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.
Measurements of photoluminescence and its dependence on hydrostatic pressure are performed on a set of InN/<i>n</i>GaN superlattices with one InN monolayer, and with different numbers of GaN monolayers (<i>n</i> from 1 to 40). The emission energies, <i>E<sub>PL</sub></i>, measured at ambient pressure, are close to the value of the band gap, <i>E<sub>g</sub></i>, in bulk GaN, in agreement with other experimental findings. The pressure dependence of the emission energies, <i>dE<sub>PL</sub>/dp</i>, however, resembles that of the InN energy gap. Further, the magnitudes of both <i>E<sub>PL</sub></i> and <i>dE<sub>PL</sub>/dp</i> are significantly higher than those obtained from <i>abinitio</i> calculations for <i>1</i>InN/<i>n</i>GaN superlattices. Some causes of these discrepancies are suggested...Detailed analysis of the electronic band structure of <i>1</i>InN/<i>5</i>GaN superlattice is performed showing that the built-in electric field plays an important role in the <i>m</i>InN/<i>n</i>GaN structures. It strongly influences the valence- and conduction-band profiles and thus determines the effective band gap.
Large bowings of the band gap and its pressure coefficient in In-containing nitride semiconductor alloys are observed.
Photoluminescence measurements for In<sub>x</sub>Ga<sub>1-x</sub>N and In<sub>x</sub>Al<sub>1-x</sub>N combined with other experimental data show large scatter of the results. A comparison with <i>ab-initio </i>calculations suggests that this scatter can be ascribed to the formation of In clusters during the sample preparation. The explanation of the observed anomalies taking into account chemical and size effects indicates a specific nature of InN, different from other nitrides and other In-based binary semiconductors.