Piezoelectric Zinc Oxide Thin Film Research Articles

Thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric zinc oxide thin films

Linear piezoelectric materials offer new opportunities for next-generation multifunctional micro/nano systems on chip aside from ferroelectrics. However, the advantage of quantitative pressure sensing of linear piezoelectric materials over ferroelectrics has not been demonstrated, while the thermal stability of linear piezoelectric zinc oxide (ZnO) is yet to be evaluated. In this work, pressure sensors based on linear piezoelectric undoped ZnO and ferroelectric vanadium doped ZnO (V-ZnO) films are studied side by side. Both films possess similar fiber textures oriented along the polar ZnO [002] direction, yet each of the V-ZnO grains consists of strip-like domains observed by SEM. Applying triangular-wave pressures, the linear piezoelectric ZnO sensor generates triangular-wave voltage signals that are one-to-one linearly correlated to the applied pressure, showing decisive advantages for quantitative pressure sensing compared to the ‘sail-like’ voltage signals of the ferroelectric V-ZnO sensor that is many-to-one correlated to the applied pressure. Superior thermal stability is demonstrated in the linear piezoelectric ZnO sensor showing stable pressure sensing properties up to 450 °C compared to the degraded sensing properties of the ferroelectric V-ZnO sensor at 150 °C. The thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric ZnO films serve as candidates for high-performance multifunctional micro/nano systems.

Tuning the Porosity of Piezoelectric Zinc Oxide Thin Films Obtained from Molecular Layer-Deposited "Zincones".

Porous zinc oxide (ZnO) thin films were synthesized via the calcination of molecular layer-deposited (MLD) “zincone” layers. The effect of the MLD process temperature (110 °C, 125 °C) and of the calcination temperature (340 °C, 400 °C, 500 °C) on the chemical, morphological, and crystallographic properties of the resulting ZnO was thoroughly investigated. Spectroscopic ellipsometry reveals that the thickness of the calcinated layers depends on the MLD temperature, resulting in 38–43% and 52–56% of remaining thickness for the 110 °C and 125 °C samples, respectively. Ellipsometric porosimetry shows that the open porosity of the ZnO thin films depends on the calcination temperature as well as on the MLD process temperature. The maximum open porosity of ZnO derived from zincone deposited at 110 °C ranges from 14.5% to 24%, rising with increasing calcination temperature. Compared with the 110 °C samples, the ZnO obtained from 125 °C zincone yields a higher porosity for low calcination temperatures, namely 18% for calcination at 340 °C; and up to 24% for calcination at 500 °C. Additionally, the porous ZnO thin films were subjected to piezoelectric measurements. The piezoelectric coefficient, d33, was determined to be 2.8 pC/N, demonstrating the potential of the porous ZnO as an, e.g., piezoelectric sensor or energy harvester.

Study of the effects of annealing temperature on the properties of piezoelectric ZnO thin film for the development of MEMS acoustic sensor

This paper reports deposition and characterization of Zinc oxide (ZnO) thin film to be used as sensing layer in an acoustic sensor. An important requirement for such devices is the presence of high c-axis orientation and low stress in the deposited ZnO film. For this purpose, annealing of the deposited films is very much necessary. Here, zinc oxide thin films have been deposited over oxidized Si(100) wafer using radiofrequency (RF) magnetron sputtering method and are annealed at different temperatures from 200-400oC in steps of 50oC. Different microstructural parameters such as grain size, surface roughness, dislocation density and stress have been determined to obtain the optimized annealing temperature for the thin film. The optimization of the annealing temperature is necessary to obtain ZnO thin film with better piezoelectric response. Surface roughness has also been determined from AFM measurements. The XRD study indicates that the intensity of (002) peak changes with annealing temperature. Grain size increases from 15.46nm to 18.24nm and dislocation density decreases to minimum value 3.006×10-3nm-2 as temperature is increased to 400oC. Stress is also minimum at temperatures of 350-400oC. An annealing temperature of approx. 350oC has been found to be optimum for ZnO film to be used in acoustic sensor.

