Intrinsic Defects and H Doping in WO3.

WO3 is widely used as industrial catalyst. Intrinsic and/or extrinsic defects can tune the electronic properties and extend applications to gas sensors and optoelectonics. However, H doping is a challenge to WO3, the relevant mechanisms being hardly understood. In this context, we investigate intrinsic defects and H doping by density functional theory and experiments. Formation energies are calculated to determine the lowest energy defect states. O vacancies turn out to be stable in O-poor environment, in agreement with X-ray photoelectron spectroscopy, and O-H bond formation of H interstitial defects is predicted and confirmed by Fourier transform infrared spectroscopy

Optimization of the hydrogen response characteristics of halogen-doped SnO2

The increasing demand for efficient sensing devices with facile low-cost fabrication has attracted a lot of scientific research effort in the recent years. In particular, the scientific community aims to develop new candidate materials suitable for energy-related devices, such as sensors and photovoltaics or clean energy applications such as hydrogen production. One of the most prominent methods to improve materials functionality and performance is doping key device component(s). This paper aims to examine in detail, both from a theoretical and an experimental point of view, the effect of halogen doping on the properties of tin dioxide (SnO2) and provide a deeper understanding on the atomic scale mechanisms with respect to their potential applications in sensors. Density Functional Theory (DFT) calculations are used to examine the defect processes, the electronic structure and the thermodynamical properties of halogen-doped SnO2. Calculations show that halogen doping reduces the oxide bandgap by creating gap states which agree well with our experimental data. The crystallinity and morphology of the samples is also altered. The synergy of these effects results in a significant improvement of the gas-sensing response. This work demonstrates for the first time a complete theoretical and experimental characterization of halogen-doped SnO2 and investigates the possible responsible mechanisms. Our results illustrate that halogen doping is a low-cost method that significantly enhances the room temperature response of SnO2

Initial stages of growth of LPCVD polysilicon films. Effect of the surface "ageing"

The initial stages of growth of polycrystalline silicon films by silane decomposition, at a pressure of 230 m Torr and a temperature of 610 °C in a conventional low pressure chemical vapor deposition reactor, have been studied. Depositions at times varying between 1 and 5 min have been made on two kinds of SiO2 covered, (100) Si substrates : i) immediately after withdrawal from the oxidation furnace ("fresh") and ii) left in the clean room for 24 hours after oxidation ("aged" oxides). The microstructural characteristics of these deposits (grain size and roughness) were studied with atomic force microscopy. It was found that grain size and surface roughness were decreasing with deposition time, and that films grown on "fresh" oxides exhibited smaller grain size and lower roughness, than films grown on "aged" oxides. After the first 5 min of deposition, the state of the substrate surface ceased to influence the grain size which became identical for films grown on both kind of surfaces, while it continued to influence the roughness of the samples. Such behaviour has not been observed in thicker films grown on "fresh" and "aged" oxides that have been studied with transmission electronic microscopy. The above observations were attributed to the increase of "activated" sites coverage with deposition time, which in turn has caused an increase of the activation energy for the surface diffusion of adsorbed silicon atoms. Surface "ageing" causes a decrease of sites available to form bonds, due to the absorption of impurities from the clean room ambien

Optical properties of ultra-thin low pressure chemically vapor deposited silicon films

Ultra-thin silicon films, with thicknesses approximately 10 nm, were low pressure chemically vapor deposited (LPCVD) on fused silica substrates at 0.23 Torr and temperatures 550, 610 and 620 °C, by silane decomposition. Transmission and reflection spectra of these films were recorded within the energy range 6.2 to 0.5 eV, throughout of which they were transparent. From these measurements the energy variation of the complex dielectric function, ε(E)=ε1(E)+iε2(E) of the films, was extracted. It was found that the overall shapes of ε1(E) and ε2(E) were similar with those for thicker LPCVD Si films. More precisely, structures attributed to the E0, E1, E2 and E1 transitions of crystalline silicon, were shown on the ε1 and ε2 spectra of samples deposited at 610 and 620 °C at nearly the same energies as in crystalline Si. The above structures were not shown on the corresponding spectra for the sample at 550 °C. Absorption threshold and gap of these ultra-thin samples, coincided with those for thicker ones deposited at similar conditions. Analysis of the dielectric function of the films with the aid of the effective medium approximation, has shown that samples deposited above 600 °C contained smaller fractions of crystalline material than thicker ones, deposited at such temperatures. It was concluded that quantum effects, related to the confinement of electronic wave functions, do not cause significant changes to the electronic band structure of ultra-thin LPCVD Si films relative to that of thicker films

