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• Cu-doped β-Ga 2 O 3 was grown via Czochralski and vertical gradient freeze methods. • Crystals displayed novel photodarkening with UV excitation, stable at room temp. • Photodarkening is reversible back to as-grown state with an anneal at 400 °C. • Dopant segregation was shown via laser ablation inductively coupled mass spec. β-Ga 2 O 3 has demonstrated insulating properties with Mg, Fe, and Zn acceptor doping. Here we investigate Cu doping (0.25 at.%) in bulk Czochralski (CZ) and vertical gradient freeze (VGF) β-Ga 2 O 3 , with significant Cu incorporation, even with the expected Cu evaporation. Representative crystals were assessed for orientation, purity, optical, and electrical properties. The solubility and electronic behavior of Cu dopants are consistent with measured concentrations of 1 × 1018–1 × 1019 atoms/cm3 and electrical measurements that show high resistivities of 109–1010 Ω∙cm. Segregation and precipitation of Cu species in part of the VGF material was determined to be Cu 2 O by analysis with Raman spectroscopy, photoluminescence microscopy, energy-dispersive X-ray spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. With sufficient Cu concentration, β-Ga 2 O 3 crystals excited with deep ultraviolet light photodarken rapidly and exhibit decreased resistivity. This darkened state remains at room temperature for several days before decaying. [ABSTRACT FROM AUTHOR]

In-situ Auger electron spectroscopy was used to determine the 1:1 flux ratios of Ga and N during growth of GaN by molecular beam epitaxy at low substrate temperatures. By linearly ramping the Ga-flux, while keeping the N-flux constant, and simultaneously measuring the chemical composition by monitoring N and Ga Auger peaks, the time of deviation from stoichiometry could be determined. The method was applied at very low substrate temperatures where reflection high-energy electron diffraction does not reveal clear growth mode changes. The importance of the N- vs Ga-rich conditions were confirmed with transmission electron microscopy which showed a distinct change in crystallinity between material at the top and bottom of the film, which are in agreement with previous findings. [ABSTRACT FROM AUTHOR]

We grew GaN films on r -plane sapphire substrates by pulsed sputtering and investigated their structural properties. X-ray diffraction measurements revealed that a-plane GaN grows epitaxially on r-plane sapphire with an epitaxial relation of [0001] GaN // [ 1 ¯ 101 ] sapphire and [ 1 ¯ 101 ] GaN // [ 11 2 ¯ 0 ] sapphire . In situ wafer curvature measurements revealed that the anisotropic compressive strain along [ 1 1 ¯ 00 ] and [0001] GaN is generated during the initial stage of growth as a consequence of the anisotropy in the lattice mismatch between a -plane GaN and r -plane sapphire. We also observed that the coalescence of the GaN islands can lead to a change of stresses from compressive to tensile. [ABSTRACT FROM AUTHOR]

• Threading dislocations in α-Ga 2 O 3 epilayers were visualized as etch pits. • The etch pits were formed by HCl gas etching. • The etch pits were found to be originated from edge dislocations by TEM. • The gas etching can clarify the distribution of dislocations in very wide area. • This technique can also enable quick feedback of the growth results. Threading dislocations in a heteroepitaxial α-Ga 2 O 3 film was visualized as etch pits on the surface. We found that etch pits were formed on a c -plane α-Ga 2 O 3 epilayer by HCl gas etching. The epilayer was prepared by using epitaxial lateral overgrowth technique with a stripe mask pattern. The etch pit density was very high in the window region, and much lower in the laterally grown area on the mask. A line of etch pits was observed at a coalesced boundary. Thus, the etch pit density had a clear correlation with the dislocation density. The correspondence between the etch pits and dislocations was confirmed by cross-sectional bright- and dark-field transmission electron microscopy (TEM). This gas-etching technique can clarify the distribution of dislocations in a wide area, which cannot be explored effectively by TEM. [ABSTRACT FROM AUTHOR]

