Influences of deuterium atoms on local bonding structures of SiO2 studied by HREELS

ELSEVIER

Thin Solid Films 313-3-l-l

( 1999,-!OS4

I1

.

Influences of deuterium atoms on local bonding structures of SiOz studied by HREELS Hiroya Ikeda”.*. Yasuyuki Nakagawa”, Kenji Sate”. Makoto Higashi”, Shigeaki Zaimab. Yukio Yasuda’

Abstract We have investigated the initial oxidationprocesses on D-wrminatedSic 100)-Z x I and - 1 X 1 surface, and influencesof Si-D bondingon SiOl localbondingstrucnnwby high-resolution electronenergylossspectroscopy. It wasfoundfromexperimental and simulated resultsthat 0 atomspreferentiallyadsorbon one of two Si-Si backbondsof a surfaceSi atom. However.the preferentialadsorptionon the Dterminated Sic LOO)-:! X 1 wface is not as significant as the 1 X I surface. In addition. from the evaluation by a central-force network model. the length of Si-0 bondsformedon the D-terminated1X I surfaceis morerelaxedthanon the 7 X I surface. This fact suggesrs that the

exihtenceof Si-Si dimerbondingchanges thebondingstatesof theiopmostSi atunisandthenthesuucturalchangein Si-0-Si

formedat

the Si back bond occurs. C 1999 Elsevier Science S.A. All rights reserved. Ke,woxfc; Deuterium atoms: SiO:: Oxidation processes

1. Introduction

of Si-D bonding and surface structures on initial oxidation processesand SiO: bonding structures.

With reduction in gate-oxide thickness due to miniaturization of ultra-large scale integrated circuits (ULSIs). the SiO? network structure and the SiOJSi interface seriously influence the electrical characteristicsof devices. especially the device reliability such as leakage current and breakdown. In order to realize a well-controlled Si02 film and SiOjSi interface. oxidation processeson H-terminated Si surfaceshave beenattracted considerableattention andhave been widely investigated by using various methodsbecause of the effectiveness of hydrogen atoms on controlling the surface reactivity [l] and the surface flatness [2]. We have examined the initial oxidation processesof Hterminated SiilOOI-1 X 1 surfaces and the relaxation processesof SiOT bonding structures using high-resolution electron energy loss spectroscopy (HREELS) and reported that 0 atomspreferentially adsorbon one of the two backbond sitesof a surface Si atom until the oxide thickness is 0.8 monolayer (ML) at temperaturesbelow 3OO’C[3,-I]. In the presentpaper, we investigated the initial oxidation processeson D-terminated SiclOOi- 1 x I and -2 x 1 surfaces at room temperature by HREELS and discussedthe effects w Corresponding 3SlS.

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2. Experimental Substratesof p-type Si( 100) (resistivity: 6-8 fl cm’)were thermally cleanedat 950°C in an ultra-high vacuum (CHV) chamber with a basepressureof 3 x IO’ I” Torr. D-terminated Si( loo)- 1 x 1 and -2 X I surfaces were obtained b> exposing clean Si(100) surfaces to D atoms at room temperature and 380’C. respectively. Deuterium atoms were produced by dissociation of their molecules using a W filament heatedat 1500°C.The amount of C and 0 atoms after the D-termination was below the detection limit in Auger electron spectroscopy (AES) measurements.Oxidation was carried out at room temperature by exposing the substratesto 0 atomsproducedby the W filament heatedat I5OO”C. HREELS measurementsH’ereperformed under the specularly reflected condition at room temperature.The incident energy and angle of electron beamswere 7.5 eV and 55” with respect to the surfacenormal, respectively. One monolayer (ML) is defined asthe ratio of the numberof adsorbing oxygen atoms to that of surface Si atoms on a Si(lO0) surface.

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S.X.

AlI rights

resend

H. lkedn

ei al. / Thin Solid Films 343-314

D-terminated

11 meV

si(loo)-2xt v D2 i p

200

0

1

Energy loss (meV) Fig. 1. HREELS spectraof after exposing to 0 atoms

D-terminared

Si( 100)-Z X 1 surfaces before and

at roomtemperature.

