In graphene channel FETs (referred to as GFETs), Si
is used as substrate. Several layers of graphene held together
by vander-waal’s forces constitute Graphite. A stack of
graphene layers of 1-5 nm thick called exfoilated graphene, is
used as channel material [9,11]. These layers are extracted
from graphite using an adhesive tape that sticks to the graphite
material, and is then peeled off. This process is done
repetitively until single or multiple layers of graphene remain
on the tape. This tape is now stuck on the chip and is removed
after some time. Raman Spectroscopy is used to detect the
graphene flakes left on the Sio2 surface .
The Source and Drain regions of a GFET are created using
PMMA and subsequent e-beam lithography. Al2O3 deposition
is done using a chemical vapor deposition method with
sequential self-terminating gas-solid reactions called Atomic
Layer Deposition (ALD) which can be achieved by the
following steps :
1. Trimethylaluminum reacts with hydroxyl group on
the surface and when concentration of hydroxyl ions
saturates the reaction is terminated.
2. Then the remaining gas and by product are let out.
3. Water vapour is the second reactant. Reaction will
terminate when surface concentration saturates.
Remaining components are let out.
Graphene has no hydroxyl group compound formed
naturally. Hence Al is first deposited using e-beam
lithography on Graphene surface and left to undergo natural
oxidation. Then ALD process is conducted over it using
Trimethylaluminum and water vapour for in-situ Al2O3
growth. Then Gate electrode is created in the similar method
as above [1,11].Electrical Properties: As we know in the conventional
…s weak Vander
Waals bond, due to which the physical and chemical
properties of graphene FETs remains unaltered after the oxide
deposition. Hence there are less chances of transport of
electron through oxide and high-k dielectric integration is
compatible with graphene.
Exfoliated graphene still offers the highest mobility
116,117,118 cm2/V s but there is a bottleneck in fabrication.
Back-gated graphene FETs with SiO2 dielectric were
typically shown to have field-effect mobilities up to around
10,000 cm2/V s .Challenges with Graphene FETs:
Graphene with interface dielectrics results in charged
carrier scattering which degrades the mobility of the carriers.
To reduce this effect, density of carriers and charged
impurities is to be reduced. Use of High K dielectric helps in
shielding the scattering effect but is still not effective [1,11].