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Mechanism of GeH4 dissociation on Si(1 1 1)-(7 × 7)
Surface Science 531 (2003) 265–271 www.elsevier.com/locate/susc
Mechanism of GeH4 dissociation on Si(1 1 1)-(7 7) J. Braun 1, H. Rauscher *, R.J. Behm Abteilung Oberfl€achenchemie und Katalyse, Universit€at Ulm, Albert-Einstein-Allee 47, 89069 Ulm, Germany Received 25 January 2003; accepted for publication 17 March 2003
Abstract Results of an STM study of dissociative GeH4 adsorption on Si(1 1 1)-(7 7) at 300 K show that GeH4 adsorbs under scission of two Ge–H bonds according to GeH4 (g) + 4db ! GeH2 (ad) + 2H(ad). GeH2 binds to two adatom dangling bonds in a bridged configuration, while the two released hydrogen atoms saturate two additional dangling bonds. The GeH4 sticking coefficient under these conditions is 1.2 106 , one order of magnitude smaller than for SiH4 . Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Hydrides; Chemisorption; Sticking; Scanning tunneling microscopy; X-ray photoelectron spectroscopy
1. Introduction GeH4 is an important precursor molecule for chemical vapor deposition (CVD) of Ge and, in conjunction with Si precursors, of thin Six Ge1x films as well. Under continuous growth conditions CVD reactions are complex processes since they involve many different parallel and sequential reactions [1,2]. However, at temperatures far below those used in CVD processes, which are usually carried out well above 700 K, many of the reactions that proceed subsequently to the primary dissociation step can be suppressed. In this case it is possible to gain information on the initial reactions that take place during and upon dissociative * Corresponding author. Tel.: +49-731-5025469; fax: +49731-502-5452. E-mail address: [email protected] (H. Rauscher). 1 Present address: Mattson Thermal Products GmbH, 89160 Dornstadt, Germany.
adsorption on the surface and to reveal, e.g., the mechanism or a site selectivity, as it had been observed, e.g., for silane adsorption [3]. Despite the importance of dissociative adsorption as an elementary process in the CVD-growth of thin layers, little is known about the initial adsorption steps of GeH4 on Si(1 1 1)-(7 7) at the atomic level. Wintterlin and Avouris [4] found that the sticking coefficient of GeH4 on Si(1 1 1)-(7 7) is very low at room temperature, without, however, quantifying it, and Van et al. [5] reported a very small amount of Ge on the surface after exposure to 104 L of GeH4 at room temperature, but the coverage was not determined quantitatively. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) results showed that formation of SiH/SiH2 species accompanies dissociative adsorption of GeH4 on the surface. However, no conclusion was reached on the formation of surface Ge hydrides [4,5]. In this work we analyze the initial mechanism of Ge deposition by GeH4 pyrolysis at the atomic
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level from atomic resolution scanning tunneling microscopy (STM). In particular, we study the formation of Ge hydrides and their adsorption sites and identify the primary adsorption step and the dissociation mechanism of the precursor.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system equipped with a homebuilt STM, XPS and facilities for annealing the sample [6]. The STM images were acquired at room temperature (300 K) in the constant current mode using tungsten tips. The samples were cut from p-doped Si(1 1 1) wafers with a resistivity between 1 and 20 X cm. They were cleaned using standard procedures [6]. GeH4 with a purity of 99.999% was obtained from Messer Griesheim. The tunnel voltage UT is the voltage applied at the sample with respect to the tip.
3. Results and discussion Exposure to GeH4 at 300 K leads to several new features on Si(1 1 1)-(7 7) in comparison with the initial surface. This is illustrated in Figs. 1 and 2, which show four STM images of the Si(1 1 1) surface after exposure to 7880 L (Fig. 1) and 23,630 L (Fig. 2) of GeH4 . Fig. 1(a) and (b) were taken at approximately the same positions, but at different tunnel voltages UT : UT ¼ þ1:0 V for Fig. 1(a) and UT ¼ þ2:0 V for Fig. 1(b). The same voltages were used for Fig. 2(a) and (b), respectively. In can be seen from Figs. 1(a) and 2(a) that with increasing GeH4 exposure the (7 7) reconstructed surface exhibits more and more adatom sites which appear darker in the constant current images as compared to the initial surface and the majority of the adatoms after exposure. These darkened adatoms are attributed to adsorption of hydrogen atoms or other dissociation products of GeH4 which bond to the dangling bonds. It is well known from other STM studies that adsorption of hydrogen on the (7 7) reconstructed surface first occurs on the adatoms and that it leads to a local decrease of the tunnel current and hence a darker
Fig. 1. STM images of a Si(1 1 1)-(7 7) surface after exposure to 7880 L GeH4 at 300 K and a GeH4 pressure of 3.5 105 mbar. Tunnel voltage UT : +1.0 V (a) and +2.0 V (b). Image sizes 35 nm 35 nm.
appearance of these adatoms in STM images [7]. Hydrogen can be provided by dissociative H2 adsorption [7], but also by dissociative adsorption of other molecules which contain hydrogen [2,3,8]. The darkening of the adatoms was attributed to a reduced local density of states (LDOS) around the Fermi level EF , by saturation of the Si adatom dangling bonds. A similar effect can hence be assumed in our case. More information on the adsorbed species can be gained from STM images recorded at different tunnel voltages UT . Increasing UT from +1.0 to
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Fig. 2. STM images of a Si(1 1 1)-(7 7) surface after exposure to 23,630 L GeH4 at 300 K and a GeH4 pressure of 3.5 105 mbar. Tunnel voltage UT : +1.0 V (a) and +2.0 V (b). Image sizes 35 nm 35 nm.