Development and characterization of confocal sputtered piezoelectric zinc oxide thin film

Thin film zinc oxide (ZnO) is often used as a piezoelectric material, but its material properties are dependent on the deposition method and parameters. Direct sputtering using reactive DC or RF sputtering are the most common deposition methods for thin film piezoelectric ZnO. This paper investigates the influence of varying sputtering parameters on piezoelectric and mechanical properties of ZnO deposited by confocal sputtering using pulsed DC on a silicon substrate. Parameters such as varying power, duty cycle and temperature were examined. Crystallinity of the films were investigated systematically by X-ray Diffraction. Mechanical properties such as hardness and Youngs Modulus of thin films were measured via nanoindentation. Piezoelectric coefficient d33 was also measured for the films. Results demonstrated that the ZnO thin film exhibited high c-axis (002) textured films with rocking curve full width half maximum values of 1.69°. It was observed that power, duty cycle and temperature significantly affected the crystallinity, mechanical properties and d33 values of the ZnO thin film. This paper demonstrates that pulsed DC confocal sputtering can be utilized to deposit piezoelectric ZnO films and demonstrates key sputtering parameters that influence the piezoelectric, mechanical and morphology properties of the film.

Development and characterization of ZnO piezoelectric thin films on polymeric substrates for tissue repair.

Currently available scaffolds for tissue repair have shown very limited success, so many efforts have being put in the development of novel functional materials capable of regulating cell behavior and enhance the tissue healing rate. Piezoelectric materials, as zinc oxide (ZnO), can be a very interesting solution for scaffold development, as they can deliver electrical signals to cells upon mechanical solicitation, allowing the development of suitable microenvironments for tissue repair. This way, it is reported the deposition of ZnO thin films on a polymer by direct current magnetron sputtering, under different conditions, in order to obtain a piezoelectric ZnO thin film with potential for tissue repair applications. The obtained ZnO thin films were characterized in terms of morphology, crystallography, electrical conductivity, transmittance, piezoelectricity, and adhesion quality. The deposition process resulted in uniform films, with a very good adhesion to the substrate. The different deposition conditions influenced the evolution of the crystalline domains and preferential growths and consequently, the electrical properties of the films. One of the conditions resulted in a thin film with a high piezoelectric coefficient and a conductor behavior, being considered the most promising to act as a bioactive coating.

Piezoelectric ZnO thin films for 2DOF MEMS vibrational energy harvesting

Zinc oxide (ZnO) is an environmental-friendly semiconducting, piezoelectric and non-ferroelectric material, and plays an essential role for applications in microelectromechanical systems (MEMS). In this work, a fully integrated two-degree-of-freedom (2DOF) MEMS piezoelectric vibration energy harvester (p-VEH) was designed and fabricated using ZnO thin films for converting kinetic energy into electrical energy. The 2DOF energy harvesting system comprises two subsystems: the primary one for energy conversion and the auxiliary one for frequency adjustment. Piezoelectric ZnO thin film was deposited using a radio-frequency magnetron sputtering method onto the primary subsystem for energy conversion from mechanical vibration to electricity. Dynamic performance of the 2DOF resonant system was analyzed and optimized using a lumped parameter model. Two closely located but separated peaks were achieved by precisely adjusting mass ratio and frequency ratio of the resonant systems. The 2DOF MEMS p-VEH chip was fabricated through a combination of laminated surface micromachining process, double-side alignment and bulk micromachining process. When the fabricated prototype was subjected to an excitation acceleration of 0.5 g, two close resonant peaks at 403.8 and 489.9 Hz with comparable voltages of 10 and 15 mV were obtained, respectively.

Tailoring Thin-Film Piezoelectrics for Crash Sensing.

Crash sensing and its assessment play a pivotal role in autonomous vehicles for preventing fatal casualties. Existing crash sensors are severely bottlenecked by sluggish response time, rigid mechanical components, and space constraints. Miniaturized sensors embedded with custom-tailored nanomaterials upholds potential to overcome these limitations. In this article, piezoelectric Zinc-Oxide thin film as a crash sensing layer is integrated onto a flexible metal-alloy cantilever. Material characterization studies are conducted to confirm piezoelectric property of sputtered ZnO film. The piezoelectric d 31 coefficient value of ZnO film was 7.2 pm V-1 . The ZnO sensing element is firmly mounted on a scaled car model and used in a crash sensing experimental set-up. A comprehensive theoretical analysis for two different real scenarios (nearly elastic and nearly inelastic collision) of crash events followed by experimental study is discussed. The crash sensor's output exhibits a linear relationship with magnitude of impact forces experienced at crash events. The response time of ZnO crash sensor is 18.2 ms, and it exhibits a sensitivity of 28.7 mV N-1 . The developed crash sensor has potential to replace bulk material sensors owing to its faster response time, high sensitivity, and compactness as the demand for crash sensors in next-generation automobile industries is progressively growing.

