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This paper reviews materials challenges and recent progress for selective area regrowth and doping for vertical gallium nitride (GaN) power devices. The purpose is to realize randomly placed, reliable, contactable, and generally useable laterally patterned p-n junctions, which are the building blocks for various advanced power rectifiers and transistors. The general regrowth process and regrowth dynamics in trenches were discussed, where the effects of trench geometries, growth methods, and bulk substrates were elucidated. Comprehensive materials characterization techniques were utilized to analyze the regrown structures, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, scanning probe microscopy, and secondary-ion mass spectrometry. Cathodoluminescence and secondary electrons in scanning electron microscopy and atom probe tomography were used to achieve lateral and vertical dopant profiling at a sub-micron scale. The regrowth interface after dry etching accumulated a high density of impurities and charges, contributing to the formation of a p+-n+ tunneling junction. This hypothesis was further confirmed by the electrostatic potential profile at the regrowth interface using electron holography. Novel etching technologies were investigated to improve the regrowth interface. It was found that low-power dry etching significantly reduced the interfacial charges and the reverse leakage currents of regrown p-n junctions. Photoelectrochemical wet etching was found to be effective in reducing deep-level defects near the regrowth interface. Atomic layer etching uses self-limiting chemical processes, thus removing the damaged layers without inducing further etching damage. Tertiarybutylchloride-based in situ etching may serve as an alternative etching method to dry etching with reduced etching damage. In terms of devices, regrown p-n junctions with low leakage currents and vertical junction field-effect transistors were demonstrated. Further improvements in selective area regrowth and associated devices can be expected using regrowth optimization and regrowth interface engineering via surface treatments and low-damage etching. These results represent an important step towards realizing selective area regrowth and doping for high performance GaN power electronics devices and systems.

We report enhancement-mode ${p}$ -channel heterojunction field-effect transistors (HFETs) without gate recess on a standard ${p}$ -GaN/AlGaN/GaN high electron mobility transistor (HEMT) platform. The ${p}$ -GaN in the gate region was partially passivated by a low-power hydrogen plasma treatment process, and the remaining active ${p}$ -GaN and the underlying AlGaN formed the ${p}$ -channel. The device showed a record low off-state leakage of $ A/mm and subthreshold swing ( SS ) of 123.0 mV/dec with a threshold voltage ( ${V} _{\text {th}}$ ) of −0.6 V and high-temperature stability up to 200 °C. These results indicate that the hydrogen plasma treatment is beneficial for suppressing leakages and preserving excellent material quality in the ${p}$ -channel. With the success of the ${p}$ -HFETs, the ${p}$ -GaN/AlGaN/GaN platform can enable the monolithic integration of GaN ${n}$ - and ${p}$ -channel transistors without the need for regrowth. This work represents a significant step towards the implementation of the GaN CMOS technology.

In situ observation of a phase transformation during annealing of Si containing diamond-cubic (dc) and metastable phases has been performed using synchrotron X-ray diffraction. Metastable body-cent...

This work has investigated the effects of etching treatment on as-grown and regrown GaN-on-GaN structures intended for vertical p-n power device applications. Surfaces of thick unintentionally doped (UID) GaN layers, grown homoepitaxially via metal-organic chemical vapor deposition on freestanding GaN bulk substrates, were subjected to RF-plasma dry etching, and additional layers of UID-GaN/p-GaN were then grown. The as-grown and regrown structures were examined in cross section using transmission electron microscopy. Two different types of etching treatment were used, fast-etch-only and multiple etches with decreasing power. Images of the fast-etch-only devices clearly showed the GaN-on-GaN interface at the etched location, and low device breakdown voltages were measured (reverse bias in the range of 45–95V). In comparison, no interfaces were visible in devices after multiple etching steps, and the corresponding device breakdown voltages were markedly higher (reverse bias ~1200–1270V). These results emphasize the importance of optimizing surface etching techniques for avoiding degraded performance in GaN-on-GaN power devices that require GaN regrowth.

Vertical GaN-on-GaN Schottky barrier diodes based on as-grown and regrown samples were fabricated to investigate the effects of the etch-then-regrow process on device performance. The surface roughness increased slightly after dry etching and decreased after regrowth. According to X-ray diffraction results, the etch-then-regrow process caused a slight increase of defect density due to increased edge dislocations. Schottky parameters extracted from forward current-voltage curves, such as turn-on voltages of 0.74 V and 0.72 V, ideality factors of 1.07 and 1.10, and barrier heights of 1.07 eV and 1.05 eV, were obtained for diodes based on the regrown and as-grown samples, respectively. The breakdown voltage of the regrown sample was much lower than the as-grown sample. The regrowth interface can be regarded as a n-doping GaN layer due to the high interface charge density after the etch-then-regrown process. This equivalent ${n}$ -doping GaN layer reduced the effective thickness of the UID-GaN under the Schottky contact thus causing lower breakdown voltage for the regrown sample. Poole-Frenkel emission and trap-assisted tunneling processes were responsible for the leakage of both as-grown and regrown samples according to the temperature dependence of the reverse currents.

