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Morphological transition of Si 1−x Ge x films growing on Si(100). II. Synchrotron- radiation-excited chemical-vapor deposition: From two-dimensional growth to growth in the Volmer–Weber mode Housei Akazawa Citation: Journal of Vacuum Science & Technology A 20, 60 (2002); doi: 10.1116/1.1421601 View online: http://dx.doi.org/10.1116/1.1421601 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/20/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effects of stress on the dielectric function of strained pseudomorphic Si1−xGex alloys from 0 to 75% Ge grown on Si (001) J. Appl. Phys. 112, 053519 (2012); 10.1063/1.4751275 Vacuum science considerations for rapid reactor recovery with extremely low oxygen in low temperature low pressure chemical vapor deposition of Si 1 − x Ge x and Si 1 − x − y Ge x C y films J. Vac. Sci. Technol. A 24, 467 (2006); 10.1116/1.2190650 Effects of temperature and HCl flow on the SiGe growth kinetics in reduced pressure–chemical vapor deposition J. Vac. Sci. Technol. B 21, 2524 (2003); 10.1116/1.1623508 Morphological transitions of Si 1−x Ge x films growing on Si(100). I. Gas-source molecular-beam epitaxy: From two-dimensional growth to growth in the Stranski–Krastanov mode J. Vac. Sci. Technol. A 20, 53 (2002); 10.1116/1.1421600 Thermal and excimer laser assisted growth of Si (1−x) Ge x alloys from Si 2 H 6 and GeH 4 monitored by on line single wavelength ellipsometry and ex situ atomic force microscopy J. Vac. Sci. Technol. A 16, 644 (1998); 10.1116/1.581082 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 131.104.62.10 On: Fri, 21 Nov 2014 20:10:05

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Page 1: Morphological transition of Si[sub 1−x]Ge[sub x] films growing on Si(100). II. Synchrotron-radiation-excited chemical-vapor deposition: From two-dimensional growth to growth in the

Morphological transition of Si 1−x Ge x films growing on Si(100). II. Synchrotron-radiation-excited chemical-vapor deposition: From two-dimensional growth to growthin the Volmer–Weber modeHousei Akazawa Citation: Journal of Vacuum Science & Technology A 20, 60 (2002); doi: 10.1116/1.1421601 View online: http://dx.doi.org/10.1116/1.1421601 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/20/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Effects of stress on the dielectric function of strained pseudomorphic Si1−xGex alloys from 0 to 75% Ge grownon Si (001) J. Appl. Phys. 112, 053519 (2012); 10.1063/1.4751275 Vacuum science considerations for rapid reactor recovery with extremely low oxygen in low temperature lowpressure chemical vapor deposition of Si 1 − x Ge x and Si 1 − x − y Ge x C y films J. Vac. Sci. Technol. A 24, 467 (2006); 10.1116/1.2190650 Effects of temperature and HCl flow on the SiGe growth kinetics in reduced pressure–chemical vapor deposition J. Vac. Sci. Technol. B 21, 2524 (2003); 10.1116/1.1623508 Morphological transitions of Si 1−x Ge x films growing on Si(100). I. Gas-source molecular-beam epitaxy: Fromtwo-dimensional growth to growth in the Stranski–Krastanov mode J. Vac. Sci. Technol. A 20, 53 (2002); 10.1116/1.1421600 Thermal and excimer laser assisted growth of Si (1−x) Ge x alloys from Si 2 H 6 and GeH 4 monitored by on linesingle wavelength ellipsometry and ex situ atomic force microscopy J. Vac. Sci. Technol. A 16, 644 (1998); 10.1116/1.581082

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Page 2: Morphological transition of Si[sub 1−x]Ge[sub x] films growing on Si(100). II. Synchrotron-radiation-excited chemical-vapor deposition: From two-dimensional growth to growth in the

Morphological transition of Si 1ÀxGex films growing on Si „100….II. Synchrotron-radiation-excited chemical-vapor deposition:From two-dimensional growth to growth in the Volmer–Weber mode

Housei Akazawaa)

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

~Received 24 April 2001; accepted 1 October 2001!

