Elimination of wing tilt laterally overgrown GaN

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Recent observations on the motion of small angle dislocation boundaries

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Anomalous growth of dislocation density in titanium during recovery

In this paper, we report on observations of the operation of secondary slip systems to relieve lattice mismatch stress in semipolar InGaN/GaN heterostructures. Two-dimensional arrays of misfit dislocations were observed. Consistent with previous reports, primary relaxation occurred along the projected c direction via primary slip on the (0001) basal plane. In addition, evidence for secondary relaxation was detected in cathodoluminescence spectroscopy, high resolution x-ray diffraction, and transmission electron microscopy (TEM) studies. The secondary misfit dislocations were determined by TEM to have a-type Burgers vectors a/3 〈2¯110〉 and line directions along 〈42¯2¯3¯〉, consistent with prismatic slip on one of the m-type planes inclined with respect to the (112¯2) growth surface. Evidence of an additional slip system with approximate misfit line direction of type 〈202¯3¯〉 is also given.

Structural characterization of gallium arsenide epitaxial layers grown on Si(001)

It is well known that the large lattice mismatch (>14%) associated with CdTe/Si, CdTe/Ge, and CdTe/GaAs composite substrates, is a great contributor to large dislocation densities and other defects that limit the performance of HgCdTe-based infrared detectors. Though thermal expansion mismatch is another possible contributor to material defects, little work has been done towards documenting and understanding its effects in these systems. Here, we perform studies to determine the relative contributions of lattice and thermal mismatch to CdTe film characteristics, including dislocation density and residual stress. Unannealed and thermally cycled films are characterized using x-ray diffraction, defect decoration, and Nomarski and transmission electron microscopy. For CdTe/Si, the residual stress is consistently observed to be tensile, while for CdTe/Ge and CdTe/GaAs, a compressive residual film stress is measured. We show based on theoretically predicted stress levels that the experimental measurements imply the dominance of thermal mismatch in the residual stress characteristics.

GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

An epitaxial laterally overgrown (ELOG) GaN layer was deposited on a (0001) sapphire substrate. Here we introduce a maskless and single-step epitaxial lateral overgrowth (ELO) process using high-dose -ion-implantation. We employed high-dose -ion-implantation as an ELO mask instead of usual dielectric material such as or . The GaN layer was laterally grown over the ion implanted array formed in a stripe pattern of width, resulting in a complete coalescence after approximately of growth in the [0001] direction. Transmission electron microscopy and photoluminescence analysis confirmed the reduced dislocation density of the ELO grown GaN layer.

We have investigated the reduction of threading dislocations and the suppression of crack formation in InxGa1-xP (x∼ 0.5) grown on Si using the multi-strained short-period superlattices (SSPSs) by transmission electron microscopy (TEM). The epitaxial layers were grown two-dimensionally and the density of threading dislocations was reduced by the generation of misfit dislocations at all hetero-interfaces. The residual strain was reduced by compensation of the lattice-mismatched stress and the thermal stress, and the formation of cracks was suppressed by controlling the In composition.

High-quality GaAs epitaxial layers have been grown by molecular beam epitaxy on nominally (100) oriented silicon substrates that were previously annealed in a hydrogen ambient at 1250°C. The growth procedure involves an in situ thermal annealing step after the initial deposition of a thin GaAs buffer layer. Rutherford backscattering and channeling of 2.1 MeV He+ ions, interference optical microscopy, transmission electron microscopy, and X-ray diffraction techniques have been used to characterize these layers. Comparative studies indicate that the epitaxial layers grown on hydrogen-ambient annealed substrates have a superior surface morphology and a lower interface disorder than those on the chemically cleaned nominal silicon (100) substrates. Furthermore, a significant reduction in the density of microtwins is observed in layers grown on the preannealed substrates. This improvement in crystalline quality may be attributed to a lower degree of disorder at the silicon surface that was achieved by high temperature hydrogen-ambient annealing. However, dislocation densities were comparable and the presence of antiphase domain boundaries were observed in both cases.

