Rare Earth Doped Gallium Nitride Powders: Synthesis, Purification, Luminescence Enhancement, Nanosizing, Luminescence Mechanism Investigation And Large Area Deposition

摘要

Gallium Nitride is a III-V compound semiconductor that has attracted a lot of interest among both applied and basic researchers because of its potential applications in optoelectronic, high power and high frequency devices. However, many questions about the material remain unanswered. In this thesis, we will present our investigation of GaN. We will first describe an ammonothermal method for the synthesis of undoped and rare earth doped GaN powders. Using careful observations and calculations, we show that the powder growth is primarily a liquid phase phenomenon. We also present a chemical method to achieve luminescence enhancement in ammonothermally grown Eu:GaN powders. Based on arguments drawn from the surface chemistry and XRD of these samples, we conclude that elimination of dark mixed oxides from the powder results in the observed luminescence enhancement. We also demonstrate a nano Eu:GaN synthesis process using a simple mechanical topdown method. The optical properties of nano Eu:GaN prepared in this manner is comparable to that of the bulk material. Based on a similar mechanical process we synthesized nano Er:GaN powders that emit in the C band (1.55 m). The mechanism involved in the luminescence of rare earth doped GaN is investigated using thermal quenching and high pressure studies. Our results suggest that an exciton bound to rare earth structured isovalent impurity (RESI) is responsible for luminescence in these materials. Luminescence quenching and pressure dependent photoluminescence enhancement in RE:GaN can be explained based on this model. Our results clearly suggest that thermal quenching can be undone by application of pressure. These powders are discovered to be fairly radiation hard as well. In the last section of this thesis, we will present an electrophoretic technique to deposit nano GaN on a fluorine doped tin oxide coated glass substrate. The technique can be easily adapted to grow layered structures that can find application in optical fibers and as a laser gain medium. Preliminary results for highly densified GaN ceramic obtained using a hot-press process are discussed. These results suggest that further densification is necessary for achieving a completely transparent GaN ceramic made out of ammonothermally synthesized GaN powders.