Jian-Hao Chen

Intrinsic and extrinsic performance limits of graphene devices on SiO2.

The linear dispersion relation in graphene gives rise to a surprising prediction: the resistivity due to isotropic scatterers, such as white-noise disorder or phonons, is independent of carrier density, n. Here we show that electron-acoustic phonon scattering is indeed independent of n, and contributes only 30 Omega to graphenes room-temperature resistivity. At a technologically relevant carrier density of 1 x1012 cm-2, we infer a mean free path for electron-acoustic phonon scattering of >2 microm and an intrinsic mobility limit of 2 x 105 cm2 V-1 s-1. If realized, this mobility would exceed that of InSb, the inorganic semiconductor with the highest known mobility ( approximately 7.7 x 104 cm2 V-1 s-1; ref. 9) and that of semiconducting carbon nanotubes ( approximately 1 x 105 cm2 V-1 s-1; ref. 10). A strongly temperature-dependent resistivity contribution is observed above approximately 200 K (ref. 8); its magnitude, temperature dependence and carrier-density dependence are consistent with extrinsic scattering by surface phonons at the SiO2 substrate and limit the room-temperature mobility to approximately 4 x 104 cm2 V-1 s-1, indicating the importance of substrate choice for graphene devices.

Charged-impurity scattering in graphene

Valuable insight into the influence of scattering from impurities on the peculiar electronic properties of graphene are gained by a systematic study of how its conductivity changes with increasing concentration of potassium ions deposited on its surface.

Defect Scattering in Graphene

Irradiation of graphene on SiO2 by 500 eV Ne and He ions creates defects that cause intervalley scattering as is evident from a significant Raman D band intensity. The defect scattering gives a conductivity proportional to charge carrier density, with mobility decreasing as the inverse of the ion dose. The mobility decrease is 4 times larger than for a similar concentration of singly charged impurities. The minimum conductivity decreases proportional to the mobility to values lower than 4e(2)/pih, the minimum theoretical value for graphene free of intervalley scattering. Defected graphene shows a diverging resistivity at low temperature, indicating insulating behavior. The results are best explained by ion-induced formation of lattice defects that result in midgap states.

Printed Graphene Circuits

we have fabricated transparent electronic devices based on graphene materials with thickness down to one single atomic layer by the transfer printing method. The resulting printed graphene devices retain high field effect mobility and have low contact resistance. The results show that the transfer printing method is capable of high-quality transfer of graphene materials from silicon dioxide substrates, and the method thus will have wide applications in manipulating and delivering graphene materials to desired substrate and device geometries. Since the method is purely additive, it exposes graphene (or other functional materials) to no chemical preparation or lithographic steps, providing greater experimental control over device environment for reproducibility and for studies of fundamental transport mechanisms. Finally, the transport properties of the graphene devices on the PET substrate demonstrate the non-universality of minimum conductivity and the incompleteness of the current transport theory.

Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering.

We reduce the dimensionless interaction strength alpha in graphene by adding a water overlayer in ultrahigh vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30%, due to the background dielectric constant enhancement leading to a reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short-range impurities is decreased by almost 40%, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron-hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.

Tunable Kondo effect in graphene with defects

Although evidence indicates that defects induce magnetism in graphite, it’s unclear whether this extends to graphene. An observation of the gate-tunable Kondo effect in ion-beam-damaged graphene suggests it does.

Atomic Resolution Imaging of Grain Boundary Defects in Monolayer Chemical Vapor Deposition-Grown Hexagonal Boron Nitride

Grain boundaries are observed and characterized in chemical vapor deposition-grown sheets of hexagonal boron nitride (h-BN) via ultra-high-resolution transmission electron microscopy at elevated temperature. Five- and seven-fold defects are readily observed along the grain boundary. Dynamics of strained regions and grain boundary defects are resolved. The defect structures and the resulting out-of-plane warping are consistent with recent theoretical model predictions for grain boundaries in h-BN.

High-fidelity conformation of graphene to SiO2 topographic features.

High-resolution noncontact atomic force microscopy of SiO2 reveals previously unresolved roughness at the few-nm length scale, and scanning tunneling microscopy of graphene on SiO2 shows graphene to be slightly smoother than the supporting SiO2 substrate. A quantitative energetic analysis explains the observed roughness of graphene on SiO2 as extrinsic, and a natural result of highly conformal adhesion. Graphene conforms to the substrate down to the smallest features with nearly 99% fidelity, indicating conformal adhesion can be highly effective for strain engineering of graphene.

Hooge’s constant for carbon nanotube field effect transistors

The 1∕f noise in individual semiconducting carbon nanotubes (s-CNT) in a field effect transistor configuration has been measured in ultrahigh vacuum and following exposure to air. The amplitude of the normalized current spectral noise density is independent of source-drain current and inversely proportional to gate voltage, to channel length, and therefore to carrier number, indicating that the noise is due to mobility rather than number fluctuations. Hooge’s constant for s-CNT is found to be (9.3±0.4)×10−3 The magnitude of the 1∕f noise is substantially decreased by exposing the devices to air.

Diffusive charge transport in graphene on SiO2

Abstract We review our recent work on the physical mechanisms limiting the mobility of graphene on SiO2. We have used intentional addition of charged scattering impurities and systematic variation of the dielectric environment to differentiate the effects of charged impurities and short-range scatterers. The results show that charged impurities indeed lead to a conductivity linear in density ( σ ( n ) ∝ n ) in graphene, with a scattering magnitude that agrees quantitatively with theoretical estimates; increased dielectric screening reduces the scattering from charged impurities, but increases the scattering from short-range scatterers. We evaluate the effects of the corrugations (ripples) of graphene on SiO2 on transport by measuring the height–height correlation function. The results show that the corrugations cannot mimic long-range (charged impurity) scattering effects, and have too small an amplitude-to-wavelength ratio to significantly affect the observed mobility via short-range scattering. Temperature-dependent measurements show that longitudinal acoustic phonons in graphene produce a resistivity that is linear in temperature and independent of carrier density; at higher temperatures, polar optical phonons of the SiO2 substrate give rise to an activated, carrier density-dependent resistivity. Together the results paint a complete picture of charge carrier transport in graphene on SiO2 in the diffusive regime.