Contribution of dangling-bond regeneration channels in the synchrotron-radiation-excited epitaxy of Si from SiH 2 Cl 2Housei Akazawa Citation: Journal of Applied Physics 89, 8321 (2001); doi: 10.1063/1.1375023 View online: http://dx.doi.org/10.1063/1.1375023 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/89/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Hydrogen induced roughening and smoothing in surface morphology during synchrotron-radiation-excited Ge H4 -source homoepitaxy on Ge(001) J. Appl. Phys. 99, 103505 (2006); 10.1063/1.2194232 Morphological transition of Si 1−x Ge x films growing on Si(100). II. Synchrotron-radiation-excited chemical-vapordeposition: From two-dimensional growth to growth in the Volmer–Weber mode J. Vac. Sci. Technol. A 20, 60 (2002); 10.1116/1.1421601 Silicon deposition from disilane on Si(100)-2×1 : Microscopic model including adsorption J. Appl. Phys. 90, 4981 (2001); 10.1063/1.1402141 Chemical vapor deposition of Si on chlorosilane-treated SiO 2 surfaces. I. Suppression and enhancement of Sinucleation J. Appl. Phys. 90, 3879 (2001); 10.1063/1.1402977 Bistable Si growth conditions on Ge(100) in synchrotron-radiation-excited atomic layer epitaxy from SiH 2 Cl 2 J. Appl. Phys. 81, 3320 (1997); 10.1063/1.364317

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Contribution of dangling-bond regeneration channelsin the synchrotron-radiation-excited epitaxy of Si from SiH 2Cl2

Housei Akazawaa)

NTT Telecommunications Energy Laboratories, 3-1 Morinosato Wakamiya, Atsugi-shi,Kanagawa 243-0198, Japan

~Received 29 January 2001; accepted for publication 2 April 2001!

The contribution of various dangling-bond regeneration channels during thesynchrotron-radiation-excited epitaxial growth of Si has been investigated by using SiH2Cl2 as asource gas. When Si films are grown through the photoexcitation of SiH2Cl2 gas molecules alone attemperatures below 300 °C, ramified chains of higher-order silicon chlorides passivate the growingsurface. The coverage of Cl decreases monotonically with increasing temperature and above 440 °Csettles at a half monolayer, and this agrees with the saturation coverage when the self-limitingchemisorption of SiH2Cl2 is completed. The removal of Cl adatoms through reaction with incomingatomic H and SiHx is thus the only channel for surface activation. When both the surface and gasmolecules are photoexcited, the coverage of Cl atoms is reduced to 20%–40% of the coveragewithout irradiation. This is due to photon-stimulated desorption, which increases the growth ratefourfold as compared with the growth rate when the surface is not excited. ©2001 AmericanInstitute of Physics.@DOI: 10.1063/1.1375023#

I. INTRODUCTION

Dichlorosilane (SiH2Cl2) is widely used as a source gasmolecule in the selective epitaxy on silicon surfaces.1 Thereactive sticking of SiH2Cl2 releases Cl atoms around the siteof chemisorption and creates a Cl passivating layer that isthermodynamically stable up to high temperatures. The pos-sibilities opened up by the self-limiting chemisorption ofSiH2Cl2 have recently been exploited with its use as the basisof a process for atomic-layer epitaxy~ALE!.2–6 Independentcontrol of the dissociative chemisorption and removal of Clatoms is achieved by having atomic H impinge on the sur-face within the ALE window at around 600 °C. When suchpurely hydride source gases as disilane (Si2H6) or silane(SiH4) gases are used in a similar process, continuous ther-mal growth occurs.

When SiH2Cl2 gas is employed in a photolytically ex-cited deposition process, however, the thermal stability ofthe Cl layer has no correlation with its stability in terms ofelectronic excitation. It has been shown that a Si–Cl bondhaving intermediate ionicity can be ruptured at a high quan-tum efficiency.7 Although the kinetics of Si growth bysynchrotron-radiation-excited chemical-vapor deposition~SR-CVD! with Si2H6 and GeH4 have been investigated,8,9 itis possible to more clearly elucidate the mechanism ofSiH2Cl2-based SR-CVD, since the number of Cl atoms canbe measured by Auger electron spectroscopy~AES!. Study-ing the correlation between the degree of coverage by Clatoms and the growth rate enables us to understand how theincoming silicon-containing products are eventually incorpo-rated into the crystal network.

