Ytterbium oxide formation at the graphene–SiC interface studied by photoemissionSomsakul Watcharinyanon, Leif I. Johansson, Chao Xia, and Chariya Virojanadara Citation: Journal of Vacuum Science & Technology A 31, 020606 (2013); doi: 10.1116/1.4792040 View online: http://dx.doi.org/10.1116/1.4792040 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/31/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Direct experimental evidence for the reversal of carrier type upon hydrogen intercalation in epitaxialgraphene/SiC(0001) Appl. Phys. Lett. 104, 041908 (2014); 10.1063/1.4863469 Charge doping of graphene in metal/graphene/dielectric sandwich structures evaluated by C-1s core levelphotoemission spectroscopy APL Mat. 1, 042107 (2013); 10.1063/1.4824038 Pentacene as protection layers of graphene on SiC surfaces Appl. Phys. Lett. 95, 093107 (2009); 10.1063/1.3224833 Absolute determination of film thickness from photoemission: Application to atomically uniform films of Pb on Si Appl. Phys. Lett. 85, 1235 (2004); 10.1063/1.1783019 Initial oxide/SiC interface formation on C-terminated β-SiC(100) c(2×2) and graphitic C-rich β-SiC(100) 1×1surfaces J. Vac. Sci. Technol. B 22, 2226 (2004); 10.1116/1.1768532

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Ytterbium oxide formation at the graphene–SiC interface studiedby photoemission

Somsakul Watcharinyanon,a) Leif I. Johansson, Chao Xia, and Chariya VirojanadaraDepartment of Physics, Chemistry, and Biology, Link€oping University, S-58183 Link€oping, Sweden

(Received 20 December 2012; accepted 30 January 2013; published 12 February 2013)

Synchrotron-based core level and angle resolved photoemission spectroscopy was used to study the

formation of ytterbium (Yb) oxide at the graphene–SiC substrate interface. Oxide formation at the

interface was accomplished in two steps, first intercalation of Yb into the interface region and then

oxygen exposure while heating the sample at 260 �C to oxidize the Yb. After these processes, core

level results revealed the formation of Yb oxide at the interface. The Yb 4f spectrum showed upon

oxidation a clear valence change from Yb2þ to Yb3þ. After oxidation the spectrum was dominated

by emission from oxide related Yb3þ states and only a small contribution from silicide Yb2þ states

remained. In addition, the very similar changes observed in the oxide related components identified

in the Si 2p and Yb 4f spectra after oxidation and after subsequent heating suggested formation of a

Si-Yb-O silicate at the interface. The electronic band structure of graphene around the �K-point was

upon Yb intercalation found to transform from a single p band to two p bands. After Yb oxide

formation, an additional third p band was found to appear. These p bands showed different

locations of the Dirac point (ED), i.e., two upper bands with ED around 0.4 eV and a lower band

with ED at about 1.5 eV below the Fermi level. The appearance of three p-bands is attributed to a

mixture of areas with Yb oxide and Yb silicide at the interface. VC 2013 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4792040]

I. INTRODUCTION

Graphene has a great potential for applications in nanoe-

lectronics due to its excellent physical and electronic proper-

ties.1 A high charge carrier mobility up to the order of 106

cm2/Vs2 (Ref. 2) and a high thermal stability3 make graphene

a new material for ultrafast electronic devices. For fabrication

of such devices, a large high quality sheet of graphene is

essential. So far, graphene grown by thermal decomposition

of SiC(0001) has been shown to be a promising way to pro-

duce such sheets4,5 on a semi-insulating substrate. This

growth process can provide uniform graphene films of con-

trolled thickness on a wafer size scale. Additionally, an

advantage of silicon carbide (SiC) is its wide band gap that

has been utilized in high-voltage and high power devices,6,7

making the combination graphene and SiC really appealing

for device applications. Especially, for metal–oxide–semi-

conductor systems that are widely used in modern electronic

devices such as field-effect transistor8 and sensor.9

In this study, we try to functionalize graphene–SiC inter-

face by creating a metal oxide after metal intercalation proc-

esses. We have recently found that ytterbium (Yb) does

intercalate at 300–500 �C on graphene grown on Si-face

SiC.10 Yb oxide can be considered as good candidate mate-

rial since it has high dielectric constant and high resistiv-

ity11–13 and therefore would provide a good insulating

barrier between the graphene and the SiC substrate. Thus,

the technique to prepare Yb oxide at interface was performed

and studied. Two intercalation steps were found necessary to

utilize, i.e., first Yb and then oxygen intercalation.

