First- and second-order Raman scattering from semi-insulating 4 H -SiC J. C. Burton, L. Sun, and F. H. Long* Department of Chemistry, Rutgers University, Piscataway, New Jersey 08854-8087 Z. C. Feng Institute of Materials Research & Engineering, S7, NUS, 119260 Singapore I. T. Ferguson EMCORE Corporation, Somerset, New Jersey 08873 Received 11 May 1998; revised manuscript received 9 October 1998 We have measured the first- and second-order Raman spectra from semi-insulating 4 H-SiC at room tem- perature under nonresonant excitation. The results are compared with Raman spectra of n-type doped 4 H- and 6 H-SiC. The second-order spectra for both 4 H- and 6 H-SiC contain an acoustic branch and an optical branch, which is clearly dependent on SiC polytype. The measured two-phonon spectra are assigned by comparison with theoretical calculations. S0163-18299903211-7 Silicon carbide SiC is currently being explored as a ma- terial for the next generation of high-power and high- temperature electronics. An important physical property of SiC is the occurrence of polytypes. 1 4 H - and 6 H -SiC are of considerable technological interest due to their large band gaps and high electron mobilities. 1 Recently high quality 4 H - and 6 H -SiC wafers have been grown. 2,3 Device isolation using insulating layers is critical to semi- conductor electronics. Progress has been made recently in the production of semi-insulating SiC for this purpose, 4 but these samples have not yet been well studied. The character- ization of semi-insulating SiC is essential for the develop- ment of the next generation of SiC materials and devices. One-phonon Raman spectra are sensitive only to phonons at the  point ( k =0) of the Brillouin zone. However, in SiC the polytype structure in the large zone allows us to see other phonons that have a pseudomomentum of zero in the Bril- louin zone. 5 The polytype dependence of one-phonon Raman spectra for doped SiC has been extensively studied. 5–10 Second-order Raman spectra reflect the entire Brillouin zone because any two phonons with opposite wavevectors will have a total momentum of zero. 11 Two-phonon Raman spec- tra are a sensitive test of theoretical models of the lattice dynamics, 11–16 because typically they are continuous spectra with peaks corresponding to regions or points of the disper- sion curve with zero gradient and hence maxima in the pho- non density of states. The second-order Raman spectrum of 3 C -SiC has previously been studied. 13,17 We present here one- and two-phonon Raman spectra of semi-insulating 4 H -SiC, and two-phonon Raman spectra for n-type doped 4 H - and 6 H -SiC. The optical branch of the second-order Raman spectra for SiC was found to be poly- type dependent and much more complex than the cubic 3 C -SiC polytype. These observations reflect clear changes in the phonon density of states with polytype, while the fre- quencies of the  point phonons vary by less than 5%. The identification of spectra was obtained by reference to experi- mental studies and theoretical calculations. The Raman spectrometer used has been described in de- tail before. 18 An Ar/Kr laser and a triple spectrometer with a charge-coupled-detector were used. The polarization was un- specified both for incident and collected light. Collection ge- ometry was in the near backscattered configuration perpen- dicular to the 0001 face of the sample. There are no selection rules for second-order Raman scattering for wurz- itic materials. 19 The doped samples of 4 H - and 6 H -SiC used were typically 1 3/8 to 2 inch wafers from a variety of sources and were n-type nitrogen doped over a nominal range of 10 18 to 10 19 cm -3 . The semi-insulating sample was a 1 3/8 inch wafer with a resistance of greater than 10 5  cm. The one-phonon Raman spectrum for semi-insulating 4 H -SiC is shown in Fig. 1. Calibration was performed with atomic emission lamps, so that the spectrum is accurate to approximately 0.5 cm -1 . The peaks visible are common to all of the 4 H -SiC samples studied. A full assignment of the peaks can be found elsewhere. 5,9,18 The major peaks are FIG. 1. Raman spectrum of semi-insulating 4 H-SiC taken at room temperature with 514.5 nm excitation. The spectrum is also shown with an enlarged y scale to show detail. PHYSICAL REVIEW B 15 MARCH 1999-I VOLUME 59, NUMBER 11 PRB 59 0163-1829/99/5911/72823/$15.00 7282 ©1999 The American Physical Society