Publications by authors named "Eric Pop"

99 Publications

Ultralow-switching current density multilevel phase-change memory on a flexible substrate.

Science 2021 09 9;373(6560):1243-1247. Epub 2021 Sep 9.

Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.

[Figure: see text].
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http://dx.doi.org/10.1126/science.abj1261DOI Listing
September 2021

Toward Low-Temperature Solid-Source Synthesis of Monolayer MoS.

ACS Appl Mater Interfaces 2021 Sep 24;13(35):41866-41874. Epub 2021 Aug 24.

Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States.

Two-dimensional (2D) semiconductors have been proposed for heterogeneous integration with existing silicon technology; however, their chemical vapor deposition (CVD) growth temperatures are often too high. Here, we demonstrate direct CVD solid-source precursor synthesis of continuous monolayer (1L) MoS films at 560 °C in 50 min, within the 450-to-600 °C, 2 h thermal budget window required for back-end-of-the-line compatibility with modern silicon technology. Transistor measurements reveal on-state current up to ∼140 μA/μm at 1 V drain-to-source voltage for 100 nm channel lengths, the highest reported to date for 1L MoS grown below 600 °C using solid-source precursors. The effective mobility from transfer length method test structures is 29 ± 5 cm V s at 6.1 × 10 cm electron density, which is comparable to mobilities reported from films grown at higher temperatures. The results of this work provide a path toward the realization of high-quality, thermal-budget-compatible 2D semiconductors for heterogeneous integration with silicon manufacturing.
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http://dx.doi.org/10.1021/acsami.1c06812DOI Listing
September 2021

Field-effect at electrical contacts to two-dimensional materials.

Nano Res 2021 Jul 28:1-7. Epub 2021 Jul 28.

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China.

The inferior electrical contact to two-dimensional (2D) materials is a critical challenge for their application in post-silicon very large-scale integrated circuits. Electrical contacts were generally related to their resistive effect, quantified as contact resistance. With a systematic investigation, this work demonstrates a capacitive metal-insulator-semiconductor (MIS) field-effect at the electrical contacts to 2D materials: The field-effect depletes or accumulates charge carriers, redistributes the voltage potential, and gives rise to abnormal current saturation and nonlinearity. On one hand, the current saturation hinders the devices' driving ability, which can be eliminated with carefully engineered contact configurations. On the other hand, by introducing the nonlinearity to monolithic analog artificial neural network circuits, the circuits' perception ability can be significantly enhanced, as evidenced using a coronavirus disease 2019 (COVID-19) critical illness prediction model. This work provides a comprehension of the field-effect at the electrical contacts to 2D materials, which is fundamental to the design, simulation, and fabrication of electronics based on 2D materials.

Electronic Supplementary Material: Supplementary material (results of the simulation and SEM) is available in the online version of this article at 10.1007/s12274-021-3670-y.
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http://dx.doi.org/10.1007/s12274-021-3670-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8316888PMC
July 2021

Uncovering Thermal and Electrical Properties of SbTe/GeTe Superlattice Films.

Nano Lett 2021 Jul 16;21(14):5984-5990. Epub 2021 Jul 16.

Department of Mechanical Engineering, Stanford University, Stanford, California 94305, United States.

Superlattice-like phase change memory (SL-PCM) promises lower switching current than conventional PCM based on GeSbTe (GST); however, a fundamental understanding of SL-PCM requires detailed characterization of the interfaces within such an SL. Here we explore the electrical and thermal transport of SLs with deposited SbTe and GeTe alternating layers of various thicknesses. We find up to an approximately four-fold reduction of the effective cross-plane thermal conductivity of the SL stack (as-deposited polycrystalline) compared with polycrystalline GST (as-deposited amorphous and later annealed) due to the thermal interface resistances within the SL. Thermal measurements with varying periods of our SLs show a signature of phonon coherence with a transition from wave-like to particle-like phonon transport, further described by our modeling. Electrical resistivity measurements of such SLs reveal strong anisotropy (∼2000×) between the in-plane and cross-plane directions due to the weakly interacting van der Waals-like gaps. This work uncovers electrothermal transport in SLs based on SbTe and GeTe for the improved design of low-power PCM.
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http://dx.doi.org/10.1021/acs.nanolett.1c00947DOI Listing
July 2021

Advanced Data Encryption ​using 2D Materials.

Adv Mater 2021 Jul 27;33(27):e2100185. Epub 2021 May 27.

