamond

films after applying correction for the absorption in the diamond

seeed crystal. Photographs of seed crystal (Seed) and doped diamond (BD6)

are embedded inside the graph. Yellow color in seed is due to

substitutional nitrogen atom in diamond lattice, and bluish color in BD6

represents boron incorporation in the diamond lattice.. ..........................................37

4.2 Optical emission spectra of H2/CH4/N2 plasma under the pressure, power

and temperature of 80 Torr, 1.5 kw and 1100oC respectively (red color)

and H2/CH4/B2H6 plasma under the pressure, power and temperature of

100 Torr, 1.5 kW and 1100oC respectively (green color). .......................................39

4.3 Optical Emission Spectroscopy (OES) results from the plasma during the

diamond deposition process. The spectra shown were obtained for various

levels of nitrogen in the plasma at a fixed concentration of diborane.. ...................39

4.4 Optical microscopy images of substrate (Seed Crystal), undoped sample (HD1)

and boron doped diamond samples (BD1, BD2, BD3, and BD4) showing

surface morphological change on epitaxial deposition.. .........................................43

4.5 Optical microscopy images of homoepitaxially deposited diamond

samples showing change in surface morphology with boron concentration

in feed gas. Film BD8 and BD13 was deposited with additional

introduction of nitrogen in feed gas (table 3.2).. .....................................................43

4.6 Histogram of growth rate of boron doped diamond as a function of B/C

ratio in gas phase. All the deposition took place at 1100oC, 1.4 kW

microwave power and about 100 Torr chamber pressure. The amount of

diborane used for BD7, BD8 and BD13 growth was same. The amount of

CH4 gas was described in table 3.2. ........................................................................44

4.7 AFM images of seed crystal (seed), the magnified view at top right corner

shows the polishing scratches and the carbon debris on the substrate before

any CVD treatment, undoped sample HD1 and boron doped single

crystal diamond samples (BD1, BD2, BD3, and BD4). The deposition time

for this study was 8 hours.. ......................................................................................45

4.8 AFM images of homoepitaxial sample (HD2), boron doped single

crystal diamond samples (BD5, BD6, BD7, BD8 and BD13). AFM images

of boron doped diamond samples (BD11) and (BD12) grown at 16 sccm

and 32 sccm of methane. AFM images of BD9 and BD10 are not shown

here. The deposition time was 5 hours.. ..................................................................47

4.9 XRD pattern of as received seed crystal (Seed), undoped film (HD)

and boron doped diamond film (BD). The inset is the rocking curve

for boron doped diamond film. ...............................................................................49

xi

4.10 (a) Raman spectra of Set-I of samples undoped film HD1, boron doped films

BD1, BD2 and BD3 and as received seed crystal (Seed). ......................................51

4.10 (b) Raman spectra of Set-II of samples of undoped film HD2, boron doped films

BD5, BD6, BD7 and BD8 and as received seed crystal (Seed). Inset is showing

the peak shift of sample BD7 with respect to that of seed. ......................................51

4.10 (c) Raman spectra at different position on sample BD7. Insets are Fano line shape

fitting with Raman curve of samples BD7 and seed crystal to measure

deformation in optical phonon line due to doping. ..................................................52

4.11 FTIR spectra of as received seed crystal (Seed), undoped diamond film HD1 and I

set of boron doped diamond films BD1, BD2, BD3. Inset is the spectra after

applying correction for the transmission of seed crystal, characteristics boron peak

is shown at 1290 cm-1

.. ............................................................................................53

4.12 Tungsten strips on doped diamond for metallic contact. .........................................56

4.13 Electrical conductivity as a function of 1000/Tof (a) Set-I boron doped diamond

films BD1, BD3 and BD4 and (b) Set-II boron doped diamond films BD6,

BD7 and BD8. ........................................................................................................57

4.14 Electrical conductivity as a function of T-0.25

of (a) Set-I boron doped diamond

films BD1, BD3 and BD4 and (b) Set-II boron doped diamond films BD6,

BD7 and BD8. .........................................................................................................58

xii

LIST OF ABBREVIATIONS

UV Ultra Violet

IR Infrared

HPHT High Presure High Temperature

CVD Chemical Vapor Deposition

MPCVD Microwave Power Assisted CVD

HFCVD Hot Filament CVD

NNH Nearest Neighbor Hopping

VRH Variable Range Hopping

sccm Standard Cubic Centimeter per Minute

ppm Parts per Million

OES Optical Emission Spectroscopy

AFM Atomic Force Microscopy

XRD X-ray Diffraction

OM Optical Microscopy

FTIR Fourier Transform IR

HD Homoepitaxial Undoped Diamond

BD Homoepitaxial Boron Doped Diamond

FWHM Full Width at Half Maximum

MWPD Micro Wave Power Density

1

CHAPTER 1

1.0 INTRODUCTION:

Diamond is a crystalline form of sp3-bonded carbon atoms. It is transparent from

deep UV to far IR radiation and owes an ultimate hardness. It has precious gem-like

quality as well as various unique physical and chemical properties. A natural diamond

forms inside the earth’s crust at vary harsh conditions of pressure and temperature and is

carried to the earth’s surface by volcanic eruption through a kimberlitic pipe. On the basis

of amount of substitutional impurities (nitrogen or boron) present in the carbon lattice

diamond is classified as Type Ia, Ib, IIa and IIb (1, 2). Type Ia and Ib contain high

percentage of nitrogen but type IIa and IIb contain almost no nitrogen. A significant

amount of nitrogen is present in Type Ia in the form of nitrogen pairs and Type Ib

contains nitrogen only as a substitutional impurity. Almost no impurity is found in Type

IIa, but due to a small amount of boron in Type IIb it shows a P-type conducting

behavior. Due to outstanding properties endowed by nature, diamond research continues

to gain popularity in science as well as societal interest. However, availability, cost and

flaws in natural diamond compelled scientists to develop methods to synthesize diamond.

At present diamond is being synthesized by two methods, the High Pressure and High

Temperature (HPHT) method and the Chemical Vapor Deposition (CVD) method. The

HPHT synthesis has a limit of producing isolated crystals of few millimeters in size and

may not be able to deposit polycrystalline diamond films or coatings over large areas.

The HPHT diamond also contains some impurities from catalytic elements like Fe, Ni,

2

and Co. On the other hand, various shapes, sizes and thickness from polycrystalline to

mono-crystalline diamond films can be grown by CVD. The major advantage of the CVD

synthesis is that the properties of diamond can be tuned easily during the deposition and

researchers have control over quality and defects. Various levels of doping in CVD

diamond opens up the possibility of its exploitation in many future electronics and

electrochemical devices (3).

The properties of diamond such as high thermal conductivity, biocompatibility,

chemical inertness and low coefficient of friction can be utilized in many tribological

applications such as cutting tools, heat sinks, and implant materials (4). Its high refractive

index, high dispersive power, and high transparency are being enjoyed in jewelry and

optical windows (5). The other excellent properties of diamond such as wide band gap,

low dielectric constant, high resistivity, and the radiation hardness lead it to exploit in the

semiconductor industries. Low capacitance of diamond due to its low dielectric constant

can be utilized in fast switching devices. Its highest carrier mobility, high saturation

velocity and doping capability makes it an outstanding candidate for high power/high

frequency electronic devices (4, 6).

1.1 Crystal Structure:

Diamond is a solid crystalline form of carbon atoms in which four sp3-hybridized

C atoms are bonded covalently in the tetrahedral geometry, as shown in Figure 1.1. The

diamond unit cell consists of two face centered cubic (fcc) lattices, offset from each other

by a quarter of its length along the body diagonal (7). The two base atoms at each lattice

point are placed at (0, 0, 0) and ),,(4

1ooo aaa , where 3567.0oa nm is the lattice

3

constant. The unit cell comprised of 8 atoms and the atomic number density can be

calculated as 233 1076.1/8 oa cm−3

. Each bond length of diamond is a quarter of its

cubic body diagonal that is 154.04/3 oad nm with the high bond energy of 711

kJ/mol. The small size of a carbon atom and the tight covalent bond of carbon atoms are

responsible for almost all of its unique mechanical, chemical and electronic properties.

Figure 1.1 Unit cell of diamond, where ao is the cubic lattice constant and d is the C-C

bond length of 0.154 nm. The four thick lines represent tetrahedral geometry.

1.2 Synthesis of Diamond:

The synthesis of diamond began with imitating the natural thermodynamic and

kinetic process of diamond formation inside the earth’s crust, the so called high pressure

and high temperature (HPHT) method. Later, researchers developed an alternative way to

4

synthesize diamond from vapor phase at relatively low pressure and low temperature, the

so-called chemical vapor deposition (CVD) method. The brief description of the diamond

synthesis has been discussed below.