The high radiation tolerance of GaAs0.86Sb0.14 based solar cells with a band gap suitable for PV is demonstrated at the low intensity low temperature (LILT) conditions. This system shows remarkable radiation hardness at AM0, and more prominently, at the conditions of several outer planetary targets. This is attributed to an irradiation induced change in the absorber band gap due to local heating and strain relaxation, and the generation of less prohibitive shallow Sb-based defects in the GaAs 1-x Sb x absorber.

The fabrication of epitaxial, ultra-thin SrTiO3 (STO) on thick SiO2 without the need for complicated wafer-bonding processes has been demonstrated. The resulting transition metal oxide (TMO)-on-glass layer stack is analogous to traditional silicon-on-insulator (SOI) wafers, where the crystalline device silicon layer of SOI has been replaced by a crystalline functional TMO layer. Fabrication starts with ultra-thin body SOI on which crystalline STO is grown epitaxially by molecular beam epitaxy. The device silicon layer is subsequently fully oxidized by ex situ high-temperature dry O2 annealing, as confirmed by X-ray photoelectron spectroscopy, X-ray reflectivity, and high-resolution electron microscopy. STO maintains its epitaxial registry to the carrier silicon substrate after annealing and no evidence for degradation of the STO crystalline quality as a result of the TMO-on-glass fabrication process is observed. The ease of fabricating the TMO-on-glass platform without the need for wafer bonding will enab...

Bismuth selenide layers and nanowires have been grown by molecular beam epitaxy on self-assembled Stranski–Krastanov InAs quantum dots of different sizes and densities on GaAs substrates. The size ...

Ga2O3 is a candidate for power electronics due to its large bandgap, controllable doping and availability of large inexpensive substrates. These include power conditioning systems, pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors and advanced power management and control electronics. There are already cases where the performance exceeds the theoretical values for SiC. Existing Si, SiC (vertical devices), and GaN (lateral devices) enjoy advantages in terms of process maturity, especially for Si, where devices such as super-junctions surpass the unipolar "limit". Continued development of low defect substrates, optimized epi growth and improved device design and processing methods for Ga2O3 are required to push the experimental results closer to their theoretical values. The actual experimental value of VB is well below the theoretical predictions. Thermal management is a key issue in Ga2O3 power devices and initial studies have appeared on both the experimental and theoretical fronts.

β-Ga2O3 Schottky rectifiers consisting of thick (10 μm) epitaxial drift regions on conducting substrates are shown to have a high tolerance to 60Co gamma ray irradiation. This is due to the low carrier removal rate of

The creation of structural defects in low-mismatched GaAs/GaAs0.92Sb0.08/GaAs(001) heterostructures and their evolution during strain relaxation have been studied using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy. These GaAsSb films had thicknesses in the range of 50–4000 nm with 50-nm-thick capping layers and were grown using molecular beam epitaxy. The strain relaxation had three distinct phases as the film thickness was increased, whereas the thin GaAs capping layers exhibited only the initial sluggish stage of relaxation in heterostructures with thick GaAsSb films. The character of the misfit dislocations at the two interfaces was determined using g.b analysis, and atomic-scale structural information was obtained using aberration-corrected electron microscopy. Stage-I relaxation took place primarily by glide of dissociated 60° dislocations. Although the films were mostly free of threading dislocations, many curved dislocations extended into the substrate side for heterostructures that had undergone Stage-II and Stage-III relaxation. Investigation of dislocation density evolution at the cap/film interface and morphological evolution of the growth surface revealed a strong correlation. The smoother growth surface in the heterostructure with 4000-nm-thick film resulted in a reduced areal density of surface troughs that acted as nucleation sites for dislocations, which explained the decreased dislocation density at the cap/film interface. Overall, these results prove that heterogeneously nucleated surface half-loops are the primary source of threading dislocations in low-mismatched heterostructures.

This work investigates defect formation and evolution associated with the deposition of GaP layers on precisely oriented Si(0 0 1) substrates. The GaP layers were grown with thicknesses ranging from ∼37 nm to ∼2 µm at a growth rate of 0.52 μm/hr using molecular beam epitaxy (MBE). The crystallinity of thin (37-nm) MBE-grown GaP layers was also compared with thin GaP layers grown by migration-enhanced epitaxy (MEE). The MBE growth procedure was shown to postpone relaxation of the epitaxial GaP layers up to a thickness of ∼250 nm. Detailed analysis of high-resolution X-ray diffraction patterns and comparison with cross-sectional transmission electron micrographs clarified the defect formation mechanism. Thin GaP layers showed very low defect densities except for anti-phase boundaries, whereas substantial threading defects predominated in the thicker, noticeably relaxed structures.