Physical Sciences and Engineering Division, King Abdullah University of Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia.

Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO and Al O , are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 2  - 1 bits), and high throughput of 1 Mbit s by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation.
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http://dx.doi.org/10.1002/adma.202100185DOI Listing
July 2021

Ultrathin Three-Monolayer Tunneling Memory Selectors.

ACS Nano 2021 May 4;15(5):8484-8491. Epub 2021 May 4.

Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States.

High-density memory arrays require selector devices, which enable selection of a specific memory cell within a memory array by suppressing leakage current through unselected cells. Such selector devices must have highly nonlinear current-voltage characteristics and excellent endurance; thus selectors based on a tunneling mechanism present advantages over those based on the physical motion of atoms or ions. Here, we use two-dimensional (2D) materials to build an ultrathin (three-monolayer-thick) tunneling-based memory selector. Using a sandwich of -BN, MoS, and -BN monolayers leads to an "H-shaped" energy barrier in the middle of the heterojunction, which nonlinearly modulates the tunneling current when the external voltage is varied. We experimentally demonstrate that tuning the MoS Fermi level can improve the device nonlinearity from 10 to 25. These results provide a fundamental understanding of the tunneling process through atomically thin 2D heterojunctions and lay the foundation for developing high endurance selectors with 2D heterojunctions, potentially enabling high-density non-volatile memory systems.
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http://dx.doi.org/10.1021/acsnano.1c00002DOI Listing
May 2021

High-Performance p-n Junction Transition Metal Dichalcogenide Photovoltaic Cells Enabled by MoO Doping and Passivation.

Nano Lett 2021 Apr 14;21(8):3443-3450. Epub 2021 Apr 14.

Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States.

Layered semiconducting transition metal dichalcogenides (TMDs) are promising materials for high-specific-power photovoltaics due to their excellent optoelectronic properties. However, in practice, contacts to TMDs have poor charge carrier selectivity, while imperfect surfaces cause recombination, leading to a low open-circuit voltage () and therefore limited power conversion efficiency (PCE) in TMD photovoltaics. Here, we simultaneously address these fundamental issues with a simple MoO ( ≈ 3) surface charge-transfer doping and passivation method, applying it to multilayer tungsten disulfide (WS) Schottky-junction solar cells with initially near-zero . Doping and passivation turn these into lateral p-n junction photovoltaic cells with a record of 681 mV under AM 1.5G illumination, the highest among all p-n junction TMD solar cells with a practical design. The enhanced also leads to record PCE in ultrathin (<90 nm) WS photovoltaics. This easily scalable doping and passivation scheme is expected to enable further advances in TMD electronics and optoelectronics.
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http://dx.doi.org/10.1021/acs.nanolett.1c00015DOI Listing
April 2021

Tuning electrical and interfacial thermal properties of bilayer MoS2 via electrochemical intercalation.

Nanotechnology 2021 Feb 18. Epub 2021 Feb 18.

Electrical Engineering, Stanford University, 420 Via Palou Mall, Allen Building 335X, Stanford, California, 94305, UNITED STATES.

Layered two-dimensional (2D) materials such as MoS2 have attracted much attention for nano- and opto-electronics. Recently, intercalation (e.g. of ions, atoms, or molecules) has emerged as an effective technique to reversibly modulate material properties of such layered 2D films. Here we probe both electrical and thermal properties of Li-intercalated bilayer MoS2 nanosheets by combined electrical measurements and Raman spectroscopy. We demonstrate reversible modulation of carrier density over more than two orders of magnitude (from 0.8×1012 cm 2 to 1.5×1014 cm-2), and we simultaneously obtain the thermal boundary conduct-ance (TBC) between the bilayer and its supporting SiO2 substrate for an intercalated system for the first time. This thermal coupling can be reversibly modulated by nearly a factor of eight, from 14 ± 4.0 MWm-2K-1 before intercalation to 1.8 ± 0.9 MWm 2K-1 when the MoS2 is fully lithiated. These results reveal electrochemical intercalation as a reversible tool to modulate and control both electrical and thermal properties of 2D layers.
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http://dx.doi.org/10.1088/1361-6528/abe78aDOI Listing
February 2021

Dynamic Hybrid Metasurfaces.

Nano Lett 2021 Feb 22;21(3):1238-1245. Epub 2021 Jan 22.

School of Electrical and Computer Engineering, Georgia Institute of Technology, 778 Atlantic Drive NW, Atlanta, Georgia 30332-0250, United States.

