Visible photoluminescence in amorphous SiNx thin films prepared by reactive evaporation

International audiencePhotoluminescence in the visible domain can be observed in amorphous silicon nitride (a-SiN x) alloys prepared by evaporation of silicon under a flow of nitrogen ions. A strong improvement of the photoluminescence intensity was obtained with annealing treatments in the range 500-1150 °C. Structural investigations were performed by infrared and Raman spectrometry experiments. The optical gap was obtained from transmission measurements in the ultraviolet, visible, and near infrared range. The evolutions of the structure and the optical properties with annealing treatments are correlated to the evolution of the photoluminescence

Improvement of the photoluminescence properties in a-SiNx films by introduction of hydrogen

International audiencePhotoluminescence properties of amorphous hydrogenated silicon nitride thin films with various compositions are presented. The as-deposited samples prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions exhibit visible photoluminescence at room temperature without any annealing treatment. The evolution of the photoluminescence properties with increasing nitrogen concentration in the films is correlated to structural investigations performed with Fourier-transform infrared spectroscopy and optical characterization obtained from transmission measurements in the ultraviolet-visible-near-infrared range. It is shown that the introduction of hydrogen is of prime importance to improve the photoluminescence intensity of the films

Local hydrogen environments in Gd 1− x Fe x thin films amorphous alloys from effusion experiments

International audienceThin films of hydrogenated Gd,-XFeX (0.25 =%< 0.90) amorphous-alloys were prepared by reactive evaporation. Effusion experiments, showing several hydrogen release peaks, allowed us to describe the site distribution of hydrogen in these alloys. The different local environments correspond to tetrahedral sites and the stability of these sites increases with the number of rare-earth neighbors

Magnetic and transport properties of evaporated Fe/SiO multilayers

International audienceFe/SiO discontinuous multilayers consisting of layers of Fe particles embedded in an insulating SiO matrix have been prepared by evaporation. Their structural, magnetic, and transport properties have been studied as a function of Fe and SiO thickness. For small iron thicknesses, magnetic measurements show a superparamagnetic behavior above a blocking temperature determined by field-cooled and zero-field-cooled magnetization curves. Negative magnetoresistance due to spin-dependent tunneling has been observed in both current-in-plane and current-perpendicular-to-the-plane geometries. For the smaller iron thickness 5 Å͒, a Coulomb blockade effect is observed at low temperature together with an increase of the magnetoresistance

Tunneling giant magnetoresistance in coevaporated Fex(SiO)1−x thin films

International audienceFe x (SiO) 1x thin films, with volume fraction x ranging from 0.04 to 0.6, have been prepared by coevaporation. Magnetization, resistivity, and magnetoresistance measurements show that the behavior of this composite is metallic like for x0.35. For 0.2x0.27, the confrontation of all measurements prove the existence of a tunneling giant magnetoresistance between ferromagnetic grains at T300 K, where the mean radius of the grains is estimated to be about 1 nm

1.54μm photoluminescence of Er-doped GeOx thin films

International audienceAmorphous Er-doped GeO x alloys were prepared by evaporation on substrates maintained at 100°C. Photoluminescence experiments were carried out in the visible and near-infrared ranges. The Er-related peak was observed at 1.54 m at room temperature in the as deposited samples. The PL of erbium is characterized by a weak temperature quenching and by a decay time in the millisecond range. The luminescence of Er is a decreasing function of the annealing temperature. Undoped GeO x layers showed a PL band at 800 nm, attributed to defects states, which disappeared when Er is introduced. The intense Er-related luminescence was assigned to an indirect excitation process from defects states to Er ions

Evidence of light-emitting amorphous silicon clusters confined in a silicon oxide matrix

International audienceAmorphous silicon oxide thin films were prepared by the coevaporation technique in ultrahigh vacuum. Different compositions were obtained by changing the evaporation rate of silicon. The samples were then annealed to different temperatures up to 950°C. The composition and the structure were investigated using energy dispersive x-ray spectroscopy, infrared absorption measurements, and Raman spectroscopy. This study attests the presence of amorphous silicon clusters in a silicon oxide matrix. Optical transmission measurements were performed and interpreted in the field of the composite medium theory. The obtained results are in good agreement with the presented structural model. The photoluminescence in the red-orange domain was studied in relation with the structure. The correlation between the photoluminescence energy and intensity and the structure shows that the light emission originates from the silicon clusters embedded in the silicon oxide matrix. Moreover the dependence of the photoluminescence energy with the silicon volume fraction suggests the origin of the light emission could be due to a quantum confinement effect of carriers in the amorphous silicon clusters

Evolution with the annealing treatments of the photoluminescence mechanisms in a-SiNx:H alloys prepared by reactive evaporation

International audienceA wide range of amorphous hydrogenated silicon nitride thin films with an excess of silicon was prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions. A strong visible photoluminescence at room temperature was observed for the as-deposited films as well as for films annealed up to 1100°C. The chemical composition and the structure of the films were investigated using x-ray photoelectron, thermal desorption, and Raman spectroscopies, infrared absorption measurements, grazing incidence x-ray diffraction experiments, and transmission electron microscopy. Two luminescence mechanisms were identified for the films depending on the annealing temperature. For annealing temperatures below 650°C, the films are made of amorphous silicon-rich phases mixed with nitrogen-rich phases. These inhomogeneities in the chemical composition, coupled with the evolution of the photoluminescence energies and intensities with the hydrogen content, suggest that the emission is due to the recombination process of the photogenerated carriers within the band-tail states. For temperatures higher than 800°C, a phase separation occurs and the films could be described as silicon nanoclusters embedded in an insulating amorphous silicon nitride matrix. The clusters are amorphous, and then crystallized when the annealing temperature is high enough. The correlation between the clusters sizes and the photoluminescence results suggests that the emission observed after annealing treatments at temperature higher than 900°C is due to the quantum confinement of the carriers inside the silicon clusters. By carefully choosing the preparation and the annealing conditions, it is possible to tune the photoluminescence energy in the visible range

Thermal stability of titanium hydride thin films

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Photoluminescence properties of size-controlled silicon nanocrystals at low temperatures

International audienceThis study attempts to clarify the origin of the temperature dependence of the photoluminescence ͑PL͒ spectra of silicon nanocrystals ͑Si-ncs͒ embedded in SiO 2 from 5 to 300 K. For this purpose, size-controlled Si-ncs with a narrow size distribution were fabricated, using the SiO/ SiO 2 multilayer structure. The PL intensity is strongly temperature dependent and presents a maximum at around 70 K, depending on the Si-nc size and on the excitation power. The origin of this maximum is first discussed thanks to PL dynamics study and power dependence study. The evolution of the PL energy with temperature is also discussed. In bulk semiconductors the temperature dependence of the gap is generally well represented by Varshni's law. Taking into account the quantum confinement energy, the PL energy of Si-ncs follows very well this law in the range 50-300 K. Below 50 K, a strong discrepancy to this law is observed characterized by a strong increase in the PL energy at low temperature, which is dependent on the Si-nc size distribution. This temperature dependence of the PL energy is correlated with a decrease in the radiative rate at low temperature and is explained by a preferential saturation effect of the bigger Si-ncs