Vacuum/volume 50/number 3–4/pages 395 to 398/1998© 1998 Elsevier Science Ltd\ Pergamon All rights reserved. Printed in Great Britain

0042–207X/98 $19.00+.00PII : S0042–207X(98)00071–2

Electrical and structural characterisation of Ni/Ge/n-GaAs interface

L David,a* B Kovacs,a I Mojzes,b B Pecz,a J Labara and L Dobos,c a Institute of Microelectronics and Technology,Kando Kalman Polytechnic, H-1084 Budapest, Tavaszmezo u. 17, Hungary ; b Department of ElectronicsTechnology, Technical University of Budapest, H-1521 Budapest, Hungary ; c Research Institute for TechnicalPhysics of the Hungarian Academy of Sciences, H-1325 Budapest, P.O. Box 76, Hungary

received for publication 11 February 1998

The structural and electrical behaviour of Ni/Ge layers deposited onto n-type GaAs by electron beamevaporation were studied. The samples have been annealed for 20 min at different temperatures in flowingforming gas—H2 : N2 (5% : 95%)—in a tube furnace. When the annealing temperature was increased theSchottky barrier heights calculated from I-V and C-V characteristics decreased, and the I-V characteristics ofthe samples heat treated at 600°C became linear. The contact resistance of the samples with linearcharacteristic was measured with Cox method. The calculated specific resistivity was 4.4×10−4 ohmcm2 inthe case of sample heat treated at 550°C and 5.1×10−5 ohmcm2 after annealing at 600°C. In the sampleannealed at 550°C protrusions appeared with the size of 20–30 nm at the interface of semiconductor-metal.Detectable amount of Ge was found in addition to Ni, As, and G in the protrusions. Moreover, those featurescontain significantly less Ga than As. The protrusions containing Ge play a role in the formation of the lowbarrier contact. © 1998 Elsevier Science Ltd. All rights reserved

Introduction

To convert the originally rectifying contact metallization toOhmic contact a heat treatment is required generally[ The Ohmiccontacts prepared by heat treatment called alloyed contacts[ Mostof the alloyed Ohmic contacts to n!type compound semi!conductor are narrow barrier contacts produced by the heavydoping of the compound semiconductor with donors "typicallywith Ge# during the heat treatment[ Although the gold basedcontact structures are widely used in the compound semi!conductor technology the research and development of new con!tact types are main topics of this _eld[ Since the gold andgermanium had an eutectic composition with a low meltingpoint*245>C*the thermal degradation of the Au:Ge based con!tacts begins at rather low temperature[

AlGeNi0Ð2PdGe3 and NeGe4\5 metallization systems are usedto making Ohmic non!gold base contact to n!GaAs[ GenerallyGe is assumed to form narrow Schottky barrier by doping thesemiconductor substrate in annealed contacts[ The resultsobtained in our investigation concerning NiGe!nGaAs contactsshowed that NiGe metallization gives a low barrier contact[ Theappearing Ni and Ge rich protrusions promote the current ~owaccording to the Braslau model[6

� Corresponding author[

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Experimental procedure

Prior to the metal deposition the GaAs surface was etched by themixture of NH3OH ] H1O1 ] H1O with the ratio 0 ] 0 ]099 at 9>Cfor 09 s followed by a rinse with 07 MV water and it was blowndry with _ltered dry nitrogen "Ni"16 nm#:Ge"12 nm# layer hasbeen deposited onto GaAs "having an n!type epitaxial layerdoped by sulphur of ND � 7[0904 cm−2 with 09 mm layer thickness#by electron beam evaporation[ The substrate temperature duringthe evaporation was kept at 049>C[ Just prior to metal depositiona movable Cu shadow mask was positioned onto the substrate[The contacts had circular shape with diameters of 79mm\ 149mm\199mm and 069mm[ After evaporation the samples were annealedfor 19 min in ~owing forming gas*H1 "4)# ] N1 "84)#*at fourdi}erent temperatures ] 349>C\ 499>C\ 449>C and 599>C[ Thepatterned samples were encapsulated using AuGe eutectic ontheir back side7 and Al wire bonding on the contacts studied[8

The current!voltage and capacitance!voltage characteristics weremeasured using Keithley 129 Programmable Voltage Source\Keithley 125 Source and Measure Unit\ and a Hewlett!Packard3160B CV Meter[ The structural characterization was carried outby cross!sectional Transmission Electron Microscopy "XTEM#equipped with Energy Dispersive System "EDS#[ The surfacewas studied by Scanning Electron Microscopy "SEM#\ and thevolatile component loss was monitored by a quadrupole massspectrometer[09\00 The method of the TEM sample preparation isdescribed in Ref[ 01[ The TEM pictures were taken at 199 keV

