#WhiteMirrorProject, cap a la reflexió activa sobre el desenvolupament sostenible

El #WhiteMirrorProject, és un projecte per a reflexionar sobre el desenvolupament sostenible i tecnològic. L'alumnat comença analitzant diversos aspectes relacionats amb el medi ambient i la tecnologia. A partir d'aquesta anàlisi, fa una predicció de com aquests elements afectaran a la societat en el futur. Com a producte final d'aquest procés, l'alumnat grava un vídeo que reflecteix l'escenari de futur que ha imaginat, seguint el format de les sèries de televisió. El projecte s'ha realitzat en el marc de la matèria de Ciències per al Món Contemporani, de 1r de Batxillerat.The #WhiteMirrorProject is a project to think about sustainable and technological development. Students begin by analyzing several subjects related to the environment and technology. Based on this analysis, they make a prediction of how these elements will affect society in the future. As a final product of this process, students record a video that reflects the future scenario they have imagined, following the format of the TV series. The project has been carried out within the framework of the subject "Ciències pel món contemporani", with 17-year-old student

Anharmonic phonon decay in cubic GaN

We present a Raman scattering study of optical phonons in zincblende (cubic) GaN for temperatures ranging from 80 to 750 K. The experiments were performed on high quality, cubic GaN films grown by molecular beamepitaxy on GaAs (001) substrates. The observed temperature dependence of the optical phonon frequencies and linewidths is analyzed in the framework of anharmonic decay theory, and possible decay channels are discussed in the light of density-functional theory calculations. The LO mode relaxation is found to occur via asymmetric decay into acoustic phonons, with an appreciable contribution of higher order processes. The TO mode linewidth shows a weak temperature dependence and its frequency downshift is primarily determined by the lattice thermal expansion. The LO phonon lifetime is derived from the observed Raman linewidth and an excellent agreement with previous theoretical predictions is foun

Phonons in III-nitride thinfilms, bulk and nanowires: a closer look into InN vibrational properties

