|
The transport properties of graphene depend sensitively on its environment. It has been shown that the intrinsic mobility
of graphene can reach 105 cm2/Vs. However, most graphene devices fabricated on a substrate show mobility one to two
orders of magnitude lower due to extrinsic scattering sources in the substrate (e.g., SiO2), including charged
impurities, surface polar phonons, etc.
We have studied the effect of the dielectric environment on the transport phenomena in graphene by fabricating graphene field effect transistors with different substrates and top gate dielectrics. Using a crystalline oxide thin film, Pb(Zr,Ti)O3 (PZT), as the gate substrate, we have observed a 10-fold increase of mobility in few layer graphene (FLG) compared to SiO2 gated single/few layer graphene, approaching the intrinsic mobility limit imposed by longitudinal acoustic phonon scattering (Fig. 1) ( PRL 102, 136808 (2009)). At high gate voltage, we also observe an unusual resistance hysteresis, with its direction opposite to that expected from the polarization switching of PZT (APL 97, 033114 (2010)). We attribute its origin to the slow dynamics of interface adsorbate layer. This robust hysteresis can potentially be used to construct graphene-ferroelectric hybrid memory devices. We have also investigated the effect of charged impurities on graphene, using the transport and quantum scattering times as the diagnose tool to identify the location and density of charged impurities, and the effect of dielectric screening. (PRB Rapid 80, 241415 (2009)). |
Fig. 1 a) AFM image of n-layer graphene (nLG) on a PZT substrate. b) Schematic PZT-gated nLG FET. c) Mobility
vs. T for different graphitic materials. Black solid line: calculated mobility dut to LA phonon scattering. (PRL 102, 136808 (2009))
|
|
Complex oxides exhibit a rich variety in their electronic and magnetic properties. Famous members include the correlated
oxides such as high temperture superconductors and colossal magnetoresistive oxides (CMR), ferroelectrics and multiferroics.
An even richer spectrum of behavior
can be envisioned by combining oxides with different functionalities into epitaixial heterostructure form.
One example is the electic field effect studies of the CMR materials, (e.g., La1-xSrxMnO3 (LSMO)). In the CMR materials, the structural, transport, and magnetic properties are closely entangled due to strong correlation effects such as Coulomb repulsion and electron-phonon coupling. One can tune the system between ferromagnetic metallic and anti-ferromagnetic insulating phases via changing the carrier density. The conventional approach to tune the charge density involves chemical doping, which also introduces structural disorder into the system. The electric field effect approach provides a unique tool to study the effect of charge density without the complication due to structural modifications associated with chemical doping. Working with a heterosture composed of a ferroelectric layer and a 3-4 nm CMR film, we have induced reversible modulation of the ferromagnetic Curie temperature (TC) in the CMR layer (Fig. 2a) (PRB 68, 134415 (2003)). In samples with optimized thickness, we have achieved reversible switching between metallic and insulating behavior (Fig. 2b) (APL 86, 142501 (2005)). |
Fig. 2 Working with a CMR film with thickness
close to the screening limit, one can a) reversiblely modulate its magnetic Curie temperature; and b) induce a metal-insulator transition by switching the polarization direction of a neighboring ferroelectric layer.
|
| The electronic and magnetic properties of complex oxide change drastically when the film thickness approaches nanoscale. For example, ferroelectric thin films often show enhanced polarization and ferroelectric TC depending on the lattice strain. A nominally dielectric material SrTiO3 can exhibit either superconductivity or ferroelectricity when the interface is properly engineered. It is of great interest to examine these novel phenomena at the microscopic level and to explore innovative applications for modern nano-electronics. An ideal tool for such studies is the scanning probe microscopy. We have carried out nanoscale study of ferroelectric thin films using atomic force and piezoelectric microscopies. By applying voltage pusles to a conducting AFM tip while it is scanning the PZT surface, we have polarized controlled arrays of domains (Fig. 3), and mapped out their piezoelectric response in PZT (APL 78, 2034 (2001)). It is also of interest to study the domain wall configuration of ferroelectric and multiferroic thin films in the presence of lattice disorder and thermal roughening. | 
Fig. 3 AFM image of a 4 by 4 array of ferroelectric domain structure on a PZT film polarized with voltage pulses with different duration.
Background: polarization up. Voltage pulse duration (from left to right): 100 ms, 50 ms, 20 ms, 10 ms. Average domain diameter
(from left to right): 100 nm, 80 nm, 70 nm, 60 nm.
|