Al Composition Range Research Articles

Observation of mixed-mode behavior of Raman active phonon modes for β-(AlxGa1−x)2O3 alloys

Raman spectroscopy has been used to investigate the phonon mode behavior of a β-(AlxGa1−x)2O3 alloy up to Al compositions of x = 0.474. The alloy displays a mixed-mode behavior, where low frequency (100–200 cm−1) modes corresponding to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains and high frequency (500–800 cm−1) modes related to stretching and bending of (Ga1O4) tetrahedral chains show a monotonic behavior throughout the studied composition range, and display one-mode behavior. On the other hand, medium frequency (300–500 cm−1) phonon modes related to the deformation of (Ga2O6) octahedra and (Ga1O4) tetrahedra chains show sudden change such as the appearance (disappearance) of new (existing) phonon modes corresponding to Al–O (Ga–O) sublattices with Al substitution, and they display a two-mode behavior. The existence of two sublattices is further supported by the observation of preferential occupation of the Al atom at the octahedral atomic site in comparison to the tetrahedral atomic site. The observation of phonon modes related to translation and libration of (Ga1O4) tetrahedral and (Ga2O6) octahedral chains up to the Al composition of x = 0.474 clearly indicates that the long-range periodicity of the monoclinic lattice is maintained throughout the investigated Al composition range. Our results provide an insight into the phonon mode behavior of β-(AlxGa1−x)2O3 alloys, which is necessary to have a better understanding of their physical properties.

Thermal conductivity of AlxGa1−xN (0≤x≤1) epitaxial layers

${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ ternary alloys are emerging ultrawide band gap semiconductor materials for high-power electronics applications. The heat dissipation, which mainly depends on the thermal conductivity of the constituent material in the device structures, is the key for device performance and reliability. However, the reports on the thermal conductivity of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ alloys are very limited. Here, we present a comprehensive study of the thermal conductivity of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ in the entire Al composition range. Thick ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ layers grown by metal-organic chemical vapor deposition on GaN/sapphire and GaN/SiC templates are examined. The thermal conductivity measurements are done by the transient thermoreflectance method at room temperature. The effects of the Al composition, dislocation density, Si doping, and layer thickness on the thermal conductivity of ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ layers are thoroughly investigated. All experimental data are fitted by the modified Callaway model within the virtual crystal approximation, and the interplay between the different phonon scattering mechanisms is analyzed and discussed.

Enhanced optical absorption of AlxGa1-xAs nanoarrays with variable Al composition structure for solar cell applications

In this study, various variable Al composition nanoarray structures are proposed and designed to achieve high-efficient light capture. The COMSOL Multiphysics package based on finite element method is utilized to investigate the optical absorption performance of cylinder, circular frustum and inverted circular frustum nanoarray structures. The effects of different geometric diameters, sub-layer thickness distribution and Al composition range on the optical properties of various nanostructures are also analyzed in detail. The results show that the designed cylinder structure can obtain optical absorption enhancement of more than 5 % and 3 % compared with the circular frustum and inverted circular frustum structure. Besides, the reasonable design of sub-layer thickness distribution can be selected to allow different nanostructures to obtain effective optical absorption enhancement performance. In addition, the larger Al composition range distribution design can effectively improve the absorption for short-wavelength photons. The design principles proposed in this work will provide a reference for selecting appropriate parameters for solar cell applications.

Direct observation of site-specific dopant substitution in Si doped (Al x Ga1− x )2O3 via atom probe tomography

In this work, the interaction of n-type dopants in Si doped (Al x Ga1−x )2O3 films with varying Al content over the entire composition range (x = 0%–100%) was analyzed using atom probe tomography. An almost uniform dopant distribution with dopant density in the range of 1018 cm−3 was obtained in (Al x Ga1−x )2O3 layers with Al contents, x < 0.60. At x , small Si segregated zones were observed which was attributed to the increased donor-acceptor transition centers or bond length differences in Al–O, Ga–O and Si–O at high Al content. We have demonstrated that for the single phase β-(Al x Ga1−x )2O3 films with Al content of x < 0.30, dopants prefer to occupy on Ga sites while Al site is preferred for high Al content (x > 0.50) (Al x Ga1−x )2O3 layers. It was also observed for Al content, x = 0.30–0.50, no specific cationic site occupancy was observed, Si occupies either Al or Ga sites. This can be attributed to highly inhomogeneous layers within this Al composition range due to which dopant Si atoms are either in the Al-rich or Al-depleted regions.

