Materials for solid-state gas sensors

The major problems in the development of inorganic gas-sensor materials are discussed. The general principle of semiconductor gas sensors is considered, and the band-structure parameters sensitive to the gas-phase composition are determined. Ways of improving sensor selectivity are examined. The composition, microstructure, and gas sensitivity of nanocrystalline SnO₂ and ZnO are investigated. The dopant content and grain size of Ni-doped SnO₂ are optimized for H₂S detection. The prospects of employing systems of two or more nanocrystalline oxides (nanocomposites) for gas detection are discussed     

MEMS气敏传感器

随着MEMS技术的飞速发展，各种MEMS器件和系统相继问世，MEMS气敏传感器是其中之一。本文重点介绍了7种MEMS气敏传感器。     

Semiconductor gas sensors

Abstract

The technology associated with the use of semiconductors in monitoring the composition of the atmosphere has been advancing rapidly in recent years. The twin demands of remote process control and concern for the environment have produced requirements for sensing different gases in a wide range of applications. In this paper the authors review the state of the art of semiconductor gas sensor development and anticipate the directions in which future progress is likely to be made.

MST/221     

Screening effect in contactless electroreflectance spectroscopy observed for AlGaN/GaN heterostructures with two dimensional electron gas

AlGaN/GaN heterostructures with a two dimensional electron gas (2DEG) at the interface have been investigated by contactless electroreflectance (CER) and photoreflectance (PR) spectroscopies. It has been shown that the 2DEG effectively screens the GaN layer and hence no signal related to a bandgap transition in the GaN layer is observed in CER spectra whereas the CER signal related to a bandgap transition in the AlGaN layer is very strong. The screening phenomenon is unimportant for PR spectroscopy due to different mechanism of the electromodulation. As a result both GaN and AlGaN related transitions are clearly observed in PR spectra. It has been proposed that the screening phenomena observed in CER can find application in contactless detection of the 2DEG in AlGaN/GaN heterostructures.     

Acoustoelastic effect in stressed heterostructures

Mechanical stresses influence the phase velocity of acoustic waves, known as the AE (acoustoelastic) effect. In order to calculate the AE effect of biaxially stressed layered systems, we extended the transfer matrix method for acoustic wave propagation by considering the change of the density, the influence of residual stress, and the modification of the elastic stiffness tensor by residual strain and by third-order constants. The generalized method is applied to the calculation of the angular dispersion of the AE effect for transverse bulk modes and surface acoustic waves on Ge(001). Our calculations reveal that the AE effect significantly depends on the propagation direction and can even change sign. The maximal velocity change occurs for transversally polarized waves propagating parallel to the [110] direction. For the layered Ge/Si(001) system, the AE effect is investigated for Love modes propagating in the [100] and [110] directions. The AE effect increases rapidly with increasing layer thickness and almost reaches its maximal value when the wave still penetrates into the unstressed substrate     

Molecular memory based on nanowire-molecular wire heterostructures

This article reviews the recent research of molecular memory based on self-assembled nanowire-molecular wire heterostructures. These devices exploit a novel concept of using redox-active molecules as charge storage flash nodes for nanowire transistors, and thus boast many advantages such as room-temperature processing and nanoscale device area. Various key elements of this technology will be reviewed, including the synthesis of the nanowires and molecular wires, and fabrication and characterization of the molecular memory devices. In particular, multilevel memory has been demonstrated using In2O3 nanowires with self-assembled Fe-bis(terpyridine) molecules, which serve to multiple the charge storage density without increasing the device size. Furthermore, in-depth studies on memory devices made with different molecules or with different functionalization techniques will be reviewed and analyzed. These devices represent a conceptual breakthrough in molecular memory and may work as building blocks for future beyond-CMOS nanoelectronic circuits.     

Acoustic sensors for analyzing binary gas mixtures

This paper concerns the development of robust, directly acting, acoustic gas sensors, which can be used for analyzing two-component gas mixtures even under harsh measuring conditions, such as high temperatures up to 300/spl deg/C; aggressive,explosive, or toxic pollutions in the gas mixture; dust; electromagnetic disturbances; nuclear radiation; or very fast concentration changes of the mixture components. These characteristics result from the predominantly mechanical structure of the sensors. An important application of the robust sensor is the in situ measurement of humidity in the hot and contaminated exhaust air of industrial driers.     

