Optical Characterization of GaN/AIGaN Quantum Well Structures
Abstract: The III-nitride material family is particularly well suited for optoelectronic applications, e.g. as UV light emitters in future lighting solutions that are expected to replace the incandescent and fluorescent lamps in use today. Here a UV primary light emitting diode (LED) excites an efficient phosphor so that the entire LED package produces white light. The LED is expected to be built upon a technology involving multiple quantum wells (MQWs) in the AIGaN/GaN system as the active region. The AIGaN barriers will only have a moderate height, not to block the current flow in the LED, i.e. a low Al composition will be used.In order to optimize the device design, it is necessary to provide knowledge on the material properties and especially to understand the recombination mechanisms in nitride QWs. A peculiarity of the wurtzite nitrides is the significant macroscopic polarization with both a spontaneous and a piezoelectric component. These polarization charges create internal fields in the QWs that have a fundamental influence on the optical properties, strongly affecting the oscillator strengths of excitons as well as the spectral position of the corresponding photoluminescence (PL) peaks. The radiative recombination processes are strongly modified by these built-inelectric fields in the  direction, which causes a substantial quantum confined stark effect (QCSE).Screening by doping and/or carrier injection strongly affects the polarization-induced properties. For instance modulation doping can be used to screen the macroscopic polarization field in nitride quantum wells. The screening of the field increases the overlap between confined electron and hole states, enhancing the radiative transition probability across the gap. This can explain the experimental findings of lower laser threshold current, reduced radiative lifetime, and increased emission intensity indoped MQWs.In this thesis, the influence of Si doping on the exciton localization in modulationdoped GaN/A10.07Ga0.93N multiple-quantum-well (MQW) structures and the radiative recombination mechanisms for the undoped and Si doped samples have been investigated. The thesis is mainly based on experimental investigations by using a combination of photoluminescence (PL), micro photoluminescence (μ-PL) and timeresolved photo-luminescence (TRPL) techniques.The results of the measurements reveal the hole localization even at highly modulation doped samples. Localization at interfaces leads to spectrally resolve well width fluctuations in the PL spectra for the undoped samples, while these appear to be screened for the case of high n-doping. The hole localization determines the residual PL linewidth in undoped samples, and the recombination kinetics in all samples at low temperatures. At elevated temperatures, the Si doping is found to improve the radiative efficiency.
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