Free-standing gallium nitride Schottky diode characteristics and stability in a high-temperature environment

Schottky diodes have been fabricated using low-resistivity n-type free-standing GaN substrates with a reduced defect density lowly doped n-type epi-layer and an Ni/Ti/Pt/Au Schottky contact metalization. A thermionic field emission current transport mechanism was identified with a Schottky barrier height of about 0.75 eV and a diode ideality of 1.1 measured at 25 deg C, both of which increase with measurement temperature up to 200 deg C. The diodes were subjected to long-term testing under forward current (1.3 A cm-2) or reverse voltage (-3.5 V) biased storage at 300 deg C in N2 for 466 h and were also monitored under non-biased storage conditions for up to 1000 h at 350 deg C and 400 deg C in N2 or at 300 deg C for 1500 h in air. Except for the non-biased storage test at 400 deg C, the diodes show < 10% drift in ideality and barrier height during the long-term storage tests. For the 400 deg C test, there is a significant increase in both barrier height and ideality over a relatively short storage period (48 h). This to be the first reported study on the long-term stability of Schottky diodes on free-standing GaN and while no catastrophic (e.g. thermal runaway) degradation of any of the diodes was observed, it is proposed that optimized thermal annealing of the Ni-based Schottky contact metalization in the temperature range 350-400 deg C is necessary for stable long-term operation at high temperature.