Microscopic Optical Characterization of Free Standing III-Nitride Substrates, ZnO Bulk Crystals, and III-V Structures for Non-Linear Optics.

摘要

GaN presents a large potential as an optoelectronic and microelectronic material. State of the art GaN devices are fabricated on epitaxial layers grown on foreign substrates, which results in a very large extended defect concentrations (typically >109 cm-2), which is detrimental to the performance and reliability of the devices, especially for high power operation. Advances in the material quality are necessary for the implementation of the next generation of light emitting diodes (LEDs), blue laser diodes (LDs), and high power electronics. The improvement of the GaN crystal quality can be achieved by the use of GaN substrates for homoepitaxial growth. A great deal of effort has been devoted to the growth of bulk GaN crystals. Among the methods used the ammonothermal growth (solvo-thermal process using supercritical ammonia as a solvent) appears as the most promising method for fabricating crystals with size and crystalline quality scalable to industrial processes. The advantage of this method consists of the low growth temperature (550-600 deg C), and the relative low pressure (100-500 MPa). The disadvantage mainly concerns the high impurity concentrations related to the use of mineralizers. A complete optimization of these methods must consider different aspects, including a full characterization of defects, both including impurities and native defects, for which high resolution spatially resolved characterization techniques are necessary. Therefore, a study of the main defects involved in these materials is essential to the understanding of their main properties and to improve the growth methodologies, and the treatments necessary to optimize them. This grant shows repeated improvements in the subject materials by working with various strains, growth conditions, temperature variation, and impurities, and studies crystal growth parameters necessary to improve the crystalline quality of these crystals suitable for obtaining high quality substrates.