Rocksalt ZnxMg1-xO alloys are theoretically and experimentally investigated for near- and deep-UV optoelectronics with a tunable band gap of 4.2-7.8 eV. Regarding the key question about the composition x, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight- to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for x < 0.5 and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of ZnxMg1-xO multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions.

Received 21 July 2020DOI:https://doi.org/10.1103/PhysRevB.102.079903©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectronic structureFirst-principles calculationsOptoelectronicsPhysical SystemsAlloysWide band gap systemsTechniquesBand structure methodsDensity functional theoryPhotoluminescenceCondensed Matter, Materials & Applied Physics

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results.

Discussion of band gap behavior based on first principles calculations of electronic band structures for various short period nitride superlattices is presented. Binary superlattices, as InN/GaN and GaN/AlN as well as superlattices containing alloys, as InGaN/GaN, GaN/AlGaN, and GaN/InAlN are considered. Taking into account different crystallographic directions of growth (polar, semipolar and nonpolar) and different strain conditions (free-standing and pseudomorphic) all the factors influencing the band gap engineering are analyzed. Dependence on internal strain and lattice geometry is considered, but the main attention is devoted to the influence of the internal electric field and the hybridization of well and barrier wave functions. The contributions of these two important factors to band gap behavior are illustrated and estimated quantitatively. It appears that there are two interesting ranges of layer thicknesses; in one (few atomic monolayers in barriers and wells) the influence of the wave function hybridization is dominant, whereas in the other (layers thicker than roughly five to six monolayers) dependence of electric field on the band gaps is more important. The band gap behavior in superlattices is compared with the band gap dependence on composition in the corresponding ternary and quaternary alloys. It is shown that for superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary alloys. The calculated values of the band gaps are compared with the photoluminescence emission energies, when the corresponding data are available. Finally, similarities and differences between nitride and oxide polar superlattices are pointed out by comparison of wurtzite GaN/AlN and ZnO/MgO.

Discussion of band gap behavior based on first principles calculations of the electronic band structures for several InN/GaN superlattices (SLs) (free-standing and pseudomorphic) grown along different directions (polar and nonpolar) is presented. Taking into account the dependence on internal strain and lattice geometry mainly two factors influence the dependence of the band gap, E g on the layer thickness: the internal electric field and the hyb wells) is more important. We also consider mIn ridization of well and barrier wave functions. We illustrate their influence on the band gap engineering by calculating the strength of built-in electric field and the oscillator strength. It appears that there are two interesting ranges of layer thicknesses. In one the influence of the electric field on the gaps is dominant (wider wells), whereas in the other the wave function hybridization (narrow wells) is more important. We also consider m In 0 . 33 Ga 0.67 N/ n GaN SLs, which seem to be easier to fabricate than high In content quantum wells. The calculated band gaps are compared with recent experimental data. It is shown that for In(Ga)N/GaN superlattices it is possible to exceed by far the range of band gap values, which can be realized in ternary InGaN alloys.

A detailed study on the ternary Zr-based intermetallic compound Zr2TiAl has been carried out using first-principles electronic structure calculations. From the total energy calculations, we find an antiferromagnetic L11-like (AFM) phase with alternating (1 1 1) spin-up and spin-down layers to be a stable phase among some others with magnetic moment on Ti being 1.22 [Formula: see text]. The calculated magnetic exchange interaction parameters of the Heisenberg Hamiltonian and subsequent Heisenberg Monte Carlo simulations confirm that this phase is the magnetic ground structure with Néel temperature between 30 and 100 K. The phonon dispersion relations further confirm the stability of the magnetic phase while the non-magnetic phase is found to have imaginary phonon modes and the same is also found from the calculated elastic constants. The magnetic moment of Ti is found to decrease under pressure eventually driving the system to the non-magnetic phase at around 46 GPa, where the phonon modes are found to be positive indicating stability of the non-magnetic phase. A continuous change in the band structure under compression leads to the corresponding change of the Fermi surface topology and electronic topological transitions (ETT) in both majority and minority spin cases, which are also evident from the calculated elastic constants and density of state calculations for the material under compression.

The group‐III nitride and the group‐II oxide semiconductors have direct band gaps, which cover the ultraviolet to infrared energy range. In this work we calculate the band gaps and built‐in electric field of the polar wurtzite GaN/AlN and ZnO/MgO Short Period Superlattices (SPSLs). In many respects GaN and AlN are similar to ZnO and MgO, respectively, especially regarding the band gaps and the lattice parameters. To realize the wider band gap based materials the superlattices (SLs) with GaN and ZnO as quantum wells and AlN and MgO as quantum barriers, that is, GaN/AlN and ZnO/MgO, are created. We found similar evolution of the GaN/AlN and ZnO/MgO band gaps with varying number of atomic layers constituting these SPSLs. Band gap bowings and strength of the internal electric field existing in these two families of SPSLs differ significantly.

