The results of molecular dynamics (MD) simulations of dislocation glide in GaN using a Tersoff potential are presented. The simulation methodology involves applying a constant shear stress to a single crystal system containing an individual dislocation, with multiple slip systems considered. Upon reaching a steady state, the dislocation velocity as a function of applied stress and temperature are determined. Edge dislocations with a-type Burgers vectors in the basal, prismatic and pyramidal planes have been analyzed over the temperature range of 300-1300K. The results from simulations of c-type edge dislocations at 1300 K are also presented.

In order to understand enhanced mechanical properties of magnesium-yttrium (Mg-Y) alloys, applied stresses which were required to operate independent plastic deformation mechanisms on various stress axes were evaluated. Moreover, for this analysis, mechanical tests including newly-established testing method “pure-shear test” were conducted to evaluate Critical Resolved Shear Stresses (CRSSes) for various plastic deformation mechanisms of Mg-Y solid solution alloy single crystals with various Y concentration. Relatively higher solid solution strengthening of dominant plastic deformation mechanisms such as basal slip and extension twin at room temperature, results in increase in the activation of non-basal slip system. By a simple analysis based on von-Mises criterion with experimental CRSS values, it is revealed that enhanced mechanical properties of Mg-Y alloys might be attributed to the decrease of difference in the activity of plastic deformation mechanisms by Y addition.

The indentation hardness and yield strength of various wurtzite-structured semiconductors, such as AlN, GaN, InN, and ZnO, were summarized together with those of 6H-SiC. From analysis of the data, the activation energy for motion of an individual dislocation was deduced to be 2–2.7 and 0.7–1.2 eV in GaN and ZnO, respectively, and the evaluated activation energy for dislocation motion showed a dependence on the dislocation energy in the minimum length. The results were evaluated in terms of homology and the basic mechanism of the dislocation process. Dislocation motion is thought to be primarily controlled by the atomic bonding character of the semiconductors.

A review is presented of Synchrotron X-ray Topography and KOH etching studies carried out on n type 4H-SiC offcut substrates before and after homo-epitaxial growth to study defect replication and strain relaxation processes and identify the nucleation sources of both interfacial dislocations (IDs) and half-loop arrays (HLAs) which are known to have a deleterious effect on device performance. We show that these types of defects can nucleate during epilayer growth from: (1) short segments of edge oriented basal plane dislocations (BPDs) in the substrate which are drawn by glide into the epilayer; and (2) segments of half loops of BPD that are attached to the substrate surface prior to growth which also glide into the epilayer. It is shown that the initial motion of the short edge oriented BPD segments that are drawn from the substrate into the epilayer is caused by thermal stress resulting from radial temperature gradients experienced by the wafer whilst in the epi-chamber. This same stress also causes the initial glide of the surface half-loop into the epilayer and through the advancing epilayer surface. These mobile BPD segments provide screw oriented segments that pierce the advancing epilayer surface that initially replicate as the crystal grows. Once critical thickness is reached, according to the Mathews-Blakeslee model [1], these screw segments glide sideways under the action of the mismatch stress leaving IDs and HLAs in their wake. The origin of the mismatch stress is shown to be associated with lattice parameter differences at the growth temperature, arising from the differences in doping concentration between substrate and epilayer.

Effects of surface morphology of buffer layers on ZnO/sapphire heteroepitaxial growth have been investigated by means of “nitrogen mediated crystallization (NMC) method”, where the crystal nucleation and growth are controlled by absorbed nitrogen atoms. We found a strong correlation between the height distribution profile of NMC-ZnO buffer layers and the crystal quality of ZnO films. On the buffer layer with a sharp peak in height distribution, a single-crystalline ZnO film with atomically-flat surface was grown. Our results indicate that homogeneous and high-density nucleation at the initial growth stages is critical in heteroepitaxy of ZnO on lattice mismatched substrates.

AlGaN-based SQW heterostructures grown by plasma-assisted molecular beam epitaxy on c-Al2O3 substrates have been studied with high resolution transmission electron microscopy (HR TEM), photoluminescence spectroscopy and x-ray diffraction. The high-temperature (780°C) synthesis of the AlN buffer layer nucleated on c-Al2O3 by a migration enhanced epitaxy and including several ultra-thin GaN interlayers grown under moderate N-rich conditions was shown to be the optimum approach for lowering the threading dislocations density down to 108-109 cm-2. HR TEM study has confirmed the fine structure of single quantum wells (SQW) formed by a sub-monolayer digital alloying technique and revealed different kinds of compositional inhomogeneities in the AlxGa1-xN barrier layers of the heterostructures, including the formation of Al-rich barriers induced by the temperature-modulated epitaxy and the spontaneous compositional disordering along the growth axis for x=0.6-0.7. The influence of these phenomena on the parameters of the mid-UV stimulated emission observed in the SQW structures has been studied as well.

A dislocation-{101̅2} twin boundary (TB) interaction model was proposed and introduced into discrete dislocation dynamics simulations to study the mechanical behavior of micro-twinned Mg. Strong strain hardening was captured by current simulations, which is associated with the strong TB’s barrier effect. In addition, twin size effects with small TB spacing leading to a strong yield stress, were observed to be orientation dependent. Basal slip orientation produces a strong size effect, while prismatic slip does a weak one.

We succeeded in photovoltaic power generation of p-i-n solar cells utilizing epitaxial ZnInON film with a wide band gap of 3.1 eV as the intrinsic layer, suitable for a top cell of tandem solar cells. The solar cell shows a high open circuit voltage (Voc) of 1.68 V under solar simulator light irradiation of 3.2 mW/cm2. The solar cell performance becomes worse under 100 mW/cm2, which is mainly attributed to the leakage current caused by crystal defects and grain boundaries. X-ray diffraction analysis reveals that the ZnInON film has rather large tilt and twist angles and a high dislocation density of 7.62×1010 cm-2. Such low crystallinity is a bottleneck for high performance of the solar cells. Our results demonstrate a potential of epitaxial ZnInON films as an intrinsic layer of wide band gap p-i-n solar cells with a high Voc.