Chemistry Reference
In-Depth Information
4. Tuning the Fermi Energy by Field Effect Gating
The conventional way to shift the Fermi energy of a system is back gating.
In Fig. 6 we have shown the schematic of a back gated field effect transistor
using 300nm SiO
2
as a dielectric material. In this geometry a gate poten-
tial (
V
BG
) is applied between the sample graphene and the gate electrode
(highly doped Si). Therefore, it act like a capacitor and amount of induced
charges in the sample depends on the capacitance of the system, given by
C
G
=
d
,where
is the dielectric constant of SiO
2
. For a 300nm thick SiO
2
12 nF/cm
2
.
the
C
G
∼
SLG
SLG
S
D
SiO
SiO
2
V
BG
Si
Fig. 6.
Schematic of a SiO
2
back gating.
V
G
) creates (i) electrostatic potential drop
between the sample and the gate electrode and (ii) shift of the Fermi level
of the sample. The first one depends on the geometrical gate capacitance
(
An applied gate voltage (
C
G
) of the system and second one depends on the quantum capacitance
µF/cm
2
which is much
(
C
Q
) of the sample. Now, for graphene
C
Q
∼
1
larger compared to
C
G
. As a result maximum voltage drop occurs across
the SiO
2
and thus, the conversion factor from
V
G
to
E
F
is very low
∼
0.003.
For example by applying
V
G
= 100V one can shift the Fermi energy by
>
V
300 meV in graphene.
) the dielectric
of SiO
2
will break down. Therefore, the conversion factor and maximum
Fermi level shift are limited by the
At higher gate voltages (
100
C
G
. One of the solutions to overcome
this problems is to either reduce the thickness of the SiO
2
which needs
sophisticated techniques like atomic layer deposition (ALD) etc or to use
high dielectric insulator like HfO
2
.
There is another way to increase the
C
G
by using the electrolyte gat-
ing.
48-52
Here we use solid polymer electrolyte gating, where polymer acts
