Ultrahigh-sensitive tin-oxide microsensors for H/sub 2/S detection

Ultrahigh-sensitivity SnO/sub 2/-CuO sensors were fabricated on Si(100) substrates for detection of low concentrations of hydrogen sulfide. The sensing material was spin coated over platinum electrodes with a thickness of 300 nm applying a sol-gel process. The SnO/sub 2/-based sensors doped with copper oxide were prepared by adding various amounts of Cu(NO/sub 3/)/sub 2/.3H/sub 2/O to a sol suspension. Conductivity measurements of the sensors annealed at different temperatures have been carried out in dry air and in the presence of 100 ppb to 10-ppm H/sub 2/S. The nanocrystalline SnO/sub 2/-CuO thin films showed excellent sensing characteristics upon exposure to low concentrations of H/sub 2/S below 1 ppm. The 5% CuO-doped sensor having an average grain size of 20 nm exhibits a high sensitivity of 2.15/spl times/10/sup 6/ (R/sub a//R/sub g/) for 10-ppm H/sub 2/S at a temperature of 85/spl deg/C. By raising the operating temperature to 170/spl deg/C, a high sensitivity of /spl sim/10/sup 5/ is measured and response and recovery times drop to less than 2 min and 15 s, respectively. Selectivity of the sensing material was studied toward various concentrations of CO, CH/sub 4/, H/sub 2/, and ethanol. SEM, XRD, and TEM analyses were used to investigate surface morphology and crystallinity of SnO/sub 2/ films.     

Fabrication and characterization of nano-sized SrTiO/sub 3/-based oxygen sensor for near room-temperature operation

Nano-sized SrTiO/sub 3/-based oxygen sensors were fabricated from synthesized SrTiO/sub 3/ and commercial SrTiO/sub 3/ using the high-energy ball milling and the thick-film screen-printing techniques. The particle sizes, microstructural properties, oxygen-sensing properties, and humidity effects of the synthesized nano-sized SrTiO/sub 3/-based oxygen sensors were characterized using X-ray diffraction (XRD), transmission electron microscope, scanning electron microscope (SEM), and gas sensing measurements. Experimental results showed that the particle size of the powders was milled down to be around 27 nm. The effect of different annealing temperatures (400/spl deg/C, 500/spl deg/C, 600/spl deg/C, 700/spl deg/C, and 800/spl deg/C) on the gas sensing properties of the synthesized SrTiO/sub 3/ sensor from nitrogen to 20% oxygen was characterized. The commercial SrTiO/sub 3/ devices annealed at 400/spl deg/C, both with 0-h and 120-h milling time, were used for comparison. The optimal relative resistance (R/sub nitrogen//R/sub 20%oxygen/) value of 6.35 is obtained for the synthesized SrTiO/sub 3/ sample annealed at 400/spl deg/C and operating at 40/spl deg/C. This operating temperature is much lower than that of conventional metal oxide semiconducting oxygen gas sensors (300/spl deg/C-500/spl deg/C) and SrTiO/sub 3/ oxygen gas sensors (>700/spl deg/C). The response and recovery times are 1.6 and 5 min, respectively. The detected range is 1-20% oxygen. The impedance of the synthesized SrTiO/sub 3/ sensor with annealing at 400/spl deg/C and operating at 40/spl deg/C (from 1 mHz to 10 MHz) in 20% oxygen ambient was found to be independent of the relative humidity (dry, 20% RH, 80% RH, near 100% RH).     

Comparisons between NO/sub 2/- and O/sub 3/-sensing properties of phthalocyanines and n-InP thin films in controlled and noncontrolled atmospheres

This paper describes two different semiconductor gas sensors devoted to the detection of oxidizing pollutants in the atmosphere. The first sensor consists of thin films of phthalocyanines as sensing layers (CuPc, ZnF/sub 16/Pc, and LuPc/sub 2/) evaporated onto alumina substrate fitted with interdigitated electrodes. The second sensor is realized with a mineral monocrystalline semiconductor: n-doped epitaxial layer grown on a semi-insulating substrate of indium phosphide. Each sensor has been submitted to low-controlled concentrations of ozone and nitrogen dioxide, and their detection characteristics, such as response time, stability, and sensitivity, are described. Comparison of these two sensors shows their complementary sensing characteristics, and NO/sub 2/ and O/sub 3/ act in the same way. Measurements under noncontrolled atmosphere (urban air) have been realized and have demonstrated the potentialities of these structures to be used as oxidizing pollutant detectors. Proposed methods to improve the detection of oxidizing species in urban air are discussed.     

