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

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

Direct and indirect excitation of Nd 3+ ions sensitized by Si nanocrystals embedded in a SiO 2 thin film

International audienceThe luminescence properties of Nd-doped SiO x layers containing silicon nanocrystals (Si-ncs) were investigated by steady state, time-dependent and power-dependent photoluminescence spectrometry and photoluminescence excitation experiments. Both direct and indirect excitation processes of Nd 3þ ions have been evidenced. The energy transfer mechanism between Si-ncs and Nd 3þ ions is favored by the overlap between the emission spectrum of confined excitons in Si-ncs and the Nd 3þ absorption from the ground state to 4 F 5=2 electronic level. The more intense Nd-related emission was obtained in samples containing 0.5 at. % of Nd and characterized by an indirect excitation cross section equal to 8 Â 10 À15 cm 2

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

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

Low-temperature photoluminescence properties of Nd-doped silicon oxide thin films containing silicon nanocrystals

International audienceThe luminescence properties of neodymium-doped silicon oxide thin films containing silicon nanocrystals (Si-nc) were studied as a function of temperature from 10 to 300 K by steady-state and time-resolved photoluminescence (PL) spectrometry. The Nd-related emission at 920 nm, induced by the 4F3/2→4I9/2 shell transitions, was obtained either with a resonant excitation at 585 nm or with an indirect excitation at 325 nm via Si-nc, which act as sensitizers. A saturation of the neodymium-related photoluminescence intensity has been evidenced for indirect excitation thanks to silicon nanocrystals at temperatures below 100 K. According to the Förster model of energy transfer, this saturation is explained by a decrease of the coupling efficiency between Si-nc and rare earth ions at low temperatures, induced by the increase of the silicon nanocrystals lifetime at low temperatures

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

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

Improvement of the stability under illumination of a-Si:H films elaborated by ion-beam-assisted evaporation using a hydrogen–argon plasma

International audienceHydrogenated amorphous silicon films were deposited by ion-beam-assisted evaporation using a hydrogen-argon plasma. The influence of the substrate temperature was studied. Light induced photoconductivity decay measurements showed that high stability materials can be obtained under well defined conditions. By combined infrared spectrometry and thermal desorption spectrometry experiments, it was demonstrated that microstructure has a great influence on the stability against light induced defects

Densification of amorphous silicon prepared by hydrogen‐ion‐beam‐assisted evaporation

International audienceHydrogenated amorphous silicon films were deposited by ion-beam-assisted evaporation onto substrates maintained at 120 °C. The influence of the substrate bias was studied. By combined infrared spectrometry and thermal desorption spectrometry experiments, it is inferred that the bombardment of the growing a-Si:H film by energetic hydrogen ions produces a densification of the material without modification of the Si:H bonding