Thermal treatment-dependent chemical composition of ternary CdS1−xSex nanocrystals grown in borosilicate glass

Variation of the chemical composition of ternary CdS_1−xSe_x nanocrystals grown in borosilicate glass depending on the thermal treatment is studied by resonant Raman spectroscopy. It is shown that only for the nanocrystals with roughly equal content of substitutive S and Se chalcogen atoms (0.4<x<0.6) the nanocrystal composition is independent of the thermal treatment parameters. In other cases an increase of the thermal treatment temperature (625–700 °C) and duration (2–12 h) results in a considerable increase of the predominant chalcogen content in the nanocrystals.     

Polarization induced 2D hole gas in GaN/AlGaN heterostructures

The generation of high density 2D hole gases is crucial for further progress in the electronic and optoelectronic nitride devices. In this paper, we present systematic theoretical studies of Mg-doped GaN/AlGaN gated heterostructures and superlattices. Our calculations are based on a self-consistent solution of the multiband k.p Schrödinger and Poisson equation and reveal that the hole 2D sheet density is mainly determined by the polarization induced interface charges. For an aluminium concentration of 30%, the induced hole density in the heterostructure can reach values up to 1.5×10¹³ cm⁻². In the GaN/AlGaN superlattices, the hole sheet density increases with the superlattice period and saturates for a period of 40 nm at a value of 1.5×10¹³ cm⁻².     

Study of Ge epitaxial growth on Si substrates by cluster beam deposition

Ge epitaxial layers with reasonable quality were grown on Si (1 1 1) substrates by cluster beam deposition (CBD) process. Molecular dynamics study of the low energy Ge clusters deposition process utilizing the Stillinger–Weber two- and three-body interaction potentials was carried out to compare the experimental results. Both experimental and simulation results prove that the substrate temperature plays a dominant role in the epitaxial growth of Ge films in CBD process. The influence mechanisms of temperature are discussed.     

Growth of epitaxial thin films of scandium nitride on 100-oriented silicon

A series of 100-oriented ScN films was grown under N-rich conditions on 100-oriented Si using different Sc fluxes. The ScN films grew in an epitaxial cube-on-cube orientation, with [0 0 1]_ScN//[0 0 1]_Si and [1 0 0]_ScN//[1 0 0]_Si, despite the high (11%) lattice mismatch between ScN and Si. The film grain size increases and the film ω-FWHM decreases with increasing Sc flux, but the film roughness increases. Films grown under similar conditions on 111-oriented Si resulted in mixed 111 and 100 orientations, indicating that the 100 orientation is favoured both due to texture inheritance from the substrate and due to the growth conditions used.     

Metamorphic growth of tensile strained GaInP on GaAs substrate

Optical and structural properties of tensile strained graded Ga_xIn_1−xP buffers grown on GaAs substrate have been studied by photoluminescence, X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements. The Ga composition in the graded buffer layers was varied from x=0.51 (lattice matched to GaAs) to x=0.66 (1% lattice mismatch to GaAs). The optimal growth temperature for the graded buffer layer was found to be about 80–100 °C lower than that for the lattice matched GaInP growth. The photoluminescence intensity and surface smoothness of the Ga_0.66In_0.34P layer grown on top of the graded buffer were strongly enhanced by temperature optimization. The relaxation of tensile GaInP was found to be highly anisotropic. A 1.5 μm thick graded buffer led to a 92% average relaxation and a room temperature photoluminescence peak wavelength of 596 nm.     

Oriented growth of benzophenone crystals from undercooled melts

The tin sulfides compounds SnS and SnS₂ nanoflakes were prepared by the microwave-assisted polyol synthetic method. The as-prepared nanoscale flakes were characterized by X-ray powder diffraction, transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy and Raman spectra. The influences of solvents and microwave irradiation on the formation of products have been studied.     

Large-scale synthesis of uniform Cu2O stellar crystals via microwave-assisted route

A convenient microwave-assisted process has been applied to prepare stellar Cu₂O crystals. The products were characterized via X-ray diffraction pattern (XRD), transmission electron microscopy images (TEM) and UV-VIS absorption spectrum. The results showed that the as-prepared Cu₂O stellar crystals have regular shape and uniform size distribution. The influences of microwave irradiation and the concentration of starting agents on the morphology of the products were also discussed.     

Large-size β-Ga2O3 single crystals and wafers

Large-size single crystals of β-Ga₂O₃ with 1 inc in diameter have been grown by the floating zone technique. The stable growth conditions have been determined by the examination of the crystal structure. Wafers have been cut and fine polished in the (1 0 0), (0 1 0) and (0 0 1) planes. These were highly transparent in the visible and near UV, as well as electrically conductive, indicating the potential use of β-Ga₂O₃ as a substrate for optoelectic devices operating in the visible/near UV and with vertical current flow.     

