Exciton Spectra of AlN Epitaxial Films
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Exciton Spectra of AlN Epitaxial Films T. Onuma1, S. F. Chichibu1, T. Sota2, K. Asai3, S. Sumiya3, T. Shibata3, and M. Tanaka3 1 Institute of Applied Physics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan 2 Department of Electrical, Electronics, and Computer Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555, Japan 3 NGK Insulators, Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan ABSTRACT Excitonic resonance energies in a C-plane AlN epilayer on the (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence (CL) measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral lineshape was fitted considering A (Γu7v Γ7c) exciton transitions. The energies of them at 0 K were obtained to Γ7c) and BC (Γ9v, Γl7v be 6.211 and 6.266 eV, giving the crystal field splitting (∆cr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature ΘE was estimated to be 580 K.
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INTRODUCTION Aluminum nitride (AlN) has the widest bandgap energy of 6.2 eV among the group-III nitride semiconductors and is one of the possible candidates for ultraviolet (UV) light emitters and detectors though it has been exclusively used for the surface acoustic wave devices or a heat sink. The bandgap energy of AlN has been determined in the 1970's to be 6.2 eV at 300 K [1] and 6.28 eV at 5 K [2] by means of optical absorption measurements. The electronic band structure has been recently investigated by Suzuki et al. [3] and Wei et al. [4] using the first-principles calculation. However, few experiments have been carried out to determine the exciton energies, since the threading dislocation (TD) density in the AlN epilayers grown on sapphire has been too high to observe the exciton structures. Bremser et al. [5] have observed a neutral donor bound exciton emission I2 in the cathodoluminescence (CL) spectra of AlxGa1-xN epilayers at 4.2 K for various AlN mole fraction (0
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