Composition dependence of photoconductivity of Al20AsxTe80−x glasses

The time dependent photocurrent of Al₂₀As_xTe_80−x glasses has been studied at low temperatures. It is found that the photocurrent of all the Al₂₀As_xTe_80−x samples studied does not decrease appreciably during illumination, which is consistent with the behavior of other narrow band gap amorphous chalcogenides. Further, the photosensitivity is found to be maximized for the composition x=25, which can be associated with rigidity percolation.     

Bismuth a new dopant for GaN films grown by molecular beam epitaxy—surfactant effects,formation of GaN1−xBix alloys and co-doping with arsenic

We have investigated the influence of an additional bismuth flux during growth on the properties of GaN films prepared by plasma-assisted molecular beam epitaxy (MBE). A wide range of bismuth fluxes have been used, the highest Bi flux was larger than the flux of Ga. We have demonstrated that using a Bi flux during the growth of GaN by MBE at a temperature ∼800°C improves the surface morphology of the films and decreases the deep emission. We have demonstrated for the first time the growth of GaN_1−xBi_x alloys by MBE, the Bi concentration was small and increased with decreasing growth temperature. We have studied the influence of an additional bismuth flux on the growth of As-doped GaN layers and observed an increase of blue emission from the layers at some optimum range of Bi fluxes. All the results allow us to conclude that Bi may be a potential new dopant for the growth of GaN by MBE.     

AlGaAs/InGaP interfaces in structures prepared by MOVPE

Al_0.3Ga_0.7As/In_1−xGa_xP structures were prepared by low-pressure MOVPE. Lattice matched and strained ones with top In_1−xGa_xP layers as well as reverse ones with top Al_0,3Ga_0,7As layers were examined. The structures were studied by photoluminescence, X-ray and atomic force microscope (AFM) methods. An additional photoluminescence peak from the Al_0.3Ga_0.7As/In_1−xGa_xP interface was observed in our samples and it was attributed to a type-II band offset. A conduction band offset of 0.121 eV was measured in the Al_0.3Ga_0.7As/In_0.485Ga_0.515P lattice-matched structure and a linear dependence of the conduction band offset on In_1−xGa_xP composition, with a zero offset in the Al_0.3Ga_0.7As/In_0.315Ga_0.685P structure, was determined. The valence band discontinuity had a nearly constant value of 0.152 eV.     

Photoluminescence and optical absorption in glassy AsxSe1−x

The photoluminescence and optical absorption spectra in glassy As_xSe_1−x with 0.08 ? x ? 0.40 are essentially independent of x. The PL spectra peak at approximately half the optical gap and the exponential slopes of the optical absorption edges are all approximately 75 meV. For x > 0.4, the PL peaks shift to higher energies, the widths of the PL spectra increase, and there is a strong component to the optical absorption well below the optical gap. Comparisons with ESR experiments in the As_xS_1−x system suggest the possibility of two PL peaks. The second PL peak and the optical absorption below the optical band gap for x > 0.4 are attributed to the presence of As-As bonds.     

Thermal diffusivity measurements on As-Te-Ga glasses by photo-thermal deflection technique: Composition dependence and topological thresholds

The thermal diffusivity (α) of As₂₀Te_80−xGa_x glasses (7.5 ? x ? 18.5) has been measured using photo-thermal deflection (PTD) technique. It is found that the thermal diffusivity is comparatively lower for As₂₀Te_80−xGa_x glasses, which is consistent with the memory type of electrical switching exhibited by these samples. Further, the thermal diffusivity of As₂₀Te_80−xGa_x glasses is found to increase with the incorporation of gallium initially (for x ? 9), which is consistent with the metallicity of the additive. This increase in α results in a maximum at the composition x = 9; beyond x = 9, a decrease is seen in α leading to a minimum at the composition x = 15. The observed composition dependence of thermal diffusivity of As₂₀Te_80−xGa_x glasses has been found to be similar to that of Al₂₀As_xTe_100−x glasses, based on which it is proposed that As₂₀Te_80−xGa_x glasses exhibit an extended stiffness transition with compositions x = 9 and x = 15 being its onset and completion, respectively. Also, the composition x = 17.5 at which a second maximum is seen in the thermal diffusivity has been identified to be the chemical threshold (CT) of the As₂₀Te_80−xGa_x glassy system, as at CT, the glass is configurationally closest to the crystalline state and the scattering of the diffusing thermal waves is minimal for the chemically ordered phase.     

