xAsx Alloys Research Articles

Improvement of the electronic, mechanical and optical properties of cubic As-doped BN alloy for energy harvesting applications

The electronic structure, elastic and optical properties with wide energy band gap (Eg) in mixed crystal semiconductors containing As-doped BN are reported. Density functional calculations (DFT) were performed for comprehensive detection and understanding of structural, optoelectronic, and elastic properties for mixed semiconductors in the cubic phase. When applying Vegard’s law for As, BN, and mixed alloys based on BN1−xAsx, where x = 0, 0.25, 0.5, 0.75, and 1, we observed that the distortion of the lattice constants was slight to different condensations. Calculations of Eg values found that the values decreased with an increase in (x) condensation. The bulk modulus, and other elastic parameters were computed and confirmed that the alloys are mechanically stable. The calculations of dielectric Ɛ(ω), refractive index n(ω), higher absorption α(ω) peaks, and other coefficients confirm that BN1−xAsx alloys are favourable for practical photovoltaics. The results confirm that mixed semiconductors have great interest in next-generation optoelectronic devices. This work provides a route for optoelectronic applications.

The Effect of Fractionation during the Vacuum Deposition of Stabilized Amorphous Selenium Alloy Photoconductors on the Overall Charge Collection Efficiency.

The general fabrication process for stabilized amorphous selenium (a-Se) detectors is vacuum deposition. The evaporant alloy is typically selenium alloyed with 0.3–0.5%As to stabilize it against crystallization. During the evaporation, fractionation leads to the formation of a deposited film that is rich in As near the surface and rich in Se near the substrate. The As content is invariably not uniform across the film thickness. This paper examines the effect of non-uniform As content on the charge collection efficiency (CE). The model for the actual CE calculation is based on the generalized CE equation under small signals; it involves the integration of the reciprocal range-field product (the schubweg) and the photogeneration profile. The data for the model input were extracted from the literature on the dependence of charge carrier drift mobilities and lifetimes on the As content in a-Se1−xAsx alloys to generate the spatial variation of hole and electron ranges across the photoconductor film. This range variation is then used to calculate the actual CE in the integral equation as a function of the applied field. The carrier ranges corresponding to the average composition in the film are also used in the standard CE equation under uniform ranges to examine whether one can simply use the average As content to calculate the CE. The standard equation is also used with ranges from the spatial average and average inverse. Errors are then compared and quantified from the use of various averages. The particular choice for averaging depends on the polarity of the radiation-receiving electrode and the spatial variation of the carrier ranges.

Electronic properties of dilute-As InGaNAs alloys: A first-principles study

The dilute-As InyGa1 − yN1 − xAsx alloys are explored by performing first-principles density functional theory (DFT) calculations, where the In-content is varied from 0% up to 18.75% and the As-content is varied from 0% up to 3.13%. The obtained band structures indicate a direct bandgap semiconductor, whose electronic properties are modified significantly by the addition of As-atoms into the InGaN-based system. The lattice parameters (a and c) are analyzed, and the bowing parameters of the dilute-As InGaNAs are analyzed and discussed. The electronic properties indicate the dilute-As InyGa1 − yN1 − xAsx alloys as a potential new material alternative for achieving longer wavelength emission, while utilizing a low In-content (<20%).

Crystallization properties of arsenic doped GST alloys

We present the enhanced properties observed in the phase change memory alloy Ge2Sb2Te5 (GST) when doped with arsenic. Although arsenic is known as a toxic element, our observations show that significant improvement can be obtained in GST systems on thermal stability, transition temperature between amorphous and crystalline phases and switching behaviors when doping with arsenic. Though both the GST and arsenic doped GST are amorphous in the as-deposited state, only GST alloy turns to crystalline NaCl-type structure after annealing at 150 °C for 1 h. Results from the resistance versus temperature study show a systematic increase in the transition temperature and resistivity in the amorphous and crystalline states when the arsenic percentage in the GST alloy increases. The crystallization temperature (Tc) of (GST)0.85As0.15 is higher than the Tc observed in GST. Optical band gap (Eopt) values of the as-deposited films show a clear increasing trend; 0.6 eV for GST to 0.76 eV for (GST)0.85As0.15. The decreases in Eopt for the samples annealed at higher temperatures shows significant optical contrast between the as-deposited and annealed samples. Though all (GST)1−xAsx alloys show memory switching behaviors, threshold switching voltages (VT) of the studied alloys show an increasing trend with arsenic doping. For (GST)0.85As0.15, VT is about 5.2 V, which is higher than GST (4.0 V). Higher transition temperature and higher threshold switching values show arsenic doping in GST can enhance the memory device properties by improving the thermal stability and data readability. Understanding the doping effect on the GST is important to understand its crystallization properties. Structure properties of amorphous GST, Ge2Sb2−0.3As0.3Te5 and (GST)0.85As0.15 models were studied using first principles molecular dynamics simulations, compared their partial radial distribution functions, and q parameter order. Arsenic doping into GST features interesting structural and electronic effects revealed by the radial distribution functions, q order parameter and band gap value, in line with the experimental findings.

