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.