Mattia Boniardi

Physical origin of the resistance drift exponent in amorphous phase change materials

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.

A physics-based model of electrical conduction decrease with time in amorphous Ge2Sb2Te5

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 m...

Common signature of many-body thermal excitation in structural relaxation and crystallization of chalcogenide glasses

The structural stability of amorphous chalcogenides used in electrical and optical phase-change devices is critically affected by structural relaxation (SR) and crystallization. We studied the temperature activation of SR and crystallization in amorphous Ge2Sb2Te5. We demonstrate that SR and crystallization coherently obey the same Meyer–Neldel (MN) rule, evidencing the key role of many-body thermal excitation in these transformations. The different activation energies for SR and crystallization are discussed based on the strength and number of bonds to be rearranged during the transitions. The MN rule provides a straightforward explanation of the unphysical pre-exponential times (10−24–10−22 s) observed in chalcogenide glasses.

Statistics of Resistance Drift Due to Structural Relaxation in Phase-Change Memory Arrays

The phase-change memory (PCM), based on the reversible phase transition in a chalcogenide material, is among the most attractive memory concepts for next-generation nonvolatile memories. Due to the metastable nature of the amorphous state, the memory can exhibit a time variation of resistance after programming as a result of two main mechanisms: 1) structural relaxation (SR), which is an atomic rearrangement to minimize the defect density, and 2) crystallization of the amorphous chalcogenide. SR has been mostly studied at the single-cell level, whereas a statistical analysis and modeling is necessary for device reliability estimation and prediction. This work studies the statistical behavior of SR in PCM devices, through experimental and modeling approaches. Statistical SR data from PCM arrays are shown, and a Monte Carlo model for SR statistics is proposed, based on previous physical modeling of the SR process. This model allows for long-term, physics-based, and array-level reliability extrapolations in large PCM arrays.

Statistical and scaling behavior of structural relaxation effects in phase-change memory (PCM) devices

The phase-change memory (PCM) technology represents one of the most attractive concepts for next generation data storage. PCM behavior is mainly limited by the structural relaxation (SR) and by the crystallization of an amorphous chalcogenide material: the ternary alloy Ge2Sb2Te5. SR is a local structural-rearrangement at the atomic/bonding scale and crystallization is the reaching of a periodic atomic structure. While the retention capabilities related to crystallization have been already extensively addressed in the literature, both at the single-cell and at the statistical level, those related to SR have been mainly studied at the intrinsic level and a statistical analysis at the device level is still lacking. The purpose of this paper is to study the statistical and scaling behavior of the SR phenomenon in PCM devices, through experimental and modeling tools, allowing for long term, physics-based, reliability extrapolations in large-scaled PCM arrays.

Impact of material composition on the write performance of phase-change memory devices

The phase-change memory (PCM) technology represents one of the most attractive concepts for next generation data storage. PCM operation is based on the particular properties of a chalcogenide alloy, the ternary compound Ge2Sb2Te5, which is able to perform fast and reversible transitions between a crystalline, high-conductive phase and an amorphous, low-conductive one, thus enabling the binary data storage. Although the ternary alloy Ge2Sb2Te5 is the best recognised solution to meet the device reliability and performance specifications, other alloys are being studied within the GeSbT e ternary compound system in order to investigate and to enlarge the possible spectrum of PCM applications. This work focuses both on the program parameters and on the write performances of a Sb-rich GST composition, suggesting a change in the physical properties of the PCM material and a transition from nucleation to growth-dominated crystallization mechanism, both controlled by the material composition engineering. This enables new challenging performance parameters.