Experimental study on piezoelectric resonator and nanogenerator devices using ZnO thin films

Abstract

Among many piezoelectric nanomaterials, the zinc oxide (ZnO) has been one of the most commonly used materials, mainly because of its superior bio-compatibility and outstanding piezoelectric as well as semiconductor characteristics for applications in optoelectronics and piezoelectricity such as light emitting diode (LED), resonator, sensor, filter, oscillator, nanogenerator, and so on. Particularly, this dissertation focuses on the experimental researches for the ZnO-based piezoelectric resonator and nanogenerator, as piezoelectric transducer. Firstly, film bulk acoustic resonator (FBAR) device, as piezoelectric resonator, shows an acceptably high resonance characteristic improvement owing to the annealing of its Bragg reflector (BR) and strong potential for FBAR-based sensor applications. We have set up five different liquid-loading conditions ($\delta$$L_0$–$\delta$$L_4$) to compare the loading effects by gradually increasing the viscosity (η) and mass density (ρ) of liquid (evenly mixed methanol–ink) loaded on the top electrode of two kinds of FBAR-based liquid sensors with non-annealed and annealed BRs (A and B sensors). And, we have investigated the effects of BR annealing on their performance factors by comparing their resonance frequency shift (-$\delta$$f_r$), measured before and after each liquid loading ($\delta$$L$). Compared with the A sensors, the liquid-loading effect (-$\delta$$f_r$) of B sensors were significantly improved by approximately two times under the same loading conditions ($\delta$$L$), resulting in twice the liquid sensitivity (S=-$\delta$$f_r$/$\delta$$L$). The improved energy trapping in the FBAR sensors owing to their BR annealing, which results in the improved resonance characteristic in terms of quality factor (Q), seems to be the critical factor that determines their -$\delta$$f_r$ and liquid sensitivities (S). It is further verified through the comparison of resonance characteristics and performance factors for A and B sensors including the return loss ($S_{11}$), Smith chart, impedance (magnitude and phase), Q, effective electromechanical coupling factor ($k_{eff} ^2$), and liquid sensitivity. In addition, the B sensors with much higher Q values can be more advantageous, in terms of the maximum and minimum detectable liquid-loading, over the A sensors with lower Q values. Overall, the B sensors show much larger detectable liquid-loading ranges than the A sensors. Consequently, the resonance characteristic improvements of FBAR-based liquid sensors, mainly due to BR annealing, favorably influence their performance factors (i.e., liquid-loading effect, liquid sensitivity, and detectable liquid-loading range). This simple BR annealing approach can be used to further improve the performance of FBAR-based liquid (or mass) sensors for bio-chemical sensing applications. Secondly, we present the comprehensive analysis and characterization for ZnO-based tandem-type vertically integrated nanogenerator (TVING) with the configuration of AlN/ZnO/AlN/ZnO/AlN. The comprehensive characterization of TVING is discussed and verified largely by three main points of view. Firstly, the effects of the AlN/ZnO-stacked hetero-structure on the performance of piezoelectric nanogenerator device are studied. TVING shows a significant performance enhancement, as compared to the conventional VING devices with the three-layer structure (AlN/ZnO/AlN). Secondly, according to the law of energy conservation, it can be interpreted that TVING’s excellent output voltage characteristics are due to the superior energy loss characteristics in the energy transfer and conversion process. It is verified by comparing quantitatively these energy loss characteristics from the $k_{eff} ^2$, Q, and dielectric dissipation factors (DFs) of VING and TVING devices with resonant frequency of approximately 300 kHz. Thus, we believe that the energy loss characteristics of devices can be largely attributed to their configurations and this tandem-type structure seems to be effective in improving the energy efficiency of the device. Thirdly, we compare the output voltages of two kinds of TVING devices: One consists of ZnO layers deposited in $O_{2}$ reactive gas and the other consists of ZnO layers deposited in $N_{2}O$ reactive gas, which could more effectively suppress the screen effect in the tandem-type ZnO-based nanogenerator devices. From this standpoint, it is concluded that the combined use of both the tandem-type structure and $N_{2}O$ reactive gas seems very promising for the realization of high-efficiency ZnO-based energy generation and harvesting. Furthermore, we present the experimental in-depth study on optimal configuration of AlN/ZnO-stacked hetero-structures for high-performance energy harvesting devices. We fabricated the eight different kind of nanogenerator devices with the configurations changed according to two variables (thickness of ZnO layer and number of stacked layers) to investigate more specifically. We have found that 2VING (i.e., TVING) device with five layers of AlN/ZnO-stacked thin films is optimal configuration with the most outstanding output voltage performance. According to the increase in number of AlN/ZnO-stacked layers, the increase rate of output voltage due to constructive integration effect is saturated, while its decrease rate due to the linearly decreasing aspect ratio characteristic of the ZnO nanorod (NR) remains constant. Thus, the output voltage characteristic by the increase in number of AlN/ZnO-stacked layers becomes strongly dependent on the aspect ratio of ZnO NRs. Moreover, most reported nanogenerator devices harvest the vibration energy from human motions or machine vibrations in low frequency range, but the flexible nanogenerator devices developed in this dissertation have the working resonant frequency of approximately 300 kHz, applicable as high-frequency vibration or ultrasonic energy harvesting, particularly for bio-implantable wireless generator.