Formation of interfacing Si-O-Si species by hydrogenated Si radical depositions onto oxidized Si(111) surfaces

129

Surface Science 203 (1988) 129-142 North-Holland, Amsterdam

FORMATION OF INTERFACING Si-0-Si SPECIES BY HYDROGENATED Si RADICAL DEPOSITIONS ONTO OXIDIZED Si(ll1) SURFACES A. NAMIKI,

K. TANIMOTO,

T. NAKAMURA

Department of Electrical and Electronic Engmeering, Toyohashi, Japan 440

N. MURAYAMA

Toyohashi University of Technology, Tempaky

and T. SUZAKI

Toyoko Kagaku Co. Ltd., Nakammakuko,

Kawasaki,

Received 6 January 1988; accepted for publication

Japan 211 27 April 1988

Early stages of Si(ll1) oxidation via photochemical decomposition of N,O has been further studied with X-ray photoelectron spectroscopy (XPS). In order to reveal the chemical nature of the oxygen atoms bonded to the top-most silicon atoms, a hydrogenated silicon radical, SiH,, which was formed via photochemical decomposition of Si,H,, was deposited on the suboxide surfaces. From the analysis of the asymmetric decrease of the O(ls) spectra, we found that the oxygen atoms in the on-top sites made a Si-0 bond preferentially with the deposited SiH, radical within a monolayer coverage, playing a role in mediating the substrate oxide and the deposited a-Si: H to form the Si-0-Si species. The corresponding Si(2p) spectra showed a peak shift towards the higher binding energy side of 1 eV with respect to the Si(2p) peak of the bulk a-Si : H film. This chemical shift also indicates the existence of the interfacing Si-0-Si species. The interfacing Si-0-Si species were unstable with respect to heating to 500-600 o C.

1. Introduction It is very important to understand the structure of amorphous silicon oxide (SiO,) not only from the technological point of view for the Si-SiO, interface structure which affects the electrical properties in MOS devices, but also from the scientific point of view for the problem how the oxygen atoms are incorporated into the surface. A great quantity of information on the early stages of Si oxidation by 0, has been accumulated using X-ray and UV photoelectron spectroscopy (XPS, UPS) [1,2], and electron energy loss spectroscopy (EELS) [3-51 including Auger electron (AES) spectroscopy. These results have established that the oxygen adsorbs molecularly on the Si(lll) surface at low temperature, but it adsorbs dissociatively above room temperature. The rapid dissociative adsorption at room temperature completes the monolayer coverage at an exposure of 0039-6028/88/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

