Thin Solid Films, 222 (1992) 141-144 141

A comparison of the behaviour of Sio.sGeo.5 alloy during dry and wet oxidation

J. P. Zhang* and P. L. F. Hemment Department of Electronic and Electrical Engineering, University of Surrey, Guildford, Surrey GU2 5XH (UK)

S. M. Newstead, A. R. Powell, T. E. Whall and E. H. C. Parker Department of Physics, University of Warwick, Coventry CV4 7AL (UK)

Abstract

We have studied the oxidation behaviour of 350 nm thick films of Si0.sGeo 5 alloy grown on Si(100) substrates by molecular beam epitaxy. The oxidation was performed at 1000 °C in both dry and wet oxygen environments. As a reference, bulk silicon oxidation was also studied. Oxidation rates and atomic redistribution were measured using Rutherford backscattering. The formation of SiO 2 bonding was indicated by IR transmission spectroscopy, and X-ray photoelectron spectroscopy was used to determine the silicon and germanium electronic states in the oxide layer.

Two stages of oxide growth can be identified in our experiment. During the initial stage the dry oxidation rates for alloy and bulk silicon are the same whilst the wet oxidation rate for the alloy is about three times faster than that for the bulk. Germanium trapped in the near-surface region and accumulated at the oxide-alloy interface during wet and dry oxidation was observed at this stage. Longer oxidation times are characterized by similar growth rates for both alloy and bulk silicon during wet oxidation, but during dry oxidation a significantly lower rate for the alloy compared with bulk silicon. The accumulated germanium diffused away from the interface of the oxide layer in the case of dry oxidation and the alloy layer transformed to a germanium-rich layer during wet oxidation. The above results demonstrate that the presence of germanium increases the rate during wet oxidation, but decreases the rate during dry oxidation. We explain these phenomena in terms of the mass transport, of either silicon or oxygen atoms, to the oxide front.

I. Introduction

It is well known that silicon and germanium are two of the most useful semiconductor materials. According to the Si-Ge phase diagram, the solid solution can be formed at any desired composition with an adjusted band gap. Improvements in epitaxial technology have made it possible to grow pseudomorphic SiGe alloy with good crystallinity and electronic properties. It is important that a good quality thermal oxide can be formed on the SiGe layers in order to utilize the SiGe for device application.

Recently, many studies have been carried out on the kinetics and mechanisms controlling the thermal oxida- tion of SiGe alloy. It has been shown that SiO2 is the dominant phase in both dry [1-3] and wet oxidation [1, 4-11] and that germanium is rejected out of this SiO2 layer and accumulated at the oxide-a l loy interface. The rate of oxidation of SiGe alloy is significantly higher during wet oxidation compared with that of pure silicon [1, 4-11], whereas in dry oxygen, the oxidation rate of SiGe is the same as that of bulk silicon [ 1-3]. It has

*On leave from Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, China.

been demonstrated that the germanium acts as a catalyst during the wet oxidation of SiGe [8, 9]. Holland et al. have suggested that the enhancement of the wet oxida- tion rate is the result of the weak S i - G e binding energy [5]. It was argued that a similar increase in oxidation rate was not observed for the case of dry oxidation [2]. LeGoues et al. [ 8] proposed that the effect of germanium is the elimination of the injection of interstitials. It has been demonstrated recently that the behaviour of SiGe oxidation is very much dependent on the temperature, environment (wet vs. dry) and composition of the silicon and germanium [9, 11]. The reasons for the effects on the oxidation rate are not yet understood.

In this paper we describe the effects of germanium on the oxidation of the Sio.sGe0.5 layer during dry and wet oxidation at a temperature of 1000 °C for various times. The experimental results are explained by consideration of the dominant species under the oxidation processes.

2. Experimental procedures

2. I. Sample preparation The samples used in this study were prepared by

molecular beam epitaxy (MBE). A 300nm silicon

0040-6090/92/$5.00 © 1992 - - Elsevier Sequoia. All rights reserved

142 J. P. Zhang et al. I Dry and wet oxidation oJ' Sio.sGeo 5 alloy

buffer layer followed by a 320 nm Sio.sGeo•5 layer was grown on a (100) n-type silicon (p = 5-20 f~cm) sub- strate. The samples were oxidized using a conventional furnace in dry flowing oxygen at atmospheric pressure E z 10000

O and at a temperature of 1000 °C for various times o ranging from 1 to 24 h. Alloy samples were also oxi- dized at 1000 °C for times of 5 min up to 45 min, also at atmospheric pressure in flowing wet oxygen which had 5000. been bubbled through water heated to a temperature of 95 °C. For comparison, thermal oxides were grown simultaneously on bulk silicon.

2.2. Rutherford backscattering spectrometry analysis Rutherford backscattering (RBS) was performed us-

ing a 1.5 MeV 4He+ ion beam from a Van de Graaff accelerator to determine the layer thickness (hence growth rates) and the redistribution of germanium. A standard silicon surface barrier detector in a 160 ° backscattering geometry was used for the RBS analysis.

