Recently, oxide semiconductors have attracted tremendous attention due to their applications in various fields from UV emitters, power electronics, nano-sensors and optoelectronics. Furthermore, global warming forces new solutions to limit greenhouse gas emissions and mitigate the negative effects of climate change. By performing thermodynamic calculations based on free-Gibbs energy and equilibrium constants of seven theoretical reactions for two oxides systems ZnO and Ga2O3, we confirm that such semiconductors could be grown by decomposition of carbon dioxide. In addition, we present a modified carbothermal method of growing ZnO nanowires. Nanowires obtained by us display excellent structural and optical properties with FWHM of photoluminescence at T = 8 K from donor bound exciton equal to 0.5 meV. We strongly believe that our modified carbothermal method is an analog of the growth process of large ZnO bulk crystals, which occurred previously in the Olawa foundry in Poland.
We study the crystal structure-mismatched, quantum dot-like (QD) composite metamaterial based on rock-salt narrow gap PbTe and zinc-blende wide gap CdTe. In the case of PbTe/CdTe QDs the quantum effects are observed for dots with dimensions of the order of 100 nm even at room temperature making this material system a promising candidate for mid-infrared applications. Using molecular beam epitaxy technique and utilizing difference in crystal structure of both semiconductors a variety of samples containing PbTe nanostructures with complicated morphology embedded in CdTe were obtained. Investigated nanocomposite PbTe/CdTe samples exhibit unusually strong and surprisingly narrow (about 5 meV) mid-infrared photoluminescence emission (250 meV) in contrast to the spectrally wide luminescence of a typical ensemble of PbTe/CdTe quantum dots measured in similar temperature (about 100 K) and excitation power conditions (400 microW). For excitation power exceeding 500 microW additional emission in energy about 15 meV lower than previous one appears, which dominates PL spectrum for excitation higher than 800 microW. This line, not reported for such kind of QDs yet, exhibit non-linear dependence of amplitude on excitation power. We discuss the observed behavior of photoluminescence considering presence of two-dimensional electron gas with high electron mobility and carrier density up to 10^19 cm-3 spontaneously formed close to the polar CdTe/PbTe interfaces. As estimated plasmon energy in our samples (240 meV) matches well the energy of observed emission, non-resonant coupling of photons with interface plasmons mediated by LO phonon is most possible explanation of unusual enhancement of PL from studied PbTe/CdTe metamaterial.
Cubic ZnxMg1-xO have been proposed as wide bandgap semiconductors for short wavelength optoelectronic applications operating in the deep UV region. By combing MBE growth and HRTEM we were able to determine conditions in which ZnO and ZnxMg1-xO alloys in the rocksalt phase can be grown on MgO substrates. It was found that the maximum ZnxMg1-xO layer thickness strongly depends on Zn concentration, decreasing with x, which reflects the alloy phase instability.
The band structures of rocksalt ZnxMg1-xO alloys were calculated in a supercell geometry by density functional theory in the Local Density Approximation (LDA). The atomic coordinates were determined using pseudopotentials implemented in the VASP Simulation Package. Then, the band structures were obtained by a Linear-Muffin-Tin-Orbital method in a full-potential version with a semi-empirical correction (LDA+C) for the band gaps.
As MgO in the rocksalt structure has a direct band gap and ZnO has an indirect one, we expected transition: direct to the indirect gap for a certain content, x, of Zn.
However, it is shown, that the ZnxMg1-xO band gaps depend strongly on the local arrangement of atoms in a 64 atoms supercell. For each concentration of Zn we obtained a set of the band gap values depending on the arrangement of atoms. Instead of two crossing lines illustrating the dependence of the direct and indirect gaps on composition, we got two crossing bands. The crossing of the two bands covers composition from 10% of Zn up to almost 70% of Zn. The results are compared with the experimental data.
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