Si (100) wafers with various doping levels were subjected to high-pressure torsion (HPT). The resistivities for all doping levels increased by one or two orders of magnitude after initial compression, but then decreased after 10 revolutions of HPT processing to ~0.1 Ω cm for normally and heavily doped samples, and to ~0.02 Ω cm for the ultraheavily doped sample. After annealing at 873 K, the resistivities increased by four orders of magnitude compared to the original Si wafers. These results indicate that the formation of metastable phases plays an important role in the electrical resistivities of HPT-processed samples.

The reaction of oxygen anions with carbon dioxide molecules is considered to be the main channel of CO3− ion formation in the Martian ionosphere. Comprehensive theoretical ion-chemical models of the Martian ionosphere require experimental data on the reactivity of negative ions with neutral gases under relevant ambient temperature and pressure conditions. The temperature dependence of the rate of the ternary reaction of O− with CO2 has been measured at temperatures relevant to the ionosphere of Mars using a VT-SIFT instrument using both helium and nitrogen as the carrier (ternary) gases. The ternary rate coefficient of the reaction was measured equal to (1.24 ± 0.06) × 10−28 (300/T)2.86±0.28 and (2.29 ± 0.05) × 10−28 (300/T)2.06±0.13 cm6s−1 for He and N2 carrier gas, respectively, in the temperature range of 230–340 K. The k3 is over two times higher in nitrogen than in helium and has a less strong inverse temperature dependence.

Unoccupied Aerial Systems (UAS) are ideal tools for responding to nuclear incidents where large outdoor areas have become contaminated with a radiological hazard. They are advantageous because rapid response radiation surveys can be conducted while the human operator remains at a safe distance and avoids direct contamination of the platform. During fieldwork within the Chernobyl Exclusion Zone (Ukraine), an airborne platform was equipped with a GNSS enabled gamma spectrometer and used to survey an area surrounding a known highly contaminated building (a ‘hot-spot’), resulting in a radiation intensity map. The detected radiation pattern, however, was ‘blurred’ since the intensity recorded at any point counted nadir emissions, but also emissions from all sources within line-of-sight; The ‘hot-spot’ had an influence far outside its ground footprint. Methods exist to correct for errors introduced by varying terrain altitude, however, they do not remove the unwanted blurring. Hence, small point sources appear as broad regions of contamination which is entirely an artefact of the measurement process. The effect is further accentuated with increasing height above ground hence understanding and correcting for this phenomenon is particularly relevant to data collected using UAS. Here, we present a novel algorithm to refine the detected pattern to more accurately recover the ground-truth.A forward model of the system is created which describes the relationship between the unknown ground-truth and the aerial measurements. Gamma ray emissions from a point source obey the inverse square law of spatial dilution and have an exponential attenuation in air. To model both effects, geometric information of the scene is required and is provided by the geotagged spectrometer data and photogrammetrically processed DEMs of the surveyed terrain. The resulting model is hyper-cube of linear equations, where every aerial measurement point is assumed to be influenced by every ground sample point. By finding the inverse solution of this system, the ground-truth radiation pattern is estimated in more detail. The Kaczmarz method is advantageous because a large system of equations can be broken down into smaller sub-routines and solved iteratively. A caveat is that the solution might settle to false positive. The refinement algorithm will be presented with simulated results, controlled laboratory experiments using robotic arms and sealed radioactive sources, and finally applied to a real-world data set collected in the Chernobyl Exclusion Zone.

The process of fusion arc welding of steel pipes in power generation plants induces residual stresses which may be detrimental to the integrity and endurance of plant pipelines. P91 is high-grade steel used in the construction of pipelines carrying hot steam at high pressure, conditions which cause creep during service. Welded P91 pipes are usually subjected to post-weld heat treatment (PWHT) to mitigate the magnitude of residual stresses and temper the material, hence improving its resistance to creep. In this paper, the finite element (FE) method of modelling residual stresses due to PWHT in a circumferentially butt-welded P91 pipe is presented. The PWHT hold temperature is 760°C. The paper describes the X-Ray Diffraction (XRD) and Deep-Hole Drilling (DHD) experimental techniques and how they are applied to measure residual stresses in the welded P91 pipe after PWHT. The material property data, necessary for the FE simulation of PWHT, has been obtained from stress-relaxation tests on P91 uniaxial tensile specimens at 760°C. Good agreements have been achieved between the results of the FE method and the two sets of experimentally-measured residual stresses.

