Efficient Deep Neural Network Accelerator Using Controlled Ferroelectric Domain Dynamics

The current work reports an efficient deep neural network (DNN) accelerator where synaptic weight elements are controlled by ferroelectric domain dynamics. An integrated device-to-algorithm framework for benchmarking novel synaptic devices is used. In P(VDF-TrFE) based ferroelectric tunnel junctions, analog conductance states are measured using a custom pulsing protocol and associated custom circuits and array architectures for DNN training is simulated. Our results show precise control of polarization switching dynamics in multi-domain, polycrystalline ferroelectric thin films can produce considerable weight update linearity in metal-ferroelectric-semiconductor (MFS) tunnel junctions. Ultrafast switching and low junction current in these devices offer extremely energy efficient operation. Through an integrated platform of hardware development, characterization and modelling, we predict the available conductance range where linearity is expected under identical potentiating and depressing pulses for efficient DNN training and inference tasks. As an example, an analog crossbar based DNN accelerator with MFS junctions as synaptic weight elements showed ~ 93% training accuracy on large MNIST handwritten digit dataset while for cropped images, a 95% accuracy is achieved. One observed challenge is rather limited dynamic conductance range while operating under identical potentiating and depressing pulses below 1V. Investigation is underway for improving the dynamic conductance range without losing the weight update linearity

Ultrafast Switching and Linear Conductance Modulation in Ferroelectric Tunnel Junctions via P (VDF-TrFE) Morphology Control

Neuromorphic computing architectures demand the development of analog, non-volatile memory components operating at femto-Joule/bit operation energy. Electronic components working in this energy range require devices operating at ultrafast timescales. Among different non-volatile, analog memories, ferroelectric tunnel junctions (FTJs) have emerged as an important contender due to their voltage-driven operation leading to extreme energy-efficiency. Here, we report a study on the switching timescale and linear conductance modulation of organic FTJs comprising a metal/ferroelectric/semiconductor (MFS) stack with different morphologies of ferroelectric copolymer P(VDF-TrFE) ultrathin films. The results show that due to different annealing temperatures and protocols, the spin-coated copolymer films are modified significantly, which can have a large effect on the switching timescales and threshold fields of the FTJs with the best quality devices having a projected switching timescale of sub-nanosecond range. An improvement in switching speed by 7 orders of magnitude can be obtained with an increase of the programming voltage by less than a factor of 2 in these devices. This ultrafast switching of ferroelectric domains in our FTJs leads to pico to femto joule range of operation energy per bit opening the pathways for energy efficient and fast operating non-volatile memories while devices with higher domain pinning sites show a route for tuning analog conductivity for bio-realistic neuromorphic architectures

Pulsed laser deposition of La1-xSrxMnO3 : thin-film properties and spintronic applications

Materials engineering on the nanoscale by precise control of growth parameters can lead to many unusual and fascinating physical properties. The development of pulsed laser deposition (PLD) 25 years ago has enabled atomistic control of thin films and interfaces and as such it has contributed significantly to advances in fundamental material science. One application area is the research field of spintronics, which requires optimized nanomaterials for the generation and transport of spin-polarized carriers. The mixed-valence manganite La1−xSrxMnO3 (LSMO) is an interesting material for spintronics due to its intrinsic magnetoresistance properties, electric-field tunable metal–insulator transitions, and half-metallic band structure. Studies on LSMO thin-film growth by PLD show that the deposition temperature, oxygen pressure, laser fluence, strain due to substrate–film lattice mismatch and post-deposition annealing conditions significantly influence the magnetic and electrical transport properties of LSMO. For spintronic structures, robust ferromagnetic exchange interactions and metallic conductivity are desirable properties. In this paper, we review the physics of LSMO thin films and the important role that PLD played in advancing the field of LSMO-based spintronics. Some specific application areas including magnetic tunnel junctions, multiferroic tunnel junctions and organic spintronic devices are highlighted, and the advantages, drawbacks and opportunities of PLD-grown LSMO for next-generation spintronic devices are discussed.Peer reviewe

