High temperature (400°C) performance of ohmic contacts to n-type GaN and GaAs

Stable high temperature ohmic contacts to III-V semiconductor materials are still rather problematic. In this work we report results on thermodynamically stable Ohmic contacts to n-type Ca-V semiconductor materials (e.g., GaN and GaAs) that at e capable of withstanding prolonged 400 degrees C exposure in untreated air ambient. In this work, Cu3Ge was studied as an ohmic contact to n-GaN and n-GaAs. Contact layers of Cu-Ge alloy with 25% Ge were prepared by sequentially depositing a Ge layer on top of the Cu layer on GaAs and GaN epitaxial layers. The deposition was performed at room temperature, followed by an annealing procedure. Specific contact resistance of Cu3Ce to n-GaAs, with doping concentration of 2.3x10(17)cm(-3), was found to be about 6.0 x 10(-7) Omega.cm(2). Unprotected Cu3Ge ohmic contacts were found to oxidize when exposed to temperatures higher than 300 degrees C. A thin TiW layer serving as a diffusion barrier between Cu3Ge and Au was found to be very effective in preventing the contacts from oxidation. The contact resistance remained constant at around 10(-6) Omega cm(2) after annealing at 300 degrees C for 150 hours. Aging tests, using Cu3Ge as an ohmic contact covered by TiW and Au showed that this contact had superior thermal stability even after it was exposed at 300 degrees C for 500 hours in air atmosphere. Furthermore, aging tests of diodes, where Cu3Ge covered with TiW and Au was used as the ohmic contact to n-type GaAs, showed stable I-V characteristics after annealing st 400 degrees C for 120 hours. Since there was no significant change in the contact resistance, Au/TiW/Cu3Ge can; be consider-ed a promising ohmic contact to n-GaAs, suitable for high temperature devices. The ohmic behavior of Cu3Ce to n-GaN is first reported in this work. It was found that, after vacuum annealing, the wetting properties and the surface morphology of Cu3Ge on n-GaN were excellent. These are very critical issues for forming a good ohmic contact. Studies on thermal stability of both Pd/Al and Ti/Al ohmic contacts to n-type GaN are also included in this work. For Pd/Al contacts, the best furnace annealing condition was found to be 610 degrees C for 30 minutes in N-2 ambient. These contacts were found to remain ohmic at temperatures up to 670 degrees C, but with severe surface oxidation. Pd/Al contacts are Schottky for annealing times less than 15 minutes at 600 degrees C. For Ti/Al contacts to n-GaN, an annealing time less than three minutes resulted in Schottky characteristics. Low specific contact resistance was achieved using conventional annealing in N-2 ambient. The lowest specific contact resistance observed was 5.4x10(-7) Omega cm(2) to n-GaN with a doping concentration of 1x10(18)cm(-3). The I-V characteristics of Pd/Al, covered with TiW/Pt changed from ohmic to Schottky after aging at 350 degrees C for 2 hours. Deposition of a very thin Cr or TIW layer between Ti/Al and Au layers, resulted in a very stable and low contact resistance Ti/Al contacts, which did not change even after an aging of 130 hours at 350 degrees C and 130 hours at 400 degrees C. However, the contact resistance increased from 2.4x10(-6) Omega cm(-2) to 3.8x10(-6) Omega cm(-2) after annealing at 500 degrees C for 2 hours.