Raman spectroscopy of InN films grown on Si

We have used Raman spectroscopy to study indium nitride thin films grown by molecular beam epitaxy on (111) silicon substrates at temperatures between 450 and 550 C. The Raman spectra show well defined peaks at 443, 475, 491, and 591 cm{-1}, which correspond to the A_1(TO), E_1(TO), E_2^{high}, and A_1(LO) phonons of the wurtzite structure, respectively. In backscattering normal to the surface the A_1(TO) and E_1(TO) peaks are very weak, indicating that the films grow along the hexagonal c axis. The dependence of the peak width on growth temperature reveals that the optimum temperature is 500 C, for which the fullwidth of the E_2^{high} peak has the minimum value of 7 cm{-1}. This small value, comparable to previous results for InN films grown on sapphire, is evidence of the good crystallinity of the films.Comment: 3 pages, 1 eps figure, RevTe

Growth of out-of-plane standing MoTe2(1-x)Se2x/MoSe2 composite flake films by sol–gel nucleation of MoOy and isothermal closed space telluro-selenization

This study describes the sol–gel processing of MoOy on Si (1 0 0) to subsequently achieve out-of-plane MoTe2/MoSe2 flake composite films by an isothermal closed space vapor transformation. The oxide precursor films have been prepared from a Mo isopropoxide solution in isopropanol and acid catalysis induced by HCl. Thermal annealing at 200, 400 and 600 °C enhanced the condensation after xerogel formation. An x-ray absorption analysis demonstrates that films condensed at 200 °C are at an intermediate chemical state between MoO3 and MoO2. To achieve MoTe2/MoSe2 composite films, the precursor oxide films were reduced in H2 and exposed to the chalcogenides by isothermal closed space vapor transport at 600 °C. The multilayered nanocomposite films grow with an out-of-plane flake-like structure and an evident integration of Se in the MoTe2 phase according to a MoTe2(1-x)Se2x alloy, with an estimation of x of 0.25. The alloy and the orientation of the flakes are consistent with the bands present in the Raman spectrum. These films are attractive for applications requiring high surface area interfaces favoring gas or ion exchange reactions with transition metal dichalcogenidesThe current research was funded by grant CTQ2017-84309-C2-2-R from Ministerio de Ciencia e Innovación (Spain). The authors acknowledge the ESRF and the Ministerio de Ciencia, Innovación y Universidades (Spain), for provision of synchrotron radiation facilities and the Consejo Superior de Investigaciones Científicas (Spain) financial support for the operation of the beamline under Grant No. PIE 2010 6 OE 01

Differences in n-type doping efficiency between Al- and Ga-ZnO films

A careful and wide comparison between Al and Ga as substitutional dopants in the ZnO wurtzite structure is presented. Both cations behave as n-type dopants and their inclusion improves the optical and electrical properties of the ZnO matrix, making it more transparent in the visible range and rising up its electrical conductivity. However, the same dopant/Zn ratio leads to a very different doping efficiency when comparing Al and Ga, being the Ga cation a more effective dopant of the ZnO film. The measured differences between Al- and Ga-doped films are explained with the hypothesis that different quantities of these dopant cations are able to enter substitutionally in the ZnO matrix. Ga cations seem to behave as perfect substitutional dopants, while Al cation might occupy either substitutional or interstitial sites. Moreover, the subsequent charge balance after doping appear to be related with the formation of different intrinsic defects that depends on the dopant cation. The knowledge of the doped-ZnO films microstructure is a crucial step to optimize the deposition of transparent conducting electrodes for solar cells, displays, and other photoelectronic devices.Ministerio de Ciencia e Innovación TEC2007-60996, MAT2008-06858-C02-02, MAT2008- 06330, TEC2010-16700FUNCOAT CSD2008-00023- CONSOLIDER INGENIOSonderforschungsbereich SFB 76

Enhancement of Rabi Splitting in a Microcavity with an Embedded Superlattice

We have observed a large coupling between the excitonic and photonic modes of an AlAs/AlGaAs microcavity filled with an 84-({\rm {\AA}})/20({\rm {\AA}}) GaAs/AlGaAs superlattice. Reflectivity measurements on the coupled cavity-superlattice system in the presence of a moderate electric field yielded a Rabi splitting of 9.5 meV at T = 238 K. This splitting is almost 50% larger than that found in comparable microcavities with quantum wells placed at the antinodes only. We explain the enhancement by the larger density of optical absorbers in the superlattice, combined with the quasi-two-dimensional binding energy of field-localized excitons.Comment: 5 pages, 4 figures, submitted to PR

Raman scattering by longitudinal optical phonons in InN nanocolumns grown on Si(111) and Si(001) substrates

Raman measurements in high-quality InN nanocolumns and thin films grown on both Si(1 1 1) and Si(1 0 0) substrates display a low-energy coupled LO phonon–plasmon mode together with uncoupled longitudinal optical (LO) phonons. The coupled mode is attributed to the spontaneous accumulation of electrons on the lateral surfaces of the nanocolumns, while the uncoupled ones originates from the inner part of the nanocolumns. The LO mode in the columnar samples appears close to the E1(LO) frequency. This indicates that most of the incident light is entering through the lateral surfaces of the nanocolumns, resulting in pure longitudinal–optical mode with quasi-E1 symmetry. For increasing growth temperature, the electron density decreases as the growth rate increases. The present results indicate that electron accumulation layers do not only form on polar surfaces of InN, but also occur on non-polar ones. According to recent calculations, we attribute the electron surface accumulation to the temperature dependent In-rich surface reconstruction on the nanocolumns sidewalls

