Initial adsorption of trimethylsilane on Ge(100) surfaces

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 113È120 (1998)

Initial Adsorption of Trimethylsilane on Ge(100)Surfaces

Y. Qi,1 J. L. Sulak,1 W. G. Durrer,1 J. H. Craig, Jr1,2 and P. W. Wang1,2,*1 Department of Physics, The University of Texas at El Paso, El Paso, TX 79968, USA2 Materials Research Institute, The University of Texas at El Paso, El Paso, TX 79968, USA

Trimethylsilane (TMSiH) was adsorbed onto a Ge(100) surface at a temperature of Ô150 » 3 ÄC and x-rayphotoelectron spectroscopy (XPS) was used to study the resulting surface species as functions of the TMSiHexposure in Langmuir (L). The core-level C 1s, Si 2p and Ge 3d photoelectrons were monitored after each dosing.It was observed that the C–C bonds are the dominant species formed at the low doses of TMSiH. The secondabundant species at the low coverage is the C–Ge bond. This indicated dissociative adsorption of TMSiH moleculesonto a clean Ge(100) surface, which is similar to the adsorption of TMSiH molecules onto an Si(100) surface. Asthe dose increases, the Si–C species gradually increase due to physisorbed TMSiH on top of the C–C- and C–Ge-covered surface. This study clearly reveals the growth processes of TMSiH on a Ge(100) surface. The electronega-tivities of C, Si and Ge and the bond strengths of C–C, C–Si and C–Ge were invoked to discuss the initialformation of C–C and C–Ge on the Ge(100) surface. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. 26, 113È120 (1998)

KEYWORDS: XPS; x-ray photoelectron spectroscopy ; Ge ; adsorption ; trimethylsilane

INTRODUCTION

Low-temperature chemical vapor deposition studieshave been initiated in our laboratory to explore thee†ects of irradiation with low-energy electrons, x-rayphotons and low-energy ions on methylated silanesadsorbed on Si(100) and Ge(100) surfaces. Adsorptionstudies of methylsilane on metal surfaces have demon-strated that this species serves as a precursor to siliconcarbide deposition.1 Our previous studies oftemperature-programmed desorption (TPD) andelectron-stimulated desorption (ESD) on the TMSiH/Si(100) system at 100 K have indicated2,3 that tri-methylsilane (TMSiH) spontaneously dissociates at lowcoverage, yielding a monohydride-terminated siliconsurface accompanied by adsorption of CÈH complexes.At high exposure, a loosely bound physisorbed stateappears, which is generally characteristic of a molecu-larly adsorbed species. Electron beam irradiationappears to enhance dissociation and deposition ofcarbon-containing species and therefore is a possibletool to remove unwanted ligands from the precursor onthe surface. Previous investigation of electron beame†ects on the same system revealed that dehydroge-nation in and bonds was induced andCÈH

nSiÈCÈH

mnew SiÈC and CÈSi bonds were formed by electron irra-diation on a TMSiH-covered Si(100) surface.4

Because TMSiH molecules and Si substrate bothcontain Si species, it is very difficult to study the originof surface Si atoms in the TMSiH/Si system and the

* Correspondence to : P. W. Wang, Department of Physics, TheUniversity of Texas at El Paso, El Paso, TX 79968, USA. E-mail :pwang=utep.edu.

interaction between substrate Si atoms and adsorbed Siatoms from the precursor. This problem would not arisefor a Ge(100) substrate. However, in order to investigatethe e†ect of irradiation of energetic particles on theTMSiH/Ge(100) system, a dose dependence studyserving as a control experiment is necessary. Therefore,a Ge(100) substrate is used in this study and the dose-dependent growth of TMSiH on Ge(100) surfaces isreported.

EXPERIMENTAL

Polished n-type single-crystal germanium(100) waferswith 0.005È0.04 ) É cm resistivity were purchased fromEagle-Picher Industries. Wafers were cut into D10 ] 10mm sample sizes, cleaned by acetone, methanol anddeionized water in an ultrasonic cleaner, blow-driedwith nitrogen gas and Ðnally mounted onto a hot/coldsample holder in a vacuum chamber that was connectedto an ultrahigh vacuum (UHV) chamber through aloadlock.

