Papers by Keyword: Planar Defects

Abstract: Germanium nanocrystals (Ge-nc) were produced by the implantation of Ge+ into a SiO₂ film deposited on (100) Si, followed by a high-temperature annealing. High-resolution transmission electron microscopy (HRTEM) has been used to investigate the defect structures inside the Ge-nc produced by different implantation doses (1×10¹⁶, 2×10¹⁶, 4×10¹⁶ and 8×10¹⁶ cm^-2). It has been found that the planar defects such as nanotwins and stacking faults (SFs) are dominant in Ge-nc (60%) for the samples with implantation doses higher than 2×10¹⁶ cm^-2, while for the sample with an implantation dose lower than 1×10¹⁶ cm^-2, fewer planar defects are observed in the Ge-nc (20%). The percentages of nanotwins in the planar defects are 87%, 77%, 67% and 60% in four samples, respectively. The twinning structures include single twins, double twins and multiple twins. We also found that there are only SFs in some nanocrystals, and in others the SFs coexist with twins. These microstructural defects are expected to play an important role in the light emission from the Ge-nc.

Abstract: Planar defects may exhibit free volumes which, in principle, are detectable using positron annihilation spectroscopy. In this contribution, we present a preliminary theoretical study of positron trapping at stacking faults in zinc oxide and at a grain boundary in zirconia. In particular, we calculate the positron lifetime and positron binding energy to such defects. In the case of the grain boundary in zirconia, the influence of the yttrium segregation on the GB structure and positron characteristics is also examined. Calculated structural and positron characteristics are critically compared with experimental and other calculated data.

Abstract: The structure of intermetallic phases and planar defects in the as-cast and the solutiontreated microstructures of a Mg-8Y-2Zn-0.6Zr (wt%) alloy are characterized using transmission electron microscopy. The alloy was produced by permanent mould casting and solution treated at 500 °C. It is found that the intermetallic particles in the as-cast microstructure have a monoclinic structure. An appreciable amount of intermetallic particles is still retained along grain boundaries after solution treatments for up to 60 hrs. However, the structure of the retained intermetallic particles changes gradually from monoclinic to hexagonal during the solution treatments. Some planar defects are also detected in the as-cast and the solution-treated (1 hr) microstructures. These defects have characteristic features of stacking faults.

Abstract: One of the problems with Si(001)/3C-SiC templates is that they involve highly defective interfaces due to the presence of misfit dislocations, voids and planar defects that degrade the SiC layer quality. A way to accommodate the high lattice mismatch between these materials and reduce the voids density is to carbonize the Si substrate before the epitaxial growth. In this contribution an alternative way to reduce planar defects density is presented by analyzing the relationship between planar defects and voids. Planar view and cross section transmission electron microscopy micrographs show a diminution of planar defects in the regions surrounding the voids. Due to the lower elastic energy over the voids and/or to a lateral growth in these regions, the generation of planar defects is partially deactivated, improving locally the crystalline quality of the SiC layer. The introduction of such cavities can be thus seen as a new parameter of Si/SiC templates design.

Abstract: Planar defects have been found in nitrogen doped 2" 4H-SiC crystals grown on off-axis seeds. The doping level was 1×1019cm-3, which is below the critical one for the thermally activated cubic stacking fault formation in the 4H matrix. Planar defects in the doped region are nucleated on the whole seed surface outside the growth facet. They are coexisting 15R and 6H lamellas of unitcell height as revealed by means of luminescence and high resolution transmission electron microscopy. These inclusions are preferably formed at the rim of the growth facet, where polytype change occurs after switching off the nitrogen flow during growth.