| Owing to its small cell size, fast read-write speed, low power consumption, simple device structure and fabrication technology, the resistance random access memory (ReRAM) has been internationally recognized as a candidate for the next generation of nonvolatile memory. Recently, the materials for ReRAM devices have been intensively studied by researchers around the world. Due to their simple crystal structures, precise chemical compositions and good compatibility with the semiconductor processes, the binary metallic oxide and nitride have become the most promising concepts of ReRAM materials. Owing to its precise thickness control, large area uniformity and high conformality, atomic layer deposition (ALD), an advanced preparation technique for ultrathin films, has been established as an excellent method for the fabrication of ReRAM devices, since the active layers of ReRAM are extremely thin in nanometers range.Recent researches have shown that ZnO and AlN thin films have good resistive switching characteristics and are very promising for the application of ReRAM. Most of these films were deposited by physical vapor-phase deposition (PVD) method. There has been limited activity in using ALD-ZnO or AlN films to make the resistive switching devices. Generally, the films deposited by traditional Thermal ALD are defective, which are not suitable for ReRAM device fabrication. Moreover, the metallic nitrides are hard to be synthesized by Thermal ALD at low temperature. The plasma-enhanced atomic layer deposition (PEALD) is a kind of energy-enhanced ALD technology. Due to the high reactivity of the radicals in plasma, it is shown that the use of a plasma leads to a wider choice in ReRAM materials and improved properties.In this work, we investigated the RS characteristics of ZnO and AlN films deposited by PEALD, and discussed the possible physical mechanism of the RS properties. Then, we implemented in-situ oxygen plasma treatment (OPT) processes and single-layer doping processes on ALD-grown thin films to improve their RS performance. The main results are as follows:(1) The growth rates, crystal structures, morphologies, optical and electrical properties of ZnO films deposited by Thermal ALD and PEALD were systematically investigated. For Thermal ALD-ZnO films, the reactants were H2O and the resistivity was typically very low,<10Ω·cm. For PEALD-ZnO films, the reactants were oxygen plasma and the resistivity was found much higher,>104Ω·m.(2) The RS properties were observed in the defect-free ZnO films deposited by PEALD. Generally, the films deposited by Thermal ALD are defective without RS characteristics. These defects were oxygen vacancies and hydrogen impurities. Due to the higher reactivity of the oxygen plasma than that of H2O, there was less defects in PEALD-ZnO films. Thus, the resistance ratio for the high and low resistance states (HRS/LRS) is more than 103 and the resistances at both the states remain unchanged for the test up to 104s. The dominant conduction mechanisms are space charge limited current and Ohmic behavior at different sweeping voltage. The resistive switching behavior was induced upon the formation/disruption of oxygen vacancies conducting filaments.(3) In-situ and ex-situ OPT were applied to treat ZnO thin films deposited by Thermal ALD, and the RS behaviors of the films were investigated. The Thermal ALD-ZnO films were defect-rich and conductive with no RS behavior. After the OPT process, the resistivity of the films increased drastically, which was believed to be mainly due to the removal of hydrogen impurities, and the films showed bipolar RS characteristics. The mean value of the resistance ratio for the in-situ OPT ZnO films is above three times higher than that of the ex-situ OPT films. According to Part (2) and (3), it was demonstrated that the oxygen plasma can improve the RS characteristics of ZnO ReRAM devices as reactants or after treating agents.(4) The RS properties were observed in the Cu/PEALD-AIN/Pt devices and the physical mechanism of the devices was investigated. The AIN-based ReRAM devices have been fabricated in a crossbar array by patterning, wet etching and using a metal shadow mask. The AlN films are hard to be synthesized by Thermal ALD at low temperature, whereas they can be synthesized by PEALD at 332℃. Other than the ZnO-based devices, the RS properties are induced upon the formation/disruption of Cu conducting filaments, as confirmed by the temperature dependent resistances relationships at different resistance states. The crossbar array is a common form of integration of memory. The RS properties were observed in AIN-based crossbar array, which provides foundation and direction for the application of the devices.(5) Single-layer doping processes were used to improve the device performance of AlN films deposited by PEALD. The physical mechanism of the RS properties for the devices was investigated. AlN is an important wide band gap semiconductor with high resistivity and is ideally suitable for applications in microelectronics. Thus, a destructive electroforming process at higher voltage is needed to initiate the switching operation for the undoped AIN-based devices, which may lead to the permanent breakdown of the devices. The single-layer TiOxNy doping using PEALD can modify the chemical environment of AIN film and improve the electrical characteristics of devices. It is believed that the TiOxNy was likely to accelerate the migration of Cu ions, reduce the forming voltage, facilitate the formation/rupture of conducting filaments and induce stable resistance switching. |
