Publications – 2012

K. L. Shepard, T. Ito, and A. J. Griffith, “Extractin energy from the inner ear,” Nature Biotechnology 30, 1204-1205 (2012)

N. Sturcken, M. Petracca, S. Warren, P. Mantovani, L. P. Carloni, A. V. Peterchev, and K. L. Shepard, “A Switched-Inductor Integrated Voltage Regulator With Nonlinear Feedback and Network-on-Chip Load in 45 nm SOI,” IEEE Journal of Solid-State Circuits, Vol. 48, No. 8, August 2012.

A four-phase integrated buck converter in 45 nm silicon-on-insulator (SOI) technology is presented. The controller uses unlatched pulse-width modulation (PWM) with nonlinear gain to provide both stable small-signal dynamics and fast response (700 ps) to large input and output transients. This fast control approach reduces the required output capacitance by 5 in comparison to a conventional, latched PWM controller at a similar operating point. The converter switches package-integrated air-core inductors at 80 MHz and delivers 1 A/mm at 83% efficiency and 0.66 conversion ratio. A network-on-chip (NoC) serves as a realistic digital load along with a programmable current source capable of generating load current steps with slew rate of 1 A/100 ps for characterization of the control scheme.

N. Petrone, C. R. Dean, I. Meric, A. M. van der Zande, P. Y. Huang, L. Wang, D. Muller, K. L. Shepard, and J. Hone, “Chemical Vapor Deposition-Derived Graphene with Electrical Performance of Exfoliated Graphene,” Nano Letters,

While chemical vapor deposition (CVD) promises a scalable method to produce large-area graphene, CVD-grown graphene has heretofore exhibited inferior electronic properties in comparison with exfoliated samples. Here we test the electrical transport properties of CVD-grown graphene in which two important sources of disorder, namely grain boundaries and processing-induced contamination, are substantially reduced. We grow CVD graphene with grain sizes up to 250 μm to abate grain boundaries, and we transfer graphene utilizing a novel, dry-transfer method to minimize chemical contamination. We fabricate devices on both silicon dioxide and hexagonal boron nitride (h-BN) dielectrics to probe the effects of substrate-induced disorder. On both substrate types, the large-grain CVD graphene samples are comparable in quality to the best reported exfoliated samples, as determined by low-temperature electrical transport and magnetotransport measurements. Small-grain samples exhibit much greater variation in quality and inferior performance by multiple measures, even in samples exhibiting high field-effect mobility. These results confirm the possibility of achieving high-performance graphene devices based on a scalable synthesis process.

N. Wang, E. J. O’Sullivan, P. Herget, B. Rajendran, L. E. Krupp, L. T. Romankiw, B. C. Webb, R. Fontana, E. A. Duch, E. A. Joseph, S. L. Brown, X. Hu, G. M. Decad, N. Sturcken, K. L. Shepard, and W. J. Gallagher, “Integrated on-chip inductors with electroplated magnetic yokes” (invited), J. Appl. Phys. 111, 07E732 (2012), DOI:10.1063/1.3679458

Thin-film ferromagnetic inductors show great potential as the energy storage element for integrated circuits containing on-chip power management. In order to achieve the high energy storage required for power management, on-chip inductors require relatively thick magnetic yoke materials (several microns or more), which can be readily deposited by electroplating through a photoresist mask as demonstrated in this paper, the yoke material of choice being Ni45Fe55, whose properties of relatively high moment and electrical resistivity make it an attractive model yoke material for inductors. Inductors were designed with a variety of yoke geometries, and included both single-turn and multi-turn coil designs, which were fabricated on 200mm silicon wafers in a CMOS back-end-of-line (BEOL) facility. Each inductor consisted of electroplated copper coils enclosed by the electroplated Ni45Fe55 yokes; aspects of the fabrication of the inductors are discussed. Magnetic properties of the electroplated yoke materials are described, including high frequency permeability measurements. The inductance of 2-turn coil inductors, for example, was enhanced up to about 6 times over the air core equivalent, with an inductance density of 130 nH/mm2 being achieved. The resistance of these non-laminated inductors was relatively large at high frequency due to magnetic and eddy current losses but is expected to improve as the yoke material/structure is further optimized, making electroplated yoke-containing inductors attractive for dc-dc power converters.

