Effect of tungsten on the phase-change properties of Ge₈Sb₂Te₁₁ Thin Films for the Phase-change device

In this study, the electrical, optical, and structural properties of tungsten (W)-doped $Ge_8Sb_2Te_{11}$ thin films were investigated. Previously, GeSbTe alloys were doped with various materials in an attempt to improve the thermal stability. $Ge_8Sb_2Te_{11}$ and W-doped $Ge_8Sb_2Te_{11}$ films with a thickness of 200 nm were fabricated by using an RF magnetron reactive co-sputtering system at room temperature on Si (p-type, 100) and glass substrate. The fabricated thin films were annealed in a furnace in the ${\sim}0-400^{\circ}C$ temperature range. The optical properties were analyzed using a UV-Vis-IR spectrophotometer, and by using Beer's Law equation, the optical-energy band gap ($E_{op}$), slope $B^{1/2}$, and slope 1/F were calculated. For the crystalline materials, an increase in the slope $B^{1/2}$ and 1/F was observed, exhibiting a good effect on the thermal stability in the amorphous state after the phase change. The structural properties were analyzed by X-ray diffraction, and the result showed that the W-doped $Ge_8Sb_2Te_{11}$ had a face-centered-cubic (fcc) crystalline structure increased crystallization temperature ($T_c$). An increase in the $T_c$ increased the thermal stability in the amorphous state. The electrical properties were analyzed using a 4-point probe, exhibiting an increase in the sheet resistance ($R_s$) in the amorphous and the crystalline states indicating a reduced programming current in the memory device.