L David et al : Characterisation of Ni/Ge/n-GaAs interface

Figure 0[ the I!V curves of NiGe!nGaAs samples after heat treatment for 19 min at 399\ 349\ 499\ 449 and 599>C[

using a Philips CM 19 microscope[ EDS analysis was done byNoran microprobe on the TEM[ To study in situ the volatilecomponent loss and the actual surface morphology\ SEM "JEOLJSM!T19# was combined with mass spectrometer system"ATOMKI Q099#[ We used TR!5545 digital multimeter for con!tact resistivity measurement[ This instrument has a resolution of9[0 mV in the 09V range at 09 mA measurement current[ In thisrange the accuracy of measurement is 9[94)[

Electrical results

The I!V "current!voltage# and C!V "capacitance!voltage#measurements were carried out on all samples[ The measured I!V characteristics of the samples are shown in Fig[ 0[ The contactarea of the samples was 9[4 mm1 The samples were heat treatedat di}erent temperatures for 19 min[ The sample heat treated at599>C shows a linear I!V characteristic "Fig[ 0#[ Increasing theannealing temperature of samples the Schottky barrier obtained

Figure 1[ The Schottky barrier height and ideality factor of NiGe:GaAscontacts vs annealing temperature calculated from I!V and C!V charac!teristics[

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from I!V and from C!V characteristics decreased while the ide!ality factor increased "Fig[ 1#[

The contact resistance of the samples heat treated at 449>Cand 599>C was measured using the Cox method[02

The results of the measurement of contact resistance are shownin Fig[ 2 in the form of plot r×R vs 0:r where r is the radius ofthe contact and R is the measured resistance[03 Each of the datapoints represents the average of at least _ve measurements[ Theslope of this plot is proportional to rc "ohmcm1#[03 The calculatedspeci_c contact resistivity of the sample heat treated at 449>Cand 599>C were 3[3×09−3 and 4[0×09−4 ohmcm1\ respectively[

Microstructural analysis

The original Ge:Ni interface could not be revealed in the as!deposited sample[ This can be due to the annealing of the sampleat 079>C during the standard XTEM sample preparation[01 The

Figure 2[ Plot of r×R versus l:r of the samples annealed at 449>C and599>C[

L David et al : Characterisation of Ni/Ge/n-GaAs interface

Figure 3[ XTEM micrograph of NiGe!nGaAs sample annealed at 449>Cfor 19 min showing a protrusion with triangular shape[

samples annealed at 449>C showed large pits grown into GaAs"Fig[ 3#[ The pits show triangular shape[ In perpendicular sectionthese formations show elongated forms[ Pe�cz and co!workers

Figure 5[ EDS spectrum of the protrusion of NiGe!nGaAs sample annealed at 449>C for 19 min[

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Figure 4[XTEM of the Ni!Ge!nGaAs sample annealed at 449>C for 19 min[

observed similar pits in GaAs with Au metallization[04 Figure 4shows that the contact layer consists of two parts ] the lower layeris considerably thick\ while the upper one is a thin layer[ The

L David et al : Characterisation of Ni/Ge/n-GaAs interface

Figure 6[ EDS spectrum of the upper metal layer of NiGe!nGaAs sample annealed at 449>C for 19 min[

composition of the pits consisted of Ni\ As\ and Ga with adetectable amount of Ge "Fig[ 5#[ Moreover\ those protrusionscontain signi_cantly less Ga than As[ The lower metal layerconsists of Ni and Ge[ The upper\ thin layer was rich in Ga "Fig[6#[ The simultaneous observation of the volatile component loss"in situ# by Evolved Gas Analysis "EGA# and the change insurface morphology "SEM# during the heat treatment using aheating rate of 29>C:min was carried out[ We found that therewas no detectable As out!di}usion during heating!up to 519>C[It is worth mentioning that the surface of the samples remainedvery smooth during the whole heating!up process[

Summary

The behaviour of Ni!Ge!nGaAs interface\ where Ge and Ni weredeposited by electron beam evaporation\ can be converted fromrectifying to ohmic by heat treatment[ The samples annealed at449>C showed large protrusions grown into GaAs[ The com!position of these protrusions was Ni\ As\ Ga and some Ge[ Thisnew phase containing Ge plays a role in the formation of the lowbarrier ohmic character "Braslau|s contact model#[ The inter!action of the contact with the GaAs substrate also resulted in thedi}usion of the excess Ga onto the top surface of annealed con!tact layer[

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