Publications
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A 1024-Channel 10-Bit 36-μW/ch CMOS ROIC for Multiplexed GFET-Only Sensor Arrays in Brain Mapping
Cisneros-Fernandez J., Garcia-Cortadella R., Illa X., Martinez-Aguilar J., Paetzold J., Mohrlok R., Kurnoth M., Jeschke C., Teres L., Garrido J.A., Guimera-Brunet A., Serra-Graells F. IEEE Transactions on Biomedical Circuits and Systems; 15 (5): 860 - 876. 2021. 10.1109/TBCAS.2021.3113556. IF: 3.833
This paper presents a 1024-channel neural read-out integrated circuit (ROIC) for solution-gated GFET sensing probes in massive muECoG brain mapping. The proposed time-domain multiplexing of GFET-only arrays enables low-cost and scalable hybrid headstages. Low-power CMOS circuits are presented for the GFET analog frontend, including a CDS mechanism to improve preamplifier noise figures and 10-bit 10-kS/s A/D conversion. The 1024-channel ROIC has been fabricated in a standard 1.8-V 0.18-mum CMOS technology with 0.012 mm 2 and 36 mu W per channel. An automated methodology for the in-situ calibration of each GFET sensor is also proposed. Experimental ROIC tests are reported using a custom FPGA-based muECoG headstage with 16times 32 and 32times 32 GFET probes in saline solution and agar substrate. Compared to state-of-art neural ROICs, this work achieves the largest scalability in hybrid platforms and it allows the recording of infra-slow neural signals. © 2007-2012 IEEE.
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A study on free-standing 3C-SiC bipolar power diodes
Li F., Renz A.B., Pérez-Tomás A., Shah V., Gammon P., Via F.L., Jennings M., Mawby P. Applied Physics Letters; 118 (24, 242101): 1ENG. 2021. 10.1063/5.0054433. IF: 3.791
A low p-n built-in potential (1.75 V) makes 3C-SiC an attractive choice for medium voltage bipolar or charge balanced devices. Until recently, most 3C-SiC had been grown on Si, and power device fabrication had, therefore, been hindered by issues, such as high defect density and limited processing temperature, while devices were necessarily limited to lateral structures. In this work, we present the fabrication and characterization of a vertical PiN diode using bulk 3C-SiC material. A p-type ohmic contact was obtained on Al implanted regions with a specific contact resistance ∼10−3 Ω cm2. The fabricated PiN diode has a low forward voltage drop of 2.7 V at 1000 A/cm2, and the on-off ratio at ±3 V is as high as 109. An ideality factor of 1.83-1.99 was achieved, and a blocking voltage of ∼110 V was observed using a single-zone junction termination design. © 2021 Author(s).
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A walk on the frontier of energy electronics with power ultra-wide bandgap oxides and ultra-thin neuromorphic 2D materials
Prez-Toms A., Chikoizde E., Rogers D. Proceedings of SPIE - The International Society for Optical Engineering; 11687 (2590747) 2021. 10.1117/12.2590747. IF: 0.450
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Bipolar self-doping in ultra-wide bandgap spinel ZnGa(2)O4
Chi Z, Tarntair FG, Fregnaux M, Wu WY, Sartel C, Madaci I, Chapon P, Sallet V, Dumont Y, Perez-Tomas A, Horng RH, Chikoidze E Materials Today Physics; 20 (100466) 2021. 10.1016/j.mtphys.2021.100466. IF: 9.298
The spinel group is a growing family of materials with general formulation AB2X4 (the X anion typically being a chalcogen like O and S) with many advanced applications for energy. At the time being, the spinel zinc gallate (ZnGa2O4) arguably is the ternary ultra-wide bandgap bipolar oxide semiconductor with the largest bandgap (∼5eV), making this material very promising for implementations in deep UV optoelectronics and ultra-high power electronics. In this work, we further demonstrate that, exploiting the rich cation coordination possibilities of the spinel chemistry, the ZnGa2O4 intrinsic conductivity (and its polarity) can be controlled well over 10 orders of magnitude. p-type and n-type ZnGa2O4 epilayers can be grown by tuning the pressure, oxygen flow and cation precursors ratio during metal-organic chemical vapor deposition. A relatively deep acceptor level can be achieved by promoting antisites (ZnGa) defects, while up to a (n > 1019 cm−3) donor concentration is obtained due to the hybridization of the Zn–O orbitals in the samples grown in Zn-rich conditions. Electrical transport, atomic and optical spectroscopy reveal a free hole conduction (at high temperature) for p-ZnGa2O4 while for n-ZnGa2O4 a (Mott) variable range hopping (VRH) and negative magnetoresistance phenomena take place, originated from “self-impurity” band located at Ev+ ∼3.4 eV. Among arising ultra-wide bandgap semiconductors, spinel ZnGa2O4 exhibit unique self-doping capability thus extending its application at the very frontier of current energy optoelectronics.
