We report quasi-particle energy calculations of the electronic bandstructure as measured by valence-band photoemission for selected II–VI compounds and group III nitrides. By applying GW as perturbation to the ground state of the fictitious, non-interacting Kohn–Sham electrons of density-functional theory (DFT), we systematically study the electronic structure of zinc-blende GaN, ZnO, ZnS and CdS. Special emphasis is put on analysing the role played by the cation semicore d-electrons that are explicitly included as valence electrons in our pseudo-potential approach. Unlike in the majority of previous GW studies, which are almost exclusively based on ground state calculations in the local-density approximation (LDA), we combine GW with exact-exchange DFT calculations in the optimized-effective potential approach (OEPx). This is a much more elaborate and computationally expensive approach. However, we show that applying the OEPx approach leads to an improved description of the d-electron hybridization compared to the LDA. Moreover, we find that it is essential to use OEPx pseudo-potentials in order to treat core–valence exchange consistently. Our OEPx-based quasi-particle valence bandstructures are in good agreement with available photoemission data in contrast to the ones based on the LDA. We therefore conclude that for these materials, OEPx constitutes the better starting point for subsequent GW calculations.

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Combining GW calculations with exact-exchange density-functional theory: an analysis of valence-band photoemission for compound semiconductors

Caracterisation optique et physique de ZnSe. Etude des macles et des deformations thermoelastiques d'une couche ZnSe sur un substrat. Etude des excitons libres dans des films de ZnSe, des impuretes et defauts dans la couche de ZnSe epitaxique, et de leurs proprietes optiques non lineaires

Optical Properties of ZnSe Epilayers and Films

Semiconductor based nanoscale heterostructures are promising candidates for photocatalytic and photovoltaic applications with the sensitization of a wide bandgap semiconductor with a narrow bandgap material being the most viable strategy to maximize the utilization of the solar spectrum. Here, we present a simple wet chemical route to obtain nanoscale heterostructures of ZnO/CdS without using any molecular linker. Our method involves the nucleation of a Cd-precursor on ZnO nanorods with a subsequent sulfidation step leading to the formation of the ZnO/CdS nanoscale heterostructures. Excellent control over the loading of CdS and the microstructure is realized by merely changing the initial concentration of the sulfiding agent. We show that the heterostructures with the lowest CdS loading exhibit an exceptionally high activity for the degradation of methylene blue (MB) under solar irradiation conditions; microstructural and surface analysis reveals that the higher activity in this case is related to the dispersion of the CdS nanoparticles on the ZnO nanorod surface and to the higher concentration of surface hydroxyl species. Detailed analysis of the mechanism of formation of the nanoscale heterostructures reveals that it is possible to obtain deterministic control over the nature of the interfaces. Our synthesis method is general and applicable for other heterostructures where the interfaces need to be engineered for optimal properties. In particular, the absence of any molecular linker at the interface makes our method appealing for photovoltaic applications where faster rates of electron transfer at the heterojunctions are highly desirable.

Nanoscale ZnO/CdS heterostructures with engineered interfaces for high photocatalytic activity under solar radiation

ZnSe nanoribbons have been synthesized using sputter-coated gold films as catalysts via metalorganic chemical vapor deposition on Si (100) substrates. Both x-ray and selected area electron diffractions determine that they have the zinc-blende structure. High-resolution transmission electron microscopic investigations show that their structure is highly ordered and contains coherent twin lamellae near one edge but is essentially free of dislocations. Photoluminescence studies at 10 K show that sharp excitonic peaks dominate their spectra, reflecting their high purity and nearly perfect stoichiometry. New excitonic peaks are observed in the nanoribbons and their possible origins are discussed.

Structure and photoluminescence of ZnSe nanoribbons grown by metal organic chemical vapor deposition

Doping limits in II–VI compounds — Challenges, problems and solutions

Analysis of strain and impurity distribution in II-VI epilayers with optical methods

Shallow impurity- and defect-related complexes in undoped ZnSe crystals

Phase transition from the cubic to the hexagonal modification in thin CdS films on InP(110)

Using reflection, photoluminescence and excitation spectroscopy the authors carried out systematic studies on the optical properties in the excitonic energy region of ZnTe/GaAs epilayers grown by hot-wall epitaxy (HWE). They calculate the strain magnitude, the light-hole (lh) and heavy-hole (hh) energy shifts, and compare the theoretical values with the experimentally observed energy positions in ZnTe films of different thicknesses. The detected energy distance between the free excitons Xlh and Xhh decreases with increasing layer thickness, indicating a relaxation of the thermally induced strain in the ZnTe layer. Additionally, the authors investigate the incorporation of different impurities, the electronic structure of the main acceptor-exciton complexes (A0,X) and the high-photon-density spectra of the ZnTe films. For increasing densities a biexciton band appears, and they determine the biexciton binding energy to amount to about 3.3+or-1.0 meV.

https://iopscience.iop.org/article/10.1088/0268-1242/6/9A/016/pdf

G. Kudlek

Papers

Optical Properties of ZnSe Epilayers and Films

Analysis of strain and impurity distribution in II-VI epilayers with optical methods

Shallow impurity- and defect-related complexes in undoped ZnSe crystals

Phase transition from the cubic to the hexagonal modification in thin CdS films on InP(110)

Photoluminescence and excitation spectroscopy of ZnTe/GaAs epilayers grown by hot-wall epitaxy