The role of electron-hole recombination in organic magnetoresistance

Magneto-electrical measurements were performed on diodes and bulk heterojunction solar cells (BHSCs) to clarify the role of formation of coulombically bound electron-hole (e-h) pairs on the magnetoresistance (MR) response in organic thin film devices. BHSCs are suitable model systems because they effectively quench excitons but the probability of forming e-h pairs in them can be tuned over orders of magnitude by the choice of material and solvent in the blend. We have systematically varied the e-h recombination coefficients, which are directly proportional to the probability for the charge carriers to meet in space, and found that a reduced probability of electrons and holes meeting in space lead to disappearance of the MR. Our results clearly show that MR is a direct consequence of e-h pair formation. We also found that the MR line shape follows a power law-dependence of B0.5 at higher fields

Giant magnetoresistance response in Sr2FeMoO6 based organic spin valves

We report the fabrication of the first Sr2FeMoO6 based organic spin valve device using Tris(8-hydroxyquinolinato) aluminum (Alq(3)) as a spin transport layer. The characterization of the device confirms hysteretic magnetoresistance with approximately 20%-30% switching between high and low resistance states at low temperatures. The results demonstrate that organic semiconductors can form a suitable interface with double perovskite, half metallic Sr2FeMoO6, for efficient low temperature operation and have a potential to improve the room temperature performance significantly in tunneling devices where decay in spin diffusion length of organic layer does not affect the transport

Single-cell analysis of hematopoietic cell fate on niche-mimetic platforms using Raman microspectroscopy

Regenerative therapies are predicated on our ability to achieve controlled and reproducible expansion of stem cells ex vivo. Hematopoietic stem cells (HSCs) are the best-studied class of tissue-restricted stem cells and their microenvironment in vivo has also been the subject of considerable research. Although a host of cellular, molecular and physical cues that regulate stem cell fate in vivo have been identified, efforts to culture HSCs for extended periods by recapitulating some of these interactions in vitro have met with limited success. Miniaturized biomaterial platforms that present tens to hundreds of putative niche components can help screen individual factors or their combinations which drive quiescence, self-renewal and differentiation in vitro while reducing the numbers of rare cell populations required for testing. Screening approaches rely on accurate identification of specific stem cell outcomes in order to correlate extrinsic cues with the fates they elicit. Traditionally, HSCs and their progeny populations have been identified using time-consuming functional assays or immunofluorescence detection of a combination of surface antigens, both of which are invasive and preclude longitudinal analysis of cell fate in culture. Methods that allow rapid, label-free and location-specific characterization of the differentiation stages of individual cells would enable the rapid development of concurrent screens of the effects of niche components and their combinations on stem cell fate specification. Raman spectroscopy has emerged as one of the few analytical approaches that can be used to noninvasively assay the biomolecular composition of cells. Combining Raman spectroscopy with multivariate methods such as partial least-squares discriminant analysis and self-organizing maps may enable objective classification of specific stem cell outcomes on screening platforms, or facilitate investigations into the clonal heterogeneity of stem cells, respectively. The work presented herein describes the use of spontaneous Raman microspectroscopy and multivariate methods as a noninvasive technique for following hematopoietic stem cells and their progeny in vitro. Chapter 1 introduces the field of tissue engineering and the role of the stem cell microenvironment in guiding stem cell fate especially as it relates to the hematopoietic stem cell and lays down the background and motivation for label-free analytical approaches for phenotypic analysis of stem cells in vitro. Chapter 2 discusses the development of a hydrogel culture platform that is compatible with Raman spectroscopy, and demonstrates the capability of multivariate methods to capture subtle changes in the biochemical make-up of cells using mammalian cell lines. Chapter 3 uses a monocytic leukemia cell line capable of differentiating into macrophages (THP-1 line) to build on the work presented in Chapter 2 and provide proof of concept for the feasibility of real-time tracking of cell differentiation using Raman spectroscopy. This chapter introduces self-organizing maps (SOMs). Finally, Chapter 4 shows the successful use of Raman spectroscopy and partial least-squares discriminant analysis for determining the differentiation stages of individual living hematopoietic stem cells and their progeny on biomaterial substrates with varying stiffness. Chapter 5 presents a short discussion of the implications of this work for the field and summarizes some of the issues and challenges that must be addressed in future work using Raman spectroscopy on HSCs.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste