Date of Completion

12-18-2015

Embargo Period

12-17-2016

Keywords

Phase Change Memory, GeSbTe, Crystallization Dynamics

Major Advisor

Prof. Helena Silva

Associate Advisor

Prof. Ali Gokirmak

Associate Advisor

Prof. Rajeev Bansal

Field of Study

Electrical Engineering

Degree

Doctor of Philosophy

Open Access

Open Access

Abstract

Phase change memory (PCM) is currently seen as the most promising candidate for a future storage-class memory with the potential to be as fast as Dynamic Random-Access Memory but with much longer retention times, and as dense as flash memory but significantly faster due to unique material properties which include strong electrical resistivity contrast, fast crystallization and high crystallization temperature. PCM devices utilize chalcogenide materials (most commonly Ge2Sb2Te5, or GST) that can be reversibly and rapidly switched between amorphous and crystalline phases (enabling storage of information) with orders of magnitude difference in electrical resistivity. Crystallization dynamics, transition temperatures, and grain size distributions depend on the material composition, interfaces and cell geometry and determine the electrical pulses required for operation, cell performance, power consumption and reliability.

This work focused on temperature dependent characterization of GeSbTe thin films and nanoscale structures with the goal of contributing to a better understanding of phase-change memory materials and devices.

The electrical resistivity of liquid GST (ρGST-Lq )was extracted from measurements on large number of individual GST nanostructures self-heated to melt by single microsecond voltage pulses, as well as on thin film samples.The crystallization behavior of GST films on silicon nitride and on silicon dioxide through slow resistance versus temperature measurements was also characterized. Silicon nitride appears to facilitate the fcc-hcp phase-transition of GST and we speculate this may be due to the hexagonal symmetry of silicon nitride. Our results also show the importance of the heating rate in determining phase transition temperatures.

Understanding the crystallization dynamics is critically important for PCM device operation. Above a certain amplitude, a baseline (or offset) voltage after a melting pulse can play an important role in the set operation by leading to retention of a molten filament and growth-from-melt templated from the surrounding crystalline regions. The effect of different baseline voltages on the set dynamics of phase change memory devices was studied by applying melting voltage pulses with varying baseline voltages. Simulations of the effect of different baseline voltages were also performed to compare to and help interpret the experimental results.

Lastly, in-situ X-Ray Diffraction (XRD) measurements up to high temperatures and ex-situ XRD measurements on pre-annealed samples were performed to characterize grain size distributions as a function of anneal temperature. The material crystallizes over time as the chuck temperature is increased and the crystallization process is monitored by the evolution of different peaks in the XRD measurement which are related to the grain sizes. These results will be used to improve and calibrate our electrothermal and crystallization models for PCM materials and devices.

Kadir Cil - University of Connecticut, 2015

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