Electrical properties and deep traps spectra in undoped M-plane GaN films prepared by standard MOCVD and by selective lateral overgrowth

Electrical properties, deep traps spectra and structural performance were studied for m-GaN films grown on m-SiC substrates by standard metalorganic chemical vapor deposition (MOCVD) and by MOCVD with lateral overgrowth (ELO) or sidewall lateral overgrowth (SELO). Standard MOCVD m-GaN films with a very high dislocation density over 10⁹ cm⁻² are semi-insulating n-type with the Fermi level pinned near E_c−0.7 eV when grown at high V/III ratio. For lower V/III they become more highly conducting, with the electrical properties still dominated by a high density (∼10¹⁶ cm⁻³) of E_c−0.6 eV electron traps. Lateral overgrowth that reduces the dislocation density by several orders of magnitude results in a marked increase in the uncompensated shallow donor density (∼10¹⁵ cm⁻³) and a substantial decrease of the density of major electron traps at E_c−0.25 and E_c−0.6 eV (down to about 10¹⁴ cm⁻³). Possible explanations are briefly discussed.     

The effect of annealing on the surface morphology of strained and unstrained InxAl1−xN thin films

In_xAl_1−xN is a particularly useful group-III nitride alloy because by adjusting its composition it can be lattice matched to GaN. Such lattice-matched layers may find application in distributed Bragg reflectors (DBRs) and high electron mobility transistors (HEMTs). However, compared with other semiconducting nitride alloys, In_xAl_1-xN has not been researched extensively. In this study, thin In_xAl_1−xN epilayers were grown by metal-organic vapour phase epitaxy (MOVPE) on GaN and Al_yGa_1−yN layers. Samples were subjected to annealing at their growth temperature of 790 °C for varying lengths of time, or alternatively to a temperature ramp to 1000 °C. Their subsequent surface morphologies were analysed by atomic force microscopy (AFM). For both unstrained In_xAl_1−xN epilayers grown on GaN and compressively strained epilayers grown on Al_yGa_1−yN, surface features and fissures were seen to develop as a consequence of thermal treatment, resulting in surface roughening. It is possible that these features are caused by the loss of In-rich material formed on spinodal decomposition. Additionally, trends seen in the strained In_xAl_1−xN layers may suggest that the presence of biaxial strain stabilises the alloy by suppressing the spinode and shifting it to higher indium compositions.     

The structure and optical properties of GeO2-GeS2 glasses

Oxy-chalcogenide glasses with compositions of xGeO₂-(100 − x)GeS₂, where 0 ? x ? 100 mol%, have been prepared and studied in terms of their structures and optical properties. X-ray fluorescence spectroscopy shows that Ge:S ratio can deviate from GeS₂ by ∼10 at.%, depending critically upon the preparation conditions. Raman scattering spectroscopy suggests that stoichiometric GeO₂-GeS₂ glasses have a heterogeneous structure in the scale of 1-100 nm. The optical gaps are nearly constant at 3.0-3.5 eV for glasses with 0 ? x ? 80 mol% and abruptly increase to ∼6 eV in GeO₂. This dependence suggests that the optical gap is governed by GeS₂ clusters, which are isolated and/or percolated. Composition-deviated glasses appear as orange and brown, and these glasses seem to have more inhomogeneous structures.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

Compositional dependence of the thermal,structural, and electrical properties of AsTeI glasses

Thermal, structural, and electrical properties of semiconducting AsTeI (and AsTe) glasses have been examined as a function of concentration. Analytical techniques have been developed for quantitative chemical analysis of all three components. Differential scanning calorimetry data indicate a broad endothermic reaction, T_min ∼ 145 °C, above the glass transition (T_g = 120 ± 8 °C) for iodine compositions of 0 to ∼ 20 at%. Above this reaction temperature the thermal data are composition dependent. For glasses with I ? 35 at %, T_g is much lower (65–70 °C) and the endothermic reaction is much sharper with a minimum at ∼ 133 °C. The wide variation in thermal properties with composition suggests that electrical effects associated with high temperatures (e.g. switching and memory phenomena) may also be composition dependent, as well as being dependent upon kinetics. Structural studies show that phase segregation above T_g is dependent upon kinetics as well as upon temperature. Thermal, structural, and electrical data give evidence that As₂Te₃ exists as a unit in the non-crystalline state. This structural unit, stable at high temperatures, is present in the molten material and is thought to be present as the supercooled liquid is quenched. The short-range order extends to long-range order upon devitrification and the first crystalline phase detected is monoclinic As₂Te₃. Apparently metastable under the conditions of formation, this phase converts to a previously unreported fcc phase upon further heat treatment. Similar crystalline structures are known to be associated with thermally- and electrically-induced memory phenomena in AsTeGe and AsTeI glasses. Density measurements at room temperature show that (1) there are no phase miscibility gaps in the glass substructure or different crystalline phases segregating as the I content is varied, and (2) there is a large change in molar volume with increasing I concentration, whereas the molar weight does not change significantly. Electrical conductivity, σ, data in the region around room temperature, show that the σ₀ values, determined from σ = σ₀e^−E/kt, range from 10² to 10³ (Ω-cm)⁻¹. A possible dependence of σ₀ on I content may be due to changes in molar volume. The more homogeneous glasses appear to show breakpoints in the σ versus 1/T data; the corresponding changes in E are only 0.02–0.06 eV, with E = 0.04 eV being most frequently observed. For As₅₀Te_50−xI_x glasses, σ at a given temperature increases as the iodine concentration is increased to about 5 at % and then decreases with further increase in I content. Correspondingly there is a minimum in the E versus I concentration data at I ∼ 5 at %. Results suggest that dependence of the σ breakpoints on glass homogeneity and the variation of E with I concentration may be due to trapping effects.     

