J.C. Alonso

Structural and Optical Characteristics of Gallium Oxide Thin Films Deposited by Ultrasonic Spray Pyrolysis

Amorphous gallium oxide thin films were prepared by the ultrasonic spray pyrolysis method using gallium acetylacetonate as source material and water as oxidizer. Samples annealed at 850°C during 1 h show the crystalline β-phase of Ga 2 O 3 . Rutherford backscattering results indicate that both as-deposited and annealed films have the stoichiometric chemical composition without incorporation of carbon impurities. Infrared (IR) spectroscopic measurements show that there is no incorporation of O-H and Ga-OH radicals in any of the studied films. The IR spectra for amorphous films show a broad absorption band from 400 to 900 cm -1 , typical for some amorphous metallic oxides. Meanwhile, for the annealed films the IR spectra show well-defined peaks located at 450 and 670 cm -1 related to the β-phase of Ga 2 O 3 . The refractive index of the films shows a strong change from 1.846 for the amorphous films to 1.935 for the annealed ones. The optical bandgap energy values are 4.94 eV for the as-deposited films and 4.99 eV for the annealed films. All these changes are associated with a different microstructure of the annealed films.

Micro-Raman characterization of WO3 and MoO3 thin films obtained by pulsed laser irradiation

Abstract Thin films of MoO3 and WO3 have been obtained by pulsed laser deposition. The structure of the films and of the splashed species were studied by Raman microscopy. In the case of WO3 a single monoclinic phase was observed. For MoO3 the deposited material corresponds to its metastable phase known as β-MoO3. In the splashed species a mixture of the α-MoO3 and β-MoO3 phases is found.

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.

Characterization of amorphous aluminum oxide films prepared by the pyrosol process

Abstract Amorphous aluminum oxide films were deposited by the pyrosol process using aluminum acetylacetonate as source material dissolved in a mixture of three parts of deionized water and one part of methanol. An accelerator ion beam analysis technique was used to obtain the areal density and the chemical composition of the aluminum oxide films, which result in being oxygen-rich when films are deposited at a substrate temperature of 480°C. The infrared spectra show a broad-absorption band from 400 to 1000 cm −1 typical of the vibrations of the Al 2 O 3 . IR analysis also shown that there are no O–H or alanol (Al–OH) groups incorporated in the films and that films kept at air atmosphere at room temperature have a high stability against water adsorption. The refractive index of the films measured by ellipsometry was 1.647. The optical transmission has a value of the order of 88% in the range from 400 to 900 nm for films deposited onto pyrex glass and fused quartz slices. There is no optical absorption edge for wavelengths around 190 nm, which indicates that the deposited films have an energy band gap of at least 6.2 eV. The current density-electric field characteristics of MIM structures, incorporating insulating aluminum oxide films, show current injection across the film for electric fields higher than 2 MV/cm. Electric breakdown was not observed for applied electric fields of the order of 4.5 MV/cm.

Sprayed SnO2 thin films for NO2 sensors

Abstract SnO 2 thin films as NO 2 sensor in low concentration range have been obtained by chemical spray deposition technique. The AFM analysis showed that the as-deposited films are constituted of islands of rectangular shape and elongated in the same direction. The electrical characterisation results and their variations in the presence of the NO 2 gas mixed in low concentration with dry air are reported. The electrical response to NO 2 gas was studied in order to find the optimum detection temperature. The results have shown a resistance change of about 6000% at a working temperature of 350°C in the presence of 500 ppm of NO 2 toxic gas and a sensitivity threshold of about 5 ppm at the same working temperature.

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.

Analysis of structural changes in plasma-deposited fluorinated silicon dioxide films caused by fluorine incorporation using ring-statistics based mechanism

Fluorinated silicon dioxide (SiOF) films were prepared by remote plasma enhanced chemical vapor deposition using SiF4, O2, H2, and He reaction gases. Fourier transform infrared spectroscopy studies accompanied by molecular orbital (MO) modeling were used to explain structural changes in SiOF films caused by F incorporation. On the basis of the results of MO modeling, it was shown that F atoms incorporated into the SiOF network only slightly affect the geometry of ring units, the main building blocks of SiOF film network, and cannot cause strong changes in the value of the average Si–O–Si angle, 〈θ〉. Ring-statistics-based mechanism is proposed to explain the increase in 〈θ〉 in SiOF films with F content. It is supposed that interaction of highly reactive F species from the incoming flux with the growing SiOF network during deposition process induces the preferential conversion of the most strained small-order ring units into those of higher order characterized by larger 〈θ〉. As a result, the 〈θ〉 in SiOF fil...

The Effect of Hydrogen Addition on the Fluorine Doping Level of SiOF Films Prepared by Remote Plasma-Enhanced Chemical Vapor Deposition Using SiF4-Based Plasmas

Fluorinated silicon dioxide films (SiOF) have been prepared by high-density remote plasma-enhanced chemical vapor deposition from silicon tetrafluoride (SiF4), oxygen and helium feedstock gas mixture at substrate temperature as low as 175°C. Addition of hydrogen has been found to control the fluorine doping level of the SiOF films over a wide range of fluorine concentrations while the SiF4-to-O2 flow rate ratio is kept constant. It has been confirmed that hydrogen addition does not lead to the incorporation of hydrogen in detectable quantities into the grown film with Si–Fx bond concentrations in the range of 0.5–4.9 at.% because of effective mutual scavenging behavior of fluorine and hydrogen in the entire range of hydrogen flow rates used. The decrease in the refractive index of the SiOF films from 1.463 to 1.410 and the increase in Si–O–Si bonding angles from about 137° to about 148° were found to be approximately linear with the increase in fluorine concentration. The hydrogen added to the process gas mixture has been found to play an active role in the film formation process affecting both the surface reactions and properties of the obtained SiOF films.

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.