The resistance of amorphous chalcogenides used in phase change memory devices increases over time due to structural relaxation (SR). The resistance drift usually follows a power law with time described by an exponent ν. Understanding the origin of may lead to engineering methods to improve the stability in memory devices. This work presents an analytical model to describe the activation energies for conduction and SR based on the Meyer–Neldel rule. The model accounts for the observed temperature and time dependence of resistance, and highlights that νis related to the ratio between conduction and SR activation energies at any given time during drift.
The time-stability of the electrical characteristics of chalcogenide materials is one of the most important issues for their use in nonvolatile solid state memory applications. In particular the electrical conduction of the glassy phase evolves with time due to two different physical phenomena: the crystallization and the so-called low conductivity drift. Despite the physics of crystallization having been extensively studied in literature, the latter is mainly described by phenomenological relationships, and its physical comprehension is still under discussion. In this paper we study the amorphous phase low-field conductivity drift and its dependence on the temperature experienced by the device. We developed an experimental procedure able to separate the reversible change in the electrical conductivity with temperature due to the material semiconductorlike behavior from the nonreversible one related to the drift mechanism. A drift model explaining such nonreversible conductivity change as a band diagram modification is also provided and calibrated on experimental data. The present work finally introduces alternative metrics for drift quantification that can be useful in order to compare different materials.
Phase change memory (PCM) device physics comprehension represents an important chapter of future development of the PCM‐based architectures and their placement into the storage class memory (SCM) segment of the memory hierarchy. Here, a reduction of SET and RESET currents by more than 60% with respect to conventional GeTe–Sb2Te3 (GST) alloys is demonstrated by using phase change memory cells containing (GeTe–Sb2Te3)/Sb2Te3 super‐lattices (SL). Further, it is demonstrated that our SL PCM devices have similar characteristics in terms of the memory transition as conventional memory cells based on GST, even though showing reduced power consumption, indicative of an efficiency augmented SET‐to‐RESET transition. The reduced power consumption may be attributed to an increased thermal resistance of the SL with respect to the bulk GST alloy. This demonstrates that it is possible to engineer PCM with enhanced performance by employing SL structures, enlarging the possibility of employing SL as SCM players.
Phase change materials based on chalcogenides are key enabling technologies for optical storage, such as rewritable CD and DVD, and recently also electrical nonvolatile memory, named phase change memory (PCM). In a PCM, the amorphous or crystalline phase affects the material band structure, hence the device resistance. Although phase transformation is extremely fast and repeatable, the amorphous phase suffers structural relaxation and crystallization at relatively low temperatures, which may affect the temperature stability of PCM state. To improve the time/temperature stability of the PCM, novel operation modes of the device should be identified. Here, we present bipolar switching operation of PCM, which is interpreted by ion migration in the solid state induced by elevated temperature and electric field similar to the bipolar switching in metal oxides. The temperature stability of the high resistance state is demonstrated and explained based on the local depletion of chemical species from the electrode region.
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