| Summary: | 博士 === 臺灣大學 === 物理研究所 === 95 === In this thesis, we perform several studies of electrical and optical properties of nitride-based semiconductor heterostructure, including InGaN thin film, InGaN/GaN multiple quantum wells, InGaN/GaN superlattices, AlGaN/GaN superlattices, and InGaN/GaN nanotips. These results are presented as follows.
The structural and optical properties of Mg-doped AlGaN/GaN superlattices have been investigated by photoluminescence (PL), scanning electron microscopy (SEM), cathodoluminescence (CL) and transmission electron microscopy (TEM). We found that the edge blue-band emission shows a strong optical anisotropy. Through the combination of the CL and TEM images, we clearly establish that the underlying microstructure responsible for the blue luminescence in Mg-doped AlGaN/GaN arises from the pyramidal defects.
We have reported an intriguing photoelastic effect in InGaN QDs for the first time. The optically modulated internal strain contributes to the blueshift in edge PL spectra, a reduction of the refractive index, and a redshift in the InGaN LO phonon mode. In addition, the change of the temperature dependence of the PL emission energy under high and low excitation density can also be explained consistently.
We have demonstrated a significant improvement of the emission from InGaN/GaN nanotip arrays compared with InGaN/GaN MQWs. The nanotip arrays were formed by a simple and low cost self-masked dry etching process, which is compatible with the current semiconductor technologies. Our unique approach is able to enhance the light output power by a factor of up to 10 times. Based on our study, we clearly demonstrate that the main underlying mechanism for the enhanced luminescence arises from the strain relaxation in the nanotip through its inherent characteristic of a large surface-to-volume ratio.
Lateral current-induced spin polarization in InGaN/GaN superlattices (SLs) without an applied magnetic field is reported. The fact that the sign of the nonequilibrium spin changes as the current reverses and is opposite for the two edges provides a clear signature for the spin Hall effect. In addition, it is discovered that the spin Hall effect can be strongly manipulated by the internal strains. A theoretical work has also been developed to understand the observed strain-induced spin polarization. Our result paves an alternative way for the generation of spin polarized current.
The correlation between optical and structure properties obtained by XRD, SEM images, EDS, and localized CL spectra provides a direct and concrete evidence to support the fact that the formation of nanoclusters is responsible for the enhanced luminescence in InGaN thin films. Our results shown here can serve as an important clue for the enhancement of the luminescent intensity in future optoelectronic devices.
Key words : PL, InGaN/GaN, Raman
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