Efficient hybrid plasmonic-photonic metasurfaces that simultaneously take advantage of the potential of both pure metallic and all-dielectric nanoantennas are identified as an emerging technology in flat optics. Nevertheless, postfabrication tunable hybrid metasurfaces are still elusive. Here, we present a reconfigurable hybrid metasurface platform by incorporating the phase-change material GeSbTe (GST) into metal-dielectric meta-atoms for active and nonvolatile tuning of properties of light. We systematically design a reduced-dimension meta-atom, which selectively controls the hybrid plasmonic-photonic resonances of the metasurface via the dynamic change of optical constants of GST without compromising the scattering efficiency. As a proof-of-concept, we experimentally demonstrate two tunable metasurfaces that control the amplitude (with relative modulation depth as high as ≈80%) or phase (with tunability >230°) of incident light promising for high-contrast optical switching and efficient anomalous to specular beam deflection, respectively. Our findings further substantiate dynamic hybrid metasurfaces as compelling candidates for next-generation reprogrammable meta-optics.
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http://dx.doi.org/10.1021/acs.nanolett.0c03625DOI Listing
February 2021

High Current Density in Monolayer MoS Doped by AlO.

ACS Nano 2021 Jan 6;15(1):1587-1596. Epub 2021 Jan 6.

Electrical Engineering, Stanford University, Stanford, California 94305, United States.

Semiconductors require stable doping for applications in transistors, optoelectronics, and thermoelectrics. However, this has been challenging for two-dimensional (2D) materials, where existing approaches are either incompatible with conventional semiconductor processing or introduce time-dependent, hysteretic behavior. Here we show that low-temperature (<200 °C) substoichiometric AlO provides a stable -doping layer for monolayer MoS, compatible with circuit integration. This approach achieves carrier densities >2 × 10 cm, sheet resistance as low as ∼7 kΩ/□, and good contact resistance ∼480 Ω·μm in transistors from monolayer MoS grown by chemical vapor deposition. We also reach record current density of nearly 700 μA/μm (>110 MA/cm) along this three-atom-thick semiconductor while preserving transistor on/off current ratio >10. The maximum current is ultimately limited by self-heating (SH) and could exceed 1 mA/μm with better device heat sinking. With their 0.1 nA/μm off-current, such doped MoS devices approach several low-power transistor metrics required by the international technology roadmap.
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http://dx.doi.org/10.1021/acsnano.0c09078DOI Listing
January 2021

Uncovering the Effects of Metal Contacts on Monolayer MoS.

ACS Nano 2020 Nov 23;14(11):14798-14808. Epub 2020 Sep 23.

Department of Electrical Engineering, Stanford University, Stanford, California 94305, United States.

Metal contacts are a key limiter to the electronic performance of two-dimensional (2D) semiconductor devices. Here, we present a comprehensive study of contact interfaces between seven metals (Y, Sc, Ag, Al, Ti, Au, Ni, with work functions from 3.1 to 5.2 eV) and monolayer MoS grown by chemical vapor deposition. We evaporate thin metal films onto MoS and study the interfaces by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and electrical characterization. We uncover that (1) ultrathin oxidized Al dopes MoS type (>2 × 10 cm) without degrading its mobility, (2) Ag, Au, and Ni deposition causes varying levels of damage to MoS (e.g. broadening Raman E' peak from <3 to >6 cm), and (3) Ti, Sc, and Y react with MoS. Reactive metals must be avoided in contacts to monolayer MoS, but control studies reveal the reaction is mostly limited to the top layer of multilayer films. Finally, we find that (4) thin metals do not significantly strain MoS, as confirmed by X-ray diffraction. These are important findings for metal contacts to MoS and broadly applicable to many other 2D semiconductors.
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http://dx.doi.org/10.1021/acsnano.0c03515DOI Listing
November 2020

Improved Current Density and Contact Resistance in Bilayer MoSe Field Effect Transistors by AlO Capping.

ACS Appl Mater Interfaces 2020 Aug 31;12(32):36355-36361. Epub 2020 Jul 31.

Viterbi Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel.

