Ž .Thin Solid Films 323 1998 212–216

Electrical and microstructure analysis of NirGern-GaAs interface

L. David a,), B. Kovacs b, I. Mojzes b, B. Pecz c, J. Labar c´ ´ ´ ´ ´a Institute of Microelectronics and Technology, Kando Kalman Polytechnic, H-1084 Budapest, TaÕaszmezo u. 17, Hungary´ ´ ´ ¨

b Dept. of Electronic Technology, Technical UniÕersity of Budapest, H-1521 Budapest, Hungaryc Research Institute for Technical Physics of the Hungarian Academy of Sciences, H-1325 Budapest P.O. Box 76, Hungary

Received 24 March 1997; accepted 4 December 1997

Abstract

Ž . Ž . Ž . Ž .Ni 27 nm rGe 23 nm and Ni 10 nm rGe 40 nm layers deposited onto n-type GaAs by electron beam evaporation were studiedŽ .structurally and electrically. The samples were annealed for 20 min at different temperatures in flowing forming gas—H :N 5%:95%2 2

—in tube furnace. The current–voltage characteristics of the samples annealed at 6008C show ohmic character. The contact resistancewas found to be a minimum of 5.1=10y5

V cm2 after annealing at 6008C. The alloying behaviour of the specimens were investigatedby electron microscopy. The contacts show a mixed structure in the case of as-deposited samples. In the sample annealed at 5508C, thereappeared deep pyramidal pits of 20–30 nm size. The structural characterization was carried out by cross-sectional transmission electron

Ž . Ž .microscope XTEM equipped with energy dispersive system EDS . The composition investigations showed that the pits contained ofGa, As, Ni and Ge. The upper layer of the metal was very thin and rich in Ga. The layer between the semiconductor and the upper layer

Ž .was a Ni–Ge Ga, As mixed one. q 1998 Elsevier Science S.A. All rights reserved.

1. Introduction

At first, Au–Ge metallization was widely used to makew xohmic contacts to n-type GaAs 1–3 . Au–Ge layer was

evaporated from an eutectic mixture and ohmic characterwas achieved after alloying at temperatures ranging be-tween 390 and 4508C, but these contacts suffered from

w xsurface non-uniformity 4 . In most cases, the Au–Ge isw xcovered by a Ni layer 5–7 . In this contact system, Ge

serves to increase the surface doping while Ni forms abarrier and a conductive NiAs compound. NirAu–Gemetallization gives a low contact resistance, but it hasseveral disadvantages: short alloying time, critical controlof the alloying process and degradation during ageing and

w xrough surface of the contacts 8,9 . Nowadays, many othermetallization systems are used to form thermally stable,low resistance, reproducible and reliable ohmic contacts.

w x w xPdGe 10 and NiGe 11 are used for ohmic contact. In thecase of NiGe, the contact becomes ohmic after annealing,but the ohmic character depends strongly on the concentra-tion of Ge and on the temperature of annealing. To reducethe contact resistance of the NiGe system, a small amount

w xof a third element is added 12 . Among others, the

) Corresponding author.

Ž .NiGe Au W system is also used to form ohmic contactw x13 . Generally, the role of Ge added to the contact metal-lization is believed to narrow the Schottky barrier due tothe doping of the GaAs. Although this phenomenon isproven in several cases, there are some metallizationswhich work in a different way. In this work, theNirGern-GaAs contacts were analysed to study the rea-son for ohmic character after annealing.

2. Experimental procedure

To prepare samples, an n-type epitaxial layer doped bysulphur of N s8=1015 cmy3 with 10 mm layer thick-D

ness was grown by Effer–Nozaki type VPE reactor onqq Ž .n -GaAs 100 . The samples were carefully degreased.

