xN Solid Solutions Research Articles

Novel boron‐rich aluminum nitride advanced ceramic materials

AbstractAluminum nitride (AlN) and boron nitride (BN) are well‐known ceramic materials with numerous valuable properties, whereas recently there is a growing field of research on the AlN/BN advanced ceramic materials. Here, we present a study on boron‐rich AlN, structural and electronic properties, and structure–property relationship. Several AlxB1−xN solid solutions (x = 1, .875, .75, and .625) have been investigated, and structure optimization has been performed for four different structure types: h‐BN, wurtzite, sphalerite, and rock salt. First‐principles calculations were performed using hybrid B3LYP functional. New modifications and compounds have been predicted as a function of boron concentration in AlN, and especially, interesting phase transitions were found at extreme pressure conditions. Electronic properties and band structures were computed, and the possibility for bandgap tuning has been discovered. The present study, and especially the structure–property relationship, gives new possibilities for bandgap engineering in boron‐rich AlN electroceramic materials.

Plasma enhanced atomic layer deposition of titanium nitride-molybdenum nitride solid solutions

As part of improving the tribological properties of TiN-based coatings, researchers have introduced additional elements to the binary TiN system. Addition of a self-lubricating and oxide-forming substitutional element such as Mo to the rock salt TiN system deposited by sputtering has been widely studied. But, the TiN-MoN solid solution system grown by atomic layer deposition (ALD) is yet to be reported. Our current work is motivated by the need to understand and probe the structure of TixMo1−xN solid solutions with respect to the ALD growth process. In this work, thin films of TixMo1−xN (0 ≤ x ≤ 1) were deposited by plasma enhanced atomic layer deposition (PEALD) at 250 °C. Tetrakis(dimethylamido) titanium, bis(tert-butylimido)bis(dimethylamido) molybdenum, and N2 plasma were used as sources for Ti, Mo, and N, respectively. X-ray diffraction revealed nanocrystalline films with a rock salt crystal structure for all compositions of TixMo1−xN except for MoNx, which consisted of multiple phases with cubic MoN being the dominant phase. The elemental composition determined by x-ray photoelectron spectroscopy deviated from the pulse ratio of TiN:MoN. This study revealed that nearly the whole solid solution of the TiN-MoN system can be accessed by PEALD.

InxAl1 –xN Solid Solutions: Composition Stability Issues

The phase diagrams of the InxAl1 –xN solid solutions and conditions for their growth by magnetron sputtering and molecular beam and metalorganic vapor phase epitaxy are analyzed. The mutual equilibrium solubility in a wide range of compositions of the thick solution layers is near zero. Elastic misfit stresses in the InxAl1 –xN thin layers narrow the unstable immiscibility gap. By optimizing the growth conditions, one can obtain high-quality homogeneous InxAl1 –xN films for fabricating barrier layers in InAlN/GaN high electron mobility transistors.

Theoretical prediction of the electronic and thermodynamic properties of YN‐ZrN solid solutions

In this study, the results of structural parameters, electronic structure, and thermodynamic properties of the ZrxY1–xN solid solutions are presented. The effect of zirconium composition on lattice constant, and bulk modulus shows nonlinear dependence on concentration. Deviations of the lattice constant from Vegard's law and deviations of the bulk modulus from linear concentration dependence were found. Our findings indicate that the ZrxY1–xN solid solutions are metallic for x = 0.25, 0.5, 0.75. The calculated excess mixing enthalpy is positive over the entire zirconium composition range. The positive mixing enthalpies for ZrxY1–xN alloys indicate the existence of miscibility gaps and spinodal decompositions. The effect of temperature on the volume, bulk modulus, Debye temperature, and the heat capacity for ZrxY1–xN alloys were analyzed using the quasi‐harmonic Debye model. Results show that the heat capacity is slightly sensitive to composition as temperature increases. © 2015 Wiley Periodicals, Inc.

