Bowing Parameter of Al x Ga 1-x N
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Bowing Parameter of AlxGa1-xN Feng Yun, Michael A. Reshchikov, Lei He, Thomas King, M. Zafar Iqbal, Hadis Morkoç, Steve W. Novak1, and Luncun Wei2 Dept. of Electrical Engineering, Virginia Commonwealth University, Richmond, VA 23284; 1 Evans East, a member of the Evans Analytical Group, East Windsor, NJ 08520; 2 Charles Evans and Associates, a member of the Evans Analytical Group, Sunnyvale, CA. ABSTRACT AlxGa1-xN samples covering the entire range of alloy compositions, 0≤x≤1, were grown by plasma-assisted MBE on c-plane sapphire substrates. The aluminum mole fraction was determined, by three different techniques, namely, x-ray diffraction, secondary ion mass spectroscopy, and Rutherford backscattering spectrometry. The energy bandgaps of the alloys were obtained from low temperature reflectance spectra. The data lead to a bowing parameter of b=1.0 eV in relating the bandgap of the AlxGa1-xN alloy to its chemical composition. A discussion of bowing parameter determination is presented along with possible causes for the large dispersion in previously reported bowing parameter values. INTRODUCTION The growth of AlxGa1-xN/GaN heterostructures are crucial in achieving light emitters, detectors, and modulation-doped field-effect transistors. The AlxGa1-xN /GaN interface band offset and refractive indices, together with polarization charges and polarization-induced barrier height, depend strongly on the Al composition. Precise determination of the Al composition from the bandgap requires knowledge of the bowing parameter, which suffers from a large dispersion in the literature up to date.1,2,3,4,5 This is mainly due to the scatter of samples by different growth techniques, growth on different substrates,6 limits of specific characterization techniques, and perhaps lack of sufficient attention to the spatial distribution (both depth and lateral) of Al composition. In this paper, a variety of characterization techniques have been employed independently in different laboratories to get dependable bowing parameters for samples grown in our laboratory. Three different techniques which include secondary ion mass spectroscopy (SIMS), high-resolution x-ray diffraction (HR-XRD), assuming Vegard’s law, and Rutherford backscattering (RBS) were used to determine the Al chemical composition of our epitaxial layer samples grown by molecular beam epitaxy (MBE). Energy bandgaps of AlxGa1-xN alloys were determined from optical reflectance spectra. EXPERIMENTAL DETAILS The AlxGa1-xN samples used in this study were grown on c-plane sapphire by plasma-assisted molecular beam epitaxy (MBE) system with nitrogen rf source. A thin AlN buffer layer about 40 nm thick was grown at about 780°C, prior to the AlGaN growth at medium temperatures (615665°C). The AlGaN layers were grown for 2-7 hours. Growth rate decreases with the increase of Al mole fraction because of the specific method employed. The thickness of the layers was in the range of 0.16-0.75 µm. The crystalline quality of the AlxGa1-xN films was examined by x-ray
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