S. Rohmfeld

Quantitative evaluation of biaxial strain in epitaxial 3C-SiC layers on Si(100) substrates by Raman spectroscopy

We present experimentally determined biaxial strain coefficients for the longitudinal optical (LO) and transversal optical (TO) Raman lines in 3C-SiC. Suspended 3C-SiC membranes with a (100) texture are deflected by a variable pressure load on one side and the strain-induced shifts of the LO and TO Raman lines are measured while the strain is simultaneously calculated from the membrane deflection vs pressure. Using these results we measure the residual strain of 3C-SiC films grown on Si as a function of preparation conditions. The largest residual strain is found in thin samples which relaxes as film thickness increases to a value imposed by the different thermal expansion coefficients of 3C-SiC and Si. As the residual strain decreases and the film thickness increases, the Raman lines narrow indicating an improved crystalline quality. We also find a reduction of the residual strain with increasing growth rate.

Influence of Stacking Disorder on the Raman Spectrum of 3C-SiC

We measured Raman spectra of as-deposited and laser-annealed polycrystalline 3C-SiC layers. Compared to single crystal 3C-SiC the Raman lines are considerably broader and an additional signal at frequencies corresponding to a high density-of-states is found. We discuss the influence of stacking disorder on the Raman spectrum by comparing our results with computer simulated Raman intensity profiles of one-dimensionally disordered SiC structures. The broadening of the linewidth and the disorder-induced peak shift, as well as the density-of-states feature can be explained with stacking disorder. After laser annealing above 1900 K the Raman lines narrow irreversibly and the density-of-states feature disappears. We ascribe both observations to a reduction of stacking disorder.

Near-thermal equilibrium growth of SiC by physical vapor transport

Abstract Silicon carbide single crystals of the 4H-, 6H- and 15R- polytype are grown by using physical vapor transport (PVT). The crystal growth is performed at conditions where the growth chamber is close to thermal equilibrium. Micropipe-free growth of 6H-SiC on both the C- and Si-face of a 6H-SiC seed is demonstrated. 4H/15R bulk material has been grown on 4H-/15R-seeds, respectively. The growth of 4H-/15R-SiC can not yet be stabilized for the entire boule. Micropipe generation is observed in the neighborhood of polytype transitions, where Raman spectroscopy reveals internal stress. Nitrogen donor concentrations and concentrations of the compensation range from 5.9×10 17 to 1.5×10 18 cm −3 and from 3×10 17 to 6×10 17 cm −3 , respectively. The maximum of the electron Hall mobility is about 200 cm 2 (Vs) −1 (6H–SiC) and 300 cm 2 (Vs) −1 (4H- and 15R-SiC).