130

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et al. / Form&on

of interfacing .S-0-Si

species

- 10 L (1 L = 10e6 Torr s), and it is followed by a very slow sorption process for further 0, exposure [3,5,6]. At an oxygen coverage of around one monolayer either one of the three back bonds or the dangling bond of the top-most silicon atom is occupied by one oxygen atom. According to Hollinger et al. [7], about 20% of the oxygen is incorporated into the on-top sites of the top-most silicon atoms, forming Si-0 species, and 80% into the bridge sites, forming Si-0-Si species. This quantitative assignment for the Si-0 or the Si-0-Si species has been based on the coexistence of the high and low binding energy band in the O(ls) XPS spectra. The oxygen in the on-top sites was easily bleached out upon heating to 500” C, while the oxygen in the bridge sites remained intact [7]. The slow adsorption process following the initial rapid oxygen uptake up to a coverage of one monolayer indicates that once the monolayer coverage is accomplished, the silicon atoms of the first layer loose rapidly the ability for incorporating oxygen into the Si-Si bonds or Si dangling bonds which still remain unoccupied. Owing to this reason it is rather hard to saturate all silicon dangling bonds with oxygen atoms at room temperature. Though the oxidation can certainly be enhanced at elevated temperature, the heating process is accompanied by a considerable atomic displacement or structural relaxation. Thus no Si-0 species exists on the heat-processed oxidized surface. For this fact the Si-0 species on the Si(ll1) surfaces had not been studied and there was no detailed knowledge of their physical and chemical properties. In the previous paper we reported XPS studies of the early stages of oxidation of the Si(ll1) surface subjected to the simultaneous exposure to N,O and an UV laser beam [8,9]. The oxygen uptake tended to saturation at monolayer coverage by N,O exposure alone [9,10]. But when radical oxygen atoms formed via photochemical decomposition of N,O were supplied onto the surface the rate of oxygen uptake increased quite rapidly [9]. Since this process does not involve any heat treatment, the oxides so formed are free from atomic relaxation or atomic displacement, and the oxygen in the on-top sites still exists even in the 10 A thick oxide film. It was also observed that the Si-0 species dissociated and the amount of Si-0-Si species increased during the direct UV laser annealing as well as during the thermal annealing around 500 0 C [9]. The so quenched Si-0 species were recovered quite reversibly by the successive radical oxygen supply, indicating that the dangling bonds of the top-most silicon atoms can still survive after an annealing process [9]. In this work we present XPS experiments for the further study of the desired amount of the Si-0 species supplied by means of photochemical decomposition of N,O. We focus our attention to the chemical properties of the Si-0 species towards the deposited hydrogenated silicon radical which is formed via photochemical decomposition of disilane, Si,H,. Si,H, has gathered great attention for laser-induced chemical vapor deposition of Si

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oJinterJacing

Si-O-.5

species

131

[11,12], and of SiO, [13] because it can be easily decomposed via electronic excitations by UV light [14]. The oxygen atoms of the Si-0 species bond with silicon supplied onto it, changing to oxygen in the bridge sites. This new type of interface structure between the substrate a-SiO, (0
2. Experimental The samples used in the present experiment were Si(ll1) wafers (p-type, 3 fi cm). The Si wafers were pretreated by etching in HF solutions and subsequently the surface was oxidized in HCl + H,O, solutions [15]. The wafers were directly subjected to ohmic heating to - 1000” C for 1 min to evaporate the oxide layer in an UHV chamber. The UHV apparatus consists of two chambers: one is for XPS measurements at a base pressure below 5 X lo-” Torr and the other, the reaction chamber, for laser-induced surface oxidation and SiH, deposition. XPS spectra were recorded using the Mg Ka line (1253.6 eV) to distinguish the entities formed in the surface. An ArF excimer laser (10 ns, 193 nm, 70 Hz, 2-3 W) was used to induce the photochemical decomposition of N,O or Si2H,. The laser beam was introduced parallel to the surface to avoid structural modification due to laser annealing. The photochemical decomposition of Si2H, was done under a pressure of 0.01 Torr. The details of the experimental procedure for the laser irradiation of N,O, the XPS measurements and smoothing the recorded spectra have been reported previously [9]. Thermally desorbed species from the a-Si : H films during the thermal annealing process were detected with a quadrupole mass spectrometer (QMS) which was faced to the sample surface. The temperature of the surface during the annealing process was measured by an electro-pyrometer.

3. Results and discussion 3.1. Preparation

of suboxide surfaces and SiH,

deposition

Based on previous results [9], three oxidized surfaces with a different extent of oxidation prior to SiH, deposition were prepared: the oxidized surface with a mono-layer coverage - we call it here stage I surface - was prepared by exposing the Si(ll1) surface to N,O up to lo5 L; the intermediarily oxidized surface - stage II surface - where all of the Si dangling bonds of the first layer are filled with oxygen atoms, was prepared by supplying radical oxygen produced via photochemical decomposition of N,O onto the clean Si(ll1)

132

A. Namiki

of intwfaring

et rrl. / Formatmn

SiLO-Si

spcres

m

III

)

