Marta De Luca

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.

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.

Determination of exciton reduced mass and gyromagnetic factor of wurtzite (InGa)As nanowires by photoluminescence spectroscopy under high magnetic fields.

Semiconductor nanowires (NWs) have the prospect of being employed as basic units for nanoscale devices and circuits. However, the impact of their one-dimensional geometry and peculiar crystal phase on transport and spin characteristics remains largely unknown. We determine the exciton reduced mass and gyromagnetic factor of (InGa)As NWs in the wurtzite phase by photoluminescence (PL) spectroscopy under very high magnetic fields. For B perpendicular to the NW ĉ axis, the exciton reduced mass is 10% greater than that expected for the zincblende phase and no field-induced circular polarization of PL is observed. For B parallel to ĉ, an exciton reduced mass 35% greater than that of the zincblende phase is derived. Moreover, a circular dichroism of 70% is found at 28 T. Finally, an analysis of the PL line shape points at two Zeeman split levels, whose separation corresponds to an exciton gyromagnetic factor |g(e) - g(h,∥)| = 5.8. These results provide a quantitative estimate of the basic electronic and spin properties of NWs and may guide a theoretical analysis of the band structure of these fascinating nanostructures.

Reduced temperature sensitivity of the polarization properties of hydrogenated InGaAsN V-groove quantum wires

We investigated the effects of hydrogen irradiation on the degree of linear polarization, ρ, of the light emitted by site-controlled, dilute-nitride InGaAsN V-groove quantum wires (QWRs). While in the as-grown sample the polarization of the QWR emission is highly sensitive to the increasing temperature (T), after sample hydrogenation the value of ρ remains nearly unchanged (and ∼25%) for T ≤ 220 K. This observation—potentially important for the development of devices based on the QWR polarization—points to a larger energy separation between hole subbands in the hydrogenated QWRs, due to the strain increase associated with the H-induced passivation of nitrogen.

Effects of dielectric stoichiometry on the photoluminescence properties of encapsulated WSe2 monolayers

Two-dimensional transition metal dichalcogenide semiconductors have emerged as promising candidates for optoelectronic devices with unprecedented properties and ultra-compact footprints. However, the high sensitivity of atomically thin materials to the surrounding dielectric media imposes severe limitations on their practical applicability. Hence, to enable the effective integration of these materials in devices, the development of reliable encapsulation procedures that preserve their physical properties is required. Here, the excitonic photoluminescence (at room temperature and 10 K) is assessed on mechanically exfoliated WSe2 monolayer flakes encapsulated with SiOx and AlxOy layers by means of chemical and physical deposition techniques. Conformal coating on untreated and non-functionalized flakes is successfully achieved by all the techniques examined, with the exception of atomic layer deposition, for which a cluster-like oxide coating is formed. No significant compositional or strain state changes in the flakes are detected upon encapsulation, independently of the technique adopted. Remarkably, our results show that the optical emission of the flakes is strongly influenced by the stoichiometry quality of the encapsulating oxide. When the encapsulation is carried out with slightly sub-stoichiometric oxides, two remarkable phenomena are observed. First, dominant trion (charged exciton) photoluminescence is detected at room temperature, revealing a clear electrical doping of the monolayers. Second, a strong decrease in the optical emission of the monolayers is observed, and attributed to non-radiative recombination processes and/or carrier transfer from the flake to the oxide. Power- and temperature-dependent photoluminescence measurements further confirm that stoichiometric oxides obtained by physical deposition lead to a successful encapsulation, opening a promising route for the development of integrated two-dimensional devices.

