SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 29, 194–200 (2000)

Adsorption studies of digermane and disilaneon Ge(100)

S. Ateca, C. Bater, M. Sanders and J. H. Craig, Jr.*Department of Physics, Materials Research Institute, University of Texas at El Paso, El Paso, TX 79968-0515, USA

Adsorption studies of digermane (Ge2H6) and disilane (Si2H6) on Ge(100) are reported. Temperature-programmed desorption (TPD) experiments suggest the existence of two hydrogen adsorption states in thesubmonolayer regime for both digermane and disilane. The TPD spectra observed for disilane on Ge(100) arequalitatively similar to previous studies of disilane adsorbed on thin epitaxial layers of germanium on Si(100).These spectra show the existence of ana-state arising from the germanium monohydride and ab1-state arisingfrom a silicon monohydride. The two-peak structure observed in the hydrogen TPD spectra for digermaneon Ge(100) suggests the existence of both germanium monohydride and germanium dihydride, about whichthere is disagreement in the literature. However, high-resolution electron energy-loss spectroscopy and low-energy electron diffraction provide additional evidence for the existence of both germanium hydrides onthe digermane/Ge(100) system. Kinetic energy distributions of electronically desorbed HY and hydrogenremoval cross-sections obtained through electron-stimulated desorption are also presented for both systems.Copyright 2000 John Wiley & Sons, Ltd.

KEYWORDS: Ge(100); digermane; disilane; adsorption; monohydride; dihydride; temperature-programmed desorption (TPD); electron-stimulated desorption (ESD)

INTRODUCTION

Within the last decade, interest in germanium as a semi-conductor has steadily risen. Alloys of Ge/Si and strained-layer superlattices exhibit novel properties beyond thoseof either semiconductor alone, and are increasingly beingused in, or considered for, tailored electronic devices.Much, however, is still not known about these compoundsystems, and the techniques involved in their manufac-ture are extremely dependent on processes occurring atthe atomic or molecular level. For example, thin filmsare currently grown principally through atomic layer epi-taxy (ALE), molecular beam epitaxy (MBE) or chemi-cal vapor deposition (CVD). These techniques require atleast a rudimentary understanding of the dynamics andkinetics of the adsorbate/substrate interaction if growth isto be adequately controlled or tailored. Furthermore, asnanotechnology advances, electronic devices continue toshrink. Tailoring these devices then requires ever moreprecise control of growth rates down to the atomic level.1

Thus, electronic device technology is becoming increas-ingly dependent on fundamental research leading to a clearunderstanding of atomic-level interactions that occur onsurfaces during device fabrication.

Numerous previous studies have investigated the roleof hydrogen as a component of molecular precursorsadsorbed on silicon, germanium and Ge/Si systems.1 – 12

However, the majority of these studies have focused

* Correspondence to: J. H. Craig, Jr., Department of Physics, Uni-versity of Texas at El Paso, 500 West University Ave, El Paso, TX79968-0515, USA.E-mail: jcraig@utep.edu

Contract/grant sponsor: National Science Foundation; Contract/grantnumber: CHE8920120.

on silicon substrates. Comparatively less work hasbeen expended on germanium substrates, and manyambiguities pertaining to molecular precursors adsorbedon germanium have not been resolved. As such,this study employs temperature-programmed desorption(TPD), electron-stimulated desorption (ESD), high-resolution electron energy-loss spectroscopy (HREELS)and low-energy electron diffraction (LEED) to investigatethe dynamics of hydrogen desorption from digermane.Ge2H6/ and disilane.Si2H6/ adsorbed on Ge(100).

EXPERIMENTAL

Two ultrahigh vacuum (UHV) stainless-steel chamberswere used in this study. The first chamber is equippedto perform AES, TPD and ESD, and so the TPD andESD experiments were performed in this chamber and theHREELS and LEED results were acquired in the secondchamber. A base pressure of 2ð 10�10 Torr was main-tained in both UHV chambers for all experiments reportedin this work.

