Temperature-dependent Optical Properties of AlN Thin Films by Spectroscopy Ellipsometry
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Temperature-dependent Optical Properties of AlN Thin Films by Spectroscopy Ellipsometry Yao Liu1,2, Ehsan Ghafari2, Xiaodong Jiang2, Yining Feng2, Zhe Chuan Feng1, Ian Ferguson3 and Na Lu2,* 1 Laboratory of optoelectronic materials & detection technology, Guangxi Key Laboratory for the Relativistic Astrophysics, School of Physical Science & Technology, Guangxi University, Nanning, 530004, China 2 Lyles School of Civil Engineering, School of Materials Engineering, Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47906, U.S.A. 3 Dept. Elect. Comp. Engineering, Missouri Science and Technology, Rolla, MO 65409, U.S.A. ABSTRACT In this work, temperature-dependent optical properties of a series of AlN thin films with different thickness are studied by spectroscopic ellipsometry (SE) ranging from 300 to 825K. The fitted refractive index at 300K is in good agreement with the reported by others, which confirms the high accuracy of the optical model used in this work. The degradation of the absorption properties and the decrease of the bandgap become more pronounced with temperature increases above 475K. A larger change of bandgap at elevated temperature is observed for the thinner AlN epi-layer (300nm) than the thicker ones (404nm). This can be attributed to the poor surface morphologies and crystal qualities in the thinner AlN epi-layer. INTRODUCTION AlN films have become a promising candidate for a wide range of applications including high-temperature power electronics, piezoelectric sensors, ultraviolet (UV) light emitting diodes (LEDs), and UV detectors. This is due to their unique material properties such as high thermal conductivity, high thermal stability, large dielectric breakdown field, high surface velocity of acoustic wave (SAW) and ultra-wide bandgap [1-6]. To optimize its high temperature application potentials, it is important to understand the optical constants and bandgap variation of AlN with the function of temperature. Several experimental methods have been employed to study the temperature-dependent optical properties of AlN thin films in the past. For instance, Watanable et al. [7] evaluated the refractive index of AlN films up to temperatures of 788K by using optical interference measurements. Sohal et al. [8] and He et al. [9] reported the bandgap of AlN films on low temperature (20-450K) by using optical transmission spectroscopy. Nam et al. [10] utilized deep UV photoluminescence spectroscopy to evaluate the temperature-dependent bandgap energy in AlN thin films from 10 to 800K. However, PL measurement cannot investigate the thermo-optic effect. Optical constants and film thickness can be estimated with very high precision by using ellipsometry, since this technique measures relative light intensities modulated by optical elements [11]. M. Röppischer et al. [12] investigated the dielectric function (DF) of zinc-blende AlN determined by ellipsometry. Feneberg et al. [13] reported the lowtemperature-dependent exciton resonances in the DF of wurtzite AlN derived by spectroscopic ellip
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