I-Chuan Yao

Fabrication and resistive switching characteristics of high compact Ga-doped ZnO nanorod thin film devices

This study investigates the resistive switching behavior of Ga-doped ZnO (GZO) nanorod thin films with various Ga/Zn molar ratios. Vertically well-aligned and uniform GZO nanorod thin films were successfully grown on Au/Ti/SiO(2)/p-Si substrates using an aqueous solution method. X-ray diffraction (XRD) results indicate that GZO nanorods have [0001] highly preferred orientation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations show the formation of highly ordered and dense nanorod thin films. These compact GZO nanorod thin films can be used to make resistive switching memory devices. Such memory devices can be reversibly switched between ON and OFF states, with a stable resistance ratio of ten times, narrow dispersion of ON and OFF voltages, and good endurance performance of over 100 cycles. The resistive switching mechanism in these devices is related to the formation and rupture of conducting filaments consisting of oxygen vacancies, occurring at interfaces between GZO nanorods (grain boundaries). Results show that the resulting compact GZO nanorod thin films have a high potential for resistive memory applications.

Nanotip fabrication of zinc oxide nanorods and their enhanced field emission properties.

The ZnO nanorods on ZnO/Si substrates were synthesized by using the low temperature growth aqueous solution method. The chemical and plasma etching treatments were carried out on the as-grown ZnO nanorods to provide the nanorods with various tip angles. The crystal structure and morphology of the ZnO nanorods were examined by x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The nanorods grew along the [0001] direction and had various tip angles formed after the above etching processes. The field emission properties of the ZnO nanorods with tip angles of 110 degrees and 85 degrees are: the turn-on electric fields (at the current density of 10 microA cm(-2)) are about 3.03 and 1.70 V microm(-1), respectively, while field enhancement factors are 1972 and 3513, respectively. The lifetime measurement result indicates the turn-on field of the ZnO nanostructures reaches a stable value during 1000 cycle times. The enhanced field emission properties are believed to benefit from decreased tip angle of the nanorod emitter.

Unipolar Resistive Switching in ZrO2 Thin Films

Unipolar resistive switching behaviors including bistable memory switching and monostable threshold switching were found in ZrO2 thin films fabricated by a simple sol–gel method with the Ti/ZrO2/Pt structure. The multilevel resistive switching behaviors were also revealed by varying the compliance current from 9 to 38 mA. Physical mechanisms based on a conductive filament model were proposed to explain the resistive switching phenomena and the device breakdown. A figure of merit Z = ρa/ρf was defined as a criterion for evaluating OFF/ON resistance ratio, where ρf and ρa represent the resistivities of the conductive filament and the fracture region of the filament, respectively. The advantages such as unipolar resistive switching, multilevel resistive switching, good scalability, low operation voltage ( 103), nondestructive readout, long retention (>104 s), and simple fabrication method make the ZrO2-based resistive switching device a promising candidate for next-generation nonvolatile memory applications.

Electrical Properties and Reliability of ZnO-Based Nanorod Current Emitters

The fabrication, optical, and field emission properties of ZnO-based nanorod emitters were studied. Ga-doped ZnO nanorods combined with the formation of tip structure on top of a ZnO nanorod by oxygen plasma treatment are employed to improve the field emission properties of the nanorod emitters. By either of these two methods, the nanorod emitters exhibit significantly reduced turn-on field and enhanced field-emission factor. The morphology, crystal structure, and composition of all the nanorods used for making emitters are characterized by a scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectrometer. The nanorods exhibit the highly preferred c -axis orientation single crystal structure. The photoluminescence spectra indicate the nanorods have better crystalline structure after doping and oxygen plasma treatment. Combining gallium doping process (Ga/Zn molar ratio of 1% in solution) and oxygen plasma treatment (etching time of 60 s), the tip-structured GZO nanorod emitters with tip angle of 100° have a turn-on field of 1.99 V/μm under a current density of 1 μA/cm2, field enhancement factor of 2465, and stable operation over 2 × 104 s. Such improved field emission properties are attributed to decreased work function and sharp nanotips morphology. In addition, the GZO nanorod emitters with tip structure are successively and stably operated between 25°C and 100°C over 3000 s based on the high-temperature field emission measurement results. They have high potential for practical applications in flat panel display and light emitting device in the future.