In this article, we report on the contribution of variousdangling bond~DB! regeneration channels which mediate in

the deposition of Si atoms by SR-CVD. In ALE, determiningthe method for the removal of Cl atoms is crucial in facili-tating the deposition of Si atoms.7 Likewise, in SR-CVD, theremoval process should be the rate-limiting step because Cland H atoms are continually being supplied to the surface.The possible DB regeneration channels include photon-stimulated desorption~PSD! and the reaction of photofrag-mentation products with Cl and H atoms on the surface.When only SiH2Cl2 molecules are excited, we find that thesurface is densely covered by a higher-order chloride adlayerand that only the latter channel is feasible. When the surfaceis excited as well, PSD plays a major role in reactivating thesurface.

II. EXPERIMENT

The experiments were performed on beamline 1C in thePhoton Factory at the National Laboratory for High-EnergyPhysics. The experimental procedure is basically similar tothat employed in SR-CVD from Si2H6.

8 Briefly, a white pho-ton beam covering the vacuum ultraviolet and soft x-rayranges was focused onto the surface of a specimen mountedin an ultrahigh-vacuum~UHV! chamber (base pressure55310210Torr). The energy range of the photons is between10 and 1000 eV and the energy which corresponds to themaximum flux of photons is 100 eV. The beam at the sam-ple’s surface is elliptical in shape with major and minor axesof 6 and 4 mm, respectively. The positron current in thestorage ring was between 250 and 400 mA and the photonflux was 2.331016s21 per 100 mA of ring current. The pho-ton beam was either perpendicular or parallel to the sub-strate’s surface. The configuration is described in detailelsewhere.8 Parallel to the surface, the beam only photoex-a!Electronic mail: akazawa@aecl.ntt.co.jp

JOURNAL OF APPLIED PHYSICS VOLUME 89, NUMBER 12 15 JUNE 2001

83210021-8979/2001/89(12)/8321/6/$18.00 © 2001 American Institute of Physics

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cites 99.9% pure SiH2Cl2 gas molecules. Perpendicular to thesurface, the beam photoexcites both SiH2Cl2 gas moleculesand the surface.

Ge~100! wafer substrates were used, and were subjectedto the standard wet treatment procedure.10 A clean surfacewas obtained by thermally desorbing the oxide overlayer in aUHV. Thin Si films ~less than 100 Å! were grown by SR-CVD on the Ge~100! substrates, and their thicknesses weremeasured by reflectance interferometry.10 When a Si film isgrown from a hydrogen-containing gas at low temperatures,segregation of Ge atoms is suppressed because of the surfac-tant effect of the hydrogen. The Si adlayer alters the surfaceproperties of the Ge substrate to those of Si. We thus con-centrate on the reactions that proceed on the Si surface dur-ing SR-CVD. The number of Cl atoms on the surface wasevaluatedin situ by AES. The current of the primary electronbeam was minimized to prevent the electron-stimulated de-sorption ~ESD! of Cl atoms. The total number of Cl atomsremoved by ESD was less than 5% of the number of atomsthat were present before the measurement. The apparent sig-nal intensities from Cl and Si were corrected by using sen-sitivity factors as listed in the literature11 then converted todetermine the respective coverage. We confirmed, by reflec-tion high-energy electron diffraction and cross-sectionaltransmission electron microscopy, that growth temperaturesdown to 170 °C resulted in an epitaxial film of Si. The con-centration of H and Cl atoms in the Si film was measuredfrom the depth profile of secondary ion mass spectroscopyobtained during sputtering by 12.5 keV O2

1 primary ions.

III. RESULTS

Figure 1 is a mass spectrum of the ion species producedby the photoionization of SiH2Cl2 molecules. The fragment

ions include H1, H21, HCl1, SiHx

1 (x50,1,2), SiHxCl1 (x50,1,2), and SiHxCl2

1 (x50,1). There are two stable iso-topes of chlorine, Cl35 and Cl37, and the ratio of their exis-tence in nature is 76:24. The ratio of the intensities of theCl351 and Cl371 signals and of the SiCl351 and SiCl371 sig-nals both coincide with the ratio of existence. While Cl1

contributes as an etchant, SiHx1 , SiHxCl1, and SiHxCl2

1 arethe precursors for the Si atoms that are incorporated into theSi film. H1 is an ambivalent species that is able to behaveboth as an etchant and a promoter for the growth process.The simultaneous breaking of the three chemical bonds be-tween the center Si atom and the H and Cl ligand atoms isneeded to produce Si1, SiH1, and SiCl1 from SiH2Cl2. Cou-lombic explosion caused by repulsion between the multipleholes localized in the valence orbital must thus be respon-sible for the production of these species. Since more neutralthan ionic particles are produced, we will consider neutralsto be the primary species that participate in the growth pro-cess.