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Carbon Incorporation in MOCVD of MoS2Thin Films Grown from an Organosulfide Precursor
Schaefer C.M., Caicedo Roque J.M., Sauthier G., Bousquet J., Hébert C., Sperling J.R., Pérez-Tomás A., Santiso J., Del Corro E., Garrido J.A. Chemistry of Materials; 33 (12): 4474 - 4487. 2021. 10.1021/acs.chemmater.1c00646. IF: 9.811
With the rise of two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors and their prospective use in commercial (opto)electronic applications, it has become key to develop scalable and reliable TMD synthesis methods with well-monitored and controlled levels of impurities. While metal-organic chemical vapor deposition (MOCVD) has emerged as the method of choice for large-scale TMD fabrication, carbon (C) incorporation arising during MOCVD growth of TMDs has been a persistent concern-especially in instances where organic chalcogen precursors are desired as a less hazardous alternative to more toxic chalcogen hydrides. However, the underlying mechanisms of such unintentional C incorporation and the effects on film growth and properties are still elusive. Here, we report on the role of C-containing side products of organosulfur precursor pyrolysis in MoS2 thin films grown from molybdenum hexacarbonyl Mo(CO)6 and diethyl sulfide (CH3CH2)2S (DES). By combining in situ gas-phase monitoring with ex situ microscopy and spectroscopy analyses, we systematically investigate the effect of temperature and Mo(CO)6/DES/H2 gas mixture ratios on film morphology, chemical composition, and stoichiometry. Aiming at high-quality TMD growth that typically requires elevated growth temperatures and high DES/Mo(CO)6 precursor ratios, we observed that temperatures above DES pyrolysis onset (â 600 °C) and excessive DES flow result in the formation of nanographitic carbon, competing with MoS2 growth. We found that by introducing H2 gas to the process, DES pyrolysis is significantly hindered, which reduces carbon incorporation. The C content in the MoS2 films is shown to quench the MoS2 photoluminescence and influence the trion-To-exciton ratio via charge transfer. This finding is fundamental for understanding process-induced C impurity doping in MOCVD-grown 2D semiconductors and might have important implications for the functionality and performance of (opto)electronic devices. ©
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Characterization of optogenetically-induced cortical spreading depression in awake mice using graphene micro-transistor arrays
Masvidal-Codina E., Smith T.M., Rathore D., Gao Y., Illa X., Prats-Alfonso E., Corro E.D., Calia A.B., Rius G., Martin-Fernandez I., Guger C., Reitner P., Villa R., Garrido J.A., Guimera-Brunet A., Wykes R.C. Journal of Neural Engineering; 18 (5, 055002) 2021. 10.1088/1741-2552/abecf3. IF: 5.379
Objective. The development of experimental methodology utilizing graphene micro-transistor arrays to facilitate and advance translational research into cortical spreading depression (CSD) in the awake brain. Approach. CSDs were reliably induced in awake nontransgenic mice using optogenetic methods. High-fidelity DC-coupled electrophysiological mapping of propagating CSDs was obtained using flexible arrays of graphene soultion-gated field-effect transistors (gSGFETs). Main results. Viral vectors targetted channelrhopsin expression in neurons of the motor cortex resulting in a transduction volume 1 mm3. 