Comprehensive study of Ohmic electrical characteristics and optimization of Ti∕Al∕Mo∕Au multilayer Ohmics on undoped AlGaN∕GaN heterostructure

The performance of Ohmic contacts is highly dependent on the metal stack contents and layer thicknesses evaporated, annealing temperature, and annealing time. A four-layer-based metal stack was used to form Ohmic contacts on an undoped AlGaN∕GaN material structure, grown by molecular-beam epitaxy technique on a sapphire substrate. The thicknesses of the overlayers Ti∕Au, or Mo∕Au, investigated were fixed. The rapid thermal annealing time and temperature in a N2 ambient are optimized for Ti∕Al∕Mo∕Au (from bottom to top) metal stack system, with bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The lowest contact transfer resistance rt of 0.4Ωmm was obtained for Ti∕Al metal at thickness ratio of 1:5. The sheet resistance Rsh associated with it was 600Ω∕sq. The edge acuity of Ti∕Al∕Mo∕Au Ohmic contacts after annealing has been discussed and examined under scanning electron microscopy for the optimized Ohmic contact transfer resistance associated with the bottom Ti∕Al metal at thickness ratios of 1:5 and 1:6. The straightness of the edge of the metal contact has also been compared with the one using Ti∕Al∕Ti∕Au as the Ohmic metal stack under the condition of same annealing environment and metal thickness.