Publications – 2013

This paper presents a compact virtual source (VS) model to describe carrier transport valid in both unipolar and ambipolar transport regimes in quasi-ballistic graphene fieldeffect transistors (GFETs). The model formulation allows for an easy extension to bi-layer graphene transistors, where a bandgap can be opened. The model also includes descriptions of intrinsic terminal charges/capacitances obtained selfconsistently with the transport formulation. The charge model extends from drift-diffusive transport regime to ballistic transport regime, where gradual-channel approximation (GCA) fails. The model is calibrated exhaustively against DC and S-parameter measurements of GFETs. To demonstrate the model capability for circuit-level simulations, the Verilog-A implementation of the model is used to simulate the dynamic response of frequency doubling circuits with GFETs operating in the ambipolar regime.

Neuroscience is at a crossroads. Great effort is being invested into deciphering specific neural interactions and circuits. At the same time, there exist few general theories or principles that explain brain function. We attribute this disparity, in part, to limitations in current methodologies. Traditional neurophysiological approaches record the activities of one neuron or a few neurons at a time. Neurochemical approaches focus on single neurotransmitters. Yet, there is an increasing realization that neural circuits operate at emergent levels, where the interactions between hundreds or thousands of neurons, utilizing multiple chemical transmitters, generate functional states. Brains function at the nanoscale, so tools to study brains must ultimately operate at this scale, as well. Nanoscience and nanotechnology are poised to provide a rich toolkit of novel methods to explore brain function by enabling simultaneous measurement and manipulation of activity of thousands or even millions of neurons. We and others refer to this goal as the Brain Activity Mapping Project. In this Nano Focus, we discuss how recent developments in nanoscale analysis tools and in the design and synthesis of nanomaterials have generated optical, electrical, and chemical methods that can readily be adapted for use in neuroscience. These approaches represent exciting areas of technical development and research. Moreover, unique opportunities exist for nanoscientists, nanotechnologists, and other physical scientists and engineers to contribute to tackling the challenging problems involved in understanding the fundamentals of brain function.

In this work we present room-temperature measurements of graphene nanoelectromechanical resonators (GN-ERs) demonstrating quality factors (Qs) greater than 200 at resonance. A nominal resonant frequency (f_o) of 200 MHz is attained by applying strain to the suspended graphene using an SU-8 polymer clamp. Additionally, the device f_o can be tuned by more than 5% by application of a DC gate bias on the order of 5V. Chemical vapor deposited (CVD) graphene is used to demonstrate the scalability of the process.

Oscillators, which produce continuous periodic signals from direct current power, are central to modern communications systems, with versatile applications including timing references and frequency modulators. However, conventional oscillators typically consist of macroscopic mechanical resonators such as quartz crystals, which require excessive off-chip space. Here, we report oscillators built on micrometre-size, atomically thin graphene nanomechanical resonators, whose frequencies can be electrostatically tuned by as much as 14%. Self-sustaining mechanical motion is generated and transduced at room temperature in these oscillators using simple electrical circuitry. The prototype graphene voltage-controlled oscillators exhibit frequency stability and a modulation bandwidth sufficient for the modulation of radiofrequency carrier signals. As a demonstration, we use a graphene oscillator as the active element for frequency-modulated signal generation and achieve efficient audio signal transmission.

Here we discuss the limits to temporal resolution in nanopore sensor recordings, which arise from considerations of both small-signal frequency response and accumulated noise power. Nanopore sensors have strong similarities to patch-clamp ion channel recordings, except that the magnitudes of many physical parameters are substantially different. We will present examples from our recent work developing high-speed nanopore sensing platforms, in which we physically integrated nanopores with custom low-noise complementary metal-oxide-semiconductor (CMOS) circuitry. Close physical proximity of the sensor and amplifier electronics can reduce parasitic capacitances, improving both the signal-to-noise ratio and the effective temporal resolution of the recordings.

Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.

A wide-field fluorescence lifetime imager capable of up to 100 frames per second (fps) is presented. The imager consists of a64-by-64 array of low-noise single photon avalanche diodes (SPADs) in a standard 0.13-micron CMOS process, 4096 time-to- digital converters, and an application specific data path to enable continuous image acquisition at a total output data rate of 42 Gbps. These features combine to enable new lifetime-based diagnostic imaging.

Four-turn coupled (flux-closed) inductor structures have been fabricated and tested for use in high-power-density integrated voltage regulator (IVR) applications. Our solenoid-like structure is comprised of a laminated magnetic core of four rungs surrounded by four Cu windings creating four coupled inductors. The magnetic core is made of laminations of ultra-high-vacuum-sputtered [5 nm Ta/200 nm Co_91.5Zr_4.0Ta_4.5 (CZT)/7 nm SiO₂ ] repeated 20 times. Individual CZT layers are deposited under a magnetic bias to induce uniaxial anisotropy. The quad-coupled inductor shows a frequency response with a measured self-inductance of 7.4 nH for one inductor sustained up to 100 MHz and roll-off to half this low-frequency value at ~450MHz. This inductance is more than 65X higher than what would be calculated from an air-core inductor of equivalent geometry.

