Thin layers of black phosphorus have recently raised interest owing to their two-dimensional (2D) semiconducting properties, such as tunable direct bandgap and high carrier mobilities. This lamellar crystal of phosphorus atoms can be exfoliated down to monolayer 2D-phosphane (also called phosphorene) using procedures similar to those used for graphene. Probing the properties has, however, been challenged by a fast degradation of the thinnest layers on exposure to ambient conditions. Herein, we investigate this chemistry using in situ Raman and transmission electron spectroscopies. The results highlight a thickness-dependent photoassisted oxidation reaction with oxygen dissolved in adsorbed water. The oxidation kinetics is consistent with a phenomenological model involving electron transfer and quantum confinement as key parameters. A procedure carried out in a glove box is used to prepare mono-, bi- and multilayer 2D-phosphane in their pristine states for further studies on the effect of layer thickness on the Raman modes. Controlled experiments in ambient conditions are shown to lower the A(g)(1)/A(g)(2) intensity ratio for ultrathin layers, a signature of oxidation.

GaAs 1−x Bi x epilayers with bismuth concentrations up to x=3.1% were grown on GaAs by molecular beam epitaxy. The Bi content in the films was measured by Rutherford backscattering spectroscopy. X-ray diffraction shows that GaAsBi is pseudomorphically strained to GaAs but that some structural disorder is present in the thick films. The extrapolation of the lattice constant of GaAsBi to the hypothetical zincblende GaBi alloy gives 6.33±0.06 Å. Room-temperature photoluminescence of the GaAsBi epilayers is obtained and a significant redshift in the emission of GaAsBi of ∼84 meV per percent Bi is observed.

The band gap of GaAsBi epitaxial layers as a function of bismuth concentration up to 3.6% is determined. The optical transitions were measured by modulated electroreflectance. The energy of the band gap decreases at a linearized rate of 88 meV/% Bi, or 83 meV/% Bi for the heavy hole to conduction band transition for GaAsBi strained to GaAs. The valence-band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs.

We report a giant bowing of the spin-orbit splitting energy Delta0 in the dilute GaAs1-xBix alloy for Bi concentrations ranging from 0% to 1.8%. This is the first observation of a large relativistic correction to the host electronic band structure induced by just a few percent of isoelectronic doping in a semiconductor material. It opens up the possibility of tailoring the spin-orbit splitting in semiconductors for spintronic applications.

We describe the growth of hexagonal GaN on Si(111) by gas source molecular beam epitaxy with ammonia. The initial deposition of Al, at 1130–1190 K, resulted in a very rapid transition to a two-dimensional growth mode of AlN. The rapid transition is essential for the subsequent growth of high quality GaN and AlGaN. This procedure also resulted in complete elimination of cracking in thick (>2 μm) GaN layers. For layers thicker than 1.5 μm, the full width at half maximum of the (0002) GaN diffraction peak was less than 14 arc sec. We show that a short period superlattice of AlGaN/GaN grown on the AlN buffer can be used to block defects propagating through GaN, resulting in good crystal and luminescence quality. At room temperature, the linewidth of the GaN exciton recombination peak was less than 40 meV, typical of the best samples grown on sapphire.

Owing to its crystallographic structure, black phosphorus is one of the few 2D materials expressing strongly anisotropic optical, transport, and mechanical properties. We report on the anisotropy of electron-phonon interactions through a polarization-resolved Raman study of the four vibrational modes of atomically thin black phosphorus (2D phosphane): the three bulk-like modes A, B, and A and the Davydov-induced mode labeled A(B). The complex Raman tensor elements reveal that the relative variation in permittivity of all A modes is irrespective of the atomic motion involved lowest along the zigzag direction, the basal anisotropy of these variations is most pronounced for A and A(B), and interlayer interactions in multilayer samples lead to reduced anisotropy. The bulk-forbidden A(B) mode appears for n ≥ 2 and quickly subsides in thicker layers. It is assigned to a Davydov-induced IR to Raman conversion of the bulk IR mode B and exhibits characteristics similar to A. Although this mode is expected to be weak, an electronic resonance significantly enhances its Raman efficiency such that it becomes a dominant mode in the spectrum of bilayer 2D phosphane.

We report a study of the luminescence properties of coherently strained GaAs1−xNx grown on GaAs by metalorganic molecular beam epitaxy. Well-defined photoluminescence was observed in samples with a nitrogen concentration up to 3%. Samples subjected to thermal anneals, investigated by x-ray diffraction and photoluminescence, show increased nitrogen incorporation and significant improvements in the luminescence efficiency. A band-gap reduction of more than 400 meV, compared to GaAs, is observed for a nitrogen concentration of ∼3%. For the range of nitrogen concentrations investigated here, the band gap follows predictions of the dielectric model of Van Vechten [J. A. Van Vechten and T. K. Bergstresser, Phys. Rev. B 1, 3351 (1970), and references therein].

Gallium selenide (GaSe) is a 2D material with a thickness-dependent gap, strong non-linear optical coefficients and uncommon interband optical selection rules, making it interesting for optoelectronic and spintronic applications. In this work, we monitor the oxidation dynamics of GaSe with thicknesses ranging from 10 to 200 nm using Raman spectroscopy. In ambient temperature and humidity conditions, the intensity of all Raman modes and the luminescence decrease rapidly with moderate exposure to above-gap illumination. Concurrently, several oxidation products appear in the Raman spectra: Ga 2 Se 3 , Ga 2 O 3 and amorphous and crystalline selenium. We find that no safe measurement power exists for optical measurements on ultrathin GaSe in ambient conditions. We demonstrate that the simultaneous presence of oxygen, humidity, and abovegap illumination is required to activate this photo-oxidation process, which is attributed to the transfer of photo-generated charge carriers towards aqueous oxygen at the sample surface, generating highly reactive superoxide anions that rapidly degrade the sample and quench the optical response of the material.