arXiv subjects

Steven J. Koester

Publications and source records attributed to Steven J. Koester.

32 records · Page 2Linked to original sources

arXiv2017

Using Programmable Graphene Channels as Weights in Spin-Diffusive Neuromorphic Computing

A graphene-based spin-diffusive (GrSD) neural network is presented in this work that takes advantage of the locally tunable spin transport of graphene and the non-volatility of nanomagnets. By using electrostatically gated graphene as spintronic synapses, a weighted summation operation can be performed in the spin domain while the weights can be programmed using circuits in the charge domain. Four-component spin/charge circuit simulations coupled to magnetic dynamics are used to show the feasibility of the neuron-synapse functionality and quantify the analog weighting capability of the graphene under different spin relaxation mechanisms. By realizing transistor-free weight implementation, the graphene spin-diffusive neural network reduces the energy consumption to 0.08-0.32 fJ per cell-synapse and achieves significantly better scalability compared to its digital counterparts, particularly as the number and bit accuracy of the synapses increases.

physics.app-ph↗

arXiv2017

Semi-analytical model of the contact resistance in two-dimensional semiconductors

Contact resistance is a severe performance bottleneck for electronic devices based on two-dimensional layered (2D) semiconductors, whose contacts are Schottky rather than Ohmic. Although there is general consensus that the injection mechanism changes from thermionic to tunneling with gate biasing, existing models tend to oversimplify the transport problem, by neglecting the 2D transport nature and the modulation of the Schottky barrier height, the latter being of particular importance in back-gated devices. In this work, we develop a semi-analytical model based on Bardeen's transfer Hamiltonian approach to describe both effects. Remarkably, our model is able to reproduce several experimental observations of a metallic behavior in the contact resistance, i.e., a decreasing resistance with decreasing temperature, occurring at high gate voltage.

cond-mat.mes-hall↗

arXiv2016

Band Alignment of 2D Semiconductors for Designing Heterostructures with Momentum Space Matching

We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II and III heterojunctions; an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center. Our study, which includes the Group IV and III-V compound monolayer materials, Group V elemental monolayer materials, transition metal dichalcogenides (TMD) and transition metal trichalcogenides (TMT) reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterojunction for 903 different possible combination of these 2D materials which establishes a periodic table of heterojunctions.

cond-mat.mtrl-sci↗

arXiv2016

Symmetric Complementary Logic Inverter Using Integrated Black Phosphorus and MoS2 Transistors

The operation of an integrated two-dimensional complementary metal-oxide-semiconductor inverter with well-matched input/output voltages is reported. The circuit combines a few-layer MoS2 n-MOSFET and a black phosphorus (BP) p-MOSFET fabricated using a common local backgate electrode with thin (20 nm) HfO2 gate dielectric. The constituent devices have linear threshold voltages of -0.8 V and +0.8 V and produce peak transconductances of 16 uS/um and 41 uS/um for the MoS2 n-MOSFET and BP p-MOSFET, respectively. The inverter shows a voltage gain of 3.5 at a supply voltage, VDD = 2.5 V, and has peak switching current of 108 uA and off-state current of 8.4 uA (2.4 uA) at VIN = 0 (VIN = 2.5 V). In addition, the inverter has voltage gain greater than unity for VDD > 0.5 V, has open butterfly curves for VDD > 1 V, and achieves static noise margin over 500 mV at VDD = 2.5 V. The voltage gain was found to be insensitive to temperature between 270 K and 340 K, and AC large and small-signal operation was demonstrated at frequencies up to 100 kHz. The demonstration of a complementary 2D inverter which operates in a symmetric voltage window suitable for driving a subsequent logic stage is a significant step forward in developing practical applications for devices based upon 2D materials.

cond-mat.mes-hall↗

arXiv2015

Ambipolar Black Phosphorus MOSFETs with Record n-Channel Transconductance

Ambipolar black phosphorus MOSFETs with record n-channel extrinsic transconductance are reported. The devices consist of multi-layer black phosphorus aligned to a local back-gate electrode with 10-nm-thick HfO2 gate dielectric. Before passivation, devices with 0.3-um gate length behaved as p-MOSFETs with peak extrinsic transconductance, gm, of 282 uS/um at VDS = -2 V. After passivation, the same devices displayed ambipolar behavior, and when tested as n-MOSFETs, had peak gm = 66 uS/um at VDS = +2 V, and similar devices on the same wafer had gm as high as 80 uS/um. These results are an important step toward realization of high-performance black phosphorus complementary logic circuits.