1.2 (a) HPHT Method:

Diamond is synthesized using HPHT method by two ways: (1) The static

compression method, and (2) The dynamic compression method. In the static

compression method about 8 to 20 GPa of pressure and (1000 - 3000)o

C of temperature

are required. A diamond anvil cell apparatus or a belt type apparatus is used to generate

such a high pressure and an electric current or a laser is used to heat the sample. In

dynamic compression about 7 to 150 GPa pressure and a variable range of temperature

are required. This way of synthesizing diamond transforms graphite directly into diamond

with the application of extreme high pressure and high temperature (8). However, such

kinetic barrier of high pressure and temperature in diamond production can be reduced by

the solvent-catalyst reaction process. Solvent catalysts make the reaction path faster with

much lower activation energy than the direct transformation kinetics. Transition metals

such as Fe, Co, Ni, Cr and Pd are generally used as solvent catalysts. These solvents

dissolve the carbon, break the bonds between individual or groups of carbon atoms and

transport the carbon into growing diamond surface (9-11). The diamond synthesis using

metal catalysts generally operate at a pressure of 5 GPa and a temperature of 1300oC.

5

1.2 (b) CVD Method:

Diamond can be deposited on diamond or non-diamond substrates by using vapor

phase of carbon at very low pressure (< 1 atm) and low temperature (600-1200oC)

condition. At such condition of pressure and temperature graphite is the most stable form

of carbon. However, the environment inside the CVD chamber is so adjusted that it

favors the diamond growth and suppress the graphitic formation. The CVD is a kinetic

controlled process rather than thermodynamics (12). Varieties of CVD systems are

available in the market for diamond deposition. Based upon their activation methods e.g

thermal, electric discharge and combustion flame, they are named as Microwave plasma

CVD, Hot Filament plasma CVD, RF plasmas CVD, DC arc jet CVD and Oxy-Acetylene

torches CVD. All CVD systems require some form of carbon precursor such as

hydrocarbon for efficient growth of diamond (13). Hydrogen plays an essential role on

diamond deposition in CVD process. Hydrogen etches graphite much faster than it does

to diamond; it prevents graphitization by terminating carbon dangling bonds. However,

hydrogen is not absolutely essential for CVD diamond growth. The growth of CVD

diamond in methane/inert gas mixtures have been reported and it is believed that C2

dimer is responsible for the diamond growth, as shown by the following reaction

mechanism (14, 15).

Any material which can withstand deposition temperature and can support nucleation and

growth of diamond can be used as a substrate for polycrystalline CVD diamond. The

substrate surface must have the following qualities for diamond deposition (16-18). The

substrate surface must be free from any catalyst which enhances graphitic formation; it

2CH4 2CH3*

+ H2 C2H2 + 3H2 C2 + 4H2

6

lies at or near the solubility limit for carbon at deposition temperature and possesses low

carbon diffusivity.

In hot filament plasma chemical vapor deposition (HFCVD) feed gasses are

dissociated by thermal energy generated by hot tungsten or tantalum filament. It can be

used for large area deposition with high growth rate but the reproducibility and

contamination of the film remains the main issue due to limited lifetime of filament. In dc

arc jet CVD the reactant gases are activated by electrical discharge (plasma) and is able

to deposit on small areas with high growth rate but the downside is the substrate

experiences very high temperature 1000o-1500

oC during the deposition. In combustion

flame CVD thermal energy from oxy-acetylene torch is used and is able to deposit

diamond at atmospheric pressure with very high growth rate however, the process is

difficult to control because of extreme heat produced by the high thermal plasma jet. In

microwave plasma chemical vapor deposition (MPCVD) microwave energy (electric

discharge) creates plasma to activate the reactant gases in very controllable manner hence

the deposition of diamond is of high quality and scalable over large areas.

1.3 Microwave Plasma Chemical Vapor Deposition (MPCVD):

In this system, microwave reactor generates plasma inside the chamber which

dissociates the hydrogen into hydrogen ions. Hydrogen ions then collide with the other

reactant gasses such as methane and produce active species such as C2, CH3, C2H2, etc.

These species then interact with diamond nuclei on the surface to grow diamond film on

the substrate (19). The schematic and photograph of MPCVD is shown in Figure 1.2 (a)

and 1.2 (b) respectively. The MPCVD is being used to deposit diamond films on various

7

substrates from non-diamond to diamond and from polycrystalline to monocrystalline.

Various micro and nanostructured forms of diamond have unique properties; hence film

properties can be tuned during deposition as per its intended application.

1.4 Homoepitaxial Growth:

Homoepitaxy is the process of growing monocrystalline films of the one material

on top of substrate of the same material. Since substrate acts as a seed crystal the

deposited film takes the same orientation and lattice structure as that of the substrate. The

homoepitaxy grows the films of high purity than the substrate itself and can deposit films

with different doping levels (20, 21). It is believed that the epitaxial growth mechanism is

based on the vapor-solid interaction. The surface mobility of the atoms, lattice misfit

between substrate and epitaxial layer, substrate electronic influence, super saturation of

the vapor and the competition between the surface energy and the interface energy are

believed to be responsible for the epitaxial growth (22, 23). Since the surface energy of

the substrate and the film is same in homoepitaxy and the interface does not play a

significant role in the growth process, the surface mobility of atoms is believed to be

mainly responsible for the homoepitaxial growth. Atoms with high surface mobility

would migrate on the substrate surface until they are hindered by the step. Those

migrating atoms when meet together form a cluster. A cluster can either decay to atoms

by breaking away or grows as nucleus when gather together. An atom with low surface

mobility would hit the substrate surface and stick to the site where it first hits (23). The

homoepitaxial growth may therefore proceed as a step-flow mechanism. The carbon

atoms that is being adsorbed from the gas phase onto the substrate surface are

8

incorporated into the growing crystal at the kinks of atomic surface steps. When the

carbon concentration in the gas phase exceeds the ability of the surface steps to

incorporate them, abnormal nucleation occurs on the surface terraces and results a non-

epitaxial growth in the form of hillocks or twins (24).

Fig. 1.2 (a) Schematic of AsTex type MPCVD system. The deposition chamber is

enclosed in a cylindrical Faraday cage.

9

Control unit

Vacuum Chamber

Pyrometer

Microwave generator unitAntenna

Fig. 1.2 (b) Microwave power reactor used in this study to deposit boron doped

semiconductor diamond films. Three main units of the system are circled as vacuum

chamber, control unit, and microwave generator.

1.5 Twins:

Twins are the symmetrical intergrowth of same crystal species. They are formed

through the process of growth, transformation and deformation (25). It obeys reflection,

rotational, and inversion symmetry. Reflection across a mirror plane defines the twins

plane, rotation about an axis of crystal defines the twins axis, inversion through a point

defines the twins center (26). There are two types of twins found in a crystal called

penetration twins and contact twins. In penetration twins the compositional surfaces

penetrate each other perpendicularly and in contact twins the compositional surfaces are

parallel. Examples of twins are given in Figure 1.3. The lattice points are shared along the

compositional surface in the twinned crystal.

10

(a)

(b)

Fig. 1.3 (a) Penetration twins, and (b) Contact twins.

1.6 Defects, Doping and Compensation:

A defect is an imperfection in the microscopic arrangement of atoms in a crystal.

Presence of defect in a crystal disturbs the crystal symmetry and hence changes the

material properties. It may be in the form of disorder, dislocation, impurities, vacancies

and grain boundaries. Presence of defect like nitrogen or boron in diamond lattice

changes its optical property. Yellow color of a diamond indicates presence of

substitutional nitrogen and blue color of diamond represents presence of substitutional

boron in a diamond lattice. However, a defect is not always detrimental to the material

property. For example, doping is essential for electronic (p-type or n-type) properties of a

diamond. A defect in the form of nitrogen-vacancy center in diamond acts as a single

photon source and retains the quantum information called qubit, is useful for quantum

computation (27). The crystal structure of diamond with impurity doping and vacancies is

shown in Figure 1.4.

It has already been observed that the level of doping can change a diamond from

an insulator to a conductor and then to a superconductor (3). However, a high quality

11

(a)

(b)

Fig. 1.4 Diamond crystals with (a) Boron and (b) presence of defect in the form of

nitrogen–vacancy center in its lattice. The spheres green, red, and pink represent boron

atom, nitrogen atom, and vacancy in carbon lattice.

extrinsic or intrinsic single crystal diamond is required for fabricating any diamond-based

device. The electrical properties of diamond can be enhanced by fabricating single

crystal doped diamond. Polycrystalline diamond limits the electrical properties of

diamond due to grain boundary scattering of the carriers. Intrinsic diamond is an insulator

at room temperature but by doping with a suitable impurity it can be turned into a

semiconductor. Boron, a natural impurity of Type IIb diamond can be doped into a

diamond lattice to give synthetic diamond a p-type behavior. The n-type behavior in

diamond can be achieved by phosphorus or nitrogen incorporation in its lattice. However,

a suitable n-type donor dopant is still a topic of intense research activity.