Atomically thin semiconductors are of interest for future electronics applications, and much attention has been given to monolayer (1L) sulfides, such as MoS, grown by chemical vapor deposition (CVD). However, reports on the electrical properties of CVD-grown selenides, and MoSe in particular, are scarce. Here, we compare the electrical properties of 1L and bilayer (2L) MoSe grown by CVD and capped by sub-stoichiometric AlO. The 2L channels exhibit ∼20× lower contact resistance () and ∼30× larger current density compared with 1L channels. is further reduced by >5× with AlO capping, which enables improved transistor current density. Overall, 2L AlO-capped MoSe transistors (with ∼500 nm channel length) achieve improved current density (∼65 μA/μm at = 4 V), a good / ratio of >10, and an of ∼60 kΩ·μm. The weaker performance of 1L devices is due to their sensitivity to processing and ambient. Our results suggest that 2L (or few layers) is preferable to 1L for improved electronic properties in applications that do not require a direct band gap, which is a key finding for future two-dimensional electronics.
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http://dx.doi.org/10.1021/acsami.0c09541DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7588022PMC
August 2020

Nonvolatile Electrically Reconfigurable Integrated Photonic Switch Enabled by a Silicon PIN Diode Heater.

Adv Mater 2020 Aug 26;32(31):e2001218. Epub 2020 Jun 26.

Department of Electrical and Computer Engineering, University of Washington, Seattle, WA, 98195, USA.

Reconfigurability of photonic integrated circuits (PICs) has become increasingly important due to the growing demands for electronic-photonic systems on a chip driven by emerging applications, including neuromorphic computing, quantum information, and microwave photonics. Success in these fields usually requires highly scalable photonic switching units as essential building blocks. Current photonic switches, however, mainly rely on materials with weak, volatile thermo-optic or electro-optic modulation effects, resulting in large footprints and high energy consumption. As a promising alternative, chalcogenide phase-change materials (PCMs) exhibit strong optical modulation in a static, self-holding fashion, but the scalability of present PCM-integrated photonic applications is still limited by the poor optical or electrical actuation approaches. Here, with phase transitions actuated by in situ silicon PIN diode heaters, scalable nonvolatile electrically reconfigurable photonic switches using PCM-clad silicon waveguides and microring resonators are demonstrated. As a result, intrinsically compact and energy-efficient switching units operated with low driving voltages, near-zero additional loss, and reversible switching with high endurance are obtained in a complementary metal-oxide-semiconductor (CMOS)-compatible process. This work can potentially enable very large-scale CMOS-integrated programmable electronic-photonic systems such as optical neural networks and general-purpose integrated photonic processors.
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http://dx.doi.org/10.1002/adma.202001218DOI Listing
August 2020

Monolithic mtesla-level magnetic induction by self-rolled-up membrane technology.

Sci Adv 2020 01 17;6(3):eaay4508. Epub 2020 Jan 17.

Department of Electrical and Computer Engineering and Micro and Nanotechnology Laboratory, University of Illinois, Urbana, IL 61801, USA.

Monolithic strong magnetic induction at the mtesla to tesla level provides essential functionalities to physical, chemical, and medical systems. Current design options are constrained by existing capabilities in three-dimensional (3D) structure construction, current handling, and magnetic material integration. We report here geometric transformation of large-area and relatively thick (~100 to 250 nm) 2D nanomembranes into multiturn 3D air-core microtubes by a vapor-phase self-rolled-up membrane (S-RuM) nanotechnology, combined with postrolling integration of ferrofluid magnetic materials by capillary force. Hundreds of S-RuM power inductors on sapphire are designed and tested, with maximum operating frequency exceeding 500 MHz. An inductance of 1.24 μH at 10 kHz has been achieved for a single microtube inductor, with corresponding areal and volumetric inductance densities of 3 μH/mm and 23 μH/mm, respectively. The simulated intensity of the magnetic induction reaches tens of mtesla in fabricated devices at 10 MHz.
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http://dx.doi.org/10.1126/sciadv.aay4508DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6968933PMC
January 2020

Localized Heating and Switching in MoTe-Based Resistive Memory Devices.

Nano Lett 2020 Feb 30;20(2):1461-1467. Epub 2020 Jan 30.

Department of Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we study bipolar switching in MoTe-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive "plugs" formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.
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http://dx.doi.org/10.1021/acs.nanolett.9b05272DOI Listing
February 2020

Publisher Correction: An electrochemical thermal transistor.

Nat Commun 2019 Sep 27;10(1):4465. Epub 2019 Sep 27.

Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.
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http://dx.doi.org/10.1038/s41467-019-12471-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6764990PMC
September 2019

Dry Transfer of van der Waals Crystals to Noble Metal Surfaces To Enable Characterization of Buried Interfaces.

