correlation between the hysteresis and the initial defect density of graphene
TRANSCRIPT
Correlation between the hysteresis and the initial defect density of grapheneChunhum Cho, Young Gon Lee, Ukjin Jung, Chang Goo Kang, Sungkwan Lim, Hyeon Jun Hwang, Hojun Choi,
and Byoung Hun Lee Citation: Applied Physics Letters 103, 083110 (2013); doi: 10.1063/1.4818770 View online: http://dx.doi.org/10.1063/1.4818770 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/8?ver=pdfcov Published by the AIP Publishing
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Correlation between the hysteresis and the initial defect density of graphene
Chunhum Cho,1 Young Gon Lee,2 Ukjin Jung,2 Chang Goo Kang,2 Sungkwan Lim,1
Hyeon Jun Hwang,2 Hojun Choi,2 and Byoung Hun Lee1,2,a)
1Department of Nanobio Materials and Electronics, Gwangju Institute of Science and Technology,Oryong-dong 1, Gwangju 500-712, South Korea2School of Materials Science and Engineering, Gwangju Institute of Science and Technology, Oryong-dong 1,Gwangju 500-712, South Korea
(Received 20 May 2013; accepted 2 August 2013; published online 20 August 2013)
The role of the initial defects of graphene characterized by Raman spectroscopy is correlated with
the physical mechanisms causing the hysteretic device characteristics of graphene field effect
transistors (FETs). Fast charging related to the tunneling-induced charge exchange is found to be
closely correlated with the initial defect density, while slow charging related to environmental
influences such as the water redox reaction showed a weak correlation. It can be concluded that the
intrinsic quality of graphene should be improved to minimize the hysteresis of graphene FETs even
in an air-tight environment. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4818770]
Device instabilities such as a wide distribution of Dirac
points and hysteretic current-voltage (I-V) characteristics
have limited the applications of graphene in electronics. The
physical origins of the instability of graphene field effect
transistors (GFETs) have been actively studied to find a way
to minimize device scattering.1–10 Many factors affecting the
stability of GFETs have been identified including metallic
residues originating from the growth process, substrate mate-
rials, capping dielectrics, process-induced defects, imperfect
cleaning processes, initial defects, and operation ambient
and temperature.1–17
Especially, the time dependent device instability of
GFETs resulted in the hysteretic device characteristics. Two
main mechanisms, charge trapping through tunneling and an
interfacial redox reaction, have been suggested as the origin
of the hysteresis in GFETs by analyzing the time domain
response of the drain current.6,9,10 The two-trap model pro-
posed by Lee et al. could explain the time dependence of
hysteresis using two time constants.10 The slow time con-
stant is related to the chemical redox reaction at the gra-
phene/SiO2 interface, which can be modulated by limiting
the oxygen supply to the test environment.9 The fast time
constant is attributed to charge trapping through quantum
mechanical tunneling because of the temperature independ-
ence and short time constant. Yet, it is not clear how the tun-
neling component could be controlled and what the physical
origin of the tunneling component could be.
The primary candidates of the tunneling sites are either
graphene lattice defects or contaminants adsorbed in the sur-
face of the graphene. Structural defects or charged impurities
introduce a screening effect and impurity scattering in gra-
phene devices.18–23 In addition, lattice defects are known to
induce mid-gap states resulting in a charge transfer from per-
fect lattices to defects.18,19 Much research has reported a
strong correlation between electrical properties and lattice
defects or charged impurities.18–23 However, no studies have
addressed the relation between defects and hysteretic device
characteristics.
In this work, GFETs with various channel defect den-
sities were fabricated using laser-grown graphene, and their
I-V hysteresis characteristics were correlated with the initial
defect density of the graphene measured using Raman spec-
troscopy. The physical mechanism of the hysteresis, espe-
cially the fast tunneling components, was found to closely
correlate with the initial defect density measured by Raman
spectroscopy.