Effect of Mn concentration and growth temperature on nanostructures and magnetic properties of Ge1−xMnx grown on Si

The nanostructures and magnetic properties of Ge_1−xMn_x thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1−4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge_1−xMn_x thin films were grown at 70 °C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 °C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films.     

Structural characterization of CdTe layers grown on (0 0 0 1) sapphire by MOCVD

Crystalline ZnO nanoparticles were synthesized on Si substrates with or without a Au catalyst by a chemical vapor deposition (CVD) method using ZnS as the source material. The average sizes are in the range of 40–200 nm and the densities of 10⁴–10¹⁰ cm⁻². In the absence of an Au catalyst, the average nanoparticle size firstly decreases and then increases with increasing substrate temperature while the nanoparticle density decreases as the substrate temperature increases. In the presence of an Au catalyst, ZnO nanoparticles only grow when the substrate temperature is higher than 300°C and the higher the substrate temperature the denser the nanoparticles are deposited. The density of the ZnO nanoparticles grown on a Si (1 1 1) substrate is higher than that on a Si (1 0 0) substrate with or without Au catalyst.     

Possibility of AlN growth using Li–Al–N solvent

The possibility of AlN growth using Li–Al–N solvent was investigated. Based on theoretical prediction, we selected Li₃N as a suitable nitrogen source for AlN growth. First, vapor phase epitaxy using Li₃N and Al as source materials was performed to confirm the following reaction on the growth surface: Li₃N+Al=AlN+3Li. The results suggest that the reaction proceeds to form AlN on the substrate under appropriate conditions. Next, AlN growth using Li–Al–N solvent was carried out. The Li–Al–N solvent was prepared by annealing of mixtures composed of Li₃N and Al. The results imply that AlN was formed under an Al-rich condition. Moreover, it was found that Li was swept out from AlN grains during growth. The results suggest that AlN growth using Li–Al–N solvent might be a key technology to obtain an AlN crystal boule.     

MOCVD growth of GaN on Si(1 1 1) substrates using an ALD-grown Al2O3 interlayer

GaN thin films have been grown on Si(1 1 1) substrates using an atomic layer deposition (ALD)-grown Al₂O₃ interlayer. This thin Al₂O₃ layer reduces strain in the subsequent GaN layer, leading to lower defect densities and improved material quality compared to GaN thin films grown by the same process on bare Si. XRD ω-scans showed a full width at half maximum (FWHM) of 549 arcsec for GaN grown on bare Si and a FWHM as low as 378 arcsec for GaN grown on Si using the ALD-grown Al₂O₃ interlayer. Raman spectroscopy was used to study the strain in these films in more detail, with the shift of the E₂(high) mode showing a clear dependence of strain on Al₂O₃ interlayer thickness. This dependence of strain on Al₂O₃ thickness was also observed via the redshift of the near bandedge emission in room temperature photoluminescence (RT-PL) spectroscopy. The reduction in strain results in a significant reduction in both crack density and screw dislocation density compared to similar films grown on bare Si. Screw dislocation density of the films grown on Al₂O₃/Si substrates approaches that of typical GaN layers on sapphire. This work shows great promise for the use of oxide interlayers for growth of GaN-based LEDs on Si.     

Reconstruction of the -GaO (1 0 0) cleavage surface to hexagonal GaN after NH nitridation

The potential use of β-Ga₂O₃ wafers as transparent conductive substrates for compounds with hexagonal structure, in particular the GaN-system, is shown. Nitridation of the main cleavage plane under high temperature and NH₃ gas results in the substitution of O by N on the surface, and a simultaneous surface reconstruction. The 2-fold symmetry of the initial (1 0 0) plane changes to a 6-fold symmetry, keeping the [0 1 0] azimuth as a symmetry direction. The RHEED pattern suggests the formation of GaN on the β-Ga₂O₃ surface, with the b-axis of β-Ga₂O₃ parallel to the direction of GaN, so that β-Ga₂O₃ can be used as a substrate with no lattice mismatch to GaN.     

Anisotropic vapor phase growth of Ga2O3 crystalline nanobelts

Ga₂O₃ nanobelts were synthesized by gas reaction at high temperature in the presence of oxygen in ammonia. X-ray diffraction and chemical microanalysis revealed that the nanostructures were Ga₂O₃ with the monoclinic structure. Electron microscopy study indicated the nanobelts were single crystalline with broad (0 1 0) crystallographic planes. The nanostructures grew anisotropically with the growth direction of . Statistical analysis of the anisotropic morphology of the nanobelts and electron microscopy investigation of the nanobelt tips indicated that both vapor–solid and vapor–liquid–solid mechanisms controlled the growth process. The anisotropic nature of crystallographic morphology is explained in terms of surface energy.     