cond-mat.mes-hall↗

arXiv2015

Capacitive Sensing of Intercalated H2O Molecules Using Graphene

Understanding the interactions of ambient molecules with graphene and adjacent dielectrics is of fundamental importance for a range of graphene-based devices, particularly sensors, where such interactions could influence the operation of the device. It is well-known that water can be trapped underneath graphene and its host substrate, however, the electrical effect of water beneath graphene and the dynamics of how it changes with different ambient conditions has not been quantified. Here, using a metal-oxide-graphene variable-capacitor (varactor) structure, we show that graphene can be used to capacitively sense the intercalation of water between graphene and HfO2 and that this process is reversible on a fast time scale. Atomic force microscopy is used to confirm the intercalation and quantify the displacement of graphene as a function of humidity. Density functional theory simulations are used to quantify the displacement of graphene induced by intercalated water and also explain the observed Dirac point shifts as being due to the combined effect of water and oxygen on the carrier concentration in the graphene. Finally, molecular dynamics simulations indicate that a likely mechanism for the intercalation involves adsorption and lateral diffusion of water molecules beneath the graphene.

cond-mat.mes-hall↗

arXiv2014

Waveguide-integrated black phosphorus photodetector with high responsivity and low dark current

Layered two-dimensional materials have shown novel optoelectronic properties and are well suited to be integrated in planar photonic circuits. For example, graphene has been utilized for wideband photodetection. Because graphene lacks a band gap, however, graphene photodetectors suffer from very high dark current. In contrast, layered black phosphorous, the latest addition to the family of 2D materials, is well-suited for photodetector applications due to its narrow but finite band gap. Here, we demonstrate a gated multilayer black phosphorus photodetector integrated on a silicon photonic waveguide operating in the near-infrared telecom band. In a significant advantage over graphene devices, black phosphorus photodetectors can operate under a bias with very low dark current and attain intrinsic responsivity up to 135 mA/W and 657 mA/W in 11.5nm and 100 nm thick devices, respectively, at room temperature. The photocurrent is dominated by the photovoltaic effect with a high response bandwidth exceeding 3 GHz.

physics.optics↗

arXiv2014

Black Phosphorus p-MOSFETs with High Transconductance and Nearly Ideal Subthreshold Slope

We report record performance for black phosphorus p-MOSFETs. The devices have locally patterned back gates and 20-nm-thick HfO2 gate dielectrics. Devices with effective gate length, Leff = 1.0 um display extrinsic transconductance, gm, of 101 uS/um at a drain-to-source bias voltage, Vds = -3 V. Temperature-dependent analysis also shows that the subthreshold slope, SS, is nearly ideal, with a minimum value of SS = 66 mV/decade at room temperature and Vds = -0.1 V. Furthermore, devices with 7-nm HfO2 dielectrics and Leff = 0.3 um displayed gm as high as 204 uS/um at Vds = -1.5 V.

cond-mat.mes-hall↗

arXiv2014

Multifunctional graphene optical modulator and photodetector integrated on silicon waveguides

For optical communication, information is converted between optical and electrical signal domains at a high rate. The devices to achieve such a conversion are various types of electro-optical modulators and photodetectors. These two types of optoelectronic devices, equally important, require different materials and consequently it has been challenging to realize both using a single material combination, especially in a way that can be integrated on the ubiquitous silicon platform. Graphene, with its gapless band structure, stands out as a unique optoelectronic material that allows both photodetection and optical modulation. Here, we demonstrate a single graphene-based device that simultaneously provides both efficient optical modulation and photodetection. The graphene device is integrated on a silicon waveguide and is tunable with a gate made from another layer of graphene to achieve near-infrared photodetection responsivity of 57 mA/W and modulation depth of 64%. This novel multifunctional device may lead to many unprecedented optoelectronic applications.