Prior to the metal deposition, the surface was etched by themixture of NH OH:H O :H O with the ratio 1:1:100 at4 2 2 2

08C for 10 s followed by a rinse with 18 MV water and itŽ . Žwas blown dry with filtered dry nitrogen. Ni 27 nm rGe 23

. Ž . Ž .nm , henceforth NiGe1, and Ni 10 nm rGe 40 nm ,henceforth NiGe2, layers were deposited onto n-type GaAsby electron beam evaporation. Two types of samples wereprepared using each composition: some were patterned tomeasure I–V, C–V characteristics and contact resistance,while others, which were used for XTEM and scanning

0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.Ž .PII S0040-6090 97 01211-X

( )L. DaÕid et al.rThin Solid Films 323 1998 212–216´ 213

Fig. 1. The I – V curves of NiGe1 and NiGe2 samples after metaldeposition and after heat treatment for 20 min at 550 and 6008C.

Ž .electron microscope SEM investigations, were devoid ofpatterns. The substrate temperature during the evaporationwas kept at 1508C. The evaporation rates were 0.4 and 0.2nmrs in the case of Ni and Ge, respectively. Pressure waskept below 4=10y4 Pa during the whole process. Justprior to metal deposition, a movable Cu shadow mask waspositioned onto the substrate. The contacts had circularshape with diameters of 800, 250, 200 and 170 mm. Afterevaporation, the samples were annealed for 20 min in

Ž . Ž .flowing forming gas—H 5% :N 95% —at four differ-2 2

ent temperatures: 450, 500, 550 and 6008C. The patternedsamples were encapsulated using AuGe eutectic on their

w xback side 14 and Al wire bonding on the contacts studiedw x15 . The current–voltage and capacitance–voltage charac-teristics were measured applying Keithley 230 Pro-grammable Voltage Source, Keithley 236 Source and Mea-sure Unit, and a Hewlett-Packard 4271B CV Meter. Thestructural characterization was carried out by cross-sec-

Fig. 2. The heat treatment dependence of Schottky barrier height and theideality factor of the NiGe1 metallization.

Fig. 3. Plot of r = R vs. 1r r of the samples NiGe1 annealed at 6008C.

Ž .tional transmission electron microscopy XTEM equippedŽ .with energy dispersive system EDS . The surface was

Ž .studied by scanning electron microscopy SEM , and thevolatile component loss was monitored by a quadrupole

w xmass spectrometer 16,17 . The method of the preparationw xof samples studied by XTEM and EDS is described in 18 .

The XTEM pictures were taken at 200 keV using a PhilipsCM 200 microscope. EDS analysis was done by Noranmicroprobe. To study in situ the volatile component loss

Ž .and the actual surface morphology, SEM JEOL JSM-T20Žwas combined with mass spectrometer system ATOMKI

.Q100 . We used TR-6656 digital multimeter for contactw xresistivity measurement by the Cox method 19 . The

accuracy of measurement was 0.05%. The contact resistiv-ity was measured at both polarities and the thermal effectwas diminished using their average.

3. Results

3.1. Electrical properties

Ž . ŽThe I–V current–voltage and C–V capacitance–volt-.age measurements were carried out for all samples. The

Fig. 4. Transmission electron microscopy micrographs of as-depositedNiGe1 sample.

( )L. DaÕid et al.rThin Solid Films 323 1998 212–216´214

Fig. 5. Transmission electron microscopy micrographs of NiGe1 sampleŽ .annealed at 5508C for 20 min. showing pits with triangular shape a ,

Ž .EDS spectrum of the pit b .

measured I–V characteristics of the samples NiGe1 andNiGe2 are shown in Fig. 1. The contact area of thesamples was 0.5 mm2. It can be seen that the sampleheat-treated at 6008C has nearly linear characteristic. In-creasing the annealing temperature of NiGe2 samples, theSchottky barrier of the interfaces obtained from I–V andfrom C–V characteristics decreased while the ideality

Ž .factor increased Fig. 2 , which means that the ohmiccharacter became stronger.

