Si-H bond production by NH3 adsorption on Si(111): An UPS study

Surface Science 183 (1987) 503-514 North-Holland, Amsterdam

503

S i - H B O N D P R O D U C T I O N BY N H 3 A D S O R P T I O N O N S i ( l l l ) : AN UPS STUDY L. K U B L E R , E.K. H L I L , D. B O L M O N T a n d G. G E W I N N E R Facult~ des Sciences et Techniques, Unioersitd de Haute Alsace, 4 rue des Fr~res Lumikre, 68093-Mulhouse cedex, France

Received 22 August 1986; accepted for publication 16 December 1986

The adsorption of NH 3 on Si(lll) has been examined using essentially ultraviolet photoemission spectroscopy (UPS) between room temperature (RT) and 400~ In this domain NH 3 molecules chemisorb dissociatively on some surface sites as deduced from the observation of Si-H monohydride units at 5.4 eV below E F. Other species labeled NH x (X= 1, 2 or 3), characterized by two NH 3 induced orbitals at 4.9 and 10.6 eV, are also adsorbed at RT with a saturation coverage ~<1/3 monolayer for a 10 L (1 L = 1 0 - 6 Torr s) exposure. A strong S2 surface state decrease results from adsorptions. With increasing substrate temperature the adsorption of the nitrided NH x species gradually decreases until 300 ~ where mainly Si-H bonds are observable. No direct conclusive assignment could be given by UPS about the exact nature of the NH x units, but XPS Nls binding energy (BE) data give arguments for partly dissociated species (NH 2 or NH). The nitride formation starts to develop only above 300 o C as evidenced by both a rapid XPS nitrogen coverage increase and new Si-N UPS features in this domain.

1. Introduction T h e r e is a c o n s i d e r a b l e interest in the e l a b o r a t i o n o f silicon n i t r i d e p r o d u c t s d e p o s i t e d at low substrate t e m p e r a t u r e s (T~) in the r o o m t e m p e r a t u r e ( R T ) - 4 0 0 ~ range. Particularly, p l a s m a deposited a m o r p h o u s S i N x : H films are p r o m i s i n g materials in the p h o t o v o l t a i c c o n v e r s i o n or as high b a n d gap m at eri al s in the a - S i : H / a - S i N x : H superlattices. A m m o n i a is the m o s t c o m m o n l y used gas for these nitridations. D e s p i t e this c o n s i d e r a b l e interest a n d the huge n u m b e r of papers d e v o t e d to the p r e p a r a t i o n of such silicon nitrides w i t h N H 3 , very little w o r k has b e e n d o n e to characterize, at low t e m p e r a t u r e , a n d u n d e r u l t rah ig h v a c u u m c o n d i t i o n s ( U H V ) , the f u n d a m e n t a l i n t e r a c t i o n s b e t w e e n a m m o n i a a n d silicon using surface techniques. S o m e workers have studied in the past the c h e m i s o r p t i o n process of N H 3 o n transition metals in relation with its catalytic d e c o m p o s i t i o n [1-9]. L E E D , X P S or A u g e r studies, dealing with the S i - N H 3 system, are m a i n l y d e v o t e d to the high T~ n i t r i d a t i o n d o m a i n ( Ts >~ 800 o C) [10-12]. T o o u r k n o w l e d g e o n l y Isu an d F u j i w a r a [13], in a letter, r e p o r t e d on U P S spectra for a d s o r b e d N H 3 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 9 Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)

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L Kubler et al. / UPS o f N H 3 adsorbed on S i ( l l l )

on S i ( l l l ) at RT and very recently Bozso and Avouris [14] presented some interesting results about the Si(100)-NH 3 interaction. The purpose of this work is to investigate more extensively the low temperature interaction range between Si(lll)-(7 • 7) surfaces and N H 3 essentially by substrate temperature dependent UPS (RT < T~ < 400 ~ C). The relevant UPS data are also compared with the correlated changes of the XPS N ls intensities and BE values. Complementary LEED and angular resolved UPS (ARUPS) results are also mentioned in order to confirm some peak assignments.

