Material and sensing properties of Pd-deposited WO3 thin films

The physicochemical and electrical properties of Pd-deposited WO3 thin films were investigated as a function of Pd thickness, annealing temperature, and operating temperature for application as a hydrogen gas sensor. WO3 thin films were deposited on an insulating material using a thermal evaporator. X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to evaluate the crystal structure, microstructure, surface roughness, and chemical property of the films, respectively. The deposited films grew into polycrystalline WO3 with a rhombohedral structure after annealing at 500 degrees C. Adding Pd had no effect on the crystallinity, but suppressed the growth of WO3 grains. The Pd was scattered as isolated small spherical particles of PdO2 on the WO3 thin film after annealing at 500 degrees C, while it agglomerated as irregular large particles or diffused into the WO3 after annealing at 600 degrees C. PdO2 reduction under H2 and reoxidation under air were dependent on both the Pd deposition thickness and annealing conditions. The WO3 thin film with a 2-nm-thick Pd deposit showed a good response and recovery to H2 gas at a 250 degrees C operating temperature.     

聚吡咯/二氧化钛复合薄膜的制备及气敏性研究

运用静电力自组装和原位化学氧化聚合相结合的方法制备了聚吡咯/纳米二氧化钛(PPy/TiO₂)复合薄膜, 并进行了紫外－可见光谱分析和原子力显微镜分析. 采用平面叉指电极制备了PPy/TiO₂复合薄膜气体传感器, 研究了其在常温下对有毒气体NH₃和CO的敏感性. 最后测试了该传感器的温度湿度特性. 结果表明, 该传感器对NH₃具有较高的灵敏度, 对CO几乎没有响应. 同时讨论了复合薄膜沉积时间对气敏特性的影响, 实验表明当沉积时间为20min时, 该传感器的NH₃敏感特性最好.     

CO2 and H2 detection with a CHx/porous silicon-based sensor

There has been great interest in the last years in gas sensors based on porous silicon (PS). Recently, a gas sensing device based on a hydrocarbon CH_x/porous silicon structure has been fabricated. The porous samples were coated with hydrocarbon groups deposited in a methane argon plasma. We have experimentally demonstrated that the structure can be used for detecting a low concentration of ethylene, ethane and propane gases [Gabouze N, Belhousse S, Cheraga H. Phy State Solidi (C), in press].In this paper, the CH_x/PS/Si structure has been used as a sensing material to detect CO₂ and H₂ gases. The sensitivity of the devices, response time and impedance response to different gas exposures (CO₂, H₂) have been investigated.The results show that current-voltage and impedance-voltage characteristics are modified by the gas reactivity on the PS/CH_x surface and the sensor shows a rapid and reversible response to low concentrations of the gases studied at room temperature.     

CNT-WO3元件的氨敏性能研究

以碳纳米管（CNT）为掺杂剂制成CNT—WO3旁热式气敏元件．采用混酸氧化法对碳纳米管进行纯化，化学沉淀法制备了纳米WO3微粉，并用TEM、FT—IR、TG—DSC、XRD等方法进行了表征．测试了元件在室温条件下对NH3的气敏性能．结果表明，碳纳米管掺杂元件在室温下对NH3的灵敏度远远高于纯WO3元件，其中0．8wt％的掺杂元件对NH3具有最高的灵敏度．另外，掺杂元件还具有检测浓度低、检测范围宽、选择性好等优点，是一种较为理想的氨敏元件．     

Co3O4中空纳米球的可控制备及气敏性能

以碳纳米球为模板,采用硬模板法制得多孔Co_3O_4中空纳米球。分别采用SEM、XRD、FTIR、BET和XPS对Co_3O_4纳米球的形貌和结构进行表征。通过改变前驱体浓度和陈化反应时间调控Co_3O_4中空纳米球的空间结构及气敏性能。结果表明:在前驱体浓度为0.1mol/L、陈化时间为48h时,得到的Co_3O_4中空纳米球的表面呈疏松多孔结构。Co_3O_4中空纳米球直径约为500nm,由40nm的Co_3O_4纳米粒子组成。室温下,由该材料组装的气敏传感器对浓度为100×10^-6～0.5×10^-6的NH_3有较好的气敏性能;对浓度为100×10^-6的NH_3响应灵敏度高达155.8%,响应时间为1.3s。该气体传感器对NH_3的最低检测限为0.5×10^-6。     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

多孔硅基热敏薄膜的制备与性能

对MEMS用具有绝热性能的多孔硅基底上沉积的热敏感薄膜进行了研究．首先用电化学方法制备多孔硅,分别在多孔硅基底和硅基底上通过溅射镀膜方法沉积氧化钒、Cu、Au热敏薄膜,测试多孔硅基底和硅基底上的氧化钒及金属薄膜电阻的热敏特性．结果表明,在多孔硅基底表面沉积的热敏薄膜具有与硅基表面热敏薄膜同样的热敏特性且表现出更高的灵敏度;此外,对沉积在不同制备条件得到的多孔硅上的氧化钒薄膜电阻热敏特性进行比较,发现随着孔隙率和厚度的增加,多孔硅的绝热性能提高,其上沉积的氧化钒薄膜电阻热敏特性增强．     

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.     

介孔硅基WO3纳米颗粒薄膜室温气敏元件特性

采用双槽电化学腐蚀法在p＋单晶硅片表面制备介孔硅层（meso-PSlayer）,然后用对向靶磁控反应溅射法在介孔硅表面沉积WO3纳米颗粒薄膜,在干燥空气中于400℃下保温4h进行退火热处理,制备出介孔硅基WO3纳米颗粒薄膜（WO3-PS）室温气敏元件.利用扫描电子显微镜（SEM）分析介孔硅层及WO3-PS的表面形貌,通过X射线衍射（XRD）研究WO3的结晶状态,测试WO3-PS气敏元件在室温下对NO2、NH3的气敏性能,并探讨了WO3-PS气敏元件的工作机理.实验结果表明,在介孔硅表面沉积WO3纳米颗粒薄膜可使介孔硅的气敏性能显著提高,其中在室温下对10×10^-6NO2的灵敏度由5提高至56,大大提高了介孔硅的灵敏度,并降低了其响应/恢复时间,提高了对NO2的选择性.     

Microsensor based on low temperature cofired ceramics and gas-sensitive thin film

A novel design of gas sensor using low temperature cofired ceramics (LTCC) and thin film technologies is presented. The LTCC structure is composed essentially of two ceramic layers with interlayer thick film Pt heater, interdigitated electrodes on top, contact pads and metallic connections realised by vias. The thin films of both SnO₂ and In₂O₃, intentionally doped and activated, were deposited on top of the structure. With some modifications of the lamination process and heat treatment parameters, the authors obtained the upper ceramic layer with the roughness not exceeding 250 nm, what was suitable for thin film technology. The films deposited onto such LTCC structure revealed the sensing properties very similar to the reference films deposited onto glass. The gas-sensitive films were tested with changing concentrations of reducing and oxidising gases in air. The necessary sensor working temperature was obtained and stabilised using a custom-built digital controller. The low heat capacity of the sensor structure enabled also a sinusoidal temperature control. The satisfactory results obtained by the authors indicate that the connection of LTCC and thin film technologies can lead to the fabrication of good quality gas sensors.     

Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO.     

磁控共溅射Ni3 Al合金薄膜的微观结构及电阻特性

研究了室温下采用直流磁控共溅射法在抛光玻璃和Si基底上沉积Ni3Al合金薄膜的制备工艺、微观结构和电阻特性.采用SEM、EDX、AFM、TEM等测试分析了不同基底、溅射功率、工作气压等因素对薄膜微观结构、成分比和电阻特性的影响.结果表明:采用大功率混合溅射可以得到多晶态Ni3Al纳米合金薄膜,且呈多层岛状生长.所得薄膜具有良好的导电性,与玻璃相比,在Si基底上的薄膜表面光滑平整,晶粒更小,电阻率略大.然而随着厚度的减小,薄膜的电阻率增加迅速,发生金属向绝缘体过渡的相变,而厚度较大时这种现象不明显,这表明Ni3Al薄膜相变与厚度及晶格中氧含量有关.     

SiO2层上沉积的纳米多晶硅薄膜及其特性

采用低压化学气相沉积(LPCVD)系统以高纯SiH4为气源,在p型10.16 cm<100>晶向单晶硅衬底SiO2层上制备纳米多晶硅薄膜,薄膜沉积温度为620℃,沉积薄膜厚度分别为30 nm、63 nm和98 nm.对不同薄膜厚度的纳米多晶硅薄膜分别在700℃、800℃和900℃下进行高温真空退火.通过X射线衍射(XRD)、Raman光谱、扫描电子显微镜(SEM)和原子力显微镜(AFM)对SiO2层上沉积的纳米多晶硅薄膜进行特性测试和表征,随着薄膜厚度的增加,沉积态薄膜结晶显著增强,择优取向为<111>晶向.通过HP4145B型半导体参数分析仪对沉积态掺硼纳米多晶硅薄膜电阻I-V特性测试发现,随着薄膜厚度的增加,薄膜电阻率减小,载流子迁移率增大.     

Low–Temperature Hydrogen Sensors Based on Au Nanoclusters and Schottky Contacts on ZnO Films Deposited by Pulsed Laser Deposition on Si and SiO2 Substrates

Unintentionally doped n-type ZnO thin films deposited on Si and SiO₂ substrates by pulsed laser deposition (PLD) techniques, were functionalized as H₂ gas sensors by a) incorporating Au nanoclusters in the surface, and b) developing Au Schottky diodes on ZnO. The influence of the catalytic action of the Au nanoclusters on the sensing properties of the devices was examined and found to provide faster response times at a reduced working temperature of 150 degC. The field-assisted sensing of the Au Schottky diodes demonstrated for the first time in this system, a more dramatic reduction in the working temperature of the sensor to nearly room temperature     

Physical and electronic properties of ZnO:Al/porous silicon

UV, violet and blue-green photoluminescence has been achieved at room temperature (RT) from ZnO:Al (AZO) films deposited by radio frequency (rf) co-sputtering. As the ZnO target power increases from 100 W, the violet luminescence vanishes and the blue and green-blue luminescences appear. The most intense UV and blue-green luminescence is obtained for the films deposited at higher sputtering powers depending upon the stoichiometry of the films as well as the crystalline quality. The as-prepared porous silicon (PS) emission band lies in the blue-green spectral region and is blue shifted due to the AZO deposition. The current–voltage characteristics of AZO/PS heterostructures have been studied. The ideality factor is found to be 19 and the series resistance as determined from the forward characteristics is 36 MΩ.     

WO3薄膜制备及其电致变色性能研究

采用直流反应磁控溅射方法在室温下制备WO3薄膜。研究溅射功率对WO3薄膜结构及电致变色性能的影响规律,考察退火后WO3薄膜的结构演变及电致变色性能变化。结果表明溅射功率为270W时薄膜表现出较好的电致变色性能,其调制幅度达78.5%,着色时间为9s,褪色时间为3.2s。将该功率下制备的WO3薄膜进行退火处理,其结构由非晶态转变为晶态,但调制幅度、响应时间特性都发生一定程度的退化。非晶态WO3薄膜相比晶态结构具有更快的响应时间和更宽的调制幅度,但晶态薄膜具有更好的循环稳定性。     

Faster response of NO₂ sensing in graphene-WO₃ nanocomposites

Graphene-based nanocomposites have proven to be very promising materials for gas sensing applications. In this paper, we present a general approach for the preparation of graphene-WO(3) nanocomposites. Graphene-WO(3) nanocomposite thin-layer sensors were prepared by drop coating the dispersed solution onto the alumina substrate. These nanocomposites were used for the detection of NO(2) for the first time. TEM micrographs revealed that WO(3) nanoparticles were well distributed on graphene nanosheets. Three different compositions (0.2, 0.5 and 0.1 wt%) of graphene with WO(3) were used for the gas sensing measurements. It was observed that the sensor response to NO(2) increased nearly three times in the case of graphene-WO(3) nanocomposite layer as compared to a pure WO(3) layer at room temperature. The best response of the graphene-WO(3) nanocomposite was obtained at 250?°C.     

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.     

Very thin poly-SiC films for micro/nano devices

We report characterization of nitrogen-doped, very thin, low-stress polycrystalline silicon carbide (poly-SiC) films suitable for fabricating micro/nano devices. The poly-SiC films are deposited on 100 mm-diameter (100) silicon wafers in a large-scale, hot-wall, horizontal LPCVD furnace using SiH2Cl2 and C2H2 as precursors and NH, as doping gas. The deposition temperature and pressure are fixed at 900 degrees C and 4 Torr, respectively. The deposition rate increases substantially in the first 50 minutes, transitioning to a limiting value thereafter. The deposited films exhibit (111)-orientated polycrystalline 3C-SiC texture. HR-TEM indicates a 1 nm to 4 nm amorphous SiC layer at the SiC/silicon interface. The residual stress and the resistivity of the films are found to be thickness dependent in the range of 100 nm to 1 microm. Films with thickness less than 100 nm suffer from voids or pinholes. Films thicker than 100 nm are shown to be suitable for fabricating micro/nano devices.     

Visible-light active photocatalytic WO3 films loaded with Pt nanoparticles deposited by sputtering

Visible-Light active photocatalytic tungsten trioxide (WO3) films were deposited at a substrate temperature of 800 degrees C by dc reactive magnetron sputtering using a W metal target. In addition, Platinum (Pt) was deposited on the WO3 film surfaces at room temperature, also by sputtering. In the early stages of Pt growth, formation of Pt nanoparticles could be expected because of the island structure observed in Volmer-Weber-type growth mode. The surface coverage of Pt on the WO3 films was estimated quantitatively by X-ray photoelectron spectroscopy and was found to be approximately 60% after 7 s deposition. High resolution electron microscopy (HREM) demonstrated that Pt nanoparticles with a diameter of about 2.5 nm were generated and dispersed uniformly on the entire surface area of the columnar polycrystalline WO3 films. These Pt-loaded films exhibited high photocatalytic activity in the decomposition of acetaldehyde (CH3CHO) under visible light irradiation.