Phase transformation in MOCVD growth of (AlxGa1−x)2O3 thin films

This paper investigated the growth of (AlxGa1−x)2O3 thin films on semi-insulating (010) Ga2O3 substrates over the entire Al composition range (0% &amp;lt; x ≤ 100%) via metalorganic chemical vapor deposition (MOCVD). For the Al composition x &amp;lt; 27%, high quality single phase β-(AlxGa1−x)2O3 was achieved. A mixture of β and γ phases existed in (AlxGa1−x)2O3 when Al composition ranged between 27% and 40%, whereas a single γ-phase was observed for the films with Al composition x &amp;gt; 40%. The transition from the β to γ phase in AlGaO alloys was observed from x-ray diffraction spectra. The growth of γ-phase AlGaO with higher Al content was further confirmed via atomic resolution scanning transmission electron microscopy imaging and nanodiffraction. Compositional and statistical analyses performed on data acquired from atom probe tomography provided insight on the local compositional homogeneity in AlGaO films with different Al compositions. For AlGaO with pure β or γ phases, the Al composition distribution showed homogeneity with similar Al composition values as extracted from the x-ray diffraction peak positions. For AlGaO films with mixed β and γ phases, inhomogeneity in the Al composition distribution became more obvious in the nm scale. A mechanism was proposed for the observed phase transformation between β and γ phases in MOCVD growth of AlGaO films.

Exploration of the microstructure space in TiAlZrN ultra-hard nanostructured coatings

Ti1−x−yAlxZryN cubic alloys within the 25–70% Al composition range have high age-hardening capabilities due to metastable phase transition pathways at high temperatures. They are thus ideal candidates for ultra-hard nano-coating materials. There is growing evidence that this effect is associated with the elasto-chemical field-induced phase separation into compositionally-segregated nanocrystalline nitride phases. Here, we studied the microstructural evolution in this pseudo-ternary system within spinodal regions at 1200 °C by using an elasto-chemical phase field model. Our simulations indicate that elastic interactions between nitride nano-domains greatly affect not only the morphology of the microstructure but also the local chemical phase equilibria. In Al-rich regions of the composition space we further observe the onset of the transformation of AlN-rich phases into their equilibrium wurtzite crystal structure. This work points to a wide palette of microstructures potentially accessible to these nitride systems and their tailoring is likely to result in significant improvements in the performance of transition metal nitride-based coating materials.

High-performance GaAs nanowire cathode for photon-enhanced thermionic emission solar converters

GaAs nanowire cathodes with exponential doping and graded Al composition structures are proposed for photon-enhanced thermionic emission (PETE) devices. The conversion efficiency models with these two nanowire cathodes are deduced on the basis of one-dimensional continuity equations. The conversion efficiency as a function of wire length, wire width, Al composition distribution, cathode temperature, emissive surface and back interface recombination velocity are also simulated, respectively. Results show that exponential doping and graded Al composition cathode structures can obviously improve the conversion efficiency of devices through introducing a built-in electric field along the growth direction of nanowire. Besides, the optimum wire length and wire width range are 300–340 nm and 5.9–6.4 μm, respectively. Moreover, wider Al composition range is beneficial to achieve higher conversion efficiency. These simulations provide an interesting attempt to explore the working mechanism of GaAs nano-based PETE devices and are expected to be verified by the experimental results in the future.

Exploration of the Microstructure Space in Tialzrn Ultra-Hard Nanostructured Coatings

Ti1-x-yAlxZryN cubic alloys within the 25-70% Al composition range have high age-hardening capabilities due to metastable phase transition pathways at high temperatures. They are thus ideal candidates for ultra-hard nano-coating materials. There is growing evidence that this effect is associated with the elasto-chemical field-induced phase separation into compositionally-segregated nanocrystaline nitride phases. Here, we studied the microstructural evolution in this pseudo-ternary system within spinodal regions at 1200 °C by using an elasto-chemical phase field model. Our simulations indicate that elastic interactions between nitride nano-domains greatly affect not only the morphology of the microstructure but also the local chemical phase equilibria. In Al-rich regions of the composition space we further observe the onset of the transformation of AlN-rich phases into their equilibrium wurtzite crystal structure. This work points to a wide palette of microstructures potentially accessible to these nitride systems and their tailoring is likely to result in significant improvements in the performance of transition metal nitride-based coating materials.

Influence of the aluminum content on structure and optical properties of Zr1-xAlxN films

The Zr1-xAlxN films have been deposited on Si (111) and glass (soda lime glass) substrates by ion beam assisted deposition. The microstructure, morphology, electrical resistivity and optical properties of the Zr1-xAlxN films were investigated by XRD, TEM, four-probe method and spectroscopic ellipsometry. In the Al composition range 0.32 ≤ x ≤ 0.7, the Zr1-xAlxN films contain crystallized ZrAlN (NaCl type). The deposited Zr0.2Al0.8N film is amorphous. The aluminum content increase in the Zr1-xAlxN films induces the film structure tends to amorphization. The refractive index of the Zr1-xAlxN films increases with Al content increase, whereas, a continuous decrease of the extinction coefficient was noticed. As the Al content increase, the electrical resistivity of films increases. The addition of Al into ZrN-based films leads to the loss of the metallic character.

Dynamics of graded-composition and graded-doping semiconductor nanowires under local carrier modulation.

Scanning photocurrent microscopy is a powerful tool for investigating charge transfer and internal fields, which strongly influence carrier statics and dynamics in semiconductor nanowires. We performed comprehensive numerical modeling of the carrier dynamics of graded-composition and graded-doping AlGaAs nanowires to achieve a greater understanding of these nanowires. The simulation results indicated that the built-in electric field changes the shape of the scanning photocurrent microscopy profiles, which helped us to judge the dopant level, Al composition range and doping type of the material. The simulation results also assess the potential of the scanning photocurrent techniques in graded-doping and graded-composition nanowire properties.

Photoemission characteristics of graded band-gap AlGaAs/GaAs wire photocathode

A photoemission model of graded band-gap AlGaAs/GaAs wire NEA photocathode is developed based on the numerical solution of coupled Poisson and continuity equations. The emission current density and integral sensitivity of graded band-gap AlGaAs/GaAs wire photocathode as a function of incident light wavelength, Al composition range, and wire length, are simulated according to the model. The simulation results show that, compared with the GaAs (Al composition 0) wire photocathode, the peak integral sensitivities for the photocathodes with wire width of 1µm and linearly graded Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4 increase by 29.5%, 38.5%, 42.1%, and 43.8%, respectively. The optimum wire lengths are 4.7, 5.9, 7.1, and 8.4µm for the wire photocathodes with Al composition ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

Effects of graded band-gap structures on spectral response of AlGaAs/GaAs photocathodes.

The effects of AlGaAs/GaAs layer thickness and Al composition range on the spectral response and integral sensitivities of reflection-mode graded band-gap AlGaAs/GaAs photocathodes have been investigated and simulated. The experimental results demonstrate that the spectral response over the wavelength region of interest for graded band-gap photocathodes is greater than that for uniform band-gap cathodes, and the increase in long-wavelength response is more pronounced. These results can be attributed to the built-in electric field in the graded band-gap AlGaAs layer. We established a spectral response model of graded band-gap photocathodes based on the numerical solution of coupled Poisson and continuity equations. According to the model, we calculated the theoretical spectral response and sensitivities of graded band-gap cathodes, and found the optimum Al(x)Ga(1-x)As layer thicknesses are 6, 10, 16, and 22 μm for the reflection-mode cathodes with linearly graded Al composition x ranges of 0 to 0.1, 0.2, 0.3, and 0.4, respectively.

