QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

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QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.

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Phonons and related crystal properties from density-functional perturbation theory

We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.

Band parameters for III–V compound semiconductors and their alloys

Diamond-like carbon (DLC) is a metastable form of amorphous carbon with significant sp3 bonding. DLC is a semiconductor with a high mechanical hardness, chemical inertness, and optical transparency. This review will describe the deposition methods, deposition mechanisms, characterisation methods, electronic structure, gap states, defects, doping, luminescence, field emission, mechanical properties and some applications of DLCs. The films have widespread applications as protective coatings in areas, such as magnetic storage disks, optical windows and micro-electromechanical devices (MEMs).

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Diamond-like amorphous carbon

A new model for band offsets in lattice-matched heterojunctions is presented along with a novel definition of the interface dipole which avoids any reference to an ideal interface. The model is derived only from the charge densities of the bulk constituents and naturally predicts the independence of the offsets on interface geometry. It is in excellent agreement with accurate first-principles pseudopotential calculations for ${(\mathrm{GaAs})}_{3}$/${(\mathrm{AlAs})}_{3}$ grown in the (001), (110), and (111) directions and with available experimental data.

Band offsets in lattice-matched heterojunctions: A model and first-principles calculations for GaAs/AlAs.

Using a combined experimental and theoretical approach, we address C 1s core-level shifts in amorphous carbon. Experimental results are obtained by x-ray photoelectron spectroscopy (XPS) and electron-energy-loss spectroscopy (EELS) on thin-film samples of different atomic density, obtained by a pulsed-laser deposition growth process. The XPS spectra are deconvoluted into two contributions, which are attributed to sp(2)- and sp(3)-hybridized atoms, respectively, separated by 0.9 eV, independent or atomic density, The sp(3) hybridization content extracted from XPS is consistent with the atomic density derived from the plasmon energy in the EELS spectrum. In our theoretical study, we generate several periodic model structures of amorphous carbon of different densities applying two schemes of increasing accuracy in sequence. We first use a molecular-dynamics approach, based on an environmental-dependent tight-binding Hamiltonian to quench the systems from the liquid phase. The final model structures are then obtained by further atomic relaxation using a first-principles pseudopotential plane-wave approach within density-functional theory. Within the latter framework. we also calculate carbon 1s core-level shifts for our disordered model structures. We find that the shifts associated to threefold- and fourfold- coordinated carbon atoms give rise to two distinct peaks separated by about 1.0 eV. independent of density, in close agreement with experimental observations. This provides strong support for decomposing the XPS spectra into two peaks resulting from sp(2)- and sp(3)-hybridized atoms. Core-hole relaxations effects account for about 30% of the calculated shifts.

sp(2)/sp(3) hybridization ratio in amorphous carbon from C 1s core-level shifts: X-ray photoelectron spectroscopy and first-principles calculation

We demonstrate the feasibility of ab initio studies of piezoelectricity within an all-electron scheme. The focus of our analysis is on wurtzite ZnO; for comparison, some results are also presented for the related materials BeO and ZnS. The comparative study is performed in order to understand the microscopic origin of the peculiar behavior of ZnO, whose piezoelectric response is the strongest among the tetrahedrally bonded semiconductors. In all such materials, the piezeoelectric effect results from two different terms of opposite sign: these are usually referred to as the ``clamped-ion'' and the ``internal-strain'' contributions. Cancellation among them is least effective in ZnO, where the dominant effect is due to a rigid-ion-like mechanism. Furthermore, we compute the spontaneous polarization of ZnO and we discuss the puzzling agreement between our calculated value and a very indirect experimental estimate of the same quantity.

Ab initio study of piezoelectricity and spontaneous polarization in ZnO.

The spontaneous macroscopic polarization of perovskite ${\mathrm{KNbO}}_{3}$ is calculated as a Berry's phase using the Bloch functions of the tetragonal crystal. The result P=0.35 C/${\mathrm{m}}^{2}$ supports the measured value 0.37 C/${\mathrm{m}}^{2}$ and implies that earlier data from hysteresis loops are too low. The polarization is linear in the ferroelectric distortion; the Born effective charges show strong variations from nominal ionic values and a large inequivalence of the O ions. Linearity posteriori demonstrates that the polarization of perovskites at finite temperature can be safely predicted assuming the ions frozen in their time-averaged positions.

Towards a quantum theory of polarization in ferroelectrics: The case of KNbO 3

Understanding the mechanisms which determine the band offsets and Schottky barriers at semiconductor contacts and engineering them for specific device applications are important theoretical and technological challenges. In this review, we present a theoretical approach to the band-line-up problem and discuss its application to prototypical systems. The emphasis is on ab initio computations and on theoretical models derived from first-principles numerical experiments. An approach based on linear-response-theory concepts allows a general description of the band alignment for various classes of semiconductor contacts and predicts the effects of various bulk and interfacial perturbations on the band discontinuities.

http://iopscience.iop.org/article/10.1088/0022-3727/31/11/002/pdf

Alfonso Baldereschi

Papers

Band offsets in lattice-matched heterojunctions: A model and first-principles calculations for GaAs/AlAs.

sp(2)/sp(3) hybridization ratio in amorphous carbon from C 1s core-level shifts: X-ray photoelectron spectroscopy and first-principles calculation

Ab initio study of piezoelectricity and spontaneous polarization in ZnO.

Towards a quantum theory of polarization in ferroelectrics: The case of KNbO 3

Band engineering at interfaces : Theory and numerical experiments