The crystallization characteristics of a ubiquitous T-shaped phase change memory (PCM) cell, under SET current pulse and very small disturb current pulse, have been investigated by finite element modelling. As analyzed in this paper, the crystallization region under SET current pulse presents first on the corner of the bottom electron contact (BEC) and then promptly forms a filament shunting down the amorphous phase to achieve the low-resistance state, whereas the tiny disturb current pulse accelerates crystallization at the axis of symmetry in the phase change material. According to the different crystallization paths, a new structure of phase change material layer is proposed to improve the data retention for PCM without impeding SET operation. This structure only requires one or two additional process steps to dope nitrogen element in the center region of phase change material layer to increase the crystallization temperature in this confined region. The electrical-thermal characteristics of PCM cells with incremental doped radius have been analyzed and the best performance is presented when the doped radius is equal to the radius of the BEC.