應用於相變化記憶元件之摻雜鍺-銻-鍗薄膜及其性質研究

Abstract

本研究探討應用於相變化記憶體（Phase-change Random Acesss Memory，PRAM）之鍺-銻-鍗（GeSbTe，GST）材料摻雜鈰（Cerium，Ce）元素後之結構與性質變化。X光繞射分析（X-ray Diffraction，XRD）顯示摻雜能穩定非晶態GST以及抑制再結晶後六方晶（Hexagonal）GST相之形成。電子顯微鏡（Transmission Electron Microscopy，TEM）之觀察顯示，Ce摻雜會使結晶態GST之晶粒細化，元素分布（Element Mapping）則發現Ce在GST中均呈勻分布，故Ce原子係以固溶態摻雜於GST中，此亦符合等升溫實驗發現相變化再結晶溫度（Recrystallization Temperature，Tc）與活化能（Activation Energy，Ea）隨著Ce摻雜濃度增加而上升之結果；但Ce摻雜達10%貼靶濺鍍濃度時，退火之GST中可能出現介金屬相（Intermetallic Compound，IMC）而使Ea值下降。實驗最重要之發現為Ce之摻雜不會改變非晶態與多晶態GST之電阻比值（R-Ratio □ 105），尤其不會使非晶態GST之電阻率下降，有助於維持訊號之對比清晰度，此一發現對PRAM材料開發將有重要意義。恆溫實驗配合Johnson-Mehl-Avrami（JMA）理論探討GST薄膜之相變化機制變化，發現摻雜使相變化維度下降，推測其為異質成核（Heterogeneous Nucleation）效應所致，但Ce摻雜大幅升高成長活化能而使恆溫相變化活化能（□H）升高。資料保存時間（Retention Time）之分析顯示發現Ce摻雜濃度越高，資料保存效果愈佳。 PRAM元件之應用發現臨界轉換電壓（Threshold Voltage，Vth）雖隨Ce之摻雜濃度升高而上升，但Ce摻雜之GST薄膜確實可應用於PRAM元件之製作。

This study investigates the microstructure and property changes of cerium (Ce)-doped GeSbTe thin films applied to phase-change random access memory (PRAM). X-ray diffraction (XRD) showed that Ce doping may stabilize the amorphous GST and inhibit the emergence of hexagonal GST phase after annealing. Transmission electron microscopy (TEM) revealed Ce doping causes the grain refinement in GST. The element mapping depicted a uniform distribution of Ce in all types of GST films, indicating that Ce atoms reside in GST in solid-solution form. Kissinger’s analysis found that the recrystallization temperature (Tc) and the activation energy (Ea) of doped-GST increase with the increase of Ce content. However, intermetallic compound (IMC) likely formed in crystalline GST with Ce concentration exceeded 10% area coverage ratio and thus led to the decrease of Ea. A significant result observed in this part of study is that the Ce doping does not alter the resistivities of amorphous and crystalline GSTs and hence the resistivity ratio (R-ratio) remains the same at about 105. This greatly benefits the preservation of signal contrast as well as the high-density signal storage and will be a distinguishing finding in recent development of PRAM materials. Isothermal experiment in conjunction with Johnson-Mehl-Avrami (JMA) analysis revealed that Ce doping suppresses the dimensionality of phase-change process in GST. This is attributed to the heterogeneous nucleation effects occurring during the phase-change process. The retention time analysis found that the retention time increases with the increase of Ce doping amount in GST. In the study of PRAM device applications, it was found that the threshold voltage (Vth) of device containing doped-GST increases with the Ce content. It nevertheless illustrates that the Ce-doped GST films are indeed feasibile to PRAM device fabrication.