Electronic structure of GaN with strain and phonon distortions

Intrinsic Room-Temperature Ferromagnetism in New Halide Perovskite AgCrX3 (X: F, Cl, Br, I) Using Ab Initio and Monte Carlo Simulations

Herein, we present a detailed comparative study of the structural, elastic, electronic, and magnetic properties of a series of new halide perovskite AgCrX3 (X: F, Cl, Br, I) crystal structures using density functional theory, mean-field theory (MFT), and quantum Monte Carlo (MC) simulations. As demonstrated by the negative formation energy and Born-Huang stability criteria, the suggested perovskite compounds show potential stability in the cubic crystal structure. The materials are ductile because the Pugh's ratio is greater than 1.75, and the Cauchy pressure (C12-C44) is positive. The ground state magnetic moments of the compound were calculated as 3.70, 3.91, 3.92, and 3.91 μB for AgCrF3, AgCrCl3, AgCrBr3, and AgCrI3, respectively. The GGA + SOC computed spin-polarized electronic structures reveal ferromagnetism and confirm the metallic character in all of these compounds under consideration. These characteristics are robust under a ±3% strained lattice constant. Using relativistic pseudopotentials, the total energy is calculated, which yields that the single ion anisotropy is 0.004 meV and the z-axis is the hard-axis in the series of AgCrX3 (X: F, Cl, Br, and I) compounds. Further, to explore room-temperature intrinsic ferromagnetism, we considered ferromagnetic and antiferromagnetic interactions of the magnetic ions in the compounds by considering a supercell with 2 × 2 × 2 dimensions. The transition temperature is estimated by two models, namely, MFT and MC simulations. The calculated Curie temperatures using MC simulations are 518.35, 624.30, 517.94, and 497.28 K, with ±5% error for AgCrF3, AgCrCl3, AgCrBr3, and AgCrI3 compounds, respectively. Our results suggest that halide perovskite AgCrX3 compounds are promising materials for spintronic nanodevices at room temperature and provide new recommendations. For the first time, we report results for novel halide perovskite compounds based on Ag and Cr atoms.

Coherent-interface-induced strain in large lattice-mismatched materials: A new approach for modeling Raman shift

Strain engineering as one of the most powerful techniques for tuning optical and electronic properties of Ill-nitrides requires reliable methods for strain investigation. In this work, we reveal, that the linear model based on the experimental data limited to within a small range of biaxial strains (< 0.2%), which is widely used for the non-destructive Raman study of strain with nanometer-scale spatial resolution is not valid for the binary wurtzite-structure group-III nitrides GaN and AlN. Importantly, we found that the discrepancy between the experimental values of strain and those calculated via Raman spectroscopy increases as the strain in both GaN and AlN increases. Herein, a new model has been developed to describe the strain-induced Raman frequency shift in GaN and AlN for a wide range of biaxial strains (up to 2.5%). Finally, we proposed a new approach to correlate the Raman frequency shift and strain, which is based on the lattice coherency in the epitaxial layers of superlattice structures and can be used for a wide range of materials.

Electronic Supplementary Material

Supplementary material (Table S1: Values of bulk phonon deformation potentials and elastic constants for GaN and AlN from each reference used in Table 1, Fig. S1: Lattice parameters of SL layers using Eq. (8), and Fig. S2: Raman mapping using Eq. (7)) is available in the online version of this article at 10.1007/s12274-021-3855-4.

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.

Molecular Dynamics Simulation on B3-GaN Thin Films under Nanoindentation

The B3-GaN thin film was investigated by performing large-scale molecular dynamics (MD) simulation of nanoindentation. Its plastic behavior and the corresponding mechanism were studied. Based on the analysis on indentation curve, dislocation density, and orientation dependence, it was found that the indentation depths of inceptive plasticity on (001), (110), and (111) planes were consistent with the Schmid law. The microstructure evolutions during the nanoindentation under different conditions were focused, and two formation mechanisms of prismatic loop were proposed. The "lasso"-like mechanism was similar to that in the previous research, where a shear loop can translate into a prismatic loop by cross-slip; and the extended "lasso"-like mechanism was not found to be reported. Our simulation showed that the two screw components of a shear loop will glide on another loop until they encounter each other and eventually produce a prismatic dislocation loop.