CO-sensing properties of In/sub 2/O/sub 3/-doped SnO/sub 2/ thick-film sensors: effect of doping concentration and grain size

In/sub 2/O/sub 3/-doped SnO/sub 2/ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl/sub 4/ 5H/sub 2/O) and indium nitrate. The doping concentration was varied from 7.718/spl times/10/sup -5/ to 3.859/spl times/10/sup -4/ moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300/spl deg/C-900/spl deg/C. Thick-film samples with a thickness of /spl sim/15 /spl mu/m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220/spl deg/C-240/spl deg/C. A blue shift in the sensing temperature and increase in sensitivity factor (S/sub f/) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of /spl sim/5 is found for the highest doping concentration (3.859/spl times/10/sup -4/ moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.     

Optical and electrical H/sub 2/- and NO/sub 2/-sensing properties of Au/In/sub x/O/sub y/N/sub z/ films

In this paper, we describe the optical and electrical gas-sensing properties of In/sub x/O/sub y/N/sub z/ films with an ultrathin gold promoter overlayer. We have fabricated In/sub x/O/sub y/N/sub z/ films with a nanocrystalline porous structure by RF-sputtering in Ar/N/sub 2/ followed by an annealing process. Gold particles with 20-30-nm diameter have been formed on top of the In/sub x/O/sub y/N/sub z/ films by dc sputtering and an annealing process. We have investigated the optical H/sub 2/and NO/sub 2/-sensing properties (change of absorbance) and also the electrical sensing effect (change of electrical resistance) for these two gases. A combined optical/electrical sensor for H/sub 2//NO/sub 2/ is proposed.     

Mg/sub x/Zn/sub 1-x/O: a new piezoelectric material

Piezoelectric ZnO thin films have been successfully used for multilayer surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. Magnesium zinc oxide (Mg/sub x/Zn/sub 1-x/O) is a new piezoelectric material, which is formed by alloying ZnO and MgO. Mg/sub x/Zn/sub 1-x/O allows for flexibility in thin film SAW device design, as its piezoelectric properties can be tailored by controlling the Mg composition, as well as by using Mg/sub x/Zn/sub 1-x/O/ZnO multilayer structures. We report the metal-organic chemical vapor deposition (MOCVD) growth, structural characterization and SAW evaluation of piezoelectric Mg/sub x/Zn/sub 1-x/O (x<0.35) thin films grown on (011~2) r-plane sapphire substrates. The primary axis of symmetry, the c-axis, lies on the Mg/sub x/Zn/sub 1-x/O growth plane, resulting in the in-plane anisotropy of piezoelectric properties. SAW test devices for Rayleigh and Love wave modes, propagating parallel and perpendicular to the c-axis, were designed and fabricated. Their SAW properties, including velocity dispersion and piezoelectric coupling, were characterized. It has been found that the acoustic velocity increases, whereas the piezoelectric coupling decreases with increasing Mg composition in piezoelectric Mg/sub x/Zn/sub 1-x/O films.     

Study of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ solid solution for low-temperature resistive oxygen gas sensors

A noble type of oxygen-sensitive and electrical-conductive material, ZrO/sub 2/-based with /spl alpha/-Fe/sub 2/O/sub 3/ thick-film gas sensor, was investigated for low operating temperature. Amorphous-like solid solutions of x/spl alpha/-Fe/sub 2/O/sub 3/-(1-x)ZrO/sub 2/ powders were derived using the high-energy ball milling technique, and their physical and microstructural properties were characterized from DTA, XRD, TEM, and XPS. The oxygen gas-sensing properties of the screen-printed thick-film gas sensors fabricated from such mechanically-alloyed materials were characterized systematically. Very good sensing properties were obtained with a relative resistance value of 82 in 20% oxygen, and at a low operating temperature of 320/spl deg/C. AC impedance spectra and thermally stimulated current were characterized to investigate the conduction properties of the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, in air and nitrogen (carrier gas), respectively. It was found that the Arrhenius plots of /spl sigma/T versus 1000/T have two distinct gradients corresponding to two activation energies in the high and low temperature regions. The transition temperature occurs at about 320/spl deg/C that corresponds to an optimal operating temperature of the gas sensor. It is believed that the high oxygen vacancy concentration present in the solid solution, 0.2/spl alpha/-Fe/sub 2/O/sub 3/-0.8ZrO/sub 2/, and the dissociation of the associated oxygen vacancy defect complexes at 320/spl deg/C are the critical factors for the high relative resistance to oxygen gas at low operating temperature.     

