L.I. Popova

Quartz resonator with SnO2 thin film as acoustic gas sensor for NH3

Abstract A quartz resonator with a thin SnO2 deposited film was investigated as a sensor for detecting the presence of ammonia in the ambient. The SnO2 film was used as a gas sensing element. Considerable variations of the resonance frequency Δf = 15 ÷ 119 Hz were observed depending on the NH3 concentrations in the range of 10 ÷ 5000 ppm. These variations were reversible at the conditions of the experiment carried out and no hysterisis was observed at these conditions. The mass Δm of the adsorbed substance was calculated as a function of the ammonia concentration c. The character of this dependence was well described with Δm = 2.4 c,0.3 that is an equation based on the Freundlich isotherm adsorption model.

Voltage dependence of gas-sensing behaviour of SnO2-gate FETs

SnO2-gate FETs with ammonia-sensing behaviour are investigated at constant ammonia concentration (100 ppm) and different bias conditions. Strong dependence of the sensing behaviour on the drain and gate voltage (Vd, Vg) applied is observed. The current response obtained could be positive, negative or zero at different bias conditions. On the basis of the interpretation of the experimental data two conclusions are deduced: that the presence of ammonia in the air results in effective contributions ΔVg to the Vg applied electrically and that the working mechanism is generally dependent on the transversal electrical field between the gate and the channel. However, there exists an additional dependence on Vd and, thus, the observed sensing behaviour cannot be explained by polarization or work function variations, or charge storage effects originating from the transversal field alone.

Conductive-oxide-gate FET as a gas sensor

Abstract An n-channel FET, called a COGFET, with a conductive SnO2 sensing layer as a gate is proposed. The room-temperature sensitivity for NH3 and ethanol is investigated as a function of the gate resistivity. COGFETs with low gate resistivity (103-104 ohm/□) are both ethanol and NH3 sensitive. Devices with high gate resistivity (≥ 1012 ohm/□) are only NH3 sensitive. Fast response time and very good reproducibility are obtained. The logarithmic current dependence versus concentration covers more than four decades. Two different slopes are observed in the NH3 curve.

Characteristics and gas-sensing behaviour of a tin-oxide-gate FET

Abstract The gas-sensing behaviour of a tin-oxide-gate FET is investigated at room temperature with a pulsed drain-voltage measuring technique. Devices are measured in two different modes: with the gate connected to a constant gate voltage supply and with a floating gate. In the two modes of operation well-defined response is observed, the threshold voltage shift amounting to several hundred mV/decade in the range of concentrations 10–1000 ppm. The existence of a process depending on VD versus time is revealed.

Oxide field enhancement corrected time dependent dielectric breakdown of polyoxides

Abstract Time-to-breakdown ( t bd ) of polysilicon/polyoxide/polysilicon structures is investigated on small and large area capacitors. The ln( t bd ) versus 1/ E ox projection lines are corrected by using an average oxide field enhancement factor for the interface polysilicon/thermally grown polyoxide. A field acceleration factor G≈320 MV / cm in the time-to-breakdown projection line is obtained. It is shown that the fast prediction of time-to-breakdown can be achieved with short stress time measurements in structures of different area.

Gas-sensitivity of SnO2 layers treated by rapid thermal annealing process

Abstract A rapid thermal annealing (RTA) treatment was used to improve the performance of ammonia sensors based on a MOSFET structure with a thin SnO2 layer used as a gate. RTA processes in vacuum with duration of 15–60 s and temperatures 600–800°C were applied. After thermal treatment samples are subjected to a cycle of successive steps with different environmental conditions in order to reveal the sensitivity, selectivity and reversibility of the response. It is found that the RTA process at 800°C, with duration of 60 s, rise time of 2 s and fall time of 4 s results in enhanced sensitivity to the active agent (NH3), reduced cross-sensitivity to water vapours (relative humidity) and improved reversibility of the devices response. This overall improvement of the performance is explained by surface changes of the SnO2 layer, provoked by the RTA process. Such changes are revealed by reflection high-energy electron diffraction (RHEED) and scanning electron microscopy (SEM).

Modelling the gas-sensing behaviour of SnO2-gate FETs

Abstract An attempt to create a first-order model of the operation mechanims of SnO 2 -gate FETs sensitive to ammonia is reported in this work. In previous work a strong dependence of the sensing effect on the relative humidity of the ambient has been reported. On these grounds, the presence of adsorbed positive NH + ions and negative OH − ions on the SnO 2 surface is assumed and the model is built up on the lateral transportation of these ions under the combined effect of the transversal gate-to-channel electric field and the surface source-drain field. The transversal field is supposed to weaken electrostatically the adsorption bonds of the OH − ions over the channel and of the NH + ions over the saturated surface area. This enables lateral transportation and redistribution of the ions along the surface source-drain field. Due to the non-uniformity of the channel this redistribution results in equivalent addition to the gate voltage applied and, thus, in channel current modulation. Expressions for the floods of positive and negative ions over the channel are derived. The calculated curves for the gate voltage variation according to these expressions are in good agreement with experimental data.

Pulse mode of operation of diode humidity sensors

Abstract The combined gas-sensing properties of a thin SnO 2 layer and an underlying surface p-n junction are used for creating a solid-state humidity sensor. The dependence of the device response behaviour on pulsed-voltage measuring conditions is investigated. A strong dependence on pulse duration and pulse/pause ratio is observed. A model for explaining the device behaviour, based on the voltage dependence of the sorption phenomena during the pulse and pause durations, is proposed. A good agreement with the experimental results is obtained.

Characteristics of diode humidity sensors based on SnO2-sensing properties

Abstract The structure investigated consists of a surface p-n junction formed by a planar n+ diffusion region in p-type Si substrate. The silicon surface is covered with thermally grown SiO2 and on top of it an SnO2 layer is deposited and electrically connected to then + region. A pulsed voltage V is applied to the n+ region contact and the substrate-to-ground current pulses are recorded at forward bias conditions. When water vapours are present in the air, significant and reversible variations of the current are observed (the current increases more than 2 to 3 times). A model is proposed for explanation of this effect based on modulation of the p-n junction barrier by polarized water molecules. A good agreement between theoretical calculations and experimental data is obtained.

TEM structural characterization of Sn–Te–O thin films on monocrystalline silicon wafers

The structural properties of Sn–Te–O thin films, deposited by co-evaporation of Sn and TeO2 on monocrystalline Si substrates at room temperature are investigated by transmission electron microscopy analysis. Instead of homogeneous amorphous films, the obtained films are inhomogeneous multilayered structures, with clearly expressed polycrystalline and monocrystalline regions. Some exotic crystalline forms of Sn, Te and SnTe are detected in the layers. The composition and microstructure of the investigated films depend mainly on the Sn/Te ratio, the thickness of the layers and the substrate surface.