We have examined the influence of flake-substrate effects that affect one and few layers of MoS ₂ in terms of their electrical and optical properties. In the measurements, we used SiO ₂ /Si substrates with etched cavities and aluminum electrodes. Suspended areas are easily identifiable both on images depicting the topography and on the surface potential maps measured with the Kelvin probe force microscopy. Compared to the SiO ₂ /Si supported material, surface potential decrease is observable at the membrane. The surface potential value of the flakes located on the electrodes is the lowest. PL measurements prove that single MoS ₂ monolayer was obtained. Suspended regions are also correlated with maps obtained as a result of Raman spectroscopy.

In this letter, the authors demonstrate large size $m$ -plane GaN substrates grown by ammonothermal method. These substrates have excellent structural quality. The concentration of threading dislocation density is below $5\times {10}^4{\text{cm}}^{-2}$ and the full width at half maximum for the symmetrical and asymmetrical peaks equals 16 and 19 arc sec, respectively. Also good optical quality, the energy gap-related transition is clearly observed at room temperature in photoluminescence and contactless electroreflectance spectra. GaN epilayers deposited on these substrates exhibit intrinsic narrow exciton lines which are very sensitive to the optical selection rules typical for hexagonal symmetry, proving truly nonpolar character of the material.

Contactless electroreflectance is applied to study direct optical transitions from the heavy hole, light hole, and spin-orbit split-off band to the conduction band in compressively strained Ge_1−xSn_x layers of various Sn concentrations at room temperature. It is shown that the energies of these transitions are in very good agreement with theoretical predictions, which take into account non-linear variation of bandgap and spin-orbit splitting plus the strain-related shifts obtained from the Bir-Pikus theory. The bowing parameter for the direct bandgap has been determined to be 1.8 ± 0.2 eV and agree with this one obtained within ab initio calculations, which is 1.97 eV (for indirect bandgap the bowing parameter is 0.26 eV).

Photoreflectance has been applied to study the exciton energies and linewidths for hetero- and homoepitaxial GaN layers (i.e., GaN layers grown on sapphire and truly bulk GaN crystal obtained by ammonothermal method). In order to modulate the built-in electric field inside the samples and eliminate photoluminescence signal from photoreflectance spectra, the surface band bending was modulated by the laser light with the photon energy smaller than the energy gap of GaN, i.e., a 532 nm laser line. The Varshni-type and Bose–Einstein-type parameters that describe the temperature dependence of the exciton transition energies have been evaluated and compared for the two GaN epilayers. It has been concluded that the residual strain in the heteroepitaxial layer influences the exciton transition energy but does not influence the Varshni and Bose–Einstein parameters. It has been found that this strain significantly influences the exciton linewidth. The broadening parameter, which is associated with the temperature-independent mechanisms (Γ0 parameter corresponding to the exciton linewidth at 0 K), has been found to be ∼1 meV for the homoepitaxial layer. For the heteroepitaxial layer this parameter is twice higher (this work) or few times higher (previous papers).