Search

One hour annealing at 300 °C changes the optical emission characteristics of InAs nanowires (NWs) from the wurtzite (WZ) phase into that of zincblende (ZB). These results are accounted for by the conversion of a small fraction of the NW WZ metastable structure into the stable ZB structure. Several paths toward the polytype transformation in the configuration space are also demonstrated using first-principles calculations. For lower annealing temperatures, emission which is likely related to WZ polytypes is observed at energies that agree with theoretical predictions. These results demonstrate severe constraints on thermal processes to which devices made from InAs WZ NWs can be exposed.

This paper reports the effects of high strains on the optoelectronic properties of 2D crystals. By realizing micro- and nano-domes made of single layer transition-metal dichalcogenides, the authors demonstrate the possibility to induce a clear-cut crossover from direct to indirect bandgap in strained monolayers. The indirect excitons can be harvested and potentially stored for long times, which is relevant for flexible photovoltaics devices and for inducing bosonic condensation.

InAs nanowires (NWs) have been grown on semi-insulating InAs (111)B substrates by metal-organic chemical vapor deposition catalyzed by 50, 100, and 150 nm-sized Au particles. The pure wurtzite (WZ) phase of these NWs has been attested by high-resolution transmission electron microscopy and selected area diffraction pattern measurements. Low temperature photoluminescence measurements have provided unambiguous and robust evidence of a well resolved, isolated peak at 0.477 eV, namely 59 meV higher than the band gap of ZB InAs. The WZ nature of this energy band has been demonstrated by high values of the polarization degree, measured in ensembles of NWs both as-grown and mechanically transferred onto Si and GaAs substrates, in agreement with the polarization selection rules for WZ crystals. The value of 0.477 eV found here for the bandgap energy of WZ InAs agrees well with theoretical calculations.

COCHISE is a 2.6 meter millimetric telescope, mainly devoted to cosmological observations, located at Concordia. In this paper we report on the installation phases and preliminary operations performed with this instrument. At present, COCHISE is the largest telescope located at Concordia, for that reason it represents a pathfinder for future telescopes.

We report on millimetric continuum observations of two bright compact H II regions, which have been observed for the first time in this frequency range. For the two observed regions (G291.6-0.5 and G291.3-0.7), we derive the flux densities at the two observed wavelengths (1.25 and 2 mm) as well as the spectral index and the temperature of the surrounding dust by fitting a modified blackbody curve to our results combined with IR values obtained from the literature. We also estimate the dust mass and the bolometric luminosity of the two regions.

COCHISE (Cosmological Observations at Concordia with High-sensitivity Instrument for Source Extraction) is a 2.6 m telescope located on the high Antarctic Plateau near the Italian-French Concordia Base. The telescope is mainly devoted to Cosmological observations, able to operate between 200 mu m and 3 mm of wavelength. In this paper we describe the main characteristics of the instrument. We also report on the first light, obtained during summer 2010-2011: this result marks the beginning of millimetre astrophysical observations at Concordia. Responsivity, noise equivalent temperature and field of view of the instrument are reported. At present COCHISE is the largest telescope located at Concordia. Beside the scientific expectations, the use of this kind of instrument in the Antarctic environment poses technological aspects of relevant interest: thus COCHISE can be considered as a pathfinder for future Antarctic telescopes.

We investigate the absorption properties of ensembles of wurtzite (WZ) InP nanowires (NWs) by high-resolution polarization-resolved photoluminescence excitation (PLE) spectroscopy at T = 10 K. The degree of linear polarization of absorbed light, ρ(abs), resulting from the PLE spectra is governed by a competition between the dielectric mismatch effect and the WZ selection rules acting differently on different optical transitions. These two contributions are deconvoluted with the help of finite-difference time-domain simulations, thus providing information about the symmetry of the three highest valence bands (A, B, and C) of WZ InP and the extent of the spin-orbit interaction on these states. Moreover, ρ(abs) shows two characteristic dips corresponding to the two sharp A and B exciton resonances in the PLE spectra. A model developed for the dip in A provides the first experimental evidence of an enhancement in the dielectric mismatch effect originating from the Coulomb interaction between electron and hole.