We utilized synchrotron radiation based photoemission

techniques to examine the intercalation and oxide formation

steps and the effects of these steps induced in the electronic

properties of graphene grown on Si-face SiC. Core level

photoemission spectroscopy (PES) data reveal the formation

of Yb oxide at the interface after Yb intercalation and subse-

quent exposures to oxygen. Angle resolved photoemission

spectroscopy (ARPES) results show clear and pronounced

changes in the electronic band structure after Yb intercala-

tion and after Yb oxide formation at the interface. These

findings are presented and discussed below.

II. EXPERIMENT

The experiments were carried out at beamline I4 at the

MAX synchrotron radiation laboratory in Lund (Sweden).

This beamline provides linearly polarized light in the energy

range from 13 to 200 eV which is suitable for studies of the

electron band structure and of shallow core levels. The

beamline is equipped with a spherical grating monochroma-

tor, a preparation chamber, and an analyzer chamber. The

preparation chamber is equipped with gas-inlet system and a

number of optional ports for evaporation sources. The ana-

lyzer chamber is equipped with an angular resolved electron

energy analyzer with a two-dimensional detector from

SPECS. The monochromator and electron analyzer were typ-

ically operated to give an energy resolution of around

0.1 eV. The Yb 4f and Si 2p levels were acquired at photon

energy of 140 eV while the electronic band structure was

investigated using mainly a photon energy of 33 eV. All

experiments were conducted in ultra high vacuum, at a base

pressure of <1� 10�10 Torr.

Graphene samples were prepared by direct current heat-

ing of Si-face 4H-SiC substrates at 1250 �C for 2 min in ultra

high vacuum. This procedure is known, and was checked, to

provide a dominant coverage of 1 monolayer (ML) graphenea)Electronic mail: somwat@ifm.liu.se

020606-1 J. Vac. Sci. Technol. A 31(2), Mar/Apr 2013 0734-2101/2013/31(2)/020606/5/$30.00 VC 2013 American Vacuum Society 020606-1

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with a mixture of small 0 and 2 ML areas/domains.5 The Yb

was deposited in situ, by resistive heating of a tungsten coil,

onto the graphene sample kept at room temperature. After

Yb deposition, the sample was heated at 300–500 �C in order

to promote Yb intercalation into graphene–substrate inter-

face. Intercalation of Yb at the interface was verified by the

typical shift of the SiC substrate core levels,10 in this case

the Si 2p doublet, to lower binding energy (BE). The Yb

intercalated samples were then exposed to pure oxygen at

1000 l (1 L¼ 10�6 Torr*s) while kept at a temperature of

260 �C, to promote oxygen intercalation and oxide formation

at the interface. The effects induced after the intercalation

and oxide formation steps were investigated using PES and

ARPES and also effects of subsequent heating of the sam-

ples at temperatures from 300 to 500 �C.

III. RESULTS AND DISCUSSION

Valence band spectra collected from the Si-face graphene

sample before and after Yb deposition and intercalation, af-

ter O2 exposure and after subsequent heat treatments are

shown in Fig. 1(a). At this photon energy of 140 eV, the

spectrum from the clean graphene sample (bottom curve)

exhibits broad features originating not only from C 2p states

of the graphene and carbon buffer layer but also features

attributed hybridized C 2p and Si 3 sþ3 p states14 from the

SiC substrate. After Yb deposition the spectrum is totally

dominated by emission from the Yb 4f levels, located in the

BE range from 0 to 4 eV. The spectrum shows two doublets

of the divalent Yb2þ 4f14 initial-state configuration with a

spin-orbit splitting of 1.3 eV and a branching ratio 3:4

(4f5/2:4f7/2). The two doublets originate from bulk and sur-

face Yb atoms15 so at this stage the Yb is located on top of

the graphene. After heating the sample at 500 �C only one

considerably broader Yb 4f doublet, and slightly shifted to

lower BE, is observed. We also observe shifts of the Si 2plevels after heating, see below. Our earlier studies10,16,17

have demonstrated that an elevated sample temperature is

sometimes required to promote intercalation of a metal

through the graphene and buffer layers into the substrate

interface. The intercalated metals10,16,17 were found to bond

to Si at the interface. We therefore interpret the shift and

broadening of the Yb2þ 4f levels and Si 2p to indicate that

after heating Yb has intercalated into the interface and inter-

acted with Si atoms in the topmost layer of the SiC substrate.