(annealed

0

Si

-0

Fig. 1. Schematic side views of the Si(l11) surfaces oxidized via photochemical decomposition of N,O at the representative stages I, II. III and the thermally annealed stage III. The stage I surface corresponds to an oxygen monolayer coverage which can be prepared by the saturated dissociative adsorption of N,O. Stage II and III surfaces can be prepared by the supply of radical oxygen via photochemical decomposition of N,O. The thermally annealed stage III surface can be prepared by thermal annealing of the stage III surface at - 600 o C. Apart from bleaching of the oxygen in the on-top sites, a structural modification due to the thermal annealing is not involved here. This concept for the oxide surface was taken from ref. 191.

surface without relatively thick incorporated

direct oxide

into

the

laser irradiation to avoid an annealing effect; and the film - stage III surface - where oxygen atoms are third

or

fourth

supply of radical

monitored

from the ratio of the chemically

energy,

BE,

were l/3 stage also

about

III surfaces,

explore

without

was

shifted Si(2p)

the intentionally

of the Si-0

also

Si(2p),, band

> 3.0 at stage III. Thermally

for comparison

the role

layer,

prepared

The stage II and stage III surfaces

103 eV to the substrate

at stage II and

prepared

oxygen.

Si

prolonged

with species

the

bleached-out non-annealed

in forming

by

a

were

band with binding around

annealed Si-0 surface

a SiO,-a-Si:

99 eV;

they

stage II and species,

were

in order

to

H interface

structure. Fig. 1 shows a schematic view of the oxidized Si(ll1) surfaces at stage I, II and III, and also of the thermally annealed surface of a stage III surface. The oxygen atoms in the on-top sites are extinguished upon heating to 600 0 C. At the same time, the suboxides may relax upon annealing towards the conformation of the vitreous

silica, but such structural

change is not drawn in the figure

because any conclusive structural model has not been given yet. Single electronic excitation of Si,H, by 193 nm laser light may be followed by decomposition smaller radicals second reaction actually formed

into SiH,

and SiH,

[16]. These species may decompose

into

by successive laser light absorption and also proceed with a with another radical or a parent molecule. Therefore, the silane radical may take various structures, and the deposited

films may thus contain Si-H bonds. This is directly verified by the QMS measurements in the last session. In this report a bundle of these radical species are thus denoted as SiH x, and the deposited films as a-Si : H. When the SiH r radicals were deposited onto the suboxide surface, the 0(1 s) around 103 eV decreased, while the band around 530 eV and the Si(2p),, Fig. 2 shows one example of the Si(2p) band around 100 eV increased.

A. Namiki

et al. / Formation

108

106 104

of interfacing Si- 0-Si

102

B. E.

100

98

species

133

96

(eV)

Fig. 2. Si(2p) XPS spectra of stage III surface after different BH, depositions. BE is the binding energy referenced to the Fermi level. The chemically shifted bands denoted as oxide decrease in intensity due to the deposited a-% : H layer, while the bulk bands around 100 eV increase with SiH, deposition.

observed Si(2p) spectra at various stages of SiH, deposition onto the stage III surface. The decrease in the intensity of the chemically shifted Si(2p),, band around 103 eV which are ascribable to the SiO, structure is due to the deposition of SiH, over the oxide surface. The concomitantly increased Si(2p) band around 100 eV corresponds with deposited a-Si : H. The bonding structure of the SiH, with the suboxide surface is studied by analyzing these Si(2p) spectra and also the corresponding O(ls) spectra. The net Si(2p) spectra ascribed to the deposited SiH, were obtained by subtracting the substrate component after normalizing the spectral intensity around 104 eV considering that the tail of the Si(2p) spectra of a-Si : H must be small. For this subtraction we assumed that the Si(2p) spectrum of the oxidized substrate does not change during the SiH, deposition. The Si(2p) spectra of the a-Si : H are shown in fig. 3 for the various stages of the SiH, deposition onto the stage III surface (A) and onto the thermally annealed surface at stage III (B). The corresponding O(ls) spectra are also shown on the left-hand side in fig. 3. Each line of the O(ls) spectra viewed in a decreasing order of spectral intensity was recorded corresponding to each line in the Si(2p) spectra viewed in an increasing order. A dent around 99 eV in the first Si(2p) spectrum (curves (b) and (c)) was not reproducible. It may be due to some artifact in the subtraction process. The spectral changes in the O(ls) and Si(2p) spectra were also obtained for the stage I and the stage 11 surface. In what follows we reveal the bond formation of the Si-0 species with depositing SiH, radicals from both an O(ls) and a Si(2p) point of view.

