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.

/pdf/phonons-and-related-crystal-properties-from-density-3mdybdn8w1.pdf

Phonons and related crystal properties from density-functional perturbation theory

Microstructures and properties of high-entropy alloys

Fossil fuels currently supply most of the world's energy needs, and however unacceptable their long-term consequences, the supplies are likely to remain adequate for the next few generations. Scientists and policy makers must make use of this period of grace to assess alternative sources of energy and determine what is scientifically possible, environmentally acceptable and technologically promising.

Alternative energy technologies

Halide perovskites have recently emerged as promising materials for low-cost, high-efficiency solar cells. The efficiency of perovskite-based solar cells has increased rapidly, from 3.8% in 2009 to 19.3% in 2014, by using the all-solid-state thin-film architecture and engineering cell structures with mixed-halide perovskites. The emergence of perovskite solar cells revolutionized the field not only because of their rapidly increased efficiency, but also flexibility in material growth and architecture. The superior performance of the perovskite solar cells suggested that perovskite materials possess intrinsically unique properties. In this review, we summarize recent theoretical investigations into the structural, electrical, and optical properties of halide perovskite materials in relation to their applications in solar cells. We also discuss some current challenges of using perovskites in solar cells, along with possible theoretical solutions.

Halide perovskite materials for solar cells: a theoretical review

The macroscopic nonlinear pyroelectric polarization of wurtzite AlxGa1-xN, InxGa1-xN and AlxIn1-xN ternary compounds (large spontaneous polarization and piezoelectric coupling) dramatically affects the optical and electrical properties of multilayered Al(In)GaN/GaN hetero-, nanostructures and devices, due to the huge built-in electrostatic fields and bound interface charges caused by gradients in polarization at surfaces and heterointerfaces. Models of polarization-induced effects in GaN-based devices so far have assumed that polarization in ternary nitride alloys can be calculated by a linear interpolation between the limiting values of the binary compounds. We present theoretical and experimental evidence that the macroscopic polarization in nitride alloys is a nonlinear function of strain and composition. We have applied these results to interpret experimental data obtained in a number of InGaN/GaN quantum wells?(QWs) as well as AlInN/GaN and AlGaN/GaN transistor structures. We find that the discrepancies between experiment and ab initio theory present so far are almost completely eliminated for the AlGaN/GaN-based heterostructures when the nonlinearity of polarization is accounted for. The realization of undoped lattice-matched AlInN/GaN heterostructures further allows us to prove the existence of a gradient in spontaneous polarization by the experimental observation of two-dimensional electron gases?(2DEGs). The confinement of 2DEGs in InGaN/GaN QWs in combination with the measured Stark shift of excitonic recombination is used to determine the polarization-induced electric fields in nanostructures. To facilitate inclusion of the predicted nonlinear polarization in future simulations, we give an explicit prescription to calculate polarization-induced electric fields and bound interface charges for arbitrary composition in each of the ternary III-N alloys. In addition, the theoretical and experimental results presented here allow a detailed comparison of the predicted electric fields and bound interface charges with the measured Stark shift and the sheet carrier concentration of polarization-induced 2DEGs. This comparison provides an insight into the reliability of the calculated nonlinear piezoelectric and spontaneous polarization of group III nitride ternary alloys.

http://iopscience.iop.org/article/10.1088/0953-8984/14/13/302/pdf

Pyroelectric properties of Al(In)GaN/GaN hetero- and quantum well structures

Structural models used in calculations of properties of substitutionally random ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$ alloys are usually constructed by randomly occupying each of the N sites of a periodic cell by A or B. We show that it is possible to design ``special quasirandom structures'' (SQS's) that mimic for small N (even N=8) the first few, physically most relevant radial correlation functions of a perfectly random structure far better than the standard technique does. We demonstrate the usefulness of these SQS's by calculating optical and thermodynamic properties of a number of semiconductor alloys in the local-density formalism.

/pdf/special-quasirandom-structures-hm0aiwelrc.pdf

Special quasirandom structures.

Structural models needed in calculations of properties of substitutionally random ${\mathit{A}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathit{B}}_{\mathit{x}}$ alloys are usually constructed by randomly occupying each of the N sites of a periodic cell by A or B. We show that it is possible to design ``special quasirandom structures'' (SQS's) that mimic for small N (even N=8) the first few, physically most relevant radial correlation functions of an infinite, perfectly random structure far better than the standard technique does. These SQS's are shown to be short-period superlattices of 4--16 atoms/cell whose layers are stacked in rather nonstandard orientations (e.g., [113], [331], and [115]). Since these SQS's mimic well the local atomic structure of the random alloy, their electronic properties, calculable via first-principles techniques, provide a representation of the electronic structure of the alloy. We demonstrate the usefulness of these SQS's by applying them to semiconductor alloys. We calculate their electronic structure, total energy, and equilibrium geometry, and compare the results to experimental data.

