Three-dimensional imaging spectroscopy of the III-nitride material system

Despite being the leading candidates for blue and ultraviolet light emitting optoelectronic devices, the III-nitrides suffer from extremely high defect densities of 108--1012 cm-2 , which cause local fluctuations in material parameters such as bandgap and internal polarization fields. In order to non-invasively probe optical properties of the III-nitrides, we have designed and built a scanning confocal microscope with visible and ultraviolet capabilities and spatial resolution on the scale of threading dislocation separation. With this microscope, we have the ability to spectroscopically image the III-nitrides in three dimensions using both well-established and newly-developed optical experimental techniques. In this work, I describe four main sets of experiments. In the first, we used micro-photoluminescence to study the emission properties of intentionally-grown inversion domain boundaries in GaN. Though inversion domain boundaries were expected to be non-radiative centers, we observed strong luminescence, polarized in the boundary plane, originating from them. This work motivated the theoretical calculations, published in 2003 by Fiorentini, that provide rationale for the enhanced luminescence behavior of inversion domain boundaries. In the second, we employed micro-Raman and micro-photoluminescence to map strain fluctuations in high quality pendeo-epitaxial and homoepitaxial GaN with an accuracy of 6 x 10-6. Strain fields in stripe and wing material, as well as those directly surrounding a single screw dislocation, are measured. In the third, we developed a technique utilizing continuous-wave two-photon absorption for optically sectioning and imaging deep into GaN structures. With this technique, we are able to study the optical properties of GaN surfaces, bulk, and interfacial material without physically altering the sample by, for example, cleaving or milling. In the fourth, GaN whiskers with threading dislocation cores are spectroscopically imaged in both standard and cross-sectional experimental geometries. The whiskers are created by a specific defect-selective photoelectrochemical etch. The incorporation of oxygen impurities during the etch process is proposed as an explanation for the observed luminescence phenomena. Our experimental observations demonstrate not only that fundamental material parameters are dominated by localized behavior, but that localization is essential to the luminescence properties of III-nitride devices.