In this experiment, forming a graphene sheet with a
wide range of defect densities was challenging. Such gra-
phene is necessary because it is best to fabricate GFETs in
the same batch to minimize the influence of fabrication vari-
ability. In this work, a pulsed KrF excimer laser was
employed to grow graphene with a non-uniform quality dis-
tribution. Other methods such as ion irradiation, plasma, and
impurity deposition were discarded because the influence of
such processes may further increase the variability.21–23
To form graphene with a wide range of defect density,
a Si-face 4H-SiC wafer with an on-axis orientation was
graphitized using a KrF laser (k¼ 248 nm) annealing pro-
cess.24 Since the surface of the SiC wafer was smoothed by
chemical mechanical polishing (CMP), an initial hydrogen
anneal to form a large area facet was not necessary. Before
the laser anneal, the sample underwent Piranha cleaning
followed by a dilute HF clean to remove organic and metal-
lic contaminants. The wafer was then loaded into a vacuum
chamber and exposed to multiple laser shots (600 shots,
repetition rate¼ 10 Hz, average fluence¼ 1.33 J/cm2, pulse
width¼ 25 ns) in high vacuum (<10�6 Torr) at room tem-
perature. The laser spot size was increased to 4 mm � 4 mm
using a laser beam homogenizer.
The graphene layer grown on the SiC substrate was
transferred to a SiO2 substrate as follows. A stack of Au thin
film/polymethyl methacrylate (PMMA)/thermal release tape
served as the transfer template. The graphene/Au/PMMA/
tape stack was detached from the SiC substrate, and the gra-
phene layer was transferred to the SiO2 substrate. Before the
transfer, the substrate was cleaned by sonication in methanol,
a)Author to whom correspondence should be addressed. Electronic mail:
0003-6951/2013/103(8)/083110/4/$30.00 VC 2013 AIP Publishing LLC103, 083110-1
APPLIED PHYSICS LETTERS 103, 083110 (2013)
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acetone, and deionized (DI) water and treated with O2
plasma to remove organic contaminants. Finally, the tape
and Au were removed, leaving a graphene layer on the SiO2.
Using the transferred graphene, back-gated GFETs were fab-
ricated to examine their hysteretic characteristics. Since the
graphene region was large enough, contact photolithography
and O2 plasma were employed to pattern the graphene. Au
electrodes were used as the source/drain/gate. After pattern-
ing the graphene channel, Raman spectra of the graphene
channel were measured using a RENISHAW inVia Raman
microscope with an Arþ ion laser (k¼ 514.5 nm). Then, a
30 nm Al2O3 capping layer was deposited on the graphene
channel by atomic layer deposition (ALD) to minimize
p-type doping effects.9 In this process, trimethylaluminium
(TMA) precursor is used with a H2O/N2 gas at 130 �C.
Fig. 1(a) is a photograph of a transferred graphene layer
showing a few voids that were formed during the transfer pro-
cess. These voids are due to particles preventing the graphene
from bonding to the SiO2 substrate. The size of graphene is
� 2.5 mm � 2.5 mm, which is smaller than the focused laser
spot size (4 mm � 4 mm) because the graphene was not
formed at the edge of the exposed region because of rapid
thermal dissipation to the unexposed region. Fig. 1(b) is
AFM image of center region showing root-mean-square
(rms) roughness of �0.32 nm in 25 lm � 25 lm region.
Surface roughness is slightly higher than a clean graphene on
SiO2 (�0.16 nm)25 due to surface particles. Fig. 1(c) is the
AFM image of edge region and (d) is the line scan data show-
ing step height �1.1–1.2 nm. This is typical step height of
monolayer graphene which is higher than the layer-to layer
distance of bulk graphite (�0.34 nm) due to ambient species
or adsorbates between graphene and SiO2 substrate.26,27
The distribution of the quality of transferred graphene
was monitored by mapping the D/G intensity ratio as shown
in Fig. 2(a). Raman spectra of the transferred graphene were
mapped to check the defect density and thickness of gra-
phene.28 The D/G intensity ratio ranged from 0.25 to 1.1,
indicating a wide spectrum of defect density due to the dif-
ference in the local annealing condition originated from the
spatial non-uniformity laser beam. Since the defect density is
influenced by the non-uniformity of the laser beam, we
assumed the local quality variation is not significant within a
local device channel area (5.6 lm� 10 lm). A representative
Raman spectrum is shown in Fig. 2(b). G/2D intensity ratio
<0.65 and 2D peak well fitted by single Lorentzian curve
indicate the transferred graphene consists of a monolayer the
graphene.