Influence of texture on the absorption threshold of LPCVD silicon films

Polycrystalline silicon films were chemically vapor deposited on oxidised silicon and quartz substrates from SiH4 decomposition in a conventional reactor at 230 mTorr and at temperatures of 550, 610 and 700 °C. Samples were characterized by optical transmission and atomic force microscopy measurements. The optical properties of these films were studied within a two-band model proposed by A. Forouhi and I. Bloomer (Phys. Rev. B34, 7018 (1986)) that is able to provide an approximate picture of the energy distribution of the density of electronic states. It was found that this model describes satisfactorily the optical properties of films near the absorption threshold. It was also shown that the energy distribution of the density of states is uifluenced by crystallization and film texture. Electronic transitions at different points of the Brillouin zone dominate the optical properties. Thus, transitions near the K point of the Brillouin zone dominate the optical properties of films grown at 610 °C, while for films grown at 700 °C the dominant role is played by transitions at the X, K and L point

Hot-wire CVD of copper films on self-assembled-monolayers of MPTMS

Self-assembled monolayers (SAM's) of (3-Mercaptopropyl))trimethoxysilane (MPTMS) were applied on Si (100) wafers covered either with low-temperature silicon oxide (LTO) or with SiLK®. Cu films were subsequently chemically vapor deposited (CVD) on the MPTMS SAMs with a novel reactor equipped with a tungsten hot-wire enabling the separate heating of the gas phase (hot-wire CVD, HWCVD). CupraSelect®, which is the commercial name of hexafluoroacetylacetonate Cu(I) trimethylvinylsilane, was used as precursor for the deposition of Cu films and delivered in the reactor with the aid of a direct-liquid injection system using ultra-pure N 2 as carrier gas. High quality Cu films were obtained on both LTO and SiLK®. The resistivities of HWCVD Cu films were found to be higher than those of thermally grown films. This is due possibly to the presence of impurities into the Cu films from the incomplete dissociation of the precursor and W impurities caused by the presence of the filament, which does not, however, degrade catastrophically the conductivity of Cu HWCVD films. The separate heating of the gas phase enhanced the deposition rate, which at filament temperature of 170°C increases by a factor of approximately one and a half. © 2008 WILEY-VCH Verlag GmbH & Co. KGaA

Fabrication of fine copper lines on AZ 5214TM patterned silicon substrates by selective chemical vapor deposition

Copper features with dimensions down to 0.5 µm were fabricated on silicon substrates by selective chemical vapor deposition. For the fabrication oxidized (100) silicon substrates were used, covered with a film grown by LPCVD at 0.1 Torr and 550 °C, from W(CO)6 decomposition. These substrates were subsequently covered with AZ 5214TM photosensitive polymer, which has been developed as both positive and negative tone resist. Copper was then chemically vapor deposited on the patterned substrates by 1, 5-cyclooctadiene Cu(I) hexafluoroacetylacetonate decomposition, at 1 Torr and temperatures of 110 and 140 °C. A vertical, cold-wall reactor was used, equipped with a UV lamp permitting photon-assisted deposition. Under UV illumination, copper was deposited on resist covered and uncovered parts of the substrate. In absence of illumination, the metal was selectively grown on the tungsten film only at relatively slow rates (1 and 3.5 nm/min at 110 and 140 °C respectively). Copper films had a granular form with a grain size increasing with temperature (150 and 550 nm at 110 and 140 °C respectively). Afer depositions the resist was removed in oxygen plasma leading to the formation of fine copper features

Characterization and stressing properties of polysilicon TFTs utilizing oxide films deposited using TEOS

In this work we investigated the structural and electrical characteristics of SiO2 films deposited by Low Pressure Chemical Vapor Deposition (LPCVD) using tetraethylorthosilicate (TEOS) under various deposition pressures and temperatures and their application as gate dielectrics in thin film transistors. It was found using Fourier Transform InfraRed spectroscopy (FTIR) that films deposited at temperatures lower than 635 °C are carbon contaminated, so this temperature was considered as the lowest limit for depositing SiO2 films with an acceptable carbon contamination. The threshold voltage of TFTs utilizing these oxides was correlated with the structural properties of the oxides. The degradation of the TFT transfer characteristics under various stressing gate bias values was studied and the evolution of the electrical parameters with stressing time was determined. The degradation of both the threshold voltage and the subthreshold swing exhibits a logarithmic dependence with stressing time, indicating a charge trapping process occurring in the oxide and at the polysilicon / SiO2 interface