ACS Appl Mater Interfaces 2019 Oct 1;11(41):38218-38225. Epub 2019 Oct 1.

Department of Electrical and Systems Engineering , University of Pennsylvania , Philadelphia , Pennsylvania 19104 , United States.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have been explored for many optoelectronic applications. Most of these applications require them to be on insulating substrates. However, for many fundamental property characterizations, such as mapping surface potential or conductance, insulating substrates are nonideal as they lead to charging and doping effects or impose the inhomogeneity of their charge environment on the atomically thin 2D layers. Here, we report a simple method of residue-free dry transfer of 2D TMDC crystal layers. This method is enabled via noble-metal (gold, silver) thin films and allows comprehensive nanoscale characterization of transferred TMDC crystals with multiple scanning probe microscopy techniques. In particular, intimate contact with underlying metal allows efficient tip-enhanced Raman scattering characterization, providing high spatial resolution (<20 nm) for Raman spectroscopy. Further, scanning Kelvin probe force microscopy allows high-resolution mapping of surface potential on transferred crystals, revealing their spatially varying structural and electronic properties. The layer-dependent contact potential difference is clearly observed and explained by charge transfer from contacts with Au and Ag. The demonstrated sample preparation technique can be generalized to probe many different 2D material surfaces and has broad implications in understanding of the metal contacts and buried interfaces in 2D material-based devices.
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http://dx.doi.org/10.1021/acsami.9b09798DOI Listing
October 2019

Ultrahigh thermal isolation across heterogeneously layered two-dimensional materials.

Sci Adv 2019 Aug 16;5(8):eaax1325. Epub 2019 Aug 16.

Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA.

Heterogeneous integration of nanomaterials has enabled advanced electronics and photonics applications. However, similar progress has been challenging for thermal applications, in part due to shorter wavelengths of heat carriers (phonons) compared to electrons and photons. Here, we demonstrate unusually high thermal isolation across ultrathin heterostructures, achieved by layering atomically thin two-dimensional (2D) materials. We realize artificial stacks of monolayer graphene, MoS, and WSe with thermal resistance greater than 100 times thicker SiO and effective thermal conductivity lower than air at room temperature. Using Raman thermometry, we simultaneously identify the thermal resistance between any 2D monolayers in the stack. Ultrahigh thermal isolation is achieved through the mismatch in mass density and phonon density of states between the 2D layers. These thermal metamaterials are an example in the emerging field of phononics and could find applications where ultrathin thermal insulation is desired, in thermal energy harvesting, or for routing heat in ultracompact geometries.
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http://dx.doi.org/10.1126/sciadv.aax1325DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6697438PMC
August 2019

Fast Spiking of a Mott VO-Carbon Nanotube Composite Device.

Nano Lett 2019 10 28;19(10):6751-6755. Epub 2019 Aug 28.

Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

The recent surge of interest in brain-inspired computing and power-efficient electronics has dramatically bolstered development of computation and communication using neuron-like spiking signals. Devices that can produce rapid and energy-efficient spiking could significantly advance these applications. Here we demonstrate direct current or voltage-driven periodic spiking with sub-20 ns pulse widths from a single device composed of a thin VO film with a metallic carbon nanotube as a nanoscale heater, without using an external capacitor. Compared with VO-only devices, adding the nanotube heater dramatically decreases the transient duration and pulse energy, and increases the spiking frequency, by up to 3 orders of magnitude. This is caused by heating and cooling of the VO across its insulator-metal transition being localized to a nanoscale conduction channel in an otherwise bulk medium. This result provides an important component of energy-efficient neuromorphic computing systems and a lithography-free technique for energy-scaling of electronic devices that operate via bulk mechanisms.
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http://dx.doi.org/10.1021/acs.nanolett.9b01554DOI Listing
October 2019

Localized Triggering of the Insulator-Metal Transition in VO Using a Single Carbon Nanotube.

ACS Nano 2019 Oct 26;13(10):11070-11077. Epub 2019 Aug 26.

