T. Clausen et O. Leistiko, THE IMPACT OF THE SURFACE-ROUGHNESS ON THE ELECTRICAL-PROPERTIES OF AUGENI N-LNP OHMIC CONTACTS/, Semiconductor science and technology, 10(5), 1995, pp. 691-697
The surface roughness of AuGeNi-based ohmic contacts to n-type InP ann
ealed using a single-step rapid thermal annealing (RTA) sequence have
been measured as a function of the annealing temperature and correlate
d to the specific contact resistance. For AuGeNi contacts it was found
that the mean surface roughness had a narrow maximum of 120 nm at 420
degrees C annealing, which is also the onset of very low specific con
tact resistance (7 x 10(-8) Omega cm(2) for N-d = 6 x 10(18) cm(-3)).
In addition, for contacts annealed in between 420 degrees C and 500 de
grees C, large outgrowths were observed. The height of the outgrowths
varied from 2-4 mu m with a 4-10 mu m diameter and a surface density o
f 6 x 10(3) outgrowths/cm(2) at 420 degrees C annealing. The outgrowth
surface density decreased with increasing annealing temperature above
420 degrees C, but outgrowths were still present. The outgrowths thus
represent a potential problem for further processing, but it was foun
d that they could be eliminated without any degradation of the electri
cal properties by employing a two-step annealing procedure. Using this
procedure the reacting species are given the necessary long time to i
nterdiffuse (annealing time 20-90 min) and react in the first step. In
the second step the contacts are activated in a short flash-annealing
(RTA). When optimizing the resulting contacts have very low values of
the specific contact resistance ((5-9) x 10(-8) Omega cm(2) for N-d =
6 x 10(18) cm(-3)) with mean values of the surface roughness between
25 and 110 nm and no visible outgrowths. A similar two-step annealing
procedure using only RTA resulted in a large surface roughness (250 nm
), while still maintaining a very low value of the specific contact re
sistance (7 x 10(-8) Omega cm(2) for N-d = 6 x 10(18) cm(-3)).