Proposal and Estimation of High Polarization and High Quantum Efficiency Semiconductor Photocathode

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1. GaAs quantum wire structure was proposed as a new photocathode of polarized electron source with high polarization and high quantum efficiency. The anodization was carried out using a (111) oriented p-GaAs substrate (1x10^<18>cm^<-3> : Zn doped) under the conditions of HF concentration of 5-30%, current density of 20-200mA/cm^2 and immersion time of 2-5min. High density and low-size triangle poles (40-90nm in width) were obtained by this simple method when HF concentration was 25%, immersion time 2mins and current density 150mA/cm^2. The shift of the band edge and the splitting of valence band were predicted on the assumption that the quantum wire had the circular cross section.2. The circular polarization of photoluminescence of the p-GaAs quantum wire samples fabricated by anodization method was measured on the excitation of circularly polarized light. Peaks appeared at the higher energy side of the band edge of bulk GaAs in the luminescence polarization spectra. The luminescence polarization exceeded 25% which was the maximum value obtained in bulk GaAs. The luminescence polarization depends on the spin polarization of the conduction band electrons and the splitting of the valence band. These peaks and the enhancement of luminescence polarization were thought to be due to by the band edge shift and the splitting of the valence band.3. The luminescence polarization method using a streak camera is applied to the measurement of the spin relaxation time and the life time of electrons of samples. The temperature dependence of the spin relaxation time of the strained GaAs sample was measured. Electron-hole scattering is thought to be the main mechanism of the spin relaxation of our strained-GaAs photocathode. The spin relaxation time of GaAs quantum wire samples fabricated by the anodization method was measured by using the same method. It was found that the anodized p-GaAs sample had the long spin relaxation time in comparison with original substrate.