J. Calderer

Fabrication of Highly Selective Tungsten Oxide Ammonia Sensors

Most of the chemical sensors described in the literature are based on inorganic semiconducting oxides. In some cases, the main oxide is modified by doping with small amounts of additives such as other oxides and/or metals. The sensing principle is based on the change in the conductance undergone by the oxide film when gases are adsorbed and react on its surface. A survey of typical sensor materials for detecting different gases in characteristic temperature ranges can be found elsewhere. 1,2 In recent years, some gas-sensitivity studies with tungsten trioxide (WO 3 ) based semiconductors have been reported. Pure or doped tungsten oxide is a promising material for the detection of nitrogen oxides (NO and NO 2 ) 3-6 and sulfur dioxide, 7 two substances which are considered to be responsible for ambient degradation together with carbon oxides and hydrocarbons. The gas-sensing properties of WO 3 are highly dependent on the deposition method. Tungsten oxide is generally deposited by reactive rf sputtering. 8-10 However, other techniques such as thermal evaporation, 11,12 sol-gel methods, 13 screen printing, 14 or photochemical production 6 have been reported to grow thin or thick active

The role of oxygen partial pressure and annealing temperature on the formation of W = O bonds in thin WO3 films

Thin films of tungsten oxide were deposited onto silicon substrates using reactive rf sputtering. The structure of the films is strongly dependent on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and of annealing temperature. We used x-ray photoelectron spectroscopy to investigate the in-depth composition of the films. The most surface sensitive O 1s core level spectra are made up of two structures, one generated by photoelectrons emitted from oxygen atoms in WO3 (O–W–O) and other at lower energy generated by the photoelectrons emitted from oxygen atoms located at the boundary of the grains (W = O). Using Raman spectroscopy, an increase of the W = O/O–W–O ratio was correlated to an increase in the oxygen partial pressure used during the deposition. A decrease of this ratio was observed while annealing temperature was increased, which was correlated to an increase in the size of the grains that form the films.

ELECTRICAL CHARACTERIZATION OF N-AMORPHOUS/P-CRYSTALLINE SILICON HETEROJUNCTIONS

n‐type amorphous silicon on p‐type crystalline silicon heterojunction diodes were fabricated and electrically characterized. The a‐Si:H film was deposited by plasma enhanced chemical vapor deposition. Electrical properties were investigated by capacitance–voltage and current–voltage measurements at different temperatures. The capacitance–voltage results confirm an abrupt heterojunction. Current–voltage characteristics show good rectifying properties (50000:1 at ±0.5 V). A detailed analysis of transport mechanisms was developed in order to establish a unified model of conduction. Two carrier transport mechanisms are believed to be at the origin of the forward current. At low bias voltage (V<0.4 V), the current is determined by recombination at the amorphous side of the space charge region, while at higher voltages (V≳0.6 V), the current becomes space charge limited.

Influence of the doping method on the sensitivity of Pt-doped screen-printed SnO2 sensors

Abstract In this work, we study the influence of the introduction method of Pt atoms on the sensitivity to traces of ethanol of Pt-doped SnO2 sensors. The tin oxide films were obtained by a screen-printing process. Two different methods were employed to introduce Pt atoms on SnO2 films. In the first one, the Pt atoms were added to the screen-printed tin oxide layer by using RF magnetron sputtering and a subsequent thermal treatment. The second method consisted of mixing SnO2 and Pt pastes before the screen-printing process. The different active layers (including un-doped tin oxide) were carefully examined relative to their sensitivity to ethanol at different working temperatures. Sensors prepared by the second method showed sensitivity to ethanol four times higher than one of the sensors prepared by the first method and 12 times higher than un-doped sensors. XPS and scanning electron microscopy (SEM) measurements showed that this behaviour could be associated with the spatial distribution of the doping elements within the tin oxide film. While in Pt-sputtered sensors most of the Pt atoms were found at the surface of the active layer, for the sensors made by mixing Pt and SnO2 pastes, a homogeneous distribution of the Pt atoms was observed. These sensors show high sensitivity and fast response time to ethanol vapours, with a detection limit in the ppb range.