The morphological evolution of Si12xGex films growing under conditions of excitation byhigh-energy photons (hn.100 eV) has been investigated by means ofin situ spectroscopicellipsometry. An atomically discontinuous wetting layer is produced by the incidence of the productsof photolysis from Si2H6 and GeH4 on a hydrogen-terminated Si~100! surface. At temperatures ofgrowth below 300 °C, a uniform Si12xGex layer grows because of the strong hydrogen surfactanteffect and the low mobility of the Si and Ge adatoms. At temperatures above 400 °C, most of the Siadatoms are in the form of monohydride and Ge adatoms are free of bonds with hydrogen atoms.On a partially hydrogen-covered surface, the number of dangling-bond terminated nucleationcenters is restricted, while the migration of the adatoms is enhanced. This results in islanding withroughness up to a maximum height of about 100 Å from the initial stages of growth. Since the grainboundaries between islands become the sources of dislocations, the strain stored in the Si12xGex

islands is completely relieved. This leads to early coalescence so that a flat overlayer surface isrecovered. ©2002 American Vacuum Society.@DOI: 10.1116/1.1421601#

I. INTRODUCTION

It is generally accepted that the growth of Si12xGex filmson Si~100! surfaces by gas-source molecular beam epitaxy~GSMBE! and solid-source MBE proceeds in either a two-dimensional ~2D! mode or the Stranski–Krastanov~SK!mode. The driving force of the SK growth mode is the partialrelief of the elastic strain stored in the wetting layer by theappearance of undulations on the surface or the nucleation ofthree-dimensional~3D! coherent islands prior to the appear-ance of misfit dislocations. With increasing growth tempera-ture, the wetting layer becomes thinner and the islands be-come larger. At lower temperatures, the adatoms are lessmobile and the surfaces of the Si12xGex films are smoother.Besides such general tendencies that are due to the effect oftemperature, the morphology is strongly affected by chemi-cal processes when the Si and Ge atoms are supplied fromgas sources.1 This is particularly so when electronically ac-tivated growth takes place under a nonthermal equilibriumcondition.

Here we report on morphological evolution in Si12xGex

films grown by synchrotron-radiation-excited chemical-vapor deposition~SR-CVD! from Si2H6 and GeH4.

2,3 SR-CVD is distinct from GSMBE, but is similar to plasma-enhanced CVD ~PECVD! in terms of the gas-phasechemistry, in that the photolysis of the source gases producesreactive fragments that contribute the greater part of thegrowth rate. The primary difference between these tech-niques is that the near-surface region of the film is electroni-cally excited by high-energy photons in SR-CVD, while thisis not the case in PECVD.3 This electronic excitation results

in the breaking of Si–H and Ge–H bonds by photon-stimulated desorption,3 and coverage of the irradiated regionof the film’s surface by H atoms is reduced significantly. Thedangling-bond-terminated sites thus created facilitategrowth. Electronic excitation also promotes crystallization ofSi12xGex . This is so even without dilution of the sourcegases in a H2 carrier gas.

In this work, thein situ measurement of the changes indielectric response by spectroscopic ellipsometry~SE! re-vealed the initial islanding of Si12xGex films under certaingrowth conditions, and this resembles growth in the Volmer–Weber~VW! mode. We explain this new growth behavior inthe Si12xGex /Si~100! system in terms of the balance be-tween partial blocking of the dangling bonds by H atoms andthe enhanced migration of Si and Ge admolecules.

II. EXPERIMENT

The Si12xGex films were grown by SR-CVD in an ultra-high vacuum chamber.4 This growth chamber is connected tobeamline 7 of the compact electron storage ring ‘‘Super-ALIS’’ at NTT’s Atsugi Research and Development Center.The synchrotron radiation emitted at the bending magnet isfocused by two toroidal mirrors onto the surface of a Si~100!substrate. The photon beam is characterized by its energydistribution ranging between 10 and 1500 eV and the energylevel with the maximum flux of photons is 100 eV. Morethan 90% of the photons have energies greater than 100 eVand are thus able to excite Si 2p and 2s core electrons. TheSR beam which excites the gas and the substrate and theultraviolet probe light for SE measurement were incident toa!Electronic mail: [email protected]

60 60J. Vac. Sci. Technol. A 20 „1…, Jan ÕFeb 2002 0734-2101Õ2002Õ20„1…Õ60Õ8Õ$19.00 ©2002 American Vacuum Society

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the substrate at 12° and 72° from the surface normal, respec-tively. The geometrical configuration is explained in detailelsewhere.5

The Si~100! wafers were pretreated in a 2.5% aqueoussolution of hydrofluoric acid to remove the surface oxidelayer. The resulting surface was terminated by a variety ofsilicon hydrides. A 50–70-nm-thick Si buffer layer was thusgrown at 700 °C to create a smooth surface. A Si12xGex

overlayer was grown by SR-CVD with various rates of GeH4

and Si2H6 flow. The flow rate of Si2H6 was varied between 1and 10 sccm and the flow rate of GeH4 was varied between10 and 30 sccm. A Si capping layer was grown on some ofthe Si12xGex layers.