Preparation, morphology, and gas-sensing behavior of Cr/sub 2-x/Ti/sub x/O/sub 3+z/ thin films on standard silicon wafers

The effect of humidity on chromium titanium oxide (Cr/sub 2-x/Ti/sub x/O/sub 3+z/, CTO), on both baseline resistance and sensitivity, is small compared to SnO/sub 2/. This has been the key to development of thick-film sensors based on CTO, for detection of carbon monoxide and ammonia in synthetic air. Thin-film structures on silicon substrates offer the possibility to use fabricating, bonding and housing equipment and, hence, a low cost gas sensor production is possible. CTO thin-film sensors on silicon substrates use conventional photolithography, sputtering and evaporation techniques. A Ta/Pt resistance layer (25/200-nm thick) for heating the device to its operating temperature and interdigital electrodes are evaporated and structured on a silicon substrate which is covered by a 1-/spl mu/m SiO/sub 2/ insulating layer. The polycrystalline p-type CTO is deposited onto the electrodes by oxidizing reactive sputtering or evaporation of Cr/Ti-sandwich structures. The resulting sensors were characterized by means of energy dispersive X-ray analysis, secondary electron microscopy, and X-ray diffraction pattern. Also, gas responses toward NO/sub 2/, NH/sub 3/, CO and CH/sub 4/, and different humidity, were investigated.     

Hall effect measurements in gas sensors based on nanosized Os-doped sol-gel derived SnO/sub 2/ thin films

The influence of dopants on the electrical properties of gas sensitive layers used in semiconductor gas sensors has to be carefully understood for getting a deeper insight in the relationship between the sensor performance and its chemical composition. In this work, undoped and Os-doped SnO/sub 2/ thin films have been prepared by the sol-gel process with an Os-Sn atomic ratio of 5%. The films have been characterized by resistivity and Hall effect measurements in a temperature range from 100 K to 500 K, both in air and in vacuum. The results have been investigated according to grain boundary scattering mechanism. We found that in air, the ambient oxygen species adsorbed on the film increase the height of the grain boundary barriers and the activation energy for the electrical conductivity increases in the doped film. In vacuum, the results showed that the height of the intergranular barrier is lower than the corresponding value in air. Both in air and in vacuum, the conductivity of the Os-doped sample is higher than the value in the undoped SnO/sub 2/ sample. The same occurs for the Hall mobility and the carrier concentration. The experimental results have been used to explain the better methane sensitivity, at low temperature, of the Os-doped films as compared with the undoped ones.     

ZnO/SnO_2,SnO_2/ZnO UPF复合膜的制备及气敏特性研究

用直流气体放电活化反应蒸发沉积技术在普通玻璃基片上制备了ZnO／SnO2及SnO2／ZnO超微粒子（UPF）双层复合薄膜。样品经扫描电子显微镜（SEM）和X射线衍射仪分析，结论为超微粒子的复合薄膜。同时提出了最佳制备工艺。气敏测试结果表明：ZnO／SnO2及SnO2／ZnOUPF复合膜较单层ZnO及SnO2UPF表现出优良的选择性，其灵敏度和最佳工作温度也得到相应的改善。     

Gas-sensing properties of catalytically modified WO/sub 3/ with copper and vanadium for NH/sub 3/ detection

Ammonia gas detection by pure and catalytically modified WO/sub 3/-based gas sensors was analyzed. Sensor response of pure tungsten oxide to NH/sub 3/ was unsatisfactory, probably due to the unselective oxidation of ammonia into NO/sub x/. Copper and vanadium were introduced in different concentrations and the resulting material was annealed at different temperatures in order to improve the sensing properties for NH/sub 3/ detection. The introduction of Cu and V as catalytic additives improved the sensor response to NH/sub 3/. Possible reaction mechanisms of NH/sub 3/ over these materials are discussed. Sensor responses to other gases like NO/sub 2/ or CO and interference of humidity on ammonia detection were also analyzed so as to choose the best sensing element.     