The contact resistance was measured on the NiGe1samples heat-treated at 550 and 6008C. The results of themeasurement are shown in Fig. 3 in the form of plot r=Rversus 1rr where r is the radius of the contact and R isthe measured resistance. Each of the data points representsthe average of at least five measurements. The slope of this

Ž 2 .plot is proportional to r V cm . Knowing the thicknesscw xand resistivity of the epitaxial layer 20 , the exact value of

r can be extracted. The specific contact resistivity ofc

NiGe1 was 4.4=10y4 and 5.1=10y5V cm2 for sam-

ples heat-treated at 550 and 6008C, respectively.

3.2. Microstructural analysis

The microstructures of the NiGe1 samples having thelowest specific contact resistance were investigated. Theas-deposited sample was heat-treated at 1808C during the

w xstandard XTEM sample preparation technique 18 . Themicrostructure of the as-deposited metalrGaAs interfacesis shown in Fig. 4.

A white line can be seen in the picture which meansthat the metal separated from the substrate during the

Fig. 6. Transmission electron microscopy micrographs of the metal layerŽ .of NiGe1 annealed at 5508C for 20 min. a , EDS spectrum of the lower

Ž . Ž .metal layer b , and the upper metal layer c .

( )L. DaÕid et al.rThin Solid Films 323 1998 212–216´ 215

preparation for XTEM investigation. The original interfacebetween the Ge and Ni disappeared, meaning that under1808C, an extensive interaction already took place betweenNi and Ge. We could not find amorphous Ge, although Lin

w xet al. 21 had observed this at 2008C. EDS measurements,carried out with small spot size in XTEM samples, provedthat the lower part of the layer—nearby the substrate—contained more Ge and less Ni than the upper layer. Thesamples annealed at 5508C showed large pits grown into

Ž .the GaAs Fig. 5a . The pits show triangular shape. Inperpendicular section, these formations show elongated

w xforms. Pecz et al. 22 observed similar pits in GaAs with´Au metallization. We investigated the composition of thepits and found that they contained Ge in measurable

Ž .amounts in addition to As, Ga and Ni Fig. 5b . The metallayer consists of two parts: the upper layer is very thin,

Ž .while the lower one is considerably thicker Fig. 6a . Thelower thick metal layer consisted of Ni and Ge with some

Ž .traces of Ga and As Fig. 6b . The composition of theŽ .upper thin layer was rich in Ga Fig. 6c . The composition

of the substrate between two pits was investigated andonly Ga and As were found in the ratio of 1:1.

The simultaneous observation of the volatile componentŽ . Ž .loss in situ by Evolved Gas Analysis EGA and the

change in surface morphology by SEM during the heattreatment using a heating rate of 308Crmin was carriedout. There was no detectable As out-diffusion during heattreatment up to 6208C. It is worth mentioning that thesurface of the samples was very smooth during the wholeheating-up process.

4. Discussion

For the III–V semiconductors under forward bias, cur-rent flows by thermionic emission, field emission, or acombination of the two. We used the thermionic emissionmodel to calculate the Schottky barrier height and theideality factor. The current density J for an applied bias Vis given by:

2JsA)T exp yqF rkT exp qVrkT y1Ž . Ž .B

sJ exp qVrkT y1 1Ž . Ž .o

w x23 , where A) is the Richardson constant; k, Boltzmann’sconstant; T , the absolute temperature; and F , the Schot-B

tky barrier height, where:

J sA)T 2 exp yqF rkT . 2Ž . Ž .o B

Because of image force lowering, the actual barrier heightis bias-dependent. It is often desirable to remove all biasdependence from J and this can be accomplished byo

introducing a diode ideality factor, n, where:

