Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userEffects of strain on the valence band structure and exciton-polariton energies in ZnO
320
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
The uniaxial stress dependence of the band structure and the exciton-polariton transitions in wurtzite ZnO is thoroughly studied using modern first-principles calculations based on the HSE+G0W0 approach, k p modeling using the deformation potential framework, and polarized photoluminescence measurements. The ordering of the valence bands [A(G7), B(G9), C(G7)] is found to be robust even for high uniaxial and biaxial strains. Theoretical results for the uniaxial pressure coefficients and splitting rates of the A, B, and C valence bands and their optical transitions are obtained including the effects of the spin-orbit interaction. The excitonic deformation potentials are derived and the stress rates for hydrostatic pressure are determined based on the results for uniaxial and biaxial stress. In addition, the theory for the stress dependence of the exchange interaction and longitudinal-transversal splitting of the exciton-polaritons is developed using the basic exciton functions of the quasi-cubic approximation and taking the interaction between all exciton states into account. It is shown that the consideration of these effects is crucial for an accurate description of the stress dependence of the optical spectra in ZnO. The theoretical results are compared to polarized photoluminescence measurements of different ZnO substrates as function of uniaxial pressure and experimental values reported in the literature demonstrating an excellent agreement with the computed pressure coefficients.
rate research
Read More
We studied the circular polarization and angular dependences of the magneto-photoluminescence spectra of the free A-exciton 1S state in wurtzite ZnO at T = 5 K. The circular polarization properties of the spectra clearly indicate that the top valence band has Gamma_7 symmetry. The out-of-plane component of the magnetic field, which is parallel to the samples c axis, leads to linear Zeeman splitting of both the dipole-allowed Gamma_5 exciton state and the weakly allowed Gamma_1/Gamma_2 exciton states. The in-plane field, which is perpendicular to the c axis, increases the oscillator strength of the weak Gamma_1/Gamma_2 states by forming a mixed exciton state.
Low-dimensional materials differ from their bulk counterpart in many respects. In particular, the screening of the Coulomb interaction is strongly reduced, which can have important consequences such as the significant increase of exciton binding energies. In bulk materials the binding energy is used as an indicator in optical spectra to distinguish different kinds of excitons, but this is not possible in low-dimensional materials, where the binding energy is large and comparable in size for excitons of very different localization. Here we demonstrate that the exciton band structure, which can be accessed experimentally, instead provides a powerful way to identify the exciton character. By comparing the ab initio solution of the many-body Bethe-Salpeter equation for graphane and single-layer hexagonal BN, we draw a general picture of the exciton dispersion in two-dimensional materials, highlighting the different role played by the exchange electron-hole interaction and by the electronic band structure. Our interpretation is substantiated by a prediction for phosphorene.
To clarify the whole picture of the valence-band structures of prototype ferromagnetic semiconductors (III,Mn)As (III: In and Ga), we perform systematic experiments of the resonant tunneling spectroscopy on [(In_0.53Ga_0.47)_1-x Mn_x]As (x=0.06-0.15) and In_0.87Mn_0.13As grown on AlAs/ In_0.53Ga_0.47As:Be/ p+InP(001). We show that the valence band of InGaMnAs almost remains unchanged from that of the host semiconductor InGaAs, that the Fermi level exists in the band gap, and that the p-d exchange splitting in the valence band is negligibly small in (InGaMn)As. In the In0.87Mn0.13As sample, although the resonant peaks are very weak due to the large strain induced by the lattice mismatch between InP and InMnAs, our results also indicate that the Fermi level exists in the band gap and that the p-d exchange splitting in the valence band is negligibly small. These results are quite similar to those of GaMnAs obtained by the same method, meaning that there are no holes in the valence band, and that the impurity-band holes dominate the transport and magnetism both in the InGaMnAs and In_0.87Mn_0.13As films. This band picture of (III,Mn)As is remarkably different from that of II-VI-based diluted magnetic semiconductors.
Soft x-ray emission and absorption spectroscopy of the O K-edge are employed to investigate the electronic structure of wurtzite ZnO(0001). A quasiparticle band structure calculated within the GW approximation agrees well with the data, most notably with the energetic location of the Zn3d - O2p hybridized state and the anisotropy of the absorption spectra. Dispersion in the band structure is mapped using the coherent k-selective part of the resonant x-ray emission spectra. We show that a more extensive mapping of the bands is possible in the case of crystalline anisotropy such as that found in ZnO.
IV-VI semiconductor SnSe has been known as the material with record high thermoelectric performance.The multiple close-to-degenerate valence bands in the electronic band structure has been one of the key factors contributing to the high power factor and thus figure of merit in the SnSe single crystal. To date, there have been primarily theoretical calculations of this particular electronic band structure. In this paper, however, using angle-resolved photoemission spectroscopy, we perform a systematic investigation of the electronic structure of SnSe. We directly observe three predicted hole bands with small energy differences between their band tops and relatively small in-plane effective masses, in good agreement with the ab initio calculations and critical for the enhancement of the Seebeck coefficient while keeping high electrical conductivity. Our results reveal the complete band structure of SnSe and help to provide a deeper understanding of the electronic origin of the excellent thermoelectric performances in SnSe.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