+2.0 V changes the contrast in the STM images. A comparison of STM images recorded at +1.0 V (Figs. 1(a) and 2(a)) with those recorded at the same position, but with UT ¼ þ2:0 V (Figs. 1(b) and 2(b)) shows that many of the adatoms that are dark at lower voltages become bright again. In fact, only a small difference in contrast as compared to the unaffected adatoms persists at UT ¼ þ2:0 V. The reason for this behavior is that at a tunnel voltage of +2.0 V the antibonding r orbital of the Si–H bond, which is centered around
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3.5 eV above the Fermi level [9], already contributes to the tunnel current, so that the contrast between reacted and non-reacted adatoms is decreased. The most prominent features in the STM images of Figs. 1(b) and 2(b), recorded at UT ¼ þ2:0 V, are small protrusions that are absent in the STM images at lower voltages (Figs. 1(a) and 2(a)). The adatoms at these positions appear also dark at low voltage. Such protrusions have not been observed after exposure to hydrogen or to silanes (SiH4 , Si2 H6 ) under similar experimental conditions, i.e., for low exposures at room temperature. They resemble, however, structures that were observed earlier after room temperature adsorption of Ge2 H6 on Si(1 1 1) [4]. We assign these protrusions, which are imaged with a size similar to that of adatoms at UT ¼ þ2:0 V, to adsorbed GeHx species. The characteristic contrast variation with changing tunnel voltage indicates that these GeHx species do not have dangling bonds. Rather, the dark appearance at low potentials and the brighter appearance at higher positive voltage points towards the presence of saturated bonds which become visible at higher voltages due to a higher density of antibonding states at these voltages. The protrusions are not located directly above adatom sites. Rather, they are exactly in the middle between two adatoms. This can also be seen in the inset in Fig. 1(b), where one of these fragments is imaged with high resolution. It indicates adsorption in a bridged position, and therefore we assign these fragments to GeH2 species bound to an adatom–restatom pair or to a pair of adatoms. Under these tunnel conditions the restatoms cannot be resolved with the STM, but it is known from other studies of reactions of silanes on Si(1 1 1)(7 7) [2,10,11] that the dangling bonds of the restatoms can also participate in such adsorption reactions. Before we analyze the adsorption process and the involved species in more detail we turn to the adsorption kinetics and a possible site selectivity of the adsorption process. Fig. 3(a) displays the fraction of reacted (darkened) Si adatoms as a function of increasing GeH4 exposure and Fig. 3(b) shows the fraction of reacted center adatoms vs. reacted corner adatoms. From Fig. 3(a) it can
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Fig. 3. (a) Fraction of reacted adatoms as a function of GeH4 exposure to Si(1 1 1)-(7 7) at 300 K. The inset shows the region for exposures of up to 1400 L in more detail. (b) Fraction of the reacted center adatoms vs. reacted corner adatoms during GeH4 exposure to Si(1 1 1)-(7 7) at 300 K.
be seen that saturation is not yet achieved even after exposure to 55 103 L. Furthermore, for large exposures the fraction of reacted adatoms increases linearly with the GeH4 exposure. Closer inspection of Fig. 3(a) with its inset for low exposures reveals that two different adsorption regimes can be distinguished. In the initial regime, for exposures up to 1350 L, the reactivity is larger by a factor of 3.3 as compared to the reactivity in the high-exposure regime (above 8000 L). In the first regime up to around 4% of the adatoms take part in the adsorption reaction. Two regimes with different sticking coefficients have also been found for room temperature adsorption of SiH4 on Si(1 1 1)-(7 7) [12]. In that system, the initial reaction channel dominates up
to Si deposition of about 2% of a monolayer (ML, 1 ML ¼ 7:78 1014 atoms/cm2 ). STM investigations showed that this reaction path was due to a strong site preference for dissociative SiH4 adsorption at corner hole/corner adatom pairs [3]. The amount of adsorbed Si (0.02 ML) corresponded exactly to the fraction of reacted adatoms (8–9%), provided that each SiH4 molecule splits off one H atom that saturates a corner adatom dangling bond while the remaining SiH3 group adsorbs at the dangling bond at the bottom of the corner hole. After saturation of these adsorption sites the sticking coefficient decreased by a factor of 10. In contrast, for the GeH4 /Si(1 1 1)-(7 7) system there is almost no selectivity between center and corner adatoms towards GeH4 dissociation. The ratio of reacted center vs. corner adatoms remains constant over the entire exposure regime, at a value of 1.16:1. This indicates that for GeH4 adsorption on Si(1 1 1)-(7 7) there is no such a fast initial reaction path as for SiH4 . However, a fast reaction may still proceed by preferential adsorption at point defects of the initial surface. The initial (7 7) reconstruction after preparation under our conditions has a typical defect density of 1–2% of the adatoms. Hence, for dissociative GeH4 adsorption at such a defect with breakage of 1–2 Ge–H bonds around 2–4% of the adatoms would be involved in such a reaction. This fraction corresponds very well to the fraction of adatoms involved in the fast adsorption reaction (4%), and therefore it appears probable that here the initial adsorption regime is defect-induced. More information on the adsorption process can be gained from STM images after longer exposures. As discussed above, the number of protrusions (observed at UT ¼ þ2:0 V) as well as the number of darkened adatoms (observed at UT ¼ þ1:0 V) both increase linearly for exposures of more than 1000 L, which indicates that GeH4 adsorption proceeds via a precursor. In the latter regime the ratio of the number of protrusions vs. the number of reacted adatoms remains constant: the number of protrusions is around 15–18% of the reacted adatoms, indicating that GeH4 adsorption proceeds under scission of two or more Ge–H bonds.
