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Register a new userResistance Drift in Ge2Sb2Te5 Phase Change Memory Line Cells at Low Temperatures and Its Response to Photoexcitation
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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.
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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.
We have measured the critical phase change conditions induced by electrical pulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing a comprehensive resistance map as a function of pulse parameters (width, amplitude and trailing edge). Our measurements reveal that the heating scheme and the details of the contact geometry play the dominant role in determining the final phase composition of the device such that a non-uniform heating scheme promotes partial amorphization/crystallization for a wide range of pulse parameters enabling multiple resistance levels for data storage applications. Furthermore we find that fluctuations in the snap-back voltage and set/reset resistances in repeated switching experiments are related to the details of the current distribution such that a uniform current injection geometry (i.e. circular contact) favors more reproducible switching parameters. This shows that possible geometrical defects in nanoscale phase change memory devices may play an essential role in the performance of the smallest possible devices through modification of the exact current distribution in the active chalcogenide layer. We present a three-dimensional finite element model of the electro-thermal physics to provide insights into the underlying physical mechanisms of the switching dynamics as well as to quantitatively account for the scaling behaviour of the switching currents in both circular and rectangular contact geometries. The calculated temporal evolution of the heat distribution within the pulse duration shows distinct features in rectangular contacts providing evidence for locally hot spots at the sharp corners of the current injection site due to current crowding effects leading to the observed behaviour.
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.
A stable suspension of nanoscale particles due to the Casimir force is of great interest for many applications such as sensing, non-contract nano-machines. However, the suspension properties are difficult to change once the devices are fabricated. Vanadium dioxide (VO$_2$) is a phase change material, which undergoes a transition from a low-temperature insulating phase to a high-temperature metallic phase around a temperature of 340 K. In this work, we study Casimir forces between a nanoplate (gold or Teflon) and a layered structure containing a VO$_2$ film. It is found that stable Casimir suspensions of nanoplates can be realized in a liquid environment, and the equilibrium distances are determined, not only by the layer thicknesses but also by the matter phases of VO$_2$. Under proper designs, a switch from quantum trapping of the gold nanoplate (on state) to its release (off state) as a result of the metal-to-insulator transition of VO$_2$, is revealed. On the other hand, the quantum trapping and release of a Teflon nanoplate is found under the insulator-to-metal transition of VO$_2 $. Our findings offer the possibility of designing switchable devices for applications in micro-and nano-electromechanical systems.
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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