AlN, GaN, AlxGa1 − xN nanotubes and GaN/AlxGa1 − xN nanotube heterojunctions

Semiconductor optoelectronic devices based on GaN and on InGaN or AlGaN alloys and superlattices can operate in a wide range of wavelengths, from far infrared to near ultraviolet region. The efficiency of these devices could be enhanced by shrinking the size and increasing the density of the semiconductor components. Nanostructured materials are natural candidates to fulfill these requirements. Here we use the density functional theory to study the electronic and structural properties of (10,0) GaN, AlN, Al_xGa_1 − xN nanotubes and GaN/Al_xGa_1 − xN heterojunctions, 0<x<1. The Al_xGa_1 − xN nanotubes exhibit direct band gaps for the whole range of Al compositions, with band gaps varying from 3.45 to 4.85 eV, and a negative band gap bowing coefficient of −0.14 eV. The GaN/Al_xGa_1 − xN nanotube heterojunctions show a type-I band alignment, with the valence band offsets showing a non-linear dependence with the Al content in the nanotube alloy. The results show the possibility of engineering the band gaps and band offsets of these III-nitrides nanotubes by alloying on the cation sites.     

Electronic structures of zigzag AlN, GaN nanoribbons and AlxGa1−xN nanoribbon heterojunctions: First-principles study

The electronic and structural properties of zigzag aluminum nitride (AlN), gallium nitride (GaN) nanoribbons and Al_xGa_1−xN nanoribbon heterojunctions are investigated using the first-principles calculations. Both AlN and GaN ribbons are found to be semiconductor with an indirect band gap, which decreases monotonically with the increased ribbon width, and approaching to the gaps of their infinite two dimensional graphitic-like monolayer structures, respectively. Furthermore, the band gap of Al_xGa_1−xN nanoribbon heterojunctions is closely related to Al (and/or Ga) concentrations. The Al_xGa_1−xN nanoribbon of width n=8 shows a continuously band gap varying from about 2.2 eV-3.1 eV as x increases from 0 to 1. The large ranged tunable band gaps in such a quasi one dimension structure may open up new opportunities for these AlN/GaN based materials in future optoelectronic devices.     

Effect of Al content on the properties of Cr1−xAlxC films prepared by RF reactive magnetron sputtering

Cr_1−xAl_xC films were deposited on high-speed steel by RF reactive magnetron sputtering. In this study, we aimed to identify the effect of the Al content on the properties of Cr_1−xAl_xC films. We found that Cr_1−xAl_xC films exhibited a fine columnar grain microstructure with some special characteristics, such as high hardness of Hv 1426, a low friction coefficient of 0.29, and a large contact angle of 90° for x = 0.18. Furthermore, an increase in Al content resulted in a decrease in film hardness and an increase in contact angle. Moreover, on annealing at 923 K, the mechanical properties of the films improved and a dense protective film of complex Cr₂O₃ and Al₂O₃ oxides was formed on the surface for better wear resistance, which will ultimately increase the lifetime of the high-speed steel substrate.     

Experimental investigation of long-wavelength optical lattice vibrations in quaternary AlxInyGa1−x−yN alloys and comparison with results from the pseudo-unit cell model

Raman and Fourier transform infrared (FTIR) spectroscopies have been utilized to measure long-wavelength optical lattice vibrations of high-quality quaternary Al_xIn_yGa_1−x−yN thin films at room temperature. The Al_xIn_yGa_1−x−yN films were grown on c-plane (0 0 0 1) sapphire substrates with AlN as buffer layers using plasma assisted molecular beam epitaxy (PA-MBE) technique with aluminum (Al) mole fraction x ranging from 0.0 to 0.2 and constant indium (In) mole fraction y=0.1. Pseudo unit cell (PUC) model was applied to investigate the phonons frequency, mode number, static dielectric constant, and high frequency dielectric constant of the Al_xIn_yGa_1−x−yN mixed crystals. The theoretical results were compared with the experimental results obtained from the quaternary samples by using Raman and FTIR spectroscopies. The experimental results indicated that the Al_xIn_yGa_1−x−yN alloy had two-mode behavior, which includes A₁(LO), E₁(TO), and E₂(H). Thus, these results are in agreement with the theoretical results of PUC model, which also revealed a two-mode behavior for the quaternary nitride. We also obtained new values of E₁(TO) and E₂(H) for the quaternary nitride samples that have not yet been reported in the literature.     