The comparative effect of two different annealing temperatures and times on the sensitivity and long-term stability of WO/sub 3/ thin films for detecting NO/sub 2/

We have deposited 150-nm-thick WO/sub 3/ films on Si/sub 3/N/sub 4//Si substrates provided with platinum interdigital electrodes and annealed in static air at 300/spl deg/C and 500/spl deg/C temperatures for 24 h and 200 h. The morphology, crystalline phase, and chemical composition of the films have been characterized using AFM, grazing incidence XRD and high resolution XPS techniques. The sensor resistance response curve has been obtained in the 0.2 -4 ppm NO/sub 2/ gas concentration range in humid air (50% relative humidity), varying the operating temperature between 25 and 250/spl deg/C. By plotting both sensor resistance and gas concentration logarithmically, the response is linear over the investigated dynamic range. Sensor sensitivities, here defined as the ratio of sensor resistance in gas to that in air (i.e., S=R/sub Gas//R/sub Air/), have been compared at a given NO/sub 2/ gas concentration (0.2 ppm). The long-term stability properties have been evaluated by recording film sensitivity for 1 yr under standardized test conditions. Increasing the annealing temperature from 300 to 500/spl deg/C causes the sensitivities to decrease. The 300/24h film is shown to be the most sensitive at S=233, but with poor long-term stability properties. The 300/200h film with S=32 is stable over the examined period. The 500/24 and the 500/200 films are shown to be less sensitive with S=16 and S=14, respectively. The longer the annealing time and the higher the temperature, the poorer the sensitivity, but with positive effects upon the long-term stability of the electrical response. The influence of the annealing conditions on sensitivity and long-term stability has been correlated with the concentration of surface defects, like reduced WO/sub 3/ phase (i.e., W/sup 4+/), which resulted in a strong effect on the sensors' response.     

Microstructure Characterization of Sol-gel Prepared MoO3-TiO2 Thin Films for Oxygen Gas Sensors

Binary metal oxide MoO3-TiO2 films have been prepared using the sol-gel technique. The thin films were annealed at several temperatures including 400℃,450℃,500℃,550℃ and 600℃ for lhour. The morphology, crystalline structure and chemical composition of the films have been analysed using SEM,XRD,RBS and XPS techniques. The SEM analysis showed that the films annealed at 450℃ are mainly smooth and uniform with 20-100nm-sized grains and with few particles as large as a micrometre or more. The XRD analysis revealed that the films annealed at 400℃ were a mixture of orthorhombic and hexagonal MoO3phases. The films annealed at 450℃ increased in hexagonal phase. The preferential orientation growth along(100) plane of the hexagonal phase and (010) plane of the orthorhombic phase has been found in both samples. RBS and XPS analysis showed that the films were stoichiometric. When the annealing temperature is increased to more than 500℃, the concentration ratio of MoO3 to TiO2 decreased due to the evaporation of MoO3. For the study of the electrical and gas sensing properties, films were deposited on sapphire substrates with interdigital electrodes on the front-side and a Pt heater on the backside. The O2 gas sensing properties of MoO3-TiO2 thin films are discussed.     

SnO/sub 2/ gas sensing array for combustible and explosive gas leakage recognition

A gas-sensing array with ten different SnO/sub 2/ sensors was fabricated on a substrate for the purpose of recognizing various kinds and quantities of indoor combustible gas leakages, such as methane, propane, butane, LPG, and carbon monoxide, within their respective threshold limit value (TLV) and lower explosion limit (LEL) range. Nano-sized sensing materials with high surface areas were prepared by coprecipitating SnCl/sub 4/ with Ca and Pt, while the sensing patterns of the SnO/sub 2/-based sensors were differentiated by utilizing different additives. The sensors in the sensor array were designed to produce a uniform thermal distribution along with a high and differentiated sensitivity and reproducibility for low concentrations below 100 ppm. Using the sensing signals of the array, an electronic nose system was then applied to classify and identify simple/mixed explosive gas leakages. A gas pattern recognizer was implemented using a neuro-fuzzy network and multi-layer neural network, including an error-back-propagation learning algorithm. Simulation and experimental results confirmed that the proposed gas recognition system was effective in identifying explosive and hazardous gas leakages. The electronic nose in conjunction with a neuro-fuzzy network was also implemented using a digital signal processor (DSP).     

A Pt/Ga/sub 2/O/sub 3/-ZnO/SiC Schottky diode-based hydrocarbon gas sensor

In this paper, a novel metal-reactive insulator-silicon carbide device with a catalytic layer for hydrocarbon gas-sensing is presented. This structure, employed as a Schottky diode, utilizes sol-gel prepared Ga/sub 2/O/sub 3/-ZnO layer as the reactive insulator. The sensor has been exposed to propene gas, which lowers the barrier height of the diode. The responses were stable and repeatable at operating temperatures between 300 and 600/spl deg/C. The response to propene in different ambients was examined. The effect of diode bias has been investigated by analyzing the sensors response to various propene concentrations when held at constant currents of 2 and 8 mA.     

