Structure and properties of resistive thick films after laser treatment

We have studied the influence of laser radiation on the structure and electrophysical properties of resistive thick films based on BaB₆-LaB₆ solid solution. It has been shown that, in the course of laser treatment in the millisecond range, barium and lanthanum diffuse into the substrate, and micro-and nanosecond laser radiation leads to the breakup of current-conducting chains and the formation of domains with discrete structure. We have carried out a differential analysis of voltampere characteristics and determined the temperature dependences of electrical resistance R and resistance temperature coefficient. Laser treatment in the micro-and nanosecond ranges slightly increases the electrical resistance, which can be used for the precision treatment of thick films. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 79–85, November–December, 2007.     

Effect of laser irradiation on structural,electrical and optical properties of SnO2 films

Thin films of SnO₂ were prepared using a spray pyrolysis technique. Films were irradiated with Nd:YAG laser pulses of various energy densities (2–50 mJ cm^–2) with varying number of pulses from 1–50. X-ray diffraction studies were made to investigate the structural changes due to laser irradiation. An improvement in crystallinity and an increase in grain size were observed in laser-irradiated films. Hall coefficient and Hall mobility studies were made in the temperature range 77–300 K for the as-grown as well as laser-irradiated films. An increase in mobility and a decrease in carrier concentration were observed in the films after laser irradiation. Optical transmission studies revealed that the refractive index increased as a result of laser irradiation.     

Effect of thickness on H2 gas sensitivity of SnO2 nanoparticle-based thick film resistors

The effect of thickness on the p.p.m. level H₂ gas sensitivity of SnO₂ nanoparticle-based thick film resistors is reported. The nanoparticles are synthesized by a sol-gel method and the films are prepared using standard screen printing technology. The thickness of the films is varied from a few micrometers to a few hundred micrometers. The results indicate that the sample having thickness 76 m gives the maximum sensitivity. The mechanism of the change in sensitivity with thickness is discussed.     

用激光烧结法制备的SnO2薄膜的气敏性质

用溶胶-凝胶法制备SnO2前驱液,然后用提拉法分别在单晶Si和Al2O3基片表面制备出SnO2前驱膜,再用脉冲Nd:YAG激光烧结前驱膜使其转变为晶体SnO2薄膜.用XRD分析了单晶Si表面的SnO2薄膜,研究激光功率对SnO2薄膜相组成的影响.TEM观察表明,激光烧结后的薄膜SnO2颗粒均匀,直径约为10 nm.用激光烧结法制备的SnO2薄膜对浓度为1.80×10-4丙酮的最高灵敏度为30～40,明显高于用传统烧结法制备的SnO2薄膜的灵敏度.激光烧结能降低薄膜具有最高灵敏度的工作温度.     

SnO2-Sb2O3基压敏陶瓷致密化及脉冲电流耐受特性

实验研究了TiO₂、Co₃O₄、Cr₂O₃、Ni₂O₃和MnO掺杂对SnO₂-Sb₂O₃基压敏陶瓷材料微观结构和电性能的影响. 研究结果表明, TiO₂和Co₃O₄促进SnO₂陶瓷烧结致密化, 根据XRD图谱分析结果, Co₃O₄与SnO₂反应形成了Co₂SnO₄晶相, TiO₂则固溶于SnO₂晶相;Sb元素的引入能够促进SnO₂晶粒的半导化;复合添加Cr₂O₃、Ni₂O₃和MnO可以有效提高材料的电压非线性特性和脉冲电流冲击耐受能力. 获得电性能接近实用化的SnO₂压敏陶瓷样品, 其压敏电压V_1mA约为350V/mm, 非线性系数α达到50, 漏电流小于5μA, 并且在8/20μs脉冲电流冲击试验中,直径14mm的样品能够经受2kA的脉冲峰值电流.     

Electrical conductivity and gas sensitivity of Zn-Sb-O thick films

The thick film of Zn-Sb-O was prepared by coating the paste of nanoparticles mixture (Sb₂O₃:ZnO=1:3) on the alumina substrate, followed by sintering at 500-900 °C for 2 h in air. The electrical resistance and gas-sensing properties to benzene, alcohol and acetone of Zn-Sb-O films were found to be dependent on the change of phase structure caused by sintering temperature.     

