Measurements of the Refractive Indices of MOCVD and HVPE Grown AlGaN Films Using Prism-Coupling Techniques Correlated wi
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Measurements of the Refractive Indices of MOCVD and HVPE Grown AlGaN Films Using Prism-Coupling Techniques Correlated with Spectroscopic Reflection/Transmission Analysisa Norman A. Sanford National Institute of Standards and Technology, Boulder, CO 80305, U.S.A. Lawrence H. Robins, Albert V. Davydov, Alexander J. Shapiro National Institute of Standards and Technology, Gaithersburg, MD 20899, U.S.A Denis V. Tsvetkov, Vladimir A. Dmitriev, Technologies and Devices International Inc. Silver Spring, MD 20904, U.S.A. Stacia Keller, Umesh K. Mishra, Steven P. DenBaars University of California, Santa Barbara, CA 93106, U.S.A. ABSTRACT Waveguide prism-coupling methods were used to measure the ordinary and extraordinary refractive indices of AlxGa1-xN films grown on sapphire substrates by HVPE and MOCVD. Several discrete wavelengths ranging from 442 nm to 1064 nm were used and the results were fit to one-term Sellmeier equations. The maximum standard uncertainty in the refractive index measurements was ± 0.005 and the maximum standard uncertainty in the self-consistent calculation for film thickness was ± 15 nm. Analysis of normal-incidence spectroscopic transmittance and reflectance measurements, correlated with the prism-coupling results, was used to determine the ordinary refractive index as a continuous function of wavelength from the band gap wavelength of each sample (between 252 nm and 364 nm) to 2500 nm. The Al compositions of the samples were determined using energy-dispersive X-ray spectroscopy analysis (EDS). HVPE grown samples had compositions x = 0.279, 0.363, 0.593, and 0.657. MOCVD samples had x = 0.00, 0.419, 0.507, 0.618, 0.660, and 0.666. The maximum standard uncertainty in the absolute EDS-determined value for x was ± 0.02. INTRODUCTION AlxGa1-xN alloys are important for the development of laser diodes and LEDs operating in the blue and ultraviolet. These applications require refractive index data correlated with Al mole fraction and estimates of the uncertainties of these quantities. Various methods have been used to measure refractive indices in AlxGa1-xN alloys. Spectroscopic reflectance and transmittance (R/T) has been used to measure the ordinary refractive index.1, 2 Spectroscopic ellipsometry (SE) has been used to measure refractive index, birefringence, and extinction coefficients.3, 4 Both SE and R/T permit quasi-continuous measurement of optical constants over a wide spectral range. Reference 5 describes transmission-electron microscopy (TEM) combined with reflectance studies and SE. Inclusion of TEM analysis enabled modeling the film/substrate interlayers and improved the SE model fitting. In order to obtain the most accurate results, both SE and R/T require measurements of film thickness that are independent of refractive index. Waveguide prism coupling (WPC) allows measurements of refractive indices and thickness of AlxGa1-xN films. If at least two TE (polarization in the plane of the film) or two TM modes (polarization perpendicular to the film) are supported at a given wavelength,
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