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

AuthorsDomènech-Amador, Núria
KeywordsThin films
Vibrational spectra
Raman spectroscopy
Nanostructured materials
Issue Date30-Oct-2015
PublisherUniversidad de Barcelona
AbstractThis 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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