The temperature dependent growth rates for the perpen-dicular and parallel beams are plotted in Fig. 2. According tothe literature,8 the SR-CVD growth rate at 350 °C under aSi2H6 pressure of 1.331023 Torr is 3 Å min21. This value isabout three times as high as that obtained by using SiH2Cl2.The explanation for this is that a Si2H6 molecule releases twosilicon-containing products (SiHx), whereas a SiH2Cl2 mol-ecule only carries the single Si atom at its center. In therange above 200 °C, the growth rates for the beam parallel tothe surface are almost constant or gradually decreasing, butthe growth rates for perpendicular incidence increase in therange from 400 to 460 °C. Such V-shaped temperature de-pendence has been observed in the epitaxy of Si from Si2H6

~Ref. 8! and of Ge from GeH4 ~Ref. 9! for both parallel and

FIG. 1. Mass spectrum of product ions resulting from the photolysis ofSiH2Cl2 gas molecules.

FIG. 2. The dependence of SR-CVD growth on the substrate’s temperaturefor perpendicularly incident~open symbols! and parallel~filled symbols!beams, normalized for an equivalent ring current of 300 mA.

8322 J. Appl. Phys., Vol. 89, No. 12, 15 June 2001 Housei Akazawa

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perpendicular irradiation. If we compare the growth rate for aparallel beam at 1.331023 Torr with the growth rate for aperpendicularly incident beam at 531024 Torr, the near-constant growth rates seen at temperatures below 400 °C dif-fer by a factor of only 1.5. Nontheless, the perpendicular-beam growth rate is thermally enhanced in the range from400 to 460 °C and above, while the parallel-beam growthrate is not. This absence of thermal enhancement in the resultfor a parallel beam is specific to the SiH2Cl2 source gas. Thetemperature that corresponds to the minimum growth rate~bottom of the V shape! is shifted toward higher tempera-tures as SiH2Cl2 pressures are increased. At 1.331023 Torrthe ratio between the growth rates for perpendicular and par-allel beam incidence is between 4 and 5. This suggests thepotential for applying this difference to selective growth.

To evaluate the elements on the surface during growth,the Si (LVV) ~92 eV! and Cl (LVV) ~180 eV! AES signalintensities were measured after approximately 30-Å-thickfilms of Si had been grown on Ge~100! at a SiH2Cl2 pressureof 1.331023 Torr. The AES signal from segregated Ge at-oms was well below the detection limit~indicating a cover-age of less than 2%!. The apparent degree of coverage by Cland Si is shown in Fig. 3, with the saturation coverage of Clafter the exposure of Ge~100! to SiH2Cl2 at 33106 L alsoplotted as control data (1L5106 Torr s). A quarter mono-layer ~ML ! of Si and a half ML of Cl atoms are introducedby the dissociative chemisorption of an SiH2Cl2 molecule onGe~100! at 200 °C according to the following reactionscheme:12

SiH2Cl2~g!142→SiCl~a!1Cl~a!12H~a!, ~1!

where ~a! and (g), respectively, denote the species at thesurface and in the gas phase, and– is a single DB-terminatedGe site. The resulting surface is illustrated in Fig. 4~a!. Thesaturation Cl coverage of 0.5 ML has been confirmed byphotoelectron spectroscopy.13,14 At temperatures above430 °C, H and Cl atoms that had been bound to Ge surfaceatoms recombine and are desorbed as HCl.12 This leads tothe appearance of DB sites on the Ge surface, and the ran-domly distributed Si admolecules are then rearranged toform a closely packed adlayer of Si, which produces newsites that serve as acceptors for further Si admolecules. Therepetition of this sequence leads to completion of a mono-atomic Si adlayer to which half a monolayer of Cl atoms isbonded. The desorption of HCl from H and Cl atoms that arebound to Si atoms requires temperatures above 550 °C, whilethe desorption of H2 is able to proceed at 500 °C.15 Thescheme which yields 0.5 ML of Cl coverage is thereforegiven by

4SiH2Cl2~g!142→2SiCl~a!12Si~a!1H2~g!