A dopant is chosen in such a way that the activation energy lies near k T where k

is the Boltzman constant ( 510617.8 k eV/K) and T is the temperature. The dopant

which has low activation energy is called shallow dopant and its level lies near the

conduction or the valance band edge in the band gap. However, a deep level dopant needs

12

high activation energy and its level lies far from conduction or valance band edge in the

band gap. The basic need to realize diamond electronics depends on the success of

doping shallow dopants at the substitutional site in a diamond lattice. To make a device

work efficiently these levels should be shallow enough to assure dopant activation at

room temperature, where kT = 0.025 eV. A low boron concentration at substitutional

sites creates an acceptor level with an activation energy of 0.37 eV. At such high

activation energy of boron in diamond, only 0.2 % holes are activated at room

temperature (27). At higher temperatures, the device conductivity increases but the

mobility of holes decreases. The increasing phonon concentration with the rise of

temperature creates more scattering centers in a crystal and reduces carriers mobility.

While higher boron concentration (~1021

cm-3

) shows metallic properties due to the

broadening of the acceptor level into a band (28-30). A superconducting behavior in

heavily boron-doped diamond below 11 K has also been reported and it seems to be

related to the strong electron-phonon coupling in a diamond lattice (31, 32).

Diborane (B2H6), Boron Nitride (BN), Boron Alkoxide B(OR)3 [R = alkyl group]

and Trimethylboron, TMB [B(C2H5)3] are mostly used precursors for boron (B) doping in

CVD diamond (31, 33-35). Instead of Boron (B) diamond thin films can also be made

conductive by substitutional doping with some other hetro-atoms (like Al, N, P, S, Si, Ni,

As, Sb, Cr, Ni, Fe, W, Mo, etc.) with slightly different electronic configurations in the

diamond lattice leading to the formation of new energy levels in the band structure that

facilitate both p- and n-type conduction depending on the type of dopant (36). Nitrogen

(N) and phosphorous (P) doping in a diamond form a deep level at 1.7eV and 0.52eV

respectively below the conduction band minimum leading to n-type conduction. Energy

13

band diagram of boron atom is shown in Figure 1.5. The electrical properties of diamond

depend on the presence of dopants or residual impurities. The presence of deep-level

impurities in doped semiconductors trapped the charge carriers when they are emitted to

the conduction or valance band and consequently reduce the free carrier concentrations.

That is the donor electron will drop to the acceptor level and will not be available as a

current carrier (37). This effect is called the compensating effect. The ratio between

dopant and compensating defect concentration should be kept at minimum in order to

produce n- or p- type material. In p-type semiconductors the ratio of concentration of

compensating centers (donors), ND to the concentration of dopant (acceptors), NA is

called compensation, K (i.e. K = ND/NA). If K = 1, then material behaves as intrinsic in

nature which will have useful applications. Vacancy and other defects in a crystal act as

compensating centers for a doped semiconductor (38, 39). In p –type diamond defects

(vacancy or nitrogen) acts as donor centers.

Fig. 1.5 Energy level diagram of a sbstitutional boron in diamond. EA is an activation

energy of boron atoms in diamond lattice and EG is an energy band gap of diamond.

14

1.7 Conduction in Semiconductors:

Semiconductor is a material whose resistivity lies between that of a conductor and

an insulator. The conductivity of semiconductor can vary by several orders of magnitude

by introducing a suitable foreign element. Semiconductors can be an elemental material

like Si and Ge or a compound material like SiC, GaN, GaAs, AlN, and InP or an alloy

like Si1-xGex, Al1-xGaxAs, Hg1-xCdxTe. Diamond, however comprises of sp3 bonded group

IV element carbon and has a tetrahedral atomic structure like Si or Ge but it is not

considered as a semiconductor, because it does not have any free charge carrier to

conduct electricity. A semiconductor establishes its conduction properties through a

complex quantum mechanical behavior within a crystalline structure. The periodic nature

of crystalline lattice possesses energy bands in semiconductor and bands have a gap in

allowed energy states which are responsible for conduction. Bloch’s theorem explains the

detailed understanding of band structure (40). The Hamiltonian of the system with its

electronic eigenstates gives a basis for understanding electronic conduction in a material.

In a crystalline material the eigenstates, )(r are spatially extended Bloch states (41).

Due to the extended nature of the Bloch states, electrons with equal energy have an equal

probability2

)(r to be in every unit cell in space and are thus extended. The real crystal

is assumed to be weakly disordered due to defects and therefore conductivity is

dependent on the scattering of carriers from defects in the lattice. The perturbative

approach or percolation network theory is used to calculate conduction in such a crystal

(41). The band conduction and hopping conduction are the charge transport mechanism

in semiconductors. The schematics of carrier excitation in bands are shown in Figure 1.6.

15

Thermal holes and electrons are created in a semiconductor when electrons

undergo electronic transition from the valence band to the conduction band at sufficiently

high temperature. This is also called an intrinsic conduction of semiconductor. Wide band

gap semiconductors cannot have such conduction. The defects in semiconductor always

take compensating position i.e. defect has a p-type effect if semiconductor is an n-type

material and vice versa. Electron has a tendency to stay in lowest possible energy states

in a material. Hence electrons from a compensating donor level drop off to the acceptor

levels and produce ionized acceptors which are filled state of holes. At high enough

temperature some of the acceptors are also occupied by the excited electrons from

valance band and form acceptors holes in a valence band and ionized acceptors in

acceptor levels. The conduction due to the transition of electrons from a valence band to

an impurity band or vice-versa is called the band conduction in semiconductors. As the

temperature is reduced electrons from ionized (filled) acceptors begin to go back to the

acceptor holes in the valence band and give rise to band conduction. With the further

decrease of temperature all the electrons recombine their holes in a valence band and

produce saturation effect for conduction. At this point no change in conductivity is

recorded with some span of temperatures. If the temperature is reduced again further

there are only empty (unoccupied) acceptors in acceptors levels except for some ionized

acceptors due to compensating donors. Hence holes begin to hop from a filled impurity

band to an empty impurity band within the band gap. Such transition of carriers gives

hopping conduction. The details of hopping conduction are described below.

16

Fig. 1.6 Schematics of excitations of carriers in a compensated p-type semiconductor.

1.7.1 Hopping Conduction:

Hopping conduction is a phonon assisted quantum mechanical tunneling of charge

carriers. In a doped semiconductor charge carriers are located in localized states around

the impurities at low temperature. When charge carriers of a material jump (hop) in these

localized states of different energy, electric conduction occurs. Hopping nature of charge

carriers in localized states can be determined by the hopping probability (42-44).

Hopping probability ( hopP ) is the product of a tunneling factor )/2( R and a Boltzmann

factor )/( TkE Bij . R is a hopping distance and is the localization length (impurity

radius) that characterize the extension of wave functions in space. ijE is the energy

difference between sub states i and j. If the electronic wavefunctions of the two localized

states overlap, hopping takes place in the form of direct tunneling between the states. On

17

the other hand, when carriers trapped in localized states absorb a phonon to jump to the

next available site then thermally assisted hopping is taking place. Schematic of carriers

hopping from a filled site to an empty site by the absorption of a phonon is illustrated in

Figure 1.7. If 0 ijij EEE , The probability of a carrier to hop from a filled site to

any of the empty site (A, B or C) by the absorption of a phonon is given by the equation

]2

exp[Tk

ERP

B

ij

hop

(1),

and if 0 ijij EEE then the probability of hoping is given by equation

)2

exp(

RPhop (2)

In equation (2) TkB makes an important contribution to hop. For localized states, when

)/2()/( RTkE Bij then hopP is maximum for R minimum and conduction is nearest

neighbor hopping (NNH) (45, 46). The number of available states for hopping is small at

this case and Boltzmann factor will decrease the total hopping probability. The

conductivity at this point can be determined as follows:

The diffusion coefficient for hopping is

21RP

fD hophop (3)

Where f is the co-ordination number and for diamond 6f .

The conductivity is given by

TkEeENne Bf )()( (4)

18

Fig. 1.7 Schematics of carrier hopping mechanism in localized states (impurity band) of a

semiconductor. Here Ef, Ej and Ei are the energy of a Fermi level, an empty and a filled

state respectively.

Here, TkENn Bf )( , is the number of carriers per unit volume taking part in hopping

and have energy E such that TkEE Bf . The density of states )( fEN at the Fermi

level is given by

])(4[

33REN

Ef

ij

(5)

The mobility from the Einstein’s relation is given as

Tk

eD

B

hop

(6)

Hence the conductivity can be written as

]2

exp[)(6

1 22

Tk

ERRewEN

B

ij

of

Or, ]2

exp[)4

3(

6

1 2

Tk

ER

RE

ew

B

ij

ij

o

(7)

19

Hence the thermally activated hops can be characterized by

)/exp( 3 TkBo (8)

Equation (8) shows the temperature dependent behavior of hopping conductivity of a

system, ow is the factor depending on phonon frequency, N(E) is the density of state and

o is the pre exponential factor. The activation energy 3 ijE , is necessary for the

carrier to surmount coulomb potential between occupied and unoccupied sites.

