Antonio Miriametro

Polarized Light Absorption in Wurtzite InP Nanowire Ensembles

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.

Temperature dependence of interband transitions in wurtzite InP nanowires

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.

Bandgap Energy of Wurtzite InAs Nanowires

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.

Resonant depletion of photogenerated carriers in InGaAs/GaAs nanowire mats

Photoluminescence excitation spectra of InGaAs in InGaAs/GaAs heterostructure nanowire mats show clear antiresonances at the critical points of the joint density of states of the GaAs barrier. This remarkable effect arises from resonant light absorption in the upper GaAs segments, with ensuing reduction of photogenerated carriers reaching the lower InGaAs segments. The extent of this effect depends strongly on the excitation geometry of the wires, as well as on their areal density. Our work suggests that a careful design is required for optimal light conversion in nanowire-based solar cell devices.

COCHISE: Cosmological Observations from Concordia, Antarctica

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.

Investigation of Compositional Disorder in GaAsN:H

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.