[cat] Aquesta tesi està dedicada a l’estudi de les interaccions dels fonons en nitrur d'indi (InN) i en semiconductors del sistema (In.Ga)N amb estructura wurtzita. Amb aquest objectiu es presenten estudis d'espectroscòpia Raman en capes primes, nanofils (NWs), i mostres bulk, que han permès abordar de manera global les interaccions dels fonons en aquests materials. Hem estudiat les interaccions anharmòniques i els canals de decaïment dels fonons de InN, tant en capes primes com en NWs. La dependència de l’amplada del pic Raman amb la temperatura de tots els modes fonònics s’ha estudiat utilitzant un model anharmònic que considera la contribució dels processos de tres i quatre fonons, i tenint en compte la densitat d'estats de fonons obtinguda mitjançant càlculs ab-initio. L'anàlisi dels temps de vida fonònics i de la dependència amb temperatura de les freqüències permet afirmar que els NWs tenen una estructura més relaxada que les capes primes. També hem estudiat el decaïment anharmònic de modes locals de vibració corresponents a complexos d'H en InN fortament dopat amb Mg. Hem estudiat les ressonàncies en el sistema (In,Ga)N i la influència de la densitat d’impureses en l’eficiència dels mecanismes ressonants. Hem demostrat que la dispersió Raman de modes òptics longitudinals en el InN es produeix a través de la doble ressonància del Martin tant en capes primer com en nanoestructures, tot i que la densitat de defectes d'aquestes últimes és significativament menor. Hem estudiat també el mecanisme de cascada mediat per impureses, a través del qual es produeix la dispersió de multifonons, en capes primes de InGaN amb diferent composició i diferent grau d’implantació d'ions d'He, i hem comprovat que les intensitats relatives dels multifonons depenen de la concentració d’indi i de la dosi de la implantació. Finalment, hem estudiat l’acoblament de fonons polars amb els plasmons mitjançant el model dielèctric de Lindhard-Mermin, amb la finalitat d’investigar la densitat d’electrons lliures utilitzant espectroscòpia Raman. Hem determinat la concentració d'electrons en NWs de InN sense dopar, dopats amb Si i dopats amb Mg. També hem fet un estudi de la distribució de la densitat de càrrega en una mostra de GaN ammonotermal mitjançant mesures de micro-Raman confocal.[eng] This thesis is devoted to the study of the interactions of phonons in indium nitride (InN) and materials of the (In,Ga)N system with wurtzite structure. For this purpose, we present Raman spectroscopy on nanowires (NWs), thin films and bulk samples, in order to adress phonon interactions in these materials. We also present Brillouin spectrsocopy measurements of InN thin films, from which a reliable set of elastic constants is proposed. We have studied the phonon anharmonic interactions and phonon decay channels of InN, both in thin films and NWs. The temperature dependence of Raman peak width of all the phonon modes has been studied using a model that considers the contribution of three- and four-phonon processes, taking into account the phonon density of states obtained by ab-initio calculations. In InN thin films, we find that the E2h phonon mode mainly decays through 4-phonon processes, whereas the extremely narrow E2l mode can decay only through up-conversion processes. The LO and the TO modes are found to decay through 3-phonon and 4-phonon interactions. In InN NWs we found the same phonon decay channels but phonon linewidths are significantly reduced, indicating a higher crystalline quality. The lifetimes of the phonon modes are derived from the measured phonon linewidths. The long-lived E2l phonon exhibits the largest lifetime, which is mainly limited by impurity scattering. We also study the anharmonic decay of high-frequency LVMs H complexes in Mg-doped InN, which can be explained by considering dephasing due to quasi-elastic acoustic phonon scattering. We have discussed the relevant electronic resonances that affect Raman scattering in the (In,Ga)N system. We show that the optical excitation of the longitudinal optical modes in InN occurs via the Martin's double resonance both in InN layers and nanostructures, even though the defect density of the latter is significantly lower. By performing wavelength-dependent measurements on InN thin films and NWs, the A1(LO) and the E1(LO) wave-vector dispersion close to zone-center have been obtained. We have also studied the impurity-mediated cascade mechanism of multiphonons in InGaN layers. To ascertain the role of the impurities we have studied as-grown samples in comparison with He+-implanted InGaN layers. UV Raman-scattering measurements allow us to measure up to fifth order multiphonon scattering due to cascade mechanism. Relative multiphonon intensities depend on the indium concentration and implantation dose. Finally, we have studied the LO-Phonon-Plasmon Coupled Modes (LOPCMs) in InN and GaN using the Lindhard-Mermin model. We have determined the electron density in undoped, Si-doped and Mg-doped NWs. We have also studied a bulk, ammonothermally-grown Si-doped GaN sample. No evidence of LOPCMs was detected in the Ga-polar face, probably due to the higher defect density existing in this sample sector. We have detected both branches of the LOPCMs in the N-polar face, and we have made a study of the distribution of the free charge density by means of confocal micro-Raman measurements

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Electron density gradients in ammonothermally grown Si-doped GaN

Charge homogeneity in heavily Si-doped ammonothermal GaN has been studied by confocal micro-Raman measurements. The polarized Raman spectra indicate a high crystalline quality of the sample. On the N-polarity growth sector we observe both branches (L+, L-) of the phonon-plasmon coupled modes. The free-electron density, which is derived from a line-shape fit to the L+ mode, displays an in-depth gradient with a decreasing density when the top surface is approached. The Raman spectra of the Ga-polarity growth sector do not show any trace of phonon-plasmon coupled modes, suggesting a lower Si incorporation and/or the presence of compensating deep levels. © 2014 The Japan Society of Applied Physics.This work has been partially supported by the Spanish Ministry under Grant No. MAT2010-16116. ND acknowledges financial support from the FPU program of the Spanish Ministry of Education.Peer Reviewe

