CO-sensing properties of hafnium oxide thin films prepared by electron beam evaporation

Thin films of hafnium oxide were deposited by electron beam evaporation. The effects of the film thickness and preparation conditions (films prepared on the heated substrate with or without the presence of oxygen environment during deposition) on the optical and carbon monoxide sensing properties of the films were studied. The films were characterized using X-ray diffraction and X-ray photoelectron spectroscopy and optical spectroscopy techniques. Films deposited on unheated substrates were amorphous, whereas those deposited on heated substrates showed a mixture of amorphous and polycrystalline structure. It was found that the sensitivity of the films to CO increased with the thickness and the porosity (as reflected by the refractive indices) of the films.     

Nanoparticles and polarons: active centers in thin film sensor devices

The spectroscopic characterization of localized charges in thin film devices is addressed. We demonstrate their existence and show their particular contribution to the operation of sensor devices. Examples in oxidic and polymeric thin films systems are given in which the localized states dominate the electronic properties.     

Hydrogen sensors with double dipole layers using a Pd-mixture-Pd triple-layer sensing structure

This paper reports on new GaN sensors using a Pd-mixture-Pd triple-layer sensing structure to enhance their sensitivity to hydrogen at the tens of ppm level. The proposed hydrogen sensor biased with a constant voltage produced relatively high sensing responses of 4.84 × 10⁵% at 10,100 ppm and 8.7 × 10⁴% at 49.1 ppm H₂ in N₂. The corresponding barrier height variations are calculated to be 220 and 168 mV. When the sensor is biased by a constant current with maximum power consumption of 0.4 mW, a sensing voltage as an output signal showed a voltage shift of more than 17 V (the highest value ever reported) at 49.1 ppm H₂ in N₂. By comparison to Pd-deposited GaN sensors, the improvement in static-state performance is likely attributed to double dipole layers formed individually at the Pd–GaN interface and inside the mixture. Moreover, voltage transient response and current transient response to various hydrogen-containing gases were experimentally studied. The new finding is that the former response time is shorter than the latter one.     

Thin polymer film based rapid surface acoustic wave humidity sensors

Polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) thin films, deposited on the surface of glass slides, were studied using transmission FTIR spectroscopy upon varying relative humidity (RH) from 2 to 70%. The obtained data revealed fast dynamics of water vapor adsorption-desorption with responses on the order of several seconds. Based on the fast FTIR signal intensity changes versus RH, it was proposed that a similar rapid response can be achieved for PVA and PVP coated SAW devices due to changes in mass-loading and film viscoelastic properties upon absorption of water vapor in the films. Sub-micron thickness films were spin-coated onto the surface of LiNbO₃ SAW substrates. Both PVA and PVP based humidity sensors revealed prompt reversible response to variations in humidity, although PVP-based device demonstrated better sensor parameters with total insertion loss variation of about 50 dB over the studied RH range and response time 1.5 s for the humidity step 5-95% (recovery time - 2.5 s), representing one of the fastest SAW-based humidity sensors reported to date.     

Evaluation of photoresponse in solution-processable ZnO thin film transistor for radiative sensor applications

Thin film transistor based on the spin-cast ZnO channel layer was fabricated with SiO₂ dielectric layer on Si substrate. The ZnO active layer grown by sol-gel spin-cast caused an increase in the field-effect mobility compared to those of the ZnO TFTs with the channel layer grown by zinc acetate precursor. Under light illumination, the ZnO-TFT in turn-off state exhibited a high drain current, which is 12.82 times higher than dark drain current, whereas in turn-on state is 9.43 times. The photosensing behavior of thin film transistor based on the spin-cast ZnO channel layer indicated more pronounced under a depletion region of 0 V gate bias. The obtained results indicate that the ZnO layer spin coated on SiO₂ gate layer can be an effective and promising way to increase factor for improving the device performance and for light detecting of ZnO thin film transistor and the studied thin film phototransistor can be used in optoelectronic applications.     

On the determination of the steady film profile for a non-Newtonian thin droplet

A shooting method is used to determine a solution to a third-order ODE modeling the steady profile of a non-Newtonian thin droplet. We compare a direct approach to an iterative approach using a secant method. We obtain a nonlinear relationship between the contact angle ? and the position of the contact line r. From this nonlinear relationship we use curve fitting to obtain an empirical law of the form tan?∝r^f(k) where k is the power law coefficient and f is a nonlinear function of k.     

射频磁控溅射功率对Ni-Mn-Ga薄膜成分与形貌的影响

采用射频磁控溅射镀膜技术在P型Si(100)基片上沉积Ni-Mn-Ga薄膜.实验结果表明,射频溅射功率对Ni-Mn-Ga薄膜成分与形貌有显著地影响.Ni含量随溅射功率的升高呈先增加后减少的趋势,Mn含量呈先减少后增加的趋势,Ga含量几乎呈线性减少的趋势.薄膜的价电子浓度(e/a)变化较小.参考英国国家物理实验室数据中有...     