Aluminum Deoxidation Equilibria in Liquid Iron: Part I. Experimental

In order to provide accurate information for refining of steel containing more than 1 mass pct Al, previously known information about Al deoxidation equilibria in liquid iron was critically reviewed. New Al deoxidation equilibria, Al and O contents in liquid iron in equilibrium with solid Al2O3 were measured at 1873 K and 1923 K (1600 °C and 1650 °C) over the whole Al composition range, 0.0027 < [pct Al] < 100. In order to secure the deoxidation equilibria, in the present study, the Al deoxidation experiments were carried out by employing three different methods: (1) traditional Al deoxidation by the addition of Al into Fe-O alloys, (2) oxidation of Al in Fe-Al alloys by the addition of Fe2O3 as an oxygen source, and (3) addition of CaO flux for an effective removal of suspended Al2O3 inclusions in liquid alloys containing high Al. In addition, in the present study, the O solubility limit in pure Al melt in equilibrium with solid Al2O3 was also measured in the temperature range from 1673 K to 1873 K (1400 °C to 1600 °C). The present experimental results provide a complete set of Al deoxidation equilibria in liquid iron which may be useful for the estimation of residual oxygen level and alumina inclusion formation in high Al steel processing. Interaction parameter formalism, which was originally proposed by Wagner and Chipman and has been widely used to interpret the Al deoxidation equilibria in liquid iron, was found to be inapplicable. Limitation of the interaction parameter formalism at high Al content in liquid Fe was discussed.

Inductively coupled plasma–reactive ion etching of c- and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry

Inductively coupled plasma (ICP)–reactive ion etching (RIE) patterning is a standard processing step for UV and optical photonic devices based on III-nitride materials. There is little research on ICP-RIE of high Al-content AlGaN alloys and for nonpolar nitride orientations. The authors present a comprehensive study of the ICP-RIE of c- and a-plane AlGaN in Cl2/Ar plasma over the entire Al composition range. The authors find that the etch rate decreases in general with increasing Al content, with different behavior for c- and a-plane AlGaN. They also study the effect of BCl3 deoxidizing plasma pretreatment. An ICP deoxidizing BCl3 plasma with the addition of argon is more efficient in removal of surface oxides from AlxGa1−xN than RIE alone. These experiments show that AlxGa1−xN etching is affected by the higher binding energy of AlN and the higher affinity of oxygen to aluminum compared to gallium, with oxides on a-plane AlGaN more difficult to etch as compared to oxides on c-plane AlGaN, specifically for high Al composition materials. The authors achieve reasonably high etch rate (∼350 nm/min) for high Al-content materials with a smooth surface morphology at a low DC bias of ∼−45 VDC.

Theoretical studies of surface phonon polariton in wurtzite AlInN ternary alloy

In this study, we report the surface phonon polariton (SPP) characteristics of wurtzite structure aluminium indium nitride (AlxIn1−xN) ternary alloys over the whole Al composition range. An anisotropic model is used to simulate the surface polariton (SP) dispersion curves of the AlxIn1−xN ternary alloys. The characteristics of these dispersion curves are discussed in detail and the effects of the composition dependence of the AlxIn1−xN on the SPs are illustrated and explained. Moreover, the relevant experimental information from the attenuated total reflection (ATR) method is also presented, namely, the corresponding ATR spectra are simulated based on the standard matrix formulation. Through this study, it has been found that the SPP mode of the wurtzite AlxIn1−xN exhibits mixed-mode behaviour.

High-resolution X-ray diffraction investigations of the microstructure of MOVPE grown a-plane AlGaN epilayers

We report a comprehensive characterization of the microstructure of non-polar ( 1 1 2 ¯ 0 ) a-plane Al x Ga 1− x N epilayers on ( 1 1 ¯ 0 2 ) r-plane sapphire substrates over the entire Al composition range using high-resolution X-ray diffraction. All a-plane AlGaN epilayers show an anisotropic in-plane mosaicity which is strongly influenced by Al incorporation and growth conditions. We use data from symmetric and skew-symmetric reflections combined with Williamson–Hall analysis to estimate the mosaic parameters and follow trends across the Al composition. We also analyze the dependence of the microstructure on thickness for a set of Al 0.5Ga 0.5N epilayers.

