Influence of the annealing temperature on the photoluminescence of Er-doped SiO thin films

International audienceEr-doped amorphous silicon suboxide thin films were prepared by the coevaporation method. The Er concentration was varied from 0.4 to 6 at. % and the samples were annealed at different temperatures up to 900°C. The samples exhibit a broad photoluminescence band in the visible range. Both energy and intensity of this band were dependent on the annealing temperature. For as-deposited films and samples annealed below 500°C, this band was assigned to defects in the oxide films. For higher annealing temperatures, this photoluminescence band shifted to higher wavelengths and was correlated to the appearance of amorphous silicon clusters. Two narrow bands in the near-infrared range at 0.98 and 1.54 m were also observed for the annealed samples. The intensity of these Er-related luminescence was maximal for an annealing temperature equal to around 700°C. The effective absorption cross section of Er was dependent on the annealing temperature and was equal to 6.6ϫ 10 −16 cm 2 for the sample annealed at 700°C. The strong Er-related photoluminescence is discussed in terms of a coupling phenomenon between Er 3+ ions and spatially confined amorphous silicon clusters which act as sensitizers. The existence of a low annealing temperature to obtain the best Er-related photoluminescence is also discussed

Luminescence efficiency at 1.5 μm of Er-doped thick SiO layers and Er-doped SiO∕SiO2 multilayers

International audienceThe luminescence from Er-doped thin films is studied in two different systems. The first one is a SiO single layer. The second one is a SiO / SiO 2 multilayer allowing us to obtain size-controlled silicon nanocrystals. In both systems, the annealing-temperature dependence of the luminescence is investigated. It is shown that the optimal annealing temperatures are equal to 700 and 1050°C for the single layer and the multilayer, respectively. Moreover the luminescence efficiency at 1.5 m is one order of magnitude higher in the single Er-doped SiO layer. These results are discussed in relation to the formation of silicon nanoparticles with annealing treatments

Tailoring the surface density of silicon nanocrystals embedded in SiOx single layers

In this article, we explore the possibility of modifying the silicon nanocrystal areal density in SiOx single layers, while keeping constant their size. For this purpose, a set of SiOx monolayers with controlled thickness between two thick SiO2 layers has been fabricated, for four different compositions (x=1, 1.25, 1.5, or 1.75). The structural properties of the SiO x single layers have been analyzed by transmission electron microscopy (TEM) in planar view geometry. Energy-filtered TEM images revealed an almost constant Si-cluster size and a slight increase in the cluster areal density as the silicon content increases in the layers, while high resolution TEM images show that the size of the Si crystalline precipitates largely decreases as the SiO x stoichiometry approaches that of SiO2. The crystalline fraction was evaluated by combining the results from both techniques, finding a crystallinity reduction from 75% to 40%, for x = 1 and 1.75, respectively. Complementary photoluminescence measurements corroborate the precipitation of Si-nanocrystals with excellent emission properties for layers with the largest amount of excess silicon. The integrated emission from the nanoaggregates perfectly scales with their crystalline state, with no detectable emission for crystalline fractions below 40%. The combination of the structural and luminescence observations suggests that small Si precipitates are submitted to a higher compressive local stress applied by the SiO2 matrix that could inhibit the phase separation and, in turn, promotes the creation of nonradiative paths

Photoluminescence of Nd-doped SnO2 thin films

Structural, optical, and electrical properties of Nd-doped SnOx thin films are reported. The atomic structure was characterized by x-ray diffraction and infrared absorption spectrometry. Investigation of the photoluminescence properties revealed Nd-related bands at 920 and 1100 nm for samples annealed at 700 degrees C, which present the tetragonal structure of the SnO2 rutile phase. Nd3+ ions can be indirectly excited and no concentration quenching was observed up to 3 at. %. It is concluded that Nd3+ ions are efficient optically active dopants in addition to be responsible of the observed electric conductivity improvement. These materials are then interesting for solar cell applications

Phenomenological quantum confinement models for excitons and phonons applied to photoluminescence and Raman spectra of silicon nanocrystals

International audienceThe photoluminescence and vibrational properties of silicon nanocrystals are studied in a multilayered system elaborated by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The multilayer systems are deposited on a glass substrate (Herasil). The photoluminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. Contrary to the same study realized with multilayer systems deposited on silicon substrate, the two confinement models (phononic and excitonic) do not lead to the same size distribution. An amorphous silicon phase was also detected in Raman spectroscopy that prevented a good fitting accuracy by the model. Contribution of the substrate to the thermal crystallization process is thus discussed, as well as the origin of the photoluminescence and vibrational properties in terms of quantum confinement or interfacial defects

Embedded Silicon Nanocrystals Studied by Photoluminescence and Raman Spectroscopies: Exciton and Phonon Confinement Effects

International audienceThe optical and vibrational properties of silicon nanocrystals are studied in two systems elaborated by evaporation. The first one is constituted by a thick SiO layers. The second one is a multilayered sample made by successive evaporations of SiO and SiO2 layers with controlled thicknesses. The luminescence and Raman spectra are fitted by phenomenological exciton and phonon confinement models accounting for the size distribution of the embedded nanocrystals. The coherence between the two models and experimental data is demonstrated and gives support to the notion of exciton and phonon confinement effect in silicon nanocrystals embedded within silica matrix

Tailoring the surface density of silicon nanocrystals embedded in SiOx single layers

Équipe 104 : NanomatériauxInternational audienceIn this article, we explore the possibility of modifying the silicon nanocrystal areal density in SiOx single layers, while keeping constant their size. For this purpose, a set of SiOx monolayers with controlled thickness between two thick SiO2 layers has been fabricated, for four different compositions (x = 1, 1.25, 1.5, or 1.75). The structural properties of the SiOx single layers have been analyzed by transmission electron microscopy (TEM) in planar view geometry. Energy-filtered TEM images revealed an almost constant Si-cluster size and a slight increase in the cluster areal density as the silicon content increases in the layers, while high resolution TEM images show that the size of the Si crystalline precipitates largely decreases as the SiOx stoichiometry approaches that of SiO2. The crystalline fraction was evaluated by combining the results from both techniques, finding a crystallinity reduction from 75% to 40%, for x = 1 and 1.75, respectively. Complementary photoluminescence measurements corroborate the precipitation of Si-nanocrystals with excellent emission properties for layers with the largest amount of excess silicon. The integrated emission from the nanoaggregates perfectly scales with their crystalline state, with no detectable emission for crystalline fractions below 40%. The combination of the structural and luminescence observations suggests that small Si precipitates are submitted to a higher compressive local stress applied by the SiO2 matrix that could inhibit the phase separation and, in turn, promotes the creation of nonradiative paths

Optical phonons as a probe to determine both composition and strain in InxAl(1-x)As quantum dots embedded in an AlAs matrix

Équipe 104 : NanomatériauxInternational audienceInxAl(1-x)As quantum dots (QDs) embedded in an AlAs matrix were studied using Raman scattering and photoluminescence spectroscopy techniques. The longitudinal optical (LO) and transverse optical (TO) In-As bond vibrations frequencies are found to depend on both composition and strain. Using different scattering geometries (allowed for LO or for TO modes) we were able to obtain the experimental values of these frequencies. By comparing these values with the calculated frequencies, one can determine both the mean composition (stoichiometry parameter x) and the biaxial strain in InxAl(1-x)As QDs embedded in an AlAs matrix. Our approach is simple, non-destructive and fast