In-Situ Investigation of Surface Stoichiometry During InGaN and GaN Growth by Plasma-Assisted Molecular Beam Epitaxy Usi
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In-Situ Investigation of Surface Stoichiometry During InGaN and GaN Growth by PlasmaAssisted Molecular Beam Epitaxy Using RHEED-TRAXS R.P. Tompkins, B. L. VanMil, E. D. Schires, K. Lee, Y. Chye, D. Lederman and T. H. Myers Multifunctional Materials Laboratory, Department of Physics, West Virginia University, Morgantown, WV 26506, U.S.A. ABSTRACT Reflection high-energy electron diffraction total-reflection-angle x-ray spectroscopy (RHEED-TRAXS) uses high-energy electrons from RHEED to excite x-ray fluorescence. Monitoring characteristic x-rays of selected elements thus allows study of surface coverage of materials. In this study, surface coverage of Ga and In during growth of GaN and InGaN was probed using this technique. Evolution of the surface layer of Ga on GaN during growth and deposition of Ga on static GaN at room temperature were studied. RHEED-TRAXS measurements were performed during growth of InGaN by measuring the ratio of the In Lα to Ga Kα intensity. A significant surface coverage of In was observed at all temperatures investigated regardless of actual In incorporation. INTRODUCTION Because small changes in thin film stoichiometry can cause drastic changes in the film’s physical properties, there exists a real need to determine material composition during growth. Reflection high-energy electron diffraction total-reflection-angle x-ray spectroscopy (RHEEDTRAXS), which uses x-ray fluorescence stimulated by RHEED electrons to measure surface composition during growth, is one approach that addresses this need. RHEED-TRAXS has been shown to be capable of measuring chemical composition of a growing surface with submonolayer sensitivity in InGaAs/GaAs and YBaCuO films. [1,2] Since the RHEED electrons just penetrate the surface, and by using a geometry that measures x-rays at the total reflection angle, RHEED-TRAXS probes primarily the top 20 to 30 Å of material. [3] Thus, the surface stoichiometry of layers of GaN can be directly probed during growth, including the amount of adsorbed excess Ga. High quality Ga-polar GaN grown by MBE is grown under gallium stable conditions, which results in the formation of an excess Ga layer on the surface. There are several models of the amount of Ga in the excess layer. [4,5] In addition, too much gallium incident on the surface during growth results in gallium droplet formation. Thus, RHEED-TRAXS can be used to test predictions and also to serve as an in-situ diagnostic to determine optimal growth conditions. A RHEED-TRAXS setup was constructed at WVU for use in a MBE system for growth of nitride materials. The setup was used to probe Ga coverage versus Ga overpressure during growth and to investigate Ga/In surface coverage. Future studies will involve investigation of surface stoichiometry of oxide materials. Currently, there are two studies experimentally investigating how the Ga surface layer evolves during growth. One study by Adelmann et. al. finds that RHEED intensity variations after shuttering suggest the Ga surface layer quickly forms a bi-layer and
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