Structure of SiC-Quantum Wells Studied by TEM and CBED
Summary and Conclusion SiC hetero-polytypic structures have been investigated by conventional and high resolution TEM and the lattice parameters of a thin cubic stripe have been determined by CBED. For the case of the stepped substrate, nearly free of defects, about 2nm thick cubic stripes are formed within the hexagonal layer. For the case of the growth on on-axis substrates, it was demonstrated that 6H/3C as well as 4H/3C stacks are formed on 6H and 4H-SiC substrates, respectively, By CBED the lattice parameters of the cubic stripe were determined to a rhom =0.4360nm a rhom = (89.78±0.02)° demonstrating the rhombohedral distortion of the 3C stripes. It might be suggested that the result is valid also for the 2nm thick cubic stripes in the multi-quantum well structures grown on the stepped SiC substrate, although such layers are generally to thin for the lattice parameter determination by CBED. Acknowledgements The authors are indebted to Dr. A. Chuvilin for the generous scientific support during his stay as guest scientist at Jena University, thank Ch. Zaubitzer for sample preparation and acknowledge the financial support of the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 196.
Optical properties of as‐grown and process‐induced stacking faults in 4H‐SiC
We present a review of recent photoluminescence, cathodoluminescence and micro‐photoluminescence studies that have been made to investigate the electronic properties of as‐grown and/or process‐induced stacking faults (SFs) in silicon carbide (SiC) epitaxial layers. For different polytypes and different acquisition conditions, we discuss the optical signature and compare with the results of model calculations. Since a SF is always a finite lamella of 3C or 8H polytype in a 4H or 6H matrix, we take successively into account the effect of the valence band offsets, internal polarizations and non‐homogeneity of the potential wells. In the case of cathodoluminescence and micro‐photoluminescence techniques, due to the higher pumping level, we show that some screening of the built‐in electric field can be reached. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Surface roughness monitoring by singular spectrum analysis of vibration signals
Abstract This study assessed two methods for enhanced surface roughness (Ra) monitoring based on the application of singular spectrum analysis (SSA) to vibrations signals generated in workpiece-cutting tool interaction in CNC finish turning operations i.e., the individual analysis of principal components (I-SSA), and the grouping analysis of correlated principal components (G-SSA). Singular spectrum analysis is a non-parametric technique of time series analysis that decomposes a signal into a set of independent additive time series referred to as principal components. A number of experiments with different cutting conditions were performed to assess surface roughness monitoring using both of these methods. The results show that singular spectrum analysis of vibration signal processing discriminated the frequency ranges effective for predicting surface roughness. Grouping analysis of correlated principal components (G-SSA) proved to be the most efficient method for monitoring surface roughness, with optimum prediction and reliability results at a lower analytical-computational cost. Finally, the results show that singular spectrum analysis is an ideal method for analyzing vibration signals applied to the on-line monitoring of surface roughness.