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Let us first assume that all protrusions imaged in the STM represent GeH2 groups, i.e., that GeH4 adsorption exclusively proceeds under scission of two Ge–H bonds. The protrusions are located at interstitial sites between the adatoms and hence one possible adsorption configuration for GeH2 is in a bridge position between an adatom–restatom pair. If this were true it would imply that three adatoms or less are saturated for each accommodated GeH4 molecule, since the two released H atoms adsorb preferentially at adatom dangling bonds [13,14]. Hence, in that case a ratio of 1:3 is expected for the number of protrusions imaged at +2.0 V relative to the reacted adatoms imaged at +1.0 V. But this scenario contradicts our experimental finding that the number of protrusions is only 15–18% of that of the reacted adatoms. Consequently, there must be another reaction channel, which leads to a larger proportion of darkened adatoms. Such a mechanism, which is in agreement with the experimental findings, involves the adsorption of GeH4 under twofold Ge–H bond scission and subsequent accommodation of GeH2 at an adatom–adatom pair. This would result in bridgebonded GeH2 , which saturates two adatom dangling bonds plus two H atoms which saturate two additional adatom dangling bonds. In total, this mechanism saturates four adatom dangling bonds per adsorbed GeH4 molecule. A configuration where GeH2 is bridge-adsorbed between two adatoms is indicated by the position of the protrusions in the STM images: they are located precisely halfway between two adatoms (see also the inset in Fig. 1(b)). If GeH2 is located at a bridge position above an adatom–adatom pair one expects to find darkened adatom pairs in the STM images obtained at UT ¼ þ1:0 V at those sites where protrusions are imaged at UT ¼ þ2:0 V. Closer inspection of Fig. 1(a) and (b) shows that this is indeed the case for most of the structures under discussion. Some examples are marked by dashed ellipses in Fig. 1(a) and (b). Sometimes more than two adatoms that are close together are darkened, indicating nearby adsorption of hydrogen atoms. If only a single Si adatom is darkened at UT ¼ þ1:0 V with the surrounding atoms unaffected there is never a pro-
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trusion imaged at UT ¼ þ2:0 V, which is a clear indication of hydrogen adsorption on that adatom. Very rarely we find a protrusion at UT ¼ þ2:0 V, but no clear darkening of the underlying adatom pair at UT ¼ þ1:0 V. In those cases the adatom structure also shows some irregularity such as an atom at an interstitial site. An example is also indicated by a dashed square in Fig. 1(a) and (b). The nature of the species that are imaged as protrusions in those exceptional cases could not be identified unambiguously. However, these protrusions appear slightly larger in size as compared to the majority and they may tentatively be assigned to GeH or Ge atoms. Further evidence that GeH2 is adsorbed at adatom–adatom pairs comes from Fig. 4(a), which shows an STM image recorded after exposure to 55,140 L GeH4 . This image displays a large number of the bright protrusions which are all located at bridge sites. Moreover, a number of them is adsorbed precisely at the border between two (7 7) unit cells. Several of these protrusions are marked by arrows in Fig. 4(a). These GeH2 groups are located directly above the dimers of the DAS reconstruction and hence for these GeH2 fragments bonding to a restatom dangling bond is impossible. This is further confirmation that the GeH2 species are adsorbed at adatom pairs. A model for the GeH2 adsorption configuration is shown in Fig. 5. To further support this mechanism and to establish a relation between the number of protrusions and the deposited amount of Ge we annealed the samples to 800 K after adsorption and STM characterization at room temperature. Such an annealing step decomposes all Si and Ge hydrides on the surface and removes hydrogen by H2 desorption [3,4]. The Ge atoms become mobile and aggregate in clusters or fill up defect sites of the substrate. Finally, islands are formed which exhibit (5 5) or (7 7) DAS (dimer adatom stacking-fault) structures. The coverage of these islands can be determined by measuring the island area or by counting the number of adatoms. This way, the amount of deposited Ge can be precisely determined by STM. Taking also into account the number of defects of the initial surface and after annealing, this results in a Ge coverage of 0.01
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Fig. 4. (a) STM image of a Si(1 1 1)-(7 7) surface after exposure to 55,140 L of GeH4 (p ¼ 3:5 105 mbar) at 300 K. Image size 35 nm 35 nm, UT ¼ þ2:0 V. Arrows indicate several GeH2 groups which are adsorbed at the borderline between neighboring (7 7) unit cells. (b) STM image of the same sample after an additional annealing step to 800 K for 2 min. Image size 25 nm 25 nm, UT ¼ þ1:0 V.