JsJ exp qVrnkT . 3Ž . Ž .o

For pure thermionic emission, the n value is close to one.Increasing the heat treatment temperature, the Schottky

barrier height calculated from thermionic emission model

decreased and the ideality factor increased. This changemay be caused by two factors: either the doping concentra-tion in the semiconductor increased or the composition ofthe metal layer at the interface changed. As the dopingconcentration is increased, the width of the depletion layeris decreased and the barrier becomes thin enough that thethermally excited carriers can tunnel through near the topof the barrier. This current transport is referred tothermionic-field emission. The deviation of the n-valuefrom unity may be used as a measure of the relativecontribution of thermionic-field tunneling to conduction.An appropriate form of the diode equation for boththermionic and thermionic-field emission is:

JsJ exp qVrnkT yexp 1rn y1 qVrkT 4� 4Ž . Ž . Ž .Ž .o

w x24 . For n)2, the diode conducts better under reversew xbias 25 . When ns2, the current–voltage characteristic is

w xsymmetrical, and field emission tunneling dominates 24 .In the semiconductor technology, the ‘ohmic’ contactmeans a contact with symmetrical, nearly linear I–V char-

w xacteristics 24 . It means that a contact with existing Schot-Ž .tky barrier can be applied as Ohmic contact see, e.g., 4 .

In our case, the samples heat-treated at 6008C had nearlysymmetrical I–V characteristic, so we can say that thesecontacts have more ohmic character than rectifying charac-

Ž .ter. Fig. 7 shows the original measured I–V character-istics of the NiGe2 samples heat-treated at 550 and 6008C

Žjust as the fitted I–V characteristics the calculated Schot-

Fig. 7. Illustration of the current–voltage relationship of the NiGe2Ž . Ž . Ž .samples heat-treated at 5508C a and 6008C b using Eq. 4 .

( )L. DaÕid et al.rThin Solid Films 323 1998 212–216´216

tky barrier heights and ideality factors are replaced in Eq.Ž ..4 . The XTEM measurements on the as-deposited NiGe1sample show that the original NirGe interface disap-peared, while, in contrast, Ni and Ge reacted with eachother forming a NiGe compound. The XTEM results provedthat even during the evaporation or at least during the1808C heat treatment, the evaporated amorphous Ge be-came polycrystalline. The contacts have Schottky charac-ter. When the annealing temperature was increased to5508C, the I–V characteristic started to straighten. TheXTEM results show that deep pits were etched into thesemiconductor. These pits contained Ga, As, Ni and Ge.The EDS results prove that at 5508C, Ni reacts with GaAsand Ga is released and diffuses out onto the upper layer.

w xWhen the Ni reacts with GaAs, Ni GaAs 26 is formed.x

During this process, more or less Ge atoms also diffuseinto this ternary layer. It is generally supposed that theformation of a nqq-GaAs layer can be attributed to thelayer, which is rich in Ge. The Ge-doped nqq-GaAsenhances tunneling probability through narrowing the bar-rier and Schottky behaviour changes to ohmic character.But, in our experiment, the barrier lowering plays a moreimportant role in making ohmic character due to the pits

w xfilled with the ternary compound As, Ga, Ni : Ge plays arole of the current-conducting points according to the

w xBraslau theory 1,2 . Ohmic behaviour was also observedin the case of NiGe2, but in this case, the contact resis-tance was higher than that for NiGe1 samples. The factthat the surface was very smooth during the heat treatmentwas due to the high melting point of the NiGe compoundŽ . w x8508C 27 .

5. Summary

The behaviour of NirGern-GaAs interface, where Geand Ni were deposited by electron beam evaporation, canbe converted from rectifying to ohmic by a heat treatment.The Schottky barrier height decreased with increasing theheat treatment temperature. The specific contact resistanceof these ohmic interfaces depends on the Ge concentrationin the NiGe layer. The microstructures of the NiGe ohmiccontact show that Ni enters into reaction with GaAs result-

Ž .ing in deep pits ternary compound , which is rich in Ge.These pits containing Ge play a role in the formation of

Ž .the low barrier ohmic character Braslau’s contact model .The exact mechanism of the barrier lowering requiresfurther investigations.

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