A historic perspective is provided to the choice of methodologies for point defects in semiconductors. A summary and commentary on the highlights of the CECAM workshop: “Which electronic structure ...method for the study of defects?” is given, attempting to provide a link between the different contributions. To this purpose different themes running through the compilation are identified and rather than discussing individual contributions one by one, the discussion is focused around these themes. The first theme is the problem of correcting for finite size effects. The second theme is the problem of the underestimate of the band gaps and how to correct for it in defect calculations. The third theme is the self‐interaction error of local density approximation (LDA) and its repercussions for polaronic defects. The fourth theme is progress in methods beyond LDA that are becoming applicable to point defects, either for ground states or excited state properties.
In this work, the spatially dependent recombination kinetics of mixed-halide hybrid perovskite CH3NH3Pb(Br1–x Cl x )3 (0 ≤ x ≤ 0.19) single crystals are investigated using time-resolved ...photoluminescence spectroscopy with one- and two-photon femtosecond laser excitation. The introduction of chloride by substituting a fraction of the bromide leads to a decreased lattice constant compared to pure bromide perovskite (x = 0) and a higher concentration of surface defects. The measured kinetics under one-photon excitation (1PE) shows that increasing the chloride addition quenches the photoluminescence (PL) lifetimes, due to substitution-induced surface defects. In stark contrast, upon 2PE, the PL lifetimes measured deeper in the bulk become longer with increasing chloride addition, until the halide substitution reaches the critical concentration of ∼19%. At x = 19% Cl concentration, a significant reversal of this behavior is observed indicating a change in crystal structure beyond the continuous trends observed at lower percentages of halide substitution (x ≤ 11%). The observed opposing trends, based on 1PE versus 2PE, highlight a dichotomy between extrinsic (surface) and intrinsic (bulk) effects of chloride substitution on the carrier dynamics in lead bromide perovskites. We discuss the physical relation between halide exchange and bulk carrier lifetimes in CH3NH3PbBr3 in terms of the Rashba effect. We propose that the latter is suppressed at the surface due to disorder in the alignment of the MA and that it increases in the bulk with Cl concentration because of the reduction in lattice parameters, which compresses the space available for the MA orientational degrees of freedom.
Recent work on heterovalent ternary nitrides, II‐IV‐N2, is reviewed. The authors first provide an overview of the relevant literature, then briefly discuss band gaps, band offsets and the effects and ...nature of disorder. The authors discuss the energies of formation and evaluate the stability or metastability with respect to competing binary compounds. The Cd‐IV‐N2 compounds are found to be only metastable. For ZnGeN2 we present a revised chemical potential stability region and discuss its effects on the point defect energies of formation. The authors briefly discuss the current status of understanding of the point defects and doping in ZnGeN2.
Recent work on the II‐IV‐N2 (with II=Be, Mg, Zn, Cd and IV=Si, Ge, Sn) semiconductors is reviewed, including their band gaps, bandosets, stability versus competing binary compounds, order‐disorder aspects, point defects and doping. The cation antisite defects are found to be the dominant defects and closely related to the disorder in these materials.
Band Gaps and Stability of CsSiX3 Halides Radha, Santosh K.; Lambrecht, Walter R. L.
Physica status solidi. A, Applications and materials science,
08/2019, Letnik:
216, Številka:
15
Journal Article
Recenzirano
Odprti dostop
There is a great current interest in lead‐free halide perovskites. While Sn based and Ge based compounds of this type have already been demonstrated experimentally, Si based ones have not. In this ...paper, the authors consider the possibility of CsSiX3 halide perovskites. While previous work for cubic CsSiI3 found the band gap to close or in fact become inverted and to lead to a topological insulator, the authors show here that in the rhombohedrally distorted structure that occurs because of Si off‐centering, the band gap becomes larger than 1.5 eV and possibly of interest for photovoltaics. On the other hand, even in the cubic structure at high temperature, fluctuations of the Si position, will lead to a sizable gap. The total energy calculations, show that the materials are unstable toward 3CsSiI3 → Si2I6 + 3CsI + Si and 2CsSiI3 → SiI4 + 2CsI + Si and are thus above the convex hull. Similar results are presented for halogens: I, Br, Cl.
The feasibility of Si‐based halide perovskites, CsSiX3, X = Cl, Br, I is investigated by evaluating their energy of formation relative to those of competing binaries. Their rhombohedrally distorted ferroelectric structures are found to be slightly metastable. Their band structures are evaluated in the GW approximation. CsSiI3 is found to have a gap of 1.69 eV suitable for photovoltaic applications.