Intrinsic germanium wafers (n-type) were obtained fromEagle-Picher Industries at a stated purity of 99.9999%.The wafers are cleaved into rectangular samples of¾26ð13 mm. No chemicalex situ cleaning is employed priorto mounting in the sample holder, other than a methanolrinse. The sample is cleanedin situ by several cycles ofArC sputtering and thermal anneals. Surface cleanlinessis monitored with AES, which shows that carbon andoxygen contamination is below the detection limits of ourequipment.

Owing to the brittle nature and relatively low meltingpoint of germanium, adequately mounting the sampleto prevent fracture or melting during anneals is not a

Copyright 2000 John Wiley & Sons, Ltd. Received 15 June 1999Revised 16 August 1999; Accepted 17 August 1999

ADSORPTION OF DIGERMANE AND DISILANE ON Ge(100) 195

trivial consideration. To overcome these problems, a novelmounting scheme was developed whereby the germaniumsample is mounted atop two rectangular pieces of siliconwith a strip of tantalum foil sandwiched in-between.Heating the silicon (which has a higher melting point thangermanium) is accomplished via contact with the tantalumstrip, through which current is passed. The germanium isthen heated through thermal contact with the silicon.

Linear temperature ramps of 3 K s�1 were generated bya Eurotherm temperature controller for all TPDs reportedin this work. A Sorensen power supply was used to passcurrent through the tantalum strip. Sample temperaturewas monitored using two devices. The first consists ofa type K thermocouple attached to the silicon using aceramic adhesive. There is, however, a temperature gra-dient between the silicon and the germanium sample dueto the mounting scheme. The actual sample temperatureduring anneals was obtained from a Raytek infrared opti-cal pyrometer whose readings are taken through a UHVchamber infrared port. The thermocouple temperature ofthe silicon, which is fed directly to data collection soft-ware, can then be calibrated to the true temperature of thegermanium sample obtained from the pyrometer.

Digermane and disilane were dosed via a molecularbeam doser located¾1 cm away from the germaniumsubstrate. Sample temperature during dosing was held to120 K for all experiments, and was achieved by passingliquid nitrogen through a copper reservoir in thermalcontact with the sample holder.

All exposures reported in this study are given in termsof a pressure–time product (ptp) equal to

(Uncorrected ion gauge pressure during dose [Torr])

ð (Duration of dose [s])ð 106

owing to the significant dose amplification inherent ina tubular doser, the ptp is considerably greater thanan exposure in terms of Langmuirs, and is used as aconvenient measure of exposure to ensure reproducibility.

All ESD spectra were acquired at a beam energy of150 eV supplied by a Kimball electron gun at an incidentangle of 60° with respect to the surface normal. Kineticenergy distribution (KED) data of electronically desorbedions were obtained with a Bessel box energy analyser13

mounted between the sample and a UTI quadrupole massfilter. During all ESD experiments, the sample temperaturewas 120 K, which is the adsorption temperature.

EXPERIMENTAL RESULTS

Temperature-programmed desorption experiments

Digermane/Ge(100).The TPD spectra for successively dou-bled exposures of digermane to Ge(100) are shown inFig. 1. At the lowest exposure, a single peak is evident at630 K. As the exposure increases, a second peak devel-ops at 565 K and begins to dominate, while the original630 K peak shifts slightly downward in temperature to615 K. No further peak shifts are evident as a functionof exposure. It is evident from Fig. 1 that saturation hasessentially occurred at an exposure of 0.14 ptp.

