~ ) Solid State Communications, Vol. 77, No. 5, pp. 341-343, 1991. Printed in Great Britain.

0038-1098/9153.00+ .00 Pergamon Press plc

a

b

X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES OF THE O Is STATE ON THE

SURFACE OF DUAL ION BEAM DEPOSITED ZrOx FILMS

Y.S.Tang • and N.K.Huang b

Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12

8QQ, Scotland

Institute of Nuclear Science and Technology, Sichuan University, Chengdu, People's Republic of

China

(Received by D. Van Dyck - October 15, 1990)

The 0 Is state on the surface of dual ion beam deposited ZrOx films were studied

by using x-ray photoelectron spectroscopy. Different from the bulk of the films, it

was found that a new O Is peak exists on the surface, which is assigned to be due

to the existence of carbon contamination.

X-ray photoelectron spectroscopy (XPS) has

widely been used in analyzing the bonding and

composition properties of materials including pre-

cision optical films, such as TiO v SiO v Ta20 ~,

ZrO 2 films etc., especially on surface and interface

studies. As one of the most important optical films,

ZrO 2 has been studied by several groups.tl-~l The

deposition methods applied were vacuum evapora-

tion, ion beam assisted evaporation and dual ion

beam deposition techniques. For the characteriza-

tion, various techniques, such as x-ray diffraction,

electron transmission spectroscopy, ellipsometry,

Rutherford backscattering, XPS etc. have been

employed. In the previous studies,t3.4~ we reported

the dual ion beam deposition process dependent

refractive index, atomic composition and Zr bond-

ing properties of the ZrOx (0 < x _< 2) films. Here

we concentrate on the O Is state on the natural

surface of the dual ion beam deposited ZrOx films.

Except for the state same as in the bulk of the

films, a new O Is state was observed, which will

be discussed in detail in this communication.

As reported before,m the samples were prepared

by a dual ion beam deposition technique. The

341

composition of the ZrOx films, depending on the

deposition parameters, is 0 _< x _< 2. The XPS

measurements were carried out on a VG ESCAL-

AB MkII system using monochromatic AI Kcz

radiation (hv = 1486.6 eV) as the x-ray source,

which has a pass energy of 20 eV and a typical

sampling depth of about 3 nm. During the experi-

ments, an ultrahigh vacuum (better than 0.4x10 -'°

Torr) was maintained in the analyzer chamber. The

sputtering beam was from a cold cathode A r ion

gun run at 4 kV, which gives an estimated etching

rate of about 3 nm/min.

Figure la shows a wide scan spectrum in the

0-1000 eV kinetic energy range of a sample on the

natural surface. The major components are Zr 3s,

Zr 3p, Zr 3d, Zr 4s, Zr 4p, 0 Is, 0 2s, C Is, C

(KLL), Ar 2,o and Si 2p lines. After 10 min of ion

etching, the approximate composition becomes

stoichiometric ZrOvm which is considered as the

bulk of the films. It can be seen in figure lb that no

apparent carbon related lines appear again. De-

tailed depth profile studies were reported else-

where.m

What is interesting is that the O l s line on the

342 X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES Vol. 77, No. 5

Io J ~ N I ~

(a)

200 400 600 800 1000

BINDING ENERGY (eV)

Fig.1 XPS surveys of a sample both on the

surface and in the bulk.

D 0

natural surface of the samples has a shoulder on

the higher energy side, as shown in figure 2a. A

number of experiments indicate that the double

peaks always exist despite the different values of x

in the films, but the energy separation of this two

peaks varies with x and is typically within a range

of 1.8-2.3 eV. This is similar to the previous

observed results on the real surface of glasses,

such as alkali silicate glasses, where two O ls

peaks were observed, which were assigned to be

.4

[.., z

o L)

0 Is A B (b)

527 529 531 533 535 537 539

BINDING ENERGY (eV)

Comparison of the O I s states on the

surface and in the bulk of a sample.

Fig.2

due to the -Si-O-Si- bridging (high energy peak)

and -=Si-O non-bridging (low energy peak) oxygen

configurations.[5-?l In our case, the situation is

slightly different from the above. Comparing fig-

ures 2a and 2b, it is obvious that the lower energy

peak A locating at 531.2 eV is resulting from the -

Zr-O-Z~ bridging oxygen configuration (shown in

figure 3a), but is the higher energy peak B locating

at 533.3 eV due to the -~Zr-O- oxygen configura-

tion? As we know, usually, the more ionic the

oxygen ions, the lower kinetic energy the XPS

peak locates at. So that the new O l s peak B

(533.3 eV) is more likely to be resolving a ---

Zr-O-Si= or a =Zr-O-C= bonding oxygen configu-

ration. From the composition and depth profile

analyses2) it seems likely to be the -Zr-O-C-

bonding oxygen configuration (see figure 3b) due

to the carbon contamination on the sample surface.

To confirm the above idea, we first clean a

sample surface by using the Ar* ion beam to etch

the sample for 5 min until XPS showing the

existence of only one O Is peak, and then intro-

duce it into a carbon contaminated chamber for

one hour. Further XPS measurements on the

surface of this sample indicate the reappearance of

the double O l s peaks, similar to that shown in

figure 2a, which gives a strong evidence of the

above discussion.

In conclusion, we have studied the O Is state on

the surface of the dual ion beam deposited ZrOx

films. Two O l s peaks were observed, which are

Zr Zr N \

0 0 N N

Z r - - O - - Z r - - O - - Z r Z r - - O - - Z r - - O - - C / /

O O / /

Zr Zr

(a) (b)

Fig.3 Bonding models of oxygen on the natural

surface of the samples.

Vol. 77, No. 5 X-RAY PHOTOELECTRON SPECTROSCOPIC STUDIES 343

common for all the samples with different values Acknowledgement - The authors are grateful to

of x, and are respectively assigned to -=Zr-O-Z~ Mr.H.Liu of the University of Surrey for his help

and ----Zr-O-C-- bonding oxygen configurations, with the XPS experiments.

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