Single-step Au-catalysed synthesis and microstructural characterization of core–shell Ge/In–Te nanowires by MOCVD

ABSTRACT We report on the self-assembly of core–shell Ge/In–Te nanowires (NWs) on single crystal Si substrates by Metalorganic Chemical Vapour Deposition (MOCVD), coupled to the Vapour–Liquid–Solid (VLS) mechanism, catalysed by Au nanoparticles (NPs). The NWs are formed by a crystalline Ge core and an InTe (II) shell, have diameters down to 15 nm and show <110> oriented growth direction. The role of the MOCVD process parameters and of the NPs size in determining the NWs core–shell microstructure and their alignment was investigated by high-resolution TEM, EDX, XRD and Raman spectroscopy. GRAPHICAL ABSTRACT IMPACT STATEMENT A novel one-step MOCVD growth procedure, yielding self-assembled, oriented and crystalline core–shell Ge/In–Te nanowires with diameters down to 15 nm, along with their chemical–physical characterization.

Quantitative determination of In clustering in In-rich InxGa1−xN thin films

We investigated atomic ordering in In-rich InxGa1−xN epilayers in order to obtain an understanding of whether a deviation from a random distribution of In atoms in the group-III sublattice could be the origin of the strong carrier localization and defect-insensitive emission of these semiconductor alloys. This phenomenon can be exploited for application in optoelectronics. By coupling In K-edge x-ray absorption spectroscopy and high resolution x-ray diffraction, we were able to discard the hypothesis of significant phase separation into InN + GaN, in agreement with previous N K-edge absorption spectroscopy. However, we found an enrichment of In neighbours in the second atomic shell of In as compared to random statistics (clustering) for x = 0.82, while this is not the case for x = 0.46. This result, which is also supported by optical spectroscopy, is likely to stimulate new theoretical studies on InxGa1−xN alloys with a very high In concentration.

Crystalline, Phononic, and Electronic Properties of Heterostructured Polytypic Ge Nanowires by Raman Spectroscopy

Semiconducting nanowires (NWs) offer the unprecedented opportunity to host different crystal phases in a nanostructure, which enables the formation of polytypic heterostructures where the material composition is unchanged. This characteristic boosts the potential of polytypic heterostructured NWs for optoelectronic and phononic applications. In this work, we investigate cubic Ge NWs where small (∼20 nm) hexagonal domains are formed due to a strain-induced phase transformation. By combining a nondestructive optical technique (Raman spectroscopy) with density-functional theory (DFT) calculations, we assess the phonon properties of hexagonal Ge, determine the crystal phase variations along the NW axis, and, quite remarkably, reconstruct the relative orientation of the two polytypes. Moreover, we provide information on the electronic band alignment of the heterostructure at points of the Brillouin zone different from the one (Γ) where the direct band gap recombination in hexagonal Ge takes place. We demonstrate the versatility of Raman spectroscopy and show that it can be used to determine the main crystalline, phononic, and electronic properties of the most challenging type of heterostructure (a polytypic, nanoscale heterostructure with constant material composition). The general procedure that we establish can be applied to several types of heterostructures.

Semiconductor Nanowires: Raman Spectroscopy Studies

Nanowires (NWs) are filamentary crystals with diameters of tens of nanometers and lengths of few microns. Semiconductor NWs have recently attracted a great interest, because they are emerging as building blocks for novel nanoscale devices. Since physical properties are size dependent, NWs display novel properties with respect to their bulk counterparts. Raman scattering is a nondestructive inelastic light scattering technique enabling the assessment of fundamental properties of NWs, such as crystal phase, electronic band structure, composition, and strain field. Here, we summarize the basic principles of Raman spectroscopy and measurement setup, with focus on the scattering geometries typically used for NWs. We show that changing experimental conditions, such as light polarization, excitation energy, and pressure, allows gaining information on specific NW properties, even in a spatially resolved manner along the NW length. As examples, we discuss Ge and GaAs NWs to highlight some differences between Raman spectra of NWs and the bulk, GaAs NWs to show how Raman permits to establish the crystal phase, and InGaAs/GaAs core/shell nanoneedles to explain how compositional homogeneity and strain field can be addressed by Raman spectroscopy. Finally, we discuss resonant Raman experiments on wurtzite InAs NWs that allowed the determination of their electronic band structure.