Temperature dependence of the photoelectric conversion quantum efficiency of 4H-SiC Schottky UV photodetectors

Ultraviolet Schottky photodetectors based on n-4H-SiC (N-d - N-a = 4 x 10(15) cm(-3)) epitaxial layers of high purity have been fabricated. Their spectral sensitivity range is 3.2-5.3 eV peaking at 4.9 eV (quantum efficiency is about similar to 0.3 electron/photon), which is close to the bactericidal ultraviolet radiation spectrum. The temperature dependence of the quantum efficiency of 4H-SiC Schottky structure has been investigated to determine the temperature stability and the mechanism of the photoelectric conversion process. At low temperatures (78-175 K) the quantum efficiency increases with increasing temperature for all photon energy values and then tends to saturate. We suppose that some imperfections in the space-charge region act as traps that capture both photoelectrons and photoholes. After some time the trapped electron-hole pairs recombine due to the tunnelling effect. At high temperatures (more than 300 K), the second enhancement region of the quantum efficiency is observed in the photon energy range of 3.2-4.5 eV. It is connected with a phonon contribution to indirect optical transitions between the valence band and the M-point of the conduction band. When the photon energy is close to a direct optical transition threshold this enhancement region disappears. This threshold is estimated to be 4.9 eV. At photon energies more than 5 eV a drastic fall of the quantum efficiency has been observed throughout the temperature interval. We propose that in this case the photoelectrons and photoholes are bound to form hot excitons in the space-charge region due to the Brillouin zone singularity, and do not contribute to the following photoelectroconversion process.