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EP01.03.21 : Instability of High Resistance Conductive Filaments in RRAM Cells During the Read Operation

5:00 PM–7:00 PM Apr 3, 2018

PCC North, 300 Level, Exhibit Hall C-E

Description
Mohammad Al-Mamun1 Marius Orlowski1

1, Virginia Tech, Blacksburg, Virginia, United States

In filamentary resistive switching memory cells (RRAM) the resistance of the on-state, Ron, is determined by the limiting (compliance) current, Icc, by the relation Ron~1/Iccn, where the exponent n in many RRAM cells with a metal atom filament is close to unity1. Our RRAM cells are MIM Cu/TaOx/I structures with 25nm TaOx and inert electrode I=Pt,Rh, or Ru. We show that at low Icc (roughly <50μA), the resulting high resistance Ron (~100kΩ) is fragile and operatively undefined as any read operation is bound to disturb its Ron value. The Ron can increase or decrease depending on the read voltage polarity, read voltage starting point, read voltage ramp rate and its sweep direction. In contrast, a set operation performed at a high Icc (>150μA) leads to a stable, low resistance Ron independent of reasonably chosen reading conditions. For Icc<50 μA, the measured Ron will return different values depending whether the measurement within a 100mV voltage interval started at 0.0V, -0.1V or +0.1V. The highly resistive conductive filament (CF) is fragile and subject to small displacement of individual atoms or defects, charging and discharging reactions. The starting point of the applied voltage determines (non)-equilibrium conditions for Cu+, oxygen O2-, and oxygen vacancy Vo charge states and electron concentrations, all of which impact the properties of CF. For highly resistive CF, most read measurements tend to decrease Ron. The underlying strengthening of the CF, ascribed to aggregation or displacement of Cu+, Vo, and O2-, is, often, impermanent and auto-reversible upon suspension of the read operation. However, the rate of atomic displacement depends strongly on the voltage ramp rate. In general, we find that the Ron resistance decreases with decreasing ramp rate which was has been varied from 10V/s to 0.1V/s. The geometric shape of CF, is approximated by that of a truncated cone. The bulk part of the resistance resides at the tip of CF. As soon as the current exceeds Icc ,used at the set operation, it generates at the tip high electric field which depending on its polarity causes new transport of Cu+, O2-, and Vo. The voltage ramp rate determines the time scale of the ionic transport at high fields. Thus ramp rate dependence gives insight into the time scales of the ionic transport. The Ron disturbances by electric fields yield insight into the transient mechanisms for CF formation and rupture.
One implication is that during reset operations the differences between high Ron values are brought to the same value before the filament is ruptured, i.e. the reset is rendered independent of the set operation conditions in contrast to a low resistance CF2. The ionic mechanisms and the time scales involved that lead to the change of Ron during the read and reset operations will be discussed in detail.

[1] T. Liu, Y. Kang, S. El-Helw, T. Potnis, M. Orlowski, Jap. J. Appl. Phys. 52, (2013) 084202
[2] G. Ghosh, M. Orlowski, IEEE Trans. Elect. Dev. 62(9) (2015) 2850-56

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