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.
많은 압전 나노소재들 중에서 무연(lead-free)의 산화아연(이하 ZnO) 물질은 우수한 인체/자연 친화성과 압전/반도체 특성 등으로 인해 LED와 같은 광전자소자, 공진기, 센서, 필터, 오실레이터 및 나노발전기와 같은 압전소자의 핵심 소재로 가장 보편적으로 사용되어 왔다. 본 논문은 압전 트랜스듀서로써 산화아연 기반의 압전 공진기 및 나노발전기 소자기술을 중점 연구하였다.
첫째, 체적음향파 박막형 공진기(이하 FBAR)는 음향파 반사층의 어닐링을 통해 공진특성을 크게 향상시켰으며, 이는 FBAR기반 액체감지 센서에 적용되었다. 반사층을 어닐링한 것과 하지 않은 2종의 FBAR 액체감지 센서의 인가효과를 비교하기 위해 상부전극에 떨어뜨리는 액체 (균등하게 혼합된 메탄올/잉크)의 점도와 밀도를 점진적으로 증가시키는 5가지 인가조건을 설정하였으며, 각 액체 인가 전·후로 공진주파수 이동(-$\delta$$f_r$)을 측정하였다. 같은 인가조건 ($\delta$$L$)에서 어닐링 미실시 센서와 비교해 어닐링 센서의 액체 인가효과(-$\delta$$f_r$)는 약 2배 향상되어, 센서감도(-$\delta$$f_r$/$\delta$$L$) 역시 2배 향상됨을 확인하였다. 결과적으로 음향파 반사층 어닐링으로 인한 FBAR 센서의 공진특성 향상(ZnO내 음향파 에너지손실 개선)은 센서감도 성능에 매우 긍정적인 영향을 미치게 된다.
둘째, 5층의 질화알루미늄/산화아연(이하 AlN/ZnO)을 갖는 탠덤 구조의 압전 나노발전기(이하 TVING) 소자 성능 및 특성을 연구하였다. 나노발전기 성능에 미치는 AlN/ZnO 적층 구조의 효과를 살펴보기 위해 3층의 AlN/ZnO 구조를 갖는 일반적인 나노발전기와 특성을 비교한 결과, TVING 소자의 출력전압은 2배 이상 향상되었다. 에너지보존법칙에 따라 TVING의 우수한 출력전압 특성은 에너지전달 및 변환 과정에서의 뛰어난 에너지 손실(효율) 특성에 기인하게 되며, 이것은 기계-전기에너지 변환효율 및 소자의 손실특성을 정량화하여 보여주는 전기-기계결합계수($k_{eff} ^2$) 및 품질계수(Q), 유전손실계수(DF)를 비교함으로써 입증하였다. 따라서, 나노발전기 소자의 에너지 손실 특성은 소자 구조에 따라 크게 달라지며, 본 연구의 탠덤 구조는 소자의 에너지 효율을 향상시키는 있어 매우 효과적이다. 또한, 각각 $O_2 $및 $N_{2}O$ 반응가스에서 증착된 ZnO 박막을 갖는 2종의 TVING 소자를 비교한 결과, $N_{2}O$는 ZnO에서 정공 농도를 증가시키는 p형 불순물로 작용하여 전자에 의한 압전전위의 손실을 효과적으로 상쇄시킬 수 있었다. 즉, 탠덤 구조와 $N_{2}O$ 반응가스를 함께 나노발전기 소자 제작에 적용하게 되면 고효율 ZnO 기반 에너지하베스팅 구현에 매우 효과적일 것으로 기대된다. 더불어, 최적의 AlN/ZnO박막 적층 구조를 찾기 위해 9층까지 확장해서 제작한 소자의 성능을 비교하였으며, ZnO 나노로드의 크기(길이/지름 비율) 특성이 최적 성능 결정에 큰 영향을 주어 5층의 탠덤 구조가 최적의 소자 구조임을 재확인하였다.