J. K. Rosenstein, M. Wanunu, C. A. Merchant, M. Drndic, and K. L. Shepard, “Integrated nanopore sensing platform with sub-microsecond temporal resolution,” doi:10.1038/nmeth.1932, March 18, 2012.

Nanopore sensors have attracted considerable interest for high-throughput sensing of individual nucleic acids and proteins without the need for chemical labels or complex optics. A prevailing problem in nanopore applications is that the transport kinetics of single biomolecules are often faster than the measurement time resolution. Methods to slow down biomolecular transport can be troublesome and are at odds with the natural goal of high-throughput sensing. Here we introduce a low-noise measurement platform that integrates a complementary metal-oxide semiconductor (CMOS) preamplifier with solid-state nanopores in thin silicon nitride membranes. With this platform we achieved a signal-to-noise ratio exceeding five at a bandwidth of 1MHz, which to our knowledge is the highest bandwidth nanopore recording to date. We demonstrate transient signals as brief as 1μs from short DNA molecules as well as current signatures during molecular passage events that shed light on submolecular DNA configurations in small nanopores.

M. L. Johnston, H. Edrees, I. Kymissis, and K. L. Shepard, “Integrated VOC Vapor Sensing on FBAR-CMOS Array,” The 25th IEEE International Conference on Micro Electro Mechanical Systems (MEMS 2012), pp. 846-849, 2012.

This paper reports first results of volatile organic compound (VOC) detection on a monolithically integrated film bulk acoustic resonator (FBAR) array on a silicon integrated circuit substrate. The combined sensor platform uses thin polymer layers as gas absorbers for individual FBAR functionalization, and frequency shifts are measured on-chip in response to changing VOC concentration. Integrating sensors, drive, and read- out functionality on a single CMOS die enables a robust, multiplex sensor platform and obviates external measurement equipment.

N. Sturcken, E. O’Sullivan, N. Wang, P. Herget, B. Webb, L. Romankiw, M. Petracca, R. Davies, R. Fontana, G. Decad, I. Kymissis, A. Peterchev, L. Carloni, W. Gallagher, and K. L. Shepard, “A 2.5D Integrated Voltage Regulator Using Coupled- Magnetic-Core Inductors on Silicon Interposer Delivering 10.8A/mm2” Proceedings to the International Solid-State Circuits Conference (ISSCC), 2012.

Energy consumption is a dominant constraint on the performance of modern microprocessors and systems-on-chip. Dynamic voltage and frequency scaling (DVFS) is a promising technique for performing “on-the-fly” energy-performance optimization in the presence of workload variability. Effective implementation of DVFS requires voltage regulators that can provide many independent power supplies and can transition power supply levels on nanosecond timescales, which is not possible with modern board-level voltage regulator modules (VRMs) [1]. Switched-inductor integrated voltage regulators (IVRs) can enable effective implementation of DVFS, eliminating the need for separate VRMs and reducing power distribution network (PDN) impedance requirements by performing dc-dc conversion close to the load while supporting high peak current densities [2-3]. The primary obstacle facing development of IVRs is integration of suitable power inductors. This work presents an early prototype switched-inductor IVR using 2.5D chip stacking for inductor integration.

N. Sturcken, R. Davies, C. Cheng, W. E. Bailey and K. L. Shepard “Design of Coupled Power Inductors with Crossed Anisotropy Magnetic Core for Integrated Power Conversion” Proceedings to the Applied Power Electronics Conference (APEC), 2012.

Design and partial microfabrication of a coupled power inductor is presented for use in high power-density integrated voltage regulators (IVR). The proposed inductor uses many laminations of uniaxial, high-permeability magnetic material where the orientation of anisotropy between successive laminations is rotated to provide an effectively isotropic core. The high permeability core allows for an inductance density of 200nH/mm2, while coupling between phases prevents magnetic saturation and allows a current density as high as 11A/mm2 according to quasi-static finite-element-analysis (FEA) simulations. The coupling factor, inductance and resistance of the device are optimized for operation in a four-phase integrated buck converter switching at 100MHz.