Te-rich Ge–As–Se–Te bulk glasses and films for future IR-integrated optics

Ge₁₅As₁₅Se_70−xTe_x materials with x = 56, 60 and 63, of potential use in IR-integrated optics, were prepared by the classical melt-quenching method. A macroscopic phase separation was observed with a crystalline phase on the top and an amorphous one at the bottom. The glasses from the bottom were transparent from 1.9 to 16 μm without any purification of the elemental precursors Ge, As, Te and Se. The higher the Te/Se content in the glasses the lower their glass transition temperature and thermal stability. Films 7–12 μm thick of the above stated compositions were deposited by thermal evaporation. The higher the tellurium content, the larger the optical band gap shift of the films in the infra-red and the higher the refractive index.     

InGaN growth with various InN mole fractions on m-plane ZnO substrate by metalorganic vapor phase epitaxy

We have demonstrated In_xGa_1−xN epitaxial growth with InN mole fractions of x=0.07 to 0.17 on an m-plane ZnO substrate by metalorganic vapor phase epitaxy for the first time. The crystalline quality of the epilayers was found to be much higher than that of epilayers grown on a GaN template on an m-plane SiC substrate.     

Miscibility gap calculation for Ga1−xInxNyAs1−y including strain effects

A method including strain effects is introduced for calculating the miscibility gap of the Ga_xIn_1−xN_yAs_1−y material system. The Gibbs free energy is computed using the delta lattice parameter model and the conventional solution model. The contribution caused by the strain energy due to the mismatch between substrate and epitaxial layer is added. The spinodal points can be calculated from this expression. The critical temperature above which the solid is metastable has been found to be increased due to strain effects. It is pointed out that the result of the calculation depends on the choice of substrate. The miscibility gaps for Ga_xIn_1−xAs, GaN_yAs_1−y and Ga_xIn_1−xN_yAs_1−y are evaluated by this method for a more realistic model of crystal growth.     

Effect of carbon on the microstructural evolution of Zr66.7−xNi33.3Cx (x = 0, 1, 3) alloys during mechanical alloying

In the present paper, the effect of carbon on the microstructural evolution of Zr_66.7−xNi_33.3C_x (x = 0, 1, 3) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that these three alloys undergo similar amorphization and crystallization processes, and the final milling product is a metastable fcc-Zr_66.7−xNi_33.3C_x phase. The carbon addition can shorten the milling time for the complete amorphization reaction and enhance the stability of the formed amorphous alloy, which can suppress the mechanically induced amorphous-crystalline phase transformation with further increasing milling time.     

Crystallization kinetics of Fe73.5−xMnxCu1Nb3Si13.5B9 (x = 0, 1, 3, 5, 7) amorphous alloys

In this study, we have investigated the effect of substituting Mn for Fe on the crystallization kinetics of amorphous Fe_73.5−xMn_xCu₁Nb₃Si_13.5B₉ (x = 1, 3, 5, 7) alloys. The samples were annealed at 550 °C and 600 °C for 1 h under an argon atmosphere. The X-ray diffraction analyses showed only a crystalline peak belonging to the α-Fe(Si) phase, with the grain size ranging from 12.2 nm for x = 0 to 16.7 nm for x = 7. The activation energies of the alloys were calculated using Kissinger, Ozawa and Augis-Bennett models based on differential thermal analysis data. The Avrami exponent n was calculated from the Johnson-Mehl-Avrami equation. The activation energy increased up to x = 3, then decreased with increasing Mn content. The values of the Avrami exponent showed that the crystallization is typical diffusion-controlled three-dimensional growth at a constant nucleation rate.     

