Effect of Growth Morphology on the Electronic Structure of Epitaxial Graphene on SiC

doi:10.4236/graphene.2013.21008

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Graphene, 2013, 2, 55-59

http://dx.doi.org/10.4236/graphene.2013.21008 Published Online January 2013 (http://www.scirp.org/journal/graphene)

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

Email: [email protected]

Received November 16, 2012; revised December 20, 2012; accepted January 18, 2013

ABSTRACT

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

M. D. WILLIAMS, D. W. HESS

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 × 10−10 Torr) analy-

sis chamber. UPS is a surface sensitive spectroscopic

(a)

(b)

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.

M. D. WILLIAMS, D. W. HESS 57

(a)

(b)

Figure 2. Ne I UPS spectra of smooth (line) and rough (dots)

thick samples under (a) unbiased and (b) −3.00 V biased

conditions.

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

M. D. WILLIAMS, D. W. HESS

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

EhWE





  (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-

tion.

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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