Structure and Electrical Properties of Ge ₂ Sb ₂ Te ₅ 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 degrees C and 610 degrees 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 degrees C, and then changes into the hexagonal close-packed (hcp) structure at about 400 degrees C. Electrical resistance of as-deposited thin film reaches as high as 1.7×108 Ω/sq. Then it decreases to 1.6×104 Ω/sq and 165 Ω/sq after the annealing process at 250 degrees C and 400 degrees 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.