Optical and Structural Investigation of AlN Grown on Sapphire with Reactive MBE Using RF Nitrogen or Ammonia
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Optical and Structural Investigation of AlN Grown on Sapphire with Reactive MBE Using RF Nitrogen or Ammonia F. Yun1, L. He1, F. Xiu1, H. Morkoç1, S. Bai2, Y. Shishkin2, R. P. Devaty2, and W. J. Choyke2 1 Department of Electrical Engineering and Physics, Virginia Commonwealth University Richmond, VA 23284 2
Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, PA 15260
ABSTRACT AlN epitaxial layers in a thickness range of 0.065 to 0.6 microns have been grown on double-side polished c-plane sapphire by molecular beam epitaxy (MBE) using either RF nitrogen or ammonia as the nitrogen source. The samples were characterized by XRD, room temperature (RT) and low temperature (2K) optical absorption (transmission) measurements. The XRD (0002) peak FWHM diminishes from 380 arcsec to 84 arcsec when the thickness of the AlN films is increased from 65 nm towards 0.6 µm. All the samples grown with ammonia (NH3), representing N-rich conditions, exhibited optical bandgap exceeding 6 eV at RT. For samples grown with RF nitrogen source, we find that higher nitrogen flow (partial pressure of 8.3-9.4 x 10-5 Torr) results in optical bandgap values larger than 6.0 eV, regardless of the XRD results. The bandgap is found to be smaller than 6.0 eV for the samples grown with lower nitrogen partial pressure (1.0-3.5 x 10-5 Torr), regardless of sample thickness and the XRD data. INTRODUCTION AlN is a semiconductor which possesses a direct bandgap (purported to be ~6.2 eV at RT). It has high thermal conductivity, high hardness, high chemical resistance, and strong piezoelectric properties.1 AlN is also the end binary of AlGaN ternary alloy on the high bandgap extreme, and is necessary for optical device applications where deep UV operation is needed. Progress in fabrication of such devices as UV light sources and photodetectors based on III-nitrides requires understandingthe basic optical properties of AlN. The studies of the optical properties of AlN have been historically hindered by the lack of good quality crystals, both bulk and epilayers. In the past few years, due to improvements in the growth techniques there have been a number of reports on studies of the optical properties of aluminum nitride grown by MOCVD2,3,4,5,6, MBE7,8,9, and DC magnetron sputtering.10 In this study, we have grown AlN epilayers by MBE in both N-rich and N-lean environment. Either reactive ammonia (NH3) or RF N2 plasma is used as the nitrogen source. Optical absorption measurements and characterization by X-ray diffraction were performed to investigate the dependence of the optical bandgap and the crystalline quality on growth parameters.
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EXPERIMENTAL AlN epitaxial layers were grown by MBE using RF nitrogen plasma or NH3 as the nitrogen source. Double-side polished c-plane
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