This work presents results of the investigation of structural and morphological features of epitaxial InSb layers in the AlAs matrix. Our research group used transmission electron microscopy (TEM). The specimens were grown by molecular beam epitaxy and prepared in the cross section (110) and plan view foils (100). We found a formation of the embedded epitaxial layer of solid solution InxAl1-xSbyAs1-y in the AlAs matrix during precipitation of In and Sb on the AlAs surface. The embedded layer had continuous area (wetting layer) and islands. The study revealed two types of islands in the epitaxial layer the first having coherent interfacing with the matrix lattice and the second a relaxed island. We estimated concentration of In, Sb in the solid solution by the indirect method. We used the method of geometric phase to analyze the distribution of misfit dislocation cores on the interface. Every misfit dislocation was formed by two close 600-dislocations with the Burgers vectors like a /2 <110>. The sum Burgers vector of the dislocation pair was in the plane of the interface.

Epitaxial layers of (111) GaAs of approximately 1 μm thickness were grown on epitaxial CaF2 buffer layers that were either 140 or 380 nm thick on Si(111) substrates. The best nucleation temperature for the GaAs on CaF2/Si(111) we have observed was 620 °C. This resulted in high quality GaAs films, which exhibited channeling minimum yields of 4%. The density of threading dislocations in the GaAs layers was observed by transmission electron microscopy (TEM) to be ∼108 cm−2. Double-crystal x-ray diffraction measurements showed that the strain (ε⊥) was less than 2.2×10−4 in both sets of GaAs samples. However, ion channeling revealed a large tetragonal strain of 3.5×10−3 (ε⊥=1.7×10−3) in the thinner (140 nm) CaF2 buffer layers. By doing ion channeling with high energy (2.5 MeV) protons, it was possible to determine strain more accurately. Using this technique, we were able to set an upper limit for the tetragonal strain of 2.5×10−4 in both the GaAs (which implies ε⊥&amp;lt;8×10−5) and CaF2 (ε⊥&amp;lt;1.5×10−4) layers for the thicker (380 nm) CaF2 buffer layer structure. These results are in good agreement with the strain predicted from previous strain measurements of CaF2 epitaxial layers on Si.

Characterization of epitaxial layers grown on 4H-SiC (0 0 0 −1) substrates

Heterogeneous material integration, especially for III-V semiconductors, has enabled more possibilities in designing optoelectronic and electronic devices, and the ability to exfoliate the bonded devices layers and reuse the expensive substrates makes it economically advantageous. In our case, we propose a method using easily fractured porous layer to transfer an InP thin film from an InP substrate to an alternate substrate; e.g., glass or a flexible structure such as PDMS. Here, we demonstrate the formation of such a porous InP layer which exhibits a surface that is nearly fully dense as well as the ability to grow InP overlayers with low (< 106 cm-2) threading dislocation density. Also, we were able to transfer large area (> 1 cm2) InP layers onto PDMS substrates and glass slides.In our studies on (001) InP, we have demonstrated that the pore formation can be controlled as a function of depth during electrochemical etching. For as-etched porous layers, AFM scans on a 40 X 40 μm2 area show the lowest surface r.m.s. less than 40 Å despite of the pore features. In order to achieve both high-quality epitaxial growth and facile layer fracture, a dual porous layer structure with a more porous (mechanically weaker) under-layer was designed and fabricated by either adjusting the etching current density or changing the switching electrolyte concentration. The structures were annealed to understand the morphological evolution of the pore structure and to test the structural stability under simulated epitaxial growth condition. Under N2-purged environments, we observed that the surface remains stable (no change in surface pore morphology) with even slight improvement on roughness up to 550 °C and the layer maintained its porous structure at 650 °C. Large voids were observed after annealing at the interface between the high porosity buried layer and substrate, which is consistent with the ease by which it is to remove top layers from the substrate after annealing. Epitaxial layers were grown about 2 μm thick on porous surfaces with different porosities and both a cross-section TEM sample and a plan-view sample (> 35 μm2) were used to characterize the epitaxial growth. Since no observable threading dislocations were found in the plan-view sample, it sets a limit on the threading dislocation density in the mid-106 cm-2 range.Samples with dual porous layers were bonded to flexible PDMS substrates and easily peeled off due to the fracture at the high porosity layer. Another set of samples were bonded to glass slides with epoxy glue and the fractured surface was characterized under SEM.