The high-frequency characteristics of graphene field-effect transistors (GFETs) has received significant interest due the very high carrier velocities in graphene. In addition to excellent electronic performance, graphene possesses exceptional mechanical properties such as high flexibility and strength. Here, we demonstrate the potential of flexible-GFETs and show that the combination of electrical and mechanical advantages of graphene result in gigahertz-frequency operation at strain values up to 2%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%.

Two-dimensional atomic sheets of graphene represent a new class of nanoscale materials with potential applications in electronics. However, exploiting the intrinsic characteristics of graphene devices has been problematic due to impurities and disorder in the surrounding dielectric and graphene/dielectric interfaces. Recent advancements in fabricating graphene heterostructures by alternately layering graphene with crystalline hexagonal boron nitride (hBN), its insulating isomorph, have led to an order of magnitude improvement in graphene device quality. Here, recent developments in graphene devices utilizing boron–nitride dielectrics are reviewed. Field-effect transistor (FET) characteristics of these systems at high bias are examined. Additionally, existing challenges in material synthesis and fabrication and the potential of graphene/BN heterostructures for novel electronic applications are discussed.

An integrated lab-on-chip capable of performing quantitative polymerase chain reaction (qPCR) is demonstrated in a high-voltage 0.35-μm CMOS process operating at a 3.3 V supply. PCR thermal cycling can be performed by physically moving droplets between three distinct temperature zones on the surface of chip or by thermal cycling a droplet in place. Droplet actuation is enabled by electrowetting-on-dielectric transport at 90 V. On-chip temperature regulation to 0.15°C is performed with on-chip resistive heaters and temperature sensors. PCR cycles are monitored by measuring the fluorescence signal of an intercalator dye using integrated single photon avalanche diodes (SPADs). Results are demonstrated for the recognition of DNA extracts from Staphylococcus aureus (S. aureus) at a detection limit of a few copies per nL target volume.

Electrons moving through a spatially periodic lattice potential develop a quantized energy spectrum consisting of discrete Bloch bands. In two dimensions, electrons moving through a magnetic field also develop a quantized energy spectrum, consisting of highly degenerate Landau energy levels. When subject to both a magnetic field and a periodic electrostatic potential, two-dimensional systems of electrons exhibit a self-similar recursive energy spectrum1.Known as Hofstadter’s butterfly, this complex spectrum results from an interplay between the characteristic lengths associated with the two quantizing fields, and is one of the first quantum fractals discovered in physics. In the decades since its prediction, experimental attempts to study this effect have been limited by difficulties in reconciling the two length scales. Typical atomic lattices (with periodicities of less than one nanometre) require unfeasibly large magnetic fields to reach the commensurability condition, and in artificially engineered structures (with periodicities greater than about 100 nanometres) the corresponding fields are too small to overcome disorder completely. Herewe demonstrate thatmoire´ superlattices arising in bilayer graphene coupled to hexagonal boron nitride provide a periodicmodulationwith ideal length scales of the order of ten nanometres, enabling unprecedented experimental access to the fractal spectrum. We confirm that quantum Hall features associated with the fractal gaps are described by two integer topological quantum numbers, and report evidence of their recursive structure. Observation of a Hofstadter spectrum in bilayer graphene means that it is possible to investigate emergent behaviour within a fractal energy landscape in a system with tunable internal degrees of freedom.

In the last two decades, new techniques that monitor ionic current modulations as single molecules pass through a nanoscale pore have enabled numerous single-molecule studies. While biological nanopores have recently shown the ability to resolve single nucleotides within individual DNA molecules, similar developments with solid-state nanopores have lagged, due to challenges both in fabricating stable nanopores of similar dimensions as biological nanopores and in achieving sufficiently low-noise and high-bandwidth recordings. Here we show that small silicon nitride nanopores (0.8- to 2-nm diameter in 5- to 8-nm-thick membranes) can resolve differences between ionic current signals produced by short (30 base) ssDNA homopolymers (poly(dA), poly(dC), poly(dT)), when combined with measurement electronics that allow a signal-to-noise ratio of better than 10 to be achieved at 1-MHz bandwidth. While identifying intramolecular DNA sequences with silicon nitride nanopores will require further improvements in nanopore sensitivity and noise levels, homopolymer differentiation represents an important milestone in the development of solid-state nanopores.