3. Results and discussion Fig. 1 shows changesin HREELS spectra of a D-tetminated Sit 100)-Z x I surfaceby exposing to atomic oxygen at room temperature. In the spectrum of the D-terminated SiClOO)-2X 1 surface, there exist two peaksof Si-D species at 60 rneV ( I’~,) and 190 meV ( TT~),which correspondwith the bending and stretching vibrational mode, respectively [5,6]. The low-energy electron diffraction (LEED) pattern of this surface showeda 2 X 1 structure. Therefore, a didueteride surface is prepared. A small peak at 88 meV originatesfrom the Si-2D scissorsmode [.5,6]. The amount of Si2D species was estimated from the peak intensity to be below 0.1 *ML,. In the initial stage of the oxidation, a peak at 128 meV appears,which correspondswith the Si-0-Si asymmetric stretching (vol) mode [7]. As for the Si-0-Si species,the f’act that only the asymmetric stretching mode is cIearly observed indicates that oxygen atomspreferentially adsorb on Si-Si back bonds of surface Si atoms rather than Si-Si dimer bonds, as reported previously [3,X]. Theoretically, a Si-0-Si structure formed on a n-bonding state which lies

above the Si-Si

(1999)

$08111

:09

Si-0-Si species, the symmetric bending (vol) and symmetric stretching (vo?) mode 171, can be observed, which meansthat the Si-Si dimer-bond sitesare also occupied by 0 atoms. With a progressof the oxidation, a splitting of the Z’D? peak and a shift of the Si-O-Si peaksoccur. As mentionedin our previous paper, the losspeak of a Si2H stretching mode on H-terminated Si( 100)-l X 1 surfaces is divided into two peaks in the HREELS spectra. as the oxidation progresses[3,4]. In the caseof the Si-D peak, the similar peak splitting takes place. That is, the lower-energy peak correspondsto the Si-D speciesfor the Si atom with zero and one adjacent 0 atom (U 5 1, !I denotesthe number of neighboring 0 atoms), and the higher-energy peak correspondsto the Si-D specieswith two and three adjacent 0 atoms in 2 2). The intensities of the Si-D peaks with the lower and higher energy are shown in Fig. 2a: as a function of oxide thickness. The intensities of the peaks are normalized by the total intensity of the Si-D peaks and the total intensity hardIy changes during the oxidation. It is found from Fig. 2a that the II 2 2 peak starts to be observed at about 0.5 ML. This thicknessis different from the caseof D(H)-terminated Si(lOO)-1 X 1 surfaces.0.8 ML [3,4,10]. Fig. 2b showsthe top view of a D-terminated Si( lOO)-2 X 1 surface. Three kinds of adsorptionsitescan be considered in this situation: (i) a back-bond site whose IWO neighboring sites are not occupied by 0 atoms, on which the reaction probability of an 0 atom is definedas unity; (ii) a back-bond site whose one of two neighboring sitesis occupied by 0 atoms,on which the reaction probability is denotedasa; (iii)

Simulation

-0

1

2

SiOa thickness

3

(ML)

@I 1

1

1

1

1

a

a

[ITO] [OOI] L. I~W

dimer bond for clean ,%(100)-Z x 1

surfaces is predicted as a metastablestructure [9]. Therefore, the reason why 0 atoms are hard to adsorb on the dimer bond may be that the n-bonding state vanishes due to the formation of Si-D bonding by the D termination [X]. A small peak of the Si-OD stretching (zton) mode [5] is also observed. The amount of Si-OD speciesevaluated was at most 0.08 ML, which is much smaller than Si-0-Si species. After further oxidation, two peaks arising from

a

u

a

u

a

u 1

1

1

Fig. 2. (a) intensities of the lower and higher Si-D peaks as a function of oxide rhiclmess. The intensities are norillalized by the total intensity of the Si-D peaks. Simuiated results are also sho\vn as parameters of CTand C.
. D-terminated Si(iOO)-2x1 2 D-terminated Si(lOO)-1 xl A Clean Si(lOO)-2x1 1 SiOz thickness

2

/ 1 I I 3

(ML)

Fig. 3. Changes in the enerp): loss of a Si-0-Si asymmetric stretching mode v.tth increasinp SiOl thickness for D-tetinated Si(lOOk3 X 1 and -I X 1. clean Sir 1001-2 X 1 surface5.

