| Driven by advances in the semiconductor technology which is approaching the traditional roadmap limit, searching for novel materials and devices has been an impending challenge. Resistive switching (RS) devices have drawn much research interests in the recent applied physics field because of their engaging properties with simple structures. And the potential application in the high-performance mass non-volatile memories is pushing the RS devices to be a new favorite of major international semiconductor storage industries like Intel, Micron, Samsung, Toshiba, and so on. Compared with other emerging storage devices, RS memories promise significant advantages in programing/erasing speed, storage density, and power dissipation. However, comprehensive understanding on the RS physics is still in need for improving the property stability of devices.Here we focus on the size and humidity dependent unipolar resistive switching (URS) of Pt/TiO2/Pt cells. The whole work can be divided into two parts, the simulations and the experiments. In the simulation section, the static and dynamic models were built, respectively, to reproduce the Joule heating process in a low resistance state (LRS) device under a constant voltage and the RESET (switching from the low resistance state to the high resistance state) process by a voltage pulse, where the effect of the device sizes was discussed emphatically. In the experiment section, RS behaviors of Pt/TiO2-x/Pt cells operated in a humid environment were studied, including the morphology evolution of the device and the dependence of RS threshold voltages on the humidity. The main achievements are summarized as follows:1. The enhanced Joule heating effect in nanoscale URS cells is reported. According to the calculated temperature and heat-flow distributions in devices with varying sizes, a higher peak temperature is reached in a smaller device driven by the same heating power. This phenomenon can be attributed to the increased equivalent heat resistance of the system induced by the size dependent electric and thermal conductivities and geometry of materials. Meanwhile, the dominant heat transmitting path is changed with the device scaled down. Thus, a lower operation voltage and heating power to trigger the thermally activated dissolution of the filament in the RESET process of a smaller device can be promised.2. The impact of device sizes on the RESET behaviors is theoretically studied by a dynamic model based on the ions migration. The simulation results show that a typical RESET process can be divided into two stages. The first stage is dominated by the thermal effect, during which the current decreases quickly with the temperature climbing to the peak value, and a high resistance region is formed in the filament. Then, the second stage is dominated by the coupled electric-thermal effect, during which the current changes a little with the temperature decaying to a steady value, and the filament is quasi-disconnected. The time of the first stage, which is inversely proportional to the speed of the current declining, can be shortened by enlarging the pulse voltage or reducing the device sizes. Moreover, the operation voltage can be lower in a smaller device to get a certain RESET speed, which also consumes less energy as predicted.3. Water electrolysis is confirmed for the first time in Pt/TiO2-x/Pt URS cells with the TiO2-x film deposited by the reactive DC sputtering of a Ti target. Some bubbles arise and grow to be larger than the range of the top electrode when the FORMING (switching from the initial state to the low resistance state) is implemented in a humid (RH≥ 10%) environment, while no bubbles appear if operated in an anhydrous environment or by a negative voltage. Different from reported O2 bubbles as the products of the oxidation of O2- from TiO2 at the anode, these bubbles are found to be under the cathode and are deduced to be filled with H2 by the reduction of protons from the adsorbed water. However, water electrolysis is suppressed and bubbles are absent if a negative voltage is applied as a result of the limited migration of hydroxyl species in the oxide film.4. The dependence of RS threshold voltages on humidity is reported in Pt/TiO2-x/Pt cells with the TiO2-x film deposited by the RF sputtering of a TiO2 target. The device shows no visible bubbles even if operated in a humid environment. The slight change of the device capacitance with humidity indicates the insufficient adsorbing capacity of water in the RF sputtered TiO2-x film, which greatly weakens water electrolysis in the device. However, the adsorbed water in the top-electrode/oxide interface can modulate the interface barrier, by which the forward current increases with higher humidity but the reverse current remains almost unchanged. The increase of the junction current leads to that of the voltage on the RS film, and then the enhancement of the electric field. The ions migration in the film is accelerated as well as the FORMING process. So the FORMING threshold voltage with the positive polarity gets lower when humidity increases, while that with the negative polarity seems to be unaffected. Moreover, threshold voltages of the subsequent operations after the FORMING are independent on humidity because of the elimination of the interface barriers.
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