Positron annihilation experiments were performed to identify native point defects in n-type GaN bulk crystals as well as in epitaxial layers. The results show that Ga vacancies are present at concentrations ${10}^{17}–{10}^{18}{\mathrm{cm}}^{-3\mathrm{}}$ in both GaN bulk crystals and layers. The Ga vacancies are negatively charged, and their concentration correlates with the intensity of the yellow luminescence. We conclude that the Ga vacancies contribute to the electrical compensation of n-type GaN and that their acceptor levels are involved in the yellow luminescence transition.

Optical emission spectrum of a resonantly (λ = 632.8 nm) excited molybdenum disulfide (MoS₂) is studied at liquid helium temperature. More than 20 peaks in the energy range spanning up to 1400 cm⁻¹ from the laser line, which are related to multiphonon resonant Raman scattering processes, are observed. The attribution of the observed lines involving basic lattice vibrational modes of MoS₂ and both the longitudinal (LA(M)) and the transverse (TA(M) and/or ZA(M)) acoustic phonons from the vicinity of the high-symmetry M point of the MoS₂ Brillouin zone is proposed.

The intriguing physics of carrier-carrier interactions, which likewise affect the operation of light emitting devices, stimulate the research on semiconductor structures at high densities of excited carriers, a limit reachable at large pumping rates or in systems with long-lived electron-hole pairs. By electrically injecting carriers into WSe₂/MoS₂ type-II heterostructures which are indirect in real and k-space, we establish a large population of typical optically silent interlayer excitons. Here, we reveal their emission spectra and show that the emission energy is tunable by an applied electric field. When the population is further increased by suppressing the radiative recombination rate with the introduction of an hBN spacer between WSe₂ and MoS₂, Auger-type and exciton-exciton annihilation processes become important. These processes are traced by the observation of an up-converted emission demonstrating that excitons gaining energy in non-radiative Auger processes can be recovered and recombine radiatively.