arXiv subjects

Emanuel Tutuc

Publications and source records attributed to Emanuel Tutuc.

47 records · Page 3Linked to original sources

arXiv2014

Chemical Potential and Quantum Hall Ferromagnetism in Bilayer Graphene

Bilayer graphene has a unique electronic structure influenced by a complex interplay between various degrees of freedom. We probe its chemical potential using double bilayer graphene heterostructures, separated by a hexagonal boron nitride dielectric. The chemical potential has a non-linear carrier density dependence, and bears signatures of electron-electron interactions. The data allow a direct measurement of the electric field-induced bandgap at zero magnetic field, the orbital Landau level (LLs) energies, and the broken symmetry quantum Hall state gaps at high magnetic fields. We observe spin-to-valley polarized transitions for all half-filled LLs, as well as emerging phases at filling factors ν= 0 and ν= +-2. Furthermore, the data reveal interaction-driven negative compressibility and electron-hole asymmetry in N = 0, 1 LLs.

cond-mat.mes-hall↗

arXiv2014

Atomistic simulation of the electronic states of adatoms in monolayer MoS2

Using an ab initio density functional theory (DFT) based electronic structure method, we study the effects of adatoms on the electronic properties of monolayer transition metal dichalcogenide (TMD) Molybdenum-disulfide (MoS2). We consider the 1st (Li, Na, K) and 7th (F, Cl, Br) column atoms and metals (Sc, Ti, Ta, Mo, Pd, Pt, Ag, Au). Three high symmetry sites for the adatom on the surface of monolayer MoS2 are examined as starting points to search for the most energetically stable configuration for each adatom-monolayer MoS2 system, as well as the type of associated bonding. For the most stable adatom positions, we characterize the emergence of adatom-induced electronic states including any dopant states.

cond-mat.mes-hall↗

arXiv2013

Silicon-based Infrared Metamaterials with Ultra-Sharp Fano Resonances

Metamaterials and meta-surfaces represent a remarkably versatile platform for light manipulation, biological and chemical sensing, nonlinear optics, and even spaser lasing. Many of these applications rely on the resonant nature of metamaterials, which is the basis for extreme spectrally selective concentration of optical energy in the near field. The simplicity of free-space light coupling into sharply-resonant meta-surfaces with high resonance quality factors Q>>1 is a significant practical advantage over the extremely angle-sensitive diffractive structures or inherently inhomogeneous high-Q photonic structures such as toroid or photonic crystal microcavities. Such spectral selectivity is presently impossible for the overwhelming majority of metamaterials that are made of metals and suffer from high plasmonic losses. Here, we propose and demonstrate Fano-resonant all-semiconductor optical meta-surfaces supporting optical resonances with quality factors Q>100 that are almost an order of magnitude sharper than those supported by their plasmonic counterparts. These silicon-based structures are shown to be planar chiral, opening exciting possibilities for efficient ultra-thin circular polarizers and narrow-band thermal emitters of circularly polarized radiation.

physics.optics↗

arXiv2012

Self-aligned graphene field-effect transistors with polyethyleneimine doped source/drain access regions

We report a method of fabricating self-aligned, top-gated graphene field-effect transistors (GFETs) employing polyethyleneimine spin-on-doped source/drain access regions, resulting in a 2X reduction of access resistance and a 2.5X improvement in device electrical characteristics, over undoped devices. The GFETs on Si/SiO$_2$ substrates have high carrier mobilities of up to 6,300 cm$^2$/Vs. Self-aligned spin-on-doping is applicable to GFETs on arbitrary substrates, as demonstrated by a 3X enhancement in performance for GFETs on insulating quartz substrates, which are better suited for radio frequency applications.