bilayers (BL) after exposure to 55,140 L of GeH4 (Fig. 4(b)). The Ge coverage determined in that way can now be related to the number of GeH2 species and reacted adatoms before annealing. Counting the number of protrusions before the annealing step (Fig. 4(a)) gives a coverage of 1.5 1013 GeH2 species/cm2 . Since each GeH2 contributes one Ge atom, it leads to a Ge coverage of 1.5 1013 Ge atoms/cm2 , which corresponds to
0.01 BL, the same as determined from the islands after annealing. This result can be cross-checked by relating the number of protrusions imaged at UT ¼ þ2:0 V to the number of reacted adatoms imaged at UT ¼ þ1:0 V. According to our model each dissociated GeH4 molecule should yield one GeH2 (protrusion at UT ¼ þ2:0 V) and four saturated adatoms (darkened at UT ¼ þ1:0 V), two from bridge-bonded GeH2 and two more from the hydrogen atoms. If the Ge coverage is 0.01 BL (1.5 1013 cm2 , as determined by STM after annealing), we should find around 6 1013 saturated adatoms per cm2 . Since at UT ¼ þ1:0 V defects cannot be distinguished from saturated adatoms, we must add the density of the initial surface defects of 0.75 1013 cm2 to the number of expected hydrogen-saturated adatoms. Doing this we arrive at a coverage of 6.75 1013 darkened adatoms per cm2 . For the STM image we determine 7.5 1013 darkened adatoms per cm2 , in very good agreement with predictions from the model above which includes GeH2 species bridge-bonded between two adatoms. From the model we can calculate the dissociative sticking coefficient for GeH4 adsorption on Si(1 1 1)-(7 7) at room temperature to 1.2 106 , which is around one order of magnitude smaller than the one for adsorption of SiH4 under the same experimental conditions [3]. Still, the ratio of darkened adatoms vs. bright spots as observed in the experiments is slightly larger than expected from the adsorption and dissociation model discussed above. This indicates that a small fraction of the GeH2 groups dissociates further even at room temperature, which shifts the ratio of saturated (darkened) adatoms vs. GeHx species (bright spots) in favor of the former. When comparing the adsorption of SiH4 [3] and GeH4 on Si(1 1 1)-(7 7) one finds that the former process shows a pronounced site selectivity. Initially, that reaction involves exclusively the corner holes and the adjacent Si adatoms of the (7 7) reconstruction, with preferential adsorption of SiH3 in the corner holes and of H atoms on one of the adjacent corner adatoms. That reaction path is missing for GeH4 adsorption on Si(1 1 1)-(7 7). However, it was found that the main SiHx adspe-
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Fig. 5. Model for the adsorption configuration of GeH2 upon GeH4 dissociation on Si(1 1 1)-(7 7) at 300 K. GeH2 is adsorbed in a bridge configuration between two adatoms, to which it bonds. The detail in Fig. 1(c) shows a GeH2 group which is adsorbed in a configuration which corresponds to the right one in the model, i.e., at a pair of center adatoms.
cies after prolonged SiH4 exposure to Si(1 1 1)(7 7) at room temperature is indeed SiH2 [15]. For prolonged adsorption of SiH4 (after saturation of the corner holes) and GeH4 it may therefore well be that the reaction paths on Si(1 1 1)-(7 7) are very similar for the two molecules. 4. Conclusion Based on our experimental results the following model can be established for GeH4 adsorption on Si(1 1 1)-(7 7) at room temperature: GeH4 dissociates under scission of two Ge–H bonds according to GeH4 (g) + 4db ! GeH2 (ad) + 2H(ad), with a sticking coefficient of 1.2 106 . The released hydrogen atoms adsorb at dangling adatom bonds, and the GeH2 group is accommodated in a bridge position above two adatoms. Acknowledgements This work was supported by the Stiftung Volkswagenwerk under Grant I/72162 and by the Fonds der chemischen Industrie. The silicon samples were donated by Wacker Siltronic.
References [1] D.W. Greve, in: Thin Films, vol. 23, Academic Press, Orlando, FL, 1998. [2] H. Rauscher, Surf. Sci. Rep. 42 (2001) 207, and references therein. [3] U. Memmert, A. Berk o, R.J. Behm, Surf. Sci. 325 (1995) L441. [4] J. Wintterlin, Ph. Avouris, J. Chem. Phys. 100 (1994) 687. [5] S. Van, D. Steinmetz, F. Ringeisen, D. Bolmont, J.J. Koulmann, Phys. Rev. B 44 (1991) 13807. [6] M. Fehrenbacher, J. Spitzm€ uller, U. Memmert, H. Rauscher, R.J. Behm, J. Vac. Sci. Technol. A 14 (1996) 1499. [7] J.J. Boland, Surf. Sci. 244 (1991) 1. [8] J. Braun, H. Rauscher, R.J. Behm, Surf. Sci. 416 (1998) 226. [9] M. Schl€ uter, M.L. Cohen, Phys. Rev. B 17 (1978) 716. [10] D. Albertini, F. Thibaudau, L. Masson, F. Salvan, Surf. Sci. 350 (1996) L216. [11] M. Fehrenbacher, H. Rauscher, R.J. Behm, Z. Phys. Chem. 198 (1997) 205. [12] S.M. Gates, C.M. Greenlief, D.B. Beach, R.R. Kunz, Chem. Phys. Lett. 154 (1989) 505. [13] K.D. Brommer, M. Galvan, A. Dal Pino Jr., J.D. Joannopoulos, Surf. Sci. 314 (1994) 57. [14] Ph. Avouris, J. Phys. Chem. 94 (1990) 2246. [15] S.M. Gates, C.M. Greenlief, D.B. Beach, P.A. Holbert, J. Chem. Phys. 92 (1990) 3144.