If the temperature reduces further then spatial term begins to affect the conductivity

because ijE becomes small and R becomes large. This condition creates large number

of available states for carriers to be hopped i.e. Boltzmann factor enhances the large

hopping probability equation (1). This new regime is called Mott variable range hopping

(VRH). With the decreasing temperature, the hopping probability of both A and B

decreases but )(AP decreases much more then )(BP due to the large energy separation

( ijE ). Therefore, it is more favorable for a carrier to hop at a longer distance to find a

lower energy state (44, 45). In this case TkE Bij / is not negligibly small compared with

/2R , hence hopP is maximum for minimum )/()/2( TkER Bij . In order to

maximize hopP , at low temperature regime the localization centers R (hopping distance)

and energy separation ijE can be expressed by eqn. (5). )(REEij is the energy for

which the number of states in a radius R , with energy )(0 REE .

Now substituting )(RE for ijE in equation (1) we get

)2

exp(3R

aRPhop

(9)

20

Here, fB ENTk

a(4

3

, and the most probable hopping distance R is found by

minimizing the exponential term, i.e.

4

1

)(8

3

fB ETNkR

(10)

And, 4

14

3

)(3

8

4

3)(

f

Bij EN

TkREE

(11)

Substituting equation (11) into equation (7), the expression for conductivity is

4

1

exp

T

Too (12)

Here,

)(

3

fB

oENk

T

(13)

Which is a famous Mott’s 4

1

T VRH law.

21

CHAPTER 2

2.0 RESEARCH OBJECTIVES:

The objectives of this study were

1. To optimize the deposition parameters for the high growth rate and high quality

boron-doped single crystal diamond,

2. To investigate the influence of nitrogen on the growth process of the boron-doped

single crystal diamond, and

3. To understand the basic mechanism of temperature dependent resistivity in boron

doped single crystal diamond.

To achieve these goals, we have designed a new water cooled copper stage which

can be favorable to high growth rate and high quality of diamond. Substrate holder design

is one of the factors for better growth rate, as it directly relates to the substrate

temperature and plasma density. The concentration of microwave field changes according

to the holder shape. The higher the concentration of microwave field, the faster is the

deposition. High power and high pressure in CVD has been demonstrated recently for

high quality and low defect of single crystal diamond (47). In a single crystal diamond

deposition, the main challenges are the process control and the deposition growth rate.

Several designs of substrate holders and stages have been examined during the

experiment to optimize the process control. Some of them are discussed here.

22

80 mm(a) 80 mm

Copper Cap over

cooling drum

(b)

(c)

Fig. 2.1 Design of water cooling stages (a) stainless steel stage, (b) copper stage, and

(c) copper cap over cooling drum.

(a) (b)

Fig. 2.2 Design of substrate holders with shallow cavity (0.7mm deep and 4.5mm square

shape) (a) narrow holder (diameter 12mm, length 25mm), and (b) wide holder (diameter

22 mm, length 25mm).

23

Figure 2.1(a) is the original stainless steel stage for the deposition chamber. Our aim was

to obtain microwave power above 1 kWatt at the deposition temperature of about 900-

1200oC with the combination of 100 Torr chamber pressure. This condition was difficult

to achieve with the original setting of the chamber stage, as the formation of soot begins

at a very early stage of deposition due to inadequate formation of radicals at low power.

Hence, in order to improve this situation we designed a copper stage as shown in Figure

2.1 (b). This design helped us to attain high power at the deposition temperature for

shorter deposition time less than 3 hours, but did not work for longer deposition time

because of overheating of the chamber and the melting of polyethylene water tubing

inside the chamber due to heat radiation. So finally, we made a copper cap which covers

the original stainless steel stage upper cooling jacket, as shown in Figure 2.1(c). This

design and set up with a narrow substrate holder improved the heat sink problem

partially, so we replaced the narrow holder with a wider one and made a shallow cavity

on top of it for the substrate, as shown in Figure 2.2. The results were promising for the

deposition after such a change in the stage and the holder design. In order to investigate

the effect of holder cavity, we tried two different cavity sizes one with 0.7 mm depth and

another with 1.2 mm depth. The films grown with the shallow cavity holders showed

some amount of roughness at the edge of the film. However, the growth rate was

relatively high, so the shallow cavity on the wide holder was chosen for all depositions.

24

CHAPTER 3

3.0 EXPERIMENTAL DETAILS:

A Type Ib (100) oriented HPHT diamond seed crystal of dimensions

3.5 × 3.5 × 1.5 mm3 was used as a substrate in a 6 kW, 2.45 GHz microwave reactor

system MPCVD (48, 49). Diborane diluted in 90% of hydrogen was used as a boron

source gas in this study (30, 50). In particular, 4 to 10% ratio of CH4 /H2, and 0.2% of O2

have been introduced in a total of 400 standard cubic centimeters (sccm) of feed gas. The

microwave power and pressure were varied from 1.35 – 2.0 kW and 95 - 140 Torr

respectively for the substrate temperature range of 900 to 1200oC. The p-type

conductivity of diamond films and their quality were tuned by introducing 0.5 to 3 sccm

of diborane in a feed gas.

3.1 Synthesis of Homoepitaxial Single Crystal Diamond:

To optimize deposition process, we first synthesized few undoped homoepitaxial

diamond films. Two of them are discussed here, the seed was ultrasonicated in acetone

before placing on a shallow cavity of molybdenum holder. The holder was mounted on a

cooling jacket. The first undoped diamond film was synthesized using 8% methane, 0.4

sccm of nitrogen, 0.8 sccm of oxygen and remaining hydrogen respectively. The

deposition temperature was set at 1100oC for 8 hours. The other one was deposited for 5

hours only with 6% CH4/H2 ratio and 0.8 sccm of O2 with the remaining hydrogen.

25

3.2 Synthesis of Boron-Doped Homoepitaxial Single Crystal Diamond:

Two sets of boron doped single crystal diamond films were synthesized

homoepitaxially with varying diborane from 0 to 750 ppm (B/Cgas from 0 to 25000 ppm)

content in a feed gas. Set-I was deposited to optimize temperature and other deposition

parameters like microwave power, chamber pressure and the deposition time. We have

deposited doped diamond films from the substrate temperature range of 900oC to 1200

oC

with the step of 100oC for 8 hours. After the optimization of the temperature, Set-II was

deposited for 5 hours at a fixed temperature of 1100oC. In Set-II, we have first optimized

the flow of diborane in the process gas and then investigate the effect of nitrogen on the

growth process of boron doping. Some of the samples in Set-II were grown in presence of

0.2% oxygen. The overview of experimental growth conditions for both the set of

depositions was summarized in Table 3.1 and 3.2.

3.3 Calculation of Boron to Carbon Ratio in Gas Phase (B/C)gas:

The dilution of diborane was 10000 ppm in hydrogen (10% in H2).Taking an

example of 6% methane in a feed gas, the B/C ratio in gas phase was calculated using

following procedure: the amount of boron atoms (twice the B2H6 concentration in ppm)

divided by the number parts of carbon (which will be 6/100 for 6% of CH4 flow). So

depending on the B2H6 flow (sccm) out of the total flow (sccm) in the reactor, the amount

of B2H6 (ppm) in the feed gas can be determined.

The B/C is equal to (2×B2H6 flow (ppm) / methane fraction.

i.e.

100/%2

4

62

CH

ppmHB

C

B

gas

(14)

26

So if there is 150 ppm B2H6, 6% CH4 in the feed gas, then B/C ratio will be (2×150/0.06)

ppm, which is 5000 ppm.

For conversion of standard cubic centimeters per minute (sccm) to parts per

million (ppm), suppose total flow of gas is 400 sccm which we distribute into 106 parts,

so if we use 0.6 sccm of diborane (B2H6) which is diluted in hydgrogen by 10% then only

0.06% of B2H6 is only going into the feed gas hence from unitary method

we have 400 sccm = 106 (1 million parts)

1 sccm = 106/400 ppm (parts per million)

Therefore, 0.06 sccm of B2H6 = (106/400)×0.06 ppm = 150 ppm.

3.4 Cleaning of Films after Deposition:

The films of Set-I were cleaned in saturated solution of CrO3 + H2SO4 solution at

150oC for 5 minutes, 10 minutes of rinsed in a 1:1 boiling solution of H2O2 and 30%

NH4OH with the final rinse in deionized water and 10 minutes of ultrasonication in

acetone (51). Finally, the samples were exposed in hydrogen plasma at 900oC for 10

minutes before further characterization. To avoid any ambiguity due to hydrogenation

electrical resistance measurement was taken during cool down procedure in a four point

probe vacuum chamber. The Set-II samples were characterized without further cleaning

after deposition. However, mechanical cleaning of boron soot at the edges of samples has

been scratched with the sharp knife and final rinsed with acetone ultrasonication. In order

to ensure ohmic contact conditions with bulk limited currents, the currents through the

samples were limited to 0.1 to 1.0 mA.

