UPS differentiation between molecular NH3 and partially dissociated NH2 fragments adsorbed at low temperature on Si(001) surfaces

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Surface Science Letters 248 (1991) L240-L244 North-Holland

Surface Science Letters

UPS differentiation between molecular N H 3 and partially dissociated N H 2 fragments adsorbed at low temperature on Si(001) surfaces J.L. Bischoff, F. Lutz, D. B o l m o n t a n d L. K u b l e r FacultO des Sciences et Techniques, Laboratoire de Physique et de Spectroscopie Electronique, 4, rue des FrOres LurniOre, 68093-Mulhouse COdex, France Received 13 November 1990; accepted for publication 8 February 1991

For the first time physisorbed non dissociated N H 3 molecules and chemisorbed partially dissociated N H 2 fragments, adsorbed at low temperature on Si(001)-2 × 1 surfaces are clearly differentiated by UPS (ultraviolet photoemission spectroscopy) He II spectra. This result is obtained by a careful scanning of the relevant UPS features as a function of Ts between 80 K where the contribution of the condensed N H 3 molecules prevails and 300 K where only that of N H 2 is prominent. Actually, at 80 K, for sufficiently high exposures, condensed molecules mainly overlay a weak coverage of dissociated species capping themselves some of the Si electronic surface states. The passage at 120 K enables us to observe simultaneously the two species, providing an indubitable proof for their differentiation. This assignment is ascertained by correlated binding energy variations of the N l s core level signals in relation with the two species.

1. Introduction The low substrate temperature (T~) ( ~ < 600 K) interaction of N H 3 on clean amorphous silicon or reconstructed Si(001)-2 × 1 and Si(lll)-7 × 7 surfaces has been extensively studied during the last years. In 1985 [1] our group was the first to suggest, on the basis of the observation of N ls core level binding energy shifts related to different nitrogen species, the occurrence of partial N H 3 dissociation on amorphous silicon held at room temperature (RT) with chemisorption of N H 2 or N H species. Later core level shift observations done on S i ( l l l ) [2] and Si(001) [3] gave arguments for similar types of chemisorption on the dangling bonds of the reconstructed surfaces. They were comforted by ultra-violet photoemission spectroscopy (UPS) experiments. The latter technique afforded us an indirect proof for ammonia dissociation in providing evidence of S i - H bonds as a by-product of the N H 3 dissociation at the surface [2,3]. Meanwhile, Bozso et al. [4] extending photoemission measurements at 100 K, showed that the dissociative adsorption process is already working

well below RT. Scanning tunneling microscopy (STM) measurements confirmed dissociative coadsorption, too [5,6] and revealed the selectivity of the adsorption as regards the nature of the surface dangling bonds. Nevertheless STM does not seem able to differentiate the coadsorbed hydronitrided fragments N H x (NH, N H 2 or NH3). Thus, at the moment, besides the aforementioned XPS, arguing with a monotone N ls BE shift related to the dissociation degree but suffering from a poor resolution, the most powerful information concerning the preceding problem, is obtained by electron-energy-loss spectroscopy (EELS), capable of separating the different vibration modes. Even if it was primarily reported in favour of non dissociative adsorption of N H 3 at R T [7], all other recent reports propose a coadsorption of H and principally N H 2 species [8-10]. The latter results are further corroborated by use of electron-stimulated desorption ion angular distribution (ESDIAD) [11,12] and temperature programmed desorption [12,13]. Concerning the characterization of the different N H x fragments, valence band photoemission ex-

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J.L. Bischoff et al. / Differentiation of N H 3 and N H e adsorbed on Si(001)-2 × 1 by UPS

cited by ultraviolet photons (UPS) would theoretically be useful, too: the change of the number of hydrogen borne by nitrogen atoms should induce modifications in the N - H bonding and the lone pair orbitals, the most prominent features observed by ourselves [2,3] and by Bozso et al. [4] at 100 K and at RT. Actually, the BE shifts of the UPS features successively observed at 100 K (where, at sufficiently high exposures, molecular N H 3 gives the prominent contribution) and RT (where a given partially dissociated NH~ fragment was suspected to prevail) are well established and are briefly recalled in this Letter. Owing to the similarity of the two observed features at 100 K and at RT we could only infer that the RT adsorbed form had to bear hydrogen atoms (NHx) and could not correspond, for instance, to wholly dissociated species (nitrogen atoms). The aim of this report is now to give by UPS an indubitable argument proving that the RT adsorbate cannot be confused with the molecular N H 3 one observed at 100 K. Our proposal is to display, simultaneously on the same UPS spectra recording, the features corresponding to two different chemical environments adsorbed on Si(001)-2 x 1 surfaces. We achieve this goal by following in detail the sensitive UPS BE domain as a function of T~ during the thermal variation between 80 K and RT. The spectra are particularly carefully recorded in the temperature transition region where the two species, the one physisorbed, the other chemisorbed, coexist in comparable amounts (110-140 K). The two contributions can now be clearly separated, discussed and attributed to molecular N H 3 and N H 2 species, respectively.