We present single-ion-channel recordings performed with biomimetic lipid membranes which are directly attached to the surface of a complementary metal−oxide−semiconductor (CMOS) preamplifier chip. With this system we resolve single-channel currents from several types of bacterial ion channels, including fluctuations of a single alamethicin channel at a bandwidth of 1 MHz which represent the fastest single-ion-channel recordings reported to date. The platform is also used for high-resolution α-hemolysin nanopore recordings. These results illustrate the high signal fidelity, fine temporal resolution, small geometry, and multiplexed integration which can be achieved by leveraging integrated semiconductor platforms for advanced ion channel interfaces.

An on-chip variability characterization system implemented in a 45-nm CMOS process is used for direct time-domain measurements of random telegraph noise (RTN) in small-area devices. A procedure for automated extraction of RTN parameters from large volumes of measured data is developed. Statistics for number of traps, N_T, and single-trap amplitudes, ΔV_th, are studied across device polarity, bias, and gate area. A Poisson distribution is used to model N_T and a log-normal distribution is used to model ΔV_th. The scaling of the two statistics across gate dimensions is discussed; the expected value of N_T is shown to scale with (L −ΔL)⁻¹, whereas the expected value of ΔV_th is shown to scale with W⁻¹(L −ΔL)^−0.5. The two statistics are combined in a compact RTN probabilistic model representing the statistics of the overall ΔV_th fluctuations because of RTN. This model is demonstrated to give accurate predictions of the tails of the measured RTN distributions at the 95th percentile level, which scale with W⁻¹(L −ΔL)^−1.5. A comparison between nMOS and pMOS devices shows that pMOS devices exhibit both a higher average number of traps and a larger average single-trap ΔV_th amplitude, leading to a comparatively larger overall impact of RTN.

We show the optimization of magnetic properties of ferromagnetic (FM)/SiO2/FM trilayer structures as potential candidates for the magnetic core in toroidal integrated inductors, with FM materials Co91.5Zr4.0Ta4.5 (CZT) and Ni80Fe20 (Py). In the single-layer parent films, we found a monotonic reduction of easy-axis coercivity (Hc down to 0.17 Oe in CZT, 0.4Oe in Py) with increasing dc magnetron sputtering voltage. In the trilayer rectangular structures, with induced easy-axis in the short lateral dimension, we found proof of dipolar coupling between the two FM layers from BH loop measurements in the CZT system, showing linear response with minimal hysteresis loss when the external field is applied in the long axis. Py elements did not show this optimized property. Further investigation of domain configurations using scanning transmission x-ray microscopy suggests an insufficient induced anisotropy in Py compared with the shape anisotropy to realize the antiparallel-coupled state.

An on-chip system for combined capacitance-voltage (C-V) and current-voltage (I-V) characterization of a large integrated transistor array implemented in a 45-nm bulk CMOS process is presented. On-chip I-V characterization is implemented using a four-point Kelvin measurement technique with 12-bit sub-10 nA current measurement resolution, 10-bit sub-1 mV voltage measurement resolution, and sampling speeds on the order of 100 kHz. C-V characterization is performed using a novel leakage- and parasitics-insensitive charge-based capacitance measurement (CBCM) technique with atto-Farad resolution. The on-chip system is employed in studying both random and systematic sources of quasi-static device variability. For the first time, combined C-V/I-V characterization of circuit-representative devices is demonstrated and used to extract variations in the underlying physical characteristics of the device, including line-edge-roughness (LER) parameters and systematic device length variations across the die.

The development of flexible electronics operating at radiofrequencies (RF) requires materials that combine excellent electronic performance and the ability to withstand high levels of strain. In this work, we fabricate graphene field-effect transistors (GFETs) on flexible substrates from graphene grown by chemical vapor deposition (CVD). Our devices demonstrate unity-current-gain frequencies, f T, and unity-power-gain frequencies, f max, up to 10.7 GHz and 3.7 GHz, respectively, with strain limits of 1.75%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%. As such, they demonstrate the potential of CVD graphene to enable a broad range of flexible electronic technologies which require both high flexibility and RF operation.

An integrated voltage regulator (IVR) is presented that uses custom fabricated thin-film magnetic power inductors. The inductors are fabricated on a silicon interposer and integrated with a multi-phase buck converter IC by 2.5D chip stacking. Several inductor design variations have been fabricated and tested. The best performance has been achieved with a set of eight coupled inductors that each occupies 0.245 mm² and provides 12.5 nH with 270 mΩ DC. With early inductor prototypes, the IVR efficiency for a 1.8 V:1.0 V conversion ratio peaks at 71% with FEOL current density of 10.8 A/mm² and inductor current density of 1.53 A/mm² . At maximum load current, 69% conversion efficiency and 1.8 V:1.2 V conversion ratio the FEOL current density reaches 22.6 A/mm² and inductor current density reaches 3.21 A/mm² .