The influence of chemical treatment and thermal annealing on AlxGa1−xN surfaces: An XPS study

The influences of chemical treatment and thermal annealing of Al_xGa_1−xN (x = 0.20) have been investigated by X-ray photoelectron spectroscopy (XPS). XPS analysis showed that successive chemical treatments and annealing produced changes in the stoichiometry of the Al_xGa_1−xN surface, with the surface concentration of N increasing and Al and Ga decreasing with increasing temperature. Band bending occurred at the Al_xGa_1−xN surface, in parallel with the observed changes in stoichiometry. These results are discussed in the context of the creation of surface states via the activation of vacancies and induced by defects. These findings point towards the possibility of selecting and/or engineering the band structure at Al_xGa_1−xN surfaces through a combination of surface preparation and annealing.     

Theoretical research on optical properties and quantum efficiency of Ga1−xAlxN photocathodes

In order to design the optimal component structure of transmission-mode (t-mode) Ga_1−xAl_xN photocathode, the optical properties and quantum efficiency of Ga_1−xAl_xN photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga_1−xAl_xN photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga_1−xAl_xN photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga_1−xAl_xN layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga_1−xAl_xN photocathodes.     

Effect of quenching wheel speed on the structure, magnetic properties and magnetoimpedance effect in Co64Fe4Ni2B19−xSi8Cr3Alx (x=0, 1 and 2) melt-spun ribbons

Melt-spun ribbons of Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al_x (x=0, 1 and 2) alloy at wheel speed of 25 m/s and Co₆₄Fe₄Ni₂B_19−xSi₈Cr₃Al₁ alloy at different wheel speeds (25, 30, 35 and 40 m/s) have been prepared and investigated for structural and magnetic properties and magnetoimpedance effect. Based on the results obtained, it was shown that replacement of B by Al can improve the magnetoimpedance ratio (MIR) and the highest value of MIR (191%) was observed for the sample with X_Al=1 at wheel speed of 25 m/s. Further, it was seen that the higher quenching wheel speed gives rise to a higher coercivity and lower magnetic permeability/MIR value.     

Magnetotransport, optical, and electronic subband properties in AlxGa1−xN/AlN/GaN heterostructures

The Shubnikov-de Haas (S-dH) results at 1.5 K for Al_xGa_1−xN/AlN/GaN heterostructures and the fast Fourier transformation data for the S-dH data indicated the occupation by a two-dimensional electron gas (2DEG) of one subband in the GaN active layer. Photoluminescence (PL) spectra showed a broad PL emission about 30 meV below the GaN exciton emission peak at 3.474 eV that could be attributed to recombination between the 2DEG occupying in the AlN/GaN heterointerface and photoexcited holes. A possible subband structure was calculated by a self-consistent method taking into account the spontaneous and piezoelectric polarizations, and one subband was occupied by 2DEG below the Fermi level, which was in reasonable agreement with the S-dH results. These results can help improve understanding of magnetotransport, optical, and electronic subband properties in Al_xGa_1−xAs/AlN/GaN heterostructures.     

The role of strain in controlling the surface morphology of AlxGa1−xN following in situ treatment with SiH4 and NH3

Treatment of GaN with SiH₄ and NH₃ increases the size of surface pits associated with threading dislocations, allowing them to be easily imaged by atomic force microscopy. Here, we assess the effect of a similar treatment on Al_xGa_1−xN surfaces for x ≤ 0.4. For relaxed Al_xGa_1−xN epilayers, an increase in the observed size and density of threading dislocation pits is observed. However, if the Al_xGa_1−xN is under tensile strain, the treatment results in the appearance of nanometre-scale surface hillocks. These hillocks may prevent observation of the dislocation pits. The hillocks are found to consist of crystalline Al_xGa_1−xN, and hence are suggested to be formed by strain driven etching or transformation of the surface by SiH₄ and NH₃.     

A study of TaxC1−x coatings deposited on biomedical 316L stainless steel by radio-frequency magnetron sputtering

In this paper, Ta_xC_1−x coatings were deposited on 316L stainless steel (316L SS) by radio-frequency (RF) magnetron sputtering at various substrate temperatures (T_s) in order to improve its corrosion resistance and hemocompatibility. XRD results indicated that T_s could significantly change the microstructure of Ta_xC_1−x coatings. When T_s was <150 °C, the Ta_xC_1−x coatings were in amorphous condition, whereas when T_s was ≥150 °C, TaC phase was formed, exhibiting in the form of particulates with the crystallite sizes of about 15-25 nm (T_s = 300 °C). Atomic force microscope (AFM) results showed that with the increase of T_s, the root-mean-square (RMS) values of the Ta_xC_1−x coatings decreased. The nano-indentation experiments indicated that the Ta_xC_1−x coating deposited at 300 °C had a higher hardness and modulus. The scratch test results demonstrated that Ta_xC_1−x coatings deposited above 150 °C exhibited good adhesion performance. Tribology tests results demonstrated that Ta_xC_1−x coatings exhibited excellent wear resistance. The results of potentiodynamic polarization showed that the corrosion resistance of the 316L SS was improved significantly because of the deposited Ta_xC_1−x coatings. The platelet adhesion test results indicated that the Ta_xC_1−x coatings deposited at T_s of 150 °C and 300 °C possessed better hemocompatibility than the coating deposited at T_s of 25 °C. Additionally, the hemocompatibility of the Ta_xC_1−x coating on the 316L SS was found to be influenced by its surface roughness, hydrophilicity and the surface energy.     