The base pressure of the UHV chamber duringexperiments was maintained at 1D 3 ] 10~9 Torr. Thesurface analysis system used was a Perkin-Elmer 560equipped with Auger electron spectroscopy (AES), sec-ondary ion mass spectroscopy (SIMS) and x-ray photo-electron spectroscopy (XPS). The XPS spectra of thecore-level photoelectrons of C, Si and Ge were collectedand investigated. Emission of photoelectrons wasinduced by Al Ka x-rays generated by 14 keV electronsimpacting on aluminum at the 225 W power level. Thepass energy of the double-pass cylindrical mirror energyanalyzer was 50 eV, which provides 1 eV resolution.The uncertainty is ^0.5% in relative concentration and^0.05 eV in binding energy (BE). All the uncertainties

CCC 0142È2421/98/020113È08 $17.50 Received 20 May 1997( 1998 John Wiley & Sons, Ltd. Accepted 23 September 1997

114 Y. QI ET AL .

Figure 1. The Auger spectra of Ge(100) surfaces : (a) before 2.8 keV Ar½ ion sputtering, where the surface contaminants C and O wereobserved; (b) after sputtering cleaning, where only Ge was observed. As well as the Ge LMM signal, all the other signals are Ge signals, too.No detectable C and O were found on the Ge(100) surface after sputtering.

Figure 2. The XPS spectrum of the Ge(100) surface after 2.8 keV Ar½ ion sputtering.

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 113È120 (1998) ( 1998 John Wiley & Sons, Ltd.

TRIMETHYLSILANE ADSORPTION ON GE(100) 115

Figure 3. The decomposed Ge 3d XPS spectrum after 17 L TMSiH dosing. The Ge 3d core electrons in Ge–C and Ge–Si bonds were clearlyresolved.

were obtained by measuring the variation in Si 2psignals 10 times from a high-purity silica sample underconstant experimental conditions. Information from theXPS spectra can be obtained by the use of a curve-Ðtting program in which the peak position is thebinding energy and the integrated area of the peak isrelated to the concentration of the element.

Figure 4. Relative concentrations of Ge–Ge, Ge–C and Ge–Sibonds as functions of the TMSiH exposure : Ge–Ge bonds grad-ually decrease ; Ge–Si bonds start forming after 3.5 L dosage andreach a saturation level after 7 L dosing; and the Ge–C bonds havean opposite trend to that of Ge–Si bonds.

Sample surface cleaning

In order to obtain a clean sample surface, the followingtreatments were carried out :

(1) The sample was mounted on a hot and coldprobe and placed into the transfer chamber wherethe pressure was kept at 2 D 3 ] 10~5 Torr. Thesample was then heated to a high temperature of450 ¡C, provided by a button heater, for half anhour in order to eliminate gross contaminants fromthe air.

(2) The sample was inserted into the UHV chamberand sputtered with a 2.8 keV argon ion beam toremove the native oxide layer. A 7 ] 7 area wassputtered to cover the area (5 mm in diameter) to beirradiated by x-rays. It was found with AES thatafter 30 min of sputtering no detectable oxygen orcarbon was observed in the sputtered region. Augerelectron spectra obtained by using 3 keV primaryelectrons and the pulse counting mode before andafter sputtering the sample are shown in Fig. 1.

(3) The sample was annealed at 450 ¡C for 1 h in theUHV chamber in order to reduce the roughnesscaused by the ion bombardment.

Trimethylsilane dosing and XPS analysis

After the above cleaning treatment, TMSiH was grad-ually dosed onto a cold ([150 ^ 3 ¡C) Ge(100) surface.

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 113È120 (1998)

116 Y. QI ET AL .

Figure 5. The decomposed C 1s XPS spectrum after 17 L TMSiH dosing, where three components of the C 1s electron signals, induced byC–C, C–Ge and C–Si bonds, were seen.

Figure 6. The evolution of the C 1s spectra after various TMSiH coverages on the Ge(100) surface, where the chemical shifts are clearlyobserved. The C–C bond is the dominant species at the low dose and the C–Si bond is the main species at high coverage of TMSiH.



Figure 7. Relative concentrations of C–C, C–Ge and C–Si bondsas functions of the TMSiH exposure. The initial dominant fractionof C–C bonds decreases as the TMSiH exposure increases. Therelative concentration of C–Ge bonds increases first and decreasesafter 3.5 L coverage. The C–Si species is continually accumulatedon the surface as the TMSiH exposure increases.

The TMSiH was dosed onto the Ge(100) surface byplacing the sample in front of the dosing tube. Thedosing pressure was 5 ] 10~8 Torr. In order to repro-duce the dosing condition, the surface exposure is givenin Langmuir (L) and the dosing pressureÈtime productis divided by 10~6. It is understood that because thedosing is accomplished through an oriÐce directly ontothe sample, rather than from the ambient air, the actualexposure is considerably higher than the exposure indi-cated in Langmuir.

