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.
Control of epitaxial layers grown on 4H–SiC: from 3C microcrystalline inclusions to type II quantum well structures
We report the results of a series of LTPL (Low Temperature PhotoLuminescence) investigation performed on n‐type, non‐intentionally doped, 4H–SiC samples. We focus on the defect structure introduced by a growth fault in the stacking sequence. We treat the fault as a 3C quantum well embedded in a 4H–SiC matrix and use a simple 2‐dimensional approximation to deduce the extent of the lattice perturbation associated with a given defect. We show that the intrinsic type‐II nature of the band alignment, combined with the effect of the spontaneous polarization, must result in a double bound‐exciton signature per well, which is resolved for the first time. The comparison with experimental results confirms also that, after a relatively small critical layer thickness, the back conversion from cubic to hexagonal remains an unsolved problem. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
HOLZ line analysis of lattice parameters in magnesium alloys.
A method of refining lattice parameters from deficit higher-order Laue zone (HOLZ) line data from large angle convergent beam electron diffraction (LACBED) data is presented, relying on distances between nearest neighbour intersections alone in order to minimize effects of distortion over the field of view. Use is made of a dynamical correction deltak to the fast electron wavevector k for kinematic analysis. This correction term is shown to depend on the specific HOLZ beam under consideration, as well as the zone axis and eigenvalue associated with the branch index of the relevant dispersion surface. This method is applied to analysis of data from magnesium alloys, where momentum filtering induced by the LACBED method facilitates HOLZ contrast from a relatively low index zone axis (where contrast is not detectable with conventional CBED), and contrast is enhanced at elevated temperatures from a higher index zone axis. Although the accuracy of refined lattice parameters from these sets of data is shown to be no better than 0.1%, it is felt that issues arising out of the analysis may be of some interest, particularly since these are non-ideal specimens. Full eigen-state analysis of the fast electron wavefunction is presented, and issues related to the influence of the dispersion surface on deficit HOLZ line behaviour are discussed.