Ion Beam Study of Early Stages of Growth of GaN films on Sapphire
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Ion Beam Study of Early Stages of Growth of GaN films on Sapphire Eugen M. Trifan and David C. Ingram Ohio University, Department of Physics and Astronomy, Athens, OH 45701, U.S.A.
ABSTRACT An innovative approach for in-situ characterization has been used in this work to investigate the composition, growth mode, morphology and crystalline ordering of the early stages of growth of GaN films grown on sapphire by MOCVD for substrate temperatures in the range of 450oC to 1050oC. We have performed in-situ characterization by Rutherford Backscattering Spectroscopy (RBS), Ion Channeling, X-ray Photoelectron Spectroscopy (XPS), and Low Energy Electron Diffraction. Ex-situ the films have been characterized by Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and thickness profilometry. The films have been grown in an in-house designed and build MOCVD reactor that is attached by UHV lines to the analysis facilities. RBS analysis indicated that the films have the correct stoichiometry, have variable thickness and for low substrate temperature completely cover the substrate while for temperatures 850oC and higher islands are formed that may cover as few as 5 percent of the substrate. From Ion Channeling and LEED we have determined the crystallographic phase to be wurtzite. The crystalline quality increases with higher deposition temperature and with thickness. The films are epitaxialy grown with the crystallographic axis and planes of the GaN films aligned with the sapphire within 0.2 degrees.
INTRODUCTION GaN is a useful material in the field of optoelectronic applications [1] and in high frequency, high temperature electronic devices. In order to achieve the level of performance needed for such devices an improvement of the quality of the films is needed. At this time there is no inexpensive substrate with a matched lattice constant and sapphire is one of the most used materials. Lowest reported defect densities are in the mid 107 cm-2 requiring complicated patterning and substrate preparation [2]. MOCVD is the commercial growth method for GaN, but most of the in-situ studies have been concentrating on MBE. We have investigated in-situ the early stages of growth of GaN films by MOCVD using an in-house designed and assembled reactor that is attached via UHV transfer lines to the analysis facilities.
EXPERIMENTAL SETUP The experiment has been designed in such a way to be able to do the analysis of the sample without exposing it to air and to maintain the samples in UHV conditions except when growth is occurring. The experimental setup (‘W.M. Keck Thin Film Analysis Facility’) consists of three chambers, connected together by UHV transfer lines. First chamber is dedicated to the pretreatment of the substrate and the actual growth of the film. The second chamber is connected to the Ohio University accelerator beam line and the third one is equipped for surface analysis
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(UPS/XPS/AES) using a Kratos XSAM800 electron spectrometer. The second chamber is equipped with the LEED system and a six axis gonio
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