Elastic properties of GaN nanowires: revealing the influence of planar defects on young's modulus at nanoscale

The elastic properties of gallium nitride (GaN) nanowires with different structures were investigated by in situ electron microscopy in this work. The electric-field-induced resonance method was utilized to reveal that the single crystalline GaN nanowires, along [120] direction, had the similar Young's modulus as the bulk value at the diameter ranging 92-110 nm. Meanwhile, the elastic behavior of the obtuse-angle twin (OT) GaN nanowires was disclosed both by the in situ SEM resonance technique and in situ transmission electron microscopy tensile test for the first time. Our results showed that the average Young's modulus of these OT nanowires was greatly decreased to about 66 GPa and indicated no size dependence at the diameter ranging 98-171 nm. A quantitative explanation for this phenomenon is proposed based on the rules of mixtures in classical mechanics. It is revealed that the elastic modulus of one-dimensional nanomaterials is dependent on the relative orientations and the volume fractions of the planar defects.

Electronic structure of biaxially strained wurtzite crystals GaN, AlN, and InN

We present first-principles studies of the effect of biaxial (0001)-strain on the electronic structure of wurtzite GaN, AlN, and InN. We provide accurate predictions for the valence band splittings as a function of strain which greatly facilitates the interpretation of data from samples with unintentional growth-induced strain. The present calculations are based on the total-energy pseudopotential method within the local-density formalism and include the spin-orbit interaction nonperturbatively. For a given biaxial strain, all structural parameters are determined by minimization of the total energy with respect to the electronic and ionic degrees of freedom. Our calculations predict that the valence band state Γ9(Γ6) lies energetically above the Γ7(Γ1) states in GaN and InN, in contrast to the situation in AlN. In all three nitrides, we find that the ordering of these two levels becomes reversed for some value of biaxial strain. In GaN, this crossing takes place already at 0.32% tensile strain. For larger tensile strains, the top of the valence band becomes well separated from the lower states. The computed crystal-field and spin-orbit splittings in unstrained materials as well as the computed deformation potentials agree well with the available experimental data.

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.

Electronic and Optical Properties of the Group-III Nitrides, their Heterostructures and Alloys

Various aspects of the electronic structure of the group III nitrides are discussed. The relation between band structures and optical response in the vacuum ultraviolet is analyzed for zincblende and wurtzite GaN and for wurtzite A1N and compared with available experimental data obtained from reflectivity and spectroscopic ellipsometry. The spin-orbit and crystal field splittings of the valence band edges and their relations to exciton fine structure are discussed including substrate induced biaxial strain effects. The band-offsets between the III-nitrides and some relevant semiconductor substrates obtained within the dielectric midgap energy model are presented and strain effects which may alter these values are discussed. The importance of lattice mismatch in bandgap bowing is exemplified by comparing AlxGa1−xN and InxGa1−xN.

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.

Diameter-dependent electromechanical properties of GaN nanowires

The diameter-dependent Young's modulus, E, and quality factor, Q, of GaN nanowires were measured using electromechanical resonance analysis in a transmission electron microscope. E is close to the theoretical bulk value ( approximately 300 GPa) for a large diameter nanowire (d=84 nm) but is significantly smaller for smaller diameters. At room temperature, Q is as high as 2,800 for d=84 nm, significantly greater than what is obtained from micromachined Si resonators of comparable surface-to-volume ratio. This implies significant advantages of smooth-surfaced GaN nanowire resonators for nanoelectromechanical system (NEMS) applications. Two closely spaced resonances are observed and attributed to the low-symmetry triangular cross section of the nanowires.

Columnar AlGaN/GaN nanocavities with AlN/GaN Bragg reflectors grown by molecular beam epitaxy on Si(111)

Self-assembled columnar AlGaN/GaN nanocavities, with an active region of GaN quantum disks embedded in an AlGaN nanocolumn and cladded by top and bottom AlN/GaN Bragg mirrors, were grown. The nanocavity has no cracks or extended defects, due to the relaxation at the Si interface and to the nanocolumn free-surface to volume ratio. The emission from the active region matched the peak reflectivity by tuning the Al content and the GaN disks thickness. Quantum confinement effects that depend on both the disk thickness and the inhomogeneous strain distribution within the disks are clearly observed.