Phase-change electronic memory utilizes the electric field-induced reversible structural change in chalcogenide materials to switch between crystalline and amorphous phases to store information, which is fast and non-volatile. In spite of extensive investigations of the field-induce phase-change phenomena, the underlying mechanisms are quite complex and their elucidation requires the continued development of new experimental tools.
Our group has demonstrated that conventional understanding of melt-quench based amorphization process needs to be revisited. While working with single-crystalline nanowires which due to their long lengths do not typically reach high temperatures required for conventional melting of the material, we realized that alternate pathways for crystal-amorphous transformation can exist. Furthermore, nanowires due to their cylindrical geometry, conventional melting should lead to the formation of an amorphous shell around the crystalline core, which cannot explain the abrupt resistance switching as observed. We have shown that crystal-amorphous transformation in phase-change materials can be achieved through a subtle and ad lower energy costing defect-based pathway. This pathway involves creation of extended defects such as anti-phase boundaries (APBs) in GeTe and dislocations in Ge2Sb2STe5, which migrate and accumulate at a region of local inhomogeneity creating a defect template. The formation of APBs leads to polar domain inversion as revealed by optical second-harmonic generation polarimetry. Due to the accumulation of defects locally, the material first undergoes a metal-to-insulator transition followed by a structural collapse leading to amorphization without conventional melting. We utilized this understanding to pre-induce defects in the crystalline phase via controlled ion bombardment to engineer carrier localization and enhance carrier-lattice coupling in order to efficiently extract work required to introduce bond-distortions necessary for amorphization from input electrical energy itself. We demonstrated that such a strategy shows 100X improvement in amorphization current densities compared to the melt-quench strategy. The existence of multiple resistance states along with ultra-low power switching makes this approach promising for low power memory and neuromorphic computation. We will also discuss our efforts to integrate defect-engineered phase change nanowires in integrated photonics platforms for designing the next generation of reconfigurable photonic devices.
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We will also discuss our efforts to explore the optoelectronic properties of MoxW1-xTe2, which are type-II Weyl semimetals, i.e., gapless topological states of matter with broken inversion and/or time reversal symmetry, which exhibit unconventional responses to externally applied fields. We have observed spatially dispersive circular photogalvanic effect (s-CPGE) over a wide spectral range from mid-IR to visible region in these materials. This effect shows exclusively in the Weyl phase and vanishes upon temperature induced topological phase change. Since the photon energy in our experiments leads to interband transitions between different electronic bands, we use the density matrix formalism to describe the photocurrent response under chiral optical excitation and obtain microscopic insights into the observed phenomena. We will discuss how spatially inhomogeneous optical excitation and unique symmetry and band structure of Weyl semimetals produces CPGE in these systems. Implications for studying band topologies in these class of materials via photogalvanic effects will also be discussed. These results provide a new approach to controlling photoresponse by patterning optical fields in certain class of broken-symmetry materials to store, manipulate and transmit information over a wide spectral range.
2D materials enable quantum well-like performance, while enjoying substrate independence. Together with their unique band-engineering potential, they pose an opportunity for exploring next generation devices. The rationale for heterogeneous integration is material function separation; that is to perform electrooptic switching in light-matter-enhanced or polaritonic material-mode combinations, while reserving the bosonic and weak-interacting character for photonics, ideally Si or SiN platforms for cost and loss competitiveness, respectively. Here we report on the first 2D material (TMD) integration into microring resonators (MRR), and demonstrated tunability to critical coupling regime. This system allows determining the TMD index via a semi-empirical approach, which is challenged by traditional ellipsometry due to the atom-thin TMD. We further discuss MRR-TMD electrooptic modulation contrasting spectrally ON versus OFF exciton tuning. We conclude by discussing optical nanocavity-TMD systems with applications in QED or LED emission, such as radiation and emission-channel engineering.
Conference Committee Involvement (3)
2D Photonic Materials and Devices III
5 February 2020 | San Francisco, California, United States
2D Photonic Materials and Devices II
6 February 2019 | San Francisco, California, United States
2D Photonic Materials and Devices
29 January 2018 | San Francisco, California, United States