CO2 laser annealing of plasma-deposited high-T c superconducting thick films

Recent advances in the fabrication of high-T _c superconducting thick films demand processing techniques which can eliminate film/substrate interdifiusion that occurs during subsequent post-annealing heat treatment after the film is deposited, thereby limiting the application of the thick films for devices. The present study evaluates laser annealing techniques for plasma-deposited Y-Ba-Cu-O thick films using a high-energy CO₂ laser (10.6Μm) in a continuous wave mode. The results are compared with those obtained by conventional furnace annealing techniques necessary for post-heat treatment of as-deposited superconducting thick films. The high-T _c superconducting phase is recovered by cationic diffusion during subsequent post-annealing heat treatment. Crystallographic phases and microstructural characterization have been performed using XRD, SEM, and EPMA analytical techniques. The significance of the technology lies in the elimination of film/substrate interdiffusion problems, thereby resulting in high-quality superconducting thick films. The technology will significantly reduce the post-annealing times usually required by conventional furnace annealing techniques.     

Room temperature Cl(2) sensing using thick nanoporous films of Sb-doped SnO(2)

Thick film resistive Cl(2) sensors were fabricated using SnO(2) doped with Sb. The nanocrystalline powders of Sb-doped SnO(2) synthesized by a sol-gel method were compressed into an 800?μm thick pellet. The fabricated sensors were tested against gases like Cl(2), Br(2), HCl, NO, NO(2), CHCl(3), NH(3) and H(2). The highest response to Cl(2) was achieved in 0.1% Sb doping where an exposure to 3?ppm of Cl(2) gas led to a 500-fold increase in device resistance. The high sensitivity to Cl(2) is accompanied by minor interference due to other gases at room temperature. It was found that the SnO(2) doped with 0.1% Sb exhibited high response, selectivity (>100 in comparison to the gases described above) and short response time (～60?s) to Cl(2) at 3?ppm level at room temperature.     

On the segregation of the conductive phase in RuO2-based thick resistive films

The relation between the resistivity and the volume fraction v of the conductive phase in RuO₂-based thick resistive films was investigated. Two series of pastes with RuO₂ particle sizes of the order of 10 and 300 nm were prepared and were used to obtain two series of thick resistive films. To describe the relations obtained between the resistivity and v, the conducting chains model, the percolation model and the modified percolation model were utilized. As a result of the approximation procedure three groups of optimal parameters were obtained. The values of all these parameters indicate the segregation (or clustering) of the conductive phase in the thick resistive films. The degree of this segregation is evidently greater in the thick resistive films containing particles of the smaller size. The observed segregation of the conductive phase is believed to be related to a high value for the ratio of the particle diameter of the glaze to that of the RuO₂. For the films investigated this ratio has been estimated to be 160 and 5 for thick resistive films containing RuO₂ particles of the smaller and larger diameters respectively.     

Enhancement of the characteristics of thin-film electromluminescent structures based on ZnS:Mn films after low-power laser irradiation

It is shown that short-term exposure of thin-film electroluminescent structures based on ZnS:Mn films to ultraviolet pulses with energy much lower than the threshold energies for laser annealing of ZnS:Mn films can enhance the brightness and efficiency of the electroluminescence, increase the slope of the brightness-voltage characteristics, improve the operating stability, and increase the electrical safety margin of the structures. It is established that the improvement in the properties of structures is caused by the photostimulation of below-threshold defect annealing processes and impurity diffusion, which lead to redistribution of the Mn ions in the ZnS film and reduce the concentration of shallow states which influence the structural characteristics. Pis’ma Zh. Tekh. Fiz. 24, 1–6 (February 26, 1998)     

High-sensitivity humidity sensor based on SnO2 nanoparticles synthesized by microwave irradiation method

Tin oxide hexagonal-shaped nanodiscs (SnO) and spherical nanoparticles (SnO₂) have been prepared by using a simple household microwave irradiation method with an operating frequency of 2.45 GHz. This technique permits us to produce gram quantity of homogeneous nanoparticles in just 10 min. The crystallite size was evaluated from powder X-ray diffraction (XRD) studies and was in the 20 to 25 nm range. Transmission electron microscopy (TEM) analysis showed that the as prepared SnO form as hexagonal-shaped nanodiscs and upon subsequent annealing at 500 °C for 5 h in air, the SnO gets converted to spherical-shaped nanoparticles of SnO₂. The SnO₂ sample shows good sensitivity towards the relative humidity. The calculated response and recovery time were found to be 32 s and 25 s respectively. These results indicate promising applications of SnO₂ nanoparticles in a highly sensitive environmental monitoring and humidity controlled electronic devices. The samples were further subjected to thermal analyses (TG–DTA) and UV–VIS diffusion reflectance spectroscopy (DRS) studies.     