Hydrogen sensor based on Pd-functionalized film bulk acoustic resonator

We present an application of the Pd-functionalized film bulk acoustic resonator for hydrogen detection. The resonator consisted of an AlN piezoelectric stack and a Bragg reflector has a working resonance near 2.23 GHz and a high performance. A 50 nm thick Pd film was coated on the top electrode as a specific layer to capture hydrogen. The absorption of hydrogen can trigger changes of elastic properties of the Pd layer, which affects the resonance frequency of the resonator. The experimental results show that the Pd-functionalized film bulk acoustic resonator can yield a rapid, sensitive, reversible and reproducible response to hydrogen in the concentrations from 0.3% to 2%. The advantages of this sensor, including the simple fabrication process, ease of detection method, rapid response and high sensitivity and working at room temperature, make this promising in the early alarm of hydrogen leaking.     

Polypyrrole thin film fiber optic chemical sensor for detection of VOCs

A fiber optic reflectance sensor (FORS) using Polypyrrole (PPy) conducting polymer to detect volatile organic compounds (VOCs) is demonstrated. The conventional interfacial polymerization method is used to synthesize a sensitive polypyrrole membrane, which shows relatively low roughness and high reflectivity. In general the changes in electrical properties of conductive polymers are explored in sensing applications, however their optical properties have been less explored. In the present study, we evaluate the optical properties of PPy and transfer on the end face of polymer optical fiber (POF) for the detection of VOCs. The change in the reflected optical signal from PPy upon interaction with the VOCs is systematically evaluated. The fabricated PPy FORS shows the excellent sensitivity to the VOCs under test with the detection limit up to 1 ppm.     

Fluid flow and heat transfer in the evaporating thin film region

The evaporating thin film region is an extended meniscus beyond the apparent contact line at a liquid/solid interface. Thin film evaporation plays a key role in a highly efficient heat pipe. A detailed mathematical model predicting fluid flow and heat transfer through the thin film region is developed. The model considers the effects of inertial force, disjoining pressure, surface tension, and curvature. Utilizing the order analysis, the model is simplified and can be numerically solved for the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate in the evaporating thin film region. The prediction shows that while the inertial force can affect the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate, in particular, near the non-evaporating region, the effect can be neglected. It is found that a maximum velocity, a maximum heat flux, and a maximum curvature exist for a given superheat, but the locations for these maximum values are different.     

Parallelization strategies for Monte Carlo simulations of thin film deposition

Atomistic simulations of thin film deposition, based on the lattice Monte Carlo method, provide insights into the microstructure evolution at the atomic level. However, large-scale atomistic simulation is limited on a single computer—due to memory and speed constraints. Parallel computation, although promising in memory and speed, has not been widely applied in these simulations because of the intimidating overhead. The key issue in achieving optimal performance is, therefore, to reduce communication overhead among processors. In this paper, we propose a new parallel algorithm for the simulation of large-scale thin film deposition incorporating two optimization strategies: (1) domain decomposition with sub-domain overlapping and (2) asynchronous communication. This algorithm was implemented both on message-passing-processor systems (MPP) and on cluster computers. We found that both architectures are suitable for parallel Monte Carlo simulation of thin film deposition in either a distributed memory mode or a shared memory mode with message-passing libraries.     

High-performance blue-green-red magneto-optical spatial light modulators based on thin film multilayers

Herein, we report high-performance thin film multilayer structures with the capability of operating as high-contrast magneto-optical spatial light modulators (MOSLMs) for simultaneously operating at blue-green-red (BGR) wavelengths. The adjustment is carried out by tuning the applied magnetic field to use the high potential of CeF₃ as a new and transparent visible Faraday rotator. For practical purposes, to avoid instabilities due to the wavelength fluctuations, we showed that achieving a suitable tunable MOLSM with flat-top responses is possible by offering appropriate approach in designing of multilayers.     

PVDF压电薄膜传感器的研制

采用28μm厚的4层聚偏二氟乙烯(PVDF)压电薄膜研制了PVDF压电传感器,该传感器表面电极形状应用剪切加丙酮腐蚀的方法制成,保证了传感器有一定的非金属化的边缘。对于电极的引出是将传感器上、下电极面引脚错开,引出电极采用比较容易做到的穿透式,并用压接端子压接和空心小铆钉铆接的2种方法。     

微型薄膜集成氢气敏感器件的研制

采用将类光激发二极管(即N-Si/Thin-SiO2/Thin-Ta/PdCr/Pt结构)、PdCr合金薄膜电阻、Pt加热和感温双电阻温控系统等集成的方式,研制了微型薄膜集成氢气敏感器件,能稳定测量0~2%体积分数的氢气,且具有较快的响应和恢复时间。     

Pd/PVDF thin film hydrogen sensor system based on photopyroelectric purely-thermal-wave interference

