First-principles calculation of the structural, electronic, and vibrational properties of gallium nitride and aluminum nitride

Non-Perturbative Determination of Isotope-induced Anomalous Vibrational Physics

In general, vibrational physics has been well described by quantum perturbation theory to provide footprint characteristics for common crystals. However, despite weak phonon anharmonicity, the recently discovered cubic crystals have shown anomalous vibrational dynamics with elusive fundamental origin. Here, we developed a non-perturbative ab initio approach, in together with spectroscopy and high-pressure experiments, to successfully determine the exact dynamic evolutions of the vibrational physics for the first time. We found that the local fluctuation and coupling isotopes significantly dictate the vibrational spectra, through the Brillouin zone folding that has been previously ignored in literature. By decomposing vibrational spectra into individual isotope eigenvectors, we observed both positive and negative contributions to Raman intensity from constitutional atoms (10B, 11B, 75As or 31P). Importantly, our non-perturbative theory predicts that a novel vibrational resonance appears at high hydrostatic pressure due to broken translational symmetry, which was indeed verified by experimental measurement under a pressure up to 31.5 GPa. Our study develops fundamental understandings for the anomalous lattice physics under the failure of quantum perturbation theory and provides a new approach in exploring novel transport phenomena for materials of extreme properties.

Pressure-induced novel nitrogen-rich aluminum nitrides: AlN6, Al2N7 and AlN7 with polymeric nitrogen chains and rings

Pressure-induced non-molecular phases of polymeric nitrogen have potential applications in the field of energetic materials. Here, through a structural search method combined with first-principles calculations, we have predicted four novel nitrogen-rich aluminum nitrides C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7. Nitrogen atoms polymerize into infinite chains in C2/m-AlN6, C2-Al2N7 and P1-AlN7 structures and form N3 chains and N4 rings in Cm-Al2N7. C2/m-AlN6 is stable in the pressure range from 30 to 80 GPa and Cm-Al2N7, C2-Al2N7 and P1-AlN7 are metastable in the pressure ranges of 35-65, 65-80 and 41-80 GPa, respectively. The present study predicts that C2/m-AlN6 has a superconducting transition temperature of 18.9 K at 0 GPa and can be quenched and recovered under ambient conditions. The energy densities of C2/m-AlN6, Cm-Al2N7, C2-Al2N7 and P1-AlN7 compounds are expected to be 4.80, 4.59, 5.46 and 5.59 kJ g-1, respectively, due to their high nitrogen content, indicating that they have potential to be high-energy density materials. The calculated Vickers hardness of C2/m-AlN6, Cm-Al2N7, Cm-Al2N7 and P1-AlN7 is 43.86, 39.32, 63.96 and 33.58 GPa, respectively, showing that the novel nitrogen-rich aluminum nitrides are potential superhard materials as well. This study may encourage further experimental exploration of high N content superhard or high-energy density nitrides.

Structural, electronic, and optical properties of the gallium nitride semiconductor by means of the FP-LAPW method

In the present paper, the structural, electronic, and linear optical properties of different phases of the gallium nitride (GaN) have been investigated. The zinc blende and wurtzite phases of the GaN have been studied using the full-potential linearized augmented plane wave method (FP-LAPW). In our study, many approximations have been used, such as the local density approximation (LDA), the generalized gradient approximation (GGA), the Engel and Vosko generalized gradient approximation (EV-GGA), and the modified Becke-Johnson (mBJ) potential exchange. As a result, we found a very good agreement with literature experimental results for the energy band gap using the mBJ approximation with a scaling factor of 98% and 80% for the zinc blende and wurtzite phases, respectively.

