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

摘要

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 N_2 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 r_t of 0.4 Ω mm was obtained for Ti/Al metal at thickness ratio of 1:5. The sheet resistance R_(sh) associated with it was 600 Ω/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.