The Study On High-performance Phase Change Alloy And Properties Of Novel Threshold Switching Alloy

With rapidly growing of Big Data,Cloud Computing,Artificial Intelligence and 5th generation Mobile Telecommunication Networks,the smart connected time has come.In the current Von Neumann structure,data storage uses three level distribution of Cache,RAM and External Memory,which was used in conventional computer system widely.This distribution is due to the incompatibility between high-speed reading/writing and large volume of storage,in which each level possesses higher speed and shorter delaying than the inferior.In order to break traditional three level distribution,IBM company put forward the concept of Storage Class Memory?SCM?,wishing to fill the performance gap between Cache and External Memory and build new storage system.Storing logic data based on reversible switching between amorphous state and crystalline state of chalcogen alloy,Phase Change Random Access Memory?PCRAM?was believed as the most potential one of next-generation storage technologies.Conventional phase-change alloy reaches nanosecond operation time and extremely high scalability but poor thermal stability and relatively large consumption.In the condition of further shrinking device size,traditional semiconductor selector unit is no longer adapted high-scalable and good performance requirement due to crosstalk,leakage and etc.For the above problems,the theme of this paper is focused on developing phase change alloy and device with high data retention,high speed and low consumption and the study on high-performance novel threshold switching alloy.The main achievements include:1.Ge Sb Se Te high data retention phase change alloy study.To improve the thermal stability of Ge-Sb-Te ternary system,we built Ge-Sb-Se-Te alloy by doping Se atom with 300? of crystallization temperature and 212? for data retention.Se atom made change of crystallization process,where we found the direct transition from amorphous structure to Hex structure and smaller grain size than Ge₂Sb₂Te₅ alloy.Small size is favor of the stability on cycle operation due to decreased volume change rate.2.Developing Hf-Sb₂Te₃ high-performance phase change alloy.Developing high-performance phase change alloy with fast speed and low consumption was taken as the goal.We started the research on doping element Hf into conventional phasechange materials.Doped Hf atom could improve amorphous stability of Sb₂Te₃ alloy with crystallization temperature for 266.4? and data retention for 160.0?.Via XRR test,the fitting value of volume change rate of Hf0.06?Sb₂Te₃?0.94 alloy was about 4.3% smaller than that of Sb₂Te₃?8.1%?.Reduced change rate is beneficial for uniform and stable unit structure increasing endurance of memory cell.Based on the good film properties of Hf-Sb₂Te₃,we fabricated memory cell with “T”-type structure to conduct electrical test.The results indicated that memory cells based on Hf-Sb₂Te₃ have fast operation speed?SET and RESET operating with 6 ns electrical pulse?and stable phase change performance?cycle times exceeding 1.8×10⁴?,which was believed as phasechange alloy candidate with high-speed,low consumption and robust stability.3.Novel high-performance threshold switch selector based on GeAsSeTe alloy study.In order to fill the requirement of large drive current,low leakage,high operation speed and good stability on PCRAM,we have developed high-performance threshold selector alloy.Deposited GeAsSeTe alloy illustrated as amorphous glass-network structure,including GeSe_4/2 tetrahedron,As Se_3/2 pyramid and a little Se-Se band connecting with other structures.The optical band gap(E_opt)of GeAsSeTe films was 1.21 eV tested by the UV/Vis/NIR Spectrophotometer.And we found the Eopt could be tuned by Ge atom content.150 nm GeAsSeTe films were annealed at 450? for 30 min without crystallization happened,implying that the GeAsSeTe film has great thermal stability and can tolerate the heating process in semiconductor processing technology without device failure.We fabricated memory cells with typical “T”-shaped structure on W bottom electrode of 190 nm diameter for electrical test.GeAsSeTe alloy has typical threshold switching characteristics,which presents “S”-shaped current-voltage characteristic curve.Driving current reached 10 m A and current density about 11.2 MA/cm²(10 m A/??×95 nm?2,supplying large driving current for memory cell.In addition,cycle operation time can exceed 10⁷ and memory cell keep relatively stable before device failure,indicating that GeAsSeTe alloy is promised to be highperformance selector for PCRAM.