THE IMPACT OF THE SURFACE-ROUGHNESS ON THE ELECTRICAL-PROPERTIES OF AUGENI N-LNP OHMIC CONTACTS/

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)).