Evaluation of oxygen in oxide materials by SIMSusing 18O2 gas

Satoshi Takeda*

Research Center, Asahi Glass Co. Ltd., 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi,

Kanagawa 221-8755, Japan

Available online 27 April 2004

Abstract

SIMS analysis using 18O2 gas was applied to investigate the effects of oxygen diffusion from the atmosphere on thermal

properties of a float glass. It was revealed that a phase separation was induced by the oxygen diffusion, resulting in degradation

of the optical properties of the glass. The same analysis was performed for fluorine-doped SnO2 films grown by atmospheric

chemical vapor deposition (APCVD). It was found that the amount of diffused 18O into the film was larger for the film with lower

resistivity when the films were heat-treated in 18O2=N2 atmosphere. Based on the results obtained, the roles of oxygen in the

electrical properties of the films are discussed.

# 2004 Elsevier B.V. All rights reserved.

Keywords: Oxygen; SIMS; Float glass; SnO2; 18O

1. Introduction

Oxide materials are widely used because of their

excellent stability under our current environment. In

order to apply the oxide materials to practical pro-

ducts, high stability and reliability under severe con-

ditions are necessary. Therefore, the evaluation of

interactions between the oxide materials and oxygen

in the atmosphere is indispensable. However, it is

difficult to distinguish the origin of the oxygen

because the majority of the components in the oxide

materials is oxygen. In such a case, SIMS analysis

using oxygen isotopes is considered to be very useful.

In the present study, we investigated oxygen diffusion

from the atmosphere into float glass and fluorine-

doped SnO2 (SnO2:F) films by SIMS using 18O2

gas. Furthermore, in order to clarify the effects of

the oxygen diffusion on their thermal or electrical

properties, the 18O diffused materials were character-

ized by various analytical techniques. Based on the

results obtained, the interactions between the oxide

materials and oxygen from the atmosphere are dis-

cussed.

2. Experimental

Commercial soda–lime–silica float glass was used

in this study. The heat treatment conditions are sum-

marized in Table 1. The experimental setup of the heat

treatment was described in Refs. [1,2]. The fluorine-

doped SnO2 films were grown onto silica-coated glass

by APCVD with a thickness of �1 mm [3]. The as-

grown film was subsequently annealed at 500 8C for

10 min in N2 or air atmosphere. Thereafter, the

Applied Surface Science 231–232 (2004) 864–867

* Tel.: þ81-45-374-8794; fax: þ81-45-374-8892.

E-mail address: satoshi-takeda@agc.co.jp (S. Takeda).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2004.03.151

annealed films were reheated at 500 8C for 10 min in

an 18O2=N2 ¼ 1=4 atmosphere. The depth profiles of

Sn and 18O were measured using SIMS (Adept 1010

and ATOMIKA 6500). Positive secondary ions were

detected using an O2þ primary ion beam operated at

8 keV, 100 nA. The angle of incidence was 608 to the

normal of the sample surface. Negative secondary ions

were detected using a Csþ primary ion beam operated

at 6 keV, 20 nA. The angle of incidence was 458 to the

normal of the sample surface. The thermal stability of

the float glass was evaluated by the haze value change

using a haze meter. The haze value is defined as Td/

Tt � 100% (Td: scattered light; Tt: transmitted light).

The resistivity (r), Hall mobility (m) and free carrier

density (n) of SnO2:F films were evaluated by the four-

point probe method and Hall-effect measurement in

the Van der Pauw method.

3. Results and discussion

3.1. Float glass

Float glass is most widely used in industrial uses for

architectures, automobiles and displays because of its

high productivity and excellent flatness over a large

area. When the glass is used for architectural or auto-

motive applications, it is often tempered or bent by heat

treatment and quenched in air. Occasionally, the

appearance of the glass becomes hazy after the process

[4–6]. This is a serious problem for the glass manu-

facturing industry because the transparency, which is

one of the most important properties of glass, is lost.