The role of exchange defects on the band structure of ZnGeN2 is investigated. Exchange defects are defined through the exchange of cations Zn and Ge starting from the ideal Pna21 crystal structure, ...which obeys the local octet rule. Each such exchange creates several nitrogen-centered tetrahedra which violate the local octet rule, although overall charge neutrality is preserved. We study several distributions of exchange defects, some with all antisites making up the exchange defect close to each other and with increasing numbers of exchange defects, and others where the two types of antisites ZnGe and GeZn are kept separated from each other. We also compare the results for these models with a fully random distribution of Zn and Ge on the cation sites. We show that for a single-nearest-neighbor exchange defect, the band gap is narrowed by about 0.5 eV due to two effects: (1) the ZnGe antisites form filled acceptor states just above and merging with the valence-band maximum (VBM) of perfect crystal ZnGeN2 and (2) the GeZn antisites form a resonance in the conduction band which lowers the conduction-band minimum (CBM). When more exchange defects are created, these acceptor states broaden into bands which can lower the gap further. When tetrahedra occur surrounded completely by four Zn atoms, states even deeper in the gap are found localized all near these tetrahedra, forming a separate intermediate band. Finally, for phase-segregated ZnGe and GeZn, the gap is significantly more reduced, but no separate band is found to occur. The ZnGe acceptorlike states now form a percolating defect band which is significantly wider and hence reaches deeper into the gap. In all cases, the wave functions near the top of the new VBM remain, to some extent, localized near the ZnGe sites. For a fully random case, the gap is even more severely reduced by almost 3 eV. The total energy of the system increases with the number of octet-rule-violating tetrahedra and the energy cost per exchange defect of order 2 eV is quite high.
This paper summarises the theory and functionality behind Questaal, an open-source suite of codes for calculating the electronic structure and related properties of materials from first principles. ...The formalism of the linearised muffin-tin orbital (LMTO) method is revisited in detail and developed further by the introduction of short-ranged tight-binding basis functions for full-potential calculations. The LMTO method is presented in both Green’s function and wave function formulations for bulk and layered systems. The suite’s full-potential LMTO code uses a sophisticated basis and augmentation method that allows an efficient and precise solution to the band problem at different levels of theory, most importantly density functional theory, LDA+U, quasi-particle self-consistent GW and combinations of these with dynamical mean field theory. This paper details the technical and theoretical bases of these methods, their implementation in Questaal, and provides an overview of the code’s design and capabilities.
Program Title: Questaal
Program Files doi:http://dx.doi.org/10.17632/35jxxtzpdn.1
Code Ocean Capsule:https://doi.org/10.24433/CO.3778701.v1
Licensing provisions: GNU General Public License, version 3
Programming language: Fortran, C, Python, Shell
Nature of problem: Highly accurate ab initio calculation of the electronic structure of periodic solids and of the resulting physical, spectroscopic and magnetic properties for diverse material classes with different strengths and kinds of electronic correlation.
Solution method: The many electron problem is considered at different levels of theory: density functional theory, many body perturbation theory in the GW approximation with different degrees of self consistency (notably quasiparticle self-consistent GW) and dynamical mean field theory. The solution to the single-particle band problem is achieved in the framework of an extension to the linear muffin-tin orbital (LMTO) technique including a highly precise and efficient full-potential implementation. An advanced fully-relativistic, non-collinear implementation based on the atomic sphere approximation is used for calculating transport and magnetic properties.
Abstract
The simplest picture of excitons in materials with atomic-like localization of electrons is that of Frenkel excitons, where electrons and holes stay close together, which is associated with ...a large binding energy. Here, using the example of the layered oxide V
2
O
5
, we show how localized charge-transfer excitations combine to form excitons that also have a huge binding energy but, at the same time, a large electron-hole distance, and we explain this seemingly contradictory finding. The anisotropy of the exciton delocalization is determined by the local anisotropy of the structure, whereas the exciton extends orthogonally to the chains formed by the crystal structure. Moreover, we show that the bright exciton goes together with a dark exciton of even larger binding energy and more pronounced anisotropy. These findings are obtained by combining first principles many-body perturbation theory calculations, ellipsometry experiments, and tight binding modelling, leading to very good agreement and a consistent picture. Our explanation is general and can be extended to other materials.
An alloy of ZnGeN2 and GaN in equal proportions can form the octet-rule-preserving quaternary heterovalent nitride semiconductor ZnGeGa2N4. Single-crystal films of the alloy targeting this ...composition were deposited on (11̅02) Al2O3 (r-plane sapphire), (0001) Al2O3 (c-plane sapphire), and (0001) GaN/Al2O3 by metal–organic chemical vapor deposition using the precursors diethylzinc, germane, trimethylgallium, and ammonia. The growth directions were along the c-axis for films grown on the c-plane sapphire and GaN templates, as well as along the orthorhombic 010 axis for films grown on r-plane sapphire. The effects of varying the growth temperature from 550 to 700 °C, choice of substrate, and trimethylgallium and germane flow rates on film composition and morphology were examined by X-ray diffraction, field-emission scanning electron microscopy, and atomic force microscopy. The Zn/Ge atomic ratios were observed to decrease with growth temperature but increase with trimethylgallium flow rate. Growth rates, which varied with growth temperature from approximately 1 to 3.5 μm/h, were observed to increase with growth temperature up to 670 °C, then decrease abruptly with further increase in temperature. The growth rates were similar for growth on r- and c-plane sapphire substrates at the lower growth temperatures. However, above 650 °C the growth rates on c- and r-plane sapphire differed by as much as 70%. A broad photoluminescence double peak was observed only for samples grown on r-plane sapphire at the highest growth temperature. Hall measurements show n-type carrier concentrations in the mid-1018/cm–3 range and mobilities of a few cm2/V-s for material grown on r-sapphire substrates at 670 °C and above.
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