Date of Completion

10-24-2014

Embargo Period

10-24-2015

Keywords

Phase Change Memory, PCM, Phase Change Memory Devices, Ge2Sb2Te5, GST, Crystallization, Resistance Drift, Metastable Phases, Amorphous Phase, High-Temperature Characterization

Major Advisor

Ali Gokirmak

Associate Advisor

Helena Silva

Associate Advisor

Rajeev Bansal

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Phase change memory (PCM) is one of the most promising non-volatile memory technologies in the marketplace today and offers tremendous potential for high speed, energy efficient computing as a non-volatile DRAM replacement or as a direct competitor to flash memory. PCM devices utilize the electrical resistivity contrast between highly resistive amorphous and highly conductive crystalline phases of phase change materials. Their operation differs significantly from conventional solid state devices: PCM devices experience melting, resolidification and crystallization in nanosecond time scales, with high current densities and strong thermal gradients. Understanding threshold switching, crystallization dynamics, resistance drift phenomena, and electrical and thermal transport in nanoscale, in conjunction with complete modeling, will significantly accelerate PCM development. Detailed characterization of material properties in the device operation time scales for a wide temperature range is critically important.

In this work, the impacts of the material parameters and thermoelectric effects are illustrated using finite element modeling. Temperature dependent electrical resistivities of the metastable amorphous and fcc (face centered cubic) phases of Ge2Sb2Te5 (GST-the most common phase change material) are measured for the first time in nanoscale device level (PCM line cells) using the high-speed electrical pump-probe characterization technique developed for this work. Electrical resistivities of liquid and hcp (hexagonal close packed) phases are also measured with the same measurement technique. In addition to these, crystallization processes immediately after amorphization are monitored at elevated temperatures and resistance drift behaviors in amorphous phase are observed in short (~2 ms) and very long (~13 months) time scales in a 300-675 K temperature range. Electrical breakdown field of amorphous GST is extracted. Carrier density and mobility are measured using the van der Pauw Hall measurement technique for various crystallinity states. The devices are cycled to demonstrate memory operation.

Our studies show that electrical resistivities of metastable amorphous and fcc GST exponentially decrease as a function of temperature with significantly higher values compared to those of the typical slow R-T measurements. Crystallization dynamics play a significant role in both resistance drift and carrier activation. Results suggest that nucleation reduces the carrier lifetimes by increasing the trapping probability and charging of these crystalline nuclei gives rise to Coulomb blockade, significantly hindering transport.

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