Investigations of the Optical Properties of GaNAs Alloys by First-Principle

We present a Density Functional Theory (DFT) analysis of the optical properties of dilute-As GaN1−xAsx alloys with arsenic (As) content ranging from 0% up to 12.5%. The real and imaginary parts of the dielectric function are investigated, and the results are compared to experimental and theoretical values for GaN. The analysis extends to present the complex refractive index and the normal-incidence reflectivity. The refractive index difference between GaN and GaNAs alloys can be engineered to be up to ~0.35 in the visible regime by inserting relatively low amounts of As-content into the GaN system. Thus, the analysis elucidates on the birefringence of the dilute-As GaNAs alloys and comparison to other experimentally characterized III-nitride systems is drawn. Our findings indicate the potential of GaNAs alloys for III-nitride based waveguide and photonic circuit design applications.

Partially polycrystalline GaN1−xAsx alloys grown on GaAs in the middle composition range achieving a smaller band gap

GaN1−xAsx alloys have been successfully grown on (100) GaAs substrates over a wide composition range (0.15 < x < 0.98) by plasma-assisted molecular beam epitaxy. In the middle composition range, the weak and broad (111) diffraction peaks are observed in the X-ray diffraction patterns. These diffraction peaks most likely come from small crystalline grains within the amorphous matrix and are unlike the entirely amorphous GaNAs alloys grown on sapphire and Pyrex glass. A transmission electron microscopy micrograph of the GaN0.50As0.50 alloy also shows a weak periodic structure consisting of small polycrystalline grains. To study the band gap and the As-affected spin–orbit band to conduction-band minimum transition, photomodulated reflectance is utilized. The band gap energies range from 0.78 to 2.15 eV (3.4 eV for end-point compounds GaN). Finally, the original and modified band anticrossing (BAC) models for GaNAs alloys were thoroughly verified over the entire composition range. Remarkably, the band gap energies of the partially polycrystalline GaNAs alloys agree well with those obtained using the original BAC model in the middle composition range because the model has been developed for crystalline materials. These results improve the growth of highly mismatched GaNAs alloys with different substrates and should expedite studies of high-efficiency multijunction solar cells fabricated using such a single ternary alloy system.

Erratum to: The Effect of Substitution of As for Ga on the Topological Phase and Structural, Electronic and Magnetic Properties of Mn2ZrGa Heusler Alloy

The structural, electronic, and magnetic properties and existence of the topological insulator and metal phase of full-Heusler Mn2ZrGa1−xAs x (x = 0, 0.25, 0.5, 0.75, 1) alloys are studied by using the first-principles full-potential linearized augmented plane wave (FP-LAPW) method. The electronic structure results indicate that the half-metallic character for the whole series exhibits a pseudo-gap for compounds with x = 0, 0.25, and 1 and a real gap for x = 0.5 and 0.75. It was found that the total magnetic moment decreases linearly with increasing As content and follows the Slater–Pauling rule. The results also predict that the whole series from x = 0 to 1 shows ferrimagnetic ordering with antiparallel alignment between Mn and Zr moments. The results of the topological band structure show that for the equilibrium lattice parameter, the Mn2ZrGa1−xAs x alloys have normal band order. Moreover, the effect of lattice parameter change on the band order of these alloys is investigated. By increasing the lattice parameter, the inverted band order occurs for the Mn2ZrAs compound.

Modulation of the band structures and optical properties of holey C2N nanosheets by alloying with group IV and V elements

The band gap and optical absorption can be tuned effectively by the alloy concentration x in the C2N1−xPx and C2N1−xAsx alloys.

