Optical Absorption of Doped and Undoped Bulk SiC
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OPTICAL ABSORPTION OF DOPED AND UNDOPED BULK SiC K. Miller1, Q. Zhou1, J. Chen1, M. O. Manasreh1, Z. C. Feng2, and I. Ferguson3 1 Department of Electrical and Computer Engineering, University of New Mexico, Albuquerque, NM 87109. 2 Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602. 3 EMCORE Corporation, 394 Elizabeth Avenue, Somerset, NJ 08873.
ABSTRACT Optical absorption spectra of undoped, n-type, and semi-insulating 6H and 4H bulk silicon carbide (SiC) were obtained in the spectral region of 200 – 3200 nm (6.20 – 0.3875 eV). Several features were observed in the absorption spectra collected for various samples. A sharp peak below the band gap was observed in 4H SiC. The intensity of this peak was observed to increase in samples that exhibit larger absorption due to free carriers, which leads us to conclude that the defect responsible for this peak is also the source of the free carriers in the materials. Additionally, a series of optical absorption peaks separated by approximately 21 meV were observed around 0.9185 eV (1350 nm). These peaks are zero phonon lines of intraband transitions in the VSi 3d shell. The optical absorption near the band edge was observed to be sample dependent. The variation of the band gap as a function of temperature is also observed to be sample dependent.
INTRODUCTION: There has been increasing interest in wide band gap semiconductor materials due to their high-power, high temperature and high frequency applications. SiC is of special interest due to its excellent thermal properties and other characteristics. The optical absorption yields important information about the vibronic and structural properties of the material. A greater knowledge of the material’s characterization and optical properties is especially necessary for advancement of electronic industry [1]. The optical properties have been studied by reflectance, optical, and photoluminescence techniques (see for example Ref. [2]). In this article, we present the results on the optical absorption measurements of the undoped, n-type, and semi-insulating 6H and 4H bulk SiC materials. The band gap was studied as a function of temperature in the range of 10 – 300 K. The phonon or Einstein temperature was obtained by fitting the band gap results as a function of temperature using an expression based on Bose-Einstein statistics.
EXPERIMENT: For this study, several SiC samples were obtained from different sources. The samples were sectioned from bulk SiC wafers. The sample surfaces were prepared by H5.23.1
lapping and successive polishing with progressively finer Al2O3 film sheets. The samples were mounted on a copper cold finger in a closed cycle refrigerator. The optical absorption measurements were conducted in the spectral region of 200 – 3200 nm (6.20 – 0.3875 eV) using a CARY 500 UV-VIS-NIR spectrophotometer. The temperature was restricted to the range of 10 – 300 K within ±1 K. Five samples were selected for the present study, which are undoped, N-doped and semi-insulating 6H and 4H bulk SiC.
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