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Recently, resistive random access memory (RRAM) has been an outstanding candidate among various emerging nonvolatile memories for high-density storage and in-memory computing applications. However, traditional RRAM, which accommodates two states depending on applied voltage, cannot meet the high density requirement in the era of big data. Many research groups have demonstrated that RRAM possesses the potential for multi-level cells, which would overcome demands related to mass storage. Among numerous semiconductor materials, gallium oxide (a fourth-generation semiconductor material) is applied in the fields of optoelectronics, high-power resistive switching devices, and so on, due to its excellent transparent material properties and wide bandgap. In this study, we successfully demonstrate that Al/graphene oxide (GO)/Ga2O3/ITO RRAM has the potential to achieve two-bit storage. Compared to its single-layer counterpart, the bilayer structure has excellent electrical properties and stable reliability. The endurance characteristics could be enhanced above 100 switching cycles with an ON/OFF ratio of over 103. Moreover, the filament models are also described in this thesis to clarify the transport mechanisms.

Barium titanate nickelate (BTN) nanorod (NR)-based resistive random access memory (RRAM) has been demonstrated by using the hydrothermal method. The prepared BTN NR materials have the advantages of ease of fabrication, low temperature application, ability to grow various materials, and relative cost effectiveness. In addition, the BTN NR-based RRAM displayed a highly repeatable and forming-free bipolar resistive switching behavior with an ON/OFF ratio of over 105. The resistive switching behavior may be related to the oxygen vacancies on the surface of the BTN NRs, giving rise to the formation of straight and extensible conducting filaments along each vertically aligned BTN NR. Compared to its counterpart of BTN thin film, superior reproducibility was also observed, demonstrating that the nanostructured material provides an effective way to improve the memory properties.

To effectively improve the uniformity of switching behavior in resistive switching devices, this study developed magnesium zirconia nickel (MZN) nanorods grown on ITO electrodes through hydrothermal method. The field emission scanning electron microscope image shows the NR formation. Al/MZN NR/ITO structure exhibits forming-free and bipolar resistive switching behaviors. MZN NRs have relatively higher ON/OFF ratio and better uniformity compared with MZN thin film. The superior properties of MZN NRs can be attributed to its distinct geometry, which leads to the formation of straight and extensible conducting filaments along the direction of MZN NR. The results suggest the possibility of developing sol&ndash, gel NR-based resistive memory devices.

A reduced high-resistance state (HRS) current assists in obtaining high ON/OFF ratio and is beneficial to operation flexibility. This study proposes that less difference in the atomic radius of alkaline earth oxide-based memory devices is beneficial to reduce the HRS current. Forming-free resistive-switching behavior in the alkaline earth oxide-based memory device using magnesium titanate (MTO), strontium titanate (STO), and barium titanate (BTO) thin films is fabricated by sol-gel method. The dependence of HRS current on the difference in atomic radius was predicted by the Hume–Rothery rule and confirmed experimentally. The hydrolyzed particles, surface roughness, and density of oxygen vacancy were decreased when the difference in atomic radius between the Ti element and alkaline earth metal was less. Compared with the BTO thin film, the MTO thin film has fewer particles, smoother surface, and less density of oxygen vacancy, resulting in lower HRS current. Thus, suitable element selection for the alkaline earth oxide-based memory devices can reduce the HRS current.

High- $\kappa $ barium zirconate nickelate (BZN) gate dielectric layer was prepared through the low-temperature sol–gel method and was used in pentacene-based organic thin-film transistor (OTFT) applications. In comparison with its barium zirconate oxide (BZO) counterpart, experimental results show that the threshold voltage of the BZN-based OTFTs decreases from −2 to −1.1 V, the gate leakage decreases from 10−8 to 10−12 A, the carrier mobility increases from 2.1 to 5.2 cm2/Vs, and the subthreshold swing decreases from 2.4 to 0.4 V/decade. The effect of Ni on device characteristics is discussed in this paper. Ni(II) acetylacetone possesses two pairs of symmetrical bidentate ligands, which can chelate with Zr ion to smoothen surface roughness. The dielectric constant can be increased. The enhancement of mobility is partly attributed to the smoother surface to reduce the scattering effects of the carrier. In addition, the improved film quality may also increase the barrier height and reduce the leakage current, which enhances the ON/ OFF ratio through the chelate effect due to the addition of Ni in BZO. Furthermore, excellent operation stability can be achieved.

The use of a threshold-switching Ni/NbO2/Ni device with a memory-switching Pt/magnesium zirconate titanate/Al device on a flexible substrate was proposed to suppress undesired sneak currents. The proposed flexible one selector and one resistor (1S1R) memory device exhibits a low operation voltage, good ON/OFF ratio of $10^{5}$ , uniform current distribution, excellent flexibility, and stable ${I}$ – ${V}$ curve at 85 °C. The good selection and memory properties of the flexible 1S1R memory device are highly promising for high-density and low-power flexible electronic applications.

A bipolar-type one diode–one resistor (1D1R) memory device is proposed and demonstrated by integrating a Ni/TiO 2 /Ti diode and an Al/Strontium Titanate Nickelate (STN)/Pt bipolar resistive random access memory cell to suppress undesired sneak current in a cross-point array. Uniform self-compliance resistive-switching characteristics can be achieved by reverse bias current of the Ni/TiO 2 /Ti diode. Experimental results show that the bipolar 1D1R memory device has reproducible, uniform, and self-rectifying resistive-switching behavior in low-resistance state. High current ON/OFF ratio (>10 5 ) and satisfactory retention (>>10 5 s) are achieved. Therefore, the proposed device exhibits high potential for high-density integrated nonvolatile memory applications.