In order to identify and quantify the elementsadsorbed onto the Ge(100) substrate and their chemicalstates, XPS spectra of C 1s, Si 2p and Ge 3d wererecorded immediately after each TMSiH dosing. Usefulinformation from the XPS spectra can be obtained byuse of a curve-Ðtting program in which the peak posi-tion is the binding energy and the integrated area of thepeak is related to the concentration of the element. Theconcentration of each chemical species was calculatedfrom the integrated area with the photoelectron yield ofthe element taken into account. All XPS peak energiesat the low dosage were charge referenced to Ge 3d,which is assumed to be 29.95 eV for pure germanium.5This procedure was necessitated by the absence of theconventional charge reference species, adventitiouscarbon (284.6 eV),6 on the clean Ge(100) surface. TheXPS spectra were decomposed into di†erent chemicalstates by using a GaussianÈLorentzian curve-Ðttingprogram and by using the electronegativities of the ele-ments involved,7 previous reports8,9 and our previousexperimental results.4

EXPERIMENTAL RESULTS

Figure 1 shows the sample surface before and after 30min of argon sputtering at 2.8 keV. As seen in Fig. 1(b),the only detectable surface element was germanium,with a predominant LMM Ge signal at 1147 eV. Thecorresponding XPS spectrum after sputtering andbefore dosing is shown in Fig. 2. No detectable C or Owas found on the Ge(100) surface in AES and XPS aftersputtering and annealing and before dosing. This

Figure 8. The decomposed Si 2p XPS spectrum after 25 L TMSiH dosing, where only two components of Si 2p electrons, induced fromSi–C and Si–Si bonds, were seen.


118 Y. QI ET AL .

Figure 9. The changes of the relative concentrations of both Si–Cand Si–Si bonds as functions of the TMSiH exposure, where Si–Cspecies continually increase and Si–Si decrease.

observation does not preclude the possibility of traceconcentrations of C and O below the detection limit ofD1012 atoms cm~2.

The decomposed Ge 3d XPS spectrum after a dose of17 L is shown in Fig. 3 and consists of three peakslocated at 31.95, 29.95 and 27.63 eV, respectively.According to the electronegativities of C, Ge and Si,7and the previous study by Sugahara et al.,8 these threepeaks can be attributed to Ge 3d electrons in GeÈC,GeÈGe and GeÈSi bonds. The detected relative concen-trations of GeÈC, GeÈGe and GeÈSi bonds as functionsof the exposure are plotted in Fig. 4. It is observed thatthe GeÈGe bond concentration decreases. At D3.5 L, apeak associated with the formation of GeÈSi bondsappears and reaches saturation at 7 L. The peak identi-Ðed with GeÈSi bonds shows the opposite trend withexposure.

Correspondingly after 17 L of dosing the three com-ponents in the C 1s spectrum were resolved from thedecomposition as shown in Fig. 5, located at 284.6,283.54 and 282.5 eV, respectively, and indicating theexistence of CÈC, CÈGe and CÈSi bonds.8 Because noCÈC, CÈGe, GeÈC or GeÈSi bonds exist in the parentTMSiH molecules, the dissociative adsorption ofTMSiH onto Ge(100) is clear.

The evolution of the C 1s spectrum as a function ofthe TMSiH exposure is shown in Fig. 6, where thegrowth of the C 1s signal as TMSiH exposure increasesis clearly seen. The peak positions continuously shift tothe lower energies because the chemical bondingenvironment around carbon changes as the coverageincreases. The relative concentrations of the CÈC, CÈGeand CÈSi bonds as functions of TMSiH exposure areplotted in Fig. 7, where it is clearly shown that the CÈCbonds are dominant at the low dose but that the CÈSibond becomes the main species at high TMSiH expo-sure.

Only two components were resolved after decomposi-tion of the Si 2p signal after 25 L of TMSiH dosing(shown in Fig. 8). These two decomposed components,SiÈC and SiÈSi bonds, were consistent with our pre-vious work.4,10 The high BE peak corresponds to SiÈCbonds and the low BE peak is either due to SiÈSi bonds

or SiÈGe bonds because the BEs of Si 2p electrons arethe same in both SiÈSi and SiÈGe bonds, as reported byArghavani et al.9 The relative concentrations of Sispecies in the whole Si 2p signal are shown in Fig. 9,where SiÈC bonds increase and Si bonded to Si or Geatoms decreases as the TMSiH exposure increases. Eventhough the SiÈGe bonds cannot be separated from Si 2pspectra, it is expected that the concentration of theSiÈGe bond should vary as a function of the exposure.

