Metal penetration and dopant redistribution beneath alloyed Ohmic contacts to n-GaAs

Abstract

In order to estimate the depth of penetration of Ohmic metals into n-GaAs during the alloying process, marker layer samples fabricated by molecular-beam epitaxy were utilized. The samples consisted of n-GaAs substrates onto which were grown a 500-nm undoped GaAs buffer layer, followed by 20 layers, each 50 nm thick, alternately doped with Si at levels of 1×1016 and 1×1018 atoms/cm3. Ohmic metals consisting of Ni/Ge/Au/Ni/Au or Ni/Ge/Au/TiB2/Au were evaporated onto the marker layer samples and alloyed on a strip heater. Secondary ion mass spectroscopy (SIMS) depth profiling showed that metal penetration extended as much as 0.5 μm into the GaAs. In addition, significant redistribution of the marker layer Si was observed, and is assumed to result from a vacancy enhanced diffusion process. In a SIMS depth profile performed from the back side of the wafer no dispersion of the metal depth penetration was observed, suggesting that differences in penetration depth of the metals observed in front side profiles are the result of ion beam mixing, surface roughness, and/or inhomogeneities within the alloyed region.