Department of Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

Vanadium dioxide (VO) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use ∼1 nm-wide carbon nanotube (CNT) heaters to trigger the IMT in VO. Single metallic CNTs switch the adjacent VO at less than half the voltage and power required by control devices without a CNT, with switching power as low as ∼85 μW at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin probe microscopy and scanning thermal microscopy. Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO by the CNT plays a key role in the IMT. These results demonstrate the ability to trigger IMT in VO using nanoscale heaters and highlight the significance of thermal engineering to improve device behavior.
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http://dx.doi.org/10.1021/acsnano.9b03397DOI Listing
October 2019

Layer-Dependent Interfacial Transport and Optoelectrical Properties of MoS on Ultraflat Metals.

ACS Appl Mater Interfaces 2019 Aug 19;11(34):31543-31550. Epub 2019 Aug 19.

School of Chemical, Biological and Materials Engineering , University of Oklahoma , Norman , Oklahoma 73019 , United States.

Layered materials based on transition-metal dichalcogenides (TMDs) are promising for a wide range of electronic and optoelectronic devices. Realizing such practical applications often requires metal-TMD connections or contacts. Hence, a complete understanding of electronic band alignments and potential barrier heights governing the transport through metal-TMD junctions is critical. However, it is presently unclear how the energy bands of a TMD align while in contact with a metal as a function of the number of layers. In pursuit of removing this knowledge gap, we have performed conductive atomic force microscopy (CAFM) of few-layered (1 to 5 layers) MoS immobilized on ultraflat conducting Au surfaces [root-mean-square (rms) surface roughness < 0.2 nm] and indium-tin oxide (ITO) substrates (rms surface roughness < 0.7 nm) forming a vertical metal (CAFM tip)-semiconductor-metal device. We have observed that the current increases with the number of layers up to five layers. By applying Fowler-Nordheim tunneling theory, we have determined the barrier heights for different layers and observed how this barrier decreases as the number of layers increases. Using density functional theory calculations, we successfully demonstrated that the barrier height decreases as the layer number increases. By illuminating TMDs on a transparent ultraflat conducting ITO substrate, we observed a reduction in current when compared to the current measured in the dark, hence demonstrating negative photoconductivity. Our study provides a fundamental understanding of the local electronic and optoelectronic behaviors of the TMD-metal junction, which depends on the numbers of TMD layers and may pave an avenue toward developing nanoscale electronic devices with tailored layer-dependent transport properties.
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http://dx.doi.org/10.1021/acsami.9b09868DOI Listing
August 2019

Contact Engineering High-Performance n-Type MoTe Transistors.

Nano Lett 2019 Sep 13;19(9):6352-6362. Epub 2019 Aug 13.

Department of Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

Semiconducting MoTe is one of the few two-dimensional (2D) materials with a moderate band gap, similar to silicon. However, this material remains underexplored for 2D electronics due to ambient instability and predominantly p-type Fermi level pinning at contacts. Here, we demonstrate unipolar n-type MoTe transistors with the highest performance to date, including high saturation current (>400 μA/μm at 80 K and >200 μA/μm at 300 K) and relatively low contact resistance (1.2 to 2 kΩ·μm from 80 to 300 K), achieved with Ag contacts and AlO encapsulation. We also investigate other contact metals (Sc, Ti, Cr, Au, Ni, Pt), extracting their Schottky barrier heights using an analytic subthreshold model. High-resolution X-ray photoelectron spectroscopy reveals that interfacial metal-Te compounds dominate the contact resistance. Among the metals studied, Sc has the lowest work function but is the most reactive, which we counter by inserting monolayer hexagonal boron nitride between MoTe and Sc. These metal-insulator-semiconductor (MIS) contacts partly depin the metal Fermi level and lead to the smallest Schottky barrier for electron injection. Overall, this work improves our understanding of n-type contacts to 2D materials, an important advance for low-power electronics.
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http://dx.doi.org/10.1021/acs.nanolett.9b02497DOI Listing
September 2019

Significant Phonon Drag Enables High Power Factor in the AlGaN/GaN Two-Dimensional Electron Gas.

Nano Lett 2019 06 23;19(6):3770-3776. Epub 2019 May 23.

Stanford Institute for Materials and Energy Sciences , SLAC National Accelerator Laboratory , Menlo Park , California 94025 , United States.