Ellipsometry measures the Fresnel reflection coefficientratio r between light that isp-polarized ands-polarized withrespect to the surface plane, which defines the ellipsometricanglesC and D as r5tanC•exp(iD). The pseudodielectricfunctions ( e&5^e1&1 i ^e2&) of Si and Si12xGex /Si~100!were measured both at the growth temperature under irradia-tion with the photon beam and at room temperature aftergrowth. The temperature of the substrate was monitored by athermocouple, which is in contact with the back side of thesubstrate wafer. The real surface temperature in the irradiatedregion was obtained after careful calibration of the substratetemperature by using the apparent absorption level in thee2

spectrum of a Si~100! surface.The Ge contentx of the Si12xGex film was determined by

analyzing the pseudodielectric function around theE1 criti-cal point~CP! energy,6 where the apparent dielectric constantreaches saturation at a small thickness because of the shal-lowness of the depth to which the light penetrates. The Gecomposition was calibrated by Auger analysis during Ar1

ion sputtering of the film.6 The crystal images of the epitaxialfilms were obtained by cross-sectional transmission electronmicroscopy~XTEM!. High-magnification images were ob-tained at an electron acceleration voltage of 300 kV.

III. RESULTS

Figure 1 shows three sets ofC–D trajectories observed at3.4 eV with a low GeH4 /Si2H6 flow-rate ratio. Since theSi12xGex film is nontransparent at this photon energy, theone-turn short spirals starting from points 01, 02, and 03quickly converge on the respective destination pointsE1,E2 , and E3. The composition of the resulting Si12xGex

films at termination pointsE1, E2, andE3 arex50.30, 0.33,and 0.35. The instantaneous shift of the~C,D! point in theCdirection just after the introduction of the source gases~trace01→X1, trace 02→X2, and trace 03→X3! indicates that theSi~100! surface is quickly and densely covered by hydridesthat are produced by the photolysis of Si2H6 and GeH4. Thisshift in the angleC ~dC! is the consequence of the formationof a surface layer that is composed of Si–H and Ge–H bondsand agrees with values reported in the literature.7

The slightly different positions of the convergence points~E1, E2, andE3! result from the temperature-dependent di-electric constants of the materials~the Si12xGex film and theSi substrate! and the temperature dependence of the incorpo-

ration rate of Si and Ge atoms in the film. We find that theSi12xGex films are strained since the XTEM images showedno misfit dislocations over the horizontal range of 2mm. Theexperimental trajectories can be fit to the result of a simula-tion in which the growth of a smooth 2D film is assumed.

GSMBE performed at 600 °C and under the same gas-flow condition produced the composition ofx50.13. MoreGe atoms are thus incorporated into the film by SR-CVDthan by GSMBE. The explanation is that once GeH4 mol-ecules have been decomposed by photons, very reactiveGeHx (x50 – 3) species are generated. The sticking prob-ability for GeHx is more than 2 orders of magnitude greaterthan for GeH4, but there is less difference between the reac-tivities of Si2H6 and SiHx . Photodecomposition of the sourcegases therefore favors the incorporation of more Ge atoms.

Figure 2 shows typicalC–D trajectories monitored simul-taneously at 1.5, 2.3, 3.4, and 4.3 eV during SR-CVD atvarious temperatures. The compositions of the resultingSi12xGex films arex50.50 at 250 °C,x50.50 at 300 °C,x50.46 at 400 °C, andx50.36 at 450 °C. This is evident fromthe redshift of theE1-CP energy in thee2& spectra shown inFig. 3. The dependence of the composition on the growthtemperature results from the marked changes in the rate ofH2 desorption at around the threshold temperature.

At photon energies of 4.3 and 3.4 eV, where Si12xGex isabsorbent, the trajectories at 250, 300, and 400 °C describeone-turn spirals that cross one another~trace O→A→B→E!, and indicate 2D growth. The growth rate at 250 °C asderived by fitting the results of simulation to the experimen-tal trajectory is 15 Å s21. This rate is comparable with thatachieved by GSMBE at 600 °C under the same gas-flow con-dition, but the growth kinetics are essentially different. At 1.5and 2.3 eV, where the Si12xGex film is semitransparent, the~C,D! points moved along multiple spirals and asymptoti-cally approached convergence points.

