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1072-G02-09

Universal glass dynamics in PCM nano-glasses I. V. Karpov1, M. Mitra1,2, D. Kau1, G. Spadini1, V. G. Karpov2, and Y. A. Kryukov2 1 Intel Corporation, 2200 Mission College Blvd, RNB 3-01, Santa Clara, CA, 95051 2 Department of Physics & Astronomy, University of Toledo, 2801 W.Bancroft Street, MS 111, Toledo, OH, 43606 ABSTRACT The classical double-well potential (DWP) model known to explain many phenomena in glasses, is extended to the nano glasses of chalcogenide phase change memory (PCM). We describe simple analytical expressions for the temporal drift of PCM reset parameters. The threshold voltage Vth and the amorphous state resistance, R, are shown to drift with the time t ≥ t o as ∆Vth ∝ v ln(t / t o ) and R ∝ (t / t o ) respectively, in broad intervals spanning many decades in time. These dependencies saturate at long enough times that can be shorten with temperature increase. All the available data on the PCM drift are shown to be fully consistent with DWP model.

INTRODUCTION We recall that PCM devices rely on the electrically induced phase change in chalcogenide materials and most notably Ge2Sb2Te5 (GST) [1, 2]. The two states correspond to the different phases in the active volume of the chalcogenide material: a low-resistive crystalline phase in a set, and a high-resistive glassy phase in a reset state. Our work follows the earlier findings [3] of the temporal drift of the PCM parameters R (reset resistance) and Vth (threshold voltage) and presents a significant compliment to our data. In addition, our work points at the important role of atomic dynamics in PCM nano glasses, where the observed drift can be attributed to inherent structural relaxations in glasses. We relate the drift of the electrical parameters to the ageing phenomena commonly observed in these glasses, whereby the metastable glass structure relaxes to its more stable state. DWP MODEL The DWP concept [4] accounts for a structural disorder (including bond angles, lengths, and coordination numbers), which makes some elements in a glass abnormally flexible and retain its mobility. Their atoms can move between two different configurations corresponding to the two energy minima in DWP (comparable transition rates between three or more configurations would appear as a sheer coincidence in a random glass structure). Structural disorder translates into the fluctuations of DWP barrier height and energy difference. A simple, commonly accepted hypothesis about their probabilistic distributions is that they are uniform within certain limits and was shown to give at least semi-quantitative descriptions in a wide variety of glasses [5-7].

The DWP concept introduces the exponentially broad distribution of relaxation times τ (WB ) = τ 0 exp(WB / kT ) because the barrier height WB is a random quantity with almost a uniform probabilistic distribution g (W B ) ≈ 1 / ∆WB ,

∆WB = WB ,max − W B ,min

(1)

between its boundary values, the maximum and minimum relaxation times in the system are estimated as τ max(min) = τ 0 exp(W B ,max(min) / kT )