16HCl~g!, ~2!

and the surface structure is as illustrated in Fig. 4~b!.As seen in Fig. 3, the apparent coverages of Si and Cl

are similar at 200 °C for parallel beam incidence. That the Clcoverage is greater than 1 ML between 200 and 300 °C isworthy of note. As the temperature increases in the rangeabove 200 °C, the Cl coverage decreases monotonicallywhile the Si coverage increases as the number of first-layer

FIG. 3. The apparent coverage of Si~solid triangles! and Cl ~solid circles!during parallel-beam SR-CVD and the coverage of Cl during perpendicular-beam SR-CVD~open circles!. The SiH2Cl2 pressure is 1.331023 Torr. TheCl coverage after exposing a Ge~100! surface to SiH2Cl2 at 33106 L isplotted as the set of open squares.

FIG. 4. Model surface structures resulting from the self-limiting chemisorp-tion of SiH2Cl2 onto Ge~100! and the Si film surfaces produced by SR-CVD.

8323J. Appl. Phys., Vol. 89, No. 12, 15 June 2001 Housei Akazawa

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Cl atoms decreases. Finally, both coverages become constantin the range above 440 °C. In this range~.440 °C!, the cov-erage of Cl atoms during parallel-beam SR-CVD and of theCl atoms that are introduced by the self-limiting chemisorp-tion of SiH2Cl2 on Ge~100! are the same. This coincidenceindicates that this coverage of a thermodynamically stable Cladlayer, which passivates the flat Si surface, is 0.5 ML andthis is independent of the process of its formation. The result@Fig. 4~c!# is an inactive surface, in terms of the chemisorp-tion of SiH2Cl2. At low temperatures, the coverage of Clatoms during SR-CVD differs significantly, however, fromthat which results from the dissociative adsorption ofSiH2Cl2. This is because either the Cl or SiHxCl products ofphotolysis can be adsorbed to a single DB site without fur-ther dissociation, while SiH2Cl2 molecules cannot. Since theradii of Cl and Si atoms are comparable, 1 ML is the upperlimit of the Cl coverage on the top layer of Si atoms, if onlythe monochloride phase is allowed.16 If the steric hindranceeffect is taken into account, the actual degree of coverageshould be even less than 1 ML. Hence, greater than 1 MLcoverage of Cl during growth under the parallel-beam con-dition at 200–300 °C suggests the existence of ramifiedchains of higher-order chlorides@Fig. 4~d!#.17

The Cl coverage during growth under the perpendicular-beam condition is only 20%–40% of that seen duringparallel-beam SR-CVD, and this is illustrated in Fig. 4~e!.Since the irradiation of the surface by the synchrotron radia-tion beam increases the temperature by only 35 °C, PSDmust be the process with primary responsibility for the sig-nificantly reduced Cl coverage. This is reasonable, becausethe cross section of PSD of Cl1 is similar to that for the PSDof H1 PSD, as has been revealed by time-of-flight massspectroscopy of desorbing ions.7 On the other hand, when theSiH2Cl2 gas molecules alone are photoexcited, the only fea-sible channel for the regeneration of DBs is the reaction be-tween the incoming products of photolysis and the Cl atomson the surface.

Figure 5 is a plot of the concentration of H and Cl atomsin Si films as grown by parallel-beam SR-CVD. The Cl at-oms incorporated in the films are those which the Si latticeincidentally failed to repel. The reason for the 1 order ofmagnitude greater concentration of H atoms than of Cl atomsis that H atoms can occupy both interstitial and substitutionalsites, while only the interstitial sites are allowed for Cl at-oms. Mazumderet al.18 suggested that the Cl atoms that areincorporated into the bulk material as a result of the surfacereaction become permanent defects. The decrease in the con-centration of Cl as the temperature increases is correlatedwith the decreasing coverage of Cl atoms on the surface. At470 °C this coverage settles at 0.5 ML and the correspondingconcentration settles at 431018cm23. As the temperatureincreases from 200 to 600 °C, the concentration of both Hand Cl atoms is reduced by 2 decades, whereas the Cl cov-erage is reduced to only 1/3. As the crystallinity improveswith increasing temperature, the Cl atoms are more effi-ciently removed from the lattice.