C. Cheng, N. Sturcken, K. Shepard, W. E. Bailey, “Vector control of induced magnetic anisotropy using an in situ quadrupole electromagnet in ultrahigh vacuum sputtering, Review of Scientific Instruments,” 2012, pp. 063903 – 063903-4

J. Chae, S. Jung, A. F. Young, C. R. Dean, L. Wang, Y. Gao, K. Watanabe, T. Taniguchi, J. Hone, K. L. Shepard, P. Kim, N. B. Zhitenev, and J. A. Stroscio, “Renormalization of the Graphene Dispersion Velocity Determined from Scanning Tunneling Spectroscopy,” Phys. Rev. Lett. 109, 116802 (2012)

In graphene, as in most metals, electron-electron interactions renormalize the properties of electrons but leave them behaving like noninteracting quasiparticles. Many measurements probe the renormalized properties of electrons right at the Fermi energy. Uniquely for graphene, the accessibility of the electrons at the surface offers the opportunity to use scanned probe techniques to examine the effect of interactions at energies away from the Fermi energy, over a broad range of densities, and on a local scale. Using scanning tunneling spectroscopy, we show that electron interactions leave the graphene energy dispersion linear as a function of excitation energy for energies within 200 meV of the Fermi energy. However, the measured dispersion velocity depends on density and increases strongly as the density approaches zero near the charge neutrality point, revealing a squeezing of the Dirac cone due to interactions.

C. Dean, A .F. Young, L. Wang, I. Meric, G.-H. Lee, K. Watanabe, T. Taniguchi, K. Shepard, P. Kim, J. Hone, “Graphene Based Heterostructures,” Solid State Communications 152, 1275-1282 (2012)

The two dimensional charge carriers in monolayer and bilayer graphene are described by massless and massive chiral Dirac Hamiltonians, respectively. These two-dimensional materials are predicted to exhibit a wide range of behavior, etc. However, graphene devices on a typical three-dimensional insulating substrates such as SiO2 are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. We have developed a novel technique for substrate engineering of graphene devices using layered dielectric materials to build graphene based vertical heterostructures. We employ hBN, an insulating isomorph of graphite, as a substrate and gate dielectric for graphen eelectronics. In this review, we describe the fabrication and characterization of high-quality exfoliated mono-and bilayer graphene devices on single-crystal hBN substrates, using a mechanical transfer process. Graphene devices on hBN substrates have mobilities and carrier in homogeneities that are almost an order of magnitude better than devices on SiO2. We use the enhanced mobility of electrons in hBN supported graphene to investigate the effects of electronic interactions. We find that interactions drive spontaneous breaking of the emergent SU(4) symmetry of the graphene Landau levels, leading to a variety of non-trivial integer and fractional quantum Hall states. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex grapheme heterostructures.

A. F. Young, C. R. Dean, I. Meric, S. Sorgenfrei, H. Ren, K. Watanabe, T. Taniguchi, J. Hone, K. L. Shepard, and P. Kim, “Electronic compressibility of gapped bilayer graphene,” Phys. Rev. B 85, 235458 (2012)

We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a one-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. ForD > 0, the additional peak is observed only for electrons, while forD < 0 the peak appears only for holes. This asymmetry can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.

A. F. Young, C. R. Dean, L. Wang, H. Ren, P. Cadden-Zimansky, K. Watanabe, T. Taniguchi, J. Hone, K. L. Shepard, and P. Kim, “Spin and valley quantum Hall ferromagnetism in graphene,” Nature Physics, 8, 553-556 (2012)

Electronic systems with multiple degenerate degrees of freedom can support a rich variety of broken symmetry states. In a graphene Landau level (LL), strong Coulomb interactions and the fourfold spin–valley degeneracy lead to an approximate SU(4) isospin symmetry. At partial filling, exchange interactions can break this symmetry, manifesting as further Hall plateaus outside the normal integer sequence. Here we report the observation of a number of these quantum Hall isospin ferromagnetic (QHIFM) states, which we classify according to their real spin structure using tilted field magnetotransport. The large activation gaps confirm the Coulomb origin of all the broken symmetry states, but the order depends strongly on LL index. In the high-energy LLs the Zeeman effect is the dominant aligning field, leading to real spin ferromagnets hosting skyrmionic excitations at half filling, whereas in the ‘relativistic’ zero LL lattice scale interactions drive the system to a spin unpolarized state.