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Register a new userFirst principles calculation of the valence band offsets for b{eta}- polytype of A3B6 layered crystals
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The valence band offsets (VBO) for the b{eta}-type A3B6 layered compounds depending on the thickness of the crystals have been investigated from the first principles, based on the density functional theory. To simulate the structure of a given thickness the periodic slab model was used. Two adjacent crystal slabs consisting of several layers were separated by a vacuum region of two-layer width. It is shown that at the crystal thickness more than 12 layers, photothreshold practically becomes independent on the thickness of the crystal.
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The band offsets between crystalline and hydrogenated amorphous silicon (a-Si:H) are key parameters governing the charge transport in modern silicon hetrojunction solar cells. They are an important input for macroscopic simulators that are used to further optimize the solar cell. Past experimental studies, using X-ray photoelectron spectroscopy (XPS) and capacitance-voltage measurements, have yielded conflicting results on the band offset. Here we present a computational study on the band offsets. It is based on atomistic models and density-functional theory (DFT). The amorphous part of the interface is obtained by relatively long DFT first-principles molecular-dynamics (MD) runs at an elevated temperature on 30 statistically independent samples. In order to obtain a realistic conduction band position the electronic structure of the interface is calculated with a hybrid functional. We find a slight asymmetry in the band offsets, where the offset in the valence band (0.30 eV) is larger than in the conduction band (0.17 eV). Our results are in agreement with the latest XPS measurements that report a valence band offset of 0.3 eV [M. Liebhaber et al., Appl. Phys. Lett. 106, 031601 (2015)].
This paper presents a numerical implementation of a first-principles envelope-function theory derived recently by the author [B. A. Foreman, Phys. Rev. B 72, 165345 (2005)]. The examples studied deal with the valence subband structure of GaAs/AlAs, GaAs/Al(0.2)Ga(0.8)As, and In(0.53)Ga(0.47)As/InP (001) superlattices calculated using the local density approximation to density-functional theory and norm-conserving pseudopotentials without spin-orbit coupling. The heterostructure Hamiltonian is approximated using quadratic response theory, with the heterostructure treated as a perturbation of a bulk reference crystal. The valence subband structure is reproduced accurately over a wide energy range by a multiband envelope-function Hamiltonian with linear renormalization of the momentum and mass parameters. Good results are also obtained over a more limited energy range from a single-band model with quadratic renormalization. The effective kinetic-energy operator ordering derived here is more complicated than in many previous studies, consisting in general of a linear combination of all possible operator orderings. In some cases the valence-band Rashba coupling differs significantly from the bulk magnetic Luttinger parameter. The splitting of the quasidegenerate ground state of no-common-atom superlattices has non-negligible contributions from both short-range interface mixing and long-range dipole terms in the quadratic density response.
In this work we present a new method for the calculation of the electrostrictive properties of materials using density functional theory. The method relies on the thermodynamical equivalence, in a dielectric, of the quadratic mechanical responses (stress or strain) to applied electric stimulus (electric or polarisation fields) to the strain or stress dependence of its dielectric susceptibility or stiffness tensors. Comparing with current finite-field methodologies for the calculation of electrostriction, we demonstrate that our presented methodology offers significant advantages of efficiency, robustness, and ease of use. These advantages render tractable the highthroughput theoretical investigation into the largely unknown electrostrictive properties of materials.
We report first-principles density-functional pseudopotential calculations on the atomic structures, electronic properties, and band offsets of BaO/BaTiO$_3$ and SrO/SrTiO$_3$ nanosized heterojunctions grown on top of a silicon substrate. The density of states at the junction does not reveal any electronic induced interface states. A dominant perovskite character is found at the interface layer. The tunability of the band offset with the strain conditions imposed by the substrate is studied. Using previously reported theoretical data available for Si/SrO, Si/BaO and BaTiO$_{3}$/SrRuO$_{3}$ interfaces we extrapolate a value for the band alignments along the whole gate stacks of technological interest: Si/SrO/SrTiO$_3$ and Si/BaO/BaTiO$_3$/SrRuO$_3$ heterostructures.
A predicted type-II staggered band alignment with an approximately $1.4 eV$ valence band offset at the $ZnGeN_2/GaN$ heterointerface has inspired novel band-engineered $III-N/ZnGeN_2$ heterostructure-based device designs for applications in high performance optoelectronics. We report on the determination of the valence band offset between metalorganic chemical vapor deposition grown $(ZnGe)_{1-x}Ga_{2x}N_2$, for $x = 0$ and $0.06$, and $GaN$ using X-ray photoemission spectroscopy. The valence band of $ZnGeN_2$ was found to lie $1.45-1.65 eV$ above that of $GaN$. This result agrees well with the value predicted by first-principles density functional theory calculations using the local density approximation for the potential profile and quasiparticle self-consistent GW calculations of the band edge states relative to the potential. For $(ZnGe)_{0.94}Ga_{0.12}N_2$ the value was determined to be $1.29 eV$, $~10-20%$ lower than that of $ZnGeN_2$. The experimental determination of the large band offset between $ZnGeN_2$ and $GaN$ provides promising alternative solutions to address challenges faced with pure III-nitride-based structures and devices.
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