The Use of Cathodoluminescence in Gallium Nitride During Growth to Determine Substrate Temperature
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The Use of Cathodoluminescence During Molecular Beam Epitaxy Growth of Gallium Nitride to Determine Substrate Temperature K. Lee, E. D. Schires and T. H. Myers Department of Physics, West Virginia University, Morgantown, WV 26506 ABSTRACT Cathodoluminescence measurements made during the growth of GaN films represent an alternate method of substrate temperature determination. The wavelength corresponding to a given growth temperature can be determined, and represents a definitive measure of temperature not subject to secondary calibrations. Cathodoluminescence spectra were measured up to 800 oC in the growth chamber. Spectra were obtained during growth of GaN, and correlated to growth rate vs. temperature. INTRODUCTION GaN is an important technological material for laser diodes, light emitting diodes and high-power electronics because of its wide band gap, hardness, and high thermal and chemical stability. Molecular beam epitaxy (MBE) continues to be a viable approach for the growth of GaN. The temperature range that MBE adopts is 700800°C, allowing minimization of thermal effects and more flexibility in choosing substrate materials. The growth rate is about 0.25 μm/hr allowing atomic level control of film thickness. Accurate determination of the substrate temperature during epitaxial growth of GaN has been a critical issue to improve the quality of the film. Studies have shown that growth temperature is one of the decisive factors that determine the success or failure of film growth, with background carrier concentrations increasing over two orders of magnitude with variations of 40oC.[1] Conventional temperature measurement methods using thermocouples near the substrate block or pyrometers require secondary calibration, leading to inaccurate reading and reproducibility of substrate temperature. Coating of viewports during growth affects the accuracy of pyrometer readings. Accurate measurement of temperature drift during growth is also an issue. As a consequence, a new technique allowing accurate measurement of the growth temperature in an absolute sense is needed.
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In-situ cathodoluminescence (CL) occurring during reflection high energy electron diffraction (RHEED) is a strong candidate for this purpose. CL can be detected up to and beyond typical growth temperatures.[2] Importantly, since it is linked to the intrinsic properties of the GaN, CL allows the determination of the growth temperature with great accuracy and reproducibility. Imaging the sample using CL from a large area electron beam with a ccd camera and interference filters also makes in-situ 2D mapping of temperature inhomogenaities and/or alloy composition fluctuations possible. EXPERIMENTAL The electron beam supplied by a specialized RHEED gun (Staib instrument) was used for in-situ CL measurements. While the energy range is 1 to 20 keV, 18 keV at 20 μA was used for our CL measurements. The electron beam was electronically chopped at a frequency of 290 Hz. Undoped HVPE GaN on (0001) sapphire from TDI Inc. both served
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