Mahajanet al.1 and Cohenet al.2 have independentlyseen a single TPD peak at 570 K after exposure of Ge(100)

Figure 1. Hydrogen TPD spectra from digermane on Ge(100).The exposure is doubled for each successive trace. Dosingtemperature was 120 K.

to atomic hydrogen. Both groups have assigned this peakto a monohydride state. However, following exposure ofGe(100) to both diethylgermane and diethylsilane, Maha-jan saw two H2 TPD peaks arising from both adsorbatesat 570 and 616 K, where the latter peak is dominant forboth systems. Yanovskiiet al.3 have seen two TPD peaksqualitatively similar to those in Fig. 1 at 570 and 610 Kfollowing exposure of Ge(100) to hydrogen. Yanovskiiassigns the 570 K peak to the dihydride state, in contrastto both Mahajan and Cohen, and assigns the 610 K peakto the monohydride. Cho4 has also seen two TPD peaksat 572 and 610 K arising from digermane adsorbed ongermanium epitaxially deposited on Si(100).

Disilane/Ge(100).Hydrogen TPD spectra for successivelydoubled exposures of Ge(100) to disilane are shown inFig. 2. It should be noted that all disilane exposuresquoted in this work are cumulative. After a specific doseand TPD acquisition, the subsequent dose occurred onthe previously exposed surface. As discussed below, itis known that Si disappears from the Ge(100) surfaceafter heating. This diffusion of Si results in a Ge surfacethat retains the.2ð 1/ reconstruction free of silicon. Atthe lowest exposure, a single peak develops at 630 K.With increasing exposure, this peak shifts slightly downto 615 K while a high-temperature shoulder develops at680 K. In contrast to the digermane/Ge(100) system, thesingle peak produced at the lowest exposure dominatesat all exposures, while the high-temperature shoulderbroadens with increasing exposure. Saturation for thissystem is observed at an exposure of¾0.28 ptp. Thesehydrogen adsorption states are designated˛ and ˇ1, asshown in Fig. 2.

The qualitative behavior of these peaks is consistentwith prior work done independently by several workers.5 – 8

Hydrogen TPD spectra obtained by Boishin and Surnev5

from thin epitaxial layers of germanium on Si(100) show aclearly defined progression in their behavior as the germa-nium concentration increases on the Si(100) surface. Atthe lowest germanium concentration of�Ge D 0.4 ML,Boishin and Surnev’s hydrogen TPDs show a peak at670 K followed by a dominant peak at 790 K. Thisbehavior parallels that of hydrogen desorbing from dis-ilane on Si(100),11 where a dihydride peak develops at

Copyright 2000 John Wiley & Sons, Ltd. Surf. Interface Anal. 29, 194–200 (2000)

196 S. ATECAET AL.

Figure 2. Hydrogen TPD spectra from disilane on Ge(100).The exposure is doubled for each successive trace. Dosingtemperature was 120 K.

670 K, followed by a dominant monohydridepeak at800K. With increasinggermaniumconcentration,BoishinandSurnev’sTPD peaksshift to lower temperaturesandreversein intensity. At the highestgermaniumconcen-tration of �Ge D 2.9 ML, their dominantTPD peakis at600K, followed by a high-temperatureshoulderat 670K.This lastbehaviorfollows closelythatshownfor thehigh-est exposuresof disilane on Ge(100) shown in Fig. 2.Ning andCrowell,6 RussellandEkerdt7 andWu andNix8

have also obtainedhydrogenor deuteriumTPDs fromGe/Si(100)epitaxialsystems.Again, our resultsshowninFig. 2 arein closeagreementbothqualitativelyandquan-titatively with hydrogenTPDsreportedby theseworkersat highergermaniumconcentrations.

Electron-stimulated desorption experiments

Digermane/Ge(100).The ESD HC kinetic energy spectrafor digermaneadsorbedon Ge(100)areshownin Fig. 3.In thesespectra,the developmentof desorptionfrom twosurfacestatesis seenclearly. At the lowest exposure,the KED peakis almostsymmetricallycenteredat 4 eV,indicating the predominanceof a single surface state.As exposureincreases,the KEDs developa low-energyfeatureat 2.5 eV, indicative of a secondsurfacestate,which then dominateswith increasingexposure.TheseKED spectracorrelatewell with our TPD data for thedigermane/Ge(100)systemshownin Fig. 1.