First principles investigation of optical properties of zinc-blende AlxGa1−xAs1−yNy materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al_xGa_1−xAs_1−yN_y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al_xGa_1−xAs and Al_xGa_1−xAs_1−yN_y with x≤0.375 and y up to 4% are presented. Al_xGa_1−xAs on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al_xGa_1−xAs has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C_MN=2.32 and a N level of E_N=(1.625+0.069x) eV. The results show that Al_xGa_1−xAs can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al_xGa_1−xAs_1−yN_y could be an alternative to Al_xGa_1−xAs when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.     

Half-metallic antiferromagnetism in the ordered Cr1 − xCaxSb alloy from first-principles calculations

Compared to half-metallic ferromagnets, half-metallic antiferromagnets (precisely called half-metallic fully compensated ferrimagnets) are more promising candidates for spintronic applications since their zero magnetization leads to lower stray fields and thus tiny energy losses. Using the first-principles calculations, we have systematically investigated the electronic and magnetic properties of the ordered Cr_1 − xCa_xSb alloy. It is found that Cr_1 − xCa_xSb with x=0.125, 0.25, 0.5 and 0.75 all are half-metals like zinc-blende CrSb and CaSb. Interestingly, Cr_0.25Ca_0.75Sb is a half-metallic antiferromagnet with complete spin polarization, and the half-metallic antiferromagnetism is robust against the lattice compression and expansion and the choice of electronic exchange and correlation functional.     

Site preference and magnetism of Fe3−xCrxAl0.5Si0.5

In order to gain better insight into the origin of the observed differences between Fe_3−xCr_xAl and Fe_3−xCr_xSi, alloys of Fe_3−xCr_xAl_0.5Si_0.5 (x=0, 0.125, 0.250, 0.375 and 0.5) were prepared and studied by means of X-ray and neutron diffraction as well as by magnetization measurements. Electronic structure calculations of these alloys have been performed by means of TB-LMTO-ASA method. It was expected, and experimentally verified, that the presence of silicon and aluminum atoms in 1:1 proportion will result in the independence of the lattice parameter on the iron/chromium concentration. All samples have been proved to be a single phase of the DO₃-type of structure. Theoretical and experimental results indicate that chromium atoms locate preferentially in B sublattice. Cr magnetic moments are oriented antiparallel to Fe magnetic moments. Neutron measurements show a linear dependence of the magnetic moments of Fe(A,C), Fe(B) and Cr(B) as a function of Cr concentration. However the calculated total magnetic moment decreases faster with chromium content than indicated by the experiment.     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

Magnetic properties and room-temperature magnetocaloric effect in the doped antipervoskite compounds Ga1−xAlxCMn3 (0≤x≤0.15)

We report the effects of Al doping on the structure, magnetic properties, and magnetocaloric effect of antiperovskite compounds Ga_1−xAl_xCMn₃ (0≤x≤0.15). Partial substitutions of Al for Ga enhance the Curie temperature (from 250 K for x=0.0 to 312 K for x=0.15) and the saturation magnetization. On increasing the doping level x, the maximum values of the magnetic entropy change (−ΔS_M) decreases while the temperature span of −ΔS_M vs. T plot broadens. Furthermore, the relative cooling power (RCP) is also studied. For 20 kOe, the RCP value tends to saturate at a high doping level (for x=0.12, 119 J/kg at 296 K). However, at 45 kOe, the RCP value increases quickly with increasing x (for x=0.15, 293 J/kg at 312 K). Considering the relatively large RCP and inexpensive raw materials, Ga_1−xAl_xCMn₃ may be alternative candidates for room-temperature magnetic refrigeration.     

Dopings in the transition-metal aluminides OsAl2 to obtain materials with high spin polarization: A first-principles study

The doping effects of 3d transition metals on the magnetic properties of Os_1 − xCr_xAl₂ are studied by first-principles calculations. The Os_1 − xCr_xAl₂ alloys have high spin-polarizations at the Fermi level with the Cr concentration in the range of 0.0<x?0.625. At higher Cr concentration, the ferromagnetic phase transforms into a ferrimagnetic metallic phase.     

