Characterization of the crystallization dynamics of phase change materials via electron diffraction
Abstract/Contents
- Abstract
- Phase change materials (PCMs) are semiconducting alloys that rapidly and reversibly switch between an amorphous, or glassy state and a crystalline, or ordered state via electrical and optical pulses. The two phases have optical and electrical properties that vary by orders of magnitude, which means that they can be used in fast, non-volatile memory devices. However, the complex nucleation and growth processes that underlie the rate-limiting step of switching from the amorphous to crystalline state remain to be understood. In the first part of this thesis I describe how the single-pulse, high temporal resolution capabilities of the MeV Ultrafast Electron Diffraction (MeV-UED) instrument at the SLAC National Accelerator Laboratory were used to explore the transient structure of laser-melt-quenched PCMs, such as pure Sb, as they crystallize. Antimony's optical properties vary as a function of its thickness, and has shown PCM-like behavior below a critical thickness of the film. Given the fact that MeV-UED gives a direct, in-situ structural observation of the transient liquid structure, we use this technique to explore the thickness dependent aspects of the crystallization process in Sb. In the second part of this thesis I demonstrate via in-situ electron diffraction that femtosecond optical excitation above a threshold fluence of the amorphous, as-deposited, PCM Ge2Sb2Te5 induces large, 100 µm-scale single crystals, more than two-orders of magnitude larger than prior reports. Transmission electron microscopy shows that these large crystals are dewetted regions with a face-centered cubic structure. Energy-dispersive X-ray spectroscopy indicates that the crystals have the same composition as the initial amorphous phase. I will present a theoretical model which shows that this arises from a crossover from a nucleation-dominated crystallization regime to a growth-dominated crystallization regime, and I show that the measured grain size is consistent with Johnson−Mehl−Avrami−Kolmogorov (JMAK) crystallization theory for temperatures near the melting temperature. The ability to grow macroscopic single crystals from an amorphous material, and on arbitrary amorphous substrates, opens up a large area of potential applications, as well as new opportunities for tuning the nucleation, growth, and switching characteristics of phase-change materials. This work provides a new understanding of the rate-limiting crystallization step in PCMs and what gives them their unique properties.
Description
| Type of resource | text |
|---|---|
| Form | electronic resource; remote; computer; online resource |
| Extent | 1 online resource. |
| Place | California |
| Place | [Stanford, California] |
| Publisher | [Stanford University] |
| Copyright date | 2021; ©2021 |
| Publication date | 2021; 2021 |
| Issuance | monographic |
| Language | English |
Creators/Contributors
| Author | Zajac, Marc E |
|---|---|
| Degree supervisor | Lindenberg, Aaron Michael |
| Thesis advisor | Lindenberg, Aaron Michael |
| Thesis advisor | Reed, Evan J |
| Thesis advisor | Reis, David A, 1970- |
| Degree committee member | Reed, Evan J |
| Degree committee member | Reis, David A, 1970- |
| Associated with | Stanford University, Department of Materials Science and Engineering |
Subjects
| Genre | Theses |
|---|---|
| Genre | Text |
Bibliographic information
| Statement of responsibility | Marc Edward Zajac. |
|---|---|
| Note | Submitted to the Department of Materials Science and Engineering. |
| Thesis | Thesis Ph.D. Stanford University 2021. |
| Location | https://purl.stanford.edu/qw391vb0968 |
Access conditions
- Copyright
- © 2021 by Marc E Zajac
- License
- This work is licensed under a Creative Commons Attribution Non Commercial 3.0 Unported license (CC BY-NC).
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