27

Table 3.1. Overview of experimental growth conditions (Set-I), growth rate (r) and boron

concentrations (calculated from four point probe measurement data above transition

temperature) in the grown diamond films. Total feed gas flow rate was 424 sccm.

Sample Temp.

(oC)

Power

(kW)

Pressure

(Torr)

r

(μm/h)

[B/C]gas

(ppm)

B2H6

(ppm)

[B]

(atom cm

-3)

HD1

BD1

BD2

BD3

BD4

1050

1100

1000

1200

1100

1.5

1.5

1.2

2.5

1.6

75

95

80

140

110

23

16

10

8

2

0

2300

4700

6200

6200

0

90

140

190

190

0

2.4×1018

-

2.2×1019

1.0×1020

*HD1 was grown in 8% of CH4 and additional 0.4 sccm of N2.

BD1 was grown in 8% of CH4 and BD4 was grown only in 318 sccm of total gas flow rate.

Table 3.2. Overview of experimental growth conditions (Set-II), growth rate (r) and

boron concentrations (calculated from four point probe measurement data above

transition temperature) in the grown diamond films. Total feed gas flow rate was 400

sccm.

Sample r

(μm/h)

B2H6

(ppm) [B/C]gas

(ppm)

%

CH4

N2 [B]

(atom cm

-3)

HD2

BD5

BD6

BD7

BD8

BD9

BD10

BD11

BD12

BD13

10

6

5

4

6

3

7

2

8

12

0

150

250

500

500

750

150

150

150

500

0

5000

8000

16000

16000

25000

5000

7500

4000

16000

6

6

6

6

6

6

6

4

8

6

0

0

0

0

1000

0

0

0

0

2000

5.5×1017

1.1×1019

6.6×1019

2.3×1020

3.9×1019

-

-

-

-

-

*all samples are grown in additional 0.8 sccm of O2, BD10 was grown for 15 hrs.

28

3.5 Characterizations:

In order to understand the vapor phase growth mechanism, surface morphology

and doping behavior of single crystal diamond by various characterization techniques

have been utilized in this study. An optical transmission spectroscopy was used to

quantify effect of boron on the appearance of diamond color. An optical emission

spectroscopy (OES) was used to analyze in situ excited species during the growth

process. The surface morphology of samples was characterized by optical microscope

(OM), and atomic force microscope (AFM). The crystal phase and purity was verified by

X ray rocking curve experiment and Raman spectroscopy. Doping level was determined

by FTIR spectroscopy and finally electrical conductivity and number of charge carriers

were calculated using four point probe and Hall measurement.

3.5.1 Optical Transmission Spectroscopy:

The effect of doping on color of the film has been analyzed using Horiba

Microscope paired with Transmission Spectrometer in the visible range.

3.5.2 Optical Emission Spectroscopy (OES):

A hot gas or plasma emits electromagnetic radiation of different wavelengths at

low pressure which is characteristics of atoms, ions and molecules within the plasma. The

spectral analysis is the sorting of this radiation as a function of wavelength and is called

an emission spectrum. Optical emission spectrum is well known in-situ and non-invasive

technique for plasma diagnostics. It can provide information about properties of materials

within the plasma such as excited species densities, energy distribution of species, and

29

temperature of plasma (52, 53). In OES, light emitted by electronically excited species is

collected and detected. The transition from electronic ground state and excited states

contain both vibrational and rotational states. An emission from a particular electronically

excited state therefore consists of a number of emission lines corresponding to transition

from the rotational/vibrational sublevels. Hence such electronic spectrum is used to

identify emitting species (54-57). The analysis done here was to find the types of species

present in the plasma rather than deducing the absolute concentration of species. The

observed species in the H2/CH4/N2 and H2/CH4/B2H6 plasma are listed in Table 3.3.

Acton Research Corporation SpectraPro 500i, 0.500 meter triple grating monochromator /

spectrograph, has been used in this study with a 1200 grating/mm blazed at 300 nm. The

details of the result are discussed in Section 4.2.

Table 3.3 Observed species in OES of H2/CH4/B2H6/N2/O2 plasma.

Species Peak Position (nm)

Hα

Hβ

Hγ

658.6

488.2

433.2

H2 602.1

CH 431.4,

388.9

CN 360.58, 388.6, 421.6

C2 472.6, 516.5, 563.5,

BH 432.8

30

3.5.3 Optical Microscopy (OM):

High resolution optical microscopy has been widely used technique to

characterize the surface morphology of the diamond deposition. It gives information on

the presence of non-epitaxial deposition, growth sectors, and growth twins and other

imperfections on the surface or at the corner of the diamond. Crystal imperfection is often

observed in high microwave plasma density (MWPD) deposition. The Wild/Leica M420

optical microscope and Fisher scientific micromaster optical microscope were used in

this study for micro-photography of the samples.

3.5.4 Atomic Force Microscopy (AFM):

It is a high resolution scanning probe microscope. It relies on mechanical force of

atoms between the tip (probe) and the sample. It is a widely used tool for the surface

topographic image of sample at nano – scale. It is composed of a cantilever with a tip at

the end that scans on the surface. AFM can be divided into two basic scanning modes,

contact mode, and non-contact mode. In contact mode when probe is brought in to the

proximity of the varying topographic features forces between the tip and samples lead to

the deflection of the cantilever according to Hook’s law (58). This deflection can then be

measured by the using reflected laser spot from cantilever into a photodetector. In contact

mode tip can acquire electrostatic charge by friction from the surface and reduces the

actual force to detect and can also damage the sample or can dull the tip. In non-contact

mode cantilever is oscillating at its resonant frequency near the sample surface which

causes to change in force between tip and sample. These changes in forces are referred to

as force gradient. The force gradient changes the amplitude and phase of oscillation of

31

the cantilever which can be measured by the detector. Thermomicroscope’s TM Explorer

AFM instrument with Veeco probe in contact and non-contact mode was used to measure

surface morphology in this study.

3.5.5 Raman Spectroscopy:

Raman spectroscopy is a non-destructive technique to characterize various carbon

phases present in CVD diamond. It is based on inelastic scattering of monochromatic

laser radiation. When sample is illuminated with monochromatic laser light, some of the

re-emitted photons from the sample change (shift) its original frequency. This change in

frequency provides information about the vibrational, rotational or other low frequency

transitions in a system. Atomic or molecular vibrational energy is used to identify sample

quality in Raman spectroscopy. Both Raman and IR spectroscopy are based on

vibrational modes in the system. We have employed both techniques in our

characterization of homoepitaxial diamond. Asymmetric stretch in system is IR active

(59). The 532 nm line of solid state laser is used in this study as an excitation source and

the detection of the Raman signal was done in 180o configuration by a Dilor XY

spectrometer combined to a Pulnix CCD camera.

3.5.6 Single Crystal X-ray Diffraction (XRD) and Rocking Curve Experiment:

The single crystal x-ray diffraction technique was utilized to study the crystalline

quality of homoepitaxial boron-doped diamond films (60). The crystal perfection was

investigated by performing the rocking curve measurement on the (400) Bragg reflection

from the CVD diamond. In rocking curve experiment scan is performed by rotating

32

sample with an angular step (Δω of 0.02 degree) and a detector is fixed at (2) Bragg

position with the incident beam. A precise value of Bragg angle for (400) position was

first obtained using powder diffraction 2 scan for rocking curve experiment. Once this

value is determined the detector is fixed at (2) Bragg position with the incident beam

and then omega scan is performed by rotating sample with 0.02o angular step. Defects in

crystal broaden the intrinsic rocking curve (61). Philips X- ray diffractometer, model

(PW3040) with monochromatic CuK radiation has been used in this study for powder

diffraction pattern and Omega scan of the samples. A nickel filter was used to eliminate

the CuK. Philips X’Pert organizer software was used for data acquisition.

3.5.7 Fourier Transform Infrared Spectroscopy:

All the atoms in a molecule or substance are vibrating in a characteristic

frequency with respect to each other above absolute temperature. If the frequency of a

specific vibration of atom is equal to the frequency of IR radiation passing through the

sample, then IR radiation gets absorbed by the substance. Measurement of such IR

frequencies absorption by a sample is studied in Infra-red spectroscopy. To analyze the

signal of different frequencies with their intensity a mathematical operation Fourier

transformation is performed via computer, hence the term FTIR. Vertex 70 FTIR

spectrometer with Bruker optics hyperion 3000 IR microscope was used in this study for

FTIR spectra of the films.

33

3.5.8 Diamond Surface and Metallic Contact:

In order to study electrical properties of semiconducting CVD diamond films,

metallic contact is necessary. Two types of metallic contact are made on semiconductor

surface: Ohmic (non-rectification) and Schottky (rectification) contacts. When metal is

brought in contact with the semiconductor a potential barrier is formed between the

surfaces due to transfer of charges to equalize the Fermi levels. This barrier is called

Schottky barrier which has a non-linear response to I-V characteristics. Hence it is very

difficult to obtain good Ohmic contact between metal-semiconductor (MS) interfaces.