New capabilities for optimizing SAW gas sensors

It is shown how the performances of SAW gas sensors can be optimized based on pure acoustic peculiarities of SAW propagation in anisotropic single crystals. For a given gas and sensitive membrane, the calibration curve (dependence of the response R versus gas concentration n), the sensitivity S (slope of the calibration curve: S=dR/dn), the detection limit n_thr, (cut-off of the curve at the threshold R_thr), and the resolution Δn of the sensor (recognition of two close concentrations) can be controlled by a proper choice of the substrate material and its crystallographic orientation (cut and direction of the SAW propagation). An experimental test of this property is performed on SAW devices implemented on different substrate materials and crystallographic orientations, both uncoated or coated, with a sorbent membrane of polycrystalline Pd or Pd:Ni film, upon exposure to humid air as a test analyte     

碳纳米管及其掺杂氧化物半导体气敏传感器

碳纳米管气敏传感器以其工作温度低和最低检出限较低等优点而备受关注,而碳纳米管掺杂氧化物半导体气敏传感器兼备了氧化物半导体气敏传感器和碳纳米管气敏传感器二者的优点,具有灵敏度较高、最低检出限低和工作温度低等特性。综述了这两类传感器的研究进展,介绍了其气敏机理,并对相应存在的问题及今后的发展趋势进行了概述。     

Conductivity model for sputtered ZnO-thin film gas sensors

The electrical properties of reactively sputtered ZnO thin films used for chemical sensors are investigated in N₂ and oxidizing atmosphere by Hall measurements. The measured carrier concentration and mobility of the polycrystalline ZnO films are well described by combining compensated electron conduction within the crystallites and thermionic emission of carriers across the grain boundaries. Results of a computer simulation are given. It is shown that the increase of resistance in the presence of oxidizing atmospheres is owing to the reduction of the electron concentration whereas the mobility remains unchanged. The ZnO films are much more sensitive to oxidizing atmosphere than predicted by the simulation. This aspect is discussed in detail.     

Potentiometric gas sensors for use at high temperatures

Abstract

The use of sensors to monitor gaseous species in various processes is discussed. Most solid electrolyte sensors that are used in gases involve electrolytes which conduct the ion of the species to be measured. However, this is not possible for sulphur and nitrogen and in order to overcome this problem, cation conducting electrolytes have been synthesised which respond to nitrogen and sulphur. For gases containing two elements, novel sensors, based upon two electrolytes, have been developed, whose potential in a multicomponent gas mixture is independent of the oxygen content and do not require high temperature gas seals or separate references.     

Chemical deposition of ZnO films for gas sensors

Zinc oxide (ZnO) thin films were prepared following a chemical, deposition technique using a sodium zincate bath. Structural characterizations by scanning electron microscopy (SEM) and X-ray diffraction (XRD) indicate the formation of ZnO film with a preferred c-axis orientation. The electrical conductance of the ZnO films became stable and reproducible in the 300–500 K temperature range with two activation energy barrier values of 0.3 eV and 0.8 eV in the low temperature (300–420 K) and high temperature (430–500 K) ranges, respectively. The ZnO films prepared by this method are highly resistive, indicating the presence of a large density of oxygen adsorbed acceptor-like trap states (O ₂ ^- , O_-, etc.). Palladium sensitized ZnO films were exposed to hydrogen (H₂) with air as a carrier gas at different operating temperatures ranging between 150–375°C and the response is evaluated.     

Low-temperature catalyst adding for tin-oxide nanostructure gas sensors

Thick-film gas sensors are successfully fabricated using the nanostructure tin-oxide powder. In order to suppress the coarsening of the nanostructure tin-oxide particles during the adding process, the low-temperature catalyst adding (LTCA) method is proposed in this paper. LTCA is an adding method of noble Pd catalyst onto the nanostructure tin-oxide particles under the lower temperature below 300/spl deg/C by excluding chloride. It turned out that the adding without particle coarsening is successfully carried out by means of LTCA. Applying LTCA to the fabrication of the thick film using nanostructure tin-oxide powder having a size smaller than 5 nm leads to an excellent performance with respect to the methane gas sensing. After aging at 400/spl deg/C, a good sensitivity (R/sub 3500//R/sub 1000/) of 0.66 is obtained for the sensor doped with 5 wt% of Pd catalyst. Also, the sensitivity of the sensor is so stable that the deviation of the electrical resistance is within 3% after 400 h of aging.     

