Crystallization and memory programming characteristics of Ge-doped SbTe materials of varying Sb : Te ratio

A phase change memory (PCM) utilizes resistivity changes accompanying fast transitions from an amorphous to a crystalline phase ( SET) and vice versa ( RESET). An investigation was made on the SET characteristics of PCM cells with Ge-doped SbTe (Ge-ST) materials of two different Sb : Te ratios (4.53 and 2.08). For the material of higher Sb : Te ( 4.53), a SET operation was completed within several tens of nanoseconds via nucleation-free crystallization whereas the material of lower Sb : Te ( 2.08) rendered a slower SET operation requiring several hundred nanoseconds for a nucleation-mediated crystallization. From measurements of nucleation and growth kinetics via laser-induced crystallization, the observed SET characteristics of the former case were found to derive from a growth time about 103 times shorter than the nucleation time and those of the latter from a much shorter nucleation time as well as a longer growth time than in the former case. The measured nucleation kinetics of the lower Sb : Te ( 2.08) material is unexpected from the existing data, which has led us to advance an interesting finding that there occurs a trend-reversing change in the nucleation kinetics of the Ge-ST materials around the eutectic composition ( Sb : Te similar to 2.6); nucleation is accelerated with the increase in the Sb : Te ratio above Sb : Te of 2.6, but with a decrease in the Sb : Te ratio below it.