FIG. 1. C–D trajectories monitored at 3.4 eV during two-dimensionalSi0.70Ge0.30 growth at 250 °C~open circles!, Si0.67Ge0.33 growth at 400 °C~open squares!, and Si0.65Ge0.35 growth at 450 °C~closed squares!, with aSi2H6 flow rate of 5 sccm and a GeH4 flow rate of 10 sccm.

61 Housei Akazawa: Morphological transition of Si 1ÀxGex films 61

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As compared with these sets of trajectories, the shapesdescribed by the trajectories at 450 °C are somewhat dissimi-lar and this suggests the onset of 3D nucleation, although theGe content in the film is lower than in the other films. At 3.4

eV, the trajectories initially move more steeply toward theregion of lowerD after passing through the left side of thepath of the trajectories for 2D growth~trace O→C→D!.Reaching a position with a lowerD angle, as compared withthat in traceO→A→B, indicates the formation of a surface-roughness layer in the initial stage of growth. From the low-est pointD, the path of the trajectories was upward~traceD→E! and the points soon converged on pointE, whichcoincides with the end points of the other sets of trajectoriesof 2D growth. This indicates the recovery of a smooth sur-face.

The paths of the trajectories at 2.3 and 1.5 eV were, on theother hand, initially downward~trace O→C!, moved up-ward, and soon merged with the multiple-turn spiral path atthe junction pointD. They then asymptotically approachedthe convergence point, which is close to the terminationpoint for growth at 250 °C. This observation suggests that thenear-surface morphology again settles into a form similar tothat seen in 2D film growth.

Figure 4 shows further sets ofC–D trajectories, obtainedwith a higher GeH4 /Si2H6 flow-rate ratio, and these plotsexhibit characteristics that reflect advanced 3D nucleation.The resulting film compositions arex50.58 at 300 °C,x50.53 at 400 °C, andx50.47 at 450 °C. At 3.4 eV and agrowth temperature of 300 °C, the short one-turn traceO→G→F again indicates 2D film growth. At 400 °C, the ini-

FIG. 2. C–D trajectories monitored at:~a! 4.3 eV, ~b! 3.4 eV, ~c! 2.3 eV, and~d! 1.5 eV during SR-CVD with aSi2H6 flow rate of 5 sccm and a GeH4

flow rate of 30 sccm. The symbolsdesignated in~b! have the same mean-ings in ~a!, ~c!, and~d!.

FIG. 3. ^e2& spectra of Si12xGex films grown at various temperatures:x50.50 at 250 °C,x50.50 at 300 °C,x50.46 at 400 °C, andx50.36 at450 °C ~corresponding to the trajectories plotted in Fig. 2!.

62 Housei Akazawa: Morphological transition of Si 1ÀxGex films 62

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tial shift toward the lowD region is rapid~traceO→K!, andthis indicates pronounced islanding, even though this is anearly stage in growth on the Si~100! surface. The path of thetrajectory was then directed toward the termination pointF~trace K→L→M→F!. The shape traced by the trajectorydoes not appear to be very similar to the two-branch trajec-tory that is characteristic of the SK mode.4 Such a largespiral in the C–D plane was never observed during theGSMBE of Si12xGex films with similar Ge contents. Obvi-ously, SR beam excitation of the materials has altered thekinetics of nucleation. The strong initial islanding behaviorsuggests the growth in the VW mode. At 450 °C, the initialshift toward the region of lowerD angles becomes morepronounced~traceO→P→Q!. From the lowestD point Q,the trajectories again start to be directed toward the termina-tion point F ~traceQ→R→F!. The smaller theD angle atthe bottom of the spiral, the greater the maximum height ofthe islands. A much thicker film is then required to recover aflat surface. We have also confirmed a similar trajectoryshape under other Si2H6 gas-flow conditions of 2 and 1 sccmunder a GeH4 flow rate of 30 sccm.

The photon energy of 2.3 eV is below theE1 and (E1

1D) CP energies that correspond to the band-to-band tran-sitions of crystalline Si but a Si12xGex film with x.0.5 isquite photoabsorptive at this energy level. TheC–D trajec-tory paths in Fig. 4~c! exhibit multiple-turn spirals, with the

centers of the spirals drifting upward. The initial shift of thetrajectories in the direction of lowD ~tracesO→K and O→P! became more pronounced as the temperature increased.At 1.5 eV @Fig. 4~d!#, a similar initial shift in the direction oflow D is again evident. The trajectories follow a quasispiralshape, the center of which gradually shifts as growth pro-ceeds. VW growth followed by coalescence is thus charac-terized by the drifting center of the multiple-turn spiral whenthe trajectories are monitored at photon energies to which thesurface is semitransparent.