We report the fabrication of rhombohedral corundum-structured indium gallium oxide (α-(InxGa1-x)2O3) alloy thin films on sapphire substrates with α-Ga2O3 buffer layers. X-ray diffraction measurement showed the formation of the alloy thin films with In compositions of x=0, 0.05, 0.08, and 0.67. The optical band gaps and resistivity systematically changed depending on x, that is, from 5.3 to 4.0 eV and from 109 to 10¹ Ω cm, respectively, suggesting future applications to actual devices. Additionally, CL measurements for α-(In0.08Ga0.92)2O3 alloy thin film suggest the emission from deep levels. [ABSTRACT FROM AUTHOR]

Abstract: An AlGaN/GaN HEMT was irradiated with 5MeV protons at a dose up to 2×1015/cm2. Photoluminescence spectra measured at 5K indicated that GaN lattice was severely damaged by collisions with high energy protons although distinct near band-edge features were still observable. An I DS–V DS measurement showed that the current level was decreased by 43% after proton irradiation, and an I GS–V GS measurement suggested that damage at the metal-contact/AlGaN interface was minimal. Transistor operation was found to be resistant to proton irradiation, which was partially attributed to a self-healing mechanism in proximity to the AlGaN/GaN interface. [Copyright &y& Elsevier]

Abstract: We report on carbon doping of cubic GaN by CBr4 using plasma-assisted molecular beam epitaxy. Cubic GaN:C samples were doped at different CBr4 beam equivalent pressures between 2×10−9 and 6×10−6 mbar. The incorporated carbon concentration of up to 1×1020 cm−3 was achieved in c-GaN:C as measured by secondary ion mass spectroscopy. The net donor/acceptor concentration was obtained by evaluation of capacitance-voltage data. Capacitance-voltage measurements on nominally undoped cubic GaN showed n-type conductivity. With an increase in CBr4 flux the conductivity type changed to p-type and for highest CBr4 flux an acceptor surplus of 1×1019 cm−3 was obtained. The electrical properties of the c-GaN:C layers were investigated by current–voltage measurements and a decrease in the serial conductance by two orders of magnitude was demonstrated in c-GaN:C. A blue shift of the 2K donor–acceptor pair luminescence with an increase in carbon concentration was obtained. [Copyright &y& Elsevier]

Abstract: The activation energies for Ga and N desorption from a GaN surface were calculated using the density functional theory to understand the detailed decomposition process of the hydrogen terminated GaN(0001) Ga and N surfaces under a hydrogen atmosphere. It was found that the Ga atoms on the hydrogen terminated GaN(0001) Ga surface desorbed as GaH molecules from the surface while the N atoms on the hydrogen terminated GaN(0001) N surface desorbed as NH3 molecules from the surface. The desorption energies of GaH and NH3 on the hydrogen terminated surface were more consistent with the previous experimental values than those on the ideal surface. These results suggest that the initial surface structure of the GaN(0001) surface is terminated with hydrogen. [Copyright &y& Elsevier]

Abstract: Non-polar carbon-doped cubic GaN (c-GaN:C) films were grown by the plasma-assisted molecular beam epitaxy (MBE) using carbon tetra-bromide (CBr4) as a carbon source. The growth was in-situ monitored by the reflection high-energy electron diffraction (RHEED) and for the atomic carbon detection quadrupole mass spectrometry (QMS) was applied. Time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was used to quantify the carbon incorporation behavior. The structural, morphological and optical properties of the epilayers were studied by high-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements at room temperature and at 4K. The electrical properties of c-GaN:C samples were determined by measuring the current–voltage (I–V) characteristics at room temperature. [Copyright &y& Elsevier]

Abstract: GaN layers were grown on (0001) sapphire substrates by saturating Ga metal with atomic nitrogen and hydrogen mixture in a microwave plasma operating at 1000Pa with the power of 380W to avoid high equilibrium pressure and temperature environment. The semi-transparent GaN layer of about 3μm was grown on the substrate at a growth rate of 0.6μm/h under hydrogen flow ratio of 50%. The film had luminescence property without a deep level yellow emission. These results indicate that the direct conversion of Ga metal into GaN layer under nitrogen plasma is a useful method. [Copyright &y& Elsevier]

Abstract: Dislocations of GaSb on GaAs substrates stripe-patterned with were reduced by using epitaxial lateral overgrowth (ELO) method. The properties of ELO GaSb layers were studied by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD). In the TEM image with magnification of 15,000, dislocations were not observed with ELO GaSb, while its density was estimated to be in seed region. Interfacial misfit arrays were formed at the openings of the mask with space of 54Å, which is close to the value expected from 7.8% lattice mismatch between GaSb and GaAs substrate. The XRD rocking curves also indicate the superior crystalline quality of ELO GaSb layers. [Copyright &y& Elsevier]