The thermal enhancement of the perpendicular-beamgrowth rate is highlighted in Fig. 6, in terms of the depen-dence of the growth rate on pressure at 350 and 470 °C.

350 °C is the temperature of the onset of H2 desorption fromSi dihydrides. This means that the contribution of thermalgrowth is negligibly small at this temperature. That thegrowth rate is proportional to the pressure suggests that mi-gration makes no actual contribution to the growth rate. At470 °C, the growth rate decreases less steeply than wouldresult from a linear relation as the pressure is lowered fromthe 1024 to the 1025 Torr range. This observation can be

FIG. 5. The concentrations of Cl and H atoms in Si films grown at a varietyof temperatures.

FIG. 6. The dependence of the growth rate produced by perpendicular-beamSR-CVD on SiH2Cl2 pressure at 350 °C~open circles! and 470 °C~filledcircles!. The data were normalized for an equivalent ring current of 300 mA.

8324 J. Appl. Phys., Vol. 89, No. 12, 15 June 2001 Housei Akazawa

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interpreted as meaning that when the amount of H and Clatoms on the surface is diminished by the lower supply rateof radicals, the thermally activated migration of Si atoms anddesorption of H2 leads to an increased growth rate.

IV. DISCUSSION

The overall mass balance that describes the thermalCVD is the production of a Si adatom and two HCl mol-ecules from a SiH2Cl2 source molecule. This requires tem-peratures above 500 °C. This threshold temperature coincideswith the temperature of the onset of HCl desorption.12 Thus,under the parallel-beam condition, growth at temperaturesbelow 500 °C is brought about by the products of photolysis.The reactive sticking of fragments which contain Si is theinitial event in the delivery of Si atoms to the surface. ADB-terminated site can accept all species that contain Siwithout dissociation:

SiHxCly~g!1 –→SiHxCly~a!

~x50,1,2 and y50,1,2!. ~3!

Sugaharaet al.3 inferred that SiHCl generated in the vicinityof the surface is the primary precursor and yields 1 ML of Siadatoms per cycle in atomic H assisted ALE. To open thechannel for the growth of the next Si layer, fresh DBs mustbe regenerated. Since Cl atoms are too large to be incorpo-rated at interstitial sites in the bulk material’s lattice, growthstops unless some Cl atoms on the surface are removed. TheH-terminated sites still have some reactivity with particularspecies although the quantum efficiency would be lower thanat a DB-terminated site; Si, SiH, and SiH2 are known toreactively stick by this insertion reaction19

SiHx~g!1H~a!→SiHx11~a! ~x50,1,2!. ~4!

When the surface is not photoexcited, the only feasible chan-nel for the regeneration of DBs is the Eley–Rideal-type re-action between incoming H or SiHx and surface Cl atomsthat produces such simple gas molecules as HCl andSiHxCl.20–22 The temperature-independent growth rate~Fig.2! under the parallel beam condition is consistent with theseexothermic reactions that involve such activated species as Hand SiHx .

The Cl-removal reactions are only effective for growthwhen the regenerated DB site is served to an incomingSiHxCly radical before the site is blocked by the readsorptionof a H or Cl atom. Another process that reduces the growthrate is etching by Cl atoms. While atomic H promotesgrowth through reaction~4!, it also etches Si surfaces.23 Theetching ability of atomic H, however, is less than that of Cl.Etching would occur by the formation of SiH3 or SiH4 fromSiHx species at the corner sites of the step edges, where theSi atom is only bound to the substrate by a single bond.

When the pressure of the SiH2Cl2 is increased, the num-ber of all fragments increases. This includes, of course, sili-con source species as well as the species that suppress thegrowth rate. At low temperatures the surface is densely pas-sivated by higher-order chloride species. As the temperatureincreases from 200 to 460 °C, the Cl coverage decreasesmonotonically. This is despite the temperature of the onset of

HCl desorption from atomically flat Si surfaces, 500 °C, andcan be explained by considering the network restructuringbetween the adjacent ramified chains that contain Si

– SiHx~a!12SiHyCl~a!→– SiHx21– SiHy–1HCl~g!. ~5!

