Optical characterization of gallium nitride

TLDR: In this article, material and optical properties of III-nitrides have been investigated using photoluminescence (PL) spectroscopy and optical reflectance techniques, in order to reveal the electronic structure, impurities and defects in GaN.

Abstract: Group III-nitrides have been considered a promising system for semiconductor devices since a few decades, first for blue- and UV-light emitting diodes, later also for high-frequency/high-power applications. Due to the lack of native substrates, heteroepitaxially grown III-nitride layers are usually considered. However, the lattice mismatch and different expansion coefficients of the epilayer and substrate generally cause a high density of defects and a biaxial strain in the nitride layers, changing its basic structural and optical properties. This thesis deals with material and optical properties of III-nitrides to gain more insight of fundamental processes in these materials. Most experiments were performed using photoluminescence (PL) spectroscopy and optical reflectance techniques, in order to reveal the electronic structure, impurities and defects in GaN. Considering the extreme importance of improving the quality of heteroepitaxial GaN layers, some examples concerning progresses and innovatives in MOCVD growth of GaN films on sapphire and silicon substrates are presented. Additionally, photoluminescence properties of homoepitaxial GaN are discussed. The growth of smooth N-face GaN films enables the comparative study of GaN of two polarities (Ga- and N-face), revealing different affinities to impurity incorporation that is also confirmed by SIMS measurements. Next, thick HVPE grown GaN layers, which are very promising for future application as substrates, show a surprisingly good optical quality making correlation between detected PL lines and structural defects in this material possible. Finally, AlGaN/GaN quantum well (QW) structures are introduced and discussed, in particular, concerning a PL investigation of polarization-induced effects and their influence on GaN-based heterostructures. Screening of the piezoelectric field in one of the QWs due to a high unintentional doping is revealed by CV measurements