Effects of cap layer on ohmic Ti/Al contacts to Si+ implanted GaN

A low resistivity ohmic contact to Si-implanted GaN was achieved using a metal combination of Ti/Al. The effect of a protection cap during post-implantation annealing is investigated, and how it affects the specific contact resistivity (ρc). Relevant differences between the protected (PR) sample with SiO2 and unprotected (UP) sample during the post-implantation annealing were observed after metal alloying at 700 °C. The lower values of ρc have been obtained for UP sample, but with very low reproducibility. In contrast, SiO2 cap layer has demonstrated its relevance in yielding a much more uniformity of a relatively low ρc around 10−5 Ω cm2. Related mechanism for the uniformity in ρc was discussed based on the results obtained from electrical measurements, XRD (X-ray diffraction) analysis, AFM (atomic force microscopy) and SEM (scanning electron microscopy) observations.