Dielectric Constants and Endurance of Chalcogenide Phase-Change Non-Volatile Memory

Abstract

AbstractInitial attempts to create memory from chalcogenide glasses (g-Ch) had limited success particularly because the first generation of these materials (labeled as CG1) has inferior endurance (about 106 SET-RESET cycles). Recent progress in phase-change non-volatile memory (PC-RAM) related to superior properties of Ge2Sb2Te5 (GST) alloy [1,2]. The paper answers the vital for PC-RAM development question: “Why is endurance of GST memory cells (about 1011 cycles) much higher than of CG1 cells?” We show that superior endurance is related to features of –U centers [3] creation in GST during RESET of PC-RAM cells. The native –U centers exist in g-Ch due to the softness of atomic potentials [3]. They play a significant role in SET process of CG1 and GST [2,4]. The –U centers behavior in GST [5] is different compared with CG1 while other properties (threshold voltage, resistivity, etc.) are basically the same [1-4]. We found that dielectric permittivities e of CG1 and GST are also different.The e values in Ge-Sb-Te alloys films have been determined from impedance measurements in sandwich samples using method described in [6] and reported in the paper. Amorphous GST has relatively high and distinct static and optical dielectric permittivities eo=16.5 and e'=15.3 to compare with CG1 where e practically independent on frequency. Hence the second term in the effective polarization potential Ep = q^2[(1-1/e')/r + (1/e' - 1/eo)/L] is strong (here r and L are the average atomic radius and interatomic bond distance, q is the electron charge). It allows to screen the Coulomb repulsion at an –U center quite effectively in GST. Therefore polarization helps –U centers creation in amorphous GST and impedes these centers formation in crystalline hexagonal GST films where eo=38 and e'=61. In contrast to CG1 [3], the creation and destruction of –U centers during RESET and SET processes in GST [4] are not accompanied by strong plastic mechanical stresses in a memory cell. This feature (higher barrier between elastic and plastic deformations) predetermines possibility of numerous SET-RESET cycles in this alloy. Therefore, the deformation mechanism of –U centers formation in CG1 leads to inferior endurance while the polarization mechanism of their creation in GST ensures decent endurance. Second factor is hybritization of –U centers with extended states can also play role in good memory alloys. Obtained experimental e values in non-stochiometric films allow to conclude about expected endurance in the framework of the proposed model.