2. Experimental All exposures and photoemission analyses were carried out in the same ultra high vacuum (UHV) chamber whose base pressure was in the 10 -1° mbar range. The XPS and UPS spectra were recorded with a VG-CLAM 100 spectrometer operating with He I (21.2 eV), He II (40.8 eV) or Mg K a X-ray (1253.6 eV) sources. The indicated BEs are all referred to the Fermi level. The substrates were nearly intrinsic Si(001) Siltronix wafers whose

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cleanliness and surface reconstruction were obtained routinely (repeated Ar ÷ sputtering cycles followed by annealings at 1100 K). They were mounted on a substrate holder combining polar rotation (0), liquid nitrogen cooling and resistive heating. So the temperature could be rapidly varied between 1100 K (needed for the surface reconstruction) and 80 K. That is the key point to perform controlled gas exposures at very low Ts (where the sticking coefficient of the residual gases, particularly H20, is near 1) on clean reconstructed Si(001)-2 × 1 surfaces. The reconstruction is ascertained by the observation of the well known UPS He I surface state related to the 2 × 1 dimers. The N H 3 exposures expressed in L (1 L = 10 -6 Torr- s) are obtained by backfilling the chamber via a leak valve at a controlled pressure during controlled times followed by rapid ammonia evacuation.

3. Results and discussion In fig. 1 we show how local nitrogen environments of concern can be inferred from the N ls core level investigations. In fig. l a a physisorbed contribution following a RT saturation adsorption is shown. The latter exhibits a component at 398.6 eV attributed to the partial dissociation of N H 3 on the surface dangling bonds in N H x fragments. The relevant signal is weak since its coverage saturates below 2 x 1014 c m - 2 when all dangling bonds are capped by NHx fragments and coadsorbed hydrogen. Spectrum (lb) corresponds to a 5 L exposure fully realized at 80 K. The condensed contribution at 400.1 eV is now prevailing but the dissociated component is still guessed at lower BE proving, in agreement with ref. [4], that the partial dissociation on the DBs, is already working at 100 K. Spectra l c and l d describe the N ls core level evolution during the desorption of the N H 3 molecules between 80 and 300 K to be compared in fig. 3 to the relevant UPS feature changes. In spectrum l c the molecular desorption starts with increasing T~. Spectrum l d results from the preceding situations after desorption of nearly all molecules, the substrate being now carried at RT: at this T~, the sole persisting configuration is the chemisorbed NH~ fragment. If all these ex-

J.L. Bischoff et aL / Differentiation of N H 3 and N H 2 adsorbed on Si(O01)-2× l by UPS

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perimental results are in agreement with previously published results on Si(001)-2 × 1 surfaces by ourselves [3,14] and by Bozso et al. [4], the interpretation of the intermediate peak near 398.6 eV is different: they attributed it to N H species, while we suggested previously [14] the presence of N H z fragments, an assumption confirmed by all more recent studies using other techniques [9,10,12]. In fig. 2 we briefly recall the present status of what is k n o w n concerning the UPS He II features in relation with this problem. Besides the Si 3p substrate contribution, peaking between 2 and 3 eV (fig. 2a), two prominent features are always observed by adsorption of N H 3, both by condensation of molecular N H 3 at 100 K whatever the underlying substrate (fig. 2b) and by adsorption at R T (fig. 2c). Nevertheless the energy separation of this doublet structure is different in both cases as well as its peak location. In the former case it is assigned to the 3a 1 nitrogen

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Fig. 1. Typical N ls XPS spectra obtained after different exposures to NH 3 of Si(001)-2× 1 surfaces held at various Ts: (a) after two successive exposures; 20 L at RT followed by 3 L at 80 K, (b) after one exposure; 5 L at 80 K, (c) after carrying the exposed substrate (lb) to 100 K, (d) after carrying the preceding substrate to RT. All these spectra are taken at polar angle O= 60 ° .

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Fig. 2. Typical Hell (40.8 eV) UPS spectra of a Si(001)-2x 1, (a) clean surface, (b) after 5 L NH 3 exposure at 80 K, (c) after 10 L NH 3 exposure at RT. The frame defines the BE domain which will be detailed in fig. 3 at intermediate ~ between 80 and 300 K.

lone-pair orbital (lowest BE) and the le N - H b o n d i n g electrons (highest BE) [2-4]. More particular attention is payed in these reports to the description of these features. The persistent observation of the latter structure, after adsorption at RT, gave us the strongest argument in favour of a R T adsorption in the N H x form [2,3], In the absence of any investigation in the evolution of these U P S features between 80 K and RT, we could not distinguish by U P S alone, whether the relevant BE shifts are due to the chemical change from the N H 3 molecule to, for instance, the N H 2 fragment or to other possible BE changes with T~, for an unchanged molecule. The focus of this report is now to clearly show, on the U P S H e II spectra recorded at intermediate (fig. 3), the simultaneous presence of two species, therefore chemically different but having in c o m m o n b o n d i n g N H electrons. To this end we follow the detail of the binding energy region corresponding to the N H 3 features after an N H 3 exposure at 80 K, scanning the temperature range