Phase stability,elastic, electronic,thermal and optical properties of Ti3Al1−xSixC2 (0≤x≤1): First principle study

The structural parameters with stability upon Si incorporation and elastic, electronic, thermodynamic and optical properties of Ti₃Al_1−xSi_xC₂ (0≤x≤1) are investigated systematically by the plane wave pseudopotential method based on the density functional theory (DFT). The increase of some elastic parameters with increasing Si-content renders the alloys to possess higher compressive and tensile strength. The Vickers hardness value obtained with the help of Mulliken population analysis increases as x is increased from 0 to 1. The solid solutions considered are all metallic with valence and conduction bands, which have a mainly Ti 3d character, crossing the Fermi level. The temperature and pressure dependences of bulk modulus, normalized volume, specific heats, thermal expansion coefficient, and Debye temperature are all obtained through the quasi-harmonic Debye model with phononic effects for T=0−1000 K and P=0−50 GPa. The obtained results are compared with other results available. Further an analysis of optical functions for two polarization vectors reveals that the reflectivity is high in the visible–ultraviolet region up to ∼10.5 eV region showing promise as a good coating material.     

High-temperature electrical properties of magnesiowustite Mg1 − xFexO and spinel Fe3 − x − yMgxCryO4 ceramics

Phase relationships, thermal expansion and electrical properties of Mg_1 − xFe_xO (x = 0.1-0.45) cubic solid solutions and Fe_{3 − x − y}Mg_xCr_yO_4 ± δ (x = 0.7-0.95; y = 0 or 0.5) spinels were studied at 300-1770 K in the oxygen partial pressure range from 10 Pa to 21 kPa. Increasing iron content enlarges the spinel phase stability domain at reduced oxygen pressures and elevated temperatures. The total conductivity of the spinel ceramics is predominantly n-type electronic and is essentially p(O₂)-independent within the stability domain. The computer simulations using molecular dynamics technique confirmed that overall level of ion diffusion remains low even at high temperatures close to the melting point. Temperature dependencies of the total conductivity in air exhibit a complex behavior associated with changing the dominant defect-chemistry mechanism from prevailing formation of the interstitial cations above 1370-1470 K to the generation of cation vacancies at lower temperatures, and with kinetically frozen cation redistribution in spinel lattice below 700-800 K. The average thermal expansion coefficients of the spinel ceramics calculated from dilatometric data in air vary in the range (9.6-10.0) × 10^− 6 K^− 1 at 300-500 K and (13.2-16.1) × 10^− 6 K^− 1 at 1050-1370 K. Mg_1 − xFe_xO solid solutions undergo partial decomposition on heating under oxidizing and mildly reducing conditions, resulting in the segregation of spinel phase and conductivity decrease.     

Structural, electrical, optical and magnetic properties of Co0.2AlxZn0.8−xO films

Co_0.2Al_xZn_0.8−xO films prepared with different molar ratio of aluminum nitrate to zinc acetate were deposited on substrates by the sol-gel technique. X-ray diffraction, photoluminescence and ferromagnetism measurements were used to characterize the Co_0.2Al_xZn_0.8−xO diluted magnetic semiconductors. The authors found that the intensity of the acceptor-related photoluminescence increased with increasing aluminum concentration and an increase in the number of the acceptor-like defects (zinc vacancies especially) in the Co_0.2Al_xZn_0.8−xO film might lead to the enhancement of the magnetic properties. This implies that controls of the aluminum concentration and the number of the acceptor-like defects are important factors to produce strong ferromagnetism Co_0.2Al_xZn_0.8−xO films prepared by the sol-gel method.     

Photoluminescence investigation of ferromagnetic Ga1−xMnxN layers with GaN templates grown on sapphire (0 0 0 1) substrates

We have investigated the temperature and composition dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x ≈ 0.1-0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. The efficient PL is peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. The band-gap energy of the Ga_1−xMn_xN layers decreased with increasing temperature and manganese composition. The band-gap energy of the Ga_1−xMn_xN layers was modeled by the Varshni equation and the parameters were determined to be α = 2.3 × 10⁻⁴, 2.7 × 10⁻⁴, 3.4 × 10⁻⁴ eV/K and β = 210, 210, and 230 K for the manganese composition x = 0.1%, 0.2%, and 0.8%, respectively. As the Mn concentration in the Ga_1−xMn_xN layers increased, the temperature dependence of the band-gap energy was clearly reduced.