Due to their extraordinary optical and electronic character at low-dimensionality, two-dimensional (2D) layered selenides and tellurides are an actively-pursued area both in fundamental research and in applications. Recently non-equilibrium growth of 2D layered materials using molecular beam epitaxy (MBE) has attracted widespread attention in the scientific community for realizing highly scaled technologies involving both novel device structures and new physics. Here we summarized our recent work on MBE growth of 2D layered selenides and tellurides, as well as their heterostructures, along with their characterization. This chapter also points out a number of challenges lying ahead in this field, and provides our viewpoint on what the future holds for MBE-grown 2D layered selenides and tellurides.

In this work, transmission electron microscopy has been used to investigate HgCdSe/ZnTe/Si (211) heterostructures grown by molecular beam epitaxy and to study the effects of chemical etchants for measurements of defect density in the HgCdSe epilayers. Both ZnTe/Si and HgCdSe/ZnTe interfaces were decorated with {111}-type stacking faults inclined at angles of ∼ 19° or ∼ 90° with respect to the interface plane. Similar stacking faults were also present in the upper regions of the HgCdSe films. High-resolution imaging and Fourier image analysis revealed dislocations, mostly with $$ \frac{a}{3}\left\langle\bar{1}11\right\rangle $$ Burgers vector, at both ZnTe/Si and HgCdSe/ZnTe interfaces. Etching solutions based on different combinations of nitric acid, hydrochloric acid and lactic acid were tried in attempts to identify an etchant that provided one-to-one correspondence between etch pits and defects in the HgCdSe layer. Focused-ion-beam milling and transmission electron microscopy were used to prepare site-specific cross-section samples from across the etch pits. However, many defects in regions surrounding the etch pits were unaffected by the various different etchants.

Dissociated 60° dislocations in low-misfit GaAs/GaAs0.92Sb0.08/GaAs(001) heterostructures are investigated using aberration-corrected scanning transmission electron microscopy. Intrinsic stacking faults bounded by 30° and 90° Shockley partial dislocations are created at the compressively-strained film/substrate interface and the tensile-strained cap/film interface. Unpaired atomic columns are clearly observed at the cores of the 30° partial dislocations for most stacking faults. Comparison with atomic structural models establishes that these dissociated 60° dislocations belong primarily to glide set. For both interface types, the leading partial is located in the constraining layer whereas the trailing partial is located in the layer undergoing strain relaxation.

The surface reconstructions and surface core level shifts of 0.25 monolayer (ML), 0.5 ML and 1.0 ML of europium (Eu) on the Ge(001) surface are investigated experimentally and theoretically, using molecular beam epitaxy (MBE), reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Starting with a clean Ge(001) surface we find that the surface reconstruction changes from the initial (2 × 1) to a (3 × 2), (8 × 1) and a final (3 × 1) pattern with increasing Eu coverage of 0.25 ML, 0.5 ML and 1.0 ML, respectively. XPS scans demonstrate that the Ge 3d core level shifts to higher binding energies with increasing Eu coverage. We find that Eu is in the 2+ oxidation state, transferring its charge to the Ge surface, resulting in the disappearance of the asymmetry of the dimer tilt at a coverage of 0.5 ML of Eu. From density functional calculations we find that the Ge dimers flatten out at 0.5 ML Eu coverage and partially break apart at 1.0 ML Eu.

Residual stress in the welds that attach Control Rod Drive Mechanism nozzles into the upper head of a PWR reactor vessel can influence the vessel’s structural integrity and initiate Primary Water Stress Corrosion Cracking. PWSCC at Alloy 600 CRDM nozzles has caused primary coolant leakage in operating PWRs. We have used Deep Hole Drilling to characterise residual stresses in a PWR vessel head. Measurements of the internal cladding and nozzle attachment weld showed that although modest tensile stresses occur in the cladding, the attachment weld contains tensile residual stresses of yield magnitude. Despite the large dispersion of residual stress data for nozzle attachments of this type, all available data suggest that assuming a residual stress profile bounded by the weld material’s yield stress would be conservative for assessment purposes.

High radiation tolerance of GaAs1-xSbx based solar cells is demonstrated for the low-intensity-low-temperature (LILT) conditions of the target planets Saturn, Jupiter, and Mars. The GaAs1-xSbx-based cells are irradiated with high energy electrons to assess the effect of harsh radiation environment on the solar cell and the response of the cell is then investigated in terms of its photovoltaic operation. This system shows significant radiation resistance to the high energy electron environment for the conditions of the planets of interest. An unusual increase of the short circuit current after irradiation is observed at low temperature, which is supported by a simultaneous increase in the external quantum efficiency of the cell under the same conditions. The open circuit voltage and fill factor of the cell are especially tolerant to irradiation, which is also reflected in unchanged dark current-voltage characteristics of the solar cell upon irradiation particularly at LILT.

