Spin-dependent properties of two-dimensional AlGaN/GaN electron systems

• Main Authors: Kai-Sin Cho, 邱凱信
• Other Authors: Yang-Fang Chen
• Format: Others
• Language: en_US
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/55114136833277012367

博士 === 國立臺灣大學 === 物理研究所 === 95 === Abstract We have investigated spin properties using optical (circular photogavinic effect) and transport measurement (Shubnikov-de Haas oscillations) in two-dimensional GaN electron systems. This dissertation contains the following parts. At the beginning, the circular photogalvanic effects (CPGE), induced by ultraviolet (325 nm) radiation, have been observed in the (0001)-oriented Al0.15Ga0.85N/GaN superlattices. The CPGE current changes sign upon reversing the radiation helicity, and it is up to two orders of magnitude larger than that obtained by far-infrared radiation. This result suggests the existence of a sizeable spin splitting in AlGaN/GaN superlattices. In order to detect the underlying mechanism behind spin splitting, low-temperature magnetotransport measurements were performed on AlGaN/GaN two-dimensional electron systems. By studying the beating pattern in the Shubnikov–de Haas oscillations in a perpendicular magnetic field, we are able to measure the zero-field spin-splitting energies in our systems. Our experimental results demonstrate that the Rashba term due to structural inversion asymmetry is the dominant mechanism which gives rise to the measured zero-field spin splitting in our wurzite AlGaN/GaN structures. In addition, to confirm our results, we also report zero-field spin splitting using microwave-modulated Shubnikov-de Hass (SdH) oscillations in an Al0.25Ga0.75N/GaN heterostructure and get the same conclusion. In order to obtain a unified physical picture, we compare CPGE and SdH measurements in the same sample. Both experiments are tested in (0001)-oriented Al0.25Ga0.75N/GaN heterostructures. Though a quantitative analysis, we establish the link between the beating of SdH oscillations and the CPGE. This shows that the underlying mechanism responsible for both phenomena is the Rashba effect. At last, we investigate spin in high magnetic regime. By measuring the positions of a pair of spin-split SdH maxima, we are able to estimate the g-factors at different Landau level (LL) indices. We find the g-factor is enhanced over its bulk value in GaN (~2) due to many-body exchange interactions. Moreover, the measured g-factor increases with decreasing LL index, indicating that many-body electron–electron interactions become stronger as the number of occupied LLs decreases.