Compositional dependence of the optical properties of amorphous Sb2Se3−xSx thin films

Thin films of Sb₂Se_3−xS_x solid solutions (x = 0, 1, 2, and 3) were deposited by thermal evaporation of presynthesized materials on glass substrates held at room temperature. The films compositions were confirmed by using energy dispersive analysis of X-rays (EDAX). X-ray diffraction studies revealed that all the as-deposited films as well as those annealed at T_a < 423 K have amorphous phase. The optical constants (n, k) and the thickness (t) of the films were determined from optical transmittance data, in the spectral range 500-2500 nm, using the Swanepoel method. The dispersion parameters were determined from the analysis of the refractive index. An analysis of the optical absorption spectra revealed an Urbach’s tail in the low absorption region, while in the high absorption region an indirect band gap characterizes the films with different compositions. It was found that the optical band gap energy increases quadratically as the S content increases.     

Thermal stability of thin InGaN films on GaN

The thermal stability of ∼200-nm-thick InGaN thin films on GaN was investigated using isothermal and isochronal post-growth anneals. The In_xGa_1−xN films (x=0.08–0.18) were annealed in N₂ at 600–1000 °C for 15–60 min, and the resulting film degradation was monitored using X-ray diffraction (XRD) and photoluminescence (PL) measurements. As expected, films with higher indium concentration showed more evidence for decomposition than the samples with lower indium concentration. Also for each alloy composition, decreases in the PL intensity were observed starting at much lower temperatures compared to decreases in the XRD intensity. This difference in sensitivity of the PL and XRD techniques to the InGaN decomposition suggest that defects that quench luminescence are generated prior to the onset of structural decomposition. For the higher indium concentration films, the bulk decomposition proceeds by forming metallic indium and gallium regions as observed by XRD. For the 18% indium concentration film, measurement of the temperature-dependent InGaN decomposition yields an activation energy, E_A, of 0.87±0.07 eV, which is similar to the E_A for bulk InN decomposition. The InGaN integrated XRD signal of the 18% film displays an exponential decrease vs. time, implying InGaN decomposition proceeds via a first-order reaction mechanism.     

Low-temperature/high-temperature AlN superlattice buffer layers for high-quality AlxGa1−xN on Si (1 1 1)

We present MOVPE-grown, high-quality Al_xGa_1−x N layers with Al content up to x=0.65 on Si (1 1 1) substrates. Crack-free layers with smooth surface and low defect density are obtained with optimized AlN-based seeding and buffer layers. High-temperature AlN seeding layers and (low temperature (LT)/high temperature (HT)) AlN-based superlattices (SLs) as buffer layers are efficient in reducing the dislocation density and in-plane residual strain. The crystalline quality of Al_xGa_1−xN was characterized by high-resolution X-ray diffraction (XRD). With optimized AlN-based seeding and SL buffer layers, best ω-FWHMs of the (0 0 0 2) reflection of 540 and 1400 arcsec for the (1 0 1¯ 0) reflection were achieved for a ∼1-μm-thick Al_0.1Ga_0.9N layer and 1010 and 1560 arcsec for the (0 0 0 2) and (1 0 1¯ 0) reflection of a ∼500-nm-thick Al_0.65Ga_0.35N layer. AFM and FE-SEM measurements were used to study the surface morphology and TEM cross-section measurements to determine the dislocation behaviour. With a high crystalline quality and good optical properties, Al_xGa_1−x N layers can be applied to grow electronic and optoelectronic device structures on silicon substrates in further investigations.     

Calculation of phase diagrams in AlxIn1−xAs/InP,AsxSb1−xAl/InP and AlxIn1−xSb/InSb nano-film systems

The models for calculation of phase diagrams of semiconductor thin films with different substrates were proposed by considering the contributions of strain energy, the self-energy of misfit dislocations and surface energy to Gibbs free energy. The phase diagrams of the Al_xIn_1−xAs and As_xSb_1−xAl thin films grown on the InP (1 0 0) substrate, and the Al_xIn_1−xSb thin films grown on the InSb (1 0 0) substrate at various thicknesses were calculated. The calculated results indicate that when the thickness of film is less than 1 μm, the strain-induced zinc-blende phase appears, the region of this phase extends with decreasing of the layer thickness, and there is small effect of surface energies of liquid and solid phases on the phase diagrams.     

InN excitonic deformation potentials determined experimentally

We report the experimental determination of the interband deformation potentials of indium nitride by combining both optical spectroscopy investigations and high-resolution X-ray measurements performed on a series of InN films grown by metal organic vapour-phase epitaxy. Our approach, which follows the one used for GaN by Shan et al. [Phys. Rev. B. 54 (1996) 13460], gives here for InN a₁=−7.66 eV, a₂=−2.59 eV, b₁=5.06 eV, and b₂=−2.53 eV.