A similar shift of Yb 4f and Si 2p spectra has earlier been

reported during formation of Yb silicide on Si(111) and has

been assigned to Yb–Si bonding.18,19 A weaker broad feature

located between 5 and 12 eV originating from emission from

a trivalent Yb3þ 4f13 initial-state configuration is also

detected after heating, see enlarged spectrum in Fig. 1(b),

bottom curve. The relative intensity ratio between the Yb3þ

and Yb2þ components (IYb3þ/IYb2þ) is found to be about 0.2

indicating that the Yb silicide formed after heating contains

mainly Yb2þ 4f14 atoms.

After intercalation of Yb the sample was exposed to

1000 L of oxygen while kept at a temperature of 260 �C.

This gave rise to pronounced changes in the Yb 4f emission

as shown in Figs. 1(a) and 1(b). The relative intensity of

divalent Yb2þ emission decreased drastically whereas the

features related to trivalent Yb3þ become really pronounced

and dominant, see in Fig. 1(b). This indicates oxidation of

the intercalated Yb transforming Yb2þ 4f14 atoms into Yb3þ

4f13 atoms, i.e., formation of Yb–O or Si–Yb–O bond-

ing,12,20 at the interface upon oxygen exposure and heating.

The significant increase in the relative intensity ratio

between Yb3þ and Yb2þ components from 0.2 to 2.5 indi-

cates also that most of the Yb at the interface has oxidized.

The relative intensity of the Yb3þ component is found to

increase and the Yb2þ component to decrease slightly after

FIG. 1. (a) Valence band spectra collected at photon energy of 140 eV from

the graphene sample before and after Yb deposition and intercalation at

500 �C, after O2 exposure at 260 �C and heating from 300 to 500 �C. (b)

Enlarged valence band spectra collected after Yb intercalation at 500 �C and

after O2 exposure at 260 �C.

020606-2 Watcharinyanon et al.: Ytterbium oxide formation at the graphene–SiC interface 020606-2

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additional heating of the sample at temperatures up to

500 �C, as seen in Fig. 1(a).

A series of the Si 2p spectra acquired at photon energy of

140 eV are shown in Fig. 2. The spectrum collected from the

clean sample shows a Si 2p doublet of the SiC substrate at a

BE of 101 eV (Si 2p3/2). After Yb deposition and heating at

500 �C, the Si 2p levels are found to shift to lower BE and a

second doublet appears on the low binding energy side. A

similar shift, broadening and appearance of an additional Si

2p doublet on the low binding energy side has earlier been

observed in the Si 2p spectrum after intercalation of H,21,22

Li,16 and Na17 when the intercalated element bonds to Si

atoms in the topmost layer of the SiC substrate. The two Si

2p components in Fig. 2 are now determined to be located at

BE:s of 98.6 and 99.5 eV, respectively. The component at

98.6 eV is interpreted to originate from Si atoms in the top-

most SiC substrate bilayer, which are directly interacting

with intercalated Yb atoms.19 The new dipole layer thereby

formed at the interface shifts also the bulk SiC substrate

component (99.5 eV) to lower BE. This thus suggests that

Yb has intercalated under the graphene and the carbon buffer

layer and into the interface. After oxygen exposure and heat-

ing at 260 �C the Si 2p spectrum exhibits an additional com-

ponent on the high binding energy side, shifted by 0.7 eV

with respect to the bulk SiC (99.5 eV) component, as seen in

Fig. 2. This component has earlier been suggested to origi-

nate from oxidized Si in the topmost SiC bilayer forming ei-

ther a Si1þ oxide (e.g., C–Si–O bonding)23,24 or a Si-Yb-O

silicate.12 The relative intensities of the different compo-

nents in this Si 2p spectrum suggest that about 80% of the Si

in the topmost layer of the SiC substrate appears as Si1þ ox-

ide or Si-Yb-O silicate while the rest 20% remains as a sili-

cide (Si-Yb). Earlier studies of oxygen intercalation of

graphene25,26 have demonstrated the formation of silicon

oxides at the interface. Recently, it was shown that oxygen

intercalation can be performed by annealing the graphene on

SiC(0001) in air. This results in formation of quasifree stand-

ing graphene.27 Heating the sample at higher temperatures

up to 500 �C, results in an increase in relative intensity of

this shifted oxidized Si component and a decrease in relative

intensity of the Si-Yb component at 98.6 eV. This corre-

sponds well to what observed in the Yb 4f spectra in Fig.

