Stopping Resistance Drift in Reset Ge₂Sb₂Te₅ Phase Change Memory Cells—Field-Induced Charge Relaxation and Electronic Transport

Multi-bit-per-cell implementations of phase change memory (PCM) require multiple stable and distinct resistance levels. However, the spontaneous increase of resistance of reset PCM cell (drift) and read noise lead to mixing of the intermediate resistance levels [1]. We performed temperature (80 K – 350 K) and electric-field (0 ~ 40 MV/m) dependent experiments on Ge₂Sb₂Te₅ PCM line-cells with 20 nm thickness, 60 nm ~ 150 nm width and ~ 500 nm length [2] reset using 100 – 500 ns voltage pulses with and without photoexcitation. The current-voltage (I-V) characteristics of the cells show a clear low-field response with high sensitivity to photoexcitation for T&lt; 250 K and a distinct high-field response that significantly accelerates resistance drift and shows no sensitivity to photo-excitation. High-field stresses (&gt; 20 MV/m) substantially accelerate resistance drift and bring it to a stop, and significantly reduce read noise. Since the changes can be induced at very low temperatures (80 K) and low current levels (pA) but at high electric fields, we attribute the changes in the observed cell characteristics to electronic processes [3,4] rather than thermally induced structural relaxation [1,5].<br/>We have constructed a low-field 2D hopping and a high-field transport model that fits the data and explains the observed characteristics. Resistance drift appears to be a result of the charge-exchanges between the crystalline (c-) GST (or TiN contacts) and amorphous (a-) GST at the junctions. The charge-exchanges are expected to take place starting immediately after reset until the steady state condition is achieved. The net charge in the a-GST region determines the electrostatic potential of the barrier between the two c-GST contacts. The time-to-escape for the trapped charges increases over time with the increasing barrier height, which manifests itself as resistance drift following a power-law trend. When a sufficiently high electric-field is applied, the trapped (and the photo-generated) holes are removed from the a-GST region, accelerating resistance drift (and making the device insensitive to photo-excitation) [6]. The band-offset between the a-GST / c-GST (or metal) interfaces suggests that conduction in a-GST is through electrons injected at the tunnel junctions at the a-GST / c-GST (or metal) interfaces.<br/>Results support an electronic origin of resistance drift caused by a potential profile formed by negative charging of the amorphous region with the escape of the holes and n-type conduction in reset PCM cells. The amorphized length, and the band-offsets between the a-GST, c-GST and metal contacts play a significant role in drift. Resistance drift can be mitigated with materials, device and waveform engineering.<br/><br/>1. Boniardi, M. & Ielmini, D. Physical origin of the resistance drift exponent in amorphous phase change materials. <i>Appl Phys Lett</i> <b>98</b>, 243506 (2011).<br/><br/>2. Dirisaglik, F. <i>et al.</i> High speed, high temperature electrical characterization of phase change materials: Metastable phases, crystallization dynamics, and resistance drift. <i>Nanoscale</i> <b>7</b>, 16625–16630 (2015).<br/><br/>3. Khan, R. S., Dirisaglik, F., Gokirmak, A. & Silva, H. Resistance drift in Ge2Sb2Te5 phase change memory line cells at low temperatures and its response to photoexcitation. <i>Appl Phys Lett</i> <b>116</b>, 253501 (2020).<br/><br/>4. Elliott, S. R. Electronic mechanism for resistance drift in phase-change memory materials: link to persistent photoconductivity. <i>J Phys D Appl Phys</i> <b>53</b>, 214002 (2020).<br/><br/>5. Sebastian, A., Krebs, D., Le Gallo, M., Pozidis, H. & Eleftheriou, E. A collective relaxation model for resistance drift in phase change memory cells. <i>IEEE Int. Reliability Physics Symp. </i>MY51–MY56 (2015).<br/><br/>6. A. Talukder, M. Kashem, M. Hafiz, R. Khan, F. Dirisaglik, H. Silva, and A. Gokirmak, “Electronic transport in amorphous Ge2Sb2Te5 phase-change memory line cells and its response to photoexcitation,” Appl Phys Lett <b>124</b>(26), (2024).