| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 99 === In this thesis, we use low-temperature photoluminescence (PL), power-dependent PL and Raman scattering spectroscopy to investigate the optical properties of high purity AlGaAs bulk layer grown by MOCVD. These AlGaAs samples have been applied to pHEMT switches for commercial mobile phones.
From low-temperature PL, we observed four bands for each sample. Two of them were the signals of AlGaAs, while the rest came from GaAs buffer layer. We attribute the low-energy band at 1.809 ± 0.001 eV to donor-to-acceptor (DA) transition. For all the samples except sample I, grown at the lowest temperature with the highest V/III ratio, we ascribe the high-energy band at 1.828 ~ 1.829 eV to bound exciton (BE) transition. In contrast, the high-energy band of sample I is assigned to band-tail localized bound exciton (LE) transition because of its lower peak energy (1.824 eV) and larger linewidth (6.3 meV). By analyzing the power-dependent PL of sample I, we found that the PL integral intensity of its AlGaAs bands is a linear function of excitation level, suggesting that carrier recombination mainly takes place in AlGaAs layer because the excitons are trapped in the tail states resulting from alloy potential fluctuation. For the contrastive samples, the PL intensity of AlGaAs saturate at high excitation intensity and the PL intensity of GaAs bands is strong and proportional to excitation intensity, indicating that the density of the impurity binding the excitons is low in the AlGaAs layer. As a result, most excitons diffuse to GaAs buffer layer and recombine there. In addition, we also utilize rate equation to verify these phenomena. In Raman spectra analysis, we used spatial correlation model to fit the AlAs-like LO band, and found that sample I has the shortest correlation length (4.7 nm), suggesting its strong alloy potential fluctuation.
Finally, on the ground of aforementioned observations, we conclude that sample I has the most serious alloy potential fluctuation which could result from its low temperature and high V/III ratio growth condition.
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