Characteristics of amorphous Ag0.1(Ge2Sb2Te5)0.9 thin film and its ultrafast crystallization

An amorphous chalcogenide Ge2Sb2Te5 film is most commonly used for phase-change data storage, in which its small volume is switched between amorphous and crystalline states by local heating with short laser or current pulses. A speed of amorphous-to-crystalline phase transformation in Ge2Sb2Te5 and Ag-0.1(Ge2Sb2Te5)(0.9) films has been evaluated on a nanosecond time scale using a 658-nm laser beam. The focused laser beam with a diameter < 10 mu m was illuminated in the power (P) and pulse duration (t) ranges of 1-17 mW and 10-460 ns, respectively, with subsequent detection of the response signals reflected from the film surface. We also evaluated the material characteristics, such as optical absorption and energy gap, crystalline phases, and sheet resistance of as-deposited and annealed films. The experiments showed that the crystallization speed of the Ge2Sb2Te5 film is largely improved by adding Ag. For example, the threshold pulse durations (t(T)) for Ge2Sb2Te5 and Ag-0.1(Ge2Sb2Te5)(0.9) films are approximately 70 and 30 ns for P=7 mW. In addition, the sheet resistance in the amorphous state of the Ag-0.1(Ge2Sb2Te5)(0.9) film is approximately ten times that of the Ge2Sb2Te5 film. This improvement can lead to a low programming current in electronic operation. To understand the ultrafast crystallization in the amorphous Ag-0.1(Ge2Sb2Te5)(0.9) film, we suggest heat confinement by electron hopping.