Phase transformation behavior of N-doped Ge2 Sb2+x Te5 thin films (x=0, 0.2) for phase change memory

Ge2 Sb2 Te5 and nitrogen-doped Ge2 Sb2.2 Te5 films were deposited by dc magnetron sputtering on Si O2 Si (100) substrates and the effects of antimony (Sb) and nitrogen (N) doping on microstructure and sheet resistance were investigated. After annealing at various temperatures between 100 and 400°C, phase transformations in Ge2 Sb2 Te5 and nitrogen-doped Ge2 Sb2.2 Te5 films were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The sheet resistance of those samples was measured by four-point probe. XRD and plan-view TEM analysis showed that the addition of Sb and N elements to pseudo-binary Ge2 Sb2 Te5 caused crystallization and phase transformation from face-centered cubic (fcc) structure to hexagonal close-packed (hcp) structure to occur at higher temperatures with grain refinement. Also, the Sb and N doping produces increased sheet resistance in Ge2 Sb2.2 Te5 films with improved phase stability of amorphous and fcc structures up to higher temperatures. N-doped Ge2 Sb2.2 Te5 with high sheet resistance is favored for phase-change random access memory application because of reduced writing current with increased crystallization speed and thermal stability.