Low-concentration NO/sub 2/ detection with an adsorption porous silicon FET

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO/sub 2/ detection by using the APSFET is demonstrated for the first time. NO/sub 2/ concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide.     

Mechanical and electrical characterization of /spl beta/-Ga/sub 2/O/sub 3/ nanostructures for sensing applications

Single crystalline /spl beta/-Ga/sub 2/O/sub 3/ nanowire and nanoribbon materials were synthesized, and electrical and mechanical properties were studied for sensing applications. The structural analysis showed that the Ga/sub 2/O/sub 3/ nanomaterials were stoichiometric and had the same crystal lattice structure as the /spl beta/ phase Ga/sub 2/O/sub 3/ crystal. The mechanical study on individual Ga/sub 2/O/sub 3/ nanowires and nanoribbons showed that they had a bending modulus of around 300 GPa, are flexible (in bending and twisting), and are easy to be cleaved along their crystal lattice. The current-voltage electrical characterization through the thickness of nanoribbon and along the length of nanowire confirmed their semiconducting characteristic. A two-terminal device fabricated with an individual Ga/sub 2/O/sub 3/ nanowire showed good sensing response to ethanol gas at low-operating temperature, which revealed the potential of using such nanostructures for effective sensing applications.     

ZnFe2O4纳米球的制备及其对丙酮的气敏性能研究

丙酮被广泛应用于工业和实验室,对丙酮浓度的检测十分重要。ZnFe₂O₄是一种尖晶石型三元金属氧化物,气敏性能优良,可广泛应用于气体传感器。本文采用简单的一步水热法制备了球状的ZnFe₂O₄气敏材料。通过XRD、XPS、SEM、TEM、N2吸附-解析仪对材料的形貌结构、化学组成、比表面积等进行分析,并以丙酮为目标气体对其气敏性能进行了综合研究。结果表明,ZnFe₂O₄纳米球是由纳米粒子自组装而成,有较大的比表面积;该ZnFe₂O₄基气体传感器在最佳工作温度150℃下对丙酮的灵敏度为65.74,并具有出色的选择性、稳定性、重复性,但随着湿度的增加其气敏性能逐渐降低。     

Microfabricated needle-type sensors for pO/sub 2/, pCO/sub 2/, and pH

In order to conduct in vivo continuous monitoring of blood gas levels, very thin disposable pO/sub 2/, pCO/sub 2/, and pH sensors, which could be incorporated into a syringe needle or a catheter, were microfabricated. Electrode patterns were formed on a polyimide substrate and stacked with intervening polyimide insulating layers. An electrolyte layer was formed, which was covered with a silicone rubber gas-permeable membrane or a passivating layer. The most critical part of the sensor was a Ag-AgCl reference electrode with a pinhole structure. Because of the structure and novel operational mode of the reference electrode, its potential stabilized immediately, and reproducible responses were realized for the respective sensors. Performance characterization revealed clear responses and linear calibration plots in the ranges of partial pressures or pH of physiological levels. No significant damages or changes in the performance of the sensors were observed after ethylene oxide sterilization.     

Nanosized Nb-TiO/sub 2/ gas sensors derived from alkoxides hydrolization

Nanocrystalline TiO/sub 2/ modified with Nb has been produced through the sol-gel technique. Nanopowders have been obtained by means of the hydrolysis of pure alkoxides with deionized water and peptization of the resulting hydrolysate with diluted acid nitric at 100/spl deg/C. The addition of Nb stabilizes the anatase phase to higher temperatures. XRD spectra of the undoped and the Nb-doped samples show that the undoped sample has been almost totally converted to rutile at 600/spl deg/C, meanwhile the doped samples present still a low percentage of rutile phase. Nanocrystalline powders stabilized at 600/spl deg/C with grain sizes of about 17 nm have successfully been synthesized by the addition of Nb with a concentration of 2% at., which appears to be an adequate additive concentration to improve the gas sensor performances, such as it is suggested by the catalytic conversion efficiency experiments performed from FTIR measurements. FTIR absorbance spectra show that catalytic conversion of CO occurs at lower temperatures when niobium is introduced. The electrical response of the films to different concentrations of CO and ethanol has been monitored in dry and wet environments in order to test the influence of humidity in the sensor response. The addition of Nb decreases the working temperature and increases the stability of the layers. Also, large enhancement of the response time is obtained even with lower working temperatures. Moreover, humidity effects on the gas sensor response toward CO and ethanol are less important in Nb-doped samples than in the undoped ones.