G. Santana

Strong white and blue photoluminescence from silicon nanocrystals in SiNx grown by remote PECVD using SiCl4/NH3

Strong white and blue photoluminescence (PL) from as-grown silicon nanocrystals (nc-Si) in SiNx films prepared by remote plasma enhanced chemical vapour deposition using SiCl4/NH3 mixtures is reported. The colour and intensity of the PL could be controlled by adjusting the NH3 flow rate. Samples with white emission were annealed at 1000 °C, obtaining a strong improvement of the PL intensity with a blue colour. The PL can be attributed to quantum confinement effect in nc-Si embedded in SiNx matrix, which is improved when a better passivation of nc-Si surface with chlorine and nitrogen atoms is obtained. The size, density and structure of the nc-Si in the as-grown and annealed films were confirmed and measured by high-resolution transmission electron microscopy.

Influence of the surrounding host in obtaining tunable and strong visible photoluminescence from silicon nanoparticles

We have investigated the influence of the microstructure and chemistry of the surrounding host on the strong visible photoluminescence (PL) from silicon nanoclusters (nc-Si) embedded in three different silicon-based dielectric compounds: SixNy:H,Cl, SixNyOz:H,Cl, and SixOz:H,Cl, obtained from silicon nitride films deposited by SiH2Cl2∕NH3∕H2 plasma-enhanced chemical vapor deposition at different growth pressures. A blueshift is found in the PL coming from the nc-Si as the content of oxygen in the surrounding host is increased, and a significant improvement in PL intensity is achieved when the nc-Si are well passivated with O instead of H. We discuss the PL behavior in terms of the quantum confinement model and passivation state of the nc-Si surface.

Modification of the nonlinear optical absorption and optical Kerr response exhibited by nc-Si embedded in a silicon-nitride film

We studied the absorptive and refractive nonlinearities at 532 nm and 26 ps pulses for silicon-nitride films containing silicon nanoclusters (nc-Si) prepared by remote plasma-enhanced chemical vapor deposition (RPECVD). Using a self-diffraction technique, we measured for the as-grown sample beta=7.7x10(-9)m/W, n(2)=1.8x10(-16)m(2)/W, and /chi(3)1111/ = 4.6x10(-10)esu; meanwhile, when the sample was exposed to an annealing process at 1000 degrees C during one hour in a nitrogen atmosphere, we obtained beta=-5x10(-10)m/W, n2=9x10(-17)m(2)/W, and /chi(3)1111/=1.1x10(-10)esu. A pure electronic nonlinear refraction was identified and a large threshold ablation of 41 J/cm(-2) was found for our films. By fitting nonlinear optical transmittance measurements, we were able to estimate that the annealed sample exhibits a response time close to 1 fs. We report an enhancement in the photoluminescence (PL) signal after the annealing process, as well as a red-shift due to an increment in size of the nc-Si during the thermal process.

Polymorphous silicon thin films obtained by plasma-enhanced chemical vapor deposition using dichlorosilane as silicon precursor.

Polymorphous silicon thin films (pm-Si) have been deposited from mixtures of dichlorosilane and hydrogen, using argon as the diluting gas by plasma-enhanced chemical vapor deposition. The deposition conditions were chosen to simultaneously obtain both Si nanocrystallites and an amorphous silicon matrix in the as-grown samples. High resolution transmission electron microscopy studies show the crystallinity of Si domains whose dimensions are in the interval of 2-14 nm. The surface passivation state of the silicon nanocrystals was inferred from Fourier transform infrared spectroscopy analysis. Two optical absorption edges, corresponding to the amorphous matrix and the Si nanocrystals, were observed for all the pm-Si thin films. Intense visible photoluminescence was observed for the as-grown samples. The possibility of using these thin films for the down-conversion effect in silicon solar cells is discussed.

TiN nanoparticles: small size-selected fabrication and their quantum size effect

Size-selected TiN nanoclusters in the range of 4 to 20 nm have been produced by an ionized cluster beam, which combines a glow-discharge sputtering with an inert gas condensation technique. With this method, by controlling the experimental conditions, it was possible to produce nanoparticles with a high control in size. The size distribution of TiN nanoparticles was determined before deposition by mass spectroscopy and confirmed by atomic force microscopy. The size distribution was also analyzed using a high-resolution transmission electron micrograph. The photoluminescence [PL] spectra of TiN nanoparticles at different sizes were also experimentally investigated. We reported, for the first time, the strong visible luminescence of TiN nanoparticles on Si (111) wafer due to the reduced size. We also discussed the PL intensity as a function of the nanoparticle size distribution.