Investigation of the role of cadmium sulfide in the surface passivation of lead sulfide quantum dots

Surface passivation of PbS nanocrystals (NC), resulting in strong photoluminescence, can be achieved by the introduction of CdS precursors. The role of CdS in the surface passivation of PbS NCs is uncertain, as the crystalline structure of CdS and PbS are different, which should impede effective epitaxial overgrowth. Absorption spectroscopy is used to show that the CdS precursors strongly interact with the PbS NC surface. Electron microscopy reveals that the introduction of CdS precursors results in an increased particle size, consistent with overcoating. However, we also find the process to be highly non-uniform. Nevertheless, evidence for epitaxial growth is found, suggesting that effective surface passivation may be possible.     

Electron-carrier generation by edge dislocations in InN films: First-principles study

Electronic structures of edge dislocations in InN films are studied using the first-principles calculation. We found that dangling-bond states of In atoms localized in the dislocation core are located above the conduction-band bottom and thus supplies the electron carriers to the conduction band of bulk InN, in agreement with the experimental suggestion by Wang et al. [Appl. Phys. Lett. 90 (2007) 151901]. Moreover, it is shown that the Fermi energy in the conduction band has the tendency to be pinned at the energy positions of N-related dangling-bond states.     

Epitaxial growth of GdN on silicon substrate using an AlN buffer layer

We report on the epitaxial growth of the intrinsic ferromagnetic semiconductor GdN on Si (1 1 1) substrates buffered by a thick AlN layer, forming a heteroepitaxial system with promise for spintronics. Growth is achieved by depositing Gd in the presence of unactivated N₂ gas, demonstrating a reactivity at the surface that is sufficient to grow near stoichiometric GdN only when the N₂:Gd flux ratio is at least 100. Reflection high-energy electron diffraction and X-ray diffraction show fully (1 1 1)-oriented epitaxial GdN films. The epitaxial quality of the films is assessed by Rutherford backscattering spectroscopy carried out in random and channelling conditions. Magnetic measurements exhibit a Curie temperature at 65 K and saturation magnetisation of 7 μ_B/Gd in agreement with previous bulk and thin-film data. Hall effect and resistance data establish that the films are heavily doped semiconductors, suggesting that up to 1% of the N sites are vacant.     

Study of group-III binary and ternary nitrides using X-ray absorption fine structure measurements

It is demonstrated that the NEXAFS spectra are a “fingerprint” of the symmetry and the composition of the binary nitrides GaN, AlN and InN, as well as of their ternary alloys In_0.16Ga_0.84N and Al_yGa_1−yN. From the angular dependence of the N-K-edge NEXAFS spectra, the hexagonal symmetry of the under study compounds is deduced and the (p_x, p_y) or p_z character of the final state is identified. The energy position of the absorption edge (E_abs) of the binary compounds GaN, AlN and InN is found to red-shift linearly with the atomic number of the cation. The E_abs of the Al_yGa_1−yN alloys takes values in between those corresponding to the parent compounds AlN and GaN. Contrary to that, the E_abs of In_0.16Ga_0.84N is red-shifted relative to that of GaN and InN, probably due to ordering and/or phase separation phenomena. The EXAFS analysis results reveal that the first nearest-neighbour shell around the N atom, which consists of Ga atoms, is distorted in both GaN and Al_xGa_1−xN for x<0.5.     

Incorporation behaviors of group V elements in GaAsSbN grown by gas-source molecular-beam epitaxy

We report the incorporation behaviors of As, Sb, and N atoms in GaAsSbN grown by gas-source molecular-beam epitaxy. We found that N atom is more reactive and competitive than Sb atom at the growth temperature ranging from 420 to 450 °C. The increment in Sb beam flux hardly changes the N composition. However, the increment in N flux retards the incorporation of Sb. In addition, the increment in As₂ flux makes the Sb and N compositions decrease at the same rate. Based on these results, we have successfully grown GaAsSbN epilayers lattice-matched to GaAs substrates. The energy gap at room temperature is as low as 0.803 eV. Negative deviation from Vegard's law in lattice constant is observed in these layers.     

Boron oxide encapsulated Bridgman growth of high-purity high-resistivity cadmium telluride crystals

High-purity semi-insulating CdTe crystals have been successfully grown by encapsulated (B₂O₃) Bridgman technique. The procedure strongly limits component losses allowing the achievement of stoichiometry control material and keeps a low level of impurity contamination as shown by mass spectroscopy analysis data. When strictly stoichiometry-controlled and high-purity polycrystalline source material has been used, high-resistivity crystals have been obtained without any intentional doping. EPD values in the range of 1–3×10⁴ cm⁻² have been observed in a wide region of the crystals. Luminescence spectroscopy confirms the purity and good structural quality of the material. The proposed method avoids the technical problems posed by the High Pressure Bridgman technique and fits the requirements for CdTe/CdZnTe crystals large-scale production.