This reaction can proceed by overcoming an activation bar-rier lower than that required for the recombinative desorptionof HCl from an atomically flat Si~100! plane. That the epi-taxial growth is maintained, even at 200 °C, is explained bythe fact that the ramified chains are easily broken by excita-tion and only those Si atoms that have set in the~100! crystallattice are successfully incorporated in the bulk material.24

When Si2H6 is used as the source gas, the growth rateincreases steeply at temperatures above 350 °C, for both par-allel and perpendicular beams.8 This thermal enhancement isbrought about by the creation of further DB-terminated sitesthrough the desorption ofb2– H2 from dihydrides andb1– H2 from monohydrides. The absence of such enhance-ment during parallel-beam growth when using SiH2Cl2 indi-cates that the desorption of H2 does not affect the net growthrate. One possible explanation is that the removal of H atomsalone leads to a surface that is exclusively covered by Clatoms. The TPD measurement done by Coonet al.12 ruledout the desorption channel of Cl2. The thermally availablechannel for the removal of Cl, SiCl(a)1Cl(a)→SiCl2(g)only appears at temperatures above 550 °C and also removesthe Si adatom that had been delivered. Removing H atomsalone thus does nothing to increase the Si growth rate. On asurface that has been passivated by a large number of Clatoms, the migration of H atoms is blocked by the surround-ing Cl atoms and the thermal rearrangement of Si atoms isthus not facilitated.

Consider next the growth mechanism when the surface isexposed to electronic excitation. In this case the primarychannel for the regeneration of DBs is PSD of Cl atoms, as isconfirmed by the markedly reduced degree of Cl coverage inthe irradiated region and the measurement of desorption spe-cies by time-of-flight mass spectroscopy.7 Impinging SiHxClradicals can stick at DB-terminated sites with high probabili-ties. According to the result in Fig. 2, the flux of SiH2Cl2required to obtain a given growth rate with a parallel beam isfrom four to five times as high as required with a perpen-dicularly incident beam. The high probability of sticking ona DB-terminated surface in the latter case produces a highgrowth rate even when the flux of SiHxCl is low.

The thermal enhancement of the growth rate broughtabout by the perpendicular incident beam is clearly illus-trated by Fig. 6. The growth rates at 350 and 470 °C aresimilar at high pressures but the difference between thembecomes greater as the pressures are lowered. This can beinterpreted as meaning that the removal of sufficient Cl at-oms by PSD leads to either the migration of Si atoms toproduce a closely packed overlayer or the creation of furtherDBs by an increase in H2 desorption and thus in the growthrate. Both processes are activated thermally. The shifting ofthe minimum growth rate to higher temperatures with in-creasing pressure~Fig. 2! is consistent with this interpreta-tion. That is, when a larger number of fragment species issupplied to the surface, a higher temperature is required for

8325J. Appl. Phys., Vol. 89, No. 12, 15 June 2001 Housei Akazawa

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the coverage of H and Cl to be low enough that a thermalenhancement of the growth rate appears. As the supply rateof radicals is increased, the number of Si admolecules alsoincreases but the migration is then blocked by the increasedCl atoms on the surface, which hinders further increases inthe growth rate.

V. CONCLUSION

The efficiency of the various surface reactivation chan-nels involved in the SR-CVD of Si from SiH2Cl2 has beeninvestigated. When SiH2Cl2 gas alone is photoexcited below300 °C, the growing surface is passivated by more than 1 MLof Cl atoms, and this suggests that ramified Si chains ofhigher-order chlorides are produced in this case. Above440 °C, the surface is still passivated by half a monolayer ofCl atoms and the surface is deactivated in terms of the reac-tive sticking of SiH2Cl2. Growth proceeds either through theremoval of Cl atoms by the incoming SiHx and atomic Hspecies, followed by the sticking of SiHxCly radicals to theDB sites. When both the surface and SiH2Cl2 gases havebeen photoexcited, PSD removes 60%–80% of the Cl atomsin the steady state, and this results in a high probability ofSiHxCly sticking to DB sites. On the Cl-depleted surface, themigration of adatoms and desorption of H2 is promoted, andthis further enhances the growth rate at temperatures above400 °C.

ACKNOWLEDGMENT

The author wished to thank A. Shibayama for supportingthis research program.

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