Thin films of PbSe have been synthesized by molecular beam epitaxy on both GaAs(1 0 0) and GaAs(1 1 1)B substrates. Despite the smaller lattice constant of GaAs, the PbSe layers are (0 0 1)-oriented and undergo in-plane tensile strain on both substrates due to the thermal expansion coefficient mismatch between the materials. High resolution transmission electron microscopy observations reveal an abrupt and highly crystalline interface, not impacted by the tensile strain. The impact of strain on the electronic band structure is computed and found to induce an increase in the energy gap by as much as 10%.

We report on the development of deep-ultraviolet (DUV) wavelength-selective top-illuminated photodetectors based on AlGaN/AlN quantum-dots-in-wells. Structures consisting of 100 AlGaN wells and AlN barriers were grown by plasma-assisted molecular beam epitaxy on sapphire substrates. Interdigitated metal–semiconductor–metal photodetector devices were formed lithographically using indium as the contact metal. The effect of variation of the group III to group V flux ratio and the use of indium as a surfactant on the UV photoresponse were determined. Growth under near-stoichiometric conditions lead to a photocurrent peak in the 210–215 nm range with a peak width of ∼20 nm, with no other additional signatures in the entire UV–visible range. Under excess group III conditions, a second red-shifted peak was observed at ∼225 nm with significantly (up to 10×) higher responsivity. This enhancement was linked to the formation of quantum dots with truncated pyramidal structures with near-uniform size distribution and density of 6 × 1010 cm−2 within the quantum wells. Their formation was attributed to the process of droplet epitaxy. Such photodetectors do not require p-type doping or growth onto UV-transparent substrates and are appropriate for monitoring DUV skin-safe germicidal radiation in the presence of ambient visible light.

This work has investigated the microstructure and magnetic properties of supersaturated melt-spun Cu–10%Co alloys by means of magnetization, magnetotransport studies, electron microscopy and electron holography. Our results demonstrate that the microstructure of the annealed supersaturated alloys is constituted by two different types of Co-rich phases: spherical Co-rich precipitates as a result of spinodal decomposition, and Co rod-like precipitates, as a result of discontinuous grain-boundary precipitation. The aspect ratio of such precipitates is quite high, exhibiting magnetic anisotropy along their long axes, which generates global anisotropy of the alloy sample. We verified that the highest giant magnetoresistance occurs concurrently with an optimum discontinuous precipitate development. Electron holography reveals an antiferromagnetic coupling of the Co rods in the remanent magnetization state. The magnetization and magnetoresistance curves as a function of applied magnetic field are well described by an independent-moment approach considering the sum of the average magnetic moments of the spherical and the rod-like Co precipitates.

Barium titanate BaTiO 3 (BTO) thin films were epitaxially grown at 225 °C on 2 × 1-reconstructed Ge(001) surfaces via atomic layer deposition (ALD). Approximately 2 nm of BTO film was grown directly on Ge(001) as an amorphous film. Electron diffraction confirmed the epitaxy of the BTO films after post-deposition annealing at 650 °C. Additional BTO layers grown on the crystalline BTO/Ge(001) film were crystalline as-deposited. X-ray diffraction indicated that the epitaxial BTO films had a c -axis out-of-plane orientation, and the abrupt BTO/Ge interface was preserved with no sign of any interfacial germanium oxide. Scanning transmission electron microscopy provided evidence of Ba atoms occupying the troughs of the dimer rows of the 2 × 1-reconstructed Ge(001) surface, as well as preservation of the 2 × 1-reconstructed Ge(001) surface. This study presents a low-temperature process to fabricate BTO/Ge heterostructures.

The nanostructure of bulk nanograined germanium (Ge) processed by high-pressure torsion (HPT) has been analyzed by high-resolution electron microscopy. Crystalline Ge disks were subjected to HPT under the nominal pressure of 24 GPa. The samples processed at room temperature consisted of diamond-cubic Ge-I and simple-tetragonal Ge-III nanograins in addition to amorphous regions. The samples contained lattice defects such as dislocations, nanotwins, and stacking faults. Subsequent annealing at 573 K led to the phase transformation from Ge-III to Ge-I, but residual Ge-III nanograins and lattice defects remained due to the low annealing temperature. It was found that Ge-I as well as residual Ge-III nanograins and some amorphous phase were present after processing by HPT at cryogenic temperature. No other metastable phases such as body-centered-cubic Ge-IV or hexagonal-diamond Ge-V were observed in the cryogenic HPT-processed sample.