Abstract: In-situ etching of GaAs and Al x Ga1− x As in LP-MOVPE has been studied using carbon tetrabromide (CBr4). This source is routinely used by us as a source for carbon doping in GaAs and AlGaAs. The etching rate decreases with increasing AsH3 partial pressure. While the etch rate is nearly constant for GaAs between 600 and 700°C it strongly drops towards higher temperatures with higher Al content in the layers. A model based on blocking of the surface by non-volatile Al–Br compounds is proposed. The observed effects allow for selective in-situ etching of GaAs against AlGaAs. [Copyright &y& Elsevier]

Abstract: We have used positron annihilation spectroscopy to study the native vacancy distribution in -plane heteroepitaxial GaN. We show that the Ga vacancy concentration is independent of the layer thickness in the range from 5 to . This is strikingly different from the behavior in -plane GaN, where the Ga vacancy concentration decreases dramatically with the distance from the GaN/sapphire interface. This difference in the native vacancy profiles is tentatively correlated with the differences in the O impurity and dislocation density profiles in the polar and non-polar materials. [Copyright &y& Elsevier]

• Classification of a large number of dislocations in β-Ga 2 O 3 using X-ray topographs. • Deformation correction for topographs with different g using affine-transformation. • Dislocation contrast analysis using topographs taken at different g . X-ray topography (XRT) was performed on a (2 ¯ 0 1)-oriented β-Ga 2 O 3 substrate using various diffraction vectors ( g -vectors). The defect-related contrasts could be grouped roughly into line, dot, and rod contrasts based on their shapes. Most line contrasts were caused by screw-type dislocations that extended in the 〈0 1 0〉 direction in the (2 ¯ 0 1) plane. The dot contrasts were attributed to dislocations that had outcrops on the (2 ¯ 0 1) surface, while the rod contrasts were attributed to three-dimensional defects. To analyze how the contrast of a given dislocation varied across topographs taken with different g -vectors, affine transformation was applied to correct image deformations caused by differing projection angles and precisely aligned dislocation coordinates were obtained for all images. This enabled us to perform statistical contrast analyses on a large number of dislocations. The results indicated the presence of several typical Burgers vectors, including 〈0 1 0〉, 〈0 0 1〉, 〈1 0 0〉, and 〈1 0 1 ¯ 〉. A large proportion of dislocations exhibited small Burgers vectors. [ABSTRACT FROM AUTHOR]

• The hexagonal bulk GaTe single crystal was grown from melt in graphite crucible. • The hexagonal to monoclinic phase was observed over a long time. • Laue patterns reflect gradual structural transformations of the crystal lattice. • The habit plane was indexed as the coherent interface between two phases. • The phase transition goes at a constant stoichiometric composition. The polymorphic transition from hexagonal to monoclinic phase was studied for the first time in melt grown GaTe crystals. The polymorphic transition goes as a non-variant process at a constant temperature and pressure without any changes in composition. Though the character of the phase transition is apparently diffusive, no interface or grain boundaries are detected. After completion of the phase transition the whole volume of the crystal remains a single crystal. The cleavage plane is preserved and proves to be a habit plane between the hexagonal and monoclinic phase. Reflections in Laue patterns are sharpened and peaks in X-ray diffraction patterns are narrowed upon the transition, which is attributed to relaxation of internal stresses. Annealing of the transformed monoclinic GaTe at 1023 K followed by quenching does not incite decomposition of the compound, which is explained by high stoichiometry of the grown crystal. Energy gap value after storage at room temperature and after the annealing is 1.65 eV, which corresponds with the data for monoclinic GaTe. The Vickers microhardness is 360 MPa. [ABSTRACT FROM AUTHOR]

