Bias polarity dependence of a phase change memory with a Ge-doped SbTe: A method for multilevel programing

We investigated the bias polarity dependence of the characteristics of a phase change memory device and found that the device showed higher resistance both at programed and erased states, extended erasing time, and higher threshold voltage (Vth) under negative bias than those under the conventional positive bias. Taking advantage of this dependence, we were able to obtain four highly distinguishable resistance states in such a reproducible manner that may be utilized for a reliable multilevel storage. We explain this polarity dependence in terms of the difference in the density of trap states at the interfaces between the phase change material and electrodes.