Mechanism of GeH4 dissociation on Si(1 1 1)-(7 7) J. Braun 1, H. Rauscher *, R.J. Behm Abteilung Oberfl€achenchemie und Katalyse, Universit€at Ulm, Albert-Einstein-Allee 47, 89069 Ulm, Germany Received 25 January 2003; accepted for publication 17 March 2003
Abstract Results of an STM study of dissociative GeH4 adsorption on Si(1 1 1)-(7 7) at 300 K show that GeH4 adsorbs under scission of two Ge–H bonds according to GeH4 (g) + 4db ! GeH2 (ad) + 2H(ad). GeH2 binds to two adatom dangling bonds in a bridged configuration, while the two released hydrogen atoms saturate two additional dangling bonds. The GeH4 sticking coefficient under these conditions is 1.2 106 , one order of magnitude smaller than for SiH4 . Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon; Hydrides; Chemisorption; Sticking; Scanning tunneling microscopy; X-ray photoelectron spectroscopy
1. Introduction GeH4 is an important precursor molecule for chemical vapor deposition (CVD) of Ge and, in conjunction with Si precursors, of thin Six Ge1x films as well. Under continuous growth conditions CVD reactions are complex processes since they involve many different parallel and sequential reactions [1,2]. However, at temperatures far below those used in CVD processes, which are usually carried out well above 700 K, many of the reactions that proceed subsequently to the primary dissociation step can be suppressed. In this case it is possible to gain information on the initial reactions that take place during and upon dissociative * Corresponding author. Tel.: +49-731-5025469; fax: +49731-502-5452. E-mail address: [email protected] (H. Rauscher). 1 Present address: Mattson Thermal Products GmbH, 89160 Dornstadt, Germany.
adsorption on the surface and to reveal, e.g., the mechanism or a site selectivity, as it had been observed, e.g., for silane adsorption [3]. Despite the importance of dissociative adsorption as an elementary process in the CVD-growth of thin layers, little is known about the initial adsorption steps of GeH4 on Si(1 1 1)-(7 7) at the atomic level. Wintterlin and Avouris [4] found that the sticking coefficient of GeH4 on Si(1 1 1)-(7 7) is very low at room temperature, without, however, quantifying it, and Van et al. [5] reported a very small amount of Ge on the surface after exposure to 104 L of GeH4 at room temperature, but the coverage was not determined quantitatively. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) results showed that formation of SiH/SiH2 species accompanies dissociative adsorption of GeH4 on the surface. However, no conclusion was reached on the formation of surface Ge hydrides [4,5]. In this work we analyze the initial mechanism of Ge deposition by GeH4 pyrolysis at the atomic
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level from atomic resolution scanning tunneling microscopy (STM). In particular, we study the formation of Ge hydrides and their adsorption sites and identify the primary adsorption step and the dissociation mechanism of the precursor.
2. Experimental The experiments were performed in an ultrahigh vacuum (UHV) system equipped with a homebuilt STM, XPS and facilities for annealing the sample [6]. The STM images were acquired at room temperature (300 K) in the constant current mode using tungsten tips. The samples were cut from p-doped Si(1 1 1) wafers with a resistivity between 1 and 20 X cm. They were cleaned using standard procedures [6]. GeH4 with a purity of 99.999% was obtained from Messer Griesheim. The tunnel voltage UT is the voltage applied at the sample with respect to the tip.
3. Results and discussion Exposure to GeH4 at 300 K leads to several new features on Si(1 1 1)-(7 7) in comparison with the initial surface. This is illustrated in Figs. 1 and 2, which show four STM images of the Si(1 1 1) surface after exposure to 7880 L (Fig. 1) and 23,630 L (Fig. 2) of GeH4 . Fig. 1(a) and (b) were taken at approximately the same positions, but at different tunnel voltages UT : UT ¼ þ1:0 V for Fig. 1(a) and UT ¼ þ2:0 V for Fig. 1(b). The same voltages were used for Fig. 2(a) and (b), respectively. In can be seen from Figs. 1(a) and 2(a) that with increasing GeH4 exposure the (7 7) reconstructed surface exhibits more and more adatom sites which appear darker in the constant current images as compared to the initial surface and the majority of the adatoms after exposure. These darkened adatoms are attributed to adsorption of hydrogen atoms or other dissociation products of GeH4 which bond to the dangling bonds. It is well known from other STM studies that adsorption of hydrogen on the (7 7) reconstructed surface first occurs on the adatoms and that it leads to a local decrease of the tunnel current and hence a darker
Fig. 1. STM images of a Si(1 1 1)-(7 7) surface after exposure to 7880 L GeH4 at 300 K and a GeH4 pressure of 3.5 105 mbar. Tunnel voltage UT : +1.0 V (a) and +2.0 V (b). Image sizes 35 nm 35 nm.