Studying electronic properties in GaN without electrical contacts using γ-γ vs e--γ Perturbed Angular Correlations

The potential use of combined e⁻-γ vs γ-γ Perturbed Angular Correlations (PAC) experiments as a possible alternative to study electronic properties of materials and/or samples where Hall effect measurements are difficult to perform due to low-quality ohmic contacts is here demonstrated using Si- and Zn-doped GaN samples as a showcase example. To do so, the lattice site of implanted ¹⁸¹Hf/¹⁸¹Ta and the recombination of Ta ionized and excited electronic states were studied as a function of temperature and sample doping in GaN. By combining the γ-γ and e⁻-γ PAC results with Density Functional Theory simulations, it was possible to assign a single stable site with a double-donor character for Ta in GaN. A metastable charge state was also identified at particular temperatures using e⁻-γ PAC. A thermally activated process was observed for the electronic recombination at high temperatures with activation energies of 15(2) meV and 12(1) meV for the Si- and Zn-doped samples, respectively, and attributed to Si shallow donors present in both samples. A reduced number of available electrons was observed in the Zn-doped sample due to donor compensation by the Zn acceptors. At low temperatures, it is suggested that the recombination process occurs via Variable Range Hopping. The doping characteristics of both samples were successfully distinguished.

Insights into Antibonding Induced Energy Density Enhancement and Exotic Electronic Properties for Germanium Nitrides at Modest Pressures

Here, the electronic and bonding features in ground-state structures of germanium nitrides under different components that not accessible at ambient conditions have been systematically studied. The forming essence of weak covalent bonds between the Ge and N atom in high-pressure ionic crystal Fd-3 m-Ge₃N₄ is induced by the binding effect of electronic clouds originated from the Ge_ p orbitals. Hence, it helps us to understand the essence of covalent bond under high pressure, profoundly. As an excellent reducing agent, germanium transfer electrons to the antibonding state of the N₂ dimer in Pa-3-GeN₂ phase at 20 GPa, abnormally, weakening the bonding strength considerably than nitrogen gap (N≡N) at ambient pressure. Furthermore, the common cognition that the atomic distance will be shortened under the high pressures has been broken. Amazingly, with a lower range of synthetic pressure (∼15 GPa) and nitrogen contents (28%), its energy density is up to 2.32 kJ·g^-1, with a similar order of magnitude than polymeric LiN₅ (nonmolecular compound, 2.72 kJ·g^-1). It breaks the universal recognition once again that nitrides just containing polymeric nitrogen were regarded as high energy density materials. Hence, antibonding induced energy density enhancement mechanism for low nitrogen content and pressure has been exposed in view of electrons. Both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are usually the separated orbitals of N_π* and N_σ*, which are the key to stabilization. Besides, the sp² hybridizations that exist in N₄ units are responsible for the stability of the R-3 c-GeN₄ structure and restrict the delocalization of electrons, exhibiting nonmetallic properties.

Molecular dynamic simulation for thermal decomposition of RDX with nano-AlH3 particles

Reactive molecular dynamic simulation of a high explosive, RDX, mixed with AlH₃ nanoparticles was performed by a newly parameterized ReaxFF force field.

A New Phase of GaN

The structural, mechanical, and electronic properties of the orthorhombic GaN (Pnma-GaN) are investigated at ambient pressure by using first-principles calculations method with the ultrasoft pseudopotential scheme. The elastic constants and phonon calculations reveal Pnma-GaN is mechanically and dynamically stable at ambient pressure. The calculated Young modulus of Pnma-GaN is 170 GPa, which is the three-fifths of wurtzite-GaN. Electronic structure study shows that Pnma-GaN is a direct semiconductor with band gap of 1.847 eV. The anisotropic calculation shows that wurtzite-GaN has a smaller elastic anisotropy than that of Pnma-GaN in Young’s modulus. In addition, when the composition of aluminum increases from 0 to 0.063 in the alloy, the band gap decreases initially and increases afterward for Pnma-Ga1−xAlxN, while, for wurtzite-Ga1−xAlxN, the band gap increases with the increasing compositionx. Due to the structural porous feature, Pnma-GaN can also be expected to be a good hydrogen storage material.

