Micro- and Nano-scale Optical Investigation of Group III-Nitride Compound Semiconductors
In this thesis, we studied three different kinds of nano- or micro-structures in GaN, AlGaN and InN materials. First, we investigated the V-shape pit growth on AlGaN with GaN buffer layer. Both micro-photoluminescence and near-field scanning optical microscopy (NSOM) results showed an extra peak Iv located at 350 nm in addition to Inbe at 335 nm. The detailed investigations were carried out by NSOM and scanning Kelvin-force microscopy (SKM). The NSOM spectra revealed that the intensity of the Iv band increased gradually from the edge to center of V-defect, while Inbe remained unchanged. The SKM results revealed further that the Fermi level decreased from flat region to V-defect center by about 0.2 eV. These results suggest that the band could be related to shallow acceptor levels, likely resulting from VGa point defects. Besides, the Raman scattering of V-defects was also performed. The Raman results indicated that the LO modes of 3.3 um V-defects show noticeable blue shift corresponding to different concentrations. For different sizes, the appearance of forbidden Raman modes A1(TO) and E1(TO) inside V-defect is due to the right angle scattering from morphology variations. Distinct blue shift of LO-like mode was also observed inside V-defects, especially in large V-defects. Simulation results based on LOPC coupling model deduced a higher dislocation density inside V-defects than that on the plain by considering effective electron density. So that larger size V-defect higher than smaller one. The second theme was about AlGaN film growth on AlN buffer layer. The AlGaN epilayer growth on AlN buffer layer will bear a compressive stress due to lattice mismatch in contrast to a tensile stress on GaN buffer layer. The compressive and tensile stress are the main reason to form such hillocks or V-defects structures. According to AFM morphology results, there are three different types of hillocks, namely pyramid-like, mesa-like, and tent-like hillock. The micro-PL showed an additional emission peak inside the hillock structure, and the energy dispersion x-ray spectrometer (EDX) results suggest that the different peaks of Inbe and IH were due to Al composition fluctuation. According to the PL results, the calculated Al fraction is about 4% and 11%. The strain free E2 mode frequency is inferred to be 568.5cm-1 and 570.2 cm-1 inside and outside the hillock, respectively. However, the experimental results of □-Raman spectra showed that E2 mode frequency is ~570 cm-1 and ~573 cm-1 inside and outside the hillock, respectively. These are blue shifted by ~1.5 cm-1 and ~3.0 cm-1 so that hillocks bear compressive stress. The Raman depth profile of hillock also gave the evidence that the formation of hillock is from the AlN buffer layer. Moreover, NSOM measurement provided good spatial resolution for pyramid-like hillock and showed varying Al concentration inside the hillock with different hillock height. The PL peak energy showed a red shift when the angle of hillock slope increased. This phenomenon may be ascribed to the stain relaxation and change from 2D to 3D growth mode that affects the growth rate in normal direction. The peak energy at the hillocks apexes is smaller and intensity is stronger than that on plain. The weak luminescence intensity of pyramid-like hillock may be due to localization effect which observed the “S-shape” phenomenon form temperature dependence photoluminescence results in the early studies. The final theme is about InN nano-dots grown on GaN buffer layer and embedded with GaN cap layer. Size-tunable emission energy was observed and was explained by the quantum confinement effect. The PL measurements showed that the PL peak energies of the dots are less sensitive to temperature as compared with bulk film, indicating the localization of carriers in the dots. A reduced quenching of the PL from the dots was also observed, implying superior emission properties for embedded InN dots. We also investigated the capping effects on InN nano-dots by different GaN capping temperature. By increasing the capping temperature we clearly observed the deformed profiles of InN nano-dots in AFM images. So the InN decreased emission intensity. Moreover, two unexpected extra visible emission bands (violet and green) were observed in PL spectra. By analyzing the X-ray diffraction results, the visible emissions are likely due to the InGaN formation from capping processes. The spatial distribution of NSOM mapping revealed the origins of PL emission bands. Finally, we propose that the visible emissions are from the capping layer and the interface layer between InN nano-dots and GaN layer. The presence of the interface layer may include InGaN formed by capping processes.
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