| Abstract [eng] |
This thesis was prepared by Seda Türkcan (academic supervisor—dr. Kazimieras Nomeika) under the title “Carrier Dynamics in n-GaN/InGaN/p-GaN Structures with Different Active Layer Widths” at Vilnius University, Institute of Photonics and Nanotechnology. The work investigates non-equilibrium charge carrier dynamics and internal quantum efficiency (IQE) in InGaN single quantum well (SQW) structures with varying active layer widths and cap-layer doping configurations. The main objective was to investigate how SQW width and cap doping influence recombination mechanisms, efficiency, and carrier transport properties in InGaN/GaN heterostructures. The study was carried out using Light-Induced Transient Grating (LITG) and Time-Integrated Photoluminescence (TIPL) techniques. Eight SQW samples—U-2, U-3, U-5, U-7, P-2, P-3, P-5, and P-7—were analyzed, where the labels represent the QW width and cap-layer type. The experimental results showed that increasing the SQW width led to the appearance of double emission peaks in the photoluminescence spectra, with the secondary long-wavelength peak attributed to the formation of In-rich clusters. This effect became evident for SQW widths of 3 nm and above. The optical analysis further demonstrated that samples incorporating a p-GaN cap layer exhibited comparatively higher QE values at larger SQW widths than their u-GaN counterparts. This behavior indicates that the p–n junction field partially screens internal polarization fields, thereby reducing the impact of the quantum-confined Stark effect (QCSE) and helping sustain radiative efficiency in wider SQW structures. However, the introduction of the p-GaN cap layer significantly reduced QE in narrower SQW structures, likely due to Mg diffusion into the active region and the resulting increase in defect-related non-radiative recombination. Although both SQW width and cap-layer configuration strongly influenced the photoluminescence spectra and spectral shifts, the measured carrier transport parameters remained nearly unchanged among the investigated samples. This suggests that variations in internal electric fields have only a limited influence on carrier transport properties within the studied structures. Overall, the results provide insight into the competing effects of QCSE screening and defect formation in InGaN/GaN SQW systems and contribute to the optimization of quantum well designs for high-efficiency optoelectronic devices. |