Miniband-related 1.4–1.8 μm luminescence of Ge/Si quantum dot superlattices

The luminescence properties of highly strained, Sb-doped Ge/Si multi-layer heterostructures with incorporated Ge quantum dots (QDs) are studied. Calculations of the electronic band structure and luminescence measurements prove the existence of an electron miniband within the columns of the QDs. Miniband formation results in a conversion of the indirect to a quasi-direct excitons takes place. The optical transitions between electron states within the miniband and hole states within QDs are responsible for an intense luminescence in the 1.4–1.8 µm range, which is maintained up to room temperature. At 300 K, a light emitting diode based on such Ge/Si QD superlattices demonstrates an external quantum efficiency of 0.04% at a wavelength of 1.55 µm

Structural changes of VO2 crystals by heating with photons and electrons

Oral presentation given at the 4th Joint Congress of the Portuguese and Spanish Microscopy Societies: microscopy at the frontiers of science (MFS2015), held in Porto (Portugal) on September 9-11th, 2015. Session MS6 (Room B001). Materials Science: Ceramic Materials, Thin Films & Surfaces

Structure and properties of (25 - x/2)Li2O Æ (25 - x/2)Na2OxPbO Æ 50P2O5 metaphosphate glasses

The structure of (25 x/2)Li2O Æ (25 x/2)Na2O Æ xPbO Æ 50P2O5 phosphate glasses (0 6 x 6 50mol%) has been investigated by Raman spectroscopy, and 31P-MAS and 207Pb-VOCS nuclear magnetic resonance. An increase in lead content affects the PO4 tetrahedra network by weakening bonds between phosphorus and non-bridging oxygens. At low concentration, lead behaves as a modifier cation, with large coordination number. For higher PbO contents, a decrease in the coordination number of the Pb2+ cations and an increase in the covalent character of the Pb–O bonds is observed. The changes in density and thermal properties of the glasses are explained through the structural evolution. The non-linear increase of the glass transition and dilatometric softening temperatures with PbO content is related to the non-linear variation of the PbO molar volume. This behavior is discussed in terms of the change in lead coordination when PbO is introduced in a mixed-alkali metaphosphate glass composition.Peer reviewe

Buried amorphous layers by electronic excitation in ion-beam irradiated lithium niobate: Structure and kinetics

The formation of buried heavily damaged and amorphous layers by a variety of swift-ion irradiations (F at 22 MeV, O at 20 MeV, and Mg at 28 MeV) on congruent LiNb O3 has been investigated. These irradiations assure that the electronic stopping power Se (z) is dominant over the nuclear stopping Sn (z) and reaches a maximum value inside the crystal. The structural profile of the irradiated layers has been characterized in detail by a variety of spectroscopic techniques including dark-mode propagation, micro-Raman scattering, second-harmonic generation, and Rutherford backscattering spectroscopychanneling. The growth of the damage on increasing irradiation fluence presents two differentiated stages with an abrupt structural transition between them. The heavily damaged layer reached as a final stage is optically isotropic (refractive index n=2.10, independent of bombarding ion) and has an amorphous structure. Moreover, it has sharp profiles and its thickness progressively increases with irradiation fluence. The dynamics under irradiation of the amorphous-crystalline boundaries has been associated with a reduction of the effective amorphization threshold due to the defects created by prior irradiation (cumulative damage). The kinetics of the two boundaries of the buried layer is quite different, suggesting that other mechanisms aside from the electronic stopping power should play a role on ion-beam damage. © 2007 American Institute of Physics.Peer Reviewe

Raman spectroscopy of the ammonium ion in NH<inf>4</inf>MnF<inf>3</inf> and NH<inf>4</inf>ZnF<inf>3</inf> perovskites: Temperature dependence

11 pags., 5 figs., 5 tabs.The Raman spectra corresponding to the internal vibrations of the ammonium ion in NH4MnF3 and NH4ZnF3 single crystals have been measured from 10 to 300 K. The spectra clearly show the existence of a structural phase transition from cubic to tetragonal symmetry. The splitting of the internal modes is compatible with effective symmetries of the ammonium ion of D2d and Cs in the cubic and tetragonal phases, respectively. The temperature dependence of the symmetric stretching mode in the cubic phase is linear with a slope of 0.231 cm-1 K-1 for NH4MnF3. This has the opposite sign to that due only to the crystal expansion as obtained by applying hydrostatic pressure. A sharp band appearing only in the spectrum of NH4MnF3 just below the crystallographic phase transition originates in a strong symmetry-induced v2+v4:v1 Fermi resonance. © 1988 IOP Publishing Ltd.This work has been partially supported by the Spanish-German Joint Research Agreement 7-b. One of us (F A-R) would like to thank the Ministerio de Educacion y Ciencia of Spain for a PFPI pre-doctoral fellowship.Peer reviewe