cond-mat.mes-hall↗

arXiv2012

Coulomb Drag and Magnetotransport in Graphene Double Layers

We review the fabrication and key transport properties of graphene double layers, consisting of two graphene monolayers placed in close proximity, independently contacted, and separated by an ultra-thin dielectric. We outline a simple band structure model relating the layer densities to the applied gate and inter-layer biases, and show that calculations and experimental results are in excellent agreement both at zero and in high magnetic fields. Coulomb drag measurements, which probe the electron-electron scattering between the two layers reveal two distinct regime: (i) diffusive drag at elevated temperatures, and (ii) mesoscopic fluctuation-dominated drag at low temperatures. We discuss the Coulomb drag results within the framework of existing theories.

cond-mat.mes-hall↗

arXiv2011

Spin-Polarized to Valley-Polarized Transition in Graphene Bilayers at $ν=0$ in High Magnetic Fields

We investigate the transverse electric field ($E$) dependence of the $ν$=0 quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic fields. The longitudinal resistivity ($ρ_{xx}$) measured at $ν$=0 shows an insulating behavior which is strongest in the vicinity of $E$=0, and at large $E$-fields. At a fixed perpendicular magnetic field ($B$), the $ν$=0 QHS undergoes a transition as a function of $E$, marked by a minimum, temperature-independent $ρ_{xx}$. This observation is explained by a transition from a spin polarized $ν$=0 QHS at small $E$-fields, to a valley (layer) polarized $ν$=0 QHS at large $E$-fields. The $E$-field value at which the transition occurs has a linear dependence on $B$

cond-mat.mes-hall↗

arXiv2010

Dielectric Thickness Dependence of Carrier Mobility in Graphene with HfO2 Top Dielectric

We investigate the carrier mobility in mono- and bi-layer graphene with a top HfO2 dielectric, as a function of the HfO2 film thickness and temperature. The results show that the carrier mobility decreases during the deposition of the first 2-4 nm of top dielectric and remains constant for thicker layers. The carrier mobility shows a relatively weak dependence on temperature indicating that phonon scattering does not play a dominant role in controlling the carrier mobility. The data strongly suggest that fixed charged impurities located in close proximity to the graphene are responsible for the mobility degradation.

cond-mat.mes-hall↗

arXiv2010

Lateral Spin Injection in Germanium Nanowires

Electrical injection of spin-polarized electrons into a semiconductor, large spin diffusion length, and an integration friendly platform are desirable ingredients for spin-based devices. Here we demonstrate lateral spin injection and detection in germanium nanowires, by using ferromagnetic metal contacts and tunnel barriers for contact resistance engineering. Using data measured from over 80 samples, we map out the contact resistance window for which lateral spin transport is observed, manifestly showing the conductivity matching required for spin injection. Our analysis, based on the spin diffusion theory, indicates that the spin diffusion length is larger than 100 μm in germanium nanowires at 4.2 K.

cond-mat.mes-hall↗

arXiv2009

Scaling Properties of Ge-SixGe1-x Core-Shell Nanowire Field Effect Transistors

We demonstrate the fabrication of high-performance Ge-SixGe1-x core-shell nanowire field-effect transistors with highly doped source and drain, and systematically investigate their scaling properties. Highly doped source and drain regions are realized by low energy boron implantation, which enables efficient carrier injection with a contact resistance much lower than the nanowire resistance. We extract key device parameters, such as intrinsic channel resistance, carrier mobility, effective channel length, and external contact resistance, as well as benchmark the device switching speed and ON/OFF current ratio.

cond-mat.mes-hall↗

arXiv2009

Realization of a High Mobility Dual-gated Graphene Field Effect Transistor with Al2O3 Dielectric

We fabricate and characterize dual-gated graphene field-effect transistors (FETs) using Al2O3 as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al2O3. Our devices show mobility values of over 8,000 cm2/Vs at room temperature, a finding which indicates that the top-gate stack does not significantly increase the carrier scattering, and consequently degrade the device characteristics. We propose a device model to fit the experimental data using a single mobility value.

cond-mat.mes-hall↗

arXiv2009

Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single layer graphene with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities as high as 4050 cm2V-1s-1 at room temperature.

cond-mat.mtrl-sci↗