appearance of these adatoms in STM images [7]. Hydrogen can be provided by dissociative H2 adsorption [7], but also by dissociative adsorption of other molecules which contain hydrogen [2,3,8]. The darkening of the adatoms was attributed to a reduced local density of states (LDOS) around the Fermi level EF , by saturation of the Si adatom dangling bonds. A similar effect can hence be assumed in our case. More information on the adsorbed species can be gained from STM images recorded at different tunnel voltages UT . Increasing UT from +1.0 to
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Fig. 2. STM images of a Si(1 1 1)-(7 7) surface after exposure to 23,630 L GeH4 at 300 K and a GeH4 pressure of 3.5 105 mbar. Tunnel voltage UT : +1.0 V (a) and +2.0 V (b). Image sizes 35 nm 35 nm.
+2.0 V changes the contrast in the STM images. A comparison of STM images recorded at +1.0 V (Figs. 1(a) and 2(a)) with those recorded at the same position, but with UT ¼ þ2:0 V (Figs. 1(b) and 2(b)) shows that many of the adatoms that are dark at lower voltages become bright again. In fact, only a small difference in contrast as compared to the unaffected adatoms persists at UT ¼ þ2:0 V. The reason for this behavior is that at a tunnel voltage of +2.0 V the antibonding r orbital of the Si–H bond, which is centered around
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3.5 eV above the Fermi level [9], already contributes to the tunnel current, so that the contrast between reacted and non-reacted adatoms is decreased. The most prominent features in the STM images of Figs. 1(b) and 2(b), recorded at UT ¼ þ2:0 V, are small protrusions that are absent in the STM images at lower voltages (Figs. 1(a) and 2(a)). The adatoms at these positions appear also dark at low voltage. Such protrusions have not been observed after exposure to hydrogen or to silanes (SiH4 , Si2 H6 ) under similar experimental conditions, i.e., for low exposures at room temperature. They resemble, however, structures that were observed earlier after room temperature adsorption of Ge2 H6 on Si(1 1 1) [4]. We assign these protrusions, which are imaged with a size similar to that of adatoms at UT ¼ þ2:0 V, to adsorbed GeHx species. The characteristic contrast variation with changing tunnel voltage indicates that these GeHx species do not have dangling bonds. Rather, the dark appearance at low potentials and the brighter appearance at higher positive voltage points towards the presence of saturated bonds which become visible at higher voltages due to a higher density of antibonding states at these voltages. The protrusions are not located directly above adatom sites. Rather, they are exactly in the middle between two adatoms. This can also be seen in the inset in Fig. 1(b), where one of these fragments is imaged with high resolution. It indicates adsorption in a bridged position, and therefore we assign these fragments to GeH2 species bound to an adatom–restatom pair or to a pair of adatoms. Under these tunnel conditions the restatoms cannot be resolved with the STM, but it is known from other studies of reactions of silanes on Si(1 1 1)(7 7) [2,10,11] that the dangling bonds of the restatoms can also participate in such adsorption reactions. Before we analyze the adsorption process and the involved species in more detail we turn to the adsorption kinetics and a possible site selectivity of the adsorption process. Fig. 3(a) displays the fraction of reacted (darkened) Si adatoms as a function of increasing GeH4 exposure and Fig. 3(b) shows the fraction of reacted center adatoms vs. reacted corner adatoms. From Fig. 3(a) it can
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20000 40000 GeH4 exposure (L)
fraction of reacted center adatoms
(a)
0.5 0.4 0.3 0.2 0.1 0.0 0.0
(b)
60000
0.1 0.2 0.3 0.4 fraction of reacted corner adatoms
Fig. 3. (a) Fraction of reacted adatoms as a function of GeH4 exposure to Si(1 1 1)-(7 7) at 300 K. The inset shows the region for exposures of up to 1400 L in more detail. (b) Fraction of the reacted center adatoms vs. reacted corner adatoms during GeH4 exposure to Si(1 1 1)-(7 7) at 300 K.
