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

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

Silicon (Oxy)Nitride Thin Films Deposited by Lpcvd From SiCl₂H₂-NH₃-N₂O Mixtures Of Variable Composition

AbstractSilicon (oxy)nitride thin films were deposited on (100) 4″ silicon substrates in a conventional LPCVD reactor at 812°C and 230 mTorr, using mixtures of SiCl2H2 -NH3-N2O of variable composition in order to evaluate their optical properties for potential applications in SOI based Si technology. The depositions were carried out keeping the flow of SiCl2H2 and NH3 constant at 20 and 60 sccm respectively while that of N2O was varying between 0 and 180 sccm. Optical reflection measurements within the range 190 to 860 nm, performed on our films were analyzed within the Forouhi-Bloomer model (Phys. Rev. B34, 7018 (1986)) for amorphous semiconductors. From the analysis of the optical spectra, the constants of the model were derived. It was shown that the refractive index of the films varies between the limits set by those of silicon nitride and silicon dioxide, the increase of the N2O flow implying a decrease of the refractive index of the films. The film thicknesses were also derived from the analysis of the optical spectra. Capacitance - voltage measurements made on Metal-Insulator-Semiconductor (MIS) structures using our films as the insulator, have shown that, the electric charge density within the dielectric initially increases and then decreases with increasing the amount of oxygen into the deposition ambiance. Our films are suitable for optoelectronic applications in SOI structures compatible with C-MOS technology.</jats:p