Tellurium n-type doping of highly mismatched amorphous GaN1−xAsx alloys in plasma-assisted molecular beam epitaxy

In this paper we report our study on n-type Te doping of amorphous GaN1−xAsx layers grown by plasma-assisted molecular beam epitaxy. We have used a low temperature PbTe source as a source of tellurium. Reproducible and uniform tellurium incorporation in amorphous GaN1−xAsx layers has been successfully achieved with a maximum Te concentration of 9×1020cm−3. Tellurium incorporation resulted in n-doping of GaN1−xAsx layers with Hall carrier concentrations up to 3×1019cm−3 and mobilities of ~1cm2/Vs. The optimal growth temperature window for efficient Te doping of the amorphous GaN1−xAsx layers has been determined.

Modeling of the atomic structure and electronic properties of amorphous GaN1−xAsx

Chemically ordered 250-atom models for amorphous GaN1−xAsx alloys in the concentration range of 0.17<x<0.75 have been studied with density functional theory simulations, starting with initial continuous random network structures. The analysis of network topology has been achieved by examining partial-pair correlation functions, bond angle distributions, ring statistics and average coordination numbers. The electronic properties of amorphous GaN1−xAsx alloys have been estimated by means of electronic density states (EDOS) and inverse participation ratios (IPR). Our calculations indicate that the introduction of As into GaN reduces the bond angle disorder. According to our ring analysis the 250atom a-GaN1−xAsx model has a disordered tetrahedral characteristic confirming the fact that continuous random network (CRN) can provide an ideal initial structure. The study of EDOS and IPR proves that the bandgap of a-GaN1−xAsx gets narrower with increasing As concentration, which is in good agreement with the experimental results and the band anti-crossing model.

Molecular beam epitaxy of highly mismatched N-rich GaN1−xSbx and InN1−xAsx alloys

GaN materials alloyed with group V anions form the so-called highly mismatched alloys (HMAs). Recently, the authors succeeded in growing N-rich GaN1−xAsx and GaN1−xBix alloys over a large composition range by plasma-assisted molecular beam epitaxy (PA-MBE). Here, they present first results on PA-MBE growth and properties of N-rich GaN1−xSbx and InN1−xAsx alloys and compare these with GaN1−xAsx and GaN1−xBix alloys. The enhanced incorporation of As and Sb was achieved by growing the layers at extremely low growth temperatures. Although layers become amorphous for high As, Sb, and Bi content, optical absorption measurements show a progressive shift of the optical absorption edge to lower energy. The large band gap range and controllable conduction and valence band positions of these HMAs make them promising materials for efficient solar energy conversion devices.

Pressure-induced variation of effective dynamic charge in InP1−xAsx alloys due to charge transfer within cation sublattice

From the measurements of Raman spectra of ternary InP1−xAsx (x=0.4, 0.6, and 0.8) under high pressure, the pressure-dependence of the ionicity was found to be strongly affected by the As mole fraction x. In particular, for x=0.6 and 0.8 ternary alloys, the effective dynamic charge of InP-like mode strictly increases with pressure, in sharp contrast to the corresponding behavior satisfied by the majority of III–V and II–VI binary compounds including InP. This can be understood by the charge transfer from As to P in the cation sublattice when the ternary alloys are under pressure.

Thermal stability of amorphous GaN1−xAsx alloys

GaN 1 − x As x alloys grown across the composition range by low temperature molecular beam epitaxy have great technological potential for photovoltaic applications owing to their strong absorption coefficient and wide tunability of band gap and band edges. We found that amorphous GaN1−xAsx alloys that are formed for the compositions x, in the range of x∼0.3–0.7 are stable up to 700 °C. This is surprising since growth of GaN1−xAsx above 400 °C results in phase segregation. At annealing temperatures higher than 700 °C the alloy phase segregates into GaAs:N and GaN:As. The relative size of the nanocrystals depends on the initial film composition and annealing conditions.

Low gap amorphous GaN1−xAsx alloys grown on glass substrate

Amorphous GaN1−xAsx layers with As content in the range of x=0.1 to 0.6 were grown by molecular beam epitaxy on Pyrex glass substrate. These alloys exhibit a wide range of band gap values from 2.2 to 1.3 eV. We found that the density of the amorphous films is ∼0.8–0.85 of their corresponding crystalline value. These amorphous films have smooth morphology, homogeneous composition, and sharp well defined optical absorption edges. The measured band gap values for the crystalline and amorphous GaN1−xAsx alloys are in excellent agreement with the predictions of the band anticrossing model. The high absorption coefficient of ∼105 cm−1 for the amorphous GaN1−xAsx films suggests that relatively thin films (on the order of 1 μm) are necessary for photovoltaic application.