Device performance of ferroelectric/correlated oxide heterostructures for non-volatile memory applications

Ferroelectric field effect devices offer the possibility of non-volatile data storage. Attempts to integrate perovskite ferroelectric materials with silicon semiconductors, however, have been largely unsuccessful in creating non-volatile, nondestructive read memory elements because of difficulties in controlling the ferroelectric/semiconductor interface. Correlated oxide systems have been explored as alternative channel materials to form all-perovskite field effect devices. We examine a non-volatile memory using an electric-field-induced metal-insulator transition in PbZr(0.2)Ti(0.8)O(3)/La(1 - x)Sr(x)MnO(3) (PZT/LSMO), PZT/La(1 - x)Ca(x)MnO(3) (PZT/LCMO) and PZT/La(1 - x)Sr(x)CoO(3) (PZT/LSCO) devices. The performance of these devices in the areas of switching time and retention are discussed.     

Tin oxide based gas sensors

The key parameters on which the function of resistance-modulating tin dioxide gas sensors depends nave been studied. A model based on the controlling influence of chemisorbed oxygen molecules is supported by electrical conductivity measurements performed both in air and in vacuo and by measurements of surface state energies evaluated by a novel method involving a solid electrolyte. It has been shown that water vapour behaves as an interference in the detection of gases over the whole range between room temperature and 600°C and as a sensor poison at temperatures below around 300°C. The application of fine particles of precious metals to the surface of the tin dioxide introduces the possibility of an oxygen-independent influence on surface energetics and this is found to substantially modify the gas sensing properties of the material.     

Surface acoustic wave-based gas sensors

A brief review of the Surface Acoustic Wave (SAW)-based gas sensors and their physical operation background in a dual-delay line oscillator system is presented together with some own achievements dealing with SAW layered sensor structures.     

Near-infrared hollow waveguide gas sensors

The development of a hollow core waveguide (HWG) gas sensor in combination with a fast and compact near-infrared (NIR) spectrometer is presented. The spectrometer operates in the spectral range of 1200-1400 nm and may thus be applied for the detection of gas-phase analytes providing NIR absorptions in that spectral window such as, e.g., methane. Since mid-infrared spectroscopy in combination with HWGs has already been successfully demonstrated for probing hydrocarbons in the gas phase, the present study investigates the achievable sensitivity in the NIR spectral regime. Methane has been selected as an exemplary analyte due to the fact that it shows strong absorption features in the mid-infrared (mid-IR) fingerprint area, but also overtone bands in the NIR. Since the HWG simultaneously serves as a miniaturized absorption gas cell and as an optical waveguide for NIR radiation, a compact yet optical and cost-efficient sensor device was established providing an interesting alternative in target sensing for mid-IR devices. The achieved limit of detection (LOD) was 5.7% (vol./vol.) methane for a 9.5 cm long HWG, 1.6% (vol./vol.) methane for a 39.1 cm long HWG, and 1.3% (vol./vol.) methane for a setup using a 77.4 cm long HWG, which provides the most practical HWG dimensions among the three investigated setups. Limit of quantitation (LOQ) values were calculated at 20.1% (vol./vol.) methane, 8.7% (vol./vol.) methane, and 5.6% (vol./vol.) methane, respectively.     

Recent progress in carbon nanotube-based gas sensors

The development of carbon nanotube-(CNTs-)based gas sensors and sensor arrays has attracted intensive research interest in the last several years because of their potential for the selective and rapid detection of various gaseous species by novel nanostructures integrated in miniature and low-power consuming electronics. Chemiresistors and chemical field effect transistors are probably the most promising types of gas nanosensors. In these sensors, the electrical properties of nanostructures are dramatically changed when exposed to the target gas analytes. In this review, recent progress on the development of different types of CNT-based nanosensors is summarized. The focus was placed on the means used by various researchers to improve the sensing performance (sensitivity, selectivity and response time) through the rational functionalization of CNTs with different methods (covalent and non-covalent) and with different materials (polymers and metals).     

Nanocrystalline diamond film as cathode for gas discharge sensors

Nanocrystalline diamond (NCD) film was deposited on a silicon substrate utilizing microwave plasma-enhanced chemical vapor deposition in a mixed flow of methane, hydrogen and argon. The deposited film had a cauliflower-like morphology, and was composed of NCD, carbon clusters and mixed sp²- and sp³-bonded carbon. Electron field emission (EFE) in vacuum and electrical discharges in Ar, N₂ and O₂ using the NCD film as the cathode were characterized. The turn-on field for EFE and the geometric enhancement factor for the NCD film were 8.5 V/μm and 668, respectively. The breakdown voltages for Ar, N₂ and O₂ increased with pressures from 1.33 × 10⁴ Pa to 1.01 × 10⁵ Pa, following the right side of the normal Paschen curve.