Optical Characteristics of Amorphous III-V Nitride Thin Films
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Optical Characteristics of Amorphous III-V Nitride Thin Films Jebreel M. Khoshman and Martin E. Kordesch Department of Physics & Astronomy, Condensed Matter & Surface Science Program, Ohio University, Athens, OH 45701 ABSTRACT The optical constants and polarized optical properties of amorphous III-V nitride thin films, a(Al, Ga, In) N, deposited by RF magnetron sputtering onto crystalline silicon, c-Si, (111) and glass substrates have been investigated over the wavelength range 300 – 1400 nm. The optical constants of a-AlN were obtained by analysis of the measured ellipsometric spectra through the Cauchy–Urbach model while the optical constants of a-(In, Ga) N were determined using the Tauc-Lorentz model. Analysis of the absorption coefficient of a-AlN (in the range 200 – 1400 nm) and a-GaN (in the range 300 – 1400 nm) show the optical bandgap to be 5.9 ± 0.05 and 3.44 ± 0.05 eV. The absorption coefficient of a-InN (in the range 300 – 1400 nm) as a function of photon energy shows the absorption edge to be about 1.74 ± 0.05 eV. From the angle dependence of the p-polarized reflectivity we deduced Brewster and principal angles of these films. Measurement of the polarized optical properties revealed a high transmissivity (70 % – 95 %) and low absorptivity (< 18 %) for all three thin films in the visible and near infrared regions. INTRODUCTION Spectroscopic Ellipsometry (SE) is a sensitive optical technique for characterizing the physical, optical and material properties of thin films and structures. It derives its sensitivity from the determination of the relative phase change in a beam of reflected polarized light [1]. In the present study, we measured the ellipsometric spectra of a series of a-(Al, Ga, In) N thin films deposited on c-Si (111) and glass substrates by a reactive RF magnetron sputtering at a temperature T < 325 K. The optical constants and the thicknesses of the films were obtained by the analysis of the measured ellipsometric spectra through the Cauchy-Urbach and Tauc-Lorentz models. The Brewster and principal angles were obtained by analysis of the polarized reflectivities. From the law of conservation of energy the polarized absorptivities of these films were calculated as a function of wavelength. The wavelength range of our measurement was 300 – 1400 nm including some portion of the infrared region. EXPERIMENTAL DETAILS The samples for the SE measurements were prepared by a reactive RF magnetron sputtering method. The method has been in use for several years to produce amorphous thin films for lighting applications [2]. An Al target , In target and Ga target of 99.999% purity were each separately sputtered in a pure nitrogen atmosphere. Aluminum nitride, Gallium nitride, and indium nitride films were simultaneously deposited independently onto cleaned c-Si (111) and glass substrates. The substrates were clamped to a thick copper block that limited the temperature during deposition to T < 325 K. The deposition system had a base pressure 4 × 10-7 torr while the RF sputtering power and deposit
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