5-10 s of continous blue light stimulation induced CSD that propagated across the cortex at a velocity of 3.0 0.1 mm min-1. Graphene micro-transistor arrays enabled high-density mapping of infraslow activity correlated with neuronal activity suppression across multiple frequency bands during both CSD initiation and propagation. Localized differences in the CSD waveform could be detected and categorized into distinct clusters demonstrating the spatial resolution advantages of DC-coupled recordings. We exploited the reliable and repeatable induction of CSDs using this preparation to perform proof-of-principle pharmacological interrogation studies using NMDA antagonists. MK801 (3 mg kg-1) suppressed CSD induction and propagation, an effect mirrored, albeit transiently, by ketamine (15 mg kg-1), thus demonstrating this models' applicability as a preclinical drug screening platform. Finally, we report that CSDs could be detected through the skull using graphene micro-transistors, highlighting additional advantages and future applications of this technology. Significance. CSD is thought to contribute to the pathophysiology of several neurological diseases. CSD research will benefit from technological advances that permit high density electrophysiological mapping of the CSD waveform and propagation across the cortex. We report an in vivo assay that permits minimally invasive optogenetic induction, combined with multichannel DC-coupled recordings enabled by gSGFETs in the awake brain. Adoption of this technological approach could facilitate and transform preclinical investigations of CSD in disease relevant models. © 2021 The Author(s). Published by IOP Publishing Ltd.
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Direct Visualization of Anti-Ferroelectric Switching Dynamics via Electrocaloric Imaging
Vales-Castro P., Vellvehi M., Perpiñà X., Caicedo J.M., Jordà X., Faye R., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Advanced Electronic Materials; 7 (12, 2100380) 2021. 10.1002/aelm.202100380. IF: 7.295
The large electrocaloric coupling in PbZrO3 allows using high-speed infrared imaging for visualizing anti-ferroelectric switching dynamics via the associated temperature change. It is found that in ceramic samples of homogeneous temperature and thickness, switching is fast due to the generation of multiple nucleation sites, with devices responding in the millisecond range. By introducing gradients of thickness, however, it is possible to change the dynamics to propagation limited, whereby a single-phase boundary sweeps across the sample like a cold front, at a speed of ≈20 cm s−1. Additionally, introducing thermostatic temperature differences between two sides of the sample enables the simultaneous generation of a negative electrocaloric effect on one side and a positive one on the other, yielding a Janus-like electrocaloric response. © 2021 Wiley-VCH GmbH
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Graphene active sensor arrays for long-term and wireless mapping of wide frequency band epicortical brain activity
Garcia-Cortadella R., Schwesig G., Jeschke C., Illa X., Gray A.L., Savage S., Stamatidou E., Schiessl I., Masvidal-Codina E., Kostarelos K., Guimerà-Brunet A., Sirota A., Garrido J.A. Nature Communications; 12 (1, 211) 2021. 10.1038/s41467-020-20546-w. IF: 14.919
Graphene active sensors have demonstrated promising capabilities for the detection of electrophysiological signals in the brain. Their functional properties, together with their flexibility as well as their expected stability and biocompatibility have raised them as a promising building block for large-scale sensing neural interfaces. However, in order to provide reliable tools for neuroscience and biomedical engineering applications, the maturity of this technology must be thoroughly studied. Here, we evaluate the performance of 64-channel graphene sensor arrays in terms of homogeneity, sensitivity and stability using a wireless, quasi-commercial headstage and demonstrate the biocompatibility of epicortical graphene chronic implants. Furthermore, to illustrate the potential of the technology to detect cortical signals from infra-slow to high-gamma frequency bands, we perform proof-of-concept long-term wireless recording in a freely behaving rodent. Our work demonstrates the maturity of the graphene-based technology, which represents a promising candidate for chronic, wide frequency band neural sensing interfaces. © 2021, The Author(s).
