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Register a new userAccelerating and Stopping Resistance Drift in Phase Change Memory Cells via High Electric Field Stress
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We observed resistance drift in 125 K - 300 K temperature range in melt quenched amorphous Ge2Sb2Te5 line-cells with length x width x thickness = ~500 nm x ~100 nm x ~ 50 nm. Drift coefficients measured using small voltage sweeps appear to decrease from 0.12 +/- 0.029 at 300 K to 0.075 +/- 0.006 at 125 K. The current-voltage characteristics of the amorphized cells measured in the 85 K - 300 K using high-voltage sweeps (0 to ~25 V) show a combination of a linear, low-field exponential and high-field exponential conduction mechanisms, all of which are strong functions of temperature. The very first high-voltage sweep after amorphization (with electric fields up to ~70% of the breakdown field) shows clear hysteresis in the current-voltage characteristics due to accelerated drift, while the consecutive sweeps show stable characteristics. Stabilization was achieved with 50 nA compliance current (current densities ~104 A/cm^2), preventing appreciable self-heating in the cells. The observed acceleration and stoppage of the resistance drift with the application of high electric fields is attributed to changes in the electrostatic potential profile within amorphous Ge2Sb2Te5 due to trapped charges, reducing tunneling current. Stable current-voltage characteristics are used to extract carrier activation energies for the conduction mechanisms in 85 K - 300 K temperature range. The carrier activation energy associated with linear current-voltage response is extracted to be 331 +/- 5 meV in 200 - 300 K range, while carrier activation energies of 233 +/- 2 meV and 109 +/- 5 meV are extracted in 85 K to 300 K range for the mechanisms that give exponential current-voltage responses.
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Resistance drift in phase change materials is characterized in amorphous phase change memory line-cells from 300 K to 125 K range and is observed to follow the previously reported power-law behavior with drift coefficients in the 0.07 to 0.11 range in dark. While these drift coefficients measured in dark are similar to commonly observed drift coefficients (~0.1) at and above room temperature, measurements under light show a significantly lower drift coefficient (0.05 under illumination versus 0.09 in dark at 150K). Periodic on/off switching of light shows sudden decrease/increase of resistance, attributed to photo-excited carriers, followed by a very slow response (~30 minutes at 150 K) attributed to contribution of charge traps. Continuation of the resistance drift at low temperatures and the observed photo-response suggest that resistance drift in amorphous phase change materials is predominantly an electronic process.
Phase-change memory devices have found applications in in-memory computing where the physical attributes of these devices are exploited to compute in place without the need to shuttle data between memory and processing units. However, non-idealities such as temporal variations in the electrical resistance have a detrimental impact on the achievable computational precision. To address this, a promising approach is projecting the phase configuration of phase change material onto some stable element within the device. Here we investigate the projection mechanism in a prominent phase-change memory device architecture, namely mushroom-type phase-change memory. Using nanoscale projected Ge2Sb2Te5 devices we study the key attributes of state-dependent resistance, drift coefficients, and phase configurations, and using them reveal how these devices fundamentally work.
We model electrical conductivity in metastable amorphous $Ge_{2}Sb_{2}Te_{5}$ using independent contributions from temperature and electric field to simulate phase change memory devices and Ovonic threshold switches. 3D, 2D-rotational, and 2D finite element simulations of pillar cells capture threshold switching and show filamentary conduction in the on-state. The model can be tuned to capture switching fields from ~5 to 40 MV/m at room temperature using the temperature dependent electrical conductivity measured for metastable amorphous GST; lower and higher fields are obtainable using different temperature dependent electrical conductivities. We use a 2D fixed out-of-plane-depth simulation to simulate an Ovonic threshold switch in series with a $Ge_{2}Sb_{2}Te_{5}$ phase change memory cell to emulate a crossbar memory element. The simulation reproduces the pre-switching current and voltage characteristics found experimentally for the switch + memory cell, isolated switch, and isolated memory cell.
Dislocations are one-dimensional (1D) topological line defects where the lattice deviates from the perfect crystal structure. The presence of dislocations transcends condensed matter research and gives rise to a diverse range of emergent phenomena [1-6], ranging from geological effects [7] to light emission from diodes [8]. Despite their ubiquity, to date, the controlled formation of dislocations is usually achieved via strain fields, applied either during growth [9,10] or retrospectively via deformation, e.g., (nano [11-14])-indentation [15]. Here we show how partial dislocations can be induced using local electric fields, altering the structure and electronic response of the material where the field is applied. By combining high-resolution imaging techniques and density functional theory calculations, we directly image these dislocations in the ferroelectric hexagonal manganite Er(Ti,Mn)O3 and study their impact on the local electric transport behaviour. The use of an electric field to induce partial dislocations is a conceptually new approach to the burgeoning field of emergent defect-driven phenomena and enables local property control without the need of external macroscopic strain fields. This control is an important step towards integrating and functionalising dislocations in practical devices for future oxide electronics.
Phase change memory (PCM) is an emerging data storage technology, however its programming is thermal in nature and typically not energy-efficient. Here we reduce the switching power of PCM through the combined approaches of filamentary contacts and thermal confinement. The filamentary contact is formed through an oxidized TiN layer on the bottom electrode, and thermal confinement is achieved using a monolayer semiconductor interface, three-atom thick MoS2. The former reduces the switching volume of the phase change material and yields a 70% reduction in reset current versus typical 150 nm diameter mushroom cells. The enhanced thermal confinement achieved with the ultra-thin (~6 {AA}) MoS2 yields an additional 30% reduction in switching current and power. We also use detailed simulations to show that further tailoring the electrical and thermal interfaces of such PCM cells toward their fundamental limits could lead up to a six-fold benefit in power efficiency.
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