ELSEVIER Nuclear Instruments and Methods in Physics Research B 136138 (1998) 719-723

Boom Intermctionr With ~6tnid6 6 At0636

Mass and energy dispersive elastic recoil detection studies of low temperature Si/Pd/GaAs and Si/Pd/Al,Ga, 1 _ ,,As interfacial

reactions

Yanwen Zhang ‘**, Mikael Hult ‘*l, Leif Persson ‘-I, Harry J. Whitlow ‘, Margaretha Andersson b, Ian F. Bubb ‘, Mohamed El Bouanani ‘, Peter N. Johnston ‘,

Scott R. Walker ‘, David D. Cohen d, Nick Dytlewski d, Mikael 6stling e, Carina Zaring e2

“ Depurttmnt qf’ Nuclwr Pl~ysicr. Lund Institute of’ Tc~hnology. P. 0. Box 11X. S-221 00 Lund, Sweden b Department oj Inorganic Chrtnistry. Uppsulu Unirersity, P. 0. Bor 531. S-751 21 Uppsulu. Swrden

’ Depurtmmt qf' Applied Physics. Rowd Melbourne Institute oj’ Technology. G. P. 0. Bos 2476 V, Melbourne 3001. Arrstruliu ’ Austrulim Nuckur Scirnce und Technology Orgunizution. PMB I, Menui 2234, Austruliu

’ Drpartmmt qf Electronic. Solid Stutr Electronics, Rowd Institute of Technology. P. 0. Box Electrum 229. S-164 40 Kim. Sweden

Abstract

Interface reactions between bi-layer films of Si (75 nm), Pd (50 nm) and GaAs or substrates of Al,Ga(, _ ,,As have been studied using mass and energy dispersive elastic recoil detection (ERD) to identify the formation and thermal sta- bility of Pd-silicides on GaAs-based systems. Samples were prepared by sequential vacuum evaporation and subse-

quently annealed in a vacuum furnace at temperatures up to 700°C. Pd and Si signals indicate that silicidation has started at 250°C and developed further as the temperature increased for both systems. The results indicate PdSi was

formed in the surface layer, and PdSiz in a region deeper than 2.5 x IO” at. cm-’ without significant reaction with the underlying substrate at temperatures up to 500°C. In the GdAs system at 700°C. all the Pd was bound into the sub- strate together with excess Si, whilst volatile As was lost from the surfdce. The results indicate that the same reactions occurred during the heat treatments of both GaAs and Al,Ga,, _ ,,As systems. 0 1998 Elsevier Science B.V.

PACS: 68.35.D~; 68.35.F~; 68.60.D~: 8 1.15.-z Kqwordx Silicides: SilPdlGaAs; Si/Pd/Al .,Ga(, _ ,,As; Recoil spectrometry; Elastic recoil detection

* Corresponding author. Tel.: 46-46-2227682; fax: 46-46-

2224709; e-mail: Yanwen.Zhdng@nuclear.lu.se.

’ Present address: CEC-JRC-IRMM. Retieseweg, B-2240

Geel. Belgium.

Introduction 1.

_-I The use of compound semiconductors in high

’ Present address: Ericsson Components AB. Opto and RF bpeed applications and optoelectronic devices has

Power Products, lsafjordsgatan 16. S-164 81 Kista-Stockholm, attracted attention to metal/compound semicon- Sweden. ductor interfacial studies [l-8]. Several studies

0168-583X/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved.

PIISO168-583X(97)00771-4

have recently focused on metallic silicides for inter-

connects and ohmic contacts [l-S] because of the potentially stable phases at elevated temperatures.

In many ohmic contact systems Pd is one of the components to both p- and n-type IIIIV semicon- ductors [5-lo]. It has been used with success for

contacts to n-GaAs with the principal benefit of an improved surface morphology and a low con- tact resistance of about 7 x 10 ’ Q cm2 [l 11.

The stability and controllability of the silicide contact process on a nm scale are crucial proper- ties. Unanticipated reactions can drastically alter

the contact properties and nonequilibrium states may eventually lead to device failure. Unlike in

the case of elemental semiconductors, most metals react readily with compound semiconductors

which may lead to the formation of new com- pounds. Since the amount of metal is limited and

heat treatment is carried out in an open system, the reactions of thin metal films with III-V semi-

conductors may differ from those expected from the equilibrium phase diagrams. Therefore, to in- vestigate the interaction and thermal stability of metallurgical junctions in contact with III-V semi-

conductors, basic data are needed. In this work, thin-film interfacial reactions of

Si/Pd/GaAs and Si/Pd/Al,Ga(, ,,As have been studied using mass and energy dispersive elastic re-

coil detection (ERD) [2,8]. In order to compare the

different phase stability and reactions, heat treat- ments of the two systems, GaAs and GaAs alloyed

with AlAs, have been subjected to identical heat

treatment processes.