Molecular beam epitaxy of GaNAs alloys with high As content for potential photoanode applications in hydrogen production

The authors have succeeded in growing GaN1−xAsx alloys over a large composition range (0&amp;lt;x&amp;lt;0.8) by plasma-assisted molecular beam epitaxy. The enhanced incorporation of As was achieved by growing the films with high As2 flux at low (as low as 100 °C) growth temperatures, which is much below the normal GaN growth temperature range. Using x-ray and transmission electron microscopy, they found that the GaNAs alloys with high As content x&amp;gt;0.17 are amorphous. Optical absorption measurements together with x-ray absorption and emission spectroscopy results reveal a continuous gradual decrease in band gap from ∼3.4 to &amp;lt;1 eV with increasing As content. The energy gap reaches its minimum of ∼0.8 eV at x∼0.8. The composition dependence of the band gap of the crystalline GaN1−xAsx alloys follows the prediction of the band anticrossing model (BAC). However, our measured band gap of amorphous GaN1−xAsx with 0.3&amp;lt;x&amp;lt;0.8 are larger than that predicted by BAC. The results seem to indicate that for this composition range the amorphous GaN1−xAsx alloys have short-range ordering that resembles random crystalline GaN1−xAsx alloys. They have demonstrated the possibility of the growth of amorphous GaN1−xAsx layers with variable As content on glass substrates.

Highly mismatched crystalline and amorphous GaN1−xAsx alloys in the whole composition range

Alloying is a commonly accepted method to tailor properties of semiconductor materials for specific applications. Only a limited number of semiconductor alloys can be easily synthesized in the full composition range. Such alloys are, in general, formed of component elements that are well matched in terms of ionicity, atom size, and electronegativity. In contrast there is a broad class of potential semiconductor alloys formed of component materials with distinctly different properties. In most instances these mismatched alloys are immiscible under standard growth conditions. Here we report on the properties of GaN1−xAsx, a highly mismatched, immiscible alloy system that was successfully synthesized in the whole composition range using a nonequilibrium low temperature molecular beam epitaxy technique. The alloys are amorphous in the composition range of 0.17&amp;lt;x&amp;lt;0.75 and crystalline outside this region. The amorphous films have smooth morphology, homogeneous composition, and sharp, well defined optical absorption edges. The band gap energy varies in a broad energy range from ∼3.4 eV in GaN to ∼0.8 eV at x∼0.85. The reduction in the band gap can be attributed primarily to the downward movement of the conduction band for alloys with x&amp;gt;0.2, and to the upward movement of the valence band for alloys with x&amp;lt;0.2. The unique features of the band structure offer an opportunity of using GaN1−xAsx alloys for various types of solar power conversion devices.

Structural and optical properties of GaN1−xAsx grown by metalorganic chemical vapor deposition

GaN-based GaN1−xAsx alloys, x⩽0.043, were grown by metalorganic chemical vapor deposition. The results of high-resolution x-ray diffraction measurements indicated that no phase separation was observed over a range of up to x=0.043 and both the lattice constants c and a increased with an increase in arsenic composition. In cathodoluminescence measurements, the GaN1−xAsx showed a large band gap bowing parameter of 22.1eV. Furthermore, we have experimentally revealed that the valence band offset for the GaN1−xAsx∕GaN is very large, and we have proposed the band gap energy diagram of GaN1−xAsx utilizing an arsenic deep donor level.

The Transition from Blue Emission in As-Doped GaN to GaNAs Alloys in Layers Grown by Molecular Beam Epitaxy

The transition from As-doped GaN showing strong blue emission (∼2.6 eV) at room temperature to the formation of GaN1—xAsx alloys for films grown by molecular beam epitaxy was investigated. This study demonstrates that with increasing N to Ga ratio there is first an increase in the intensity of blue emission at about 2.6 eV and then a transition to the growth of GaN1—xAsx alloy films. Several possible models, which can explain how this might occur are presented.

Thermoelectric Properties of SmSl−xAsx Alloys

ABSTRACTThe electrical resistivities and the Seebeck coefficients of SmS1−xAsx alloys (0 ≤ x ≤ 1) have been measured between room temperature and 1273 K as part of a continuing search for high temperature materials having a high thermoelectric figure of merit and suitable for space power applications. SmS is an n-type semiconductor with an energy gap measured to be 0.16 eV. SmAs and SmS0.2As0.8 are itinerant electron conductors. Hysteresis between high and low resistivity states has been observed in our SmS0.9As0.1 and SmS0.8As0.2 samples. Alloys with x ≤ 0.05 appear to be the most promising high temperature thermoelectric materials.