We report on high-resolution transmission electron microscopy observations of bulk nanograined silicon processed by severe plastic deformation through high-pressure torsion (HPT). Single crystalline Si(100) was subjected to HPT processing under a nominal pressure of 24 GPa at room temperature. The HPT-processed samples contained lattice defects such as dislocations and nanotwins in diamond-cubic Si-I, and metastable phases such as body-centered-cubic Si-III and hexagonal-diamond Si-IV. The grain size ranged from several nanometers up to several tens of nanometers. Subsequent annealing at 873 K led to the phase transformation to Si-I. No appreciable grain coarsening occurred after annealing while dislocations and nanotwins remained in the Si-I nanograins. The Si-I nanograin structure was retained even after annealing for 12 h.

A circular disc containing a partial ring weld was devised to create high levels of residual stress in a relatively small specimen. The aim of the study was to utilise the complex residual stress generated within the weld and to extend the recently developed novel application of the deep-hole drilling technique in measuring residual stresses well over yield stress. This paper presents (1) finite element analysis (FEA) simulation of the residual stresses due to partial welding in an austenitic stainless steel circular disc, (2) measurement of residual stress using non-destructive and semi-destructive techniques and (3) simulation of the semi-destructive residual stress measurement technique. Comparison is made between the FEA predicted residual stress in the weld, the measurements and the reconstructed residual stresses of the measurements. The purpose of the residual stress measurement was to validate the FEA predicted weld residual stress in the circular disc. The FEA simulation of the measurement method was used (1) to explain any discrepancy between the measured and FEA predicted stresses and (2) to further modify and extend the present deep-hole drilling technique and improve its accuracy. The close correlations confirmed the suitability of new modifications made in the deep-hole drilling technique to account for plasticity when measuring near yield residual stresses present in a component.

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

While the heteroepitaxial growth of gallium nitride-based materials and devices on substrates such as SiC, sapphire, and Si has been well-documented, the lack of a cost-effective source of bulk GaN crystals has hindered similar progress on homoepitaxy. Nevertheless, freestanding GaN wafers are becoming more widely available, and there is great interest in growing GaN films and devices on bulk GaN substrates, in order to take advantage of the greatly reduced density of threading dislocations, particularly for vertical devices. However, homoepitaxial GaN growth is far from a trivial task due to the reactivity and different chemical sensitivities of N-polar (000 1 ) and Ga-polar (0001) GaN surfaces, which can affect the microstructure and concentrations of impurities in homoepitaxial GaN layers. In order to achieve high quality, high purity homoepitaxial GaN, it is necessary to investigate the effect of the ex situ wet chemical clean, the use of in situ cleaning procedures, the sensitivity of the GaN surface to thermal decomposition, and the effect of growth temperature. We review the current understanding of these issues with a focus on homoepitaxial growth of GaN by molecular beam epitaxy (MBE) on c -plane surfaces of freestanding GaN substrates grown by hydride vapor phase epitaxy (HVPE), as HVPE-grown substrates are most widely available. We demonstrate methods for obtaining homoepitaxial GaN layers by plasma-assisted MBE in which no additional threading dislocations are generated from the regrowth interface and impurity concentrations are greatly reduced.

One of the many challenges in the behaviour of structures is to understand if the presence of residual stress plays an important role in contributing to failure of a structure operating at high temperature. Assessments of the high-temperature integrity of practical structures are based on experiments carried out using standard laboratory-scale creep test specimens tested under either load- or displacement-controlled conditions. In practice, structures are subjected to combinations of residual and applied stresses which in turn lead to mixed boundary conditions. Conventional laboratory creep tests do not represent these circumstances. The purpose of this paper is to describe the design of a novel test rig to introduce long-range residual stresses at high temperature. The concept of rig is based on three-bar structure with an initial misfit introduced into the central bar to represent a long-range residual stress and could be characterised easily without using time-consuming residual stress measurement techniques. Initial results demonstrated that the magnitude and the interaction of the residual stress with the applied loading is a function of the initial misfit displacements and the relative stiffness of the components of the system. Additionally, the subsequent behaviour of the system, with and without the application of additional loading, is governed by (a) the degree to which the misfit is accommodated by plastic and creep strain and (b) the elastic follow-up provided by the system. The paper describes the design of a test rig and laboratory tests conducted to validate the concept.