A novel sensitive solid-state sensor system for trace hydrogen gas detection has been developed as a next generation device to earlier photopyroelectric (PPE) hydrogen sensors. The basic principle of the sensor is based on the technique of PPE purely-thermal-wave interferometry recently developed in this Laboratory. The active element of the sensor is a thin polyvinylidene fluoride (PVDF) pyroelectric film, sputter-coated with Pd on one surface and with a Ni–Al alloy electrode on the other surface. Unlike the conventional PPE hydrogen sensors, this new sensor produces a coherent differential PPE signal in a single detector, rather than using two detectors (one active, the other reference) and complicated electronics. The measurement results show that the signal noise level, the detectivity and the signal dynamic range are improved by more than one order of magnitude compared with the conventional single-beam method. The operating characteristics have been examined for three different thicknesses of Pd coating on the same thickness PVDF-film detector. The signal generating mechanism, attributed to the change of the optical absorptance of the Pd coating when exposed to hydrogen, and/or the shift in the Pd work function, is also discussed.     

Three-dimensional thin film and droplet flows over and past surface features with complex physics

This paper describes, in the main, the benefits of using a general Newton globally convergent solver within an adaptive multigrid framework for solving discretised forms of lubrication models of three-dimensional, free-surface flow over and/or past substrate topography, involving complex physics. The two illustrative gravity-driven problems considered address solute mixing in a continuous thin film flow and droplet migration down an inclined substrate. The computational price paid for the flexibility offered by the solver is investigated alongside the overall benefits of adaptive local mesh refinement and multigridding.     

Design and experimental implementation of an effective control system for thin film Cu(InGa)Se2 production via rapid thermal processing

Copper indium gallium diselenide, Cu(InGa)Se₂ (CIGS) solar cells have achieved efficiencies of 22.3% at the cell level and 17.5% at the module level. CIGS-based modules are also in the early stages of commercialization, with >1 GW annual production capacity. The most common method for producing CIGS in industry is via precursor reaction, which consists of depositing Cu-In-Ga precursor films and reacting them with gas-phase H₂Se at 450–550 °C for 60 min or longer, and is commonly called selenization. Recently, interest has been growing in selenization by Rapid Thermal Processing (RTP), which is characterized by rapid temperature ramping, approximately 550 °C or higher temperature reactions, and improved process throughput. However, it has been difficult to commercialize RTP for CIGS film production in part because implementing a rapid, linear temperature ramp in the reacting thin film is complicated by two intrinsic process characteristics: (i) the temperature of the CIGS film cannot usually be measured directly; and (ii), the process is significantly nonlinear due to the dominance of radiative heat transfer at high temperatures. In this paper, we present the design and modeling, construction, and successful operation of a pilot-scale RTP selenization reactor utilizing a novel temperature control system.Our two-fold approach to the unique temperature control challenges involves the design and implementation of (i) a first-principles, model-based observer to estimate the desired surface temperature; and (ii) a specialized controller to enable effective tracking of the desired linear temperature ramp set point. Our experimental results demonstrate that the control system is effective in tracking rapid temperature ramps accurately, with performance limited only by the physical constraints of the experimental system.     

Pulsed laser deposition of Y2O3 : Eu thin film phosphors

Thin films of Y₂O₃ : Eu cathodoluminescent (CL) phosphors were deposited using pulsed laser deposition using deposition temperature between 250°C and 800°C, O₂ pressures between residual vacuum (2×10⁻⁵ Torr) and 6 Torr, and post annealing up to 1200° for 1 h in air. The CL efficiency of the best thin film was about one third that of the starting powder. The brightness and efficiency of the thin films improved as the deposition temperature, O₂ pressure and post annealing temperature were increased, except that O₂ pressures above 600 mTorr did not significantly improve the CL properties. At deposition temperatures >600°C, the surface morphology changed from a smooth film to a nodular deposit for O₂ pressures >200 mTorr, with nodule dimensions ≈100 nm. Simultaneously, the CL properties improved dramatically because of enhanced optical scattering out of the thin film. Optical scattering was discussed in terms of anomalous diffraction. The CL properties also improved dramatically with high temperature post annealing. This effect was interpreted in terms of improved crystallinity and activation of the Eu. The low brightness and efficiency of thin films versus powder was affected by depletion of the Eu in the thin films owing to the deposition process.     

厚膜压力传感器的结构优化

为提高厚膜压力传感器的稳定性,借助有限元分析软件Ansys,研究了厚膜压力传感器弹性体的应力分布,进而设计了应变电阻的尺寸,并优化了其布局。采用Ф18mm的96%A l2O3弹性体瓷件研制了一种新型厚膜力敏芯片,实验结果表明:它的热零点温漂小于3×10-4FS/℃,过载达120%,非线性误差及迟滞误差都小于±0.2%FS,精度由±0.4%FS提高到±0.2%FS,其线性度和稳定性得到很大提高,可广泛应用于仪器仪表、测控和导航等领域。     

Effects of different adsorbed species on ultrafine CO sensors

A low temperature CO sensor has been made by the use of ultrafine tin oxide as the base material. The gas sensor performance is correlated with the effects of different adsorbed CO species. A mechanism for the low temperature sensing is proposed and discussed.