Low temperature reflectivity study of ZnO/(Zn,Mg)O quantum wells grown on M-plane ZnO substrates

We report growth of high quality ZnO/Zn0.8Mg0.2O quantum well on M-plane oriented ZnO substrates. The optical properties of these quantum wells are studied by using reflectance spectroscopy. The optical spectra reveal strong in-plane optical anisotropies, as predicted by group theory, and marked reflectance structures, as an evidence of good interface morphologies. Signatures ofc onfined excitons built from the spin-orbit split-off valence band, the analog of exciton C in bulk ZnO are detected in normal incidence reflectivity experiments using a photon polarized along the c axis of the wurtzite lattice. Experiments performed in the context of an orthogonal photon polarization, at 90^{\circ}; of this axis, reveal confined states analogs of A and B bulk excitons. Envelope function calculations which include excitonic interaction nicely account for the experimental report

The thermal conductivity of silicon nitride membranes is not sensitive to stress

We have measured the thermal properties of suspended membranes from 10 K to 300 K for two amplitudes of internal stress (about 0.1 GPa and 1 GPa) and for two different thicknesses (50 nm and 100 nm). The use of the original 3 \omega -Volklein method has allowed the extraction of both the specific heat and the thermal conductivity of each SiN membrane over a wide temperature range. The mechanical properties of the same substrates have been measured at helium temperatures using nanomechanical techniques. Our measurements show that the thermal transport in freestanding SiN membranes is not affected by the presence of internal stress. Consistently, mechanical dissipation is also unaffected even though Qs increase with increasing tensile stress. We thus demonstrate that the theory developed by Wu and Yu [Phys. Rev. B 84, 174109 (2011)] does not apply to this amorphous material in this stress range. On the other hand, our results can be viewed as a natural consequence of the "dissipation dilution" argument [Y. L. Huang and P. R. Saulson, Rev. Sci. Instrum. 69, 544 (1998)] which has been introduced in the context of mechanical damping.Comment: 15 pages, 6 figures. Submitted to PR

Oriented Attachment of ZnO Nanocrystals

Self-organization of nanoparticles is a major issue to synthesize mesoscopic structures. Among the possible mechanisms leading to self-organization, the oriented attachment is efficient yet not completely understood. We investigate here the oriented attachment process of ZnO nanocrystals preformed in the gas phase. During the deposition in high vacuum, about 60% of the particles, which are uncapped, form larger crystals through oriented attachment. In the present conditions of deposition, no selective direction for the oriented attachment is noticed. To probe the driving force of the oriented attachment, and more specifically the possible influence of the dipolar interaction between particles, we have deposited the same nanocrystals in the presence of a constant electric field. The expected effect was to enhance the fraction of domains resulting from the oriented attachment due to the increased interaction of the particle dipoles with the electric field. The multiscale analytical and statistical analysis (TEM coupled to XRD) shows no significant influence of the electric field on the organization of the particles. We therefore conclude that the dipolar interaction between nanocrystals is not the prominent driving force in the process. Consequently, we argue, in accordance with recent theoretical and experimental investigations, that the surface reduction, possibly driven by Coulombic interaction, may be the major mechanism for the oriented attachment process

Thermoelectric properties of high quality nanostructured Ge:Mn thin films

We report on the elaboration of germanium manganese nanostructured thin films and the measurement of their thermoelectric properties. We investigate the growth of Ge:Mn layers along with a thorough structural characterization of this materials at the nanoscale. The room temperature thermoelectric properties of these layers containing spherical inclusions are discussed regarding their potential as a model of "electron crystal phonon glass material". We show that the thermal conductivity can be decreased by a factor of 30, even if the electronic properties can be conserved as in the bulk. The thermoelectric performance ZT of such material is as high as 0.15 making them a promising thermoelectric p-type material for Ge related application

Influence of defects on optics and electronics properties of ZnO nanoparticles

L’objectif de cette étude est de mieux comprendre le rôle joué par les défauts dans les propriétés optiques et électroniques des nanostructures d’oxyde de zinc. Pour ce faire, nous avons synthétisé des nanoparticules d’oxyde de zinc de 6 à 18 nm de diamètres pouvant être considérées comme modèle en terme de stœchiométrie, de cristallinité et de qualité de surface par une méthode physique originale : la Low Energy Cluster Beam Deposition.La caractérisation optique des défauts présents dans les nanoparticules de ZnO a été faite grâce à l’analyse des spectres d’émission visible et UV à différentes températures [10K-300K]. En particulier la luminescence excitonique à 3,31 eV, qui est un sujet controversé, a été étudiée en comparant la luminescence excitonique d’échantillons structurés à différentes échelles (nanoparticules, microcristaux et monocristal). Les temps de déclins très rapides des défauts donneurs ont été étudiés par spectroscopie à décalage de fréquence au CELIA à Bordeaux révélant une dépendance en fonction de la taille des NPs du type Giant Oscillator Strenght.Les propriétés de transport électronique des couches minces de NPs, naturellement dopées n, ont été caractérisées grâce à des expériences σ(T). Différents scénarios sont proposés pour expliquer les résultats des expériences de conductivité, et discutés en fonction des propriétés optiques des couches et de leur morphologie. En particulier, il est montré que la surface des NPs, très réactive, influence fortement le transport, ce qui laisse entrevoir la possibilité d’utiliser ces films nanostructurés comme capteurs de gaz.This study deals with the influence of defects on the electronic properties of ZnO nanoparticles (NPs).In order to perform this study we have synthesized ZnO NPs using an original physical way : the Low Energy Cluster Beam Deposition. The NPs size can be adjusted between 6 and 18 nm depending the synthesis parameters and their analysis shows that the NPs have a good stoichiometry, cristallinity and surface quality.The photoluminescence properties of different ZnO samples structured at different scales (i.e. nanoparticle, microcrystal and monocrystal) have been analyzed at different temperature (10K-300K). The good cristallinity of the NPs is confirmed by the lack of visible luminescence. The comparison of the 3,31 eV excitonic emission of ZnO samples structured at different scales shows that this band is due to extended defect and/or exciton-phonon coupling rather than a surface luminescence. Moreover the lack of the 3.31 eV in the NPs luminescence shows that these defect does not occurs in ZnO NPs. At last, the time resolved study of the donor bounded exciton emission shows a variation of the decay time with the size of NPs suggesting a Giant Oscillator Strenght phenomenon.The transport properties of ZnO NPs assembled thin film are determined by conductivity measurements at variable temperatures. Different possibilities are considered in order to explain the temperature dependence of the conductivity and correlated with the optical properties of the NPs, showing a hopping type conductivity. The huge reactivity of the nanostructured film strongly influences the conductivity showing a possibility to use ZnO NPs as a gas sensor

Donor and acceptor levels in ZnO homoepitaxial thin films grown by molecular beam epitaxy and doped with plasma-activated nitrogen

International audienceDeep level transient spectroscopy of both majority and minority carrier traps is performed in a n-type, nitrogen doped homoepitaxial ZnO layer grown on a m-plane by molecular beam epitaxy. Deep levels, most of them being not detected in undoped ZnO, lie close to the band edges with ionization energies in the range 0.12-0.60 eV. The two hole traps with largest capture cross sections are likely acceptors, 0.19 and 0.48 eV from the valence band edge, able to be ionized below room temperature. These results are compared with theoretical predictions and other experimental data