Study On The Properties Of Sb-rich ZnSb-based Phase Change Films

In recent years, phase change memory(PRAM) is considered to be one of the candidates for nonvolatile memory, which are widely investigated in industry and academia due to the non-volatile, fast operation speed, durable reversibility, high integration, and the advantages of compatible with the CMOS technology. Nowadays, the traditional Ge2Sb2Te5(GST) as the storage medium of the PRAM has been used in some digital products. However, there are some drawbacks in GST film, such as poor amorphous thermal stability, low phase change speed, low crystalline resistance, and the high melting temperature. As we know, amorphous thermal stability and phase change speed is a pair of contradiction. How to balance the two parameters is always a curial problem needed to be solved. Therefore, this article embarks from the material and through the composition optimization, developing some phase change materials that possess both rapid phase transition and good data retention ability. The main content of this thesis can be listed as follows:(1) With the high crystallization temperature(Tc) and large crystalline activation energy(Ea), Zn-Sb films exhibit good data retention due to the formation of Zn-Sb bonds, which is much better than that of conventional GST. Films with low Zn content crystallized in a single-step process with Sb, while the film whose Zn/Sb ratio is about 1:1 exhibited two-step crystallization process from amorphous to Zn Sb metastable-phase and then to Zn Sb stable-phase. Importantly, it has higher Tc(~257 oC), larger Ea(~5.63 e V), better 10-yr data-retention ability(~201 oC) and lower melting temperature(~500 oC). These results all reveal Zn Sb film can be one of the good candidates for phase change material.(2) In order to combine good thermal stability of Zn Sb with fast crystallization speed of Sb2 Te, the Sb-rich Zn-Sb-Te films have been prepared. These films generally exhibit two different crystallization characteristics as follows: amorphousâ†'Sb2Te crystalline phase if Zn content in the film is less than ~10 at. % and amorphousâ†'Sb crystalline phase if Zn content is more than ~10 at. %. With the increase of Zn Sb doping content, the thermal stability was enhanced obviously. Furthermore, the quantitative crystallization speed of some these films was measured by the static tester, it is evident that an appropriate dopant can accelerate the phase transition speed. The above results reveal that Sb-rich Zn-Sb-Te material inherits both merits of Zn Sb and Sb2 Te, e.g., good thermal stability and fast crystallization speed, respectively. Especially, Zn28.62Sb53.69Te17.69 film was found to possess a higher crystallization temperature(~255 °C), better 10-yr data retention ability(~165.9 °C), shorter crystallization time(~58 ns at 70 m W) with a stable rhombohedral Sb phase, and thus promising for the applications in phase change memory.The other Sb-rich Zn-Sb-Te films with the different Sb/Te ratio, such as Zn Sb-Sb2Te3 and Zn Sb-Sb3 Te, were also investigated. Their optical, electrical, and thermal properties, as well as phase change ability, have been researched.(3) We present a new reversible phase-change medium Sb-rich Zn-Sb-Se film, which possess a large difference in both optical and electrical constant. The doped-Zn Sb, sub-formed Zn-Se, and exhausted Sb-Se3/2 co-influence the physical properties. Typically, there is ~105 resistance ratio and ~14% relative reflectivity change in Zn19Sb45.7Se35.3 film when switched by electricity or laser pulses between amorphous and crystalline states. The higher Tc(~250 °C), larger Ea(~8.57 e V), better 10-yr data retention(~200.2 °C), higher crystallization resistance(~3×103Ω/â–¡ at 300 °C-annealled) and relative lower melting temperature(~550.2 °C) are exhibited in Zn19Sb45.7Se35.3 film. Importantly, a short crystalline time(~80 ns at 70 m W) of the ideal Zn19Sb45.7Se35.3 film can be obtained without sacrificing room-temperature stability.(4) The effect of some metal elements, such as Sn, Bi, Al, Ag, and In, doping into Zn Sb-based thin film has been investigated. Optimally, Zn Sb-3Sn, Zn Sb-5Sn, Zn Sb-Al and Zn Sb-In films, which can be candidates of novel phase change material, have been studied systemly for their structures and properties. Some results show that the speed of phase transition is increased but 10-yr data retention is slightly deteriorated. The introductions of these metal elements broke the balance of Zn-Sb bonds, change original Zn Sb crystallization behavior, and refrain Zn Sb phase precipitation. In addition, the Zn Sb-Al and Zn Sb-5Sn films show multilevel storage ability. To sum up, Zn Sb-3Sn, Zn Sb-5Sn, Zn Sb-Al and Zn Sb-In films can be as ideal phase change materials owing to their outstanding performances. Especially, the Zn Sb-Al(Zn35.0Sb30.3Al34.7) film posseses smaller surface roughness(RMS=1.654 nm) and slight film’s thickness change(7.5%) which is similar to that of GST.