介面敏感之金屬氧化物奈米元件

Abstract

此論文主要研究氧化鋅的能帶彎曲與半徑的關係，藉由縮小氧化鋅奈米線的直徑，所增加的表面位壘可以完全空乏奈米線，進而縮小光放鬆的時間。此外，藉由加熱鍍有金奈米粒子的氧化鎳/氧化鋅使得光響應增加到3×108，這是由於氧離子的吸附所造成。再者，使用紫外光照射3D憶組式記憶體也觀察到有類似的現象。以上這些實驗，再次證明表面效應與元件表現是有非常密不可分的關係。

The thesis consists of an introduction, four chapters and a conclusion, which each chapter covering a different topic. In the introduction, we review applications of interface-sensitive metal oxide nanodevices that we further study in the later chapters. In chapter 2, we study the surface effect on the electronic and optoelectronic properties of ZnO nanowires (NWs) through surface treatment and electro-optical characterization of field effect transistors (FET) and ultraviolet (UV) photodetectors. The pronounced surface effect leads fully depleted NW with diameters thinner than critical dimension and incompletely depleted NW when the diameter is thicker than critical dimension. The recovery of photocurrent is found also to be strongly related to the diameter of NWs, indicating that photocarrier relaxation behavior is dominated by surface band bending (SBB). Moreover, the electrical tunability of the threshold voltage via surface treatments (i.e. annealing or Au-nanoparticles decoration) indicates strong impact of the oxygen adsorbates on SBB. These surface-related phenomena should be generic to metal oxide nanostructures. Our study is greatly beneficial for the NW-based device design of sensor and optoelectronic applications via surface engineering. In chapter 3, the surface effect on the photodetection of metal oxide nanostructures acting as a double-edged sword achieves ultrahigh photogain but unavoidably prolongs the response time due to slow oxygen adsorption/desorption processes. In this study, we break the compromise to enhance the UV photogain by 3 orders of magnitude as well as increase the photoresponse speed by 5 times via incorporating open-circuit p-n nanoscale heterojunctions (NHJs) by forming single-crystalline p-NiO nanoparticles on n-ZnO nanowires. This is because the formation of NHJs enhances surface band bending of ZnO nanowires, improving the spatial separation efficiency of photogenerated electrons and holes, and passivates the ZnO surfaces by minimizing the interaction of photocarriers with chemisorbed oxygen molecules. The concept using NHJs explores a new pathway toward ultrafast and supersensitive photodetection. In chapter 4, we investigate the photoelectrical and resistive switching properties of Pt/ZnO/Pt capacitor operated in unipolar mode under UV illumination. The oxygen photodesorption under UV illumination explains the photoconduction observed in initial and high resistance states. Meanwhile, oxygen readsorption at surface-related defects justifies the different photoresponse dynamics in both states. Finally, UV illumination significantly reduces the variations of resistance in high resistance state, set voltage and reset voltage by 58 %, 33 % and 25 %, respectively, stabilizing Pt/ZnO/Pt capacitor. Our findings in improved switching uniformity via UV light give physical insight into designing resistive memory devices. In chapter 5, a 3D double-layer HfO2-based vertical-resistive random access memory (VRRAM) with low-resistivity C54-TiSi2 as horizontal electrodes is demonstrated using CMOS processing. The electrical measurements show for the first time bipolar resistive switching by using C54-TiSi2 as novel electrodes for resistive switching applications. The statistical analysis exhibits cycle-to-cycle and cell-to-cell stable non-volatile properties with robust endurance (100 cycles) and long term data retention (104 s), suggesting that the ultrathin sidewall of C54-TiSi2 nanoscale electrodes serve to confine and stabilize the random nature of the conducting nanofilaments. The superior resistive switching characteristics demonstrated here highlight the applicability of C54-TiSi2 sidewall-confinement nanoscale electrodes to VRRAM.