Longer InN phonon lifetimes in nanowires

We present a Raman scattering study of the anharmonic phonon decay of the E low 2, E high 2 and E1(LO) phonons in InN nanowires over the 80-400 K temperature range. While the temperature-dependent anharmonic decay in the nanowires is similar to that found for bulk InN, the background contribution to the phonon lifetime is strongly reduced as a result of the improved crystalline quality. High-resolution measurements reveal a remarkably long lifetime of the E low 2 mode. From the comparison between the E low 2 frequencies measured in the nanowires with those of the thin film we obtain the deformation potentials for the E low 2 mode. © 2012 IOP Publishing Ltd.This work has been supported by the Spanish Ministry of Science and Innovation under contract MAT2010-16116.Peer Reviewe

Brillouin scattering determination of the surface acoustic wave velocity in In xGa 1-xN: A probe into the elastic constants

We have determined the surface acoustic wave velocity in In xGa 1-xN layers for 0.34 x 0.75 by means of high resolution Brillouin spectroscopy. The sagittal dependence of the surface acoustic velocity has been analyzed by comparing the experimental results with theoretical simulations based on the Green's function formalism. We find the best agreement with our data when the bowing parameters for the elastic constants recently reported from density functional theory calculations are taken into account. The dependence of the surface acoustic wave velocity on alloy composition is given. © 2012 American Institute of Physics.This work has been partially supported by the Spanish Ministry under Grants MAT2010-16116 and MAT2009-08786.Peer Reviewe

Temperature dependence of Mg-H local vibrational modes in heavily doped InN:Mg

We have studied the temperature dependence and anharmonic coupling of the local vibrational modes (LVMs) associated with Mg-H complexes in heavily doped InN:Mg. Two main LVM peaks are observed which are probably related to two different H-impurity bond lengths. The temperature dependence of the higher-frequency mode, which exhibits a monotonic frequency downshift and broadening with increasing temperature, can be explained by LVM dephasing due to acoustic phonon scattering. The lower-frequency mode displays an anomalous behavior as its frequency decreases initially and then starts to increase linearly above room temperature. The anharmonic coupling of the lower-frequency mode to a molecular mode of the impurity complex is suggested as a possible cause for this behavior. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4749266]This work has been supported by the Spanish Ministry of Science and Innovation under Contract No. MAT2010-16116Peer Reviewe

Lattice dynamics of a mist-chemical vapor deposition-grown corundum-like Ga<inf>2</inf>O<inf>3</inf> single crystal

© 2015 AIP Publishing LLC. The lattice dynamical properties of the corundum-like α-phase of Ga2O3 are investigated by means of Raman scattering experiments and ab-initio calculations. A high-quality, single-crystal thick epilayer was grown on sapphire by the mist-chemical vapor deposition method. The phonon frequencies at the Brillouin zone center of all the Raman-active modes are determined by polarized Raman scattering measurements on an α-Ga2O3 single crystal. By performing backscattering measurements from (0001) and (101¯0) faces, all Raman active modes are unambiguously identified. Density functional perturbation theory calculations were carried out to determine the symmetry and the frequency of the α-Ga2O3 lattice modes. We find a good agreement between the theoretical predictions and the Raman spectra. The relative intensity of the different modes and their polarizability are discussed. The Raman spectrum is dominated by a narrow A1g peak which indicates the high crystalline quality of the layers grown by the mist chemical vapor deposition method.This work has been supported by the Spanish Ministry of Economy and Competitiveness under Contract Nos. MAT2010-16116 and MAT2014-59199. N.D. acknowledges financial support from the FPU program of the Spanish Ministry of Education. T.H. and T.Y. acknowledge financial support from the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT) under the Strategic Research Foundation Grant-aided Project for Private Universities 2011-2015 (S1101005) and the Grants-in-Aid for Scientific Research Nos. 25420341 and 25706020, as well as from the Advanced Low Carbon Technology Research and Development Program, JST, Japan.Peer Reviewe