First-principles Study On Phase Change Memory Alloys: Electronic Excitation Mechanics Of Ge₂Sb₂Te₅and Origins Of Stability Of Amorphous Ge₁Cu₂Te₃

Phase-change memory (PCM) has been considered the most promising candidatefor the next generation nonvolatile memory technology.[1]This technology has beenwidely studied and came into commercial applications. Yet, there are still manyproblems in PCM filed. For example, the high temperature application is limited bythe insufficient amorphous stability, the device speed is limited by relative slowcrystallization speed, the recyclability is limited by phase separation, and so on.[2,3]This microscopic phase transition mechanism is the key of improvement of phasechange materials and corresponding devices. For example, the role of laser orelectrical pulse has always been known as thermal effect. But recently reports indicatea solid-to-solid phase transition suggesting an athermal effect induced by electronicexcitation.[4-7]This type of phase change is meaningful to enhance phase change speedand recyclability. So we are intended to investigate the effect of electronic excitationon phase change materials and the mechanism of amorphous phase stability of PCMmaterials. We hope our theoretical investigation can be a reference to enhance thePCM performance such as phase change speed and data retention.We study the effect of electronic excitation on Ge2Sb2Te5by first-principlescalculations. The electronic excitation can induce stresses in the material. The effectof hole excitation will make Ge2Sb2Te5compress while electron excitation will makeit expand. The phase transition induced by hole effect is similar to that by hydrostaticpressure (HP)[8-10]. But the excitation stress is more effective on compressingmaterials which may be used to acquire new phase, such as the structure obtained byde-excitaion from the body center cubic (bcc) phase under40GPa hole excitationstress. When the holes and electrons are coexist in system, the stress will bedrastically cancelled out. But there will still exists considerable atom forces in thesystem that do not be cancelled. What is more, the atom forces will be larger if thestructure is more asymmetry.In the second part of the thesis, the physical origins of thermal stability ofamorphous Ge1Cu2Te3are investigated by first-principles calculations. We find the crystal Ge1Cu2Te3has a special bonding character in form of nonequivalent sp3hybridization. Part of the bondings are formed by Te lone-pair electrons and Cuunoccupied orbitals. In crystal, the Cu d electrons are not involved in bonding. Incontrast, the amorphous Ge1Cu2Te3contains cage-like dense clusters composed oftriangle structures induce by Cu element. The structure contrast could contribute tobarrier of crystallization thus enhance the stability of amorphous Ge1Cu2Te3. Moreinsightful physical origin is attributed to the bonding character reconfiguration of Cud electrons modulated by the Te lone-pair electrons. The present mechanism may be areference for other transition-metal alloyed Te-based phase change materials.