Figure 1 shows the RBS spectra of Sio.5Ge0. 5 alloy oxidized in dry oxygen at 1000 °C for 1-24 h. Figure 2 shows the spectra of Si0.sGe0.5 alloy oxidized in a wet oxygen environment at 1000 °C for 5-45 min. Peaks b in Figs. 1 and 2 indicate that germanium was trapped at the surface during both dry and wet oxidation. The amount of germanium trapped during wet oxidation is about four times greater than that trapped during dry oxidation. No further trapping of germanium was ob- served during both wet and dry oxidation when the time was prolonged. The change of shape of the germa- nium peak indicates that germanium was rejected from the oxide layer and accumulated at the front of the moving oxide during oxidation (Figs. 1 and 2, peaks a). Therefore, for during dry oxidation the germanium peak a disappeared and the slope of the back edge of the germanium signal increased with increasing oxida- tion time after oxidation for 4 h. This implies that

4 0 0 0 c -

O

0

2000-

$0 $Si $ Ge

¢.

• , ~..',:~ ,i ['

\L~ "!

t tr~- a

i ? I

-b

. . . . I . . . . I . . . . I . . . . I . . . . I . . . . I . . . . 100 150 200 250 300 350 400 450

Channel Number Fig. 1. 1.5 MeV 4He+ RBS r a n d o m spectra f rom the dry oxide grown

on Sio.sGeo. 5 a l loy at 1000 °C: - - unoxid ized sample, ox ida t ion for l h , . . . . for 9 h a n d . - for 2 4 h .

$o Ssi + Oe

0 100

i i : : i : : i i

. . , i ;

i - ' , ~ i t

150 200 250 300

,¢, ,'t - a

,i/ 350

Channel Number

~-b

4 0 0 4 5 0

Fig. 2. 1.5 MeV 4He+ RBS r a n d o m spectra f rom the wet oxide grown

on Sio.sGe0. 5 a l loy at 1000 °C: unoxid ized sample, ox ida t ion for 5 min, • • • • for 20 min and - • for 45 min. Peaks a

and b are discussed in the text.

germanium diffused away from the oxide-SiGe alloy interface to the alloy layer and then to the silicon substrate. The width of the oxygen peak, which is proportional to the thickness of the oxide layer, in- creases as the oxidation time increases. The dependence of the thickness of the oxide layer on time during dry and wet oxidation of Sio.sGe0.5 alloy is shown in Figs. 3(A) and (B) separately. For comparison the theoretical and experimental results from bulk silicon are included. The theoretical curve was calculated using the well known linear parabolic equation x 2+ Ax = Bt; here A =0.165 jam, B = 0.0117 jam 2 h -~ and A = 0.226 jam, B = 0.287 jam 2 h-J for dry and wet oxidation respectively [12].

2.3. IR transmission spectroscopy and X-ray photoelectron spectroscopy

The formation of silicon dioxide was confirmed by IR transmission spectroscopy, using a Perkin-Elmer 577 grating spectrometer. The optical transmission of the oxidized samples was measured in the wavenumber

6OO

400 "6

200 Z I"-

A d r y o x i d a t i o n a t 1000°C

T i m e ( h r s . )

B w e t o x i d a t i o n a t 1000~C

/ i 6 " "

10 20 30 40 50

T i m e ( r a i n s I

Fig. 3. Var ia t ion of the oxide th ickness aga ins t time: [] Sio.sGeo. 5 alloy; + b u l k silicon; - - bu lk sil icon ( theoret ical) . Ox ida t ion t empera tu re 1000 °C, (A) dry ox ida t ion and (B) wet oxidat ion.

J. P. Zhang et al. / Dry and wet oxidation of Sio.sGeo 5 alloy 143

range 4000-200 cm -t using a SiGe alloy sample from the same wafer as a reference. The spectra show three absorption bands due to the vibrational modes of the Si-O-Si local bonding unit at 1000-1100cm -I, at about 800 cm-1, and at 450 cm -~. Within experimental sensitivity we only observed the Ge-O bonding at 850cm -~ for wet oxidation for 5 min. Therefore the oxide layers formed in this study are principally SiO2.

For selected samples the electronic states of silicon, oxygen and germanium in the oxide layer were studied by X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCALAB MklI system. It was observed that in the surface region the Ge 3d peaks were located at a binding energy of 33.4 eV while the Si 2p peaks were located at 104 eV. The results of XPS suggest that in the surface region the oxygen atoms are bonded to silicon atoms and germanium atoms, which were trapped, to form GeO2 [13].

3. Discussion

The experimental results indicate that there are two stages in both dry and wet oxidation processes.

3.1. Initial stage: oxidation time less than 2 h for dry oxidation and lOmin for wet oxidation

The germanium atoms trapped in the region near the surface and the germanium which was rejected from the oxide layer are accumulated at the interface between the oxide and SiGe alloy during this stage for both dry and wet oxidation (as shown in Figs. 1 and 2). This phe- nomenon can be attributed to the oxidation rate being fast enough that germanium cannot diffuse away from the moving oxide front; in addition not enough silicon is supplied to react with oxygen, owing to the lower silicon concentration in the Sio.sGeo.5 alloy (approxi- mately 2.3 x 1022 Si atoms cm -3) compared with bulk silicon (approximately 5 x 1022 Si atoms cm-3). In this case germanium is trapped in the oxide to form GeO2. This phenomenon is also associated with the solubility of germanium in SiO2. The volume of trapped germa- nium within this region will depend on the oxidation conditions (wet or dry) and the composition of the alloy [11].

