Low Energy Order-To-Disorder Transition Pathways In Phase-Change Nanowires

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Doctor of Philosophy (PhD)
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Materials Science & Engineering
Indium Selenide
Phase change memory
Transmission electron microscope
Condensed Matter Physics
Mechanics of Materials
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Modi, Gaurav

Phase change memory (PCM) can reversibly transform between the amorphous and crystalline phase within nanoseconds, making it a promising candidate for non-volatile memory (NVM) applications. The conventional method to realize the crystal−amorphous transformation in PCM is to heat the material above its melting point by an electrical pulse, after which it is quenched by sudden removal of the pulse. However, such a method requires very large current densities, which results in massive parasitic heat losses and also accelerated failure of the device. On the other hand, it has been recently shown that resistance switching in PCM systems such as superlattices and nanowires can occur at much lower current densities, where the temperature of the material always stays below its melting point. In-situ transmission electron microscopy (TEM) studies on nanowires, with a growth direction aligned with the dislocation slip system, have shown that the resistance change occurs by a dislocation-templated amorphization process. However, the resistance switching mechanism in superlattices, which consist of periodically alternating layers of two different PCM materials, has been much debated with many studies claiming it to be an order-to-order transition.In this study, we attempt to further lower the current density for the crystal-amorphous transformation by multiple strategies: pre-inducing defects in the nanowire by a size-mismatched dopant, synthesizing self-assembled compositionally modulated superlattice nanowires, and lastly by utilizing an unconventional flat phonon mode based amorphization mechanism in In2Se3 nanowires. Furthermore, we utilize TEM extensively to correlate the structural and electrical resistance changes in these nanowires, which reveal several interesting characteristics. The resistance switching mechanism in the superlattice nanowires is found to be an order-disorder transition, contrary to what is proposed in literature. Ab-initio simulations indicate that such an amorphization occurs with the aid of large concentration of vacancies in this material. On the other hand, In2Se3 PCM are found to undergo a collapse of long-range order by a d.c. voltage, but at intermediate voltage range, nucleation of topological dislocation occurs suggesting a possible charge density wave in this material. Overall, in this work, synthesis of novel nanowires is combined with ex-situ TEM experiments to uncover the structure-property correlations in the PCM materials, which is otherwise difficult to perform in thin-film PCM devices.

Ritesh Agarwal
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