Figure 5 is a pair of XTEM images of 2D Si12xGex filmsgrown at 300 °C. The Si0.57Ge0.43 film shown in Fig. 5~a! isentirely epitaxial and without dislocation in the image’s fieldof view. The film must therefore be strained. Even with theproportion of the Ge very high atx50.80@Fig. 5~b!#, the 2Dmorphology is maintained because the growth is at low tem-perature. Since the mismatch between the lattices is verygreat, the film is forced to relax by many dislocations whichextend from the Si/Si0.2Ge0.8 interface.

Figure 6~a! is a XTEM image of a Si0.47Ge0.53 film whichexhibited VW growth and coalescence~it corresponds to thetrajectory paths at 400 °C in Fig. 4!. At 3.4 eV it is evidentthat, no matter how extensive the trajectory path during theintermediate stages of growth, coalescence eventually recov-ers a flat Si0.47Ge0.53 surface. The resulting coalescedSi0.47Ge0.53overlayer surfaces are so flat that a 2D Si capping

FIG. 4. C–D trajectories monitored at:~a! 4.3 eV, ~b! 3.4 eV, ~c! 2.3 eV, and~d! 1.5 eV during SR-CVD at a Si2H6

flow rate of 3 sccm and a GeH4 flowrate of 30 sccm. The symbols desig-nated in~a! have the same meanings in~b!, ~c!, and~d!.

63 Housei Akazawa: Morphological transition of Si 1ÀxGex films 63

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Page 6: Morphological transition of Si[sub 1−x]Ge[sub x] films growing on Si(100). II. Synchrotron-radiation-excited chemical-vapor deposition: From two-dimensional growth to growth in the

layer was successfully grown. Although 60° spiral disloca-tions extend toward the upper surface region, they do notbuild up.8 The dislocations near the interface divide the filminto triangular domains. As shown by the illustration in Fig.6~b!, the grain boundaries between islands become the en-trenched sources of dislocations. When the strains are com-pletely relieved, maintenance of the vicinal surfaces of theislands is energetically disadvantageous. This leads to a rapidcoalescence of the strain-relieved Si0.47Ge0.53 islands duringthe deposition of further Si and Ge atoms so that a 2D mor-phology is recovered by the film. The initial islanding thusserves to relieve the misfit strain efficiently and early coales-cence is thus attained.

Judging from the experimental results, the photon energylevel of 3.4 eV, which coincides with theE1 CP energy of aSi crystal, is the most sensitive and appropriate in terms ofthe analysis and observation of morphological evolution.Since Si12xGex films are absorbent at 3.4 eV, the~C,D!angles primarily reflect the dielectric response of the near-surface layers and are immune to the underlying film. TheC–D trajectories resulting from VW growth followed by thecoalescence can apparently be reproduced by a simple ana-lytical model based on the assumption of the deposition of agraded layer in which the void-volume fraction varies withdepth. The dielectric function of each slab layer is given as amixture of the component dielectric functions of Si12xGex

with that of a void under Bruggeman effective-medium ap-proximation. We assume that the void–volume fractionf vdecreases linearly over time from the initial void–volumefraction f v0 , according to the relationf v5 f v02Rvt (0%t% f v0 /Rv), and that a 2D Si12xGex film without voids (f v50) grows when the coalescence is complete. This graded-layer model is depicted in Fig. 6~c!. The film’s thicknessd,on the other hand, increases over time according to the rela-tion d5Rdt. From these equations, we obtaind5( f v02 f v)•Rd /Rv (0%t% f v0 /Rv). The changes in the optical re-sponse are uniquely determined by the input parameters thatare supplied:f v0 andRd /Rv .

Figures 7~a! and 7~b! show a fit of results of simulationsto the experimental trajectories at 400 and 450 °C which ap-pear in Fig. 4~b!. As the void content of the near-surfaceregion of the film decreases, the~C, D! point approaches thetermination point of the trajectory that corresponds to 2Dfilm growth. The overall trajectories are found to be close tothose from the simulation when the initial void-volume frac-tion is between 40% and 50%. The material density ofSi12xGex at the surface is therefore only 50%–60%, and thismeans that the wetting layer is atomically discontinuous andconfirms the formation of islands in the initial stage ofgrowth.