be seen that saturation is not yet achieved even after exposure to 55 103 L. Furthermore, for large exposures the fraction of reacted adatoms increases linearly with the GeH4 exposure. Closer inspection of Fig. 3(a) with its inset for low exposures reveals that two different adsorption regimes can be distinguished. In the initial regime, for exposures up to 1350 L, the reactivity is larger by a factor of 3.3 as compared to the reactivity in the high-exposure regime (above 8000 L). In the first regime up to around 4% of the adatoms take part in the adsorption reaction. Two regimes with different sticking coefficients have also been found for room temperature adsorption of SiH4 on Si(1 1 1)-(7 7) [12]. In that system, the initial reaction channel dominates up
to Si deposition of about 2% of a monolayer (ML, 1 ML ¼ 7:78 1014 atoms/cm2 ). STM investigations showed that this reaction path was due to a strong site preference for dissociative SiH4 adsorption at corner hole/corner adatom pairs [3]. The amount of adsorbed Si (0.02 ML) corresponded exactly to the fraction of reacted adatoms (8–9%), provided that each SiH4 molecule splits off one H atom that saturates a corner adatom dangling bond while the remaining SiH3 group adsorbs at the dangling bond at the bottom of the corner hole. After saturation of these adsorption sites the sticking coefficient decreased by a factor of 10. In contrast, for the GeH4 /Si(1 1 1)-(7 7) system there is almost no selectivity between center and corner adatoms towards GeH4 dissociation. The ratio of reacted center vs. corner adatoms remains constant over the entire exposure regime, at a value of 1.16:1. This indicates that for GeH4 adsorption on Si(1 1 1)-(7 7) there is no such a fast initial reaction path as for SiH4 . However, a fast reaction may still proceed by preferential adsorption at point defects of the initial surface. The initial (7 7) reconstruction after preparation under our conditions has a typical defect density of 1–2% of the adatoms. Hence, for dissociative GeH4 adsorption at such a defect with breakage of 1–2 Ge–H bonds around 2–4% of the adatoms would be involved in such a reaction. This fraction corresponds very well to the fraction of adatoms involved in the fast adsorption reaction (4%), and therefore it appears probable that here the initial adsorption regime is defect-induced. More information on the adsorption process can be gained from STM images after longer exposures. As discussed above, the number of protrusions (observed at UT ¼ þ2:0 V) as well as the number of darkened adatoms (observed at UT ¼ þ1:0 V) both increase linearly for exposures of more than 1000 L, which indicates that GeH4 adsorption proceeds via a precursor. In the latter regime the ratio of the number of protrusions vs. the number of reacted adatoms remains constant: the number of protrusions is around 15–18% of the reacted adatoms, indicating that GeH4 adsorption proceeds under scission of two or more Ge–H bonds.
J. Braun et al. / Surface Science 531 (2003) 265–271
Let us first assume that all protrusions imaged in the STM represent GeH2 groups, i.e., that GeH4 adsorption exclusively proceeds under scission of two Ge–H bonds. The protrusions are located at interstitial sites between the adatoms and hence one possible adsorption configuration for GeH2 is in a bridge position between an adatom–restatom pair. If this were true it would imply that three adatoms or less are saturated for each accommodated GeH4 molecule, since the two released H atoms adsorb preferentially at adatom dangling bonds [13,14]. Hence, in that case a ratio of 1:3 is expected for the number of protrusions imaged at +2.0 V relative to the reacted adatoms imaged at +1.0 V. But this scenario contradicts our experimental finding that the number of protrusions is only 15–18% of that of the reacted adatoms. Consequently, there must be another reaction channel, which leads to a larger proportion of darkened adatoms. Such a mechanism, which is in agreement with the experimental findings, involves the adsorption of GeH4 under twofold Ge–H bond scission and subsequent accommodation of GeH2 at an adatom–adatom pair. This would result in bridgebonded GeH2 , which saturates two adatom dangling bonds plus two H atoms which saturate two additional adatom dangling bonds. In total, this mechanism saturates four adatom dangling bonds per adsorbed GeH4 molecule. A configuration where GeH2 is bridge-adsorbed between two adatoms is indicated by the position of the protrusions in the STM images: they are located precisely halfway between two adatoms (see also the inset in Fig. 1(b)). If GeH2 is located at a bridge position above an adatom–adatom pair one expects to find darkened adatom pairs in the STM images obtained at UT ¼ þ1:0 V at those sites where protrusions are imaged at UT ¼ þ2:0 V. Closer inspection of Fig. 1(a) and (b) shows that this is indeed the case for most of the structures under discussion. Some examples are marked by dashed ellipses in Fig. 1(a) and (b). Sometimes more than two adatoms that are close together are darkened, indicating nearby adsorption of hydrogen atoms. If only a single Si adatom is darkened at UT ¼ þ1:0 V with the surrounding atoms unaffected there is never a pro-
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trusion imaged at UT ¼ þ2:0 V, which is a clear indication of hydrogen adsorption on that adatom. Very rarely we find a protrusion at UT ¼ þ2:0 V, but no clear darkening of the underlying adatom pair at UT ¼ þ1:0 V. In those cases the adatom structure also shows some irregularity such as an atom at an interstitial site. An example is also indicated by a dashed square in Fig. 1(a) and (b). The nature of the species that are imaged as protrusions in those exceptional cases could not be identified unambiguously. However, these protrusions appear slightly larger in size as compared to the majority and they may tentatively be assigned to GeH or Ge atoms. Further evidence that GeH2 is adsorbed at adatom–adatom pairs comes from Fig. 4(a), which shows an STM image recorded after exposure to 55,140 L GeH4 . This image displays a large number of the bright protrusions which are all located at bridge sites. Moreover, a number of them is adsorbed precisely at the border between two (7 7) unit cells. Several of these protrusions are marked by arrows in Fig. 4(a). These GeH2 groups are located directly above the dimers of the DAS reconstruction and hence for these GeH2 fragments bonding to a restatom dangling bond is impossible. This is further confirmation that the GeH2 species are adsorbed at adatom pairs. A model for the GeH2 adsorption configuration is shown in Fig. 5. To further support this mechanism and to establish a relation between the number of protrusions and the deposited amount of Ge we annealed the samples to 800 K after adsorption and STM characterization at room temperature. Such an annealing step decomposes all Si and Ge hydrides on the surface and removes hydrogen by H2 desorption [3,4]. The Ge atoms become mobile and aggregate in clusters or fill up defect sites of the substrate. Finally, islands are formed which exhibit (5 5) or (7 7) DAS (dimer adatom stacking-fault) structures. The coverage of these islands can be determined by measuring the island area or by counting the number of adatoms. This way, the amount of deposited Ge can be precisely determined by STM. Taking also into account the number of defects of the initial surface and after annealing, this results in a Ge coverage of 0.01
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Fig. 4. (a) STM image of a Si(1 1 1)-(7 7) surface after exposure to 55,140 L of GeH4 (p ¼ 3:5 105 mbar) at 300 K. Image size 35 nm 35 nm, UT ¼ þ2:0 V. Arrows indicate several GeH2 groups which are adsorbed at the borderline between neighboring (7 7) unit cells. (b) STM image of the same sample after an additional annealing step to 800 K for 2 min. Image size 25 nm 25 nm, UT ¼ þ1:0 V.