2. Sample preparation

50 nm Pd and 75 nm Si were sequentially vacu-

um deposited on GaAs and Al,Ga, I _,,As wafers using an electron beam source. Pairs of Si/Pd/

GaAs and Si/Pd/Al,Ga( I ,) As samples were then

heat treated side by side for 30 min at 25O”C, 4OO”C, 500°C and 700°C in a vacuum furnace at

a pressure of 10 ~’ Pa. Depth profiles of different elements were mea-

sured using Time of Flight-Energy (ToF-E) ERD. These measurements were carried out at Lu- cas Heights, Australia, using the Australian Na-

tional Tandem for Applied RESearch

(ANTARES) FN tandem accelerator to produce a 77 MeV “‘I”” ion beam as projectiles. A sche- matic representation of the experimental set-up is

given in Refs. [12.13]. The ToF-E detector tele-

scope consisted of two carbon-foil time detectors with a flight path in between. and followed by an

energy detector. The length of the flight path for the measurements was 437.5 mm. The energy de- tector was a 10 x 10 mm’ ion implanted silicon de-

tector (SiTeK) placed 25 mm behind the second timing detector. Recoiling target atoms were de-

tected at an angle of 45” relative to the incoming

beam. Time and energy channel numbers deter-

mined from the ToF and the energy signals for each recoil were stored using a data acquisition system in list mode and analysed off-line using

CERN Physics Analysis Workstation (PAW) soft- ware. The energy calibration was established fol-

lowing El Bouanani et al. [14]. Conversion of the

recoil energy spectra to elemental depth profiles was based on Ziegler et al.‘s stopping code [15] and the assumption of Rutherford scattering cross sections.

3. Results and discussion

The normalized ERD energy distributions of Si

and Pd recoils from SilPdlGaAs and Si/Pd/ Al,Ga ,, ,,As samples are shown in Fig. 1 for both reference and heat-treated samples. One should bear in mind that the energy spectra from different

elements cannot be compared directly due to the different stopping cross sections. In order to estab- lish the stoichiometry, it is necessary to perform numerical calculations based on the energy loss in small slabs of the samples. The depth scale is

given in units of at. cm ‘. This unit is used because

it does not require assumptions about the density of the thin film or substrate (5 x 10’” at. cm ’ cor-

responds to 10 nm in bulk Si). The relative changes in stoichiometric composition with increasing tem-

perature as a function of depth extending from the surface down to 1.6 x 10” at. cm ’ are shown in

Figs. 2 and 3. In Figs. 2(a) and 3(a). the Si layer, Pd layer and

the substrate of GaAs or Al,Ga,r ,,As, can be

Y. Zhang et al. I Nucl. Instr. and Meth. in Phys. Res. B 136-138 (1998) 719-723 721

(a) Si

I 700 “C SW

Energy (ch.)

ib) Si . ,

0 1500

Energy (ch.)

(cl Pd

,t!Z!%=k 0

Energy (ch.)

(d)Pd

t 500 “C 00

0 td!!!zJ 0

Energy (ch.)

Fig. I. Elemental Si and Pd energy distributions for as-deposited samples and the samples heat treated at 250°C. 400°C. 500°C and

7OO”C, respectively: (a) Si in SilPdlGaAs; (b) Si in Si/Pd/A1,Gall_ ,,As: (c) Pd in SilPdlGaAs: (d) Pd in Si/Pd/Al,Ga,I_ ,,As.

clearly seen. For samples annealed at a tempera- ture of 250°C the energy spectra of Si and Pd re- coils of both sample types were different from those of the reference samples (Fig. 1). The Si and Pd layers became mixed. As shown in Figs. 2(b) and 3(b), Pd atoms have moved towards the surface. The surface concentration of Si de- creased as Si atoms migrated deeper into the Pd layer indicating a reaction between Pd and Si had occurred at this temperature. The profiles of Al, Ga and As indicate that these elements have not moved closer to the surface.