DISCUSSION

The decompositions of Ge 3d and C 1s signals intoGeÈC, GeÈGe, GeÈSi, CÈC, CÈGe and CÈSi bonds arebased on the previous report by Sugahara et al.8 andthe electronegativities of C, Si and Ge. Even though theelectronegativities of Si and Ge were believed to be thesame (1.8) in the past,11 many new results indicate thatGe indeed has a higher electronegativity than Si.7 Eventhough the reported electronegativity values of Gevaried from 2.02 to 2.31, the order of electronegativityamong C, Si and Ge is C[ Ge[ Si. The paperpublished by Sugahara et al.8 showed that there isindeed a lower BE peak in their Ge 3d spectrum, whichwe attribute to GeÈSi bonds. There is still the questionof why there are no SiÈGe bonds in our Si 2p spectra ifthere is a di†erence in electronegativity between Si andGe. Our observation of no SiÈGe bonds in the Si 2psignal is consistent with the results reported by Argha-vani et al.8 One possible reason to explain our observ-ation is that the 3d electrons are located farther awayfrom the nucleus and therefore are more sensitive to thebonding environment than 2p electrons. Hence, GeÈSibonds were resolved in Ge 3d spectra and no SiÈGebonds are seen in Si 2p spectra.

The binding energies of CÈC, CÈGe and CÈSi bondsare equally separated (1.0 eV), as shown in Fig. 5, whichimplies equally separated electronegativities among C,Ge and Si atoms. Therefore, the BE di†erence betweenGeÈSi and GeÈGe bonds is expected to be very close tothe BE di†erence between GeÈC and GeÈGe bonds. It isindeed the case that the BE di†erence between GeÈSiand GeÈGe bonds is 2.3 eV and the BE di†erencebetween GeÈC and GeÈGe bonds is 1.9 eV, as shown inFig. 3. As mentioned above, 3d electrons are more sensi-tive to the environment than 1s electrons, so the chemi-cal shifts of Ge 3d electrons are larger than those of C1s electrons. Therefore, the BE positions of the GeÈCand GeÈSi peaks are reasonable.

Figures 3 and 5 both show the presence of GeÈC andGeÈSi bonds in the Ge 3d spectrum and CÈGe bonds inthe C 1s signal, which are non-existent in TMSiH mol-ecules. The observation of these bonds on the Ge(100)surfaces demonstrates that TMSiH molecules dissociateupon adsorption, probably into methyl and rad-CH

xicals. Interaction between species may give rise toCHxthe formation of the CÈC bonds. Similar dissociative

adsorption was observed not only in the TMSiH/Si(100)system2h4 but also on trimethylgallium-covered Si(100)surfaces.12,13 Because the number of dissociated methyland radicals is much higher than the number ofCH

xdissociated Si species, some methyl and radicalsCHxmay be responsible for the formation of the GeÈC



bonds at low dose. Hence, GeÈC bonds, and not GeÈSibonds, are detected when the dosage is less than 3.5 L.Once the dosage of the TMSiH exceeds 3.5 L, Si atomswith dangling bonds resulting from dissociative adsorp-tion start to form GeÈSi bonds, as shown in Fig. 4. Asmore TMSiH is dosed onto the surface, it is expectedthat the detected GeÈGe bond concentration shoulddecrease.

The chemical shift in the C 1s spectrum as a functionof TMSiH exposure shown in Fig. 6 indicates that theCÈC bonds are the dominant species formed on the Gesurface during dissociative adsorption at low exposure,followed by CÈGe bonds. However, as the dosagefurther increases, CÈSi bonds start growing, which mayresult from the physisorbed TMSiH species. Therefore,the relative concentration of CÈC bonds decreases dueto physisorbed TMSiH species continually adsorbingon the surface. The increase in CÈSi bonding and thedecrease in CÈC bonding are clearly seen in Fig. 7,where the relative concentrations of CÈC, CÈGe andCÈSi bonds in C 1s spectra are plotted vs. the TMSiHexposure. The growth pattern of CÈGe species on theGe surface shows an increase when the exposure isbelow 3.5 L and a decreasing trend when the dosing isabove 3.5 L. This observation implies that some disso-ciated methyl groups or radicals from the parentCH

xmolecules bond to surface Ge atoms, and CÈGe bondsincrease in relative numbers until the surface has beenexposed to 3.5 L of TMSiH. It should be noted that theconcentrations of CÈC, CÈSi and CÈGe bonds are rela-tive. The sum of these three relative concentrations isalways equal to unity. Because of the dramatic drop inCÈC concentration when the dosage is less than 3.5 L, arelative increase of CÈGe bonds was seen, whereas aconstant relative concentration of the GeÈC bond isobserved in Ge 3d spectra, as shown in Fig. 4. Never-theless, the behaviour of GeÈC and CÈGe bonds is cor-related under the same exposure conditions.