Semiconductor nanowires (NWs) formed by non-nitride III-V compounds grow preferentially with wurtzite (WZ) lattice. This is contrary to bulk and two-dimensional layers of the same compounds, where only zincblende (ZB) is observed. The absorption spectrum of WZ materials differs largely from their ZB counterparts and shows three transitions, referred to as A, B, and C in order of increasing energy, involving the minimum of the conduction band and different critical points of the valence band. In this work, we determine the temperature dependence (T = 10-310 K) of the energy of transitions A, B, and C in ensembles of WZ InP NWs by photoluminescence (PL) and PL excitation (PLE) spectroscopy. For the whole temperature and energy ranges investigated, the PL and PLE spectra are quantitatively reproduced by a theoretical model taking into account contribution from both exciton and continuum states. WZ InP is found to behave very similarly to wide band gap III-nitrides and II-VI compounds, where the energy of A, B, and C displays the same temperature dependence. This finding unveils a general feature of the thermal properties of WZ materials that holds regardless of the bond polarity and energy gap of the crystal. Furthermore, no differences are observed in the temperature dependence of the fundamental band gap energy in WZ InP NWs and ZB InP (both NWs and bulk). This result points to a negligible role played by the WZ/ZB differences in determining the deformation potentials and the extent of the electron-phonon interaction that is a direct consequence of the similar nearest neighbor arrangement in the two lattices.

Photoluminescence and photoreflectance spectroscopy are used to investigate the electronic properties of InxGa1−xAs1−yNy/GaAs heterostructures treated with low-energy atomic hydrogen. With increasing hydrogen doses, the InxGa1−xAs1−yNy optical gap blue-shifts until it reaches the value of a corresponding N-free reference sample. This effect is ascribed to the formation of N–H bonds, which leads to an electronic passivation of N and, hence, to a reopening of the material band gap. Thermal annealing of the samples restores the band gap the InxGa1−xAs1−yNy had before hydrogenation and gives an activation energy for the dissociation of N–H complexes equal to 2.28 eV.

The electron effective mass, me, has been determined by magnetophotoluminescence in as-grown and hydrogenated Ga As1-x Nx samples for a wide range of nitrogen concentrations (from x

Photoluminescence (PL), micro-PL, and PL excitation (PLE) spectroscopy for different light polarizations have been used to investigate the electronic properties of GaAs characterized by a dominant wurtzite (WZ) phase that forms in bare GaAs and in InGaAs/GaAs heterostructure (HS) nanowires (NWs). In both cases, the GaAs luminescence exhibits very narrow emission lines, which persist up to room temperature. At 10 K, the energy of the exciton ground state recombination of GaAs NWs is equal to 1.522–1.524 eV. In HS NWs, micro-PL combined with transmission electron microscopy pinpoints the tip of the GaAs section, with a dominant WZ phase, as the origin of that emission. In PLE, two very narrow excitonic absorptions at 1.523 and 1.631 eV involve different critical points of the WZ valence band (� 9v and � 7vu). The low-energy peak shows a negligible Stokes shift with respect to PL. At 10 K, a further weak and broad PLE signal is found at 1.59 eV. The possible attribution of these lines within the present knowledge of the WZ band structure is critically discussed.

Photoluminescence (PL), micro-PL, and PL excitation (PLE) spectroscopy for different light polarizations have been used to investigate the electronic properties of GaAs characterized by a dominant wurtzite (WZ) phase that forms in bare GaAs and in InGaAs/GaAs heterostructure (HS) nanowires (NWs). In both cases, the GaAs luminescence exhibits very narrow emission lines, which persist up to room temperature. At 10 K, the energy of the exciton ground state recombination of GaAs NWs is equal to 1.522-1.524 eV. In HS NWs, micro-PL combined with transmission electron microscopy pinpoints the tip of the GaAs section, with a dominant WZ phase, as the origin of that emission. In PLE, two very narrow excitonic absorptions at 1.523 and 1.631 eV involve different critical points of the WZ valence band (Gamma(9 nu) and Gamma(7 nu mu)). The low-energy peak shows a negligible Stokes shift with respect to PL. At 10 K, a further weak and broad PLE signal is found at 1.59 eV. The possible attribution of these lines within the present knowledge of the WZ band structure is critically discussed.