134

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et al. / Formation oftnterfncing

I

Si-0-Si

species

A. Stage Ill Si (2p)

0 (1s)

(nonarmealed)

9. E. (eV) Fig. 3. O(ls) and Si(2p) XPS spectra after different

SiH, depositions (A) on the stage III surface and (B) on the thermally annealed stage 111 surface. The substrate components in the Si(2p) spectra have been eliminated by subtracting the substrate Si(2p) line recorded before deposition from each Si(2p) spectrum after normalizing the spectral intensity around the peak of the oxide in increasing order of SiH, deposition. band. Each line is denoted by (a), (b), (c),

3.2. The O(ls) point of view It is quite clear that the O(ls) spectra observed in the thermally annealed stage III surface (fig. 3B) decrease almost symmetrically upon SiH, deposition. Since this suboxide contains only the Si-0-Si species, this symmetrical decrease is merely due to the attenuation by the deposited a-Si: H layer. On the other hand, the O(ls) spectra for the stage III surface (fig. 3A) decreases asymmetrically at the early stages of SiH, deposition. In particular, preferentially the higher binding energy part diminishes as recognized in the spectral change, (a) + (b) in fig. 3A. It is interesting to note that this spectral change is quite similar to the case of laser annealing or thermal annealing of the suboxide surface; in those case the higher binding energy component was well ascribed to the oxygen in the on-top sites, i.e., the Si-0 species, and the lower binding energy component to the oxygen in the bridge sites, i.e., the Si-0-Si species [9] (as will be seen in fig. 4). Thus, the selective decrease in the higher binding energy band in fig. 3 may be understood as due to the reaction of the Si-0 species with the deposited SiH, radical. Owing to the attenuation effect by the deposited a-Si : H layer the higher binding energy band arising from the Si-0-Si species might decrease also. On

A. Namikr et al. / Formation

of inter/acing St-O-Si

species

135

Thermal annealing Before -

540

538

536

534

B. E.

532

530

528

(eV)

Fig. 4. Decomposition of the O(ls) spectrum into two basic spectral functions for the Si-0 (bleached part) and the Si-0-Si species (remained part) for the stage II surface. The thermal annealing leads to bleaching the higher binding energy band and enhances the lower binding energy one. The dashed line in the figure has been normalized at the peak of the O(ls) spectrum obtained before annealing.

the contrary, such decrease cannot be recognized in the O(ls) spectra obtained in the very early stages of deposition (curve (b) in fig. 3A). This fact suggests that the Si-0 species are converted into the Si-0-Si species which compensate the attenuation due to the deposited a-Si : H layer. The conversion from the Si-0 species to the Si-0-Si species may be qualitatively evaluated by means of the spectral deconvolution. To do this we need basic spectral line functions for the O(ls) band for the Si-0 and the Si-0-Si species. They were obtained by thermal bleaching of the higher binding energy component and then by subtracting the thermally bleached part from the entire Si(2p) spectrum. Fig. 4 shows the so obtained basic spectral functions of the higher energy band of the Si-0 species (designated as bleached part) and the lower binding energy one (designated as remained part) of the Si-0-Si species on the stage II surface. These lineshapes were slightly different among the stage I, II and III surfaces. Using these actual line functions, the O(ls) spectra obtained after the SiH, deposition were deconvoluted into the two components, using a least mean squares method. The ratios of the higher binding energy band ascribed to the Si-0 species (Zs,_,) to the entire O(ls) spectrum ( Itotal) were plotted in fig. 5 as a function of the nominal thickness of the deposited a-Si : H layer. The thickness of the a-Si : H layer was estimated from the decrease of the O(ls) intensity following the well known formula Z = Z, exp[ -da.,,&

sin 01,

136

Stage I 0.1

l

3

0 5 2 0.2 \

Stage II

0.1 = _

2’0 ‘b

. ?