However, one can minimize the barrier height by selecting the metal work function

nearly similar to semiconductor on which metallic contact has to be made. The Table 3.4

and Figure 3.1 are given below to explain the formation of barrier between metal-

semiconductor interfaces. Charges are trapped in the depletion region between metal-

semiconductor interfaces by forming Schottky junction as shown in Figure 3.1 (d) but

Ohmic contact is formed with no charge traps between the interfaces as shown in Figure

3.1 (c). Space charge limited region is formed on the interface if work function of metal

( m ) is less than that of p-type semiconductor ( s ) (62). Boron doped diamond has a

work function about 5.16 eV and Tungsten (W) has about 4.65 to 5.70eV depending upon

its crystal orientation (63) and also tungsten shows good adhesion on diamond surface

(64). We deposited tungsten on the surface of Set-I diamond by magnetron sputtering for

electrical characterization. Electrical characterization of Set-II samples was performed

using silver epoxy on the surface of diamond film.

34

Table 3.4 Electrical nature of ideal MS contacts.

Work function n-type semiconductor p-type semiconductor

sm Rectifying Ohmic

sm Ohmic Rectifying

(a)

(b)

(c)

(d)

Fig. 3.1 Energy band in metal-semiconductor systems before contact (a) sm , and (b)

sm ; metal-semiconductor junction after contact (c) sm , and (d) sm .

35

3.5.9 Four Point Probe and Hall Measurements:

The electrical properties of the samples were determined by four point probe and

Hall measurement techniques. Measurements were taken in linear configuration in four

point probe. The resistivity of the sample was determined by the following formula

)/(.2

stFtLn

R

, (15)

Where ,, R and t are resistivity, resistance, and thickness of the film and s is spacing

between two probes (65). Since st , the correction factor )/( stF is assumed to be equal

to unity. MMR technology four probes system with the help of Keithly 237 instrument

was used in this study.

Hall measurements were performed at room temperature in van der Pauw

configuration with Ecopia HMS-3000 instrument. The I-V characteristics were

maintained during experiment by limiting the current supply within 1mA at zero –

magnetic field. The value of sheet resistance was measured in magnetic field of 0.55

Tesla. The measurements were taken for both positive and negative current with both

polarities of magnetic field. P-type of doping was determined by Hall measurements.

36

CHAPTER 4

4.0 RESULTS AND DISCUSSION:

4.1 Optical Transmission Spectra:

A series of homoepitaxial boron doped single crystal diamond films were

synthesized with different level of boron doping and characterized in this study. The

color of seed crystal was observed to change from yellow to pale blue then to dark blue

and finaly opaque as boron concentration was increased in the films. Optical transmission

spectra of the samples in the visible range were taken in order to measure the effect of

doping on the color of the films. The spectra indicate small downshift of peak and

decrease in transmitted intensity as the doping level of the films increases. The peak of

low doped film (sample BD5) appeared at 454 nm in the optical transmission spectra, as

shown in Figure. 4.1.

37

Fig. 4.1 Optical transmission spectra of seed crystal (Seed) and doped diamond films

after applying correction for the absorption in the diamond seed crystal. Photographs of

seed crystal (Seed) and doped diamond (BD6) are embedded inside the graph. Yellow

color in seed is due to substitutional nitrogen atom in diamond lattice and bluish color in

BD6 represents boron incorporation in the diamond lattice.

4.2 Optical Emission Spectra:

To detect excited species within the plasma during the diamond growth process,

an optical emission spectroscopy was used. The spectral peak(s) observed during the

diamond growth process with and without diborane are labeled corresponding to the

excited species and is shown in Figures 4.2. Visible range of emissive scan showed very

intense Balmer series of atomic hydrogen and diatomic hydrogen and carbon, CH and CN

molecules. The atomic hydrogen lines produce singlets while diatomic species such as

C2, H2, and CH produce doublets. Several C2 bands from Swan band system were also

38

detected throughout the visible range. The strongest C2 band is in the green region at

516.5 nm. The observed species in OES of H2/CH4/B2H6/N2 /O2 plasma have been

summarized in Table 3.3. Addition of oxygen in source gas did not significantly change

the emission spectrum. Change in spectrum has been observed when nitrogen gas was

removed and the diborane gas was introduced for the deposition as shown in Figure 4.2.

High power and pressure may reduce the intensities of the peaks and absence of nitrogen

removed the nitrogen related peaks. Literature shows that atomic boron OES peaks

appear at around 250 nm and BH radicals appear at 432.8 nm (52). We were unable to

observe peak at 250 nm but peak at 432.8 nm seemed to be overlapping with other peaks

as shown in Figure 4.2. Because the high percentage of hydrogen gas was used as the

source gas a high concentration of atomic (H and molecular (H2) hydrogen was visible

in the spectrum. High amount of atomic hydrogen is needed to produce active carbon

radicals during diamond deposition.

The increase in flow rate of nitrogen was observed to increase the intensity of CN

band but did not show noticeable effect on the other species such as C2 dimer as shown in

Figure. 4.3. The increase in growth rate suggests that the nitrogen causes surface catalytic

reaction. The addition of nitrogen increases the surface kink density that provides the

sites to incorporate active carbon species. Atomic hydrogen increases the growth rate by

increasing the number of active surface sites or shifting the balance among adsorption,

desorption and incorporation steps (66). Locher et al and Wolden et al suggests that

atomic hydrogen affects the growth rate by enhancing the sp3

componet. In contrast,

nitrogen additon affects the growth by inhibiting the sp2 component. However, excess

nitrogen creates negative effects on the sp3

component (67, 68).

39

Fig. 4.2 Optical emission spectra of H2/CH4/N2 plasma under the pressure, power and

temperature of 80 Torr, 1.5 kw and 1100oC respectively (red color) and H2/CH4/B2H6

plasma under the pressure, power and temperature of 100 Torr, 1.5 kW and 1100oC

respectively (green color).

10000

8000

6000

4000

2000

0

Inte

nsi

ty (

a.u

.)

700650600550500450400350300

Wavelength (nm)

N2 = 1.2 sccm

N2 = 0.8 sccm

N2 = 0.4 sccm

N2 = 0 sccm

CN

CN

C2

C2

C2

H

H

BH/H

H2

CH

H2

Pressure ~ 100 TorrPower ~ 1.4 kWatt B2H6 = 16000 ppm

Fig. 4.3 Optical Emission Spectroscopy (OES) results from the plasma during the

diamond deposition process. The spectra shown were obtained for various levels of

nitrogen in the plasma at a fixed concentration of diborane.

40

4.3 Optical Microscopy Images:

The surface morphology of diamond films had been observed using optical

microscope. The untreated HPHT substrate (seed crystal) has shown macroscopically flat

surface with <100> crystal plane. The image of homoepitaxially deposited diamond film

(HD1) shows round hillocks and twins on the film surface. This film was deposited in 8%

of CH4-H2 gas, 900 ppm of N2 and 1800 ppm of O2 with a total 434 sccm of feed gas. The

deposition temperature, microwave power, and chamber pressure was maintained at

1050oC, 1.5 kW, and 75 Torr respectively. The edge of the film was observed to be very

rough. The roughness at the edge of film may be due to the formation of bright ring of

plasma at the edge of the substrate. Formation of twins and hillocks may be assumed to

be the effect of instability in plasma due to process control. The problem in process

control in MPCVD is normal because the leakage of e-m wave inside the chamber is

obvious, lack of uniform cooling system of substrate during the long deposition time and

the high thermal conductivity of hydrogen gas which can heat the chamber wall in long

run of deposition time. If the substrate is mounted in a holder above the cavity then a very

bright rim of plasma is formed around the edge of the sample may cause the edge

roughening. This explanation was verified with nearly smooth edge of sample deposited

by placing the substrate in the cavity as flushed where substrate edges are not exposed to

plasma. Optical microscope images of some of the samples are shown in Figure 4.4 and

4.5. The flow of diborane in source gas causes the formation of carbon-boron soot and

darkening of chamber wall in long deposition time. In the absence of plasma activation,

enhanced soot formation is noted (49). The growth condition of Set-I and Set-II is given

41

in Table 3.2. The experiment was performed at microwave power density (MWPD) of 90

to 100 W/cm3.

In Set-II, the experiment deposition temperature was fixed at 1100oC with 1.4 -

1.5 kWatt microwave power and 100 Torr chamber pressure. The deposited

homoepitaxial film HD2 shows some non-epitaxial deposition on its surface but no twins.

The ratio of methane and hydrogen was 6% with a total feed gas flow rate of 400 sccm.

