Graphene, 2013, 2, 55-59 Published Online January 2013 (
Effect of Growth Morphology on the Electronic Structure
of Epitaxial Graphene on SiC
Michael D. Williams1, Dennis W. Hess2
1Center of Excellence in Microelectronics and Photonics, Department of Physics, Clark Atlanta University, Atlanta, USA
2School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, USA
Received November 16, 2012; revised December 20, 2012; accepted January 18, 2013
Ultraviolet photoemission spectroscopy is used to investigate the electronic structure of epitaxial graphene grown by the
thermal decomposition of the carbon face of 4H SiC. We find that the growth of the film on the chemical mechanically
polished and hydrogen etched surface enhances spectral features in the valence band structure compared to the film
grown on an unpolished hydrogen etched substrate. This result is indicative of a more highly ordered surface structure
compared to the morphologically rough material and shows that substrate preparation plays an important role in the
quality of the film. The work function of the smooth surface film is found to be 0.4 eV higher than that for graphite and
0.1 eV less than for the rough surface growth.
Keywords: Graphene; SiC; UPS; Work Function; Electronic Structure
1. Introduction
Graphene, the 2-D crystalline form of graphite is the
subject of much interest in the fields of optoelectronics,
sensors, and hydrogen storage. The high carrier mobility
and room temperature ballistic transport of carriers in
graphene suggest that this material may be a viable re-
placement for copper interconnects in electronic device
structures. Most recently, its electronic properties have
been shown to be tunable from metallic to semiconduct-
ing with hydrogen intercalation [1-3]. Many potential
applications for graphene require ordered growth on an
insulating substrate. One successful methodology to pro-
duce graphene layers has been to thermally decompose
SiC in vacuum [4]. More recently larger grain sizes have
been reported via thermal decomposition of SiC in an
inert gas atmosphere [5,6]. Other reports have explored
the processing and growth of epitaxial graphene (EG)
layers using biological and chemical functionalization
methodologies [7,8]. The latter approach is particularly
attractive for sensor and hydrogen storage applications.
Due to the critical dependence of graphene properties
on the material quality, it is imperative that appropriate
characterization methods be employed to assess struc-
tural and the associated electronic properties. The 2-D
nature of this material makes analysis amenable to tradi-
tional surface science techniques. In this work, we use
angle integrated ultraviolet photoemission spectroscopy
(UPS) to ascertain the effect of the substrate on the per-
fection of graphene samples vis-à-vis their electronic
valence band structure. Spicer and co-workers in the
early 70 s showed conclusively in comparison studies of
amorphous Si and Ge samples to corresponding crystal-
line material that UPS analysis is a definite indicator of
material crystallinity [9]. More recently, a UPS study of
few layer graphene grown by chemical vapor deposition
on polycrystalline nickel reported a strong correlation to
the graphene quality and structure [10]. This study inves-
tigates the effects of surface morphology on the elec-
tronic structure of EG layers grown by thermal decom-
position of the C face of SiC.
2. Experiment
The two samples used in this study were grown by the
thermal graphitization of the C-face of 4H SiC substrates
purchased from Cree, Inc. The substrates were graph-
itized by confinement controlled sublimation of Si atoms
from SiC in a graphite furnace at the Georgia Institute of
Technology [11]. In the confinement controlled process
the SiC substrate is encapsulated in a graphite enclosure
that maintains a high Si vapor pressure background such
that the graphene layer growth proceeds in a near equi-
librium fashion. The resulting carbon rich sufaces that
result from the Si sublimation nucleate to form an epi-
taxial graphen layer. Graphene grown by this methodol-
ogy has a much lower occurrence of defects than the ma-
terial grown at relatively low growth temperatures and
high graphitization rates in the non-equilibium ultra-high
vacuum Si sublimation process. The samples were
opyright © 2013 SciRes. Graphene
transported under ambient conditions to Clark Atlanta
University for electronic structure analysis. The AFM
image shown in Figure 1(a) is from the sample that is
optically smooth with a thickness of 52 Å and has large
areas of contiguous domains that are nominally 2.0 m in
size. The wrinkles delineating the domains are typical of
these multiple layer films [6]. It was grown at 1560˚C for
7 minutes on a chemical mechanical polished substrate of
high quality. The sample shown in Figure 1(b) is 64 Å
thick and is optically rough with a stepped morphology
characterized by overlapping domains. The latter filmwas
grown at 1565˚C for 7 minutes on a substrate that was
not chemical mechanical polished. This substrate also
has a high density of micropipe defects as observed in
AFM images. Both substrates were hydrogen etched
prior to graphitization.
