  Oxygen is widely used to tune the performance of chalcogenide phase-change
materials in the usage of phase-Change random access memory (PCRAM) which is
considered as the most promising next-generation non-volatile memory. However,
the microscopic role of oxygen in the write-erase process, i.e., the reversible
phase transition between crystalline and amorphous state of phase-change
materials is not clear yet. Using oxygen doped GeTe as an example, this work
unravels the role of oxygen at the atomic scale by means of ab initio total
energy calculations and ab initio molecular dynamics simulations. Our main
finding is that after the amorphization and the subsequent re-crystallization
process simulated by ab initio molecular dynamics, oxygen will drag one Ge atom
out of its lattice site and both atoms stay in the interstitial region near the
Te vacancy that was originally occupied by the oxygen, forming a
"dumbbell-like" defect (O-VTe-Ge), which is in sharp contrast to the results of
ab initio total energy calculations at 0 K showing that the oxygen prefers to
substitute Te in crystalline GeTe. This specific defect configuration is found
to be responsible for the slower crystallization speed and hence the improved
data retention of oxygen doped GeTe as reported in recent experimental work.
Moreover, we find that the oxygen will increase the effective mass of the
carrier and thus increases the resistivity of GeTe. Our results unravel the
microscopic mechanism of the oxygen-doping optimization of phase-change
material GeTe, and the present reported mechanism can be applied to other
oxygen doped ternary chalcogenide phase-change materials.