Electronic properties of random alloys : special quasirandom structures

Using the all-electron mixed-basis approach to the density-functional formalism for crystals, we calculate from first principles the electronic structure of zinc-blende ZnS, ZnSe, and ZnTe as well as that of their ordered pseudobinary alloys ${\mathrm{Zn}}_{2}$SSe, ${\mathrm{Zn}}_{2}$SeTe, and ${\mathrm{Zn}}_{2}$STe. For the latter we use as a model a CuAu I-like structure (space group $P\overline{4}m2$), and analyze the observed optical bowing in terms of three contributions: (i) a volume deformation of the band structure due to the replacement of the lattice constants of the binary constituents by that of the alloy, (ii) a chemical-electronegativity contribution due to charge exchange in the alloy relative to its constituent binary subsystems, and (iii) a structural contribution due to the relaxation of the anion-cation bond lengths in the alloy. The total bowing effect [the sum of (i)-(iii) above] agrees well with observations, yet the present analysis suggests a physical mechanism for optical bowing which differs profoundly from that offered by the popular virtual-crystal approach. The maximum contribution of disorder to the optical bowing is calculated for $\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$ using a cluster-averaging method, resulting in a reduction in the bowing of the fundamental gap. We further discuss the band structures, x-ray scattering factors, charge distribution, and deformation potentials of the binary zinc chalcogenides and their ordered alloys.

Electronic structure of ZnS, ZnSe, ZnTe, and their pseudobinary alloys

Alloys of zinc chalcogenides exhibit both some of the smallest (for $\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$) and the largest (for $\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Te}}_{1\ensuremath{-}x}$) optical bowing observed in isovalent semiconductor systems. A theoretical analysis of this effect, by use of self-consistent band-structure techniques for ordered 50%-50% alloys in the CuAu-I structure, predicts correct bowing parameters and chemical trends, yet suggests a physical model which is altogether different from the virtual-crystal model.

Optical bowing in zinc chalcogenide semiconductor alloys.

Whereas substitutional adamantine compounds ${A}_{n}$${B}_{4\mathrm{\ensuremath{-}}n}$${C}_{4}$ (e.g., ${\mathrm{ABC}}_{2}$ chalcopyrites for n=2, or the AC and BC zinc-blende compounds for n=4 and 0) have four metal atoms around each nonmetal atom and vice versa, ordered-vacancy compounds OVC's) ${\mathrm{AB}}_{2}$${C}_{4}$ have but three metal atoms (one A and two B's) around each nonmetal site (C) while the fourth (unoccupied) site forms an ordered array of vacancies. An example for OVC's is ``pseudocubic'' ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ which can be structurally derived from the layered alternate monolayer superlattice of CdSe and InSe (along the [001] direction) by removing half of the Cd atoms from each Cd plane. Such OVC's form a natural bridge between crystal and impurity physics. Much like the metal vacancy in II-VI compounds (e.g., CdSe), the vacancy in ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ has associated with it (nonmetal) ``dangling bonds'' and ``lone-pair'' electrons, which, however, form a dispersed band in the crystal. Using all-electron mixed-basis electronic-structure techniques, we study the properties of such an ordered array of vacancies in ${\mathrm{CdIn}}_{2}$${\mathrm{Se}}_{4}$ vis-a$ga----vis the experimental data. We find vacancy-induced atomic relaxations (Se moves towards the vacant site), resonant broadening of the lone-pair dangling-bond states into a \ensuremath{\approxeq}3-eV band, and that the total charge density around the vacant site has little density and shows scant evidence of dangling bonds. We discuss the nature of the bonding in this system, comparing it to other covalent selenides and to the observed photoemission and optical data. A number of possible order-disorder transitions, including the disordering of cations on the vacant sites, are identified.

James E. Bernard

Papers

Special quasirandom structures.

Electronic properties of random alloys : special quasirandom structures

Electronic structure of ZnS, ZnSe, ZnTe, and their pseudobinary alloys

Optical bowing in zinc chalcogenide semiconductor alloys.

Ordered-vacancy-compound semiconductors: Pseudocubic CdIn 2 Se 4