Short communication

Noble gas ion effects on the XPS valence band spectra of silicon

Elaine Walkera,*, Christopher P. Lunda, Philip Jenningsa,John C.L. Cornisha, Craig Klauberb, Glenn Heftera

aPhysics and Energy Studies, Division of Science and Engineering, Murdoch University, South Street, Murdoch, WA 6150, AustraliabCSIRO Division of Minerals, P.O. Box 90, Waterford, WA 6152, Australia

Received 30 April 2003; received in revised form 30 April 2003; accepted 18 August 2003

Abstract

X-ray photoelectron spectroscopy (XPS) has been used to study crystalline silicon (c-Si) (1 0 0) surfaces bombarded with

argon, xenon and neon to examine the interaction of core peaks from these noble gases with the valence band region of silicon.

XPS valence band spectra of xenon- and argon-bombarded silicon were found to have prominent peaks at binding energies of

approximately 6 eV for the xenon (5p1/2, 5p3/2) and 9.3 eV for the argon (3p) core levels, respectively. These core level peaks are

within the silicon valence band energy range.

Attempts to compensate for the interfering peaks are reported but it is concluded that it is better to select a bombarding ion

whose core levels do not overlap with the silicon valence band. Results for the ion bombardment are reported for neon, which has

a peak at approximately 15.5 eV that does not significantly interfere with the photoelectron valence band spectrum of silicon.

# 2003 Elsevier B.V. All rights reserved.

PACS: 33.70.Jg

Keywords: Amorphous silicon; Noble gases; Ion bombardment; XPS valence band; Lineshape analysis

1. Introduction

The heavier noble gases (Ar, etc.) are often used in

the ion bombardment of surfaces during surface ana-

lysis studies for disordering, cleaning [1,2] and depth

profiling. In photoelectron spectroscopies (PES) such

as ultraviolet photoelectron spectroscopy (UPS) and

X-ray photoelectron spectroscopy (XPS), low inten-

sity core level lines of some of these noble gases have

significant magnitudes. Their intensities are compar-

able with those of typical valence band spectra and

their binding energies may coincide with the valence

band spectra of some materials. This can cause con-

siderable confusion and lead to incorrect interpreta-

tion of the valence band data if the interference is not

recognised and properly accounted for [3–5].

Although both XPS and UPS can be used to study

the valence band region, UPS is more commonly used

and so the literature available on the effects of noble

gases on the valence band is primarily in UPS studies

[6–10].

Since noble gases are chemically unreactive, they

tend not to bond with the surface and their outer

electrons do not become part of its valence band.

Consequently, the residual noble gas atoms produce

their characteristic discrete core level peaks in the

Applied Surface Science 222 (2004) 13–16

* Corresponding author. Tel.: þ61-8-9360-2866;

fax: þ61-8-9310-1711.

E-mail address: e.walker@murdoch.edu.au (E. Walker).

0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.08.018

spectrum. For some substances the outer core level of

the bombarding noble gas ion overlaps the valence

band of the substance. In particular when studying the

valence band spectra of amorphous Si (a-Si) produced

by ion bombardment of crystalline silicon (c-Si) this

problem can become critical. This can cause difficul-

ties in analysing the valence band spectrum, especially

if the presence of the noble gas is not recognised. The

core levels of the noble gases have different binding

energies which tend to decrease as the atomic number

increases. So by the appropriate selection of a bom-

barding ion it should be possible to eliminate the

distortions in the photoelectron valence band spectrum

of a substance, which are due to the overlap of the ion

core levels with the valence band. The results of a

study to determine the appropriate bombarding ion to

use for Si surfaces are presented in this paper.

2. Experimental

The samples of conventional crystalline silicon

(1 0 0) were cleaned and degreased [11] before being

placed in the ultra high vacuum (UHV) system, then

bombarded with Ar, Xe and Ne ions, respectively. The

samples were reannealed at high temperatures

(1200 K) to desorb any previously implanted ions

before being bombarded with a different ion.

The fluences of the ions were approximately

1:8 � 1017 ions cm�2 for Ar, 3:0 � 1017 ions cm�2

for Xe and 3:3 � 1017 ions cm�2 for Ne, with a kinetic

energy for all the ions of 10 keV. A VG ESCALAB

MK II concentric hemispherical analyser was used to

collect the XPS VB spectra using the integral N(E)

mode. Photoemission was produced by Mg Ka X-rays

from a conventional unmonochromated X-ray source.

The XPS VB spectra were taken after each bombard-

ment and also after reannealing the sample by flashing

it to >1200 K using an electron beam heater.

The raw spectra were numerically treated as

detailed by Walker [12]. The procedure consisted of

some smoothing, deconvolution to remove the effects

of instrument and X-ray lineshape broadening, filter-

ing to remove deconvolution artefacts and background

subtraction. The energy scale was also converted into

binding energy by subtracting the kinetic energy from

the photon (Mg Ka) energy (1253.6 eV), the zero

coinciding with the top of the localised states. Because

of the unmonochromated X-ray source used in this

work the deconvolution file was a convolution of the

instrument broadening function represented by a 1 eV

FWHM Gaussian and a function representing the Mg

Ka lineshape using the method of Klauber [13]. The

spectra were then normalised to the highest peak in the

reannealed crystalline Si spectrum for comparison.

Although normalisation to area is normally preferred,

in this study because of the extra area supplied by the

core peaks of the noble gases this would not have

given a consistent or reliable result.

3. Results

The spectra for the numerically-treated reannealed

c-Si and ion-bombarded (amorphous silicon) samples

are shown in Fig. 1.

