Study Of Sb-Te Based And Sb Based Phase-change Materials For Phase-change Random Access Memory

With the rapid development of portable electronic equipments, there is an increasing demand for nonvolatile memory technology. As current mainstream nonvolatile memory, Flash memory has been widely used in stand-alone and embedded chips with great commercial success. But Flash memory is not qualified as an ideal memory for future technology development owing to its relatively long programming time and limited endurance. The scaling of Flash memory is also a bottleneck because of some physical limitations. Therefore, novel nonvolatile memories are being actively studied, among which phase-change random access memory (PRAM) has been regarded as the promising candidate for the next-generation nonvolatile memory due to its excellent characteristics such as fast write/read speed, good endurance, low power consumption and good compatibility with complementary metal-oxide semiconductor (CMOS) technologies.The research and development of PRAM technology have obtained great progress during the last decade. However, several critical problems still remain to be improved and resolved, such as a relatively large RESET current, a relatively short data retention life in high-temperature environment and a lack of comprehensive understanding of device failure mechanisms. The former two problems are closely related to the phase-change material properties, thus the optimization of phase-change material is very important. Most of studies are underway in two ways: One is to improve Ge₂Sb₂Te₅ material by doping other elements and another one is to develop novel phase-change materials. Here we try to look for solutions in terms of development and optimization of novel phase-change materials and have done the following works:Si-Sb-Te films with various Si concentrations (10, 20 and 30 at.% in the case of Sb/Te=2:3) and Sb/Te ratios (2:3, 1:1 and 2.8:1 in the case of 20 at.% Si ) were investigated. The amorphous/crystalline resistivity ratio is more than 105 and the film thickness reduction upon crystallization is less than 3%. With the increase of Si concentration or the decrease of Sb/Te ratio, the crystalline resistivity increases significantly, and the crystallization temperature and amorphous state stability also increase. Compared with conventional Ge₂Sb₂Te₅ film, Si-Sb-Te films exhibit lower melting temperature, higher crystalline resistivity and better amorphous state stability. The results of both experimental measurement and thermal simulation of device have shown that the RESET current can be reduced obviously when Si-Sb-Te film is used in the PRAM device.Nitrogen-doped Ge₁₅Sb₈₅ films were investigated for better PRAM data retention at high temperature. The doped N atoms tend to bond with Ge to mainly form Ge3N4, which precipitates into the grain boundaries, suppressing the grain growth and reducing the grain size. The amorphous and crystalline resistivities of Ge₁₅Sb₈₅ film increase significantly after N doping. The maximum temperature at which the amorphous state of nitrogen-doped Ge₁₅Sb₈₅ films can be retained for 10 years is higher than 147 oC. This indicates that nitrogen-doped Ge₁₅Sb₈₅ films have much better amorphous state stability than conventional Ge₂Sb₂Te₅ film, which expects better data retention of device. I-V and R-V curves of the device have demonstrated that the device with N-doped Ge₁₅Sb₈₅ films has typical storage features and has great potential in high speed memory application due to fast SET switching.Novel Sb-rich Si-Sb films were developed. Si-Sb films crystallize into hexagonal phase similar to Sb. With the increase of Si concentration, the crystallization temperature and amorphous and crystalline resistivities increase, and the grain size decreases. The amorphous state of Si₁₅Sb₈₅ film can be retained for 10 years at a maximum temperature of 140 oC and hence has much better thermal stability than conventional Ge₂Sb₂Te₅ film.Nitrogen or oxygen was doped into Si₁₅Sb₈₅ film and the doping effects were investigated. For N-doped Si₁₅Sb₈₅ films, the doped N atoms tend to bond with Si to form Si3N4, which suppresses the grain growth. With the increase of N concentration, the amorphous and crystalline resistivities increase. The crystallization temperature and amorphous state stability also increase after N doping. For O-doped Si₁₅Sb₈₅ films, the doped O atoms prefer to bond with Si to form SiO2. When the O concentration is relatively low (14.5 at.% in this study), the amorphous and crystalline resistivities increase, while the crystallization temperature and amorphous state stability decrease slightly. But when the O concentration increases further (32.8 at.% or higher), the crystallization temperature increases. I-V and R-V curves of the device have demonstrated that the device with N- or O-doped Si₁₅Sb₈₅ films has typical storage features and has great potential in high speed memory application due to fast SET switching.