In typical thermoelectric energy harvesters and sensors, the Seebeck effect is caused by diffusion of electrons or holes in a temperature gradient. However, the Seebeck effect can also have a phonon drag component, due to momentum exchange between charge carriers and lattice phonons, which is more difficult to quantify. Here, we present the first study of phonon drag in the AlGaN/GaN two-dimensional electron gas (2DEG). We find that phonon drag does not contribute significantly to the thermoelectric behavior of devices with ∼100 nm GaN thickness, which suppresses the phonon mean free path. However, when the thickness is increased to ∼1.2 μm, up to 32% (88%) of the Seebeck coefficient at 300 K (50 K) can be attributed to the drag component. In turn, the phonon drag enables state-of-the-art thermoelectric power factor in the thicker GaN film, up to ∼40 mW m K at 50 K. By measuring the thermal conductivity of these AlGaN/GaN films, we show that the magnitude of the phonon drag can increase even when the thermal conductivity decreases. Decoupling of thermal conductivity and Seebeck coefficient could enable important advancements in thermoelectric power conversion with devices based on 2DEGs.
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http://dx.doi.org/10.1021/acs.nanolett.9b00901DOI Listing
June 2019

Quasi-Ballistic Thermal Transport Across MoS Thin Films.

Nano Lett 2019 04 7;19(4):2434-2442. Epub 2019 Mar 7.

Department of Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

Layered two-dimensional (2D) materials have highly anisotropic thermal properties between the in-plane and cross-plane directions. Conventionally, it is thought that cross-plane thermal conductivities (κ ) are low, and therefore c-axis phonon mean free paths (MFPs) are small. Here, we measure κ across MoS films of varying thickness (20-240 nm) and uncover evidence of very long c-axis phonon MFPs at room temperature in these layered semiconductors. Experimental data obtained using time-domain thermoreflectance (TDTR) are in good agreement with first-principles density functional theory (DFT). These calculations suggest that ∼50% of the heat is carried by phonons with MFP > 200 nm, exceeding kinetic theory estimates by nearly 2 orders of magnitude. Because of quasi-ballistic effects, the κ of nanometer-thin films of MoS scales with their thickness and the volumetric thermal resistance asymptotes to a nonzero value, ∼10 m K GW. This contributes as much as 30% to the total thermal resistance of a 20 nm thick film, the rest being limited by thermal interface resistance with the SiO substrate and top-side aluminum transducer. These findings are essential for understanding heat flow across nanometer-thin films of MoS for optoelectronic and thermoelectric applications.
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http://dx.doi.org/10.1021/acs.nanolett.8b05174DOI Listing
April 2019

Spatial Separation of Carrier Spin by the Valley Hall Effect in Monolayer WSe Transistors.

Nano Lett 2019 02 2;19(2):770-774. Epub 2019 Jan 2.

Departments of Applied Physics and Photon Science , Stanford University , Stanford , California 94305 , United States.

We investigate the valley Hall effect (VHE) in monolayer WSe field-effect transistors using optical Kerr rotation measurements at 20 K. While studies of the VHE have so far focused on n -doped MoS, we observe the VHE in WSe in both the n - and p -doping regimes. Hole doping enables access to the large spin-splitting of the valence band of this material. The Kerr rotation measurements probe the spatial distribution of the valley carrier imbalance induced by the VHE. Under current flow, we observe distinct spin-valley polarization along the edges of the transistor channel. From analysis of the magnitude of the Kerr rotation, we infer a spin-valley density of 44 spins/μm, integrated over the edge region in the p -doped regime. Assuming a spin diffusion length less than 0.1 μm, this corresponds to a spin-valley polarization of the holes exceeding 1%.
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http://dx.doi.org/10.1021/acs.nanolett.8b03838DOI Listing
February 2019

An electrochemical thermal transistor.

Nat Commun 2018 10 30;9(1):4510. Epub 2018 Oct 30.

Department of Materials Science and Engineering, Stanford University, Stanford, CA, 94305, USA.

The ability to actively regulate heat flow at the nanoscale could be a game changer for applications in thermal management and energy harvesting. Such a breakthrough could also enable the control of heat flow using thermal circuits, in a manner analogous to electronic circuits. Here we demonstrate switchable thermal transistors with an order of magnitude thermal on/off ratio, based on reversible electrochemical lithium intercalation in MoS thin films. We use spatially-resolved time-domain thermoreflectance to map the lithium ion distribution during device operation, and atomic force microscopy to show that the lithiated state correlates with increased thickness and surface roughness. First principles calculations reveal that the thermal conductance modulation is due to phonon scattering by lithium rattler modes, c-axis strain, and stacking disorder. This study lays the foundation for electrochemically-driven nanoscale thermal regulators, and establishes thermal metrology as a useful probe of spatio-temporal intercalant dynamics in nanomaterials.
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http://dx.doi.org/10.1038/s41467-018-06760-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6207649PMC
October 2018

High-Field Transport and Velocity Saturation in Synthetic Monolayer MoS.