All KED spectrawerefitted with two peaksusing theleast-squaresmethod.The theoreticalbasisfor the fit isbasedon a model originally proposedby Nishijima andPropstusingone-dimensionalpotentialenergy surfaces.14

The KED equationresultingfrom this model is given by

N.E/ D(c1

E

)exp

[�c2

(lnE

E0

)2]

.1/

whereN.E/ is the numberof ions desorbingwith kineticenergy E, and c1, c2 and E0 are adjustableparameters.Figure 4 shows three KEDs taken from Fig. 3, fittedaccordingto this model.

Electron-stimulateddesorptiondecaycurvesfor threee-beam emissioncurrentswere obtainedfor the diger-mane/Ge(100)system.A typical decaycurve is shown

Figure 3. The HC ESD kinetic energy distributions from diger-mane on Ge(100). All spectra were obtained with 150 eVelectrons and a current density of 25 µA cm�2.

Figure 4. Three HC ESD kinetic energy distributions from diger-mane on Ge(100) taken from Fig. 3 and fitted using Eqn (1). Therapid evolution of the low-energy hydride state with increasingdigermane exposure is evident in these plots.

in Fig. 5. Becausethe spatialprofile of the electronbeamis Gaussian,the signaldecaycannotbe representedby asimple exponentialfunction. The signal decayresultingfrom surfacedesorptioninducedby a Gaussianelectronbeamprofile is thusgiven by

I.t/ D Ib C ˛[

1� exp.�ˇt/ˇt

].2/

where I.t/ is the ion current at time t, ˇ is the decayconstant,Ib is the backgroundsignal and ˛ is a fitted

Surf. InterfaceAnal. 29, 194–200 (2000) Copyright 2000JohnWiley & Sons,Ltd.

ADSORPTION OF DIGERMANE AND DISILANE ON Ge(100) 197

Figure 5. The HC ESD decay curve from digermane on Ge(100)(upper trace) fitted with the sum of two functions of the formof Eqn (2) (lower traces). Inset shows eˇ vs. maximum currentdensity for three decay curves, where ˇ is the decay constantobtained from three decay curve fits. The total cross-sectionshown in the inset corresponds to the lower curve labeled ‘1’.

amplitudeparameter.15 Total cross-sectionsfor hydrogenremoval are obtainedby noting that the decayconstantˇ is a linear function of the emissioncurrentdensityJaccordingto the relation

ˇ D JQ

e.3/

wheree is theelectronicchargeandQ is thecross-section.Plotting eˇ versusJ for severaldecaycurvesthenyieldsa line of slopeQ.

In the presentcase,it wasfound that HC decaycurveswerebestfitted as the sumof two equationsof the formof Eqn (2). Decayconstantswereextractedfrom the fit-ted decaycurves, from which cross-sectionswere thenobtained.The e-beamcurrentdensitywasobtainedfroma Faradaycup with a known entranceaperture.The insetto Fig. 5 showsa total cross-sectionfor hydrogenremovalof Q1 D 4.6 š 0.6 ð 10�16 cm2, correspondingto thelower fit labeled‘1’ in Fig. 5. For the upperfit labeled‘2’ in Fig. 5, the total cross-sectioncalculation givesQ2 D 1.7š0.2ð10�17 cm2. In acquiringthedecaycurves,the passenergy of the ion energy analyzerwassetat thepeakmaximum of the kinetic energy distribution. Fromthecurvefits in Fig. 4, it is apparentthatthelargestcontri-bution to the total hydrogensignalresultsfrom the lowerenergy component,which implies that this componentisassociatedwith thestatethatdesorbsat 565K in theTPDspectraof Fig. 1, andthus the total cross-sectionfor thisstateis assumedto beQ2. Accordingly,Q1 thenpertainsto the statethat desorbsat 615 K.

