Effects of strain on the valence band structure and exciton-polariton energies in ZnO (original) (raw)
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Strain influence on valence-band ordering and excitons in ZnO: An ab initio study
Applied Physics Letters, 2007
Modern parameter-free methods to treat single-and two-particle electronic excitations are applied to compute the band structure and the lowest optical transitions of wurtzite ZnO under biaxial strain. The calculations are based on density functional theory with a spatially nonlocal exchange and correlation functional and include spin-orbit interaction. Quasiparticle shifts and excitonic effects are computed. In addition to the band parameters, also their dependence on biaxial strain and the ordering of the A, B, and C excitons are investigated. While the crystal-field splitting is very sensitive to strain, the spin-orbit splittings and the exciton binding energies remain unaffected.
Intensity of Optical Absorption Close to the Band Edge in Strained ZnO Films
Journal of the Korean Physical Society, 2008
Besides other one of the remarkable properties making wurtzite ZnO such an interesting material is its large exciton binding energy of about 60 meV, leading to stable excitons at room-temperature. Also, the Curie temperature of this wide-gap material has been predicted to lie above room temperature, making ZnO alloyed with magnetic ions a possible material for spintronics applications. One big challenge in the fabrication of ZnO-based heterostructure devices is the lattice mismatch between the ZnO lms and the substrates and the dierent thermal expansion coecients inducing biaxial strain. This work reports on the electronic band structure of biaxially strained ZnO for strains along the a-or the c-axis ranging from 1 % to 1 %, as calculated by means of the empirical pseudopotential method. Thereby, we also account for relativistic eects in the form of the spinorbit interaction, as well as for the energy dependence of the crystal potential through the use of nonlocal model potentials. Moreover, the application of a variable plane wave basis set allows us to directly obtain the strain-induced variations of the electronic and the optical properties of wurtzite ZnO.
High-pressure Raman spectroscopy study of wurtzite ZnO
Physical Review B, 2002
The high pressure behavior of optical phonons in wurtzite zinc oxide (w-ZnO) has been studied using room temperature Raman spectroscopy and ab-initio calculations based on a plane wave pseudopotential method within the density functional theory. The pressure dependence of the zonecenter phonons (E2, A1 and E1) was measured for the wurtzite structure up to the hexagonal→cubic transition near 9 GPa. Above this pressure no active mode was observed. The only negative Grüneisen parameter is that of the E low 2 mode. E1(LO) and (TO) frequencies increase with increasing pressure. The corresponding perpendicular tensor component of the Born's transverse dynamic charge e * T is experimentally found to increase under compression like e * T (P) = 2.02 + 6.4 • 10 −3 .P whereas calculations give e * T (P) = 2.09−2.5•10 −3 .P (in units of the elementary charge e, P in GPa). In both cases, the pressure variation is small, indicating a weak dependence of the bond ionicity with pressure. The pressure dependence of the optical mode energies is also compared with the prediction of a model that treats the wurtzite-to-rocksalt transition as an homogeneous shear strain. There is no evidence of anomaly in the E2 and A1 modes behavior before the phase transition.
Structural and electronic properties of ZnO under high pressures
Solid State Communications, 2006
In this work, we use first-principles calculations based on density-functional theory within the local-density approximation (LDA) to investigate the structural and electronic properties of ZnO under high-pressure. We have calculated the ground-state energy, the lattice constant, the bulk modulus, and its pressure derivative of the B4 (wurtzie), B3 (zinc blende), B2 (CsCl) and B1 (rocksalt) phases of ZnO. Moreover, the electronic structure, density of states (DOS) of the B4 (wurtzite) and B1 (rocksalt) phases of ZnO have been calculated. We show that our calculated values compare acceptably well with values reported in the literature.
Tunable Rashba effect on strained ZnO: First-principles density-functional study
Applied Physics Express, 2014
We investigated the Rashba effect on the conduction band of strained ZnO using first-principles calculations. We found that the Rashba spin rotations can be inversed by applying biaxial strain. This rotation inversion is due to the biaxial strain changing the direction of the electric polarization around the Zn atom. We also found that the amount of Rashba splitting can be controlled by tuning the strain. These findings suggest that the strained ZnO is suitable for spintronics applications.
