Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5x10(17) cm(-3)) were studied. The lowest contact resistivity derived from the annealed contact was rho(S)=3.0x10(-6)Omega cm(2). The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti-Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to the formation of Ti-Al alloys with the remaining Al content. Consequently, both the Al and the Ti (from the second Ti layer) contents were almost fully consumed, and none of them were left to appear on the contact surface to form oxides. (C) 2003 American Institute of Physics.