Abderrezak Belabbes

We investigate heterostructures composed of hexagonal Ge quantum wells embedded within hexagonal Si 0.25 Ge 0.75 alloy layers, systematically considering variations in crystallographic orientation, well thickness, and external biaxial strain. This study employs density functional theory together with accurate functionals for calculations of the quasiparticle electronic structure. Overall, the heterostructures exhibit type-I band alignment, with the localization of electron and hole wave functions predominantly within the hexagonal Ge regions. The quantum confinement of both charge carrier types is determined by the orientation and thickness of the Ge layer for three different external strains. The results obtained can be interpreted within a rectangular potential-well model, with characteristic parameters derived from band structures of hexagonal Ge and Si 0.25 Ge 0.75 , as well as their strained multiquantum well structures, properly aligned via the branch point energies. Besides confinement energies and subbands, optical transition strengths are also discussed.

By means of ab initio calculations we demonstrate that perturbed hexagonal germanium is a superior material for active optoelectronic devices in the infrared spectral region. Perfect lonsdaleite germanium is a pseudodirect semiconductor, with a direct fundamental band gap but almost vanishing optical transitions. Perturbing the system by replacing a germanium atom with a silicon atom in the primitive cell increases the oscillator strength at the onset gap by orders of magnitude, with a concurrent blue shift of the transition energies. This effect is mainly due to the increased s character of the lowest conduction band because of the perturbation‐induced wave function mixing. A structural distortion of pure lonsdaleite Ge can enhance as well optical oscillator strengths, but their magnitude significantly depends on the specific details of the resulting atomic geometry. In particular, moderate tensile uniaxial strain can induce an order inversion of the two lowest conduction bands, leading to an extremely efficient enhancement of the dipole matrix elements at the minimum gap. In general, chemical and/or structural perturbations are shown to make lonsdaleite germanium a suitable material for light emitting devices.

By means of an approximate quasiparticle description we study the electronic structure of wurtzite‐GaP in more detail for the two lowest and conduction bands and the three highest , , and valence bands. We conclude that the corresponding three gaps between the valence bands and the conduction band are quasi‐direct, while the ones involving the s‐like conduction band are direct. The optical oscillator strengths are also calculated outside the point. Their influence on the observability of excitons in absorption and emission spectra is investigated. The almost dipole‐forbidden transitions into the lowest p‐like conduction band become substantial for finite wave vectors perpendicular to the c‐axis. Therefore, the finite extent of the exciton envelope functions gives rise to non‐vanishing transition strengths, which explain the intense photoluminescence observed experimentally by forbidden p‐type excitons.

We report results of first-principles calculations based on the projector augmented wave (PAW) method to explore the structural, thermodynamic, and electronic properties of cubic (3C) and hexagonal (6H, 4H, and 2H) polytypes of II-VI compounds: ZnS, ZnSe, and ZnTe. We find that the different bond stacking in II-VI polytypes remarkably influences the resulting physical properties. Furthermore, the degree of hexagonality is found to be useful to understand both the ground-state properties and the electronic structure of these compounds. The resulting lattice parameters, energetic stability, and characteristic band energies are in good agreement with available experimental data. Trends with hexagonality of the polytype are investigated.

Recently several hexagonal polytypes of III-V compounds have been discovered during growth of nanowires. They exhibit a spontaneous polarization. We calculate the polarization using the ab initio density functional theory within the Berry phase and the electrostatic method. We demonstrate its clear relationship to the crystal field characterized by internal-cell parameters and polytype hexagonality. Sign and magnitude of the polarization field in wurtzite follow the deviation of the internal-cell parameter from its ideal value, thereby explaining the opposite behavior of III-nitrides and the corresponding phosphides, arsenides, and antimonides. DOI: http://dx.doi.org/10.1103/PhysRevB.87.035305 ©2013 American Physical Society

The quasiparticle band structures of four polytypes 3C, 6H, 4H, and 2H of GaP, GaAs, GaSb, InP, InAs, and InSb are computed with high accuracy including spin-orbit interaction applying a recently developed approximate calculation scheme, the LDA-1/2 method. The results are used to derive band offsets $\Delta${}Ec and $\Delta${}Ev for the conduction and valence bands between two polytypes. The alignment of the band structures is based on the branch-point energy EBP for each polytype. The aligned electronic structures are used to explain properties of heterocrystalline but homomaterial junctions. The gaps and offsets allow to discuss spectroscopic results obtained recently for such junctions in III-V nanowires.