a dimer-bond site. on which the reaction probability is denoted as c. Simulated results are also shown in Fig. 2a for (n.c) = il. 1). (0.1.0.1) and iO.02.0.02). In the case of (a. c) = (0.1.0.1). the simulation can reproduce the experimental results well. This finding supports the conclusion that 0 atoms preferentially adsorb on back bonds rather than dimer bonds. It should be noticed that the suppression of the 0 adsorption by Si-0 bondings becomes somewhat weak in comparison with n = 0.01 estimated for the oxidation on D-terminated 1 x 1 surfaces [lo]. This difference is closely related with the relaxation of Si-0-Si structures. as mentioned later. Moreover. the good reproduction of experimental results by the simulation indicates that the oxidation hardly progresses into the Si substrate. since the simulation model in Fig. 2b includes only the first and second Si-layer. Hattori et al. [ 1l] reported that Si-Si bonds between the third and fourth Si-layer are oxidized even in the initial stage of the oxidation on H-terminated SiClOOj-3 x 1 surfaces. In their experiments, the oxidation was performed in dry oxygen under a pressure of 1 Torr at 300°C. The osidation does not progress into the Si substrate on D-terminated Si(lOO)-1 x 1 surfaces at 300°C. as reported in our previous paper [-!I. Therefore. the oxidation pressure and the oxidant species strongly influence the progress of the oxide formation. Fig. 3 shows changes in the energy loss of a Si-0-Si asymmetric stretching mode (~02) with increasing Si02 thickness for D-terminated Si(lOOj-2 X 1 and -1 x 1 surfaces at room temperature. The experimental results for clean Si( lOO)-2 X 1 surfaces [3.-l] are also shown. A higher energy of the ~0~ peak means a more relaxed structure of Si-0-Si species [3.-l]. Therefore. it is found from the

results on D-terminated Si( lOO)-2 X I that the 0 adsorption accompanied with the Si-0-Si structural relaxation occurs above 0.5 ML. which corresponds to the 0 adsorption on the dimer bonds and the back bonds neighboring a Si-0 bond. In addition. the Si-0-Si structure at thicknesses below 0.5 ML is not so relaxed as that on the Dterminated 1 x 1 surface. This fact is considered to be due to the change in the bonding state of surface Si atoms by the existence of Si-Si dimer bonding. which leads to a change in the adsorption probability of an 0 atom on Si-Si backbond sites adjacent to Si-0 bonds, n. The force constant and bond angle evaluated from energy losses of vol and z/o1 using a central-force-network model [ 12,131 are shown in Fig. la and Fig. -lb, respectively. as a function of SiO? thickness. From Fig. 4b. the Si-0-Si bond angles on D-terminated surfaces are smaller than those on clean surfaces below 3 ML. This fact is considered to be due to the difference in the 0 adsorption site, that is, the Si-0-Si species are formed at Si-Si back bonds on Dterminated surfaces while Si-Si dimer bonds act as an Oadsorption site on clean surfaces. On the other hand. it is found from Fig. -&athat the force constants on D-terminated Si(lOO)-2 x 1 surfaces are close to those on clean surfaces rather than D-terminated 1 x 1 surfaces. II can be said that change in the bonding state of topmost Si atoms by forming dimer bonds has influence on the Si-0 force constant. that is. the Si-0 bond length.

---

1(4

’

_-1

’

D-terminated S1(100)-2x1 G D-terminated Si(lOO>lxl A Clean Si(lOO>Zxl

l

1

1 SD2

0

140

thickness

2 (ML)

I

I

~iI--~~--~4~~ J

-.

3

(b) E H 22 z

135

-y-

130

--T

(..I -L--V

-v Q

I

1

I-A I-4

-7:

Si0z

h

2 thickness

(ML)

Fi,o. 1. (a) Force constants and (b) bond angles evaluated force-net\\ork model, as a function of SiO: thickness Sic 100)-Z x 1 and -I X I. clean Sit l&l)-? X 1 surfxeb.

using the crntralfor D-terminiid

4. ConcIusions We have investigated initial oxidation processes and SiOz bonding structures during the oxidation of D-terminated Si(lOO)-I x I and -2 x 1 surfaces by I-REELS. Ln the initia1 stage of the oxidation on D-terminated Si( 100)-2 X 1 surfaces, 0 atoms preferentially adsorb on Si-Si back bonds rather than dimer bonds. However, the preferential adsorption on one of two back-bond sites, which is obsewed in the oxidation of Sill OOj- 1 X 1 surfaces, is not so significant. This difference is thought to be due to the difference in the bonding stale of surface Si atoms concerned with the Si-Si dimer bonding. Moreover, this bonding-state difference has influence on the Si-0 force constant from the estimation using the central-force-network model. On the other hand, the Si-0-Si bond angle is mainly influenced by rhe existence of Si-D bonding. Acknowledgements This work was partly supported by a Grant-in-Aid for Scientific Research iB) iNo. 08455019) from the ,Ministry of Education, Science, Sports and Culture, Japan.

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