In the case of dry oxidation, the oxidation rate is the same for both Sio.sGeo.5 alloy and bulk silicon, as shown in Fig. 3(A). It is found that the lower silicon concentration in Si0.sGeo.s alloy does not influence the oxidation rate. Enough silicon atoms, from broken Si-Ge bonds, are supplied to maintain the silicon flux as in the case of bulk silicon. This implies that the lack of about 50% silicon atoms for SiGe alloy compared with bulk silicon is balanced by the weak Si-Ge binding energy compared with the Si-Si binding energy. For

wet oxidation, the oxidation rate for SiGe alloy is about three times faster than that for bulk silicon during the initial period of oxidation. This result agrees with previ- ous reports. It seems that not only is breaking the Si-Ge bonds easier for SiGe alloy but also the presence of germanium speeds up the decomposition of H20 , OH + H during wet oxidation. We suggest that during this stage the reactions Si + O2 , SiO2 (for dry oxidation) and Si + H20 , SiO2 + 2H (for wet oxidation) are dominated by the oxygen flux.

3.2. Second stage: oxidation time longer than 2 h for dry oxidation and lOmin for wet oxidation

The germanium peaks of the RBS spectra indicate that the position and the amount of trapped germa- nium do not change during both wet and dry oxidation whilst the thickness of the oxide layer increases (as shown by peaks b in Figs. 1 and 2). This means that all of the germanium atoms are rejected from the oxide and a pure SiO 2 layer is formed from the oxide-alloy interface during this stage. This result shows that the new oxide is formed by long-range migration of oxygen (diffusion) and interfacial reaction. This is the same model as for the oxidation of bulk silicon. The IR measurements also confirm the presence of SiO2. It is evident that the formation of SiO2 is always easier than that of GeO 2 owing to their heats of formation ( - 8 5 0 k J m o l -I for SiOz and -480kJmo1-1 for GeO2). This can be attributed to the slowing down of the oxidation rate with increasing thickness of the oxide layer and because the diffusion of germanium atoms is faster than the moving rate of the SiO-SiGe interface. Meanwhile the accumulated germanium peak at the SiO2-SiGe interface is reduced with increased time for dry oxidation (Fig. 1) and the concentration of germa- nium in the alloy layer is increased from Si05Ge0. s to Sio.3Ge0.7 in the case of wet oxidation for 45 min (Fig. 2). This can be explained by the different times and environments.

The oxidation rate of the alloy is much slower com- pared with bulk silicon for dry oxidation (Fig. 3(A)) and is similar for both alloy ~.nd bulk silicon during wet oxidation (Fig. 3(B)). The reason for the retarda- tion during dry oxidation at this stage can be ex- plained in terms of silicon diffusion from the alloy layer to the oxide-alloy interface being suppressed by germanium diffusion from the SiOz-SiGe alloy inter- face through the SiGe layer to the silicon substrate. The germanium diffusion is clearly shown in Fig. 1. For wet oxidation it is the same as bulk silicon: the greater oxide thickness slows down the diffusion of oxygen and therefore slows down the oxidation rate. We propose that during this stage silicon is the dominant species for dry oxidation and oxygen is the dominant species for wet oxidation.

144 J. P. Zhang et al. / Dry and wet oxidation of SiosGeo. 5 alloy

4. Conclusions

The above discussion can be summarized as follows. (1) The oxidation of SiGe alloy can be described by diffusion and interfacial reaction modelling, which has been confirmed for the case of bulk silicon oxidation. (2) There are two stages during both dry and wet oxidation of Sio.sGe0. 5 alloy at 1000 °C. In the initial period of oxidation the near-surface layer of oxide consists of SiO2 and GeO2. The volume of GeO2 is higher for wet oxidation than for dry oxidation. The germanium atoms, which are ejected from the oxide, are accumu- lated at the oxide-alloy interface. Compared with bulk silicon the oxidation rate is the same for dry oxidation and enhanced for wet oxidation. The dominant species is oxygen at this stage. For extended oxidation times a pure SiO2 layer, from which germanium is rejected, is formed. The germanium concentration increases in the alloy layer for the case of wet oxidation whilst the germanium diffuses away from the SiO2-alloy interface during dry oxidation. Compared with bulk silicon the rate of oxidation is reduced for dry oxidation and is similar for wet oxidation, The dominant species are silicon for dry oxidation and oxygen for wet oxidation.

Acknowledgments

The authors thank the staff of the D. R. Chick Accelerator Laboratory, University of Surrey, for their

help in running the accelerator, Dr. G. J. Buist for his assistance in making the infrared measurements, Hengda Liu for the XPS analyses and A. Royle and F. J. Manning for their assistance in carrying out the dry and wet oxidation. This work is partially sup- ported by the UK Science and Engineering Research Council.

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