Optical Characterization of SiC Wafers
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*Chemistry Dept., Rutgers University, Piscataway, NJ 08854, frhlongarutchem.rutgers.edu
**EMCORE Corp., Somerset, NJ 08873 ABSTRACT Raman spectroscopy has been used to investigate wafers of both 4H-SiC and 6H-SiC. The two-phonon Raman spectra from both 4H- and 6H-SiC have been measured and found to be polytype dependent, consistent with changes in the vibrational density of states. We have observed electronic Raman scattering from nitrogen defect levels in both 4H- and 6H-SiC at room temperature. We have found that electronic Raman scattering from the nitrogen defect levels is significantly enhanced with excitation by red or near IR laser light. These results demonstrate that the laser wavelength is a key parameter in the characterization of SiC by Raman scattering. These results suggest that Raman spectroscopy can be used as a noninvasive, in situ diagnostic for SiC wafer production and substrate evaluation. We also present results on timeresolved photoluminescence spectra of n-type SiC wafers. INTRODUCTION Silicon carbide (SiC) has recently been the subject of renewed interest as an important material for a wide variety of high-power and high-temperature electronic applications. SiC exhibits a large number (250) of polytypes with different structural and physical properties. The polytypes have the same chemical composition but exhibit different crystallographic structures and stacking sequences along the principal crystal axis. Several important polytypes of SiC such as 4H- and 6H- have C6, crystallographic symmetry. In the a-direction 4H- and 6HSiC are almost identical (< 1 % change); however, the 4H- polytype consists of 4 units in the cdirection and the 6H- consists of 6 units. Different polytypes have different band gaps, electron
mobilities, and other physical properties; for example, 4H-SiC has attracted significant attention due to its high electron mobility and excellent thermal properties. Recently high quality wafers of both 4H- and 6H-SiC have been grown. 2 Wafers of SiC are also a promising substrate for nitride semiconductor growth due to their compatible lattice structure and similar thermal expansion coefficients. In this paper we discuss second-order Raman scattering and resonance enhancements of the electronic Raman scattering from SiC in the near IR. EXPERIMENT Raman spectra were recorded using both confocal Raman microscopy and a bulk Raman spectrometer. The bulk Raman system consisted of a Coherent Model INNOVA 90 Ar/Kr laser, a SPEX Model 1877E triple monochromator, and a CCD (charge-coupled-detector) cooled with liquid nitrogen. A liquid nitrogen cryostat was used to take low temperature data. This system has been described elsewhere. All confocal data was collected at room temperature using both a Dilor LabRam system and a Renishaw Series 1000 Raman microscope. Micro-Raman spectra were obtained with laser excitation at 785 nm (1.58 eV) and 633 nm (1.96 eV), which was compared with data taken at 514 nm (2.41 eV), 568 nm (2.18 eV) and 647 nm (1.92 eV) with the bulk Raman spectrometer. The s
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