First-principles quantum molecular dynamics study of TixZr1−xN(111)/SiNy heterostructures and comparison with experimental results

The heterostructures of five monolayers B1–TixZr1−xN(111), x = 1.0, 0.6, 0.4 and 0.0 (where B1 is a NaCl-type structure) with one monolayer of a Si3N4-like Si2N3 interfacial layer were investigated by means of first-principles quantum molecular dynamics and a structure optimization procedure using the Quantum ESPRESSO code. Slabs consisting of stoichiometric TiN and ZrN and random, as well as segregated, B1–TixZr1−xN(111) solutions were considered. The calculations of the B1–TixZr1−xN solid solutions, as well as of the heterostructures, showed that the pseudo-binary TiN–ZrN system exhibits a miscibility gap. The segregated heterostructures in which Zr atoms surround the SiyNz interface were found to be the most stable. For the Zr-rich heterostructures, the total energy of the random solid solution was lower compared to that of the segregated one, whereas for the Ti-rich heterostructures the opposite tendency was observed. Hard and super hard Zr–Ti–Si–N coatings with thicknesses from 2.8 to 3.5 μm were obtained using a vacuum arc source with high frequency stimulation. The samples were annealed in a vacuum and in air at 1200 °C. Experimental investigations of Zr–Ti–N, Zr–Ti–Si–N and Ti–Si–N coatings with different Zr, Ti and Si concentrations were carried out for comparison with results obtained from TixZr1−xN(111)/SiNy systems. During annealing, the hardness of the best series samples was increased from (39.6 ± 1.4) to 53.6 GPa, which seemed to indicate that a spinodal segregation along grain interfaces was finished. A maximum hardness of 40.8 GPa before and 55 GPa after annealing in air at 500 °C was observed for coatings with a concentration of elements of Si  (7–8) at.%, Ti  22 at.% and Zr  70 at.%.

Ultraviolet photodiodes based on AlGaN solid solutions

We present the characteristics of ultraviolet photodiodes based on Schottky barriers to the Aluminum Gallium Nitride (AlGaN) solid solution. The paper is devoted to the creation of ohmic and rectifying contacts to AlxGa1−xN solid solutions with a high (x > 0.5) proportion of AlN. The most important property of (AlxGa1−xN) materials is their solar blindness: the wide bandgap (Eg = 3.4 eV for GaN; 6.2 eV for AlN) makes these materials insensitive to the visual and IR light. The study of solid solutions with various mole fractions of Al has created solar-blind and see-blind ultraviolet photodetectors.

Special features of the electron energy spectrum of hexagonal GaN and its Y x Ga1–x N (Y ≡ B, Al, In) solid solutions

A computational scheme based on the density functional theory modified for structures with translation symmetry is used to investigate the electron energy spectrum of hexagonal gallium nitride and its Y x Ga1–x N (Y ≡ B, Al, In) solid solutions. Dependences of the position of the chemical potential, band heads, widths of valence bands, and energies corresponding to state density maxima on the substituting atom concentration are discussed.

Polarization, piezoelectric properties, and elastic coefficients of In x Ga1−x N solid solutions from first principles

III-nitrides GaN and InN, and InxGa1−xN solid solutions are polarizable semiconductors that crystallize in the prototypical wurtzite (W) structure. We present here the results of a density functional theory study carried out to determine the spontaneous polarization, piezoelectric coefficients, and elastic coefficients of InxGa1−xN alloys as a function of In the concentration x. To calculate these properties, we construct three distinct hexagonal/orthorhombic equivalent InxGa1−xN supercells that are derived from the disordered W unit cell of GaN and InN. These include an ordered W lattice (P63mc/Pmc21) and orthorhombic O-16 and O-32 lattices with Pmn21/Pna21 or P21 symmetry, respectively. Depending on the crystal structure, spontaneous polarization as a function of the In concentration x shows a downward bowing (W), a linear interpolation (O-16), and an upward bowing (O-32) between −0.033 C/m2 and −0.043 C/m2, the spontaneous polarizations of the end components GaN and InN, respectively. The composition dependence of the effective basal plane and out of plane (along the [0001] direction) piezoelectric coefficients (e// and e33, respectively) in the W and O-16 structure is non-linear and varies between e// = −0.287 C/m2 and e33 = 0.598 C/m2 for GaN, and e// = −0.455 C/m2 and e33 = 1.044 C/m2 for InN. While the bulk modulus of InxGa1−xN in the W and O-16 structures follows Vegard’s law from 170 GPa (x = 0) to 124 GPa (x = 1), in the O-32 structure it shows a strong downward bowing for compositions 0 < x < 0.5.

