High temperature alloyed Ag based ohmic contacts to pseudomorphic high electron mobility transistor (p-HEMT)

The quality of ohmic contacts in pseudomorphic high electron mobility transistors (p-HEMTs) formed by sequential e-beam evaporation of Ni/Ge/Au/Ag/Au was investigated as a function of alloying cycles in the range from 390degC to 460degC for duration of 30 sec. The contact resistance (R_c) and the specific transfer resistance (ohm-mm) values of ohmic contacts so formed were measured by Transmission line method (TLM) and compared with the contacts formed without Ag interlayer. XRD technique revealed the presence of Au-Ga phase at alloying temperatures 420degC and 440degC indicative of the formation of good quality and low R_c ohmic contacts. The Ag based contacts showed improved specific transfer resistance values and smooth surface morphology at wider alloying temperatures compared to the contacts formed without using Ag as a barrier layer. This factor is beneficial in the fabrication of lower resistance ohmic contacts in HEMTs, where a deeper penetration of ohmic metal upto the 2DEG channel is desirable.