Study of GaN-based Materials for Light-emitting Applications

Abstract

The purpose of this study was to explore the possibility of fabricating phosphor-free white-emitting LED's based in the gallium nitride material system. The structures were to be grown using metal-organic chemical vapor deposition (MOCVD). Toward this end, a Thomas Swan Scientific close-coupled showerhead reactor was installed.

The first experimental step in this project was the optimization of nominally undoped GaN. This was achieved successfully, as smooth, non-compensated, optically-active films were demonstrated. Additionally, a full on- and off-axis x-ray diffraction study showed that the crystal quality of this material compared favorably to that of published standards.

Successful n- and p-type doping of GaN were also demonstrated. Device-worthy mobility and carrier concentration values were demonstrated. Atomic force microscopy of n-type material verified that the films was sufficiently smooth as to serve as a layer upon which active-layer quantum wells could be grown. Photoluminescence of both n- and p-type material was examined as well.

An extensive indium gallium nitride growth study was carried out. The effects of several growth parameters on emission characteristics were presented. PL emission wavelengths as high as 561nm were demonstrated. The issues of uniformity and indium platelet formation were also addressed.

This InGaN experimental work was complemented with a series of calculations which gave the expected emission wavelength of an InGaN⁄GaN quantum well structure based on In content and well width. Strain, the quantum size effect, and the quantum-confined Stark effect were all factored into these calculations in order to study their individual contributions to emission wavelength values.

This work concluded with an examination of white device structure and fabrication. Both two- and three-color devices were considered. Monochromtic devices emitting in the green and yellow were fabricated. The yellow device, emitting at 575nm, yielded the longest reported wavelengths for Al-free InGaN⁄GaN multiple quantum-well LED's grown by MOCVD. Finally, white emission was demonstrated from a two-color MQW structure emitting blue and yellow light.

The research presented herein demonstrates the first step toward achieving phosphor-free solid-state white lighting. Continuation of this study will hopefully lead to future commercial, industrial, and residential applications of this technology.