Disilane/Ge(100).TheESDHC kinetic energy distributionsfor disilane adsorbedon Ge(100)are shown in Fig. 6,wherespectracontaina peakat 3 eV with a shoulderat¾5.5 eV. Figure7 showsthreespectratakenfrom Fig. 6andfittedusingEqn(1). In contrastto thefittedKED spec-tra in Fig. 4 for the digermane/Ge(100)system,the low-energy peakseenin Fig. 7 dominatesatall exposures.TheESDdecaycurvesfor four e-beamemissioncurrentswereobtainedfor the disilane/Ge(100)system,where it was

Figure 6. The HC ESD kinetic energy distributions from disilaneon Ge(100). All spectra were obtained with 150 eV electrons anda current density of 25 µA cm�2.

Figure 7. Three HC ESD kinetic energy distributions from disi-lane on Ge(100) taken from Fig. 6 and fitted using Eqn (1).

alsofoundthat two equationsof the form of Eqn(2) werenecessaryto fit the data.A typical decaycurve is shownin Fig. 8, wheretheinsetshowsthatthetotal cross-sectionfor hydrogenremovalis Q˛ D 4.9š 0.6ð 10�17 cm2 forthefit labeled‘˛’ in Fig. 8. For thefit labeled‘ˇ’ in Fig. 8,Qˇ D 9.4š 0.9ð 10�17 cm2. Applying the argumentout-lined abovein referenceto cross-sectionassignmentsfor

Copyright 2000JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 29, 194–200 (2000)

198 S. ATECAET AL.

Figure 8. The HC ESD decay curve from disilane on Ge(100)(upper trace) fitted with the sum of two functions of the formof Eqn (2) (lower traces). Inset shows eˇ vs. maximum currentdensity for three decay curves, where ˇ is the decay constantobtained from four decay curve fits. The total cross-sectionshown in the inset corresponds to the upper curve labeled ‘˛’.

thedigermane/Ge(100)system,we concludefrom inspec-tion of Figs 7 and 8 thatQ˛ is associatedwith the stateproducingthe low-temperaturepeak in the TPD spectraof Fig. 2, and that Qˇ pertainsto the stateproducingthehigh-temperatureshoulder.

High-r esolution electron energy-lossspectroscopyexperiments

TheHREELSspectrafor digermaneadsorbedon Ge(100)areshownin Fig. 9. Spectrum1 was takenfrom a cleanGe(100)surface.Spectrum2, which showstwo prominentlossesat 850and2100cm�1, wastakenfrom theGe(100)surfacesaturatedwith 0.2ptp of digermaneat 120K. TheTPDdataacquiredat thisexposurerevealthatasignificantlayer of physisorbeddigermaneis presenton the surface.Spectrum3 wastakenafterannealingthedosedsurfaceto300 K —a temperaturewell below the onsetof the low-temperaturepeak shown in Fig. 1—and showsthat thelossesat850and2100cm�1 haveappreciablydecreasedinintensity.Spectrum4 wastakenafterannealingthedosedsurfaceto 600 K —midway in temperaturebetweenthelow- and high-temperaturepeaksshown in Fig. 1—andshows that the loss at 850 cm�1 has vanishedbut the2100cm�1 losshasremainedessentiallyunchanged.Thestrong loss peaks in spectrum2 in the region around800 and 2100 cm�1 are predominantlydue to Ge–HdeformationandGe–H stretchvibration, respectively,ofthephysisorbeddigermanemolecule.16,17 After heatingthesubstrateto 300 K, physisorbedspeciesare no longerpresenton the surface.Consequently,the loss peak at850 cm�1 in spectrum3 is dueto adsorbedhydrogenandis assignedto the GeH2 scissorsmode associatedwiththe presenceof the dihydride. The small loss featureinspectrum1 at ¾1000–1100 cm�1 is likely to be due totraceoxygencontamination.

