Structure and Electrical Properties of Ge₂Sb₂Te₅ Thin Film Used for Ovonic Unified Memory

Annealing temperature effects on the structure and electrical resistance of Ge₂Sb₂Te₅ thin film were studied. The crystallization and melting temperature of the thin film are about 175°C and 610°C respectively. The structure of the as-deposited and the annealed film at different annealing temperatures are identified by the x-ray diffraction method. The thin film changes from an amorphous state to the fcc structure at about 200°C, and then changes into the hexagonal close-packed (hcp) structure at about 400°C. Electrical resistance of as-deposited thin film reaches as high as 1.7 x 10⁸ Ω/sq. Then it decreases to 1.6 x 10⁴ Ω/sq and 165 Ω/sq after the annealing process at 250°C and 400°C respectively. The current-voltage curve of the Ge₂Sb₂Te₅ thin film is also investigated. Due to the large resistance difference between the amorphous, the fcc crystalline and the hcp crystalline states, Ge₂Sb₂Te₅ film may be used to realize multilevel storage for Ovonic unified memory.