ALMALKI, Abdulaziz
(2023)
Investigation of structural, electrical and optical properties of advanced wide bandgap semiconductor materials and devices.
PhD thesis, University of Nottingham.
Abstract
This thesis reports an investigation of deep level defects in p-i-n Al0.6Ga0.4N/Al0.5Ga0.5N multi quantum wells (MQWs) based deep ultraviolet light emitting diodes (DUV-LED), In0.09Ga0.91N/GaN MQWs based ultraviolet photodetector (UV-PD) and NiO/β-Ga2O3 heterojunction diodes. Firstly, different analytical techniques were used to examine the structural, electrical, and optical properties of AlGaN-based on DUV LEDs with 50% Al content before and after electrical stress process at 100 mA over 150 hours. The cathodoluminescence (CL) intensity of the DUV LED after stress process decreased. Current-voltage (I-V) characteristics demonstrated that leakage current and ideality factor increased from 4.18 × 10-9 cm-3 to 4.50 × 10-9 A and 4.96 to 6.14, respectively ,following electric current stress. Capacitance-voltage (C-V) measurements indicated that the stress process causes the charge carriers to be redistributed in the active region of the MQWs. The Deep Level Transient Spectroscopy (DLTS) measurements at reverse bias VR=-0.5V revealed two and one electron traps with higher concentration in fresh (not subjected to the electrical current process) and stressed (subjected to the electrical current process) LEDs, respectively. Scanning Transmission Electron Microscopy (STEM) images and Energy-Dispersive X-ray (EDX) Spectroscopy mapping showed that there are not many dislocations or defects in the MQWs before electrical stress is applied. As a result of the stress, two dislocation lines appeared and extended throughout the quantum wells.
The InGaN/GaN MQWs UV-PDs, which consisted of a thick passivation layer of Al2O3, were subjected to high electrical current stress of 200 mA over 340 hours and investigated by employing different combined optical and electrical measurements. As a consequence of electrical stress, it was noticed that the Electroluminescence (EL) intensity decreased substantially (~ 48 %), suggesting that most of the charge carriers are captured by the newly induced defects in the active region. In addition, the parasitic current leakage paths increased significantly which in turn reduced the photocurrent generation process largely. Furthermore, within the temperature range of 100 K – 440 K, the values of the ideality factor and junction potential are found to be always higher and lower in treated PDs, respectively. This indicates the presence of generation recombination centres caused by defects. Two trap levels are identified in treated PDs via Laplace DLTS analysis. The obtained results indicate that degradation is mainly associated with the newly generated defects, mostly Mg-related shallow acceptors, including MgGa and Mg-H2 complexes, which can form acceptor levels as a result of the breaking of Mg-H chemical complexes due to high heating levels during the stress treatment.
The effect of Rapid Thermal Annealing (RTA) at 225 ℃ in N2 atmosphere for 15 minutes on the electrical and optical properties of NiO/β-Ga2O3 heterojunction diodes was investigated using Photoluminescence (PL) and micro-Raman spectroscopy, C-V, I-V, DLTS, L-DLTS, techniques, and SILVACO-TCAD numerical simulator. Micro-Raman results revealed no frequency shift, no linewidth variation, or relative intensity change. After RTA, the PL intensity was five times greater than that of the fresh sample as a result of an increase in the density of gallium and oxygen vacancies (VGa + VO) in the annealed samples. A comparison of the I-V characteristics of annealed and fresh samples revealed that the ideality factor decreased from 2.33 to 1.74 and the junction potential increased from 0.68 eV to 0.76 eV after the thermal annealing process. DLTS measurements showed that the number of electrically active traps differed between the two samples. In particular, three and one electron traps were detected in fresh and annealed samples, respectively. Additionally, the RTA process reduced the concentration of trap E3 of more than two times. SILVACO-TCAD simulator achieved a fairly good agreement between simulation and measurements by considering a surface NiO acceptor density of about 11019 cm-3 and E2 trap depth into the surface of β-Ga2O3 layer of about 0.220 µm. The results of the investigation of the NiO/β-Ga2O3 heterojunction diodes indicate that RTA improves their performance significantly. Specifically, RTA enhances the PL intensity by increasing gallium and oxygen vacancies, reduces the reverse leakage current and trap concentrations. These results confirm that RTA plays a critical role in improving the performance and functionality of NiO/β-Ga2O3 based devices.
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