Ca(BH 4 ) 2 compounds were investigated above room temperature by anelastic spectroscopy (AS) and concomitant measurements of thermogravimetry and mass spectrometry (TGA/MS). Both AS and TGA/MS indicate that even after a thermal treatment at 125 °C for 20 h, a non-negligible residual of THF adduct is still present in the sample, which can be removed on a subsequent thermal treatment at temperatures lower than 250 °C. Above 250 °C dehydrogenation takes place. Moreover, AS sensitively detects the occurrence of the α → α’ structural phase transition around 180 °C, and the α’ → β transformation, which is completed around 330 °C. We also show that both transitions are irreversible and are not accompanied by a latent heat.

The compositional disorder in as‐grown and hydrogen irradiated GaAs1−x N x has been investigated by photoluminescence (PL) and PL excitation spectra. The static disorder introduced by N atoms in the GaAs host lattice is removed upon H irradiation of the samples, as shown by a significant decrease in the free‐exciton broadening. A theoretical model developed for a purely random alloy well accounts for the dependence of the free‐exciton PLE linewidth on N concentration.

Compositional disorder is investigated by means of photoluminescence (PL) and PL excitation (PLE) measurements in as-grown and hydrogen-irradiated $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ samples $(x\ensuremath{\leqslant}0.21%)$. The dependence of the linewidth of the PLE free-exciton band on N concentration agrees well with that predicted by a theoretical model developed for a purely random alloy. We also find that hydrogen irradiation and ensuing nitrogen passivation reduce significantly the broadening of the free-exciton band. This result is consistent with a removal by hydrogen of the static disorder caused by nitrogen. Finally, an analysis of the dependence of the Stokes shift on the free-exciton linewidth shows that free carriers are thermalized even at low temperature, another indication of a low degree of disorder in the investigated samples.

The electron effective mass, ${m}_{\mathrm{e}}$, has been determined by magnetophotoluminescence in as-grown and hydrogenated $\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ samples for a wide range of nitrogen concentrations (from $xl0.01%$ to $x=1.78%$). A modified $\mathbf{k}\mathbf{∙}\mathbf{p}$ model, which takes into account hybridization effects between N cluster states and the conduction band edge, reproduces quantitatively the experimental ${m}_{\mathrm{e}}$ values up to $x\ensuremath{\leqslant}0.6%$. Experimental and theoretical evidence is provided for the N complexes responsible for the nonmonotonic and initially puzzling compositional dependence of the electron mass.

The electronic properties of nitrogen pairs have been investigated in dilute $\mathrm{Ga}{\mathrm{P}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ samples $(x\ensuremath{\leqslant}0.24%)$ by excitation photoluminescence (PLE) spectroscopy and by temperature-dependent photoluminescence (PL). PLE spectra show that three channels are mainly responsible for populating N pairs: (i) exciton tunneling from the isolated nitrogen atom level toward energetically shallow N pairs; (ii) capture of a free exciton by all pairs; and (iii) capture of a free electron and subsequent binding of a hole by energetically deep pairs. On this basis, the quenching with increasing temperature of PL lines associated with different N pairs is described quantitatively.

COCHISE is a 2.6 meter millimetric telescope devoted to cosmological observations. It is located near the Concordia Station, on the high Antarctic plateau, probably the best site in the world for (sub)millimetric observations. At present time, COCHISE is the largest telescope installed at Concordia: besides the scientific expectations, it is of great interest as a pathfinder for future Antarctic telescopes. The main characteristics of the telescope will be presented, including the scientific goals and the technical aspects related to the use of such an instrument at the extreme conditions of the Antarctic environment. Key aspects of the atmospheric transmission will be also discussed, by showing the preliminary results of site testing experiments.