Piezoelectric Response Evaluation of ZnO Thin Film Prepared by RF Magnetron Sputtering

The most important parameter of piezoelectric materials is piezoelectric coefficient (d33). In this study, the piezoelectric ZnO thin films were deposited on the SiNx/Si substrate. The 4 inches substrate is diced into 8 cm× 8 cm piece. During the deposition process, a zinc target (99.999 wt%) of 2 inches diameter was used. The vertical distance between the target and the substrate holder was fixed at 5 cm. The piezoelectric response of zinc oxide (ZnO) thin films were obtained by using a direct measurement system. The system adopts a mini impact tip to generate an impulsive force and read out the piezoelectric signals immediately. Experimentally, a servo motor is used to produce a fixed quantity of force, for giving an impact against to the piezoelectric film. The ZnO thin films were deposited using the reactive radio frequency (RF) magnetron sputtering method. The electric charges should be generated because of the material’s extrusion. This phenomenon was investigated through the oscilloscope by one shot trigger. It was apparent that all ZnO films exhibit piezoelectric responses evaluated by our measurement system, however, its exhibit a significant discrepancy. The piezoelectric responses of ZnO thin film at various deposition positions were measured and the crystal structures of the sputtering pressure were also discussed. The crystalline characteristics of ZnO thin films are investigated through the XRD and SEM. The results show the ZnO thin film exhibits good crystalline pattern and surface morphology with controlled sputtering condition. The ZnO thin films sputtered using 2 inches target present various piezoelectric responses. With the exactly related position, a best piezoelectric response of ZnO thin film can be achieved.

Zinc oxide/aluminum-based self-powering storage system fabricated using selectively direct-write ultraviolet-curable resin method

This article presents a flexible piezoelectric harvester based on a polyethylene terephthalate cantilever system to scavenge vibration energy. It comprises a low-voltage drop rectifying circuit and an electrolytic capacitor to store the induced voltage. The harvester was made in the form of a composite plate that consists of flexible aluminum/polyethylene terephthalate vibration plate, piezoelectric zinc oxide thin film, and selectively deposited ultraviolet-curable resin lump structures as proof mass, which were directly constructed on the rear side of the polyethylene terephthalate-based composite plate using electrospinning with a stereolithography technique. Aluminum was sputtered on the polyethylene terephthalate substrate as the bottom electrode because of superior adhesion with the polyethylene terephthalate and zinc oxide film, compared with indium tin oxide film. Finite element analysis simulations were used to analyze the operation frequency and output voltage of the piezoelectric composite plate. The experimental results show that the maximal open-circuit voltage was 2.4 V and the close-circuit voltage was 2.18 V, which is 0.56 µW/cm2. After rectification, a direct current voltage of 2 V across the capacitor can be achieved. The self-powered storage system can drive the warning signal of the light-emitting diode module in both resonant and nonresonant conditions.

Study of broad bandwidth vibrational energy harvesting system with optimum thickness of PET substrate

In this paper, we present the development of a flexible PET-based (polyethylene terephthalate; PET) vibrational energy harvesting system with broad bandwidth. This broad bandwidth harvesting system comprises of four units of individual ZnO (zinc oxide) piezoelectric harvester in the form of a cantilever structure connected in parallel, and rectifying circuit with storage module. This system has ability to convert mechanical energy into electrical energy from the varying ambient vibration. The design and simulation of a piezoelectric cantilever plate was described by using commercial software ANSYS FEA (Finite Element Analysis) to determine the optimum thickness of PET substrate, internal stress distribution, operation frequency and electric potential. With the optimum thickness predicted by developed accurate analytical formula analysis, the one-way mechanical strain that is efficient to enhance the induced electric potential can be controlled within the piezoelectric ZnO layer. In addition, the relationship among the model solution of piezoelectric cantilever plate equation, vibration-induced electric potential and electric power was realized. An individual piezoelectric harvester consists of flexible PET substrate, piezoelectric ZnO thin film with (002) c-axis preferred orientation, and selectively deposited UV-curable resin lump structure which is used to change the resonant frequency of the harvester. In combination with multi-harvesters and rectifying with storage module together, an energy harvesting system with broad bandwidth can be fabricated. One individual harvester achieves a maximum OCV (open-circuit voltage) up to 4 V with power density of 1.247 μW/cm3. So far, we succeeded in accomplishing a broad bandwidth system with operating frequency range within 100 Hz–450 Hz to enhance powering efficiency. When the DC voltage (direct current voltage) across a storage module is charged up to 1.55 V after rectification, a flash LED (light emitting diode) is driven.

