Anisotropic Dielectric Properties of GaN Epilayers on Sapphire
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Anisotropic Dielectric Properties of GaN Epilayers on Sapphire N. L. Rowell, G. Yu, and D. J. Lockwood National Research Council, Ottawa, Ontario, Canada K1A 0R6 ABSTRACT Polarized infrared reflectance from 300 to 1200 cm-1 at different incidence angles was measured at room temperature for sapphire and GaN/sapphire samples. A film of GaN was grown approximately 1.5 µm thick on c-plane sapphire by MOCVD with a thin GaN buffer layer. Because of the hexagonal structure of sapphire and GaN, their uniaxial optical properties are anisotropic. In the p-polarized reflective spectrum, we observed the mixed effect between the distinct infrared-active modes with dipole-moment oscillation perpendicular (E-mode) and parallel (A-mode) to the c axis. The contribution of the A-mode increased with increasing incidence angle. Therefore, we were able to obtain simultaneously the infrared dielectric functions parallel and perpendicular to the c axis, by fitting simultaneously three polarized reflectance spectra at three different incident angles with a suitable model. In the procedure, we adopted a new fitting technique, i.e., fitting the first numerical derivative of the polarized reflectance spectra to improve the accuracy of the phonon parameters and to overcome the inconsistency between the model and measurement in the whole frequency range. Excellent agreement has been obtained between the measured and fitted first derivative reflectance spectra for both the sapphire and GaN/sapphire samples. The dielectric information thus obtained for sapphire and GaN is of greater accuracy than those reported previously.
INTRODUCTION The technology of GaN and related compounds on sapphire substrates has enabled the fabrication of optoelectronic devices operating in the blue and near-ultraviolet wavelength region.[1,2] As such compounds are grown on sapphire c-plane substrates (c-axis perpendicular to the sample surface) with a hexagonal wurtzite structure (α-phase), the materials exhibit uniaxial optical anisotropy. In the visible, the two independent refractive index constants, n⊥ (electric field vector E⊥c) and n// (electric field vector E//c) have been obtained for GaN using polarized reflectance at two incident angles.[3] In the infrared, Kasic et al. [4] have studied the optical phonons of doped hexagonal GaN films on sapphire using infrared ellipsometry but needed Raman measurements to obtain the frequency of the transverse optic (TO) A1 (electric field vector E//c) phonon mode.[4] Here we have used a relatively new method to simultaneously fit the reflectance spectra at three different incident angles with a factorized model for the dielectric constant, in which the contributions of both the E1 (electric field vector E⊥c) and A1 phonon modes are included. Since, in p-polarized reflectance spectra, the relative contributions of the E1 and A1 modes change with incident angle, it was expected that the fitting at multiple incident angles would considerably improve the accuracy of the parameters obtained. Furthermore, for improved f
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