Low-energy electrondiffraction (LEED) patternstakenin conjunctionwith HREELSexperimentsshowa typical2ð 1 patterncharacteristicof dimerizationon the cleanGe(100)surface.After asaturationexposureof 0.36ptpofdigermanefollowedby annealingto 300K, LEED patterns

Figure 9. The HREELS spectra of digermane on Ge(100). Priorto data acquisition for curves 2 4, the sample was cooled to120 K then exposed to 0.2 ptp of digermane. The full width athalf-maximum (FWHM) for the elastic peak shown is 160 cm�1.

of theGe(100)surfaceshoweda predominant1ð1 recon-struction,indicatingthepresenceof germaniumdihydride.

Papagnoetal.9 appliedHREELSto hydrogenadsorbedon Ge(100)and saw loss peaksat 840 and 2096 cm�1

after 2000L of exposureto hydrogen.Thesepeakscoin-cide quite closelywith thoseof Fig. 9. Papagnoassignedthe 840 cm�1 losspeakto the germaniumdihydridescis-sors mode and the 2096 cm�1 loss peak to the Ge–Hstretch.This is in contrastto Chabal,10 who wasunabletodetectadihydridescissorsmodeonGe(100)atanyhydro-gen exposureusing surfaceinfrared spectroscopy(SIS).Chabaldid, however,suggestthat if a germaniumdihy-dridehadbeendetectedin his investigations,it wouldhaveshownasa losspeakwithin the 750–1000cm�1 range.

DISCUSSION

OurTPD,ESD,HREELSandLEED datasuggestthattwohydrogenadsorptionstates—a monohydrideand a dihy-dride—existon Ge(100)exposedto digermane.Basedonthis evidence,analysisof TPD datain Fig. 1 showsthatatthe lowest exposuredigermaneadsorbsdissociativelytoform a germaniummonohydride,which desorbsat 630K.As exposureincreases,thehigh-temperaturemonohydridepeakshifts slightly downwardto lower temperaturesanddesorbsat615K while asecondpeak,presumablytheger-maniumdihydride,evolvesto desorbat 565K. In contrastto hydrogendesorbingfrom Si(100),11 thedihydridepeakdominateswith increasingexposure.

With regardto Ge(100)exposedto disilane,the mech-anisms responsiblefor hydride formation and prefer-ential desorptionsites on Ge/Si epitaxial systemsarepresentlyopento debate.Ning andCrowell6 suggestthatgermaniumon the Si(100) surfaceproduceslong-rangeelectronicinteractionswith silicon atoms,resulting in alower activationenergy for silicon mono- and dihydridedesorption,and hence lower TPD desorption maximacomparedwith the unalloyedSi(100) surface.Moreover,Ning and Crowell proposethat during the TPD process,hydrogenmigratesfrom germaniumsitesto the exposedsilicon substrateand recombinativehydrogendesorptionthenproceedsfrom siliconsites.RussellandEkerdt,7 how-ever, show insteadthat the most probablepathway for

Surf. InterfaceAnal. 29, 194–200 (2000) Copyright 2000JohnWiley & Sons,Ltd.

ADSORPTION OF DIGERMANE AND DISILANE ON Ge(100) 199

hydrogen desorption from Ge/Si alloys is via a short-lived GeH intermediate. This species forms as hydrogenmigrates from silicon to germanium sites due to an activa-tion barrier that is somewhat lower for Si to Ge migrationthan is the case for direct silicon monohydride desorption(25 vs. 32 kcal mol�1). Evidence provided below tends tosupport the latter model.

Particularly relevant to the interpretation of our resultsare three relatively recent studies of silicon18 and/or disi-lane12,19 on Ge(100). Hoevenet al.19 and Linet al.18 haveobserved significant diffusion of silicon into the Ge(100)surface at a temperature as low as 50°C, with Ge segrega-tion to the surface. Indeed, after deposition of three atomiclayers of Si on Ge(100) at 450°C the top layer continued toexhibit a.2ð1/ reconstruction consisting only of Ge atoms.Tsu et al.12 report that exposure of Ge(100) to disilane atroom temperature results in dissociative adsorption, the pri-mary species being SiH2 and GeH. Some undissociatedSiH3 was also suggested. Upon annealing, they reportedincreased surface ordering and occurrence of Si island-ing prior to hydrogen desorption. Subsequent to hydrogendesorption, spectral characterization of the surface wasidentical to the initial Ge substrate. The latter observationsuggests Si diffusion into the Ge surface, as observed pre-viously. The desorption scenario envisaged by Tsuet al.12

and relevant to our studies can be described in the follow-ing manner. As the Ge substrate temperature is increased,SiH2 species decompose according to the reaction