bilayers (BL) after exposure to 55,140 L of GeH4 (Fig. 4(b)). The Ge coverage determined in that way can now be related to the number of GeH2 species and reacted adatoms before annealing. Counting the number of protrusions before the annealing step (Fig. 4(a)) gives a coverage of 1.5 1013 GeH2 species/cm2 . Since each GeH2 contributes one Ge atom, it leads to a Ge coverage of 1.5 1013 Ge atoms/cm2 , which corresponds to
0.01 BL, the same as determined from the islands after annealing. This result can be cross-checked by relating the number of protrusions imaged at UT ¼ þ2:0 V to the number of reacted adatoms imaged at UT ¼ þ1:0 V. According to our model each dissociated GeH4 molecule should yield one GeH2 (protrusion at UT ¼ þ2:0 V) and four saturated adatoms (darkened at UT ¼ þ1:0 V), two from bridge-bonded GeH2 and two more from the hydrogen atoms. If the Ge coverage is 0.01 BL (1.5 1013 cm2 , as determined by STM after annealing), we should find around 6 1013 saturated adatoms per cm2 . Since at UT ¼ þ1:0 V defects cannot be distinguished from saturated adatoms, we must add the density of the initial surface defects of 0.75 1013 cm2 to the number of expected hydrogen-saturated adatoms. Doing this we arrive at a coverage of 6.75 1013 darkened adatoms per cm2 . For the STM image we determine 7.5 1013 darkened adatoms per cm2 , in very good agreement with predictions from the model above which includes GeH2 species bridge-bonded between two adatoms. From the model we can calculate the dissociative sticking coefficient for GeH4 adsorption on Si(1 1 1)-(7 7) at room temperature to 1.2 106 , which is around one order of magnitude smaller than the one for adsorption of SiH4 under the same experimental conditions [3]. Still, the ratio of darkened adatoms vs. bright spots as observed in the experiments is slightly larger than expected from the adsorption and dissociation model discussed above. This indicates that a small fraction of the GeH2 groups dissociates further even at room temperature, which shifts the ratio of saturated (darkened) adatoms vs. GeHx species (bright spots) in favor of the former. When comparing the adsorption of SiH4 [3] and GeH4 on Si(1 1 1)-(7 7) one finds that the former process shows a pronounced site selectivity. Initially, that reaction involves exclusively the corner holes and the adjacent Si adatoms of the (7 7) reconstruction, with preferential adsorption of SiH3 in the corner holes and of H atoms on one of the adjacent corner adatoms. That reaction path is missing for GeH4 adsorption on Si(1 1 1)-(7 7). However, it was found that the main SiHx adspe-
J. Braun et al. / Surface Science 531 (2003) 265–271
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Fig. 5. Model for the adsorption configuration of GeH2 upon GeH4 dissociation on Si(1 1 1)-(7 7) at 300 K. GeH2 is adsorbed in a bridge configuration between two adatoms, to which it bonds. The detail in Fig. 1(c) shows a GeH2 group which is adsorbed in a configuration which corresponds to the right one in the model, i.e., at a pair of center adatoms.
cies after prolonged SiH4 exposure to Si(1 1 1)(7 7) at room temperature is indeed SiH2 [15]. For prolonged adsorption of SiH4 (after saturation of the corner holes) and GeH4 it may therefore well be that the reaction paths on Si(1 1 1)-(7 7) are very similar for the two molecules. 4. Conclusion Based on our experimental results the following model can be established for GeH4 adsorption on Si(1 1 1)-(7 7) at room temperature: GeH4 dissociates under scission of two Ge–H bonds according to GeH4 (g) + 4db ! GeH2 (ad) + 2H(ad), with a sticking coefficient of 1.2 106 . The released hydrogen atoms adsorb at dangling adatom bonds, and the GeH2 group is accommodated in a bridge position above two adatoms. Acknowledgements This work was supported by the Stiftung Volkswagenwerk under Grant I/72162 and by the Fonds der chemischen Industrie. The silicon samples were donated by Wacker Siltronic.
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