Optical properties of graphene/GaN hybrid structure were investigated by using a variety of optical spectroscopy techniques including low-temperature photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, confocal scanning micro-Raman spectroscopy. Single-layer graphene grown by chemical vapor deposition was transferred to GaN epilayer surface, which is verified by the Raman spectrum with a sharp characteristic peak at ~2690 cm-1and a homogeneous Raman image. Three main band-edge emissions including the free exciton A transition (denoted as FXA), the donor bound exciton transition (denoted as DX) and the third peak (denoted as Ix) were well resolved in the PL spectra of the hybrid structure as well as the as-grown GaN epilayer at low temperatures. Interestingly, the FXA transition and Ix line of the GaN epilayer were found to be dramatically altered by the top graphene layer while the DX is almost unaffected. The intensity of Ix line substantially drops after the transfer of graphene layer on GaN, indicating surface defect nature of the Ix line. More interestingly, an unpredictable dip structure develops in the FXA peak when the temperature is beyond 50 K. Similar spectral structure change also occurred in the emission of free exciton B (referred as FXB)with higher transition energy .A free exciton dissociation and electron transfer model was proposed to explain the “dip effect”. More supporting evidence to the model was found in the time-resolved PL spectra of the hybrid structure and the control sample. The results showed the significant influence of graphene monolayer on the fundamental optical properties of GaN. / published_or_final_version / Physics / Master / Master of Philosophy

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published_or_final_version / abstract / toc / Physics / Doctoral / Doctor of Philosophy

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The growth, electronic structure and doping of the semiconductor InN has been explored and analysed. InN thin films were grown by plasma assisted molecular beam epitaxy. The significance of the relative fluxes, substrate temperature and buffer layers was explored and related to the electrical and structural properties of the films. An exploration of the effect of active nitrogen species on InN films found that excited molecular nitrogen was preferred for growth over atomic and ionic species. An optimised recipe for InN was developed incorporating all explored parameters. The bandgap of InN was explored using the techniques of optical absorption, photoluminescence and photoconductivity. All three techniques identified a feature near 0.67 eV as the only dominant and reproducible optical feature measurable from InN thin films. No evidence for any optical features above 1 eV was discovered. The effect of the Burstein-Moss effect is discussed and the debate over the relative impact of the effect is related to problems with precisely measuring electron concentrations. Photoluminescence from mixed phase InN films containing significant zincblende content is presented, with tentative evidence presented for a zincblende band gap near 0.61 eV. Native defects within InN were studied by near edge X-ray absorption fine structure spectroscopy. Nitrogen related defects were found to be unlikely candidates for the high as-grown n-type conductivity. The most likely candidate remains nitrogen vacancies. Ion implantation was shown to cause substantial damage to the InN lattice, which could not be fully repaired through annealing. The limitation on annealing temperatures may limit the use of implantation as a processing tool for InN. Mg was shown to exhibit great promise as a potential p-type dopant. Photoluminescence from Mg doped films was found to quench at high Mg concentrations, consistent with a depletion region near the surface. The potential dilute magnetic semiconductor In1-xCrxN was explored. All of the In1-xCrxN films were found to be ferromagnetic at room temperature and exhibited saturated magnetic moments of up to 0.7 emu/g. An interesting correlation between background electron concentration and remnant moment is presented and the consequences of theoretical exchange models discussed. The bandgap of chromium nitride was also investigated and found to be an indirect gap of 0.7 eV.