H—Si—HC H—Si—H���! H—Si Si—HC H2

in which two dihydride units react to form a dimer. Ata temperature near 600 K, hydrogen is mobile on thesurface and diffuses to Ge sites prior to desorption. Asthe temperature is raised further, hydrogen evolves fromthe SiH species.

This scenario proposed by Tsuet al.12 is in generalagreement with our TPD spectra shown in Fig. 2. At thelowest exposures a single peak appears, corresponding toevolution of hydrogen from germanium dangling bondsites resulting from migration of hydrogen from isolatedsilicon species. As exposure increases, theˇ1-state evolvesas the silicon islands grow in size and desorption occursfrom Si–H species. The-GeH state ultimately saturatesas the Ge sites become increasingly occupied.

Within the context of this adsorption model, it is likelythat the two states yielding HC in ESD kinetic energydistributions (Fig. 7) are Ge–H and Si–H2. Because theelectron desorption experiments were carried out at theadsorption temperature it is unlikely, based on Tsu’smodel, that a significant concentration of Si–H specieswere present on the surface prior to heating. Thus, weidentify the two cross-sections measured asQ˛ andQˇ todenote that the higher energy state is associated with theˇ-states of hydrogen.

Tsu et al.12 have reported a maximum coverage obtain-able for silicon on Ge(100) as 0.5 ML. Using AES, we

Figure 10. Relative surface concentrations of silicon and germa-nium as a function of the Ge(100) surface to disilane exposure.As the disilane exposure increases beyond the range shownin the plot, the relative concentrations remain approximatelyconstant at 0.4 and 0.6 for silicon and germanium, respectively.

havecalculatedthe relativeconcentrationsof silicon andgermaniumon Ge(100)asa functionof disilaneexposure.This resultis shownin Fig. 10.At saturationexposure,themaximumrelativeconcentrationof silicon on Ge(100)isseento be¾0.4. This result is in generalagreementwithTsu’s work.

CONCLUSIONS

Dataobtainedfrom TPD, ESD,HREELSandLEED sug-gesttheexistenceof germaniummonohydrideandgerma-nium dihydride on Ge(100)exposedto digermane.Totalcross-sectionsfor hydrogenremovalfor both stateshavebeenfoundto beQ1 D 4.6š0.6ð10�16 cm2 for thehigh-temperaturepeakin Fig. 1 andQ2 D 1.7š0.2ð10�17 cm2

for thelow-temperaturepeak.If thehigh-temperaturepeakin Fig. 1 is assignedto the germaniummonohydride,asour data suggest,then this state possessesa relativelylarge ESD cross-sectionon the digermane/Ge(100)sys-tem. The TPD data further suggestthe existenceof agermaniummonohydride(˛-state) and a silicon mono-hydride (ˇ1-state)on Ge(100)exposedto disilane.Totalcross-sectionsfor hydrogenremovalfor thesestateshavebeencalculatedasQ˛ D 4.9š 0.6ð 10�17 cm2 andQˇ D9.4š 0.9ð 10�17 cm2. Hydrogendepopulationvia elec-tronicexcitationis thuscomparativelylessefficient for thegermaniummonohydrideon the disilane/Ge(100)system.

Acknowledgement

This work wassupportedin partby theScienceandTechnologyCenterProgramof the NationalScienceFoundation,GrantNo. CHE8920120.

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Surf. Interface Anal. 29, 194–200 (2000) Copyright 2000 John Wiley & Sons, Ltd.