The discrepancy between the results of the simulation andof the experiment, which are particularly obvious as the finalpoint of the trajectory path is approached, suggests the limi-tations of the simplistic model, that is, of the linearly chang-ing void-volume fraction over time. At 400 °C the film’sthickness reached 110Å at the bottom of the spiral~point L!and 220 Å at the termination pointF. The voids between the110-Å-thick islands must thus have been almost completelyfilled during the deposition of another 110-Å-thick overlayer.Likewise, at 450 °C, the spaces between the islands, withtheir maximum height of 120 Å at pointQ, were buriedwhen the total thickness reached 270 Å.

Table I summarizes the instantaneous shift of the angles inthe direction ofC ~udCu! immediately after the admission ofSi2H6 and GeH4 gases to the chamber under SR beam irra-diation, along with the initial slope of theC–D trajectory(2dD/dC) when the growth of bulk Si12xGex began. Witha Si2H6 flow rate of 5 sccm and a GeH4 flow rate of 10 sccm,2D growth was maintained at temperatures up to 450 °C. Inthis case, thedC is larger than 0.176. Under the other gas-

FIG. 5. XTEM images of:~a! Si0.57Ge0.43 and~b! Si0.20Ge0.80 films grown at300 °C.

FIG. 6. ~a! XTEM image of a Si0.47Ge0.53 film grown at 400 °C,~b! anillustration of the growth of islands followed by coalescence, and~c! thegraded-layer model used to simulateC–D trajectories.

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flow conditions, theudCu value was greater than 0.15 at tem-peratures below 300 °C, but decreased abruptly to 0.02–0.03between 400 and 450 °C.

udCu reflects the degree to which hydrides are present.This indicates that, at temperatures above the threshold tem-perature for the desorption of H2 from Ge hydrides, the hy-drides decomposed within short periods. This tendency be-comes more pronounced as the proportion of GeH4 in themixture of gases is increased. When the rate of Si2H6 supplyis 1 sccm and the rate of GeH4 supply is 30 sccm, significantislanding was observed even though thedC values are rela-tively high at 300 and 350 °C. The explanation is that, as the

Si~100! surface is covered by a Si12xGex overlayer that con-tains a high proportion of Ge, the hydrogen coverage isdecreased.9 The C angle offset disappears although it is noteasy to identify this on theC–D plot.

The value of 2dD/dC is generally larger at higherGeH4 /Si2H6 flow-rate ratios. This is due in part to the loca-tion of the convergence point in the region of lowerD angles.The 2dD/dC value for 2D growth is typically between 3and 5. When a critical point in the formation of islands isexceeded,2dD/dC suddenly becomes large, with a valuebetween 8 and 10. The2dD/dC values obtained by simu-lation for a Si2H6 flow rate of 3 sccm and GeH4 flow rate of30 sccm were 11.0 at 400 °C and 12.0 at 450 °C~Fig. 7!, andthis agrees with the experimental data. TheC–D trajectoryfor VW growth is therefore discriminated from 2D growth byits distinctly greater initial slope. This observation furtherconfirms the formation of Si12xGex islands in the initialstage of growth.

IV. DISCUSSION

SR-CVD at temperatures below the threshold for H2 de-sorption from Ge hydrides is able to produce 2D films ofSi12xGex containing a high proportion of Ge because of thesufficiently low mobility of the admolecules.10 The initialislanding that occurs at intermediate temperatures is, in con-trast, classified as growth in the VW mode. The trajectoriesfrom the VW mode can be clearly distinguished from thetwo-branch trajectory paths that are characteristic of the SKmode that is typical during GSMBE.10 The steeper initialslope of the trajectory path and the more extensive range oftrajectories provide evidence of a roughened surface.

It is possible to explain this morphological evolution,which is specific to SR-CVD, by comparing the growthmechanism with that of GSMBE; in GSMBE, the primaryspecies participating in growth are hydride molecules. Attemperatures above 600 °C that are typically used forGSMBE, H atoms are almost entirely depleted from thegrowing surface. This is because H atoms are desorbed im-mediately when they are delivered to the surface throughreactive sticking of Si2H6 and GeH4. The dangling-bond-terminated surface is readily covered by a wetting layer inwhich the network of constituent atoms is completely crys-talline. The morphology is controlled by the mobility of ada-toms, which is a function of the Ge content and the tempera-ture.

FIG. 7. SimulatedC–D trajectories at a photon energy of 3.4 eV, made to fitthe two trajectories shown in Fig. 4~b!.

TABLE I. Instantaneous shift of angle in the direction ofc immediately after the introduction of gas, and the slope of thec–D plot in the initial stage of growth(udcu/2dD/dc) as functions of temperature and flow rates of Si2H6 and GeH4. The photon energy is 3.4 eV. Underline indicates the island growth.