Design and fabrication of flexible piezo-microgenerator by depositing ZnO thin films on PET substrates

In this study, a new design of PET-based (polyethylene terephthalate) flexible power harvesting system is proposed. It consists of flexible PET vibration substrate, piezoelectric ZnO (zinc oxide) thin film, design of lump structures and electrodes. The relationship between the dynamic response of PET substrate and its power output is realized. High electromechanical transformation of piezoelectric material and efficient energy transfer of mechanical structure make the piezoelectric generator a high performance. The flexible devices of cantilevers with lump structures and piezoelectric ZnO thin film is designed. A layer of aluminum (Al) thin film is placed as a top electrode and a copper (Cu) foil as a bottom electrode. The piezoelectric cantilever plate is designed by using ANSYS ® software. Both modal analysis and harmonic response analysis are performed to obtain the structural modal parameters and frequency response functions, respectively. Then, imprinting process is applied to transfer the configuration of geometric lump structure, based on the simulated result, onto the flexible substrate to improve the power output. The lump material is ultraviolet-curable (UV) resin. The high-quality ZnO piezoelectric thin film has an even distribution of crystal grains. The angle obtained from diffraction diagram approaches 34.5°. It was deposited to the Al/PET flexible substrate by using RF (radio frequency) magnetron sputtering under 75 W power. In comparison of simulation and experiments with and without the lump structure, the lump structures on the flexible generator do improve the power output. The generator achieves maximum OCV (open-circuit voltage) up to 2.25 V and CCV (closed-circuit voltage) up to 1.55 V which is 0.276 μW per centimeter square. In addition, the devices are fabricated by a new process where the piezoelectric active areas on the cantilever are supported to achieve a large displacement with low residual stress. With low elastic constant of the PET-based layer, the cantilever structure can achieve better displacement compared with the conventional SOI (silicon on insulator) structures or other metal substrates.

RF sputtered piezoelectric zinc oxide thin film for transducer applications

This paper demonstrates the substrate dependency of the c-axis zinc oxide growth in radio-frequency sputtering system. Different deposition conditions were designed to study the influences of Si, SiO2/Si, Au/Ti/Si, and Au/Ti/SiO2/Si substrates on the piezoelectric and crystalline qualities of the ZnO thin films. Experimental results showed that the multilayer of Au/Ti/SiO2/Si-coated silicon substrate provided a surface that facilitated the growth of ZnO thin film with the most preferred crystalline orientation. The 1.5 μm-thick thermally grown amorphous silicon dioxide layer effectively masked the crystalline surface of the silicon substrate, thus allowing the depositions of high-quality 20 nm-thick titanium adhesion layer followed by 150 nm-thick of gold thin film. The gold-coated surface allowed deposition of highly columnar ZnO polycrystalline structures. It was also demonstrated that by lowering the deposition rate at the start of sputtering by lowering RF power to less than one-third of the targeted RF power, a fine ZnO seed layer could be created for subsequent higher-rate deposition. This two-step deposition method resulted in substantially enhanced ZnO film quality compared to single-step approach. The influence of stress relaxation by annealing was also investigated and was found to be effective in releasing most of the residual stress in this layered structure.

Piezoelectric zinc oxide by electrostatic spray pyrolysis

Zinc oxide (ZnO) possesses many interesting properties, such as a wide energy band gap, large photoconductivity, and high excitonic binding energy. Piezoelectric (ZnO) film has a high electro-mechanical coupling coefficient, which makes it a promising material for high frequency and low surface acoustic wave (SAW), bulk acoustic wave, and microelectromechanical system devices. In this work, we present the first results obtained for piezoelectric ZnO thin films prepared on corning 7059 glass substrate by electrostatic spray pyrolysis (ESP), a simple, cheap and efficient technique not widely used for ZnO deposition, using zinc nitrate in a mixture of deionised water and isopropyl alcohol. The structural morphologic and optical properties of the films have been studied and the effect of the preparation conditions, such as substrate temperature and substrate–nozzle distance, discussed. Highly c-axis preferred orientation, which is critical for piezoelectric applications (ultrasonic oscillators and transducers devices), ZnO thin films are obtained at 350 °C growth temperature and at 4.5 cm substrate–nozzle distance.