• GaN growth on oxidized Si(0 0 1) and quartz substrates is demonstrated. • Silicon seed layers are fabricated by aluminum-induced crystallization. • GaN on glass with uniform c-axis orientation can be realized over cm-scale areas. • An island-like seed layer limits GaN stress and cracking. • Lower Si annealing temperatures produce more aligned GaN. The integration of gallium nitride with low-cost, transparent substrates such as glass and crystallographically incompatible substrates such as Si(0 0 1) has been long desired for III-N electronics. Here the authors demonstrate how GaN growth on uniformly 〈1 1 1〉 oriented silicon seed layers fabricated by aluminum-induced crystallization provide a straightforward method for realizing large area GaN growth on these substrate by adopting well-developed GaN on Si(1 1 1) growth processes. Critical factors for promoting high quality GaN growth are identified, and remaining steps necessary for realizing GaN devices are discussed. [ABSTRACT FROM AUTHOR]

Abstract: We report the growth of dilute GaSbN layers on GaSb substrates by liquid-phase epitaxy using polycrystalline GaN as the source of nitrogen in the growth melt. Atomic force microscopy shows uniform layer surface with root mean square roughness of 2.6nm. Energy dispersive X-ray measurements confirm the presence of nitrogen in the grown layer. The material is further characterized by high-resolution X-ray diffraction and Fourier transform infrared spectroscopy measurements. Nitrogen incorporation up to 1.7% is obtained with a corresponding band gap reduction of 0.37eV. [Copyright &y& Elsevier]

Abstract: A series of 1-μm-thick undoped In0.53Ga0.47As with different substrate growth temperature (T g) or different beam flux pressure (BFP) of As were grown on lattice-matched semi-insulating InP (001) substrates by molecular beam epitaxy (MBE). Van der Pauw Hall measurements were carried out for these In0.53Ga0.47As samples. The residual electron concentration decreased with increasing temperature from 77 to 140K, but increased with increasing temperature from 140 to 300K. Rapid thermal annealing (RTA) can reduce the residual electron concentration. The residual electron mobility increased with increasing temperature from 77 to 300K. All these electrical properties are associated with As antisite defects. [Copyright &y& Elsevier]

• Growth of cubic GaN on MgF 2 by plasma assisted molecular beam epitaxy. • Possible on both tested orientations, (0 0 1) and (1 1 0), of MgF 2. • Better layer quality by prior wetting of the surface with GaN. The growth of gallium nitride (GaN) on magnesium fluoride (MgF 2) by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated under different conditions. GaN films on MgF 2 substrates are a promising material combination for UV-detectors in space applications. Two different crystallographic orientations of MgF 2 were used to analyse the influence on the formation of GaN. The GaN layers on (0 0 1) MgF 2 had better quality in comparison to a (1 1 0) surface orientation. Under a high Ga flow only cubic β -GaN without hexagonal α -GaN was formed at the growth on (0 0 1) MgF 2. The layers, grown under other conditions, consisted mainly of the meta-stable cubic GaN with inclusions of the hexagonal phase. With higher substrate temperatures a rougher surface and a higher grain size could be observed. The best results were obtained with a substrate temperature of 450 °C and a prior wetting of the surface with only gallium. In this case the relative low substrate temperature could be compensated. In X-ray diffraction (XRD) analysis mainly (1 1 1), (2 2 2) and (2 2 0) peaks were observed indicating that mostly cubic GaN was present. [ABSTRACT FROM AUTHOR]

• FeGa 2 O 4 nanowires were obtained from the GaN: Fe thermal decomposition. • Nanowires were obtained after high-energy ball milling and subsequent annealing. • DSC-TGA analysis confirms the GaN: Fe thermal decomposition. • TEM analysis shows that the nanowires are straight with nanometric diameters. • The annealing treatment causes unidirectional growth of the nanowires. In this work, FeGa 2 O 4 nanowires were prepared using high-energy ball milling and subsequent annealing from GaN samples doped with Fe. The structural and chemical characterization were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC-TGA), and transmission electron microscopy (TEM). The XRD patterns showed that GaN samples doped with Fe reach nanocrystalline sizes after the process of high-energy ball milling. The DSC-TGA studies confirmed the GaN thermal decomposition and weight loss of the milled samples. XRD patterns reveal the formation of the FeGa 2 O 4 compound with an fcc crystal lattice in GaN: Fe milled and subsequently annealed at 1100 °C. The TEM analysis of annealed powders showed the formation of FeGa 2 O 4 nanowires with diameters of around 50 to 60 nm and lengths up to 2 μm. High-resolution transmission electron microscopy (HRTEM) images provide additional evidence of the fcc lattice structure of the nanowires. The decomposition of GaN produced FeGa 2 O 4 nanowires since molten Ga at high temperature is combined with the doping element of Fe and then with the O 2 in the air. [ABSTRACT FROM AUTHOR]