Structure and optical properties of silicon nanocrystals embedded in amorphous silicon thin films obtained by PECVD

Silicon nanocrystals embedded in amorphous silicon matrix were obtained by plasma enhanced chemical vapor deposition using dichlorosilane as silicon precursor. The RF power and dichlorosilane to hydrogen flow rate ratio were varied to obtain different crystalline fractions and average sizes of silicon nanocrystals. High-resolution transmission electron microscopy images and RAMAN measurements confirmed the existence of nanocrystals embedded in the amorphous matrix with average sizes between 2 and 6nm. Different crystalline fractions (from 12% to 54%) can be achieved in these films by regulating the selected growth parameters. The global optical constants of the films were obtained by UV-visible transmittance measurements. Effective band gap variations from 1.78 to 2.3 eV were confirmed by Tauc plot method. Absorption coefficients higher than standard amorphous siliconwere obtained in these thin films for specific growth parameters. The relationship between the optical properties is discussed in terms of the different internal nanostructures of the samples.

Spatially controlled growth of highly crystalline ZnO nanowires by an inkjet-printing catalyst-free method

High-density arrays of uniform ZnO nanowires with a high-crystal quality have been synthesized by a catalyst-free vapor-transport method. First, a thin ZnO film was deposited on a Si substrate as nucleation layer for the ZnO nanowires. Second, spatially selective and mask-less growth of ZnO nanowires was achieved using inkjet-printed patterned islands as the nucleation sites on a SiO2/Si substrate. Raman scattering and low temperature photoluminescence measurements were applied to characterize the structural and optical properties of the ZnO nanowires. The results reveal negligible amounts of strain and defects in the mask-less ZnO nanowires as compared to the ones grown on the ZnO thin film, which underlines the potential of the inkjet-printing approach for the growth of high-crystal quality ZnO nanowires.

Intense white luminescence in ZnTe embedded porous silicon

Porous silicon layers were embedded with ZnTe using the isothermal close space sublimation technique. The presence of ZnTe was demonstrated using cross-sectional energy dispersive spectroscopy maps. ZnTe embedded samples present intense room temperature photoluminescence along the whole visible range. We ascribe this PL to ZnTe nanocrystals of different sizes grown on the internal pore surface. Such crystals, with different orientations and sizes, were observed in transmission electron microscopy images, while transmission electron diffraction images of the same regions reveal ZnTe characteristic patterns.

Ultrasonic Spray Pyrolysis Deposition and Characterization of Tantalum–Aluminum Oxide Thin Films

Tantalum oxide and tantalum-aluminum oxide films have been prepared by pyrosol using alcoholic start solution of tantalum chloride and aluminum acetylacetonate at substrates temperatures of 250°C. All films are amorphous independent of the deposition conditions. X-ray diffraction shows that aluminum incorporation in deposited materials stabilizes the amorphous phase even after annealing at 525°C and 900°C. Energy dispersive spectroscopy, Rutherford backscattering, refractive index, and FTIR results indicate that the relative chemical composition of the deposited films is about [Ta] = 20 atom % and [0] = 80 atom %, probably containing hydroxide or oxohydroxide groups. The concentration of aluminum in films depends on the concentration of aluminum acetylacetonate in start solution with a ratio [Al atom % in film]/[Al atom % in solution] ≈0.1. The rms roughness has values of < 1 nm. The effect of incorporation of aluminum in deposited material and its annealing at 525°C on the electrical characteristics of metal-oxide-metal structures increases the breakdown electric field and decreases the current density. The dielectric constant decreases as the aluminum concentration increases and annealing has a densification effect on the TAO films.

Development of Optimal Nanocrystalline Absorption Layer for Thin Film Silicon Solar Cell Applications

To obtain an optimum absorption layer based on hydrogenated polymorphous and nanocrystalline silicon thin films in a plasma-enhanced chemical vapor deposition, radio frequency (RF) power was varied from 25W to 100W using a mixture of dichlorosilane and hydrogen. By Raman spectroscopy, the crystalline fraction was found to be varied from 7% to 69%, and RF power value of 75W was found to be suitable with an appropriate mixture of amorphous and crystalline phases, respectively. Thickness measurements performed by profilometry were cross-checked with the value obtained from the cross-sectional scanning electron microscopy micrographs. Micrographs obtained using high-resolution transmission electron microscopy confirmed the presence of silicon nanocrystals in the range of 2–5nm with a strong probability of confinement effect. B and gap value of 1.55eV at 75W upheld the suitability of this particular RF power for active absorption layer, which has also shown maximum photosensitivity.