Temperature~°C!

Si2H6

5 sccmGeH4

10 sccmSi2H6

5 sccmGeH4

30 sccmSi2H6

3 sccmGeH4

30 sccmSi2H6

2 sccmGeH4

30 sccmSi2H6

1 sccmGeH4

30 sccm

250 0.201/3.6 0.171/5.0 ¯ ¯ ¯ ¯ ¯ ¯

300 0.176/3.6 0.162/5.0 0.149/5.5 0.165/6.1 0.171/8.5350 ¯ ¯ ¯ ¯ 0.140/7.2 0.117/6.7 0.122/9.2400 0.201/3.8 0.217/5.5 0.032/9.0 0.025/8.8 0.029/10.0450 0.214/4.3 0.039/8.3 0.026/10.0 0.019/9.5 ¯ ¯

65 Housei Akazawa: Morphological transition of Si 1ÀxGex films 65

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When Si2H6 and GeH4 gas molecules are electronicallyexcited by photoabsorption, Si and Ge hydride radicals andatomic H are generated. H2 molecules are not released, evenfrom Ge hydrides, at temperatures below 300 °C. Most of theH atoms initially bonded to the impinging SiHx and GeHx

radicals remain on the surface. The surface of the overlayeris therefore densely covered by terminating with H atoms. Siand Ge admolecules are fixed in the vicinity of the positionsat which admolecules are initially adsorbed, so the wettinglayer is atomically discontinuous. Nevertheless, the resultingfilms of Si12xGex are epitaxial because core-electronic exci-tation of Si and Ge atoms promotes rearrangement of thechemical bonds so that the atoms settle at crystalline latticepositions. The H atoms incorporated within the interface andat interstitial sites are repelled and ejected from the film asthe crystalline Si12xGex lattice is completed. The higher-order silicon hydrides that terminate the growing surface arereadily decomposed into monohydrides by the photon-stimulated desorption of H atoms.11 This action is importantin sustaining the (131) arrangement of the crystal plane.

When the temperature is raised to well above the thresh-old for H2 desorption from Ge hydrides, islanding is initiallyprevalent, even immediately after growth has started and isfollowed by coalescence. This is a universal phenomenawhen Si12xGex films are grown by SR-CVD, as has beenconfirmed by an experimental series at different gas-flowrates. This 2D–VW transition occurs abruptly. Thetemperature-programmed desorption of H2 shows that mostof the Si adatoms are in the form of the monohydride and theGe adatoms are already free of bonds with H atoms at400 °C.12 When the average number of H atoms that arebonded in admolecules decreases, the migration of admol-ecules is no longer suppressed. The situation is that the mo-bility of the admolecules is sufficiently high, while theSi12xGex surface remains covered by the ample hydridesthat are supplied ceaselessly from the gas phase. It is inferredthat admolecules gather exclusively at dangling-bond-terminated sites and nucleate there. The greater tendency toinital islanding at higher temperatures is explained by thelarger mean-free path for adatoms. The other possible factorpromoting the formation of islands is migration driven by thephotoexcitation of admolecules. Some articles have sug-gested enhacing the surface migration of atoms by using anelectron beam to excite the surface.13,14 If a similar mecha-nism applies to the photoexcited system presented here, thepositive charging of the adatoms by the depletion of valenceelectrons may be responsible.

Hydrogen atoms have been shown to strongly affect themobility of Si and Ge adatoms. The hydrogen surfactant ef-fect was originally considered in terms of its action in sup-pressing the migration of adatoms.15 Maintenance of asmooth film surface and suppression of interdiffusion acrossthe interface have been interpreted as evidence of thiseffect.16–19These results apply to the thermal growth system.In the SR-CVD process under discussion here, because themorphology is governed by the nonequilibrium kinetics ofgrowth, the appearance of the VW mode is not necessarily

inconsistent with a hydrogen surfactant effect. Under a par-ticular condition, the apparent promotion of the migration ofadatoms to enhance surface undulations was revealed whenthe Si12xGex films grown by MBE were exposed to atomicH.20

As judged from the lower temperature required to sustainepitaxy when the film contains a higher proportion of Ge, theGe atoms are more mobile than the Si atoms. Ge atoms tendto assemble themselves to form Ge islands. Such preferentialnucleation of Ge islands could occur in the initial stage ofgrowth so that the strain energy is reduced.21 The enhancedmigration with higher proportions of Ge contents originatesfrom the greater misfit strain in the Si12xGex film on aSi~100! substrate. Once the nucleation centers have been cre-ated, the incoming atoms preferentially attach themselves tosuch sites. In this manner, islands surrounded by high-indexfacet planes develop. The troughs between strain-relieved is-lands are then embedded by admolecules, and the grainboundaries between the islands become sources of disloca-tions. This process effectively relaxes the film. A flat surfacesoon appears when the islands coalesce, one with another.Extending the misfit dislocation efficiently relieves the misfitstrain while increasing the film’s thickness.