1(a) where the relative intensity of the Yb3þ component

increased and the Yb2þ component decreased somewhat

upon further heating. The similar changes in relative inten-

sity of the Si 2p and the Yb 4f levels after oxygen exposure

and heating therefore suggest that the oxygen most likely

forms a Si-Yb-O silicate at the interface. However, also after

the additional heating Yb silicide is still present at the inter-

face but in a minor quantity.

The p-band structure of graphene collected around the �K-

point and perpendicular to the �C! �K direction is shown in

Fig. 3. The clean graphene sample, Fig. 3(a), exhibits a sin-

gle p-band with linear dispersion and the Dirac point (ED)

located at �0.4 eV below the Fermi level, which are charac-

teristics of 1 ML graphene on Si-face SiC.28 After Yb inter-

calation, i.e., after Yb deposition and heating at 500 �C, two

p-bands appear as shown in Fig. 3(b). These two p-bands

have been interpreted10 to indicate a transformation and

decoupling of the carbon buffer layer into a second graphene

layer. The two p-bands exhibit different Dirac points, i.e.,

one located at 0.4 eV and the other at 1.5 eV below the Fermi

level. The downward shift of one of the p-bands (ED

¼ 1.5 eV) is interpreted to originate from electron transfer

from Yb to unoccupied graphene p states. After oxygen ex-

posure, distinct changes of the p-band structure are detected

around the �K-point, as seen in Fig. 3(c). Three p-bands can

now be distinguished. The dominant p-band with the Dirac

point at 0.4 eV is now accompanied by a fainter band on ei-

ther side. The line profile across the bands at a BE of 1.0 eV,

see Fig. 3(d), shows shoulders (labeled 2) outside the main

bands (labeled 1). The separation between these two p bands

labeled 1 and 2 is 0.06 A�1 which is similar to the separation

between the two bands of pristine bilayer graphene on Si-

face SiC.28,29 The p-band with the Dirac point shifted to

1.5 eV (labeled 3) is still present. The line profile across the

bands at a BE of 2.2 eV, in Fig. 3(e), shows the presence of

these three p-bands (labeled 1, 2, and 3) of different relative

intensities. The appearance of three p-bands after oxygen

exposures can be interpreted to be due to a mixture of differ-

ent oxides (C–Si–O, Yb–O, and Si–Yb–O bonding) and Yb

silicide at the interface, contributing to different doping lev-

els of the graphene layers at different areas. The additional

band with the Dirac point located close to 0.4 eV is attributed

to oxides at the interface while the band with the Dirac point

shifted down to around 1.5 eV is attributed to areas where

Yb silicide contributes to additional electron doping. This

corresponds fairly well to what is observed in the Si 2p and

FIG. 2. (Color online) Si 2p spectra collected at photon energy of 140 eV

from the graphene sample before and after Yb deposition and intercalation

at 500 �C, after O2 exposure at 260 �C and heating from 300 to 500 �C.

020606-3 Watcharinyanon et al.: Ytterbium oxide formation at the graphene–SiC interface 020606-3

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Yb 4f spectra after oxygen exposures that show presence of

both silicide and oxides on the sample.

IV. SUMMARY AND CONCLUSIONS

Formation of Yb oxide at the graphene–SiC interface has

been investigated using the photoemission techniques, PES

and ARPES. The Yb oxide was prepared via two intercala-

tion processes. First, the rare earth metal Yb was deposited

and intercalated into the interface by heat treatment at

500 �C. Intercalation was concluded not only from the

observed shift to lower BE of the Si 2p levels and the shift

and broadening of the Yb 4f levels but also from the distinct

changes observed in the p-band structure close to the�K-point. Transformation of monolayer graphene to bilayer

graphene is visualized by the conversion from a single p-

band into two p-bands with different Dirac point locations,

due to different electron doping of the graphene layers. For-

mation of oxides was then performed by oxygen exposure of

the Yb intercalated sample while heated at 260 �C. Yb oxide

formation was clearly identified by the valence change from

Yb2þ to Yb3þ and Yb3þ 4f13 emission was found to give the

dominant contribution after oxygen exposures. From the

similar changes in relative intensity of the oxide related

components in the Si 2p and the Yb 4f spectra, it appeared

that oxygen exposure and heating result in formation of

Si-Yb-O silicate at the interface. The formation of Yb3þ and

Si1þ oxides at the interface contributed to the appearance of

one additional p-band with ED at around 0.4 eV below the

Fermi level, so a fainter band was then discernable on either

side of the main p-band. Upon further heating at tempera-

tures up to 500 �C, the oxides are found to slightly increase.

ACKNOWLEDGMENTS

The authors gratefully acknowledge supports from the

European Science Foundation and the Swedish Research

Council.

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