Structural, elastic constant, and vibrational properties of wurtzite gallium nitride: a first-principles approach

Perdew-Wang proposed generalized gradient approximation (GGA) is used in conjunction with ultrasoft pseudopotential to investigate the structural, elastic constant, and vibrational properties of wurtzite GaN. The equilibrium lattice parameters, axial ratio, internal parameter, bulk modulus, and its pressure derivative are calculated. The effect of pressure on equilibrium lattice parameters, axial ratio, internal parameter (u), relative volume, and bond lengths parallel and perpendicular to the c-axis are discussed. At 52 GPa, the relative volume change is observed to be 17.8%, with an abrupt change in bond length. The calculated elastic constants are used to calculate the shear wave speeds in the [100] and [001] planes. The finite displacement method is employed to calculate phonon frequencies and the phonon density of states. The first- and second-order pressure derivative and volume dependent Gruneisen parameter (γ(j)) of zone-center phonon frequencies are discussed. These phonon calculations calculated at theoretical lattice constants agree well with existing literature.

Native Defects in Wurtzite GaN And AlN

The results of an extensive theoretical study of native defects in GaN and of vacancies in AlN are presented. We have considered cation and anion vacancies, antisites, and intersti-tials. The computations were carried out using quantum molecular dynamics, in supercells containing 72 atoms. Due to the wide gap of nitrides, the formation energies of defects depend strongly on the position of the Fermi level. The N vacancy in GaN introduces a shallow donor level that may be responsible for the n-type character of as-grown GaN.Other defects introduce deep states in the gap, with strongly localized wave functions.

Atomistic modeling of III-V nitrides: modified embedded-atom method interatomic potentials for GaN, InN and Ga(1-x)In(x)N

Modified embedded-atom method (MEAM) interatomic potentials for the Ga-N and In-N binary and Ga-In-N ternary systems have been developed based on the previously developed potentials for Ga, In and N. The potentials can describe various physical properties (structural, elastic and defect properties) of both zinc-blende and wurtzite-type GaN and InN as well as those of constituent elements, in good agreement with experimental data or high-level calculations. The potential can also describe the structural behavior of Ga(1-x)In(x)N ternary nitrides reasonably well. The applicability of the potentials to atomistic investigations of atomic/nanoscale structural evolution in Ga(1-x)In(x)N multi-component nitrides during the deposition of constituent element atoms is discussed.

Theoretical Study on the Assembly and Stabilization of a Magic Cluster Al4N-

We report the first attempt to assemble the magic cluster Al4N- on the basis of the density functional theory calculations on a series of pi-stacked dimers (Al4N-)2, sandwich-like compounds [D(Al4N)M]q- (where D = Al4N-, Cp-(C5H5-); M = Li, Na, K, Be, Mg, Ca) and extended compounds (Cp-)m(Li+)n(Al4N-)o (where m, n, and o are integers). For the six metals, the magic Al4N- can only be assembled and grow up in our newly proposed "hetero-decked sandwich" scheme (e.g., [CpM(Al4N)]q-) so as to avoid cluster fusion. The ground-state hetero-decked sandwich species (Cp-)(M)q+(Al4N)- (M = Li, Na, K, q = 1; M = Be, Mg, Ca, q = 2) and the extended sandwich species (Cp-)m(Li+)n(Al4N-)o are mainly ionically bonded, cluster-assembled "polyatomic molecules", grown from the combination of Cp-, M-atoms, and Al4N-. As a prototype for ionic bonding involving intact Al4N- subunits, [CpM(Al4N)]q- may be a stepping stone toward forming ionic, cluster-assembled AlN compounds.

Stress dependence of optically active diamagnetic point defects in silicon oxynitride

The cathodoluminescence (CL) spectrum arising from diamagnetic point defects of silicon oxynitride lattice was analyzed to extract quantitative information on local stress fields stored on the surface of a silicon nitride polycrystal. A calibration procedure was preliminarily made to obtain a relationship between CL spectral shift and applied stress, according to the piezo-spectroscopic effect. In this calibration procedure, we used the uniaxial stress field developed in a rectangular bar loaded in a four-point flexural jig. Stress dependence was clearly detected for the most intense spectral band of a doublet arising from diamagnetic ([triple bond]Si-Si[triple bond]) defects, which was located at around 340 nm. The shallow nature of the electron probe enabled the characterization of surface stress fields with sub-micrometer-order spatial resolution. As applications of the PS technique, the CL emission from [triple bond]Si-Si[triple bond] defects was used as a stress probe for visualizing the residual stress fields stored at grain-boundary regions and at the tip of a surface crack propagated in polycrystalline silicon nitride.