24 research outputs found
Graphene field effect transistors with ferroelectric gating
Recent experiments on ferroelectric gating have introduced a novel
functionality, i.e. nonvolatility, in graphene field effect transistors. A
comprehensive understanding in the non-linear, hysteretic ferroelectric gating
and an effective way to control it are still absent. In this letter, we
quantitatively characterize the hysteretic ferroelectric gating using the
reference of an independent background doping (nBG) provided by normal
dielectric gating. More importantly, we prove that nBG can be used to control
the ferroelectric gating by unidirectionally shifting the hysteretic
ferroelectric doping in graphene. Utilizing this electrostatic effect, we
demonstrate symmetrical bit writing in graphene-ferroelectric FETs with
resistance change over 500% and reproducible no-volatile switching over 10^5
cycles.Comment: 5 Pages; 4 figures; two column forma
Optospintronics in graphene via proximity coupling
The observation of micron size spin relaxation makes graphene a promising
material for applications in spintronics requiring long distance spin
communication. However, spin dependent scatterings at the contact/graphene
interfaces affect the spin injection efficiencies and hence prevent the
material from achieving its full potential. While this major issue could be
eliminated by nondestructive direct optical spin injection schemes, graphenes
intrinsically low spin orbit coupling strength and optical absorption place an
obstacle in their realization. We overcome this challenge by creating sharp
artificial interfaces between graphene and WSe2 monolayers. Application of a
circularly polarized light activates the spin polarized charge carriers in the
WSe2 layer due to its spin coupled valley selective absorption. These carriers
diffuse into the superjacent graphene layer, transport over a 3.5 um distance,
and are finally detected electrically using BN/Co contacts in a non local
geometry. Polarization dependent measurements confirm the spin origin of the
non local signal
van der Waals Bonded Co/h-BN Contacts to Ultrathin Black Phosphorus Devices
Due to the chemical inertness of 2D hexagonal-Boron Nitride (h-BN), few
atomic-layer h-BN is often used to encapsulate air-sensitive 2D crystals such
as Black Phosphorus (BP). However, the effects of h-BN on Schottky barrier
height, doping and contact resistance are not well known. Here, we investigate
these effects by fabricating h-BN encapsulated BP transistors with cobalt (Co)
contacts. In sharp contrast to directly Co contacted p-type BP devices, we
observe strong n-type conduction upon insertion of the h-BN at the Co/BP
interface. First principles calculations show that this difference arises from
the much larger interface dipole at the Co/h-BN interface compared to the Co/BP
interface, which reduces the work function of the Co/h-BN contact. The Co/h-BN
contacts exhibit low contact resistances (~ 4.5 k-ohm), and are Schottky
barrier free. This allows us to probe high electron mobilities (4,200 cm2/Vs)
and observe insulator-metal transitions even under two-terminal measurement
geometry
Gate-controlled non-volatile graphene-ferroelectric memory
In this letter, we demonstrate a non-volatile memory device in a graphene FET
structure using ferroelectric gating. The binary information, i.e. "1" and "0",
is represented by the high and low resistance states of the graphene working
channels and is switched by controlling the polarization of the ferroelectric
thin film using gate voltage sweep. A non-volatile resistance change exceeding
200% is achieved in our graphene-ferroelectric hybrid devices. The experimental
observations are explained by the electrostatic doping of graphene by electric
dipoles at the ferroelectric/graphene interface.Comment: 4 papes, 4 figure
Transport properties of pristine few-layer black phosphorus by van der Waals passivation in an inert atmosphere
Ultrathin black phosphorus is a two-dimensional semiconductor with a sizeable band gap. Its excellent electronic properties make it attractive for applications in transistor, logic and optoelectronic devices. However, it is also the first widely investigated two-dimensional material to undergo degradation upon exposure to ambient air. Therefore a passivation method is required to study the intrinsic material properties, understand how oxidation affects the physical properties and enable applications of phosphorene. Here we demonstrate that atomically thin graphene and hexagonal boron nitride can be used for passivation of ultrathin black phosphorus. We report that few-layer pristine black phosphorus channels passivated in an inert gas environment, without any prior exposure to air, exhibit greatly improved n-type charge transport resulting in symmetric electron and hole transconductance characteristics.B.O. acknowledges support by the National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP9-2011-3) and the SMF-NUS Research Horizons Award 2009-Phase II. A.H.C.N. acknowledges the NRF-CRP award 'Novel 2D materials with tailored properties: beyond graphene'. The calculations were performed at the GRC computing facilities. A.Z. and D.F.C. acknowledge the NSF grant CHE-1301157. (NRF-CRP9-2011-3 - National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Programme (CRP); SMF-NUS Research Horizons Award-Phase II; NRF-CRP award 'Novel 2D materials with tailored properties: beyond graphene'; CHE-1301157 - NSF)Published versio