Effects of Oxygen Partial Pressure and Annealing Temperature on the Formation of Sputtered Tungsten Oxide Films

Thin films of tungsten oxide were deposited on silicon substrates using reactive radio frequency sputtering. The structure of the films strongly depends on the conditions of deposition and post-treatment. Important issues are the influences of oxygen pressure during deposition and annealing temperature on the morphology. Atomic force microscopy and scanning electron microscopy revealed that films were formed by grains. The sample deposited with an Ar:O 2 partial pressure ratio of 1:1 showed the highest roughness and the smallest grains when annealed at 350°C. X-ray photoelectron spectroscopy analysis revealed that the films were close to their stoichiometric formulation irrespective the oxygen partial pressure used during film deposition. The number of W=O bonds at the grain boundaries was found to he dependent on the oxygen partial pressure. Analysis by Raman spectroscopy suggested that the structure of the films was monoclinic. On the basis of these results an annealing temperature of 350°C was selected as post-treatment for the fabrication of WO 3 gas sensors. These sensors were highly sensitive, highly selective to ammonia vapors, and moderately responsive to humidity.

Quantitative analysis of NO2 in the presence of CO using a single tungsten oxide semiconductor sensor and dynamic signal processingElectronic Supplementary Information (ESI) available: NIPALS algorithm, the PLS algorithm for one C variable, backpropagation learning algorithm, RBF network training algorithm, ART1 and Fuzzy ART mathematical models. See http://www.rsc.org/suppdata/an/b2/b205009a/

We demonstrate that NO2 can be quantitatively analysed in the presence of CO using a single tungsten oxide based resistive gas sensor. The working temperature of the sensor was modulated between 190 and 380 degrees C and its dynamic response to different concentrations of CO, NO2, and CO + NO2 mixtures was monitored. Either the fast Fourier transform (FFT) or the discrete wavelet transform (DWT) was used to extract important features from the sensor response. These features were then input to different (statistical and neural) pattern recognition methods. The species considered can be discriminated with a success rate higher than 90% using a Fuzzy ARTMAP or a radial basis function neural network. The concentrations of the gases studied can be accurately predicted, by using the DWT coupled to partial least squares (PLS) models. The correlation coefficients of the predicted versus real concentrations were 0.923, 0.870 and 0.866 for CO, NO2, and NO2 in CO + NO2 mixtures, respectively.

Numerical simulation of the electrode geometry and position effects on semiconductor gas sensor response

Abstract In this work we have studied the coupled effects of electrode geometry and active film thickness on the sensitivity of semiconductor gas sensors to reducing gases. This study discusses a model that takes into account gas diffusion, adsorption and chemical reaction, to obtain the charge carrier concentration profile in the sensor bulk. The result is used in a two-dimensional semiconductor device simulation package to obtain the electrical conductance of the sensor in the presence of a reducing gas. Results show that all the parameters studied have an important effect on sensor response, especially when a highly reactive gas is being measured. Both electrode placement and film thickness must be considered if sensor sensitivity and selectivity are to be increased. For instance, if these parameters are chosen well, a poorly reactive gas can be detected in the presence of a highly reactive gas.

Synthesis and characterisation of metal suboxides for gas sensors

Abstract Deposition of tin monoxide thin films by reactive magnetron sputtering is investigated. The analysis by XRD and XPS have proved the presence of SnO as the main compound in the layers, which also contain SnO 2 and metallic Sn. A set of process parameters for the obtention of these films is found. The material, used as sensitive layer in resistive type gas sensor, shows an increase in its resistivity when exposed to vapours of ethanol (p-type conductivity).

Influence of the deposition method on the morphology and elemental composition of SnO2 films for gas sensing: atomic force and X-ray photoemission spectroscopy analysis

Abstract The performance of metal oxide gas sensors is affected by their surface states, elemental composition, electronic and morphologic structures. Films of tin oxide were deposited onto silicon substrates using reactive radio frequency sputtering and drop-coating. In order to understand how the deposition procedure affects the morphology of the films, a structural characterisation based on atomic force microscopy was performed. The differences in elemental composition were analysed by X-ray photoemission spectroscopy. For sensors deposited by sputtering, a granular morphology and the presence of stannic sub-oxide (SnO) was observed. Sensors deposited by drop coating had a granular morphology but no stannic sub-oxide was present. The sensitivity of the drop-coated sensors to ethanol was found to be up to five times higher than the one of sputtered sensors. This difference can be associated to the presence of the stannic sub-oxide, grain size and inter-granular coupling.

Space charge recombination in PN junctions with a discrete and continuous trap distribution

Abstract Space charge Shockley-Read-Hall recombination currents in the presence of discrete or continuous distributions of recombination centres are analysed. For a single level trap, depending on its position inside the forbidden band, asymptotic values both for the ideality factor of the current-voltage characteristic and for the activation energy of the saturation current are obtained. The analysis is extended to continuous trap distributions and the current-voltage characteristics obtained are explained in terms of the simple theory developed for single level traps.