Structural and Electronic Properties of Zinc Blende-type Nitrides BxAl1–xN

First principles total energy calculations were carried out to investigate structural and electronic properties of zinc blende-type AlN, BN and BxAl1−xN solid solutions. We have calculated the lattice parameters, bulk modulus, pressure derivative, and BxAl1−xN band-gap energy for zinc blende-type crystals of the compositions x = 0, 0.25, 0.5, 0.75, 1. The results show that the direct energy gap Γ15 V →Γ1 c shows a strong nonlinear dependence on the concentration x. For high boron contents (x &gt; 0.71), these materials have a phase transition from direct-gap semiconductors to indirect-gap semiconductors (Γ15 V → X1 c). This essential feature indicates that these materials should have very good optical properties at high concentrations of boron compared to those of AlN. Further discussions concern a comparison of our results with results obtained with other available theoretical and experimental methods.

X-ray spectra and electronic band structure of nitrogen in Al x Ga1−x N solid solid solutions

The electronic energy structure of the valence band and the x-ray absorption near edge structure (XANES) region of nitrogen in AlxGa1−xN solid solutions and binary crystals of gallium nitride GaN and aluminum nitride AlN are calculated using the local coherent potential method and the cluster version of the muffin-tin approximation within the framework of the multiple scattering theory. It is demonstrated that the calculated electron densities of states correlate with the nitrogen K x-ray emission and nitrogen K x-ray absorption spectra. The electronic energy structure of the top of the valence band and the XANES region in AlxGa1−xN solid solutions are compared with those in the binary crystals of the GaN and AlN nitrides, and an interpretation of their specific features is proposed. An analogy is drawn between the evolution of the electronic energy structure of the valence band and the XANES region in the alloys under investigation and the evolution of the electronic band structure in the AlxB1−xN and BxGa1−xN alloys. General trends in the transformation of the structure and variations in properties of these alloys are discussed.

Stress and mechanical properties of Ti–Cr–N gradient coatings deposited by vacuum arc

This paper reports on the investigation of Ti–Cr–N gradient coatings deposited by cathodic arc vapor deposition. The Ti–Cr–N coatings were formed by the method of condensation from a plasma phase in a vacuum with ion bombardment of sample surfaces with combined Ti and Cr plasma flows in a residual nitrogen atmosphere. Auger Electron Spectroscopy (AES) investigations showed that the deposited coatings exhibited compositional gradient: a Cr-rich transition layer was formed during pretreatment of the substrate with Cr ions and a main Ti–Cr–N layer of varying Ti and Cr composition along the coating thickness was obtained by changing the arc current of Ti and Cr cathodes during deposition. Transmission Electron Microscopy (TEM) observations showed that TixCr1−xN solid solutions, with 0.60<x<0.84 and 0.25<x<0.67 were formed, depending on the relative cathode current change. The grains size of coating was ∼ 10 nm.The biaxial stress, determined from X-ray Diffraction using the sin2ψ method, was compressive and varied from −2.2 to −2.7 GPa for gradient coatings grown on silicon and from −4.1 to −4.3 GPa for the samples grown on carbon steel substrates. Gradient coatings exhibited reduced internal stresses in comparison with coatings of constant composition (−3.9 and −5.6 GPa, for samples grown on Si and steel substrate, respectively). It is established that the mechanical properties (hardness and the modulus of elasticity) of gradient coatings are the superposition of the solid solutions TixCr1−xN of different concentration (0.60<x<0.84 and 0.25<x<0.67).