The differences in the spectra are clear, with the Ar

core level producing a peak at a binding energy of

9.25 eV, the Xe core level producing an unresolved

doublet at 5.9 eV and the Ne core level a peak at

15.5 eV. The Ar and Xe peaks are the 3p and 5p1/2/5p3/2,

respectively, and clearly overlap with the VB of the Si.

The Ne peak is the 2p Ne line which only partially

overlaps the tail of the spectrum. By subtracting the

normalised spectra point by point (Fig. 2), an indica-

tion of the actual shape of each core peak can be

obtained. This was achieved by subtracting the neon

0

500

1000

1500

2000

-505101520

c-Si

Xe

Ar

Ne

Binding energy (eV)

N(E

) ar

b. u

nits

Fig. 1. Comparison of XPS spectra of c-Si (solid line), Xe (long

dashes), Ar (short dashes) and Ne (double chained) bombarded

silicon.

14 E. Walker et al. / Applied Surface Science 222 (2004) 13–16

bombarded spectrum from the Ar- and Xe- bombarded

spectra, this provided shapes for the Xe and Ar peaks.

Reversing the procedure produced the shape for the Ne

peak. This allowed us to numerically remove the core

level peaks of the noble gases from the spectra to get

the true shape of the disordered c-Si (amorphous Si)

valence band spectrum.

The numerically-treated c-Si and amorphous silicon

(a-Si), spectra were normalised to the same area and

fitted with Gaussians to represent their component ss-,

sp- and pp-like peaks, in order to discover what

changes occur in the DOS on disordering (Fig. 3).

The valence band lineshapes for crystalline and

amorphous silicon were significantly different, with

the pp-like peaks enhanced in the crystalline spectrum.

The fitted peaks for the amorphous silicon spectrum

tended to have higher FWHM than the crystalline,

especially in the sp and ss areas. These will be

analysed further in a subsequent paper.

4. Discussion

The spectra of Figs. 1 and 2 clearly show that the

core levels from the bombarding ions coincide with

the valence band of the substance and so distort the VB

spectra. This can cause errors and, if not taken into

account, these peaks are often misidentified or ignored

[4,5,14]. Even if they are recognised, they may not be

easy to compensate for. Some researchers have noted

the Ar peak before [15,16] but did not develop meth-

ods for removing its influence. References to the use of

other noble gases could only be found in UPS litera-

ture [6–10].

Subtracting the spectra from each other gave an

indication of what the core lines were, but since the Xe

and Ar lines overlapped, only those subtraction spec-

tra using the Ne data could be considered reliable. The

location of the neon peak, which is mostly outside the

valence band, makes it ideal for determining a-Si VB

spectra without interference. The Xe and Ar ions are

not as useful: to identify the Xe and Ar peaks well

enough to use them to reconstruct the a-Si spectra

would require the use of a Ne spectrum for the

subtraction, so it is much simpler to use Ne as the

bombarding ion.

Attempts were made to numerically remove the

interfering Xe and Ar core peaks from the valence

band spectra by decoupling (fitting the spectrum with

Gaussian peaks). Core peak shapes determined earlier

from the subtractions (see Fig. 2) were used as starting

parameters for the Xe and Ar peaks. These Xe and Ar

core level peaks lie very close to, or actually coincide

with, component peaks in the Si VB itself and their

intensities are not accurately known, which means the

fitting process tended to distort the true a-Si spectrum.

With the Ne bombarded spectra however this proce-

dure works well.

It is clear from the results that it is better to use

Ne as the bombarding ion of choice for silicon VB

0

500

1000

1500

-505101520

a-Si

c-SiNe

Ar

Xe

Binding energy (eV)

N(E

) ar

b. u

nits

Fig. 2. Core peaks of Xe (long dashes), Ar (short dashes) and Ne

(double chained) with c-Si (solid line) and a-Si (chained) spectra

for comparison.

0

300

600

900

1200 (a)

N(E

)

0

200

400

600

800

-505101520

(b)

Binding energy (eV)

N(E

)

Fig. 3. Comparison of c-Si (solid line) and a-Si (dashed line) (a)

XPS valence band spectra with (b) fitted components of each.

E. Walker et al. / Applied Surface Science 222 (2004) 13–16 15

PES studies. For other materials the choice of ion

may be different depending on the width of the

valence band. Helium, with its electronic binding

energy being about 3 eV higher than the neon peak

(�18.5 eV) [17], is well away from the valence band

but its low mass is a serious disadvantage. It has been

known to successfully clean low to significant con-

tamination levels of carbon and oxygen, but the

amount required is significantly more than for the

heavier noble gases [18].

5. Conclusions

Noble gases have XPS core levels with binding

energies close to or coinciding with the valence bands

of many substances. If noble gases are used to ion

bombard the sample they can distort the measured

photoelectron VB spectra. Argon has a peak at a

binding energy of 9.25 eV below the top of the

VB, xenon has a doublet peak with a maximum at

5.9 eV, and neon a peak at 15.5 eV. The argon and

xenon peaks cannot be easily compensated for with

confidence and therefore cannot be removed by

numerical means without leading to distortion of

the VB spectra of silicon. The neon core level peak

which lies only on the edge of the VB spectrum of Si

can be easily subtracted numerically from the

observed spectra to obtain true amorphous (a-Si)

XPS VB spectra. Neon is therefore the most suitable

ion with which to sputter silicon or to produce a-Si.

This would also apply to other materials with similar

VB widths.

Acknowledgements

This work was funded in part by the Minerals and

Energy Research Institute of Western Australia.

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