Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity

S R Elliott

doi:10.1088/1361-6463/ab795e

Abstract

‘Phase-change’ memory materials, such as the canonical composition Ge2Sb2Te5, are being actively researched for non-volatile resistive random-access memory applications. In these devices, ultra-rapid reversible transformations between metastable highly electrically conducting (degenerate-semiconducting) crystalline and more electrically resistive (semiconducting) glassy phases are produced by the application of appropriate voltage pulses. Multilevel programming, wherein more than two metastable resistance states can be stored in the memory material as different proportions of partially glassy/crystalline regions, allows more than one bit to be stored per memory cell. However, this route to increasing data density, without recourse to device-size down-scaling, is threatened by the phenomenon of ‘resistance drift’, wherein the electrical resistance of the glassy phase slowly increases with time, following a weak power-law dependence, after being written with a voltage pulse. In this paper, we propose an intrinsic electronic mechanism for the resistance drift, particularly valid at ambient temperature and below, by identifying it with the phenomenon of persistent photoconductivity that is commonly observed in a wide range of disordered semiconductors. We develop a model for it in terms of the long-time, deep-trap release and subsequent recombination of charge carriers, akin to that which is believed to be responsible for the long-time photocurrent decay in amorphous semiconductors, such as hydrogenated amorphous silicon. In this case, the parameters controlling the resistance drift are the widths of the (localized) valence- and conduction-band tails in the vicinity of the bandgap. Hence, there is the potential for mitigating resistance drift in the amorphous state of phase-change memory materials by suitable material engineering (e.g. via compositional or fabricational control) to control the extent of band-tailing, thereby facilitating the future introduction of multistate memory.

Highlights

Phase-change random-access memory (PCRAM), a form of electronic non-volatile resistive random-access memory (RRAM) technology, is based on Joule-heating
These challenges include: i) decreasing the rate-limiting SET switching time down to the ~1ns timescale needed for DRAM replacement by non-volatile PCRAM, while still retaining sufficiently long-time stability of the glassy logic state {0} [6]; ii) increasing the write-erase cycling durability to the value of ~1016 cycles needed for DRAM operation; and iii) ideally, increasing the datastorage density, e.g. by device down-size scaling
The light-induced fast response in resistance was attributed to photo-generated carriers, while the much slower changes suggest the presence of slow trap-to-trap transitions, similar to those that can give rise to phosphorescence observed in high-trap-density materials [43]

Summary

Introduction

Phase-change random-access memory (PCRAM), a form of electronic non-volatile resistive random-access memory (RRAM) technology, is based on Joule-heating. An alternative extrinsic model for resistance drift focuses on structural ageing, but instead concentrates on the time-dependent relaxation of the elastic compressive strain induced in the g-phase in a PCRAM cell after RESET, as a result of the ~5% density difference between c- and g-phases of GST [11]. This strain is assumed to affect the bandgap, and the electrical resistance, of the material [11]. The light-induced fast response in resistance was attributed to photo-generated carriers, while the much slower changes suggest the presence of slow trap-to-trap transitions, similar to those that can give rise to phosphorescence observed in high-trap-density materials [43]

An electronic model for resistance drift

Discussion

Conclusions