The as-received samples were mounted side by side
with In (99.9999% purity) at 160˚C onto a single Mo
MBE wafer block in a nitrogen filled glove box at at-
mospheric pressure. The wafer block was then placed
into a sealed container and removed from the glove box
for transport in the same laboratory space to the load lock
of the UPS analysis system. The block was removed
from the container, placed (<1 minute) into the nitrogen
purged load lock, sealed, then pumped down to <2 × 10–9
Torr. UPS analysis of the sample was performed after the
magnetically coupled transfer of the sample from the
load lock chamber through a gate valve into an adjacent
ultrahigh vacuum (base pressure 4 × 1010 Torr) analy-
sis chamber. UPS is a surface sensitive spectroscopic
Figure 1. AFM height and phase images of (a) smooth and
(b) rough samples.
technique that yields electron distribution curves that are
in one-to-one correspondence to the joint density of filled
electronic states of the material under study. The surface
sensitivity ensures that generated spectra are from only
the topmost layers of the sample. The ultra-high vacuum
ensures that the sample integrity is maintained.
The optical source for UPS was the Ne I (16.87) line
from a differentially pumped VSW UV-10 discharge
lamp. The He discharge pressure and an Acton type D
filter were employed to discriminate against the Ne II
(26.9 eV) line present in the Ne discharge. During the
UPS measurements, the analysis chamber pressure was 2 -
4 × 10–9 Torr. The pressure rise is due to the introduction
of inert Ne into the growth chamber during the lamp op-
eration. The angle integrated kinetic energy distribution
of the photoemitted electrons was measured with a PHI
15 - 255 GAR double pass cylindrical mirror analyzer
operated in the retarded mode with an instrumental reso-
lution of ±0.05 eV. The kinetic energy distribution of the
electrons provides a surface sensitive (4 - 5 Å) meas-
urement of the joint density of states of the valence band.
The reported 3.3 Å distance between the layers ensures
that we are probing only the first two layers [12]. The
samples were outgassed in situ at 160˚C using radiative
heating from a resistive filament mounted behind the
wafer block. UPS spectra were obtained at ground poten-
tial and under negative bias voltage conditions. The
negative biased spectra show that the work function of
the samples determines the low energy threshold of the
photoemission electron distribution curves (EDCs).
3. Results and Discussion
Results of the photoemission analyses of the smooth and
rough EG samples are shown in Figures 2(a) and (b),
respectively. The spectra are normalized to the low ki-
netic energy (KE) peak associated with the low energy
scattered electrons. The linear dispersion of the spectra at
the maximum KE edge or valence band maximum (VBM)
indicates that the layers in both samples are predomi-
nately rotationally stacked and electronically decoupled
[13]. Some discrepancies of the spectral features are
noted between the samples. These are not due to sample
charging as the photon flux is low and the semi-insulat-
ing substrate is mounted on a grounded block with me-
tallic In completely surrounding the periphery of the EG
samples’ edges up to the top surfaces. Further the sam-
ples have been mounted adjacent to each other so that
any distortion of the spectral features due to charging
effects would be present for both EG samples.
The discrepancies between the two spectra are the en-
hancement of the peak at 2 eV KE and the significant
increase in the density of states in the 3 to 10 eV KE
range of the valence band for the smooth substrate. It is
instructive to note that there is no variation in the VBM.
Copyright © 2013 SciRes. Graphene
Figure 2. Ne I UPS spectra of smooth (line) and rough (dots)
thick samples under (a) unbiased and (b) 3.00 V biased
It is electrostatically rigid with sample bias. The sp2 fea-
tures at 2.00 eV KE in the spectra of the samples also
shift rigidly with sample bias. There is, however, a rela-
tive shift in the photoemission threshold at the lower KE
edge after taking into consideration the applied bias for
each sample. The shift in the threshold is noted from a
comparison of the low KE edge in the spectra of the bi-
ased EG samples. The energy of the VBM for the two are
unchanged and identical but as can be seen in the lower
KE edge of Figure 2(b), the width of the rough EG sam-
ple spectrum is narrower, indicating a higher work func-
tion. For the smooth EG sample, the threshold is shifted
0.11 eV to lower kinetic energy. The discrepancies noted
are of necessity due to the morphological differences
between the samples.