physics.optics↗

arXiv2013

Graphene-based quantum capacitance wireless vapor sensors

A wireless vapor sensor based upon the quantum capacitance effect in graphene is demonstrated. The sensor consists of a metal-oxide-graphene variable capacitor (varactor) coupled to an inductor, creating a resonant oscillator circuit. The resonant frequency is found to shift in proportion to water vapor concentration for relative humidity (RH) values ranging from 1% to 97% with a linear frequency shift of 5.7 +- 0.3 kHz / RH%. The capacitance values extracted from the wireless measurements agree with those determined from capacitance-voltage measurements, providing strong evidence that the sensing arises from the variable quantum capacitance in graphene. These results represent a new sensor transduction mechanism and pave the way for graphene quantum capacitance sensors to be studied for a wide range of chemical and biological sensing applications.

cond-mat.mes-hall↗

arXiv2012

Optical absorption in graphene integrated on silicon waveguides

To fully utilize graphene's remarkable optical properties for optoelectronic applications, it needs to be integrated in planar photonic systems. Here, we demonstrate integration of graphene on silicon photonic circuits and precise measurement of the optical absorption coefficient in a graphene/waveguide hybrid structure. A method based on Mach-Zehnder interferometry is employed to achieve high measurement precision and consistency, yielding a maximal value of absorption coefficient of 0.2 dB/μm when graphene is located directly on top of the waveguide. The results agree with theoretical model utilizing the universal ac conductivity in graphene. Our work provides an important guide for the design and optimization of integrated graphene optoelectronic devices.

physics.optics↗

arXiv2012

High-speed waveguide-coupled graphene-on-graphene optical modulators

An electro-absorption optical modulator concept based upon a dual-graphene layer is presented. The device consists of a silicon-on-insulator waveguide upon which two graphene layers reside, separated by a thin insulating region. The lower graphene acts as a tunable absorber, while the upper layer functions as a transparent gate electrode. Calculations based upon realistic graphene material properties show that 3-dB bandwidths over 100 GHz (30 GHz) are possible at near- (λ=1.55 μm) and mid- (λ=3.5μm) infrared bands. The effect of background doping and potential fluctuations on the bandwidth, modulation depth and insertion loss are also quantified.

physics.optics↗

arXiv2010

On the possibility of obtaining MOSFET-like performance and sub-60 mV/decade swing in 1D broken-gap tunnel transistors

Tunneling field-effect transistors (TFETs) have gained a great deal of recent interest due to their potential to reduce power dissipation in integrated circuits. One major challenge for TFETs so far has been achieving high drive currents, which is a prerequisite for high-performance operation. In this paper we explore the performance potential of a 1D TFET with a broken-gap heterojunction source injector using dissipative quantum transport simulations based on the nonequilibrium Green's function formalism, and the carbon nanotube bandstructure as the model 1D material system. We provide detailed insights into broken-gap TFET (BG-TFET) operation, and show that it can indeed produce less than 60mV/decade subthreshold swing at room temperature even in the presence of electron-phonon scattering. The 1D geometry is recognized to be uniquely favorable due to its superior electrostatic control, reduced carrier thermalization rate, and beneficial quantum confinement effects that reduce the off-state leakage below the thermionic limit. Because of higher source injection compared to staggered-gap and homojunction geometries, BG-TFET delivers superior performance that is comparable to MOSFET's. BG-TFET even exceeds the MOSFET performance at lower supply voltages (VDD), showing promise for low-power/high-performance applications.

cond-mat.mes-hall↗

arXiv2010

Scalability of Atomic-Thin-Body (ATB) Transistors Based on Graphene Nanoribbons

A general solution for the electrostatic potential in an atomic-thin-body (ATB) field-effect transistor geometry is presented. The effective electrostatic scaling length, λeff, is extracted from the analytical model, which cannot be approximated by the lowest order eigenmode as traditionally done in SOI-MOSFETs. An empirical equation for the scaling length that depends on the geometry parameters is proposed. It is shown that even for a thick SiO2 back oxide λeff can be improved efficiently by thinner top oxide thickness, and to some extent, with high-k dielectrics. The model is then applied to self-consistent simulation of graphene nanoribbon (GNR) Schottky-barrier field-effect transistors (SB-FETs) at the ballistic limit. In the case of GNR SB-FETs, for large λeff, the scaling is limited by the conventional electrostatic short channel effects (SCEs). On the other hand, for small λeff, the scaling is limited by direct source-to-drain tunneling. A subthreshold swing below 100mV/dec is still possible with a sub-10nm gate length in GNR SB-FETs.

cond-mat.mes-hall↗