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Novel Graphene Electrode for Retinal Implants: An in vivo Biocompatibility Study
Nguyen D., Valet M., Dégardin J., Boucherit L., Illa X., de la Cruz J., del Corro E., Bousquet J., Garrido J.A., Hébert C., Picaud S. Frontiers in Neuroscience; 15 (615256) 2021. 10.3389/fnins.2021.615256. IF: 4.677
Evaluating biocompatibility is a core essential step to introducing a new material as a candidate for brain-machine interfaces. Foreign body reactions often result in glial scars that can impede the performance of the interface. Having a high conductivity and large electrochemical window, graphene is a candidate material for electrical stimulation with retinal prosthesis. In this study, non-functional devices consisting of chemical vapor deposition (CVD) graphene embedded onto polyimide/SU-8 substrates were fabricated for a biocompatibility study. The devices were implanted beneath the retina of blind P23H rats. Implants were monitored by optical coherence tomography (OCT) and eye fundus which indicated a high stability in vivo up to 3 months before histology studies were done. Microglial reconstruction through confocal imaging illustrates that the presence of graphene on polyimide reduced the number of microglial cells in the retina compared to polyimide alone, thereby indicating a high biocompatibility. This study highlights an interesting approach to assess material biocompatibility in a tissue model of central nervous system, the retina, which is easily accessed optically and surgically. © Copyright © 2021 Nguyen, Valet, Dégardin, Boucherit, Illa, de la Cruz, del Corro, Bousquet, Garrido, Hébert and Picaud.
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Novel transducers for high-channel-count neuroelectronic recording interfaces
Guimerà-Brunet A., Masvidal-Codina E., Cisneros-Fernández J., Serra-Graells F., Garrido J.A. Current Opinion in Biotechnology; 72: 39 - 47. 2021. 10.1016/j.copbio.2021.10.002. IF: 9.740
Neuroelectronic interfaces with the nervous system are an essential technology in state-of-the-art neuroscience research aiming to uncover the fundamental working mechanisms of the brain. Progress towards increased spatio-temporal resolution has been tightly linked to the advance of microelectronics technology and novel materials. Translation of these technologies to neuroscience has resulted in multichannel neural probes and acquisition systems enabling the recording of brain signals using thousands of channels. This review provides an overview of state-of-the-art neuroelectronic technologies, with emphasis on recording site architectures which enable the implementation of addressable arrays for high-channel-count neural interfaces. In this field, active transduction mechanisms are gaining importance fueled by novel materials, as they facilitate the implementation of high density addressable arrays. © 2021 The Authors
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Origin of large negative electrocaloric effect in antiferroelectric PbZr O3
Vales-Castro P., Faye R., Vellvehi M., Nouchokgwe Y., Perpiñà X., Caicedo J.M., Jordà X., Roleder K., Kajewski D., Perez-Tomas A., Defay E., Catalan G. Physical Review B; 103 (5, 054112) 2021. 10.1103/PhysRevB.103.054112. IF: 4.036
We have studied the electrocaloric response of the archetypal antiferroelectric PbZrO3 as a function of voltage and temperature in the vicinity of its antiferroelectric-paraelectric phase transition. Large electrocaloric effects of opposite signs, ranging from an electrocooling of -3.5 K to an electroheating of +5.5K, were directly measured with an infrared camera. We show by calorimetric and electromechanical measurements that the large negative electrocaloric effect comes from an endothermic antiferroelectric-ferroelectric switching, in contrast to dipole destabilization of the antiparallel lattice, previously proposed as an explanation for the negative electrocaloric effect of antiferroelectrics. © 2021 American Physical Society.