Fabrication of resistive CO gas sensor based on SnO2 nanopowders via low frequency AC electrophoretic deposition

A resistive CO gas sensor has been fabricated using AC electrophoretic deposition (ACEPD) technique. SnO₂ thick films are deposited by applying low frequency (0.01–1,000 Hz) AC electric field to a stable suspension of SnO₂ nanoparticles in acetyl acetone. A carbon film base electrode is used as deposit substrate. Effect of CO gas exposure on conductivity of the SnO₂ film at 300 °C is investigated. Results show that the sensor is sensitive and its response is repeatable. This work shows that ACEPD can be used as an easy and cheap technique for fabrication of electronic devices such as ceramic gas sensors.     

纳米SnO_2及分子筛封装纳米SnO_2簇的湿敏性能研究

本文以液相法制备的纳米ＳｎＯ２微粉和分子筛封装的纳米ＳｎＯ２簇微粉为原料，采用涂膜法制作成湿敏元件，并且分别对这两种元件进行了湿敏性能的测试。研究表明纳米ＳｎＯ２微粉和分子筛封装的纳米ＳｎＯ２簇微粉，其湿敏元件灵敏度明显高于普通ＳｎＯ２微粉，阻湿线性关系良好，具有较好的湿敏性能。     

Fabrication and characterization of X-ray array detectors based on polycrystalline PbI2 thick films

Polycrystalline PbI₂ thick films were grown by using close spaced vapor deposition method on glass substrate with a conducting indium–tin-oxide coating. The morphology shows a uniquely oriented film structure with hexagonal platelets accurately being upright on substrate surface. An array detector with 12 pixels was fabricated based on this structure of the thick film. It is shown that the dark current is lower than 3 nA at bias voltage below 500 V, and the dark resistivity is as high as 10¹¹ Ω cm. A mapping of dark current density of the sensitive area of the array detector exhibits a better uniformity in the central area than the fringe area. A quick photocurrent response to X-ray excitation was obtained. The photoresponse rise time about 250 μs was obtained from the detectors and the photocurrent decays in a few seconds. The values of photocurrent are higher about two orders of magnitude than the values of dark current. The distribution of photocurrent is more uniform than that of dark current in the sensitive area of the detector.     

Effect of laser irradiation on microstructure and electromigration in aluminum films

Optical and transmission electron microscopy were employed to study the effect of ruby laser irradiation on the microstructure and electromigration properties of thin aluminum films. At low energy levels a single pulse creates multiple regions of localized grain growth; grain size in the irradiated area can be an order of magnitude larger than the matrix. Vaporization, melting, and grain growth are observed and related to the input laser energy. In some cases, the morphology of the laser-damaged film resembles that of cast ingots, i.e., a columnar structure radiating from an equiaxed boundary network and bounded by a circular chill zone.     

Preparation of nanocrystalline SnO2 thin films used in chemisorption sensors by pulsed laser reactive ablation

Nanocrystalline SnO₂ thin films were fabricated by pulsed laser reactive ablation using a metallic Sn target. Oxidation of Sn to SnO₂ occurred principally on the substrate surface and was negligible during transportation of Sn atoms in the ablated plume from the target to the film. Therefore, the substrate temperature was the most important parameter to influence the phase constitution of the films. When the substrate temperature was higher than the melting point of metal Sn (230 °C), SnO₂ phase was obtained. Otherwise the films were β-Sn dominant. X-ray diffraction and transmission electron microscopy techniques were used to determine the grain size in the films, which was in the range 10–30 nm, depending upon the substrate temperature and the subsequent annealing. For chemisorption performance, films with a thickness up to 24 nm showed a higher sensitivity than the films 38 nm and 96 nm thick. Excellent chemisorption properties have been achieved on the very thin nanocrystalline films. This revised version was published online in July 2006 with corrections to the Cover Date.