Direct observation of phase transition of GeSbTe thin films by Atomic Force Microscope

GeSbTe (GST) thin films were deposited on quartz substrates using electron beam evaporation system and then annealed in nitrogen atmosphere at different temperatures, ranging from 20 °C to 300 °C. X-ray diffraction (XRD) and Atomic Force microscope (AFM) measurements were used to characterize the as-deposited and post-annealed thin films. Annealing treatment was found to induce changes on microstructure, surface roughness and grain size, indicating that with the increase of annealing temperature, the amorphous GST films first changed to face-centered-cubic (fcc) phase and then the stable hexagonal (hex) phase. Meanwhile, conductive-AFM (C-AFM) was used to produce crystallized GST dots on thin films. I–V spectroscopy results show that GST films can switch from amorphous state to crystalline state at threshold voltage. After switching, I–V curve exhibits ohmic characteristic, which is usually observed in crystallized GST films. By applying repeated I–V spectroscopies on the thin films, crystallized nuclei were observed. As the times of I–V spectroscopies increases, the area of written dots increases, and the center of the mark begin to ablate. The AFM images show that the shape of marks is an ablated center with a raised ring surrounding it.