To obtain short period superlattices (SPLS) of InxGa1−xN/GaN in most cases molecular beam epitaxy has been applied. In this work the metal‐organic vapor phase epitaxy was used for obtaining similar structures and their quality as well as light emission features are studied. Thanks to control of growth parameters it was possible to fabricate InxGa1−xN/GaN SPSLs with structural quality of structures grown by MBE. They contain around 30% of indium in quantum wells (QWs) and different number m of atomic monolayers in QWs, and n. in the barriers. X‐ray diffraction and transmission electron microscope measurements have confirmed that the designed SPSLs structures were obtained. An agreement between the experimental results of photoluminescence measurements and theoretically predicted band gap behavior of SPSLs composed of mInxGa1−xN/nGaN was achieved. Built‐in electric field in quantum wells and barriers of the selected structures were determined using a simplified method of calculation.

The general trends in the behavior of the band gaps in short period superlattices (SLs) composed from InAlN and GaN layers and grown along the wurtzite c axis have been analyzed for different alloy compositions and several thicknesses of quantum wells and barriers. The composition dependence of the gaps is compared to that of the quaternary alloys InyAl1-x-y GaxN with the same effective contents of In, Al, and Ga as the SLs. The differences in the gaps are explained mainly by the built-in electric fields in the SLs caused by spontaneous and piezoelectric polarizations. Other factors, such as internal strain caused by lattice mismatch between wells and barriers and wave function hybridization, are taken into account. A simplified approach to determine the built-in electric fields directly from the dielectric properties of the constituent materials has been applied and verified by comparison to ab initio calculations for selected SLs. The calculated band gaps are compared with existing experimental data.

First principles calculations predict the promising thermoelectric material ZnGeSb2with a huge power factor (S2σ/τ) on the order of 3 × 1017W m−1K−2s−1, due to the ultra-high electrical conductivity scaled by a relaxation time of around 8.5 × 1025Ω−1m−1s−1, observed in its massive Dirac state.

A detailed study of the high-pressure structural properties, lattice dynamics and band structures of perovskite structured fluorides KZnF3, CsCaF3 and BaLiF3 has been carried out by means of density functional theory. The calculated structural properties including elastic constants and equation of state agree well with available experimental information. The phonon dispersion curves are in good agreement with available experimental inelastic neutron scattering data. The electronic structures of these fluorides have been calculated using the quasi particle self-consistent [Formula: see text] approximation. The [Formula: see text] calculations reveal that all the fluorides studied are wide band gap insulators, and the band gaps are significantly larger than those obtained by the standard local density approximation, thus emphasizing the importance of quasi particle corrections in perovskite fluorides.

Structural and electronic properties of MgZnO and BeMgZnO alloys are studied by the ab-initio Density Functional Theory method. Large band gap bowings are found for both kinds of alloys. The total energies as functions of the lattice constants are calculated and used to determine the ranges of composition in which the alloys are stable in the wurtzite structure. It is shown that the addition of 6% of Be can already help in stabilization of the MgZnO alloy in the wurtzite structure. The band gap can reach 7 eV for the wurtzite BexMg0.5Zn0.5-xO alloys with x approaching 0.5 and about 5.0 eV for Be0.125MgxZn0.875-xO type alloys for x approaching 0.6. Varying the alloy composition according to the presented stabilization diagram showing ranges of the x, y, for which BexMgyZn1-x-yO is stable in the wurtzite phase, one may tune band gaps over a wide spectral range, which provides flexibility in band gap engineering.

First principles density functional calculations were carried out to study the electronic structure and thermoelectric properties of LnN (Ln = La and Lu) using the full potential linearized augmented plane wave (FP-LAPW) method. The thermoelectric properties were calculated by solving the Boltzmann transport equation within the constant relaxation time approximation. The obtained lattice parameters are in good agreement with the available experimental and other theoretical results. The calculated band gaps using the Tran-Blaha modified Becke-Johnson potential (TB-mBJ), of both compounds are in good agreement with the available experimental values. Thermoelectric properties like thermopower (S), electrical conductivity scaled by relaxation time (sigma/tau) and power-factor (S-2 sigma/tau) are calculated as functions of the carrier concentration and temperature for both compounds. The calculated thermoelectric properties are compared with the available experimental results of the similar material ScN.

The electronic structure of pure and Ta-doped ZrSiO4 in the tetragonal I4(1)/amd phase with andwithout defects has been studied using the ab initio full-potential linear augmented plane wave plus local orbitals method. From the determined charge densities, the electric field gradient tensor at native Zr sites and at Ta impurities localized on cation sites of ZrSiO4 were derived and compared to experimental data obtained using hyperfine techniques. The effects of the Ta probe atom, including its different charge states on the lattice, are investigated. In addition, different types of defects, such as O or Si vacancies, Ta replacing Si, and Ta enclosed in microstructures of SiO2 phases, are examined. The combination of experiments and theory enables us to identify the different interactions observed in Ta-doped ZrSiO4 and to elucidate the role played by different defects.

