Epitaxial growth of gallium nitride/aluminum gallium nitride by a rotating disk MOCVD reactor

Tuoh-Bin Ng, Purdue University

Abstract

In this work, the epitaxial growth of the group III nitride compounds, GaN and AlGaN, was studied using a rotating disk MOCVD reactor (RDR). Majority of the growth studies have been conducted on (0001) sapphire substrates using the standard two-step growth procedure. Extensive use of in situ optical reflectance monitoring of the growth has not only enabled better control of the process, but also enabled a new interpretation of the two-step growth process through observing the morphological evolution of the growing sample surface. In particular, this study established the importance of the morphological evolution, starting from the low temperature GaN buffer layer deposition up to the high temperature GaN nucleation, in determining the resulting dislocation density and material quality of the final GaN epilayers. The interpretation of the low temperature buffer layer as a nucleation template for the high temperature growth was proposed, and the critical role of a stage of morphological roughening during the high temperature nucleation in leading to reduced dislocation density was highlighted. Partly through the understanding accorded by this interpretation, the significant effect of the hydrogen carrier gas in affecting the morphological evolution, and hence the material quality, was reported for the first time. The use of the in situ reflectance monitoring in studying the growth conditions through the growth rate variations with growth parameters was also described. The growth of AlGaN in the RDR was complicated by parasitic gas phase reactions between TMA1, NH$\sb3$ and possibly TMGa. Modifications in the gas injection scheme of the reactor, in addition to operation in reduced reactor pressures, were found necessary in order to achieve controllable Al incorporation in the AlGaN epilayers. Basic growth conditions for AlGaN were established in this study. Some preliminary results in the growth of nanometer-scale GaN dots on AlGaN, by using SiH$\sb4$ as a surfactant to induce 3-dimensional GaN on AlGaN nucleation, were also presented.

Degree

Ph.D.

Advisors

Gunshor, Purdue University.

Subject Area

Materials science|Electrical engineering|Condensation

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