Thermal stability of diamond photodiodes using tungsten carbide as Schottky contact

Deep-ultraviolet (DUV) photodiodes are fabricated using tungsten carbide (WC) Schottky and Ti/WC ohmic contacts on lightly boron-doped homoepitaxial diamond thin films. The thermal stability of the electrical and optical properties of the photodiodes upon isothermal annealing at 500 degrees C for 5 h in argon/air ambient is demonstrated. The ideality factor is improved to unity after annealing for 1 h and increases to around 1.5 after subsequent annealing for longer time periods. The leakage current for at least 30 V reverse bias is lower than 10(-14) A before and after annealing for 4 h. The photoresponsivity at 220 nm is enhanced markedly by a factor of 10(3) after annealing, resulting in a DUV/visible blind ratio as large as 10(6) at 2 V reverse bias. In addition, the effects of annealing and applied bias on decay times and photoresponse spectra are examined, respectively. These results are discussed in terms of surface modification of the initially oxidized diamond epilayer.