Because three methyl groups are bonded to onesilicon atom in a TMSiH molecule, carbon-containingspecies such as C, CH, and may be producedCH2 CH3by the dissociation processes, and will have higherprobabilities of bonding to Ge atoms, as discussedabove. However, observation showed that the CÈCbonds are the dominant species in the whole C 1s signalat low coverage, which means that carbon-containingspecies tend to form more CÈC bonds Ðrst instead offorming CÈGe and CÈSi bonds. It is expected thatcarbon-containing species would form CÈC bonds Ðrstrather than forming CÈGe and CÈSi bonds because theCÈC bond has a greater bond strength, i.e. 145 ^ 5 kcalmol~1, than either the CÈGe (110 ^ 5 kcal mol~1) orthe CÈSi bond (107.9 kcal mol~1).14

Further evidence of dissociative adsorption is thatboth CÈGe and GeÈSi bond concentrations increaseunder low exposure, as shown in Figs 4 and 7, eventhough the growth of the CÈGe concentration starts atmuch lower exposure than that of GeÈSi bonds. Again,this demonstrates that more carbon-containing speciesare available to form bonds to the surface Ge atoms.

That the number of CÈSi bonds increases as TMSiHexposure increases was reconÐrmed by the Si 2p datashown in Fig. 9. As discussed previously, TMSiH maydissociate into methyl groups or radicals duringCH

xadsorption, resulting in dangling bonds on the Si atoms.

These Si atoms with the dangling bonds can bond tosurface Ge atoms as well as to each other. As the molec-ularly physisorbed layer increased in thickness, or if itsincrease rate was much higher than the formation rateof SiÈSi and SiÈGe bonds, it would be expected that thesignals corresponding to SiÈSi and SiÈGe bonds woulddecrease on the surface because no such bonds existedin the parent molecules.

It should also be noted here that low-energy ion sput-tering of the Si surface, followed by annealing to800 ¡C15 or D1200 ¡C,16h18 can restore the crystallinityof Si. It is believed that the surface defects created by 2.8keV Ar ion sputtering of a Ge surface will not beannealed out by 450 ¡C treatment, even though nodetectable surface contaminant was observed in Figs 1and 2. Undoubtedly, surface defects, as well as danglingbonds on the germanium surface, play an importantrole in the surface chemistry of the TMSiH/Ge(100)system. For example, Avouris et al.19 reported that aclean Si(100) surface is reconstructed to a 2] 1 super-lattice. This reconstructed Si(100)È2 ] 1 surface shows6.8] 1014 highly reactive dangling bonds per squarecentimeter.20 Therefore, the SiÈC species, CÈH speciesand GeÈH species are expected to exist on a Ge(100)surface immediately after dosing with TMSiH.However, because the UHV chamber used for this studyis not equipped with a low-energy electron di†raction(LEED) capability, no surface structure investigationwas conducted.

CONCLUSION

On the basis of the evolution of C 1s, Si 2p and Ge 3dsignals as functions of the TMSiH coverage on aGe(100) surface, we have reached the following conclu-sions :

(1) Dissociative adsorption of TMSiH on the Ge(100)surface was observed, similar to adsorption pro-cesses in the TMSiH/Si(100) system.

(2) The initial dominant carbon species on the Ge(100)surface during adsorption is the CÈC bond. Thesecond most abundant species at low coverage is theCÈGe bond; the CÈSi bond is the main species afterhigh doses, which may result from physisorbed(non-dissociated) TMSiH molecules.

(3) Germanium bonds to silicon were resolved from theGe 3d spectra but no SiÈGe bonds were observedfrom Si 2p spectra. Further studies such as ultraviol-et photoelectron spectroscopy, Fourier transforminfrared spectroscopy or high-resolution electronenergy-loss spectroscopy are needed to identify theexistence of SiÈGe bonds in the TMSiH/Ge(100)system.

Acknowledgement

This work was supported by the Materials Research Center of Excel-lence at the University of Texas at El Paso under NSF contractHRD-9353547.


120 Y. QI ET AL .

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Initial adsorption of trimethylsilane on Ge(100) surfaces