Very recently, the 2D form of MoSi ₂ N ₄ has been successfully fabricated (Hong et al 2020 Science 369 670). Motivated by these recent experimental results, we investigate the structural, mechanical, thermal, electronic and optical properties of the MoSi ₂ N ₄ monolayer. The mechanical study confirms the stability of the MoSi ₂ N ₄ monolayer. The Young’s modulus decreases by ∼30%, while the Poisson’s ratio increases by ∼30% compared to the corresponding values of graphene. In addition, the MoSi ₂ N ₄ monolayer’s work function is very similar to that of phosphorene and MoS ₂ monolayers. The electronic structure shows that the MoSi ₂ N ₄ monolayer is an indirect semiconductor with a band gaps of 1.79 (2.35) eV using the GGA (HSE06) functional. The thermoelectric performance of the MoSi ₂ N ₄ monolayer has been revealed and a figure of merit slightly larger than unity at high temperatures is calculated. The optical analysis shows that the first absorption peak for in-plane polarization is located in the visible range of the spectrum, therefore, the MoSi ₂ N ₄ monolayer is a promising candidate for advanced optoelectronic nanodevices. In summary, the fascinating MoSi ₂ N ₄ monoloayer is a promising 2D material for many applications due to its unique physical properties.

We derive a dielectric function tensor model approach to render the optical response of monoclinic and triclinic symmetry materials with multiple uncoupled infrared and far-infrared active modes. We apply our model approach to monoclinic $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ single-crystal samples. Surfaces cut under different angles from a bulk crystal, (010) and $\left(\overline{2}01\right)$, are investigated by generalized spectroscopic ellipsometry within infrared and far-infrared spectral regions. We determine the frequency dependence of 4 independent $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ Cartesian dielectric function tensor elements by matching large sets of experimental data using a point-by-point data inversion approach. From matching our monoclinic model to the obtained 4 dielectric function tensor components, we determine all infrared and far-infrared active transverse optic phonon modes with $A_u$ and $B_u$ symmetry, and their eigenvectors within the monoclinic lattice. We find excellent agreement between our model results and results of density functional theory calculations. We derive and discuss the frequencies of longitudinal optical phonons in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. We derive and report density and anisotropic mobility parameters of the free charge carriers within the tin-doped crystals. We discuss the occurrence of longitudinal phonon plasmon coupled modes in $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ and provide their frequencies and eigenvectors. We also discuss and present monoclinic dielectric constants for static electric fields and frequencies above the reststrahlen range, and we provide a generalization of the Lyddane-Sachs-Teller relation for monoclinic lattices with infrared and far-infrared active modes. We find that the generalized Lyddane-Sachs-Teller relation is fulfilled excellently for $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$.

The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga₂O₃ thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga₂O₃ thin films. Their concentrations are high enough to fully account for the electrical compensation of Si doping. This is in clear contrast to another n-type transparent semiconducting oxide In₂O₃, where recent results show that n-type conductivity is not limited by cation vacancies but by other intrinsic defects such as O_i.

For the first time, n-type homoepitaxial semiconducting β-Ga₂O₃ layers were attained by MOVPE.