J.L. Bischoff et al. / Differentiation of N H 3 and N H 2 adsorbed on Si(001)-2 x 1 by UPS

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Fig. 3. Detailed part of the UPS HelI spectra, taken at polar angle 0 = 60 °, in the BE domain where the changing NHx features appear. Spectrum (a) is obtained by a 5 L NH 3 exposure of the reconstructed Si(001)-2× 1 surface held at 80 K. The following spectra (b-f) result from the same surface after progressive desorption of the physisorbed NH 3 species by raising the temperature to the indicated values.

between 80 K and RT by increasing annealing steps. A similar behaviour (not represented) could be obtained starting with a RT exposure and followed by subsequent ammonia freezing. Fig. 3a displays the spectrum corresponding to the starting situation at 80 K with a high coverage of condensed molecular N H 3 giving the strong features previously described. They would almost completely mask the much weaker contributions of the underlying N H 2 species chemisorbed on the surface DBs, if it were not for an asymmetry of the 3a I peak towards the lower BEs and a relatively broad le line. When we increase the temperature to 110 K (fig. 3b) the N H 3 molecules start to desorb as revealed by the rapid decrease of the molecular UPS features and the core level component at 400.1 eV (fig. 1).

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Between 110 and 150 K (figs. 3c-3e) we simultaneously observe the resolved UPS signatures of two species in similar amounts on the surface. The N H 3 desorption lets the features attributed to the underlying chemisorbed N H 2 fragments to come in view near 5 and 11 eV. The le contribution is now the broadest ( > 3 eV) while the non bonding 3a 1 contributions are well resolved into two components (figs. 3b-3c). Increasing now the temperature up to RT allows us to select the N H 2 component only (fig. 3f) presenting a much narrower le line for the N H electrons and a non bonding contribution near 5 eV as noticed in the previous report for the RT adsorption [2,3]. These UPS assignments are ascertained by a correlated N ls core line evolution with T~ of the features at 400.1 and 398.6 eV, respectively (figs. l b - l d ) . These observations indicate that the valence band electrons as obtained by UPS are able to differentiate low concentrations ( < 2 × 10 -14 cm -2) of adsorbed N H 3 and N H 2 molecules. Furthermore, the BE difference induced by the passage from the physisorbed N H 3 to the chemisorbed N H 2 form is more pronounced for the Np~ electrons than for the bonding N - H electrons, probably less affected by the interaction with the silicon substrate. Nevertheless the question how to recognize the bonding orbitals between the N H 2 fragments with the silicon dimer orbitals (giving probably much weaker contributions than the molecular orbitals) remains open.

References [1] L. Kubler, R. Hang, J.J. Koulmann, D. Bolmont, E.K. Hlil and A. Jaegle, J. Non-Cryst. Solids 77/78 (1985) 945. [2] L. Kubler, E.K. Hlil, D. Bolmont and G. Gewinner, Surf. Sci. 183 (1987) 503. [3] E.K. Hlil, L. Kubler, J.L. Bischoff and D. Bolmont, Phys. Rev. B. 35 (1987) 5913. [4] F. Bozso and Ph. Avouris, Phys. Rev. B 38 (1988) 3937. [5] R.J. Hamers, Ph. Avouris and F. Bozso, Phys. Rev. Lett. 59 (1987) 2071. [6] R. Wolkow and Ph. Avouris, Phys. Rev. Lett. 60 (1988) 1049. [7] D.G. Kilday, G. Margaritondo, D.J. Frankel, J. Anderson and G.J. Lapeyre, Phys. Rev. B 35 (1987) 9364. [8] S. Tanaka, M. Onchi and M. Nishijima, Surf. Sci. 191 (1987) L 756.

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[9] M. Fujisawa, Y. Taguchi, Y. Kuwahara, M. Onchi and M. Nishijima, Phys. Rev. B 39 (1989) 12918. [10] C.U.S. Larsson and A.S. Flodstr~Sm, ECOSS 11 Surf. Sci., in press. [11] A.L. Johnson, M.M. Walczak and T.E. Madey, Langmuir 4 (1988) 277.

[12] M.J. Dresser~ P.A. Taylor, R.M. Wallace, W.J. Choyke and J.T. Yates, Jr., Surf. Sci. 218 (1989) 75. [13] B.G. Koehler, P.A. Coon and S.M. George, J. Vac. Sci. Technol. B 7 (1989) 1303. [14] J . L Bischoff, L. Kubler and D. Bolmont, J. Non-Cryst. Solids 97/98 (1987) 1407.