A schematic of the GFET and its scanning electron
microscope (SEM) image are shown in Fig. 3(a). Hysteresis
was measured using a pulsed current-voltage (I-V) measure-
ment system. The pulse Id-Vg curves were primarily meas-
ured at the electron conduction branch (right side of the
Dirac point) while applying a pulsed bias to the back gate.
The rise and fall time of the pulse was 300 ls and the pulse
width was 1 ms. Note that same gate bias (Vg-VDirac¼ 10 V)
was applied to restrain the effect of the voltage sweep range
on the hysteresis. A detailed explanation on the set-up of the
pulsed I-V measurement system can be found elsewhere.6,10
Fig. 3(b) shows representative Id versus (Vg-VDirac)
curves of GFETs. The drain current at the same Vg-VDirac
decreased as D/G ratio increased from 0.45 to 0.67. The hys-
teresis is defined as the difference in the Dirac points,
DVDirac, of two I-V curves generated by a gate pulse with a
negative Vg base bias and a positive Vg peak bias as shown
in Fig. 3(b). Since the charge traps gathered during a short
pulse bias cycle are easily dissipated during an off-cycle, the
hysteresis does not disappear or decreases even after multi-
ple cycles of I-V measurement. The drain current decreases
at þVg-VDirac are due to the time-dependent Dirac point shift
during the pulse width. Since DVDirac is due to the charge
generation during the gate pulse cycle, the equivalent num-
ber of trap charges, nt, generating DVDirac can be calculated
as follows:
FIG. 1. (a) Optical image of graphene transferred to SiO2. (b) AFM image
of center region showing rms roughness �0.32 nm in 25 lm � 25 lm region,
(c) AFM image of edge region showing the pinholes in the graphene due to
the insufficient graphitization of SiC, and (d) line scan data at the edge of
graphene show 1.1–1.2 nm of step height, which is close to the thickness of
monolayer graphene on SiO2 substrate.26,27
FIG. 2. (a) Raman mapping image
of D/G intensity ratio within 2 mm
� 2 mm region. (b) A representative
Raman spectrum of mono layer region.
083110-2 Cho et al. Appl. Phys. Lett. 103, 083110 (2013)
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nt ¼ COX � DVDirac=q ; (1)
where COX is the capacitance of the SiO2. q is the charge of
electron. The charge density can be correlated with the initial
defect density, ndo, measured by Raman spectroscopy. ndo is
calculated using the D/G ratio as shown as28
ndo ¼1:8 � 1022
k4L
ID
IG
� �; (2)
where kL is the excitation laser wavelength. To illustrate the
statistical correlation between nt and ndo as shown in Fig. 4,
more than 15 GFETs were tested with various defect den-
sities ranging from 8.78 � 1010 to 1.84 � 1011 cm�2, which
corresponds to 0.34 and 0.72 of the D/G ratio. nt appears to
be proportional to the ndo of the graphene channel, although
the data distribution within the batch is still wide. The trend
line shown in Fig. 4 indicates that the electrically measured
trap density, nt, is �2.5 times the initial defect density, ndo,
measured by Raman spectroscopy. Since the electrically
measured nt extracted from DVDirac detects the charges from
the trap sites, which exist outside the graphene or on the sur-
face of the graphene, as well as the charges from the initial
defect sites, a one-to-one correspondence with the initial
defect density is not expected, but the rough correlation
between nt and ndo implies that the initial defects play an im-
portant role in the charge trapping process.