We observe that the sp2 feature centered at 2.0 eV KE
for the grounded sample is essentially quenched in the
rough sample. The spectral emission associated with this
feature is evident in the smooth EG sample and is dimin-
ished with the rough EG sample. This peak is associated
with the 2 p crystalline state of the material [14] and can
be correlated to the minima in the E versus K band
structure approximately 13 eV below the valence band
maxima for graphene. Likewise the features near the
VBM associated with mixed 2 s and 2 p states are essen-
tially non-existent in the rough EG sample. As noted, this
reduction in the density of states with the rough EG sam-
ple is also accompanied by a narrowing of the spectral
band. A multi-component structure characteristic of patch
effects (non-uniform work function) is not evident in the
peak of the scattered energy tail in the 3.00 V spectra of
either sample and is indicative of a uniform work func-
tion for the 1 mm diameter imaging spot of the CMA.
In addition, we observe effects that are strictly due to
the sample biasing. The EDC emissions for the upper 5
eV or so of the valence band for each sample increases
with increased negative sample bias to the extent that the
EDCs are comparable for the 3.00 V bias. The 2.0 eV
peak is not visible beyond a 4.00 V bias as it is indis-
tinguishable from the scattered electron tail of the spectra.
Both of these effects are reversible. This band modifica-
tion is reminiscent of that associated with the negative
electron affinity material properties needed for cold
cathode emission in diamond and cesiated III-V semi-
conductors. In these systems a strong dipole is estab-
lished at the surface of the material. Alternatively, the
biasing may induce shifts in the bands or coupling be-
tween the layers. This characteristic has been explored in
detail elsewhere [12].
The data shows that the sp2 band structure associated
with the VBM is affected by the morphology of the sur-
face. These phenomena affect the width and the photo-
emission threshold. The width of a particular EDC was
determined by subtracting the VBM of the spectrum
Copyright © 2013 SciRes. Graphene
from its low energy photoemission threshold. The VBM
ismeasured by performing a linear extrapolation of the
high kinetic energy edge of the EDC from one half of its
maximum intensity to the spectral baseline. The thresh-
old of photoemission is determined by the work function
of the electrically biased material and is measured by
linearly extrapolating the low kinetic energy edge of the
EDC from the full width at half maximum of the low
energy spectral peak to the spectral baseline. Assuming a
constant band gap,
E, the electron affinity,
, and the
work function, , for a degeneratively doped p-type
surface or semi-metal are related by
  (1)
where W is the width of the VB EDC, is measured
from the vacuum level (photoemission threshold) to the
top of the valence band,
is measured from the top of
the vacuum level to the bottom of the conduction band,
and h
is the photon energy. The change in the electron
affinity can be written as
 . (2)
Using a linear extrapolation of the scattering energy
tail peaks in Figures 2 (a) and (b) to the baseline we get
vacuum levels of 2.2 eV with 3.0 Volt bias and –0.8 eV
for the smooth EG grounded sample. Assuming that the
material has no band gap, a Fermi level of 13.8 eV and
10.8 eV is obtained by a linear extrapolation of the VB
maxima for the respective biases. This implies a total
width of 11.6 eV and thus a work function of 5.1 eV for
the smooth sample which is about 0.4 eV higher than
bulk graphite. The analysis yields a work function of
5.21 eV for the rough EG sample. It should be noted that
in situ Auger analysis revealed no contamination of the
probed areas of the samples.
4. Summary and Conclusion
This work was undertaken to determine if angle inte-
grated UPS using common line sources can differentiate
between the electrical properties of different epitaxial EG
films. We have demonstrated conclusively that UPS can
differentiate between the electronic properties of epi-
taxial EG films with different growth morphologies.
Such is the case for graphene derived from the thermal
decomposition of the C-face of SiC and this result is
consistent with that reported in the literature for other
material systems. The spectral features associated with
the crystalline state of the material are significantly
quenched when the EG film is graphitized on a rough
substrate with a high density of defects present. The re-
sults show that the electronic structures are unique for
different degrees of order in the studied films and that the
ordering is strongly dependent on the substrate prepara-
5. Acknowledgements
The authors thanks J. E. Rowe for fruitful discussions
and gratefully acknowledges the support of the NSF
PREM at Clark Atlanta University, Award # DMR-
0934142, and the NSF MRSEC at the Georgia Institute
of Technology, Award # 0820382. We thank Walter de
Heer, Claire Berger and Yike Hu of the Georgia Institute
of Technology MRSEC for providing the EG samples for
this study and Biswasjit Sannigrahi of the NSF CREST at
Clark Atlanta University, Award # HRD-0630456, for
the AFM images.
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