Reverse breakdown voltages larger than 1 kV have been reported for both unterminated Ga2O3 vertical rectifiers (1000- 1600 V) and field-plated Schottky diodes (1076-2300 V) with an epi thickness of 8-20 μm. If the doping is in the 1016 cm-3 range, the breakdown is usually in the 500-800V regime. Furthermore, the switching characteristics of discrete Ga2O3 vertical Schottky rectifiers exhibited reverse recovery times in the range of 20 to 30 ns. Large area (up to 0.2 cm2 ) Ga2O3 rectifiers were fabricated on a Si-doped n-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+ Ga2O3 (001) substrate. A forward current of 2.2 A was achieved in single-sweep voltage mode, a record for Ga2O3 rectifiers. The on-state resistance was 0.26 Ω·cm2 for these largest diodes, decreasing to 5.9 × 10-4 Ω·cm2 for 40x40 μm2 devices. We detail the design and fabrication of these devices. In addition, an inductive load test circuit was used to measure the switching performance of field-plated, edge-terminated Schottky rectifiers with a reverse breakdown voltage of 760 V (0.1 cm diameter, 7.85x10-3 cm2 area) and an absolute forward current of 1 A on 8 Μm thick epitaxial β-Ga2O3 drift layers. These devices were switched from 0.225 A to -700 V with trr of 82 ns, and from 1 A to -300 V with trr of 64 ns and no significant temperature dependence up to 125°C. There was no significant temperature dependence of trr up to 150°C.

The development of gallium nitride (GaN) power devices requires a reliable selective-area doping process, which is difficult to achieve because of ongoing challenges associated with the required etch-then-regrow process. The presence of silicon (Si) impurities of unclear physical origin at the GaN regrowth interface has proven to be a major bottleneck. This paper investigates the origin of Si contamination at the epitaxial GaN-on-GaN interface and demonstrates an approach that markedly reduces its impact on device performance. An optimized dry-etching approach combined with UV-ozone and chemical etching is shown to greatly reduce the Si concentration levels at the regrowth interface, and a significant improvement in a reverse leakage current in vertical GaN-based p–n diodes is achieved.

Epitaxial GaP/Si heterostructures grown by molecular beam epitaxy (MBE) and migration-enhanced epitaxy (MEE) have been studied, primarily using aberration-corrected scanning transmission electron microscopy (AC-STEM). Atomically-resolved structure images, which are sensitive to atomic-column intensity revealed the detailed geometry of antiphase boundaries that were both parallel and also inclined to the growth direction. The polar/non-polar GaP/Si interfaces were neither atomically flat laterally nor abrupt vertically. Measurements of intensity profiles using both bright-field and dark-field AC-STEM images, as well as chemically-sensitive g002 dark-field imaging, indicated substantial interfacial intermixing, which increased significantly from ∼1.3 nm (MEE growth at 440 °C) to ∼2.1 nm (MBE growth at 600 °C). The finite interface width will impact theoretical predictions of charge imbalance and strong electric fields across the heterointerface.

Heterovalent structures consisting of group II-VI/group III-V compound semiconductors offer attractive properties, such as a very broad range of bandgaps, large conduction band offsets, high electron and hole mobilities, and quantum-material properties such as electric-field-induced topological insulator states. These properties and characteristics are highly desirable for many electronic and optoelectronic devices as well as potential condensed-matter quantum-physics applications. Here, we provide an overview of our recent studies of the MBE growth and characterization of zincblende II-VI/III-V heterostructures as well as several novel device applications based on different sets of these materials. By combining materials with small lattice mismatch, such as ZnTe/GaSb (Δa/a ∼ 0.13%), CdTe/InSb (Δa/a ∼ 0.05%), and ZnSe/GaAs (Δa/a ∼ 0.26%), epitaxial films of excellent crystallinity were grown once the growth conditions had been optimized. Cross-sectional observations using conventional and atomic-resolution electron microscopy revealed coherent interfaces and close to defect-free heterostructures. Measurements across CdTe/InSb interfaces indicated a limited amount (∼1.5 nm) of chemical intermixing. Results for ZnTe/GaSb distributed Bragg reflectors, CdTe/MgxCd1−xTe double heterostructures, and CdTe/InSb two-color photodetectors are briefly presented, and the growth of a rock salt/zincblende PbTe/CdTe/InSb heterostructure is also described.

β-Ga2O3 was deposited in thin film form by plasma-assisted molecular beam epitaxy at 670 °C and 630 °C onto a γ-Al2O3 (111) buffer layer grown at 840 °C by e-beam evaporation on a clean Si (001) surface. The β-Ga2O3 film was 66 nm thick, stoichiometric, and strongly textured, as determined by x-ray reflectivity, x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, x-ray diffraction, and transmission electron microscopy, with three basal growth planes (2̄01), (101), and {310}, including one twin variant {31̄0}. The observed basal growth planes correspond to the close-packing planes of the distorted face-centered cubic oxygen sublattice of β-Ga2O3. Local structural ordering can be thought to occur due to a continuation of the oxygen sublattice from the γ-alumina buffer layer into the β-gallia film. Each β-Ga2O3 growth plane further gives rise to 12 symmetry-derived rotational in-plane variants, resulting in a total of 48 domain variants. Atomistic models of possible gallia–alumina interfaces are presented.

