| Gallium nitride (GaN) and its alloys are used to manufacture green-to-ultraviolet range light emitting diodes (LEDs) for the solid-state lighting industry. However, heteroepitaxial growth on substrates such as 6H-SiC or alpha-Al 2O3 results in LEDs with large densities of crystal defects. Cathodoluminescence (CL) and electron-beam-induced current (EBIC) are SEM-based techniques that are used to probe the optoelectronic behavior of GaN LEDs and defects at the sub-micrometer scale.; This work examines the optoelectronic properties of defects in GaN-based LED devices. First, computer modeling of the polarization fields in quantum wells was performed, and quantitative predictions of cathodoluminescence peak shifts during electron injection, under varying conditions of electrical bias, were made. Results indicate that both polarization and InN-GaN immiscibility strongly influence the device properties, and that polarization fields of ≈1.33+/-0.15 MV/cm are present in the quantum wells. Experimental conditions and mathematical treatments for accurate cross-sectional EBIC quantification of the minority carrier diffusion length in GaN LEDs were developed and refined, which allowed quantification of hole and electron diffusion lengths of Lh≈92+/-15 and Le≈42+/-6 nm, respectively; these short values of L help explain the anomalously high quantum efficiencies of GaN layers despite their high dislocation densities. Combined CL and EBIC techniques were developed for the study of defect populations in GaN LEDs, and results show that large densities of threading defects are present in these devices. Additionally, the effects of focused-ion-beam (FIB) milling as a cross-sectional sample preparation technique for GaN were studied by CL and EBIC. It was found that preparation of GaN devices for CL or EBIC microscopy by FIB causes significant damage and modifies the CL and EBIC response of the devices. By using SEM-CL/EBIC to pinpoint defects in LED devices, site-specific FIB microsampling has been used to prepare samples of defected areas for transmission electron microscopy (TEM). Analyses of these samples have shown how the identity of crystal defects within the devices directly relates to the optoelectronic behavior observed in CL and EBIC. Densities of defects measured in CL or EBIC correspond with dislocation densities measured via TEM; this indicates that the dislocations are optoelectronically active and influence CL or EBIC behavior. These optoelectronic measurements, in conjunction with SEM and TEM microscopy, indicate a conjunction of high defect density and short diffusion length contribute to anomalously high light emission efficiency, and that the techniques developed and refined in this work can be used to study device performance and optimization. |