0

1

1;

j..;_

0.1

0.2 0.3 da-s+ / A

0.L

0.5

Fig. 5. Ratios of the spectral intensity of the Si-0 components (I,,_,) to the entire O(ls) spectral intensity (I,,,,,) as a function of the nominal thickness of the a-Si : H layer da_,,, normalized to the photoelectron mean free path X.

where I, and I are the O(ls) intensity before and after the SiH, deposition, respectively, A is the mean free path of photoelectrons in the deposited a-5 : 1-I layer, and 19(= 20”) is the take-off angle. Since the value of X has not been evaluated, the a-Si : H thicknesses are normalized by X. The result plotted in fig. 5 indicates that the Si-0 species reacts with the deposited SiH. radical rather preferentially in the early stages of deposition in the stage I and II surfaces as well as in the stage III surface. The Si-0 species tends to zero around dams,” = 0.1X. If we take h = 30 A [17], da_sIH for the full conversion of the Si-0 species to the Si-0-Si species is estimated to be - 3 A. If we consider the thickness of the deposited SiH, to be 3 A, this da_SiH may correspond approximately to a monolayer coverage of the SiH, radical over the suboxide surface. Thus. we can easily imagine that the oxide-a-Si :H interface is mediated by the oxygen atoms which are initially sitting in the on-top sites of the suboxide surface. 3.3. The Si(2p) point of view Returning to fig. 3, some evidence for the bond formation of oxygen in the on-top sites with the deposited silicon atom may be also found in the Si(2p) spectra. At first glance, it is easy to notice that for the thermally annealed stage III surface, the Si(2p) spectra increase almost symmetrically even in the early stages of SiH., deposition. Since there is no Si-0 species in the present case, this fact indicates that the deposited SiH, radical does not make a polar bond with the oxygen atoms in the bridge sites.

A. Namiki

et al. / Formation

I _

of rnterfacing Si-O-Si

Si (2p)

137

specres

( Stage II )

Thermallyannealed substrate

3 d

I

108

106

104

102

100

98

96

B. E. (eV) Fig.

6. Comparison

between the Si(2p) XPS spectra of the very thin a-Si: H layers non-annealed stage II and the annealed stage II surface.

on the

On the contrary, for the stage III surface the observed Si(2p) spectra increase quite asymmetrically. The peak of the spectra apparently shifts towards the lower energy region and the spectral width becomes narrow as the deposited a-Si : H layer grows thick. Particularly, there exists a prominent component as a tail in the high energy side. In fig. 6, we compare the two Si(2p) spectra in a very thin regime of a-Si : H on both the non-annealed and annealed stage II surface. It is clear that the Si(2p) spectrum obtained from the non-annealed surface is much broader than that in the annealed one, and shifts considerably towards the high energy side, indicating the existence of the bonding of Si atoms to oxygen. The full width at half maximum, FWHM, of the Si(2p) spectra for stage I, II and III surfaces together with the thermally annealed stage III surface are summarized in table 1. These data were obtained for the Si(2p) spectra when the thickness of each a-Si : H layer was approximately 0.15 A, i.e., when the higher energy component due to the Si-0 species disappeared in the corresponding O(ls) spectra. Considering the surface structure in fig. 1, there are at least three possible sites where SiH., can be deposited: (a) the site of the dangling bond of the Table 1 Full width at half maximum stage I, II, III and thermally