The profiles of Si and Pd in the Al,Ga,l_,YjAs samples changed significantly for heat-treatments at 400°C and 500°C (Figs. l(b), l(d) and 3(c) and (d)). The concentration of Si dropped to 48% from surface to a depth of 2.5 x 10” at. cm-’ where Pd had almost the same concentra- tion. This suggests that PdSi had been formed in this layer. Deeper than 2.5 x lOi at. cm-*, there was more Si than Pd suggesting that PdSi and PdSiz had been formed. Comparing the Pd and Si depth distributions for the samples heat treated at 400°C and 500°C (Fig. 3(c) and (d)), a slight decrease in the Si concentration and a slight in-

crease in the Pd concentration in the surface re- gion of the 500°C sample was found, while the Al. Ga and As signals did not change. For GaAs samples at 500°C similar reactions occurred as shown in Fig. 2(c). The spectra for Ga and As showed no significant reaction with the overlying silicides at temperatures up to 500°C. A descrip- tion of the reaction processes is shown schemati- cally in Fig. 4.

At 700°C for the GaAs sample, the concentra- tion of Pd was much lower at depths correspond- ing to the original Pd layer as compared to the case for lower temperatures (Figs. l(c) and 2(d)). Pd had apparently diffused deeper into the sample. Si had also moved into the substrate (see Figs. l(a) and 2(d)). Unlike all the samples heat treated at lower temperatures, Ga and As had moved right out to the surface. Furthermore the As concentra- tion is significantly lower than that for Ga near the surface. This is consistent with loss of volatile As into the vacuum. The decomposition might also have taken place at all temperatures, but was only large enough to be observed at a temperature of 700°C. The same reactions are expected in the Al,Ga(l_.Y,As samples for annealing at 700°C.

122 Y. Zhang et al. I Nucl. Instr. and Meth. in Phys. Rex. B 136-138 (1998) 719-723

as-dep. +I--Qi

-*--Pd -A-Ga --,-As

l ,..&=t-.-‘-.-.-.

b) 1 r. 250 "C

*08+17 Kb+17 1.2@18 l.Wll

Oapth (at.cm")

im-_Si -*-_Pd

+A1 -A-Ga

b) I -

d)

O.OE+OO 4.OEt17 ROE+17 l.PE+lB l.BE+lB D.pth@t.cmz)

Fig. 2. The depth distributions of SilPdiGaAs sample: (a) as-

deposited: (b) 250°C; (c) 500°C: (d) 700°C. respectively. Fig. 3. The depth distributions of SiiPd/Al,Ga,, ,,As samples:

(a) as-deposited; (b) 35O’C; (c) 400°C: (d) 500°C. respectively.

4. Conclusion Evidently, the data presented in Figs. 2 and 3

show that for heat treatment up to 5OO”C, Al, Ga and As from the substrate have not reacted with the Pd-Silicide layer. Wang et al. [9] found that for Pd/GaAs interface tertiary reactions of the form:

xPd + GaAs -+ Pd,GaAs,

can take place. Comparison this finding with our results suggests that the silicide phases form pref- erentially and these are in equilibrium with the GaAs and Al,Ga( I _.,)As substrates. This inhibits the formation of Pd,,Ga,As, phases with x N 4, J’” l,ZN 1, y 2 z observed for the PdlGaAs sys- tem [16,17].

The formation and the stability of metallic phases in SiiPdlGaAs and Si/Pd/Al.,Gacr _,,As samples subjected to different heat treatments have been studied. Pd began to diffuse to the surface and react with Si during the heat treatment at a temperature of 250°C. As the temperature in- creased, stable silicides started to form and reac- tions were probably completed at 500°C. The stable compounds up to this temperature are con- sistent with PdSi at the surface and PdSiz close to the substrate interface. Heat treatment of the Sil PdlGaAs sample at 700°C resulted in interdiffu- sion of Pd, Si, Ga and As with loss of As from the surface region because the system was not closed. The heat treatments of the GaAs and

Y. Zhamg et ul. I Nucl. Instr. and Meth. in Phys. Rex B 136-138 (I 998) 719-723 723

Si/PdlGaAs as-deposited

Si/Pd/AI,Ga+& as-deposited

t

Pd-50 run _-______--__ A&4,.&

250 “C 250 “C

Si _p62si______

_-_______-_ Pd

___-__-----

GaAs

500°C 4OOT

7oo”c 500°C

Fig. 4. Tentative schematic description of the reaction processes

for 30 min heat-treatments.

Al,VGa( 1 _,,As were carried out under identical con- ditions, but no difference in the reactions could be seen between the two systems.

Acknowledgements

The authors would like to express their grati- tude to the ANSTO staff of the ANTARES facility for their help. Also the support of the Australia In- stitute of Nuclear Science and Engineering (AIN- SE) and its staff, as well as the Australian

Department of Industry, Technology and Com- merce (DITAC) are gratefully acknowledged. YZ is grateful for support from the Swedish Institute (SI). HJW gratefully acknowledges a travel grant from the Royal Physiographical Society in Lund.

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