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Single and Multisite Graphene-Based Electroretinography Recording Electrodes: A Benchmarking Study
de la Cruz J., Nguyen D., Illa X., Bousquet J., Pérez-Marín A.P., del Corro E., Picaud S., Garrido J.A., Hebert C. Advanced Materials Technologies; 2021. 10.1002/admt.202101181. IF: 7.848
Electroretinography (ERG) is a clinical test employed to understand and diagnose many retinopathies. ERG is usually performed by placing a macroscopic ring gold wire electrode on the cornea while flashing light onto the eye to measure changes in the transretinal potential. However, macroscopic gold electrodes are severely limiting since they do not provide a flexible interface to contact the sensitive corneal tissue, making this technique highly uncomfortable for the patient. Another major drawback is the opacity of gold electrodes, which only allows them to record the ERG signal on the corneal periphery, preventing central ERG recordings. To overcome the limitations of metal-based macroscopic ERG electrodes, flexible electrodes are fabricated using graphene as a transparent, flexible, and sensitive material. The transparency of the graphene is exploited to fabricate microelectrode arrays (MEAs) that are able to perform multisite recording on the cornea. The graphene-based ERG electrodes are benchmarked against the widely used gold electrodes in a P23H rat model with photoreceptor degeneration. This study shows that the graphene-based ERG electrodes can faithfully record ERGs under a wide range of conditions (light intensity, stage of photoreceptor degeneration, etc.) while offering additional benefits for ERG recordings such as transparency and flexibility. © 2021 Wiley-VCH GmbH
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Status and prospects of cubic silicon carbide power electronics device technology
Li F., Roccaforte F., Greco G., Fiorenza P., La Via F., Pérez-Tomas A., Evans J.E., Fisher C.A., Monaghan F.A., Mawby P.A., Jennings M. Materials; 14 (19, 5831) 2021. 10.3390/ma14195831. IF: 3.623
Wide bandgap (WBG) semiconductors are becoming more widely accepted for use in power electronics due to their superior electrical energy efficiencies and improved power densities. Although WBG cubic silicon carbide (3C-SiC) displays a modest bandgap compared to its commercial counterparts (4H-silicon carbide and gallium nitride), this material has excellent attributes as the WBG semiconductor of choice for low-resistance, reliable diode and MOS devices. At present the material remains firmly in the research domain due to numerous technological impediments that hamper its widespread adoption. The most obvious obstacle is defect-free 3C-SiC; presently, 3C-SiC bulk and heteroepitaxial (on-silicon) display high defect densities such as stacking faults and antiphase boundaries. Moreover, heteroepitaxy 3C-SiC-on-silicon means low temperature processing budgets are imposed upon the system (max. temperature limited to ~1400 °C) limiting selective doping realisation. This paper will give a brief overview of some of the scientific aspects associated with 3C-SiC processing technology in addition to focussing on the latest state of the art results. A particular focus will be placed upon key process steps such as Schottky and ohmic contacts, ion implantation and MOS processing including reliability. Finally, the paper will discuss some device prototypes (diodes and MOSFET) and draw conclusions around the prospects for 3CSiC devices based upon the processing technology presented. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
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Study of Spin-Orbit Interactions and Interlayer Ferromagnetic Coupling in Co/Pt/Co Trilayers in a Wide Range of Heavy-Metal Thickness
Ogrodnik P., Grochot K., Karwacki Ł., Kanak J., Prokop M., Chȩciński J., Skowroński W., Ziȩtek S., Stobiecki T. ACS Applied Materials and Interfaces; 13 (39): 47019 - 47032. 2021. 10.1021/acsami.1c11675. IF: 9.229
The spin-orbit torque, a torque induced by a charge current flowing through the heavy-metal-conducting layer with strong spin-orbit interactions, provides an efficient way to control the magnetization direction in heavy-metal/ferromagnet nanostructures, required for applications in the emergent magnetic technologies like random access memories, high-frequency nano-oscillators, or bioinspired neuromorphic computations. We study the interface properties, magnetization dynamics, magnetostatic features, and spin-orbit interactions within the multilayer system Ti(2)/Co(1)/Pt(0-4)/Co(1)/MgO(2)/Ti(2) (thicknesses in nanometers) patterned by optical lithography on micrometer-sized bars. In the investigated devices, Pt is used as a source of the spin current and as a nonmagnetic spacer with variable thickness, which enables the magnitude of the interlayer ferromagnetic exchange coupling to be effectively tuned. We also find the Pt thickness-dependent changes in magnetic anisotropies, magnetoresistances, effective Hall angles, and, eventually, spin-orbit torque fields at interfaces. The experimental findings are supported by the relevant interface structure-related simulations, micromagnetic, macrospin, as well as the spin drift-diffusion models. Finally, the contribution of the spin-orbital Edelstein-Rashba interfacial fields is also briefly discussed in the analysis. © 2021 The Authors. Published by American Chemical Society
