Interface Reactions and Electrical Behaviour of Ni-AuGe Contacts on GaAs
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Ohmic contact resistivities depend
on the carrier concentration of the GaAs semiconductor [7] and interface structures that form during alloying [8]. The reactions between metal films and GaAs have been studied by Auger Electron Spectroscopy (AES) [2,3,9,10,11, 12], Rutherford Backscattering Spectrometry (RBS) [5,6], Transmission Electron Microscopy (TEM) [6,8,13], Scanning Electron Spectroscopy (SEM) [6,11, 12] and electrical measurements [1,2]. In this investigation the role of the individual metal layers in the GaAs/AuGe, GaAs/AuGe/Ni, GaAs/Ni, GaAs/Ni/AuGe and GaAs/Ni/AuGe/Ni metallization systems was studied by combining AES depth profiling and RBS for the first time. The well-known GaAs/AuGe/Ni system and the new system developed by this laboratory, GaAs/Ni/AuGe/Ni, were compared with respect to the diffusion of Ge in the layers to the metal/GaAs interface from where it effectively dopes the GaAs. EXPERIMENTAL Thin film structures were prepared on (100) oriented GaAs substrates by resistive heating evaporation in a vacuum of better than 10-6 torr. Prior to evaporation, the wafers were degreased, etched in a 5:1:1 solution of H2S0 4 : H20 2 :H 20 for 3 minutes, rinsed and etched in concentrated HCI for 90 seconds. Annealing was performed in forming gas (90% N2 :10% H2 ) at 450 °C for 5 minut es. Auger analysis was carried out using a 3 keV, 0.08 ýLAelectron beam. Continuous sputtering with an Ar+ beam at 1.5 keV and an ion current density of
150
pAcm-2
was
used
to
obtain
the
depth
profiles
at
a
2xl1- 7 torr. The Auger transition energies used were: Au 69 eV; Ga 1070 eV; Ge 1147 eV and As 1228 eV.
Mat. Res. Soc. Symp. Proc. Vol. 54. ý1986 Materials Research Society
pressure
of
Ni 848 eV;
422
Rutherford backscattering analysis was carried out, using 3 MeV alpha particles from a Van de Graaff accelerator. The backscattered particles were detected at an angle of 1650 and typical beam currents of 100 nA were used. Current-Voltage characteristics were determined to establish the ohmicity of the annealed contacts in the case of the GaAs/Ni/AuGe/Ni structure. RESULTS AND DISCUSSION GaAs/AuGe System 100
100
Asdpstd100
0 8o 670 "
435"C-5rimn ,
-00 -0 z
0 050
08o801
40
1 23
6000 0
45 67689 Sputtertime1(min)
2008
86000 0 1
00
Go
8,0 0
....... . .
.......... 024
46 0 12 14 Sputter time,6,mm)
IS20
8~o1000 80
300
0
0 1200
.b after annealn et
450C- 580i
6000 0
20000
28000
E
0
56 7 60910 34 Sputter time, (min)
12
GA
A8.6.40 -As 6E30
.
10
10
58
601200000
deposited, FI.I
,950• Go
.........., 200 G,0
0
0 860 , 050
90 8s80o0O.7 0 060' 5
prflsad
10
80
3000
ocf4t/I35C5m B
pcr
60A20
000
80
mntsad
of th
a
280 30
cmfe-° 450i Auesse;
(a)as
for 5 minutes. Figure 1(a)
shows the Auger depth profile (top)
and RBS spectrum
(bot-
tom) for the as-deposited sample. It is clear from this information that no separation of Au and Ge occurs during evaporation. After heat treatment at 435 oC for 5 minutes, Ge diffuses to the surface. This temperature is above the A
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