• As-grown and annealed zincblende GaN nucleation layers are crystalline. • Substrate coverage reduces when annealed to higher epilayer growth temperature. • Reduction in substrate coverage due to material desorption and ripening of islands. • Incomplete substrate coverage of nucleation layer results in wurtzite formation. Cubic zincblende (zb-)GaN nucleation layers (NLs) grown by MOVPE on 3C-SiC/Si substrates were studied to determine their optimal thickness for subsequent zb-GaN epilayer growth. The layers were characterised by atomic force microscopy, X-ray diffraction and scanning transmission electron microscopy. The as-grown NLs, with nominal thicknesses varying from 3 nm to 44 nm, consist of small grains which are elongated in the [1 −1 0] direction, and cover the underlying SiC surface almost entirely. Thermal annealing of the NLs by heating in a H 2 /NH 3 atmosphere to the elevated epilayer growth temperature reduces the substrate coverage of the films that are less than 22 nm thick, due to both material desorption and the ripening of islands. The compressive biaxial in-plane strain of the NLs reduces with increasing NL thickness to the value of relaxed GaN for a thickness of 44 nm. Both the as-grown and annealed NLs are crystalline and have high zincblende phase purity, but contain defects including misfit dislocations and stacking faults. The zb-GaN epilayers grown on the thinnest NLs show an enhanced fraction of the wurtzite phase, most likely formed by nucleation on the exposed substrate surface at elevated temperature, thus dictating the minimum NL thickness for phase-pure zb-GaN epilayer growth. [ABSTRACT FROM AUTHOR]

Abstract: We report on the molecular beam epitaxy (MBE) growth and luminescence properties of tensile-strained type II GaAs/GaSb1− x As x (0<x<0.05) nanostructures. Depending on the nominal thickness of a GaAs layer, we observed either growth of ultra-thin GaAs1− y Sb y alloyed quantum wells (QWs) or self-organized formation of quantum dots (QDs). Both QW and QD nanostructures exhibit bright photoluminescence in the 1.5–2.2μm range. AlGaSbAs laser diodes with GaAs/GaSb nanostructures in the active region have also been fabricated. The laser diode incorporating a 1.1ML-GaAs/13ML-GaSb superlattice in the active region shows room-temperature lasing at a wavelength of 1.85μm under pulsed mode operation. [Copyright &y& Elsevier]

Abstract: In order to study the effect of pyramidal structure in reducing dislocations in mismatched heteroepitaxy, pyramidal In0.06Ga0.94Sb epilayers with mirror-like {111} facets were grown on two kinds of GaSb (001) circular patterned substrates by liquid phase epitaxy. The resultant epilayers were evaluated by distribution of etch pit density. It was found that epilayer of high quality could be attained on a patterned substrate with an extra SiN x amorphous film on the center of circular pattern. [Copyright &y& Elsevier]

Abstract: Ammonothermal growth of GaN was studied through three types of experiments: synthesis of powder GaN from metallic Ga with various kinds of mineralizers, a weight loss measurement of polycrystalline GaN, and growth of GaN via fluid transport. Hexagonal (wurtzite) GaN was exclusively synthesized with basic mineralizers, whereas neutral and acidic mineralizers resulted in a mixture of hexagonal and cubic (zinc blende) phases, confirming that basic mineralizers are suitable for growing hexagonal GaN. Through a dissolution study of GaN in basic supercritical ammonia, it was discovered that the solubility of GaN has negative temperature dependence. Based on these results, the Ga nutrient was placed in a lower temperature zone and GaN was precipitated on single crystalline GaN seeds. Seeds placed in a higher-temperature zone showed more weight gain than those in a lower-temperature zone. Spontaneously nucleated precipitates on the GaN seeds showed clear crystallographic facets. The maximum size obtained in 24-h growth was about m. Deposition of planar GaN on seed surfaces was also confirmed. [Copyright &y& Elsevier]