The morphology of GaN-on-GaN vertical p-i-n diode devices after reverse-bias electrical stressing to breakdown has been investigated. All failed devices had irreversible structural damage, showing large surface craters that were ∼15–35 μm deep with lengthy surface cracks. Cross-sectional electron micrographs of failed devices showed substantial concentrations of threading dislocations around the cracks and near the crater surfaces. Progressive ion-milling across damaged devices revealed high densities of threading dislocations and the presence of voids beneath the surface cracks; these features were not observed in any unstressed devices. These results should serve as a useful reference for future reliability studies of vertical high-power GaN devices.

TiFe 1-x Mn x intermetallics (x = 0, 0.15 and 0.3) were severely deformed by high-pressure torsion (HPT) to enhance their activation and air resistivity for hydrogenation. While the as-cast ingots hardly absorbed hydrogen (TiFe 0.7 Mn 0.3 exhibited slow activation after an incubation period), the HPT-processed samples absorbed hydrogen quickly at room temperature even after air exposure. The improvement of hydrogen storage performance was due to the formation of lattice defects and amorphous regions, which act as channels for hydrogen diffusion. Rietveld analyses and first-principles calculations showed that Mn addition expands the lattice and reduces the hydride formation energy, and thus decreases the hydrogenation/activation pressure.

Tin selenide thin films have been grown by molecular beam epitaxy on GaAs (111)B substrates at a growth temperature of 150 °C, and a microstructural study has been carried out, primarily using the technique of transmission electron microscopy. The Se:Sn flux ratio during growth was systematically varied and found to have a strong impact on the resultant crystal structure and quality. Low flux ratios (Se:Sn=3:1) led to defective films consisting primarily of SnSe, whereas high flux ratios (Se:Sn > 10:1) gave higher quality, single-phase SnSe 2 . The structure of the monoselenide films was found to be consistent with the Space Group Pnma with the epitaxial growth relationship of [011] SnSe // [ 1 1 ¯ 0 ] GaAs , while the diselenide films were consistent with the Space Group P 3 ¯ m1 , and had the epitaxial growth relationship [ 2 1 ¯ 1 ¯ 0 ] SnSe2 // [ 1 1 ¯ 0 ] GaAs .

Quantitative numerical simulations and qualitative evaluations are conducted to elucidate thermo-mechanical behaviour during pouring and solidification of molten glass into a stainless-steel cylindrical container. Residual stress and structural integrity in this casting/vitrification process is important because it can be used for long-term storage of high-level nuclear wastes. The predicted temperature and stress distributions in the glass and container agree well with previous measurements of the temperature histories and residual stresses. Three different thermal-stress models are developed using the finite-element method and compared. Two simple slice models were developed based on the generalized plane strain assumption as well as a detailed two-dimensional axi-symmetric model that adds elements according to the stages of pouring glass into the stainless steel container. The results reveal that mechanical interaction between the glass and the wall of the stainless steel container generates residual tensile stresses that approach the yield strength of the steel. Together, these results reveal important insights into the mechanism of stress generation in the process, the structural integrity of the product, and accuracy of the modelling-tool predictions.

Nitrogen-doped multiwalled carbon nanotubes (CNx-MWNTs) have been decorated with γ-Al 2 O 3 nanoparticles by a novel method. This process involved a wet chemical approach in conjunction with thermal treatment. During the particle anchoring process, individual CNx-MWNT nanotubes agglomerated into bundles, resulting in arrays of aligned CNx-MWNT coated with γ-Al 2 O 3 . Extensive characterization of the resulting γ-Al 2 O 3 /CNx-MWNT bundles was performed using a range of electron microscopy imaging and microanalytical techniques. A possible mechanism explaining the nanobundle alignment is described, and possible applications of these materials for the fabrication of ceramic composites using CNx-MWNTs are briefly discussed.

This work reports the growth of crystalline SrHfxTi1-xO3 (SHTO) films on Ge (001) substrates by atomic layer deposition. Samples were prepared with different Hf content x to explore if strain, from tensile (x = 0) to compressive (x = 1), affected film crystallization temperature and how composition affected properties. Amorphous films grew at 225 °C and crystallized into epitaxial layers at annealing temperatures that varied monotonically with composition from -530 °C (x= 0) to -660 °C (x= 1). Transmission electron microscopy revealed abrupt interfaces. Electrical measurements revealed 0.1 A/cm2 leakage current at 1 MV/cm for x= 0.55.