On-axis and vicinal GaN/AlN/6H-SiC structures grown under identical conditions have been studied by x-ray diffraction and transmission electron microscopy to demonstrate the distinctive features of vicinal surface epitaxy (VSE) of nitrides on SiC. In VSE, the epilayers are tilted from the substrate due to the out-of-plane lattice mismatch (Nagai tilts), and the in-plane mismatch strains are more relaxed. The majority of misfit dislocations (MDs) at the vicinal AlN/6H-SiC interface are found to be unpaired partial MDs that are geometrically necessary to correct the stacking sequences from 6H to 2H. This mechanism indicates that it is possible to develop "step-controlled-epitaxy" strategies to control strain relaxation by adjusting the substrate offcut angles.
We performed an experiment to study the role of anisotropy in interfacial energy during the coarsening of a complex oxide CaCu3Ti4O12 (CCTO) material system. When we altered the interface anisotropy, we observed that evolution and sizes of spherical grains completely altered to facetted cubes. When interface became anisotropic, crystal growth appears to occur by the surface attachment/detachment, the motion of ledges, and the nucleation of new layers. Our observations showed that new layers grew by screw dislocation mechanism. This suggests that nucleation limited coarsening is occurring by the development of a transient bimodal grain size distribution consisting of large growing grains with step. In presence of PbO we always observed the development of shaped nucleus which grew larger. Sometimes very complex shaped crystals grew as a metastable shape. These shapes became uniform as the growth progressed and size of grains became larger. This process produces smaller grains that act as a source of material for the growing less defective larger grains.
The microstructure and luminescence properties of a series of GaN epilayers grown on sapphire and SiC substrates with various misorientations have been correlated to assess the origins of the luminescence features in the misoriented samples. Samples grown on 3.5° offcut SiC and 5° and 9° offcut sapphire substrates both exhibit photoluminescence peaks near ∼3.2 and ∼3.4eV, which are absent in the on-axis SiC and sapphire cases. Transmission electron microscopy shows that the misoriented samples have configurations of I1 basal plane stacking faults, which fold into prismatic stacking faults with stair-rod dislocations at their intersections. The luminescence features are proposed to possibly arise from transitions involving the prismatic stacking faults and∕or the stair-rod dislocations associated with their mutual intersections and their intersections with the basal plane stacking faults.
A systematic study of intersecting planar boundary structures observed in a GaN epifilm grown on a vicinal 6H-SiC substrate (offcut towards [12¯10]) with an AlN buffer is presented. These structures are shown to comprise stacking faults that fold back and forth from the basal plane [I1 basal plane stacking faults (BSFs)] to the prismatic plane [prismatic stacking faults (PSFs)]. The PSFs, with fault vector 12⟨101¯1⟩ nucleate at steps on the substrate surface as a consequence of the different stacking sequences exposed on either side of the step. Once nucleated, PSFs intersecting the vertical step risers in the AlN buffer and eventually in the GaN film are replicated during the predominantly step-flow growth and propagate into the growing crystal. As a consequence of the different growth rates experienced on either side of the intersection of a PSF with a vertical step riser, the PSF may be redirected onto an equivalent {112¯0} plane, leaving an I1 BSF between the bottom of the redirected section of PSF and the top of that portion of the original PSF which was below the terrace. This leads to the formation of folded PSF/BSF fault structures which exhibit various configurations. Such folded stacking fault configurations form walls which enclose domains of different stacking sequence. Stair-rod dislocations (SRDs) are observed to form at the intersections of these various faults and the Burgers vectors of some of these are discussed. In some cases, reconstruction of these SRDs occurs through the formation of partner dislocations which serve to minimize the energy associated with the lattice disconnections located at the cores of the SRDs.
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