As shown in Table 1, the haze value significantly

increases for the glass heat-treated in 18O2=N2 atmo-

sphere. This phenomenon is not observed when heat-

treating in Ar atmosphere. This result suggests that the

increase in haze value may be induced by oxygen from

the atmosphere. Fig. 1 shows a SIMS depth profile of

tin for the glass heat-treated in Ar or 18O2=N2 atmo-

sphere. Here, it is known that tin is present at the glass

surface as a result of direct contact with the molten tin

bath, and that the tin is not uniformly distributed in the

glass [4,6]. The significant diffusion of tin to the

surface is distinctly observed after the heat treatment

Table 1

Heat treatment conditions and haze value for a float glass

Samples Temperature

(8C)

Atmosphere Haze

(%)

Soda–lime–silica

glass

740 18O2=N2 ¼ 1=4 3.2

" " Ar ¼ 100% 0.3

" Without heat

treatment

– 0.1

Fig. 1. SIMS depth profile of Sn for the bottom face of soda–lime–

silica float glass heat-treated in Ar or 18O2=N2 atmosphere.

Fig. 2. SIMS depth profile of 18O for the bottom face of soda–

lime–silica float glass before and after heat treatment in Ar or18O2=N2 atmosphere.

S. Takeda / Applied Surface Science 231–232 (2004) 864–867 865

in 18O2=N2 atmosphere. In addition, the tin depth

profile shallower than the hump position markedly

changes. These results suggest that the tin is supplied

from a shallower region than the hump. However, no

marked change in the tin depth profile is observed for

the glass heat-treated in Ar atmosphere, indicating that

the significant diffusion of tin is induced by oxygen

diffusion from the atmosphere.

As is seen in Fig. 2, the depth of diffused 18O is

�300 nm of the surface, suggesting that the effect of

oxygen diffusion on the oxidation states of tin is

within �300 nm and this oxidation should be related

with a significant change in the tin depth profile. TEM

observations revealed that the crystalline SnO2 nano-

particles were precipitated in the tin-enriched layer

[1,2]. This fact indicates that a phase separation is

induced by the oxygen diffusion from the atmosphere

into the glass. Consequently, Sn2þ may be supplied to

the surface from the inner region in order to compen-

sate for the marked decrease in Sn2þ concentration in

the glass system, resulting in the formation of a tin-

enriched layer.

3.2. SnO2:F films

Table 2 shows the resistivity (r), carrier density

(n) and Hall mobility (m) of the films, as-grown,

annealed in N2 and annealed in air. It is found that rof the annealed films decreases compared to that of

as-grown film, indicating that the annealing is an

effective way to reduce the resistivity of the films

[7]. In addition, the decrease is considered to be

primarily due to the increase in m because the mincreases with decreasing r although the change in n

is very small.

Fig. 3 shows SIMS depth profile of 18O for the films

reheated in 18O2=N2 atmosphere. It is revealed that the

amount of diffused 18O is larger for the films with

lower resistivity. Assuming that the diffusion path of

O2 gas is primarily via grain boundaries [3,8,9], the

amount of the incorporated 18O is considered to be

corresponding to the amount of the removed oxygen

adsorbed at grain boundaries by the annealing. That is,

the role of the annealing may be to remove the oxygen

adsorbed at the grain boundaries. Consequently, the

potential barrier height at the boundaries reduces,

resulting in the increase in Hall mobility.

4. Conclusion

In this paper, the effects of oxygen diffusion from

the atmosphere on thermal and electrical properties

of float glass and SnO2:F films were investigated by

SIMS using 18O2 gas. It was clearly revealed that the

optical properties of the glass were strongly influ-

enced by oxygen diffusion from the atmosphere.

Also, it was considered that adsorbed oxygen at

the grain boundaries of the SnO2:F films was effec-

tively removed by annealing process. These results

indicate that SIMS analysis using 18O2 gas is a

powerful method to investigate not only interactions

Table 2

The electrical properties of SnO2:F films grown by APCVD

Samples Resistivity, r (O cm) Hall mobility, m (cm2/V s) Carrier density, n (cm�3)

As-grown film 2.34 � 10�3 19.8 1.32 � 1020

Air-annealed film 1.94 � 10�3 24.3 1.33 � 1020

N2-annealed film 0.63 � 10�3 55.6 1.78 � 1020

Fig. 3. SIMS depth profile of 18O for SnO2:F films heat-treated in18O2=N2 atmosphere.

866 S. Takeda / Applied Surface Science 231–232 (2004) 864–867

between oxygen in the atmosphere and oxide

materials but also the role of oxygen in oxide

materials.

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