This paper investigated the performance of flexible resistive random access memory devices based on simple spin-coated sol–gel-derived strontium titanate nickelate (STN) thin films on polyethylene terephthalate substrate. A high on/off ratio of $10^{5}$ and a uniform current distribution were demonstrated. The strong bonding between bidentate ligands of nickel (II) acetylacetone and titanium metal ion enabled the chelation effect, which contributed to the stability of the STN thin film, especially for moisture resistivity. Fourier transform infrared spectroscopy analysis was utilized to examine the effects on the resistive switching behaviors after 90 days under an atmospheric environment according to the chelation effect of the STN thin films. The devices were fabricated on a flexible plastic substrate, and they exhibited excellent durability upon repeated bending tests. They demonstrated good potential application for flexible and low-cost memory devices.

High-resistance state (HRS) current has significant effect on the reliability and power consumption of resistive switching memories. Low HRS current is helpful for obtaining ultra-low power and for high ON/OFF ratio nonvolatile memory application. The reduced HRS current of a sol-gel magnesium titanate nickelate-based resistive random access memory by using nickel (II) acetylacetone as substitute for acetylacetone in magnesium titanate (MTO) was presented. Forming-free, high ON/OFF ratio of over 106, excellent current distribution and good retention at 85 °C were achieved. Moreover, the effect of nickel (Ni) on the surface roughness, operation voltage, switching cycles, HRS current, ON/OFF ratio, current distribution, and switching behavior was explored. These results indicate that the incorporation of Ni in sol-gel MTO is an effective way to achieve high-performance memory devices.

The modification of multilayered barium titanate nickelate (BTN) thin films on ITO/glass substrate is adopted to enhance the device performance. The XPS analysis reveals the concentration of oxygen vacancies can be reduced by the increasing number of the BTN layer. In addition, the Ni acts as clusters and plays a critical role in the formation/rupture of cone-shaped conductive paths during resistive switching, which promotes the local alignment of oxygen vacancies among metallic-Ni clusters. Thus, the sol-gel BTN memory devices with high ON/OFF ratio and excellent current distribution can be achieved.

Bipolar resistive switching random access memory (RRAM) devices on a plastic substrate are investigated. Strontium titanate nickelate (STN) thin film prepared by sol-gel method served as insulator on an Al/STN/ITO/PET structure. The STN-based flexible RRAM shows a high ON/OFF resistance ratio (≥ 105) and a retention ability of over 105 s. The characteristics of Ni in the STO thin films demonstrate that spin casting without doping other elements or any complex processes can be used to fabricate thin films with higher density of oxygen vacancies, less particles, and smoother surface. In addition, the fabricated devices on a flexible plastic substrate exhibit excellent durability upon repeated bending tests, demonstrating the potential for flexible and low-cost memory applications.

A one-transistor and one-resistor (1T1R) architecture with a resistive random access memory (RRAM) cell connected to an organic thin-film transistor (OTFT) device is successfully demonstrated to avoid the cross-talk issues of only one RRAM cell. The OTFT device, which uses barium zirconate nickelate (BZN) as a dielectric layer, exhibits favorable electrical properties, such as a high field-effect mobility of 2.5 cm2/Vs, low threshold voltage of −2.8 V, and low leakage current of 10−12 A, for a driver in the 1T1R operation scheme. The 1T1R architecture with a TiO2-based RRAM cell connected with a BZN OTFT device indicates a low operation current (10 μA) and reliable data retention (over ten years). This favorable performance of the 1T1R device can be attributed to the additional barrier heights introduced by using Ni (II) acetylacetone as a substitute for acetylacetone, and the relatively low leakage current of a BZN dielectric layer. The proposed 1T1R device with low leakage current OTFT and excellent uniform resistance distribution of RRAM exhibits a good potential for use in practical low-power electronic applications.

Strontium titanate nickelate (STN) thin films on indium tin oxide (ITO)/glass substrate were synthesized using the sol-gel method for resistive random access memory (RRAM) applications. Aluminum (Al), titanium (Ti), tungsten (W), gold (Au) and platinum (Pt) were used as top electrodes in the STN-based RRAM to probe the switching behavior. The bipolar resistive switching behavior of the set and reset voltages is in opposite bias in the Al/STN/ITO and Pt/STN/ITO RRAMs, which can be partly ascribed to the different work functions of top electrodes in the ITO. Analyses of the fitting results and temperature-dependent performances showed that the Al/STN/ITO switching was mainly attributed to the absorption/release of oxygen-based functional groups, whereas the Pt/STN/ITO switching can be associated with the diffusion of metal electrode ions. The Al/STN/ITO RRAM demonstrated a high resistance ratio of &gt, 106 between the high-resistance state (HRS) and the low-resistance state (LRS), as well as a retention ability of &gt, 105 s. Furthermore, the Pt/STN/ITO RRAM displayed a HRS/LRS resistance ratio of &gt, 103 and a retention ability of &gt, 105 s.

A normal amorphous silicon-based separate absorption and multiplication avalanche photodiode (SAMAPD) with very high optical gain of the avalanche photodiode has been developed successfully by plasma-enhanced chemical vapor deposition (PECVD). Based on experimental results, using undoped /spl alpha/-Si:H as avalanche layer material and /spl alpha/-Si/sub 1-x/Ge/sub x/:H as absorption layer material, the hole-injection (HI) type SAMAPD yields a very high optical gain of 686 at a reverse bias of 16 V under an incident light power of P/sub in/=1 /spl mu/W and has a rise time of 145 /spl mu/s at a load resistance R=10 k/spl Omega/. Thus the amorphous silicon-based SAMAPD is a good candidate for the long-range optical communication applications. >