ZnSb掺杂的Ge2Sb2Te5薄膜的相变性能研究

本文采用双靶(ZnSb靶和Ge₂Sb₂Te₅靶)共溅射制备了系列ZnSb掺杂的Ge₂Sb₂Te₅(GST)薄膜. 利用X射线衍射、透射电子显微镜、原位等温/变温电阻测量、X射线光电子能谱等测试研究了薄膜样品的非晶形态、电学及原子成键特性. 利用等温原位电阻测试表明ZnSb掺杂的Ge₂Sb₂Te₅薄膜具有更高的结晶温度. 采用Arrhenius 公式计算发现ZnSb掺杂的Ge₂Sb₂Te₅薄膜的十年数据保持温度均高于传统的Ge₂Sb₂Te₅薄膜的88.9℃. 薄膜在200, 250, 300和350℃ 下退火后的X射线衍射图谱表明ZnSb的掺杂抑制了Ge₂Sb₂Te₅薄膜从fcc态到hex态的转变. 通过对薄膜的光电子能谱和透射电镜分析可知Zn, Sb, Te原子之间键进行重组, 形成Zn–Sb 和Zn–Te 键, 且构成非晶物质存在于晶体周围. 采用相变静态检测仪测试样品的相变行为发现ZnSb掺杂的Ge₂Sb₂Te₅薄膜具有更快的结晶速度. 特别是(ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7薄膜, 其结晶温度达到250℃, 十年数据保持温度达到130.1℃, 并且在70 mW激光脉冲功率下晶化时间仅～64 ns, 远快于传统Ge₂Sb₂Te₅薄膜的晶化时间～280 ns. 以上结果表明(ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7薄膜是一种热稳定性好且结晶速度快的相变存储材料.

Phase change properties of ZnSb-doped Ge2Sb2Te5 films

ZnSb-doped Ge₂Sb₂Te₅ films have been deposited by magnetron co-sputtering using separated ZnSb and Ge₂Sb₂Te₅ alloy targets. The concentrations of ZnSb dopant in the ZnSb-added Ge₂Sb₂Te₅ films, measured by using energy dispersive spectroscopy (EDS), are identified to be 5.4, 9.9, 18.7 and 24.3 at. %, respectively. X-ray diffraction (XRD), in situ sheet resistance measurements, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), are used to analyze the relationships among the composition, structures and properties of the films. The sheet resistance as a function of the temperature (R-T) is in situ measured using the four-probe method in a home-made vacuum chamber. It is found that the crystallization temperature of ZnSb-doped Ge₂Sb₂Te₅ films are much higher than that of conventional Ge₂Sb₂Te₅ (～168℃). The higher crystallization temperature is helpful to improve the amorphous thermal stability. Data retention can be obtained by the extrapolated fitting curve based on the Arrhenius equation. It is shown that the values of 10-yr data retention for ZnSb-doped Ge₂Sb₂Te₅ films are higher than that of conventional Ge₂Sb₂Te₅ film (～ 88.9℃). XRD patterns of the as-deposited films when annealed at 200℃, 250℃, 300℃, and 350℃ show that ZnSb-doping can suppress the phase transition from fcc phase to hex phase. XPS spectra are further used to investigate the binding state of (ZnSb)_18.7(Ge₂Sb₂Te₅)_81.3, suggesting that the Zn–Sb and Zn–Te bonds may exist in an amorphous state. In addition, we have measured the dark-field TEM images, selected area electron diffraction patterns, and high-resolution transmission electron microscopy images of the (ZnSb)_18.7(Ge₂Sb₂Te₅)_81.3 films. Apparently, the films show a uniform distribution of crystalline phase with the dark areas surrounded by bright ones (Zn–Te or Zn–Sb domain). A static tester using pulsed laser irradiation is employed to investigate the phase transition behavior in nanoseconds. Results show that the ZnSb-doped Ge₂Sb₂Te₅ films exhibit a faster crystallization speed. Among these samples, the (ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7 film exhibits a higher crystallization temperature of 250℃ and the 10 years data retention is 130.1℃. The duration of time for crystallization of (ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7 is revealed to be as short as ～64 ns at a given proper laser power 70 mW. A reversible repetitive optical switching behavior can be observed in (ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7, confirming that the ZnSb doping is responsible for a fast switching and the compound is stable with cycling. These excellent properties indicate that the (ZnSb)_24.3(Ge₂Sb₂Te₅)_75.7 film is a potential candidate as the high-performance phase change material.