The impact of the surface roughness on the electrical properties of AuGeNi/n-InP ohmic contacts

Abstract

The surface roughness of AuGeNi-based ohmic contacts to n-type InP annealed using a single-step rapid thermal annealing (RTA) sequence have been measured as a function of the annealing temperature and correlated 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 contact resistance (7*10-8 Omega cm2 for Nd=6*1018 cm-3). In addition, for contacts annealed in between 420 degrees C and 500 degrees 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 of 6*103 outgrowths/cm2 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 found that they could be eliminated without any degradation of the electrical properties by employing a two-step annealing procedure. Using this procedure the reacting species are given the necessary long time to interdiffuse (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)*10-8 Omega cm2 for Nd=6*1018 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 resistance (7*10-8 Omega cm2 for Nd=6*1018 cm-3).