The deposition conditions have been shown in Table 3.2. The films either grown at high

percentage of diborane or longer deposition time show polycrystalline in nature as shown

of sample BD9 (3 sccm of diborane) and BD10 (0.6 sccm of diborane for 15 hours

deposition) respectively in Figure 4.5. More hillocks were seen on the surface of the

samples grown at both low and high methane content as shown in samples BD11 and

BD12. The optical microscope images below are showing the gradual improvement in

quality. The deposition temperature, flow of gases, cleaning of substrate with H2/O2

plasma before deposition and other deposition parameters have been selected after

performing a series of experiment in Set-I. All the substrates have been exposed to 4% of

O2/H2 plasma for 10 minutes before deposition begins. The growth rate and surface

morphology was found to be dependent on B/C ratio in the gas phase. High B/C ratio

with low methane in feed gas decreases the growth rate as summarized in Figure 4.6. The

optimal growth with good quality had been observed with 2 sccm (16000 ppm) of

diborane in feed gas. The quality was improved with the additional supply of 1000 ppm

of nitrogen in feed gas as shown of sample BD7 (16000 ppm of B2H6) and BD8

(additional 1000 ppm of N2) in Figure 4.5. However, the quality degraded by forming

hillocks with the addition of further increase of nitrogen to 2000 ppm, but it improved the

42

growth rate of diamond films by a factor of two as compared to 1000 ppm of nitrogen as

shown of samples BD8 and BD13 in Figure 4.6. Addition of a little amount of nitrogen

and low (B/C)gas ratio in feed gas may be the cause of high growth rate of sample BD1.

On the other hand, the same (B/C)gas ratio in samples BD3 and BD4 but less supply of

hydrogen and methane in feed gas for sample BD4 may have caused the reduction in

growth rate of sample BD4. It can be concluded from the above discussion that a proper

amount of diborane and nitrogen is favorable for improving the structural quality and

growth of boron-doped single crystal diamond.

43

Fig. 4.4 Optical microscopy images of substrate (Seed Crystal), undoped sample (HD1)

and boron doped diamond samples (BD1, BD2, BD3, and BD4) showing surface

morphological change on epitaxial deposition.

Fig. 4.5 Optical microscopy images of homoepitaxially deposited diamond samples

showing change in surface morphology with boron concentration in feed gas. Film BD8

and BD13 was deposited with additional introduction of nitrogen in feed gas (table 3.2).

44

Fig. 4.6 Histogram of growth rate of boron doped diamond as a function of B/C ratio in

gas phase. All the depositions took place at 1100oC, 1.4 kW microwave power and about

100 Torr chamber pressure. The amount of diborane used for BD7, BD8 and BD13

growth was same. The amount of CH4 gas was described in table 3.2.

4.4 Atomic Force Microscopy Images:

AFM images were taken of the flat surfaces of the epitaxial films over scanned areas of

20 × 20 µm2 (Set-I) as shown in Figure 4.7. The surface morphology of the undoped and doped

films was seen modified after deposition. The polishing scratches of the seed crystals

disappeared and step flow morphological features were observed after deposition. Groove shaped

features seen on the surface of sample BD4 may be the result of step bunching as shown in Figure

4.7. Step asymmetry during deposition may be the cause of step bunching in CVD (69-72).

In order to compare growth rate and surface morphology of doped and undoped diamond film the

sample (HD1) was grown without using diborane in the feed gas. Presence of diborane in feed

gas prevent formation of microtwins and hillocks on diamond film, as shown in Figure 4.4 and

45

Fig. 4.7 AFM images of seed crystal (seed), the magnified view at top right corner shows

the polishing scratches and the carbon debris on the substrate before any CVD treatment,

undoped sample HD1 and boron doped single crystal diamond samples (BD1, BD2, BD3,

and BD4). The deposition time for this study was 8 hours.

46

4.6. In sample (seed) polishing lines (which are trace of mechanical polishing in HPHT

production) are seen periodic and parallel to each other. The formation of steps, kinks and

terraces (sample HD1 in Figure 4.7) are normal phenomena in step flow growth

mechanism of diamond. Additional information in step flow growth and step bunching

can be found in (71-75).

In order to study the effect of boron and nitrogen on the growth morphology of

diamond, few samples were grown at varying diborane and nitrogen contents in feed gas

for 5 hours deposition. The growth conditions are summarized in Table 3.2. The contact

mode AFM images over scanned areas of 10 × 10 µm2 for Set-II are shown in Figure 4.8.

The rms roughness of the samples was observed to decrease with increasing boron

concentration in feed gasses (samples BD5, BD6 and BD7). Introduction of 1000 ppm of

nitrogen during boron doping improved the surface morphology and increased the growth rate of

film (sample BD8). However, when higher amount of nitrogen 2000 ppm along with the same

16000 ppm of B/C ratio was used, the growth rate of film was shown to double than that using

1000 ppm of nitrogen. On the other side, higher amount of nitrogen reduces the film quality as

some non-epitaxial features are seen on the surface of the film as shown in sample BD13 of

Figure 4. 5.

Samples BD11 and BD12 were grown with 4 and 8% of methane by keeping a

constant diborane flow at 150 ppm. As we see here low methane gives smooth surface

but reduces the growth rate and on the other hand high methane produced very rough

surface but high growth rate in the grown experimental condition. From OM image of

sample BD12 in Figure 4.5 we can see that it has high hillocks density and non-epitaxial

crystallite on its surface. When high diborane 750 ppm or high methane 10% was

47

Fig. 4.8 AFM images of homoepitaxial sample (HD2), boron doped single crystal

diamond samples (BD5, BD6, BD7, BD8 and BD13). AFM images of boron doped

diamond samples (BD11) and (BD12) grown at 16 sccm and 32 sccm of methane. AFM

images of BD9 and BD10 are not shown here. The deposition time was 5 hours.

48

introduced in the deposition chamber amorphous carbon and polycrystalline nature of

diamond film was grown on seed crystal as shown in OM images of samples BD9 and

BD10 in Figure 4.5. Overall, there is a significant decrease in diamond growth rate with

an increase in boron content in the plasma as summarized in Tables 3.1 and 3.2.

4.5 X-ray Diffraction Pattern and Rocking Curve:

The quality of the film was also tracked with an x-ray powder scan and rocking

curve experiment as shown in Figure 4.9. The intense peak of (400) Bragg reflection was

observed at 119.8o in powder scan and a single intense peak of (400) was observed in the

rocking curve experiment from 30-62o omega scans indicating high quality film inset of

Figure 4.9. The FWHM of the Bragg peak was observed to be broadened and varied from

0.07o to 0.10

o with increasing boron content in the films. Few week peaks due to trace

elements from seed crystal are also observed in the powder scan. Transition metals or

their alloy have been used in the HPHT synthesis process to grow seed crystal. Those

trace metals may invite the non-epitaxial growth in CVD diamond deposition and have

detrimental effect for single crystal diamond.

49

Fig. 4.9 XRD pattern of a as received seed crystal (seed), undoped film (HD) and boron

doped diamond film (BD). The inset is the rocking curve for boron doped diamond film.

4.6 Raman Spectra:

The Raman spectra of doped and undoped single crystal diamonds are shown in Figure

4.10 (a, b and c). No graphitic carbon related peak was observed in the spectrum indicating high

quality of homoepitaxial layer. Doping was found to be uniform in its thickness as we

investigated the spectrum inside the film for few microns depths, as shown in Figure 4.10 (c). An

intense zone-center optical phonon mode of diamond is visible at 1334 cm-1

along with additional

broad bands at 580, 900, 1042, 1233 cm-1

. At low doping, no asymmetry in Raman line and no

additional band were observed in the spectrum. The observed additional bands at lower

wavenumber were previously reported mainly on polycrystalline boron-doped diamond (3, 76,

77). The significant modification in zone center optical phonon line was observed. The

50

asymmetry in optical phonon Raman line was seen increased with doping level. A small

downshift of bands 1233 and 580 cm

-1 with increasing boron concentration was also observed in

this study (77). The downshift of optical phonon line and broadening of full width at half

maximum (FWHM) with a significant change in intensity of bands 580 and 1233 cm-1

were also

observed. The origin of those additional bands is not completely understood yet but the literature

shows that bands around 1233 and 580 cm-1

are attributed to the presence of large cluster of boron

atoms in diamond (30, 76, 79). The downshift of optical phonon line with increasing boron

content has also been described by Gheeraert et al (80). The asymmetric broadening and

downshift of Raman peak of boron-doped diamond films could be explained by Fano-effect (81-

85). Fano effect is the quantum mechanical interference between discrete zone-center optical

phonon state and continuum of electronic states induced in presence of boron. The Raman peak

position and FWHM was calculated by Lorentzian fit of the spectrum. The Fano asymmetry of

diamond was derived by excellent fits of the Raman curves, as shown in inset of Figure 4.10 (c).

The Fano asymmetry of doped diamond BD7 was calculated to be (q = 9.2) and for seed crystal

q = 492. The Fano profile used in this fitting was,

Cq

II o

)1(

)(2

2

(16)

here C is constant, oI is prefactor and

o ; where and o are Raman peak position

of doped diamond and seed crystal respectively and is FWHM of a Raman curve of sample

BD7 (84).

