| Summary: | 碩士 === 國立交通大學 === 電子物理系所 === 100 === This study investigates the photo-capacitance and photo-current of light-induced excess carriers in GaAsN/GaAs quantum wells. Initially, we establish a escaping model for light-induced excess carriers in GaAsN quantum wells (QWs), and utilize this escaping model to understand the generation mechanism of photo-capacitance and photo-current in GaAsN QWs. According to our escaping model for light-induced excess carriers, the generation of photo-capacitance and photo-current in GaAsN QWs is correlated to the generation rate of light-induced excess carriers in GaAsN QWs、recombination rate of light-induced excess carriers in GaAsN QWs、electron and hole emission rates of GaAsN QWs, and these behaviors of light-induced excess carriers in GaAsN QWs are different under various conditions. Therefore, we utilize photo-capacitance and photo-current analysis combined with our escaping model to probe the light-induced excess carriers in GaAsN QWs under various conditions.
During the increase of temperature, the photoluminescence (PL) efficiency of GaAsN QWs is decreased, and this result is also indicated that the recombination rate of light-induced excess carriers is reduced during increasing temperature. Thus, during increasing temperature, the amount of electron-hole pairs escaping from GaAsN QWs is increased, resulting in the enhancement of photo-capacitance and photo-current in GaAsN QWs. When the thickness of GaAsN QWs is increased, the overlapping of electron and hole wave functions in GaAsN QWs is reduced, and the overlapping of electron and hole wave functions in GaAsN QWs is also correspond to the recombination rate of light-induced excess carriers in GaAsN QWs. Hence, as increasing the thickness of GaAsN QWs, the amount of electron-hole pairs escaping from GaAsN QWs is increased simultaneously, leading to the enhancement of photo-capacitance and photo-current in GaAsN QWs.
Furthermore, the electron emission rate of GaAsN QWs electron states is determined by the electric field in the bottom GaAs, which depends on the applied bias. Therefore, as the applied bias increasing, the electron emission rate of GaAsN QWs electron states is also increased, leading to the enhancement for photo-capacitance of GaAsN QWs. However, further increasing the applied bias, the hole emission rate of GaAsN QWs will increase to the extent close to the electron emission rate of GaAsN QWs, resulting in the enhancement of photo-current and the diminution of photo-capacitance. In addition, the presence of N-related localized states effectively suppresses the tunneling emission of GaAsN QW electron states, leading to a slow electron emission rate for GaAsN QWs； thermal annealing can reduce the number of N-related localized states, resulting in a recovery of the tunneling emission of GaAsN QWs electron states, leading to a fast electron emission rate for GaAsN QWs. Therefore, during thermal annealing, the electron emission rate of GaAsN QW is also increased, resulting in the enhancement of photo-capacitance in GaAsN QWs.
Moreover, we also use the transient measurement to probe the decay rate of photo-capacitance in GaAsN QWs. According to our results, comparing with the photo-capacitance induced by the defect states, because the photo-capacitance in GaAsN QWs is induced by the holes confined in GaAsN QWs, the decay rate of photo-capacitance in GaAsN QWs is relatively faster than the decay rate of photo-capacitance induced by the defect states, which is attributed to the relatively fast hole emission rate for GaAsN QWs.
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