Chemistry Reference
In-Depth Information
(FQHE) in graphene
38,39
and new broken symmetry phase at
= 0 Landau
level in case of bilayer graphene.
40
Although, there are several emerging
phenomena, some basic questions about graphene's electronic properties
need to be addressed. For example, there is no clear understanding about
the scattering mechanism, which limits the mobility, and very little under-
standing about transport properties near n=0 Landau level.
41
One of the important parameter that needs better understanding is
the source of disorder. There are three sources of disorder that are often
discussed in the literature: (1) Intrinsic lattice imperfection and defects,
such as dislocations, point vacancies etc., (2) Rippling of graphene
42
when
it is supported by an underlying substrate, which also creates local curva-
ture,
43
and hence fluctuation in local chemical potential and effective gauge
fields.
2
Both intrinsic ripples and extrinsic roughness of surface have been
considered. Finally, (3) a major source of disorder arises from the Coulomb
potential from the trapped charges buried in the oxide substrate (the ad-
sorbed atom on the graphene surface, such as the water molecules, have
also been shown to have adverse effects on its mobility).
The conventional time-averaged transport measurements seem to be in-
adequate in understanding the effect of disorder in graphene transistors.
Being directly sensitive to the ability of an electronic device to screen ex-
ternal potential fluctuations, the low frequency noise in electrical transport
has recently been shown to reflect the low-energy band structure in single
and BLG devices.
44,45
The noise in both cases was found to originate pri-
marily from the fluctuating charge traps inside the SiO
2
substrate, similar
to carbon nanotube field effect devices.
46
Also, the dependence of noise
magnitude on the gate electric field was found to be opposite for single
and BLG, and was attributed to a field-induced gap formation in the lat-
ter. Noise experiments on multilayer graphene
47
also shows the similar gate
voltage dependence like bilayer graphene, which ensure that the bandgap
is not the only reason for the increase in noise with gate voltage.
Here, we propose an analytical model of noise which is based on the
number and correlated mobility fluctuation model described earlier. Then
we show a systematic study of low-frequency noise measurements in four
categories of graphene devices: single layer graphene (SLG), BLG, FLG
with three to five atomic layers, and many-layer graphene (MLG) with
greater than five layers in the device. The gate voltage dependence of noise
in all kind of graphene devices seem to be consisting with our proposed
model. For BLG,
45
we have used double gated FET devices to tune the
zero gap and charge neutrality points independently, which offers a unique
n
