Papers by Keyword: Misfit Dislocation

Abstract: The defect evolution on 90 μm-thick heavily Al-doped 4H-SiC epilayers with Al doping level higher than 10²⁰ cm^-3 was studied by tracing back to initial growth stage to monitor major dislocations and their propagations in each growth stage. Results from X-ray topography and KOH etching demonstrate that all existing dislocations on the surface of 90 μm-thick epilayer can be identified as the defects originating from substrate. In other words, there seems no new dislocation generated after a long-term growth. Nevertheless, a high density of misfit dislocation was found appearing near the substrate/epilayer interface for epilayer with Al doping level of 3.5×10²⁰ cm^-3, while misfit dislocation cannot be seen on epilayer with Al doping level of 1.5×10²⁰ cm^-3.

Abstract: The Bragg peak position of a homogeneous solid solution epitaxial film is directly related to the solid solution concentration, film strain and, consequently, residual stress. The peak shape contains information about defects present in the sample. In the case of compositionally graded epitaxial films the situation is more complex since instead of a single Bragg peak there is a continuous diffracted intensity distribution which can be measured by means of recording high resolution reciprocal space maps. We analyse the thin film residual stress based not only on peak positions, but taking into account the defect-induced peak shape as well. Consideration of the peak shape enables the determination of the stress depth profile in the case of graded films and to imporves the accuracy in the case of homogeneous films.

Abstract: We have elucidated the elastic interaction between the 90° domains and misfit dislocations in PbTiO₃/SrTiO₃ (001) epitaxial thin films by the local strain mapping based on high-resolution transmission electron microscopy. The novel mechanism of the nucleation and growth of 90 ̊ domain has been proposed based on the results, including the introduction of the additional dislocation perpendicular to the misfit dislocation and the dissociation of these dislocations into two pairs of partial dislocations with an anti-phase boundary.

Abstract: The performances of heterostructural devices are often limited by misfit dislocation. In this paper, a theoretical approach for misfit dislocation reduction of wurtzite In_xGa_1-xN/GaN is presented. The linear and exponential grading techniques have been modeled for the reduction of dislocation. An energy balance model has been taken into consideration and modified for wurtzite structure to evaluate the misfit dislocation density. The value of misfit dislocation has been reduced from 7.112×10¹⁰ cm^-2 to 6.19×10⁶ cm^-2 and 7.039×10¹⁰ cm^-2 to 6.121×10⁶ cm^-2 at the plane 1/3<> {} and 1/3<>{} respectively for linear grading. In case of exponential grading the dislocation density has been reduced to 2.762×10⁵ cm^-2 for both slip systems. Because of tapered grading coefficient a tapered dislocation profile has been reported in case of exponential grading technique. Finally, a comparative study has been shown among without graded, linear and exponential grading.

Abstract: Thermal annealing experiments were performed to determine the critical conditions of misfit dislocation formation in 4H-SiC epilayers in a temperature range of 1400-1800 °C. Misfit dislocations were observed to form at a given annealing temperature if the temperature gradient across the epi-wafer exceeded a critical value. It was also found that two types of interfacial dislocations could form under different stress conditions. Their formation mechanisms are discussed.

Abstract: This paper presents a theoretical calculation of misfit dislocation and strain relaxation in compositionally step graded In_xGa _1-x N grown on GaN using the total dislocation energy at each interface. The results also compared with uniform layer of In _0.17 Ga _0.83 N and In _0.14 Ga _0.86 N grown differently on GaN. Due to having residual strain and a step increase in indium composition a lower misfit strain in upper layers and hence larger critical thickness at each interface has been reported. These effects significantly reduced the misfit dislocations from 2.6×10⁵ cm^-1 to 9.5×10⁴ cm^-1 in step graded In _0.14 Ga _0.86 N(500nm)/In _0.09 Ga _0.91 N(100nm)/In _0.05 Ga _0.95 N(100nm)/GaN layers instead of a uniform In _0.14 Ga _0.86 N(700nm)/GaN. A small residual strain of 0.0007 after 700 nm graded layer thickness has been reported with 87.04% strain relaxation.

Abstract: We study the initial growth of 10 monolayers (MLs) of GaSb on a (001) GaP substrate. Transmission electron microscopy and reflection high energy electron diffraction analysis show that an Sb-rich GaP surface promotes the formation of a 90° misfit dislocation array at the epi-substrate interface. Using atomic force microscopy, we investigate the influence of the growth temperature and the growth rate on the formation and the shape of GaSb islands.

Abstract: This study focuses on the entire shape of the  twin-precipitate in two phase stainless steel, Fe-25Cr-6Ni in terms of the interphase and elastic strain energies generated between the precipitate and matrix. An investigation of this precipitation is important not only in terms of microstructure control but also for improving the mechanical properties of materials. Firstly, the three-dimensional near-coincidence site lattice (3D-NCS) model, which is based on the atomic matching model, is applied for estimating the preferred habit planes by evaluating the result of geometrical atomic matching. Subsequently, the precipitate shape is determined from candidates of the preferred habit plane produced by the 3D-NCS model, and the elastic strain surrounding and within the precipitate is investigated by FEM analysis, which can calculate the anisotropic elastic strain depending on the shape of the precipitate. The results are compared with observed precipitate by TEM.

Abstract: An orientation relationship between the cubic 3C and nH hexagonal structures was developed to identify low surface energy, close packed, conducive to growth hexagonal semi-polar planes. This was done to identify planes on which micropipe free SiC crystals, and/or hetero-epitaxial AlGaN structures with a reduced piezoelectric field and a smaller lattice mismatch could be grown. The nH (33 0 2n)H and (33 0 n )H hexagonal planes, which correspond to the (100)C and (111)C cubic planes are identified, and equations are developed to determine their planar structure and packing density to assess whether there is a reasonable chance that high quality material can be grown on these surfaces and whether it is likely that stacking faults will be formed. It is shown that stacking faults will likely form in SiC crystals; it is also shown that good epitaxy of AlGaN films should be possible on these planes although it is likely stacking faults will be formed.

Abstract: 4H-SiC substrate wafers with epi-layers were observed using monochromatic synchrotron X-ray topography in grazing incidence geometries, to investigate the defects in the epi-layer. Misfit dislocations with b=+1/3[11 2 0] caused by the difference in lattice parameter between the epi-layer and the substrate were observed. The misfit dislocations are located near the interface as edge dislocations, and appear at the top surface as screw dislocations on basal planes. It was observed that more than half of them were introduced from the growing epi-layer surface. The misfit dislocations and some screw dislocations with b=+1/3[11 2 0] are observed to remain as basal plane dislocations at the surface, while other basal plane dislocations were converted to threading edge dislocations in the epi-layer.