Short period superlattices of the form mGaN/nAlN, where m, n denote integer numbers of monolayers, and with growth direction along the wurtzite c-axis are studied by ab initio calculations. The dependence of the band gaps on composition is compared with results obtained previously for mInN/nGaN superlattices. The strain caused by mismatch to the substrate leads to significant deformations of bonds in InN/GaN superlattices, whereas this effect is smaller in GaN/AlN superlattices. The general trends in gap behavior can to a large extend be related to the strength of the internal electric field, E, in the respective GaN and InN quantum wells. In the GaN/AlN superlattices E reaches values as high as 8 MV/cm, while in the InN/GaN superlattices E approximate to 15 MV/cm may be reached. The strong electric fields are caused by spontaneous and piezoelectric polarizations. The latter contribution dominates in InN/GaN superlattices. (C) 2015 Elsevier Ltd. All rights reserved.

The electronic structures and internal electric fields of semipolar short-period mInN/nGaN superlattices (SLs) have been calculated for several compositions (m, n). Two types of SL are considered, (112¯2) and (202¯1), corresponding to growth along the wurtzite s2 and s6 directions, respectively. The results are compared to similar calculations for polar SLs (grown in the c-direction) and nonpolar SLs (grown in the a- and m-directions). The calculated band gaps for the semipolar SLs lie between those calculated for the nonpolar and polar SLs: For s2-SLs they fall slightly below the band gaps of a-plane SLs, whereas for s6-SLs they are considerably smaller.

The electronic structures of nonpolar short-period InN/GaN superlattices grown in the wurtzite a-direction, have been calculated and compared to earlier calculations for polar superlattices (grown in the c-direction). For the nonpolar superlattices it is found that the calculated band gaps and their pressure coefficients are quite similar to those of bulk InGaN alloys with an equivalent In/Ga composition ratio. Also, they are much closer than the values calculated for polar superlattices to the photoluminescence emission energies and their pressure coefficients measured on polar structures. Possible explanations of the observed phenomena are suggested and discussed. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Quantitative high resolution transmission electron microscopy studies of intentionally grown 1InN/nGaN short-period superlattices (SLs) were performed. The structures were found to consist of an InxGa1−xN monolayer with an Indium content of x = 0.33 instead of the intended x = 1. Self-consistent calculations of the band structures of 1In0.33Ga0.67N/nGaN SLs were carried out, including a semi-empirical correction for the band gaps. The calculated band gap, Eg, as well as its pressure derivative, dEg/dp, are in very good agreement with the measured photoluminescence energy, EPL, and its pressure derivative, dEPL/dp, for a series of 1In0.33Ga0.67N/nGaN samples with n ranging from 2 to 40. This resolves a discrepancy found earlier between measured and calculated optical emission properties, as those calculations were made with the assumption of a 1InN/nGaN SL composition.

The electronic structures, densities of states, Fermi surfaces and elastic properties of AB3 (A =La, Y; B =Pb, In, Tl) compounds are studied under pressure using the full-potential linear augmented plane wave (FP-LAPW) method within the local density approximation for the exchange–correlation functional and including spin–orbit coupling. Fermi surface topology changes are found for all the isostructural AB3 compounds under compression (at V/V0 = 0.90 for LaPb3 (pressure = 8 GPa), at V/V0 = 0.98 for AIn3 (pressure = 1.5 GPa), at V/V0 = 0.80 for ATl3 (pressure in excess of 18 GPa)) apart from YPb3, although its electronic structure at zero pressure is very similar to that of LaPb3. For LaPb3 a softening of the C44 elastic constant under pressure (equivalent to 8 GPa) may be related to the appearance of a new hole pocket around the X point. From the calculated elastic properties and other mechanical properties, all the compounds investigated are found to be ductile in nature with elastic anisotropy. The states at the Fermi level (EF) are dominated by B p states with significant contributions from the A d states. For the La compounds, small hybridizations of the La f states also occur around EF.

Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/nGaN superlattices with one InN monolayer, and with different numbers of GaN monolayers (n from 1 to 40). The emission energies, EPL, measured at ambient pressure, are close to the value of the band gap, Eg, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, dEPL/dp, however, resembles that of the InN energy gap. Further, the magnitudes of both EPL and dEPL/dp are significantly higher than those obtained from abinitio calculations for 1InN/nGaN superlattices. Some causes of these discrepancies are suggested...Detailed analysis of the electronic band structure of 1InN/5GaN superlattice is performed showing that the built-in electric field plays an important role in the mInN/nGaN structures. It strongly influences the valence- and conduction-band profiles and thus determines the effective band gap.