FWHM

(eV)

(FWHM) annealed

of the Si(2p) spectra stage III surface

for the monolayer

a-Si

: H film on the

Stage I

Stage II

Stage III

Stage III (annealed)

2.3 f 0.2

2.5 i 0.2

2.6 + 0.3

1.850.2

top-most silicon atoms, (b) the site on the oxygen in the on-top sites. and (c) the site above the substrate oxygen in the bridge site. It is well established that the energy levels of core electrons are determined depending on the valence charge population. When silicon atoms bond with the oxygen in the on-top sites, a charge transfer from Si to 0 atoms takes place, forming a polar bond. The deficiency of the valence charge diminishes the Coulomb repulsive energy. This results in the shift of the binding energy of the core electrons towards the high energy side. When the Si atom is bonded to one oxygen atom, the chemical shift of Si(2p) band with respect to the substrate band is approximately 1.0 eV [18]. On the other hand, if the deposited SiH, sits on the Si dangling bond and then makes a Si-Si bond, no chemical shift may be expected because charge transfer does not take place. When the deposited SiH, sits above the oxygen in the bridge sites, the Si(2p) chemical shift may also be small because the Si-0 bond does not form. The FWHM of the Si(2p) spectra in table 1 may be basically understood by the coexistence of the three types of silicon adsorption on sites (a), (b) and (c) mentioned above. Since all silicon dangling bonds of the top-most silicon atoms are saturated with oxygen atoms in the stage II or III surface, the chemically shifted band due to the interfacing Si-0-Si species contributes for the most part to the Si(2p) spectrum. Since in the stage I surface only 20% of the Si dangling bonds are occupied by oxygen, the contribution of the chemically shifted band must be small compared with the stage I1 or stage III surface. This is reflected in the relatively’ small FWHM in table 1. As a concluding remark, the apparently broadened Si(2p) spectra obtained in the stage 1, II, and III surfaces involve the chemically shifted band which can be ascribed to the silicon atoms bonded to the oxygen located in the on-top sites. This is quite consistent with the result for the O(ls) point of view mentioned above. It might also be possible to make Si-OH species via the reaction between the Si-0 species and the hydrogen produced among the photochemical decomposition of Si,H,. But the contribution of this species to the XPS spectra must be small when we consider the fact that the core level shift due to OH is almost same to that due to 0 [19]. If the surface were covered with OH, SiH, radicals could not bond with oxygen atoms, resulting in no chemical shift. But we could observe the large chemical shift after the SiH, deposition. 3.4. Thermal annealing

effect

The deposited a-Si : H films on the oxide surfaces are very unstable with respect to heat treatment. Ejected species upon heating up to 500-600 o C were detected with the QMS. H, and SIN, were predominantly vaporized from the surface. Si, SiH. SiH, and SiH, were also vaporized, but the yield of these species was smaller by one order of magnitude than that of H, or SiH,. Other

A. h’amiki et al. / Formation

of interfacingSt-0-Si

species

139

Annealing Time (Sec.) Fig. 7. Time course of the SiH, partial pressures after heating the thick a-Si: H film to 6OO’C. The a-Si : H thickness is close to the case of the deposition (i) in fig. 3A.

