Correlation between structure and photoluminescence in amorphous hydrogenated silicon nitride alloys

As-deposited a-SiN_x:H (0.1<x<0.9) thin films prepared by evaporation of silicon under a flow of nitrogen and hydrogen ions exhibit visible photoluminescence at room temperature without any posttreatment. The nitrogen concentration was determined by X-ray photoemission spectroscopy. The structural characterization was performed with Fourier transform infrared absorption spectroscopy. The optical gap was obtained from transmission measurements in the ultraviolet–visible–near infrared range. These studies were correlated to the evolution of the photoluminescence properties.     

Influence of the temperature on the photoluminescence of silicon clusters embedded in a silicon oxide matrix

Amorphous silicon oxide thin films were prepared by co-evaporation of Si and SiO in ultra-high vacuum. Different compositions were obtained by changing the evaporation rate of silicon. After thermal annealing treatments, the dissociation of the silicon oxide in pure silicon and silicon dioxide leads to the formation of silicon clusters embedded in a silicon oxide matrix. Thus the samples were annealed to different temperatures up to 950°C. Depending on the annealing temperature and on the composition, different cluster sizes were obtained. The photoluminescence (PL) energy depends on the cluster size and a large range of wavelengths is obtained from 500 to 750 nm. The PL, attributed to a confinement effect of the electron–hole pairs in the silicon particles, is studied as a function of the temperature. It is demonstrated that the continuous decrease of PL intensity with the temperature from 77 to 500 K depends on the structure of the samples. For samples with well-separated clusters, the PL decreases rapidly with the temperature. For samples containing clusters separated by a small distance, the PL weakly depends on the temperature. No shift of the energy is observed. The results are discussed by taking into account the competition between the radiative recombination in the silicon clusters and the non-radiative escape of the carriers via a hopping mechanism.     

Influence of the SiO thickness on the photoluminescence properties of Er-doped SiO/SiO2 multilayers

Er-doped SiO single layer and Er-doped SiO/SiO₂ multilayers with different SiO thicknesses were prepared by evaporation. In the as-deposited samples, the erbium ions exhibit a very weak photoluminescence emission at 1.54 μm. This luminescence is strongly enhanced after annealing treatments between 500 and 1050 °C, with an optimal annealing temperature which is dependent from the SiO thickness. For the SiO single layer, this optimal temperature is around 700 °C while it is shifted at highest temperature for the multilayers. The origin of the higher luminescence intensity in the SiO layer is also discussed.     

Applying an improved phonon confinement model to the analysis of Raman spectra of germanium nanocrystals

The improved phonon confinement model developed previously [11] is applied for definition of germanium nanocrystal sizes from the analysis of its Raman scattering spectra. The calculations based on the model allow determining the sizes of germanium nanocrystals more precisely from the analysis of their Raman spectra. In some cases, the comparative analysis of Raman data and electron microscopy data is carried out, and good agreement is observed.     

Modulation and thermal stability of hydrogen in amorphous silicon

Hydrogenated amorphous silicon can be obtained by reactive evaporation of silicon under a flow of hydrogen atoms. By modulating the pressure of hydrogen, it is possible to prepare multilayers of type Si/Si:H/Si/Si:H/ … with thicknesses of bilayers less than 90 Å. Low-angle X-ray scattering shows diffraction peaks corresponding only to the modulation of atomic density, but low-angle neutron scattering permits to see the modulation of the isotopes hydrogen and deuterium. The thermal stability of the silicon atomic density modulation and of the hydrogen modulation is discussed.     

Polarized neutron reflection study of an Er/Fe multilayer

Low angle polarized neutron scattering experiments have been performed on an Er91 Å/Fe39 Å multilayer. The evolution of the intensity of the first peak with the temperature and the external magnetic field shows that the average magnetizations of Fe and Er layers are in the applied field direction. The study of the intensities of the second and the third peaks indicates that there is an interface effect: some erbium atoms remain antiferromagnetically coupled to iron at the interface.     

Structure and optical properties of amorphous silicon oxide thin films with different porosities

Amorphous silicon oxide thin films were prepared by evaporation of a silicon oxide powder. Samples were prepared under ultrahigh vacuum, under a flow of hydrogen ions or under a molecular hydrogen atmosphere. Two others sets of samples were prepared using deuterium instead of hydrogen. These five groups of samples were then annealed to different temperatures up to 950 °C and were exposed to the ambient air. The samples present different densities and microstructures. The sample prepared under ultrahigh vacuum is dense, hydrogen free and OH-bond free. Samples prepared under atomic hydrogen and deuterium flows contain Si–H and Si–D bonds, respectively, and are OH-bond free. The sample prepared under a molecular hydrogen atmosphere is very similar to that prepared under a molecular deuterium atmosphere. Both samples are porous and contain Si–H bonds and OH-groups coming from the exposure to the air. All the samples show visible photoluminescence attributed to isolated silicon clusters. The photoluminescence intensity increases with thermal annealing post-treatments up to an optimal annealing temperature. This maximum value is equal to 650 °C for the unhydrogenated sample and the sample prepared under an atomic hydrogen flow and to 800 °C for the sample prepared under a molecular hydrogen atmosphere. This difference is correlated to the different microstructures of the samples. Moreover the strongest photoluminescence intensity is obtained for the porous sample.