Photodetection characteristics of heterojunctions comprising p-type ultrananocrystalline diamond films and n-type Si substrates at low temperatures

0.06 at.% boron-doped p-type ultrananocrystalline diamond/hydrogenated amorphous carbon composite (UNCD/a-C:H) films were deposited on n-type Si substrates by pulsed laser deposition, and the resultant heterojunctions were evaluated as photodiodes in the deep-ultraviolet range. The rectifying action of the heterojunctions were improved accompanied by a reduction in the leakage current with decreasing temperature. The leakage current was decreased by more than three orders of magnitude at 60 K as compared with that at 300 K. Although the detectivity for 254 nm monochromatic light was gradually enhanced with decreasing temperature, the illumination current at reverse voltages was decreased along with the decrease in the leakage current. This might be because a spike due to a heterojunction band offset gets higher in the conduction band at low temperatures and it act as a barrier for photogenerated electrons flowing from the UNCD/a-C:H film to the Si substrate.