Effect of Pressure on the Stability and Electronic Structure of ZnO0.5S0.5 and ZnO0.5Se0.5
Journal of Electronic Materials, 2017
Structures and high-pressure phase transitions in ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 have been investigated using density functional theory calculations. The previously proposed structures of ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 which are chalcopyrite (I 42d), rocksalt (Fm3m), wurtzite (P6 3 mc) and CuAu-I (P 4m2) have been fully investigated. Stabilities of these materials have been systematically studied up to 40 GPa using various approaches. We have confirmed the stability of the chalcopyrite structure up to 30 GPa for which the CuAu-I structure has been previously proposed. However, our calculation revealed that CuAu-I is not a stable structure under 32 GPa and 33 GPa for both ZnO 0.5 S 0.5 and ZnO 0.5 Se 0.5 , respectively, which could explain the failure in several attempts to fabricate these materials under such conditions. We have also examined the pressure-dependence of the bandgap and electronic structure up to 30 GPa. We can conclude from our PDOS analysis that the applied pressure does not change the atomic state characters of electronic states near the top of valence and the bottom of conduction bands, but mainly modifies the dominant Zn-3d atomic state of the deep Bloch state at À1 eV below Fermi level.
Optical properties of wurtzite and rock-salt ZnO under pressure
Microelectronics Journal, 2005
This paper reports on the pressure dependence of the optical absorption edge of ZnO in the wurtzite and rock-salt phase, up to 14 GPa. Both vapor-phase monocrystals and pulsed-laser-deposition thin films have been investigated. In both types of samples the wurtzite to rocksalt transition is observed at 9.7G0.2 GPa. The absorption tail of the fundamental gap, as measured in monocrystals, exhibits a pressure coefficient of 24.5G2 meV/GPa. The evolution under pressure of the full absorption edge of the wurtzite phase is studied with thin film samples, yielding a slightly lower pressure coefficient (23.0G0.5 meV/GPa for the A-B exciton). Rock-salt ZnO is shown to be an indirect semiconductor with a bandgap of 2.7G0.2 eV. At higher photon energy a direct transition (E gd -4.5 eV) can be also identified in thin films transited to the rock-salt phase. Results on the high-pressure phase are interpreted on the basis of density-functional-theory (DFT) electronic structure calculations. q
Electronic and optical properties of ZnO quantum dots under hydrostatic pressure
Physical Review B, 2013
In the present work, we studied the electronic and optical properties of ZnO quantum dots (QDs) subjected to externally applied hydrostatic pressure. Our single-particle calculations are based on the empirical pseudopotential method and the excitonic effects are considered by employing the configuration interaction approach. The optical band gap, Stokes shift, and optical emission polarization have been investigated as a function of the applied pressure. It is found that the applied pressure causes a linear increase in the optical band gap. The pressure coefficient appears to be highly size dependent, exhibiting a monotonic increase with increasing QD size. In contrast to this monotonic behavior, the applied pressure induces a nonmonotonic Stokes shift which presents a minimum value at a critical pressure. For pressures larger than this critical value, the optical emission polarization exhibits a sharp transition from in-plane to out-of-plane polarization. Finally, it is found that the critical pressure at which the crossing takes place strongly depends on the QD size, showing larger values for larger QD sizes. Beyond this crossing point, the lowest optically bright exciton state mainly originates from one Slater determinant, where both the single-particle electron and hole states have an S-type envelope function and the hole state originates mainly from the bulk Bloch C band.
Chemical Physics, 2017
The influence of pressure on elastic, piezoelectric (total and clamped-ion contribution), dielectric constants, Infrared and Raman spectra, and topological properties of ZnO wurtzite structure was carried out via periodic DFT/B3LYP methodology. The computational simulation indicated that, as the pressure increases, the structure becomes more rigid and an enhancement of the direct piezoelectric response along the z-direction was observed. Bader topological analysis and Hirshfeld-I charges showed a slight increase in the ionic character of Zn-O bond. Besides that, changes in the piezoelectric response are mainly due to the approach between Zn and O than to charge transfer phenomena among the two atoms. Pressure induces a sensitive displacement in the Infrared and Raman frequencies and a decrease of the E 2 mode. Nevertheless, the increase of pressure does not lead to a change in the semiconductor character, which proves that the ZnO support high pressures and can be applied in different devices.
Journal of Alloys and Compounds, 2009
Structural and mechanical properties of ZnO in four different phases, namely, B4 (wurtzite), B3 (zincblende), B2 (CsCl) and B1 (rocksalt) are determined using ab initio density functional theory (DFT) calculations. The equations of state for these structures are used in determining the transition pressures from wurtzite to zinc-blende, from wurtzite to rocksalt and zinc-blende to rocksalt structures were also determined. In particular, we report the computed anisotropic elastic properties under high pressures for the phases of B3 and B1 of ZnO. Our results for structural and mechanical properties of ZnO in four phases and the predicted values of the phase transitions for B4 → B3, B4 → B1 and B3 → B1 are compared with the available experimental and theoretical values. It is shown that the results determined in this study are compatible with the experimental and other theoretical calculations.