Infrared Reflectance Characterization of a GaAs-AlAs Superlattice
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INFRARED REFLECTANCE CHARACTERIZATION OF A GaAs-AlAs SUPERLATTICE J. M. ZAVADA*, G. K. HUBLER**, H. A. JENKINSON***, W. D. LAIDIG**** *US Army Research Office, Research Triangle Park, NC 27709 "**Naval Research Laboratory, Washington, DC 20375 ***US Army Armament R&D Center, Dover, NJ 07801 "****North Carolina State University, Raleigh, NC 27695 ABSTRACT The optical properties of a GaAs-AlAs superlattice have been examined using the non-destructive technique of infrared reflectance spectroscopy. Through this technique, the absorption edge, the effective superlattice refractive index, the thickness, and the optical grading of the superlatticesubstrate were determined. Location of the absorption edge was made from the reflectance spectrum and showed general agreement with photoluminescence measurements. A more detailed analysis of the infrared spectra indicated the presence of a transition region between the substrate and the superlattice. Based on a non-linear least squares method for fitting the experimental data, a dispersion relation for the dielectric function was obtained. This dielectric function yielded a value for the superlattice refractive index that was lower than that of the corresponding, homogeneous, AlGaAs alloy film for wavelengths between 1.0 and 2.5 micrometers. INTRODUCTION The synthesis of high quality GaAs-AlAs superlattices has initiated a new class of infrared materials with important consequences in the area of optoelectronics. These materials have found a wide variety of applications in optical components such as sources, photodiodes, modulators, and switches. For many of these applications it is critical to accurately characterize the optical properties of these materials in frequency regions of interest. In the present investigation the non-contact technique of infrared reflectance spectroscopy was used to determine the optical properties of a GaAs-AlAs superlattice in the near infrared region (4,000 to 13,000 cm-1). While reflectance spectroscopy has been used with considerable success to examine optical properties of bulk and ion implanted GaAs specimens [1,2], little prior work has centered on applying this method to the characterization of superlattice films. Here it is shown that reflectance spectroscopy can be used to determine the absorption edge, the effective refractive index, the thickness, and the optical grading of the GaAs-AlAs multilayer film. EXPERIMENTAL PROCEDURES AND RESULTS The superlattice for this study consisted of alternating layers of GaAs and AlAs grown on an undoped, semi-insulating GaAs substrate. The total thickness of the superlattice was estimated to be 6 micrometers with the individual layers being 73 A for GaAs and 80 A for AlAs. From these thicknesses the average aluminum concentration x of the superlattice was 0.523. Growth conditions relating to the superlattice as well as characterization of the layer parameters have been previously reported [3].
Mat. Res. Soc. Syup. Proc. Vol. 90. ' 1987 Materials Research Society
258 The infrared reflectance measure
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