Electronic Energy Structure and X-ray Spectra of Wide-Gap AlN and BN Crystals and B[sub x]Al[sub 1 – ][sub x]N Solid Solutions

The electronic energy structure of 2H and 3C AlN and BN crystals and BxAl1−x N solid solutions is calculated on the basis of the local coherent potential method using the cluster version of the MT approximation and the theory of multiple scattering. The features of the electronic structure of 2H-AlN crystals are compared with x-ray K and L absorption and emission spectra of aluminum and nitrogen. An interpretation of these features is given. The concentration dependences of the width of the upper subband of the valence band and the band gap in BxAl1−x N solid solutions (x = 0.25, 0.5, 0.75) are investigated. Charge transfer from aluminum to nitrogen atoms is shown to occur and increase with boron doping in both crystallographic modifications.

Electronic and vibrational states in InN and InxGa1−x N solid solutions

The review presents the results of optical studies of the fundamental physical characteristics of InN, the material which remains the least studied among nitrides of Group-III elements. The results of early optical studies of InN are analyzed and compared with recent data. New experimental facts reported in the review refer to hexagonal single-crystal epitaxial InN layers with an electron concentration of (1−2)×1018 to 6×1020 cm−3, which are grown by molecular beam epitaxy (MBE) and metal-organic vapor-phase epitaxy (MOVPE) on Al2O3 substrates. The aim of this review is to make a joint analysis of optical spectra (absorption, photoluminescence (PL), PL excitation, and photomodulated reflection) near the fundamental band gap. Furthermore, basic structural and electrical characteristics that have been obtained by a whole range of techniques are given for epitaxial layers of hexagonal InN. The principal result of recent studies is that the hexagonal InN crystal is a narrow-gap semiconductor with a band gap of 0.65–0.7 eV. Previously, the band gap of this material was considered to be 1.89 eV. It is shown that the Burstein-Moss effect accounts for the strong difference between the band gap and the optical absorption threshold in InN samples with a high concentration of electrons. The small value of the band gap of hexagonal InN is confirmed by optical studies of InxGa1−xN solid solutions at high concentrations of In. Theoretical calculations of the band structure of hexagonal InN crystals are briefly reviewed.

Raman spectroscopy of disorder effects in Al xGa 1− xN solid solutions

The results of Raman spectroscopic studies of the disorder effects in hexagonal Al x Ga 1− x N epitaxial layers grown by MBE and HVPE on different substrates for a large range of Al concentrations are presented. The width of the nonpolar phonon line with E 2 symmetry results from the inhomogeneous broadening due to spatial fluctuations in the Al content. The abnormally small broadening of the A 1(TO) polar phonon mode for x or (1− x)≪1 and the large broadening for x≅0.5–0.7 are attributed to the specific frequency dependence of the density of states for the branch with the directional dispersion in pure crystals. Thus the Raman spectrum is found to be highly sensitive to the composition of Al x Ga 1− x N epitaxial layers and its inhomogeneity. It is shown that in the estimation of the crystal composition, on the basis of Raman data, the influence of the homogeneous strain effects could be excluded via measuring a linear combination of two Raman line frequencies.

Raman study of Ga1−xAlxN solid solutions

Long wavelength optical phonons of AlxGa1−xN solid solutions have been identified in the whole compositional range by Raman spectroscopy. The frequencies of A1 and E1 polar phonons increase continuously with x from one-member crystal to the other. A generalization of the dielectric model of Hon and Faust is used to treat the coupling of the longitudinal optic (LO) mode. This approach accounts for the observed frequencies and confirms the so-called one-mode behaviour of polar LO phonons. Moreover,a signature of the coupling of a discrete mode (tentatively associated to silent q=0 B1 mode) with an unidentified continuum has been obtained.

Structure cristallographique et magnetique de Cr 1−xTi xN a basse temperature

Cr 1−xTi xN solid solutions have at room temperature a crystallographic structure of NaCl type. The space group is Fm3m. The compounds with x ⩽ 0.15 present at low temperature an antiferromagnetic order of the fourth kind and show an orthorhombic distorsion. The space group of the deformed phase is Pnmm. The magnetic moment measured by neutron diffraction decreases with increasing titanium concentration. The compounds with x ⩽ 0.20 do not show any magnetic order and any crystallographic distortion at very low temperature.