Kayoung Lee

Gate-tunable resonant tunneling in double bilayer graphene heterostructures.

We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

Chemical Potential and Quantum Hall Ferromagnetism in Bilayer Graphene

Breaking down graphene degeneracy Bilayer graphene has two layers of hexagonally arranged carbon atoms stacked on top of each other in a staggered configuration. This spatial arrangement results in degenerate electronic states: distinct states that have the same energy. Interaction between electrons can cause the states to separate in energy, and so can external fields (see the Perspective by LeRoy and Yankowitz). Kou et al., Lee et al., and Maher et al. used three distinct experimental setups that clarify different parameter regimes of bilayer graphene. Science, this issue p. 55, p. 58, p. 61; see also p. 31 One graphene bilayer is used to tune and study the electronic properties of another. [Also see Perspective by LeRoy and Yankowitz] Bilayer graphene has a distinctive electronic structure influenced by a complex interplay between various degrees of freedom. We probed its chemical potential using double bilayer graphene heterostructures, separated by a hexagonal boron nitride dielectric. The chemical potential has a nonlinear carrier density dependence and bears signatures of electron-electron interactions. The data allowed a direct measurement of the electric field–induced bandgap at zero magnetic field, the orbital Landau level (LL) 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.

Band Alignment in WSe2–Graphene Heterostructures

Using different types of WSe2 and graphene-based heterostructures, we experimentally determine the offset between the graphene neutrality point and the WSe2 conduction and valence band edges, as well as the WSe2 dielectric constant along the c-axis. In a first heterostructure, consisting of WSe2-on-graphene, we use the WSe2 layer as the top dielectric in dual-gated graphene field-effect transistors to determine the WSe2 capacitance as a function of thickness, and the WSe2 dielectric constant along the c-axis. In a second heterostructure consisting of graphene-on-WSe2, the lateral electron transport shows ambipolar behavior characteristic of graphene combined with a conductivity saturation at sufficiently high positive (negative) gate bias, associated with carrier population of the conduction (valence) band in WSe2. By combining the experimental results from both heterostructures, we determine the band offset between the graphene charge neutrality point, and the WSe2 conduction and valence band edges.

Bilayer Graphene-Hexagonal Boron Nitride Heterostructure Negative Differential Resistance Interlayer Tunnel FET

We present the room temperature operation of a vertical tunneling field-effect transistor using a stacked double bilayer graphene (BLG) and hexagonal boron nitride heterostructure. The device shows two tunneling resonances with negative differential resistance (NDR). An analysis of the electrostatic potential drop across the heterostructure indicates the resonances are associated with the relative alignment of the lower or upper bands of the two BLG. Using the NDR characteristic of the device, one-transistor latch or SRAM operation is demonstrated. The device characteristics are largely insensitive to temperature from 1.5 to 300 K.

Quantum Hall effect in Bernal stacked and twisted bilayer graphene grown on Cu by chemical vapor deposition

We examine the quantum Hall effect in bilayer graphene grown on Cu substrates by chemical vapor deposition. Spatially resolved Raman spectroscopy suggests a mixture of Bernal (A-B) stacked and rotationally faulted (twisted) domains. Magnetotransport measurements performed on bilayer domains with a wide 2D band reveal quantum Hall states (QHSs) at filling factors ν = 4, 8, 12, consistent with a Bernal stacked bilayer, while magnetotransport measurements in bilayer domains defined by a narrow 2D band show a superposition of QHSs of two independent monolayers. The analysis of the Shubnikov‐de Haas oscillations measured in twisted graphene bilayers provides the carrier density in each layer as a function of the gate bias and the interlayer capacitance.

Magnetotransport Properties of Quasi-Free Standing Epitaxial Graphene Bilayer on SiC: Evidence for Bernal Stacking

We investigate the magnetotransport properties of quasi-free-standing epitaxial graphene bilayer on SiC, grown by atmospheric pressure graphitization in Ar, followed by H(2) intercalation. At the charge neutrality point, the longitudinal resistance shows an insulating behavior, which follows a temperature dependence consistent with variable range hopping transport in a gapped state. In a perpendicular magnetic field, we observe quantum Hall states (QHSs) both at filling factors (ν) multiples of four (ν = 4, 8, 12), as well as broken valley symmetry QHSs at ν = 0 and ν = 6. These results unambiguously show that the quasi-free-standing graphene bilayer grown on the Si-face of SiC exhibits Bernal stacking.

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, as well as at large E fields. At a fixed perpendicular magnetic field (B), the ν=0 QHS undergoes a transition as a function of the applied 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 follows a linear dependence on B.

Transport Gap in Dual-Gated Graphene Bilayers Using Oxides as Dielectrics

Graphene bilayers in Bernal stacking exhibit a transverse electric (E) field-dependent band gap, which can be used to increase the channel resistivity and enable higher on/off ratio devices. We provide a systematic investigation of transport characteristics in dual-gated graphene bilayer devices as a function of density and E field and at temperatures from room temperature down to 0.3 K. The sample conductivity shows finite threshold voltages along the electron and hole branches, which increase as the E field increases, similar to a gapped semiconductor. We extract the transport gap as a function of E field and discuss the impact of disorder. In addition, we show that beyond the threshold, the bilayer conductivity shows a highly linear dependence on density, which is largely insensitive to the applied E field and the temperature.

Gate capacitance scaling and graphene field-effect transistors with ultra-thin top-gate dielectrics

Graphene has emerged recently as an attractive channel material for high frequency analog device applications. High carrier mobility and large gate capacitance are both desirable attributes for such devices. A main obstacle however in depositing thin dielectrics on graphene, with high dielectric constant is its chemical inertness. This obstacle can be overcome by either directly depositing the dielectric, e.g. using sputtering or e-beam evaporation, or by using a seed layer which provides nucleation sites for atomic layer deposition (ALD). The interfacial layer however reduces the gate capacitance and can also impact the quality of the ALD dielectric subsequently grown. Here we provide a systematic study of gate capacitance scaling of graphene field effect transistors with Al2O3 gate dielectric with two seed layers, oxidized aluminum and oxidized titanium. Our results show the oxidized Ti film on graphene provides a smooth surface, which allows us to use a Ti nucleation layer as thin as 6Å, and achieve uniform coverage required for the subsequent ALD. The k-value of the ALD Al2O3 grown on graphene using oxidized Ti as nucleation layer is 12.7, a value 2.5 times larger than the ALD Al2O3 grown using oxidized Al. We demonstrate graphene devices with ultra-thin top gate dielectrics, with EOT values as low as 3.5 nm.