V. CONCLUSION

The epitaxial growth of Si12xGex films on Si~100! fromSi2H6 and GeH4 under excitation by synchrotron radiationstarts in either the 2D or the VW mode, according to thedegree of passivating H atoms, which is followed by coales-cence. Initial growth is in the 2D mode at temperatures be-low the onset of H2 desorption from Ge hydrides. In this casedangling bonds are fully tied to H atoms and the migration ofadatoms is suppressed. At temperatures higher than thethreshold for H2 desorption from Ge hydrides, the mobilityof Ge adatoms and Si hydrides is enhanced on theH-terminated surface. This causes initial islanding that rep-resents growth in the VW mode.

The mobility of adatoms is increased by at higher propor-tion of Ge content and higher temperatures as well as by theelectronic excitation of admolecules. The VW mode isclearly indicated by the initial steep reduction in theD angle,and coalescence can be identified by its return toward thetermination point for 2D growth. This behavior is distinctfrom the two-step behavior in the SK growth mode. There isa threshold H coverage or adatom mobility. When the do-main boundaries between islands are formed, they becomethe sources of dislocations, and early coalescence isachieved.

ACKNOWLEDGMENTS

The author thanks Y. Kato, E. Sato, and N. Yabumoto forXTEM image measurement. He also thanks A. Shibayamafor supporting this research program.

66 Housei Akazawa: Morphological transition of Si 1ÀxGex films 66

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1N.-E. Lee, D. G. David, and J. E. Greene, Phys. Rev. B53, 7876~1996!.2H. Akazawa and Y. Utsumi, J. Appl. Phys.78, 2740~1995!.3H. Akazawa, Phys. Rev.59, 3184~1999!.4H. Akazawa, J. Vac. Sci. Technol. A20, 53 ~2002!.5H. Akazawa and J. Takahashi, Rev. Sci. Instrum.69, 265 ~1998!.6H. Akazawa, Thin Solid Films369, 157 ~2000!.7H. Akazawa, Surf. Sci.427–428, 214 ~1999!.8M. A. Lutz, R. M. Feenstra, F. K. LeGoues, P. M. Mooney, and J. O. Chu,Appl. Phys. Lett.66, 724 ~1995!.

9J. Vizoso, F. Martin, J. Sune, and M. Mafria, J. Vac. Sci. Technol. A15,2693 ~1997!.

10H. Akazawa, Appl. Surf. Sci.130–132, 292 ~1998!.11A. Yoshigoe, M. Nagasono, K. Mase, and T. Urisu, Jpn. J. Appl. Phys.,

Part 134, 6894~1995!.12G. Lu and J. E. Crowell, J. Chem. Phys.98, 3415~1993!.

13A. G. Fedorus, E. V. Klimenko, A. G. Naumovets, E. M. Zasimovich, andI. N. Zasimovich, Nucl. Instrum. Methods Phys. Res. B116, 355 ~1996!.

14H. J. Jansch, J. Xu, and J. T. Yates, J. Chem. Phys.99, 721 ~1993!.15M. Copel and R. M. Tromp, Appl. Phys. Lett.58, 2648~1991!.16G. Ohta, F. Fukatsu, Y. Ebuchi, T. Hattori, N. Usami, and Y. Shiraki, Appl.

Phys. Lett.65, 2975~1994!.17A. Sakai and T. Tatsumi, Appl. Phys. Lett.64, 52 ~1994!.18K. Nakagawa, A. Nishida, Y. Kimura, and T. Shimada, J. Cryst. Growth

150, 939 ~1997!.19M. Okada, T. Shimizu, H. Ikeda, S. Zaima, and Y. Yasuda, Appl. Surf. Sci.

113Õ114, 349 ~1997!.20C. Silvestre, G. G. Jernigan, M. E. Twigg, and P. E. Thompson, J. Vac.

Sci. Technol. B16, 1933~1998!.21T. Walther, C. J. Humphreys, and A. G. Cullis, Appl. Phys. Lett.71, 809

~1997!.

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