| Summary: | 博士 === 中原大學 === 應用物理研究所 === 92 === This thesis studies the optical properties of the rare-earth doped InGaAsP epilayers and mesoporous materials. Different optical techniques such as photoluminescence (PL), photoconductivity (PC), contactless electroreflectance (CER), micro-Raman, polarized PL, PL excitation measurements are carried out to investigate the physical properties of the rare-earth doped InGaAsP epilayers and mesoporous materials. These results are presented in the following parts:
(1) Influence of rare-earth elements doping on the optical properties of quaternary InGaAsP epitaxial layers:
The PL, PC, and CER measurements were used to study the influence of rare-earth doping on the optical properties of InGaAsP layers grown by liquid phase epitaxy (LPE). Both the full width at half maximum (FWHM) of PL and the broadening parameter of CER were found to reduce as the doping amount of Ho element increases. The absorption tails were analyzed with the Urbach tail model and the Urbach energies were obtained from these fits. It is found that the Urbach energy decreases with increasing the doping amount of Ho elements, indicating the Ho doping leads to the decrease of impurity concentrations. The Nd-deoped InGaAsP layers exhibit the similar results and the narrowest value of the FWHM of PL peak is 7.5 meV with Nd of 0.031 wt%. We demonstrate that the introduction of the rare-earth elements can greatly reduce the residual impurities of LPE-grown layers.
(2) Large-lattice-relaxation model for persistent photoconductivity in quaternary InGaAsP epitaxial layers:
We report the first observation of persistent photoconductivity (PPC) in In1-xGaxAsyP1-y epilayers. Under the excitation-energy, temperature, and alloy composition dependence of the PPC effects, it is found that the lattice relaxation of DX-like impurity is responsible for PPC in In1-xGaxAsyP1-y. PPC was also investigated in Ho-doped InGaAsP epilayers with Ho concentrations in the range of 0-0.15 wt%. As the Ho doping increases, the decay-time constant and the electron-capture barrier were found to decrease. We suggest that the introduced Ho elements may chemically react with donor impurities, suppressing lattice relaxation and hence reducing the electron-capture barrier. Also, the rare earth doping is demonstrated to be an effective method of improving the quality of InGaAsP epilayers.
(3) Raman scattering study of rare-earth elements doped InGaAsP epilayers:
Raman scattering measurements have been used to study the structural properties of the rare-earth doped InGaAsP epilayers. Using a spatial correlation model, we found the asymmetric broadening of lineshape of the Raman signal is not influenced by the rare-earth doping. It indicates that no large amounts of the rare-earth elements are being incorporated into the epitaxial layers during the purification.
(4) Red-light emission in MCM-41 and MCM-48 meso-porous nanostructure:
PL was used to study the emission of light from siliceous MCM-41 and MCM-48 that has undergone rapid thermal annealing (RTA). Two PL bands were observed at 1.9 and 2.16 eV and assigned to the non-bridging oxygen hole centers (NBOHCs) and the NBOHCs associated with broken bonds, respectively. The PL intensity is enhanced after RTA. Based on the surface chemistry, the enhancement is explained by the generation of NBOHCs that originates from the hydrogen-bonded and single silanol groups on the MCM-41 and MCM-48 surfaces. The PL intensity degrades with time during photoexcitation. The dominant mechanism of PL degradation involves the formation of the chemisorbed oxygen-related complexes (probably O2－ molecules) on the surface, which are adsorbed onto the surface and act as an efficient quencher of PL.
(5) Blue-green photoluminescence in MCM-41 meso-porous nanotubes:
Different PL techniques have been used to study the blue-green emission from siliceous MCM-41. It is found that the intensity of the blue-green PL is enhanced after RTA. This enhancement is explained by the generation of the two-fold coordinated Si centers and the non-bridge oxygen hole centers according to the surface properties of MCM-41. Through the analysis of PL with RTA, polarized PL, and PL excitation, we suggest that the triplet-to-singlet transition of two-fold coordinated silicon centers is responsible for the blue-green PL in MCM-41. In addition, we suggest a model to explain the temperature dependence of the carrier time constant and the PL intensity.
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