materials such as SiO and Si,H, were not detected within the experimental uncertainty. The time profiles of the ejected species had two components as shown in fig. 7. This two-step emission was a common characteristic among ail ejected species. When the heating was turned off just at the end of the first band, the ejection of the second band did not take place. But when this sample was heated again to 600 o C, only the second component was ejected. The corresponding Si(2p) and O(ls) XPS spectra before and after this heat treatment were also measured. The O(ls) spectra which had been attenuated by the a-Si : H layer were recovered considerably after heating owing to the thinning of the a-Si : H layer. The Si(2p) spectra of the a-Si : H, on the other hand, decreased and shifted towards the lower binding energy side, as will be seen in fig. 8. Among the Si(2p) spectra recorded after the emission of the first and the second band in fig. 7, no differences in the spectral lineshapes were recognized, but only the decrease in the spectral intensity was observed after the emission of the second band. Thus the well relaxed a-Si : H films over the oxide surfaces were obtained by heating the substrates at 600°C for 30 s. In the next paragraph we study the annealing effect on the a-Si : H layers after the emission of the second band for various a-Si : H thicknesses. The right-hand side of fig. 8 shows the shift of the net Si(2p) spectra of the a-Si : H films on the stage II1 surfaces before (solid lines (b)) and after (dashed lines (c)) the thermal annealing. The corresponding O(ls) spectra of the oxidized substrates before (b’) and after (c’) the annealing are shown on the left-hand side. For an intuitive indication of the deposited a-Si : H film

140

A. Namikl

et al. / Formatron

oJinterJacmg

SI-O-S{

species

Si (2p)

(B)

(6) -.

L

38

536 534 532 530 528

B. E.

104 102 100 98

(eV)

Fig. 8. Spectral changes of the O(ls) and Si(2p) spectra before (b’. b) and after (c’, c) heating for (A) the very thin, (B) mono-layer coverage, (C) and very thick a-Si : H films. The curves (a’) shows the substrate

O(ls) curves before a-%

: H deposition.

thicknesses, the O(ls) spectra of the substrates before the a-5 : H deposition are also shown (dotted lines (a’)). As a matter of fact, the curves (b) and (b’) in (A), (B) and (C) should approximately correspond to the curves (b), (c) and (i) in fig. 3A, respectively. Since the a-Si : H thickness in (C) must be thicker than a few tens of %ngstroms, the (b) --) (c) spectral change in Si(2p) spectra may indicate the net relaxation effects in the bulk a-Si: H layers, e.g., the ejection of stoichiometrically unfavourable species such as H, or SiH, as observed in the QMS measurements, and bond rearrangement towards the homogeneous structure. The peak of the annealed spectrum of the curve (c) appears at the position close to the bulk Si(2p) bands. This fact suggests that the hydrogen content must be reduced considerably. On the other hand, however, the Si(2p) curves in figs. 8A or 8B may be essentially made up of the interfacing Si-0-Si species, because the a-Si : H thickness in (A) or in (B) is smaller than or almost equal to the monolayer coverage as mentioned above. Then, the spectral change from (b) to (c) during the annealing process must involve the annealing effect on the Si-0-Si species. The annealed spectra (c) for both figs. 8A and 8B appear also almost similarly to the curves (c) in fig.

A. Namiki

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of interfucrng Si-0-Si

species

141

8C. This fact suggests that the interfacing Si-0-Si species are destroyed during the annealing process. But the (b’) --, (c’) spectral shift for the O(ls) spectra in fig. 8A shows no loss of spectral intensity when we compare the intensity of (a’) and that of (c’). This fact indicates that the amount of oxygen in the oxide substrate is conserved upon annealing. Although we cannot make a direct comparison of the O(ls) intensity before and after annealing for (B) and (C), the absence of Si-0 species in the QMS measurements supports also the oxygen mass conservation in the thicker a-Si : H films. Therefore, it may be highly plausible that the interfacing Si-0-Si species relax upon heating in such a way that the bridging oxygen in this Si-0-Si species is taken into the oxide substrate which relaxes also at the same time.

Acknowledgements The authors express their sincere thanks to K. Uno, A. Takubo, T. Matsuo and Dr. M. lshida for their encouragements throughout this work. One of the authors (A. Namiki) also expresses his thanks to Dr. C. Satoko for his valuable discussion and suggestion on the Si-0 structure and also to P.J. van den Hoek for the discussion of the hole relaxation effect. This work was supported by a research fund for Joint Research between University and Industry from the Ministry of Education, Science and Culture of Japan.

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spews

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Appl.