| Summary: | 碩士 === 長庚大學 === 光電工程研究所 === 94 === This thesis concerned with the studies on the optical properties of AlGaN/GaN heterostructure and InGaN/InGaN multiple quantum wells, the silicon doping effect on InGaN/InGaN quantum wells. The content had three major parts.
(1) Optical properties in AlGaN/GaN heterostructure
We used Williamson-Hall plot to obtain the threading dislocation densities for different aluminum fraction of AlxGa1-xN epitaxial layers. As the aluminum fraction increased, the threading dislocation densities expanded. We found that higher aluminum fraction would lead to smaller Hall mobility. As the polarization induced sheet charge was stronger, the PL intensity of two dimensional electron gases (2DEG) peak was weaker. The PL energy separation between the 2DEG peak and the GaN FE emission decreased with increasing temperature. The result was attributed to the screening effect of electrons on the bending of the conduction band at the heterointerface.
(2) Optical properties in InGaN/InGaN multiple quantum wells(MQW)
We found that the degree of blue shift in temperature-depend photoluminescence (TDPL) was greater in higher indium composition samples. From T-x diagram, the higher indium composition samples had higher probability to prevail phase separation. Using Arrhenius plot in TDPL, we could confirm that higher indium samples had stronger carrier localization effect. Based on the two conjectures, the quantum dots-like might be formed in the in-rich regions. Thus, the higher indium composition samples had greater polarization field, which caused the variation of energy gap which was induced by screening effect greater as temperature increased. The reason of the obviously blue shift of the TDPL in higher indium composition could be obtained by the carrier localization effect or the polarization effect.
(3) Current and optical properties in Si-doped InGaN/InGaN MQW
Silicon doping in barriers reduced the mismatch between barriers and wells, and silicon doping in barriers could fill the defects or change the dislocation mobility. Therefore, Si-doped samples had reduced threading dislocation densities. By the calculated results of polarization field and PL spectra, we found that the enhancement of PL intensity and the blue shift of the PL spectra with increasing silicon doping concentrations were explained by a decrease in potential fluctuations and/or screening effect of the internal piezoelectric field. By comparing T-PL and current-depend electroluminescence, we found that the quantum well related signals in PL spectra weren’t the band-to-band emission, but the indium localized states induced by the potential fluctuations. According to the C-V measurement, we found that carriers were accumulating in the wells which were near surface because the width of depletion had changed by silicon doing in the barriers near substrate. The current properties showed that silicon doping could improve samples’ quality, reduce leakage current and turn-on voltage, and increase light efficiency.
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