51

Fig. 4.10 (a) Raman spectra of Set-I of samples undoped film HD1, boron doped films

BD1, BD2 and BD3 and as received seed crystal (Seed).

Fig. 4.10 (b) Raman spectra of Set-II of samples as received seed crystal (Seed), undoped

film HD2 and boron doped films BD5, BD6, BD7, BD8 and BD9. Insert (a) and (b) are

showing the peak shift and FWHM of samples in unit of cm-1

.

52

(c)

Fig. 4.10 (c) Raman spectra at different depth in sample BD7. Insets are Fano line shape

fitting with Raman curve of samples BD7 and Seed crystal to measure deformation in

optical phonon line due to doping.

4.7 Fourier Transform Infrared Spectra:

The boron concentration of Set-I samples were determined from absorption

spectrum of FTIR spectra. Strong absorption was seen in IR Spectra of the films above

2000 cm-1

wavenumber as shown in Figure 4.11. The one–phonon absorption peak at

about 1290 cm-1

was used to calculate boron concentration (82)

)1290(

1731)101.2()(][

cmcmB (17)

where tA/ , is the linear absorption coefficient at 1290 cm-1

band, and A and t

being absorbance and thickness of the film. In order to calculate concentration FTIR

spectrum first converted into absorption spectrum after that seed (reference) spectrum

53

was subtracted from it. The boron concentration in film of samples BD1, BD2 and BD3

was found to be 8.6×1018

, 1.5×1019

and 3.6×1019

cm-3

respectively. Due to strong

absorption to infrared signal, the FTIR spectrum of sample BD4 was not obtained hence

its dopant level could not be estimated using equation (17).

Characteristics Boron Peak

Fig. 4.11 FTIR spectra of as received seed crystal (seed), undoped diamond film HD1

and I set of boron doped diamond films BD1, BD2, BD3. Inset is the spectra after

applying correction for the transmission of seed crystal, characteristics boron peak is

shown at 1290 cm-1

.

54

4.8 Electrical Conductivity Measurements:

Conductivity of the grown diamond films have been measured as a function of

temperature using four point probe measurement system. The four point probe measurement

was conducted in vacuum with a pressure less than 5 mTorr. Nitrogen gas was used to cool the

samples in a four point probe system. To measure electrical conductivity, tungsten strips were

deposited on the surfaces of the diamond films using a magnetron sputtering technique. An

optical microscope image of the tungsten strips on a film surface is shown in Figure 4.12. The

currents through the samples were limited to 0.1 to 1.0 mA in order to minimize self-heating of

the samples and ensure ohmic contact conditions with the bulk limited currents. The variation in

conductivity, (σ) of the films with temperature (T) was determined from four point probe

measurement as shown in Figure 4.13. The room temperature resistivity of the most heavily

doped sample BD4 was determined to be 0.12 Ωcm. Activation energies of the samples have been

obtained by best fit Arrhenius plot of the conductivity data at high and low room temperatures

from the transition point. The activation energies of samples are listed in Table 4.1. The

difference in activation energy from high to low temperature indicates that the two different

conduction mechanisms are responsible for carrier transport in the film (81, 82). At higher

temperatures, carrier is transported via band conduction and at low temperatures, carrier hops in

the localized states via hopping conduction. At high temperatures, the activation energy of

carriers was found to decrease with increasing doping concentration. Increasing doping

concentration increases the acceptor band width which ultimately reduces the activation energy of

acceptors (85). A transition in the conduction mechanism from localized hopping to band

conduction was observed to shift towards higher temperature as the amount of doping increased.

However, in Set-II of samples this transition is seen to shift towards lower temperature with

increasing boron content in the film. The reason for such shift is not much clear yet, but the one

possibility could be the change in carrier mobility in the diamond film with increasing boron

55

concentration. Scattering centers for carriers, residual impurities, non-epitaxial growth and grains

all have diverse effect on carrier mobility. The variable range hopping conductivity of Set-I and

Set-II is shown in Figure 4.14 (a and b). The conductivity of films at lower temperature region

obeys Mott T-0.25

law.

The Pearson and Bardeen formula was also taken into account to estimate the boron

concentration, aN of the films from the calculated activation energy, aE (86).

3/1)( aoa NEE (18)

here eVEo 37.0 , is the activation energy of an isolated dopant, and is a material dependent

constant whose value for diamond is 6.7 × 10-8

eV/cm. The boron concentrations in samples BD1,

BD3 and BD4 were estimated using equation (18) to be 2.4×1018

, 2.2×1019

, and 1.0×1020

cm-3

respectively. These values lie close to the estimated values of the dopant level determined from

the FTIR data at high temperatures. The boron concentrations in samples BD6, BD7 and BD8

calculated from room temperature Hall measurement were 6.6×1019

, 2.3×1020

, and 3.9×1019

atom

cm-3

respectively and that calculated from a four point measurement above transition temperature

were 3.6×1019

, 0.6×1020

and 2.7×1019

respectively.

56

Table 4.1 Activation energies of doped films measured in four point probe experiment.

Samples Above Transition Temperature. Below Transition Temperature

BD1

BD2

BD3

BD6

BD7

BD8

0.27 eV

0.18 eV

0.05 eV

0.19 eV

0.10 eV

0.16 eV

0.02 eV

0.05 eV

0.03 eV

0.02 eV

0.02 eV

0.01 eV

BD

Fig. 4.12 Tungsten strips on doped diamond for metallic contact.

57

Fig. 4.13 Electrical conductivity as a function of 1000/Tof (a) Set-I boron doped diamond

films BD1, BD3 and BD4 and (b) Set-II boron doped diamond films BD6, BD7 and BD8.

58

Fig. 4.14 Electrical conductivity as a function of T-0.25

of (a) Set-I boron doped diamond

films BD1, BD3 and BD4 and (b) Set-II boron doped diamond films BD6, BD7 and BD8.

59

CHAPTER 5

5.0 CONCLUSIONS:

Boron-doped single crystal diamond has been synthesized using microwave

plasma chemical vapor deposition on commercially available synthetic Type-Ib diamond

substrates. The homoepitaxial diamond films were grown with boron-doping levels of

1018

to 1020

cm-3

. The color of seed crystal was seen to change from yellow to pale blue,

then to dark blue, and finally to completely opaque as the doping concentration in the

film increased. The effect of nitrogen in the growth plasma on boron doping was also

investigated during this study. Nitrogen shows both positive and negative effects on

boron doped diamond film. Surface morphology and growth rate of diamond deposition

improved with the use of 1000 ppm of nitrogen, but when the amount of nitrogen was

increased to 2000 ppm, non-epitaxial growth appeared on the surface of the doped film.

On the other hand, either the boron incorporation in diamond lattices decreases in the

presence of nitrogen in the process gas or due to the compensating nature of nitrogen in

p-type, we observed a decrease in the electrical conductivity of diamond films. Doping

concentrations of boron in films estimated by the FTIR measurements and from the four-

probe electrical resistance data are in reasonable agreement with each other. P-type

conductivity of the film is verified by an independent set of Hall measurements. The

crystalline quality of the film has been confirmed by an x-ray rocking curve and the

Raman scattering. The Raman spectrum of boron-doped diamond films contains

additional bands in the lower wavenumber regions, along with the first order Raman

60

peak. The downshift and broadening of Raman lines are also observed with the

increasing boron concentration in the crystal. The diamond growth rate was observed to

decrease with increasing boron content in the film. Temperature dependent resistivity

measurements show that the current conduction mechanism depends upon the doping

level and obeys semiconducting behavior in the temperature range between 90 to 680 K.

A significant difference was found in the activation energies of the films over the given

experimental temperature range. A transition in the conduction mechanism from

localized hopping to band conduction was observed to shift towards higher temperatures

as the amount of doping increased. The high growth rates of 5-16 μm/hr were obtained in

this study for high quality boron doped single crystal diamond. The lowest room

temperature electrical resistivity of one of the samples (BD4) was determined to be 0.12

Ωcm which is suitable for the fabrication of high power electronic devices.

61

CHAPTER 6

6.0 FUTURE WORK:

All doped diamonds selected in this study were transparent to visible light and of

p-type. Transparent p-type semiconductor can be very useful in touch panel, solar cell,

electrochemical electrodes and field effect transistor.

Upon the availability of good quality heavily boron-doped diamond samples uni-

junction diamond devices such as Schottky Barrier Diode (SBD) or Metal Semiconductor

Field Effect Transistor (MESFET) can be fabricated for industrial applications. The

nitrogen effects in the present study are intriguing and future work can emphasize n-type

doping possibilities with nitrogen in the diamond lattice. Free standing doped diamond

films are useful in many high frequency power devices, so further research efforts should

be directed to synthesize few mm thick boron-doped single crystal diamond films.

62

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