Current?voltage characteristics of high-resistive ZnO thin films deposited by RF magnetron sputtering

Current–voltage properties along the c-axis of high-resistive Ni-doped zinc oxide (ZnO) thin film were examined. Zinc oxide thin film capacitor with Al/Ti top and bottom electrodes was fabricated on SiO2/Si substrate. Current saturation due to acoustoelectric effect in piezoelectric ZnO thin film was observed. The current saturation indicates that the drift velocity of electron reaches the sound velocity of ZnO and the electron mobility of ZnO along the c-axis was estimated to be 1–3 cm2V−1 s−1. The electric field where the current saturates depended on the direction of the applied field and the internal residual stress in ZnO thin film, which indicate that the polarity of ZnO thin film deposited on Al/Ti is negative.

Current–voltage characteristics of high-resistive ZnO thin films deposited by RF magnetron sputtering

Current–voltage properties along the c-axis of high-resistive Ni-doped zinc oxide (ZnO) thin film were examined. Zinc oxide thin film capacitor with Al/Ti top and bottom electrodes was fabricated on SiO 2/Si substrate. Current saturation due to acoustoelectric effect in piezoelectric ZnO thin film was observed. The current saturation indicates that the drift velocity of electron reaches the sound velocity of ZnO and the electron mobility of ZnO along the c-axis was estimated to be 1–3 cm 2V −1 s −1. The electric field where the current saturates depended on the direction of the applied field and the internal residual stress in ZnO thin film, which indicate that the polarity of ZnO thin film deposited on Al/Ti is negative.

The significance of parallel electric field on the preferred orientation and surface morphology of ZnO thin films

Piezoelectric zinc oxide thin film, with its high electromechanical coefficient, has found applications in surface and bulk acoustic wave devices. A suitable polycrystalline zinc oxide thin-film transducer should possess the following two characteristics. First, structurally, the normals to the basal planes (0 0 2) must be predominantly oriented perpendicular to the plane of the substrate (c-axis oriented). Second, electrically, the resistivity of the film should be as high as possible. Having these requirements, polycrystalline zinc oxide can be satisfactorily used in place of ZnO single-crystal thin film in veryand ultra-high-frequency acoustic applications. Accordingly, if we could manipulate the c-axis orientation of the film, we would be able to tailor the device’s performance. We have investigated the effects of parallel electrical fields (detached electrodes) on the preferred c-axis orientation and surface morphology of polycrystalline zinc oxide thin films. Zinc thin films are oxidized at different temperatures with the application of parallel electrical fields. We have found that the application of parallel electrical fields slightly improves the c-axis orientation. The better c-axis–oriented structure is manifested by a less needle-shaped forest crystal morphology on the surface of the film.

Microfluidic motion generation with acoustic waves

This paper presents a novel approach for producing microfluidic motion by loosely focused acoustic waves (generated by piezoelectric zinc oxide thin film). Results show that the acoustic waves generated by radio frequency (r.f.) sources with frequencies corresponding to the thickness-mode resonances of the piezoelectric film are very effective in moving liquid around when the waves are loosely focused. The device operates without any significant temperature increase in the liquid, and will be very attractive for mixing or transporting temperature-sensitive fluids.

A 100-MHz ultrasonic transducer array using ZnO thin films

A 100-MHz ultrasonic linear transducer array made from a piezoelectric zinc oxide thin film on a sapphire substrate was developed and evaluated. Epitaxial, high-acoustic quality 10-/spl mu/m-thick ZnO film layers were produced by rf-magnetron sputter deposition onto a (111)-oriented gold film (with a chromium adhesion layer) that was vacuum-evaporated onto a (0001) sapphire surface. We found that, in well-oriented growth of gold, it is important to control the chromium sublayer thickness (less than 5 nm). An array was constructed by photolithography with an appropriate etch. V-shaped grooves between adjacent elements were formed by using an anisotropic etchant (HCl and HNO/sub 3/-based) that preferentially etched the c-plane of ZnO. Typical array elements were 90 /spl mu/m wide, 3.2 mm long, and 10 /spl mu/m thick, and the pitch of an array was typically 100 /spl mu/m. Our fine uniform array resulted in uniform ultrasonic response of individual elements throughout the array. For a 32-element array, the ultrasound beam in the azimuth plane in water could be electronically focused in the 100 MHz range to obtain a half-amplitude width of 60 /spl mu/m at the focal depth, agreeing well with theoretical predictions. Besides the use demonstrated with this present transducer, piezoelectric thin films should also lead to fabrication of various other kinds of ultrasonic transducers that can operate at high frequencies and should provide opportunities for miniaturizing transducers and making integrated ultrasonic devices. >