To further investigate the role of initial defects in the
graphene, the time dependence of the drain current decrease
at the peak bias of the pulse was analyzed using the two-trap
model as follows:6,10
Id ¼ I0 � A � exp�t
sA
� �þ B � exp
�t
sB
� �� �; (3)
where I0 is the initial drain current at the beginning of the
pulse peak, A and B, and sA and sB represent the magnitude
and speed of two different mechanisms. A and sA are respon-
sible for the fast time constant of the tunneling mechanism,
while B and sB are responsible for the slow time constant of
the chemical reaction. The procedure to extract the time con-
stants and their relative contributions from the transient char-
acteristics of the drain current is detailed elsewhere.10
Representative curves showing the time dependence of
the drain current under a pulsed gate bias are shown in the
left panel of Fig. 5(a). The transient portion of Id for GFETs
with three different initial defect densities is shown in the
right panel of Fig. 5(a). The fitting parameters for Eq. (3) are
calculated for the 30 ls to 2 ms range of transient drain cur-
rents. sA and sB obtained for the two-trap model are approxi-
mately fixed to average values (sA¼ 74 ls, sB¼ 628 ls),
assuming the time constants of the two-trap mechanism are
similar in devices from the same batch. Then, the amounts of
the contribution from fast and slow traps (parameter A and B
in Eq. (3)) are calculated as shown in Fig. 5(b). In general,
the contribution of A, i.e., the tunneling component, is much
higher than the chemical reaction component.
Interestingly, A and B showed significantly different
trends with ndo. Parameter A appears to be correlated with
the ndo, while parameter B appeared nearly independent of
it. This result means that the initial defects in the graphene
influence the charge trapping caused by the tunneling mecha-
nism, but do not affect charge generation caused by the re-
dox reaction. The transient Id curves showing a large
difference in a short time scale support the result of this
modeling. The correlation between the initial defect density
and tunneling-induced charging is intuitively understandable
because the preferred charge transfer from the defect sites
has been observed using scanning tunneling microscopy
(STM).19,29 Such charge transfer is promoted by the change
in the energy level of the defect states, which usually shifts
to a midgap state. On the other hand, the weak correlation
between the slow charging mechanism, i.e., the chemical
reaction component, and the initial defect density is more
difficult to explain because the chemical reaction at the
FIG. 3. (a) Schematic image and SEM
image of graphene FETs. Scale bar is
40 lm. (b) Representative pulsed Id-Vg
curves of graphene FETs with various
D/G ratios. Because of different con-
ductivity, drain currents are normal-
ized by a value of the drain current at
the Dirac point. Vd¼ 0.5 V, rise and
fall time¼ 300 ls, pulse width¼ 1 ms.
FIG. 4. Defect density dependence of hysteresis represented by charge
density.
083110-3 Cho et al. Appl. Phys. Lett. 103, 083110 (2013)
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surface of the graphene can be catalyzed by impurity mole-
cules that bond at defect sites as a result of the low formation
energy of the chemisorption compared to perfect lattices.30
One possible explanation is the supply limited reaction due
to the Al2O3 passivation of the graphene channel. Lee et al.reported that the limited oxygen supply can suppress the sur-
face reaction component.10
The implication of this finding is that the initial quality
of graphene is critically important in stabilizing graphene
devices even after optimizing other factors, including the
substrate material, integration process, and device structure,
to minimize the oxygen exposure as suggested by Lee
et al.10 If the quality of the graphene cannot be improved to
minimize the hysteresis, an alternative way to passivate the
influence of the initial defect sites at the graphene, such as a
forming gas anneal for silicon devices, will be necessary.
In summary, the charge trap density of GFETs and the
initial defect density of graphene are found to correlate to
each other. In particular, the tunneling component of the
charge trapping has shown a strong correlation with initial
defect density. While the chemical reaction component of
the hysteresis could be minimized by limiting the oxygen
supply to the graphene, the majority of hysteresis can be
eliminated only by reducing the initial defect density of the
graphene.
This work was supported by the Pioneer Research
Center Program (2012-0009462) and by the Inter-ER
Cooperation Projects funded by the MKE and KIAT
(R0000499).
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FIG. 5. (a) In the left panel, drain cur-
rent reduction as function of time dur-
ing the pulse width. Drain current is
fitted by the two-trap model formu-
lized by Eq. (3). In the right panel, rep-
resentative fitting lines with various
defect densities. To compare each
other, fitting lines are aligned by the
value of the current at saturation.
(b) Defect density dependence of A
and B extracted from fitting lines by
Eq. (3).
083110-4 Cho et al. Appl. Phys. Lett. 103, 083110 (2013)
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