Nano Lett 2018 07 29;18(7):4516-4522. Epub 2018 Jun 29.

Two-dimensional semiconductors such as monolayer MoS are of interest for future applications including flexible electronics and end-of-roadmap technologies. Most research to date has focused on low-field mobility, but the peak current-driving ability of transistors is limited by the high-field saturation drift velocity, v. Here, we measure high-field transport as a function of temperature for the first time in high-quality synthetic monolayer MoS. We find that in typical device geometries (e.g. on SiO substrates) self-heating can significantly reduce current drive during high-field operation. However, with measurements at varying ambient temperature (from 100 to 300 K), we extract electron v = (3.4 ± 0.4) × 10 cm/s at room temperature in this three-atom-thick semiconductor, which we benchmark against other bulk and layered materials. With these results, we estimate that the saturation current in monolayer MoS could exceed 1 mA/μm at room temperature, in digital circuits with near-ideal thermal management.
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http://dx.doi.org/10.1021/acs.nanolett.8b01692DOI Listing
July 2018

Unipolar n-Type Black Phosphorus Transistors with Low Work Function Contacts.

Nano Lett 2018 05 12;18(5):2822-2827. Epub 2018 Apr 12.

Department of Electrical Engineering , Stanford University , Stanford , California 94305 , United States.

Black phosphorus (BP) is a promising two-dimensional (2D) material for nanoscale transistors, due to its expected higher mobility than other 2D semiconductors. While most studies have reported ambipolar BP with a stronger p-type transport, it is important to fabricate both unipolar p- and n-type transistors for low-power digital circuits. Here, we report unipolar n-type BP transistors with low work function Sc and Er contacts, demonstrating a record high n-type current of 200 μA/μm in 6.5 nm thick BP. Intriguingly, the electrical transport of the as-fabricated, capped devices changes from ambipolar to n-type unipolar behavior after a month at room temperature. Transmission electron microscopy analysis of the contact cross-section reveals an intermixing layer consisting of partly oxidized metal at the interface. This intermixing layer results in a low n-type Schottky barrier between Sc and BP, leading to the unipolar behavior of the BP transistor. This unipolar transport with a suppressed p-type current is favorable for digital logic circuits to ensure a lower off-power consumption.
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http://dx.doi.org/10.1021/acs.nanolett.7b05192DOI Listing
May 2018

Probing the Optical Properties and Strain-Tuning of Ultrathin MoW Te.

Nano Lett 2018 04 29;18(4):2485-2491. Epub 2018 Mar 29.

SLAC National Accelerator Laboratory , Menlo Park , California 94025 , United States.

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin MoWTe, a semiconducting alloy of MoTe, using Raman, photoluminescence (PL), and optical absorption spectroscopy. MoWTe transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe counterpart. We apply tensile strain, for the first time, to monolayer MoTe and MoWTe to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton-phonon intervalley scattering. Strained MoTe and MoWTe extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.
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http://dx.doi.org/10.1021/acs.nanolett.8b00049DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7243468PMC
April 2018

Microstructural origin of resistance-strain hysteresis in carbon nanotube thin film conductors.

Proc Natl Acad Sci U S A 2018 02 12;115(9):1986-1991. Epub 2018 Feb 12.

Department of Mechanical Engineering, Stanford University, Stanford, CA 94305

A basic need in stretchable electronics for wearable and biomedical technologies is conductors that maintain adequate conductivity under large deformation. This challenge can be met by a network of one-dimensional (1D) conductors, such as carbon nanotubes (CNTs) or silver nanowires, as a thin film on top of a stretchable substrate. The electrical resistance of CNT thin films exhibits a hysteretic dependence on strain under cyclic loading, although the microstructural origin of this strain dependence remains unclear. Through numerical simulations, analytic models, and experiments, we show that the hysteretic resistance evolution is governed by a microstructural parameter [Formula: see text] (the ratio of the mean projected CNT length over the film length) by showing that [Formula: see text] is hysteretic with strain and that the resistance is proportional to [Formula: see text] The findings are generally applicable to any stretchable thin film conductors consisting of 1D conductors with much lower resistance than the contact resistance in the high-density regime.
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http://dx.doi.org/10.1073/pnas.1717217115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5834699PMC
February 2018
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