The value and the nature of the bandgap of In4Se3 are still not well defined, with a large spread of the experimental data between 0.42 and 1.68 eV and an uncertain nature, predicted to be indirect by ab initio band structure calculations. Here we report on the optical transmission and photoluminescence (PL) performed in In4Se3 thin films grown by coevaporation on (0001)-oriented sapphire wafers. The quality of the polycrystalline layers allows the first detection of the excitonic-like transition in the optical absorption of this compound at low temperature. The PL detected under weak laser excitation shows a bound exciton emission at 0.75 eV. Strong laser irradiation reveals a quadratic dependence of the PL intensity on the optical excitation, which demonstrates a stimulated emission at 0.79 eV in relation with an exciton–exciton scattering process. On the basis of a reasonable estimate of the exciton energy, equal to 10 − 15 meV, we evaluate the direct bandgap of In4 Se3 to 0.82 ± 0.01 eV at low temperature.

We study the deep-ultraviolet emission in Bernal boron nitride as a function of temperature. The quasidegeneracy of indirect and direct excitons in Bernal boron nitride leads to their simultaneous recombination, allowing a comparison of phonon-assisted broadening in the two cases. Temperature-dependent measurements reveal that below 200 K, the efficiency of phonon-assisted broadening is one order of magnitude lower for the direct transition at 6.035 eV than for the phonon replicas of the indirect exciton. This striking effect results from the inhibition of quasielastic acoustic phonon scattering in the strong-coupling regime of the light-matter interaction, where the density of final states is reduced by the curvature of the polaritonic dispersion.

InGaN/GaN single quantum wells were grown by molecular beam epitaxy on the silicon substrate onto thin AlN and GaN buffer layers. The InGaN/GaN structure is porosified using a combination of SixNy nanomasking and sublimation and compared with a non-porous reference. The photoluminescence efficiency at room temperature of the porosified sample is improved by a factor reaching 40 compared with the reference sample. Plan-view and cross-sectional transmission electron microscopy images reveal that the remaining material is free of dislocation cores. The regions around dislocations are, thus, preferentially sublimated. This explains the strong photoluminescence improvement of nanoporous InGaN/GaN samples.

In this work we investigate the properties of negatively charged nitrogen-vacancy (NV−) centres created during single crystal diamond growth by chemical vapour deposition (CVD) on [113]-oriented substrates and with N2O as a dopant gas. The use of spin echo and double electron-electron resonance (DEER) allows us to assess NV− ratio with respect to substitutional nitrogen impurities (N_S0) incorporated during growth, a critical figure of merit for quantum technologies. We demonstrate that, at moderate growth temperatures (800 °C), dense NV− ensembles of several hundreds of ppb (800 ppb for 50 ppm of added N2O) and with exceptionally high NV−/N_S0 ratios of up to 25% can be achieved. This NV− creation yield is higher by at least an order of magnitude to that typically obtained on standard [100]-grown diamonds and comparable to the best values reported for electron-irradiated diamonds. The material obtained here thus advantageously combines a high NV− density, high creation yield, long coherence times of several tens of μs together with a partial preferential orientation. These are highly desirable requirements for diamond-based quantum sensors that may spur new developments with this crystalline orientation leading to improved performance and sensitivity.

A porous InGaN/GaN blue light-emitting diode isdemonstrated using selective area sublimation. Transmissionelectron microscopy reveals that the structure is porous down tothe Si substrate; however, the porosity is higher in the GaN buffer,while smaller pores are observed in the active region. This changeof porosity between the active region and the buffer is explained bythe modification of the dislocation pattern in the heterostructure,which is evidenced by weak beam transmission electronmicroscopy on a nonporosified reference sample. Cathodolumi-nescence mapping and electron beam-induced current microscopy(EBIC) analyses are used to probe the impact of porosification onthe optical and electrical properties of the structure at nanoscaledimensions. It is observed that neither the quantum well emissionnor the p−n junction EBIC spatial profile was degraded after porosification with respect to the nonannealed reference sample. A light-emitting diode with a fully porous active region is fabricated using a parylene pore filling for electrical insulation, and its electroluminescence is analyzed.