Electrical and optical properties of Fe doped AlGaN grown by molecular beam epitaxy

Electrical and optical properties of AlGaN grown by molecular beam epitaxy were studied in the Al composition range 15%–45%. Undoped films were semi-insulating, with the Fermi level pinned near Ec-0.6–0.7 eV. Si doping to (5–7)×1017 cm−3 rendered the 15% Al films conducting n-type, but a large portion of the donors were relatively deep (activation energy 95 meV), with a 0.15 eV barrier for capture of electrons giving rise to strong persistent photoconductivity (PPC) effects. The optical threshold of this effect was ∼1 eV. Doping with Fe to a concentration of ∼1017 cm−3 led to decrease in concentration of uncompensated donors, suggesting compensation by Fe acceptors. Addition of Fe strongly suppressed the formation of PPC-active centers in favor of ordinary shallow donors. For higher Al compositions, Si doping of (5–7)×1017 cm−3 did not lead to n-type conductivity. Fe doping shifted the bandedge luminescence by 25–50 meV depending on Al composition. The dominant defect band in microcathodoluminescence spectra was the blue band near 3 eV, with the energy weakly dependent on composition.

Growth and Characteristics of Epitaxial AlxGa1−xN by MOCVD

AlxGa1-xN epilayers with a wide Al composition range (0.2 ≤ x ≤ 0.68) were grown on AlN/sapphire templates by low-pressure metalorganic chemical vapour deposition (LP-MOCVD). X-ray diffraction results reveal that both the (0002) and (101̄5) full widths at half-maximum (FWHM) of the AlxGa1−xN epilayer decrease with increasing Al composition due to the smaller lattice mismatch to the AlN template. However, the surface morphology becomes rougher with increasing Al composition due to the weak migration ability of Al atoms. Low temperature photoluminescence (PL) spectra show pronounced near band edge (NBE) emission and the NBE FWHM becomes broader with increasing Al composition mainly caused by alloy disorder. Meanwhile, possible causes of the low energy peaks in the PL spectra are discussed.

Characterization of Al and Mg Alloys from Their X-Ray Emission Bands

The valence states of Mg-Al alloys are compared to those of reference materials (pure Mg and Al metals, and intermetallics). Two methods based on X-ray emission spectroscopy are proposed to determine the phases and their proportion: first, by analyzing the Al valence spectra of the Mg-rich alloys and the Mg valence spectra of the Al-rich alloys; second, by fitting with a linear combination of the reference spectra the Al spectra of the Al-rich alloys and the Mg spectra of the Mg-rich alloys. This enables us to determine that Al and Al3Mg2 are present in the 0-43.9 wt% Al composition range and Mg and Al12Mg17 are present in the 62.5-100 wt% Al composition range. In the 43.9-62.5% Al range, the alloy is single phase and an underestimation of the Al content of the alloy can be estimated from the comparison of the bandwidth of the alloy spectrum to the bandwidths of the reference spectra.

Zr–(Cu,Ag)–Al bulk metallic glasses

In this paper, we report the formation of a series Zr–(Cu,Ag)–Al bulk metallic glasses (BMGs) with diameters at least 20 mm and demonstrate the formation of about 25 g amorphous metallic ingots in a wide Zr–(Cu,Ag)–Al composition range using a conventional arc-melting machine. The origin of high glass-forming ability (GFA) of the Zr–(Cu,Ag)–Al alloy system has been investigated from the structural, thermodynamic and kinetic points of view. The high GFA of the Zr–(Cu,Ag)–Al system is attributed to denser local atomic packing and the smaller difference in Gibbs free energy between amorphous and crystalline phases. The thermal, mechanical and corrosion properties, as well as elastic constants for the newly developed Zr–(Cu,Ag)–Al BMGs, are also presented. These newly developed Ni-free Zr–(Cu,Ag)–Al BMGs exhibit excellent combined properties: strong GFA, high strength, high compressive plasticity, cheap and non-toxic raw materials and biocompatible property, as compared with other BMGs, leading to their potential industrial applications.