2. Experimental All UPS or XPS analyses and exposures were performed in the same U H V chamber whose base pressure was in the 10-10 mbar range. The photoelectron spectra were recorded with a VG-CLAM 100 spectrometer operating with He I (21.2 eV), H e l l (40.8 eV) or Mg K a X-ray (1253.6 eV) sources. The substrates were c - S i ( l l l ) wafers (p-type 15 fl cm) whose T~ could be varied by direct Joule heating between RT and 1000 o C. In this chamber the substrate holder could be rotated in order to change the emission angle 0 measured from surface normal. However, since the photon incidence angle a could not be varied independently from 0 in the CLAM geometry, and also because of partial angle integration of the photoelectrons, no effective angular UPS could be carried out in this way. Therefore some complementary experiments were carried out in another chamber having effective ARUPS and LEED facilities. The latter results will be published in more detail elsewhere. The Si substrates were cleaned in situ by repeated cycles of argon ion bombardments, followed by annealing at 900 ~ for several minutes. This procedure generates reconstructed c-Si(lll)-(7 x 7) surfaces, characterized by L E E D or the well known [15] UPS S 1, $2, $3 surface states at 0.2, 0.8 and 1.9 eV below the Fermi level respectively. The cleaning procedure was repeated until the impurity level for O ls, N ls and C ls, was below the XPS detection limit corresponding to coverages below 0.02, 0.04 and 0.06 monolayers (ML), respectively. Only a residual nickel coverage estimated at - 0.02 M L depending on the ion etching and annealing conditions could be detected. It must originate from some ions etched from the chamber walls and deposited on the sample region surrounding the etching zone. These species then rediffuse towards the probed sample surface during annealing. The coverages can be estimated by determining the core level intensity ratios between O ls, N ls, C ls or Ni 2p3/2 and Si 2p and assuming an impurity density of 7.8 • 1014 atoms cm -2 at monolayer completion. Ammonia (99.995% purity) was introduced into the chamber via a leak valve providing a range of controlled exposures. We also condensed molecular ammonia on a c-Si surface amorphized by ion

L. Kubler et al. / UPS of N H 3 adsorbed on Si(111)

505

etching on a substrate holder which can be cooled down to 100 K but not heated above RT.

3. Results 3.1. Molecular N H 3 condensed at low T on a-Si substrates

In order to recognize with certitude the UPS spectra of molecularly physisorbed ammonia on silicon, we condensed at - 1 0 0 K N H 3 on c-Si substrates, cleaned by ion etching and therefore amorphized. Besides the clean surface He II spectrum (a) we report in fig. 1 spectrum b relevant to first layer condensation (exposure < 1 L, sticking coefficient near 1). Two prominent features at 5.85 and 11.1 eV reflect the presence of condensed N H 3 species. Except a progressive shift with exposure to higher BE values due to surface charging, no essential modification occurs between first and multilayer condensation spectra on a-Si surfaces. Only a slight increase from 5.25 to 5.4 eV in the energy separation of the two peaks can be observed. The N ls BE related to these molecularly adsorbed forms is, without charging effect, 400.0 eV. The difficulty in recording exclusively NH3 spectra is to avoid the simultaneous condensation of residual water vapor on the cooled substrate. Water also exhibits two main features in the same energy domain. Nevertheless they can be distinguished from their N H 3 counterparts by a slight shift to higher BE values, a higher energy separation between the two peak (5.7 and 5.25 eV for H 2 0 and N H 3, respectively) and inverted peak heights. 3.2. U P S spectra for ammonia adsorption on S i ( l l l ) - ( 7 • 7) at R T < T < 400 ~

The main modification induced by adsorption of N H 3 at R T on clean c-Si(lll)-(7 • 7) is again the addition of two structures now centred at 4.9 and 10.6 eV below the Fermi level (fig. ld). A rapid saturation coverage can be achieved for these peaks with a few L ( - 10 L) in agreement with the XPS results. They are similar to those observed by condensation of molecular N H 3 at low T (fig. lb), but the corresponding energy separation is now higher (5.7 eV at RT and 5.25 eV at low T ) and the positions are shifted to lower BEs (from 5.85 to 4.9 eV and from 11.10 to 10.6 eV, respectively). The only report devoted to the same subject is by Isu and Fujiwara [13] who also observe the feature at 4.9 eV, but find the second one at 9.4 eV in their He I data. This value is lower than ours inferred from He II spectra. With He I excitation we have indeed also obtained some scattering about a lower BE value for this second peak. But we have deliberately rejected all data obtained with He I in this BE domain: An instrumental cut-off was actually observed b y analyzing electrons with very low kinetic energies. In contrast to He I, with He II

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L. Kubler et al. / UPS of NH~ adsorbed on S i ( l l l )

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excitation, the structure at 10.6 eV is sharp a n d very well d e f i n e d a n d its value r e p r o d u c i b l e . F o r this reason, we use generally H e II s p e c t r a if we need i n f o r m a t i o n f r o m the second a m m o n i a i n d u c e d p e a k at higher B E values (10.6 eV) (fig. 1) a n d H e I s p e c t r a in the low BE d o m a i n (fig. 2). T h e s e l a t t e r are p a r t i c u l a r l y useful to characterize the clean surfaces: I n g o o d a c c o r d a n c e with the literature (see, for example, U h r b e r g et al. [15]) the c h a r a c t e r i s t i c surface

L. Kubler et aL / UPS of N H 3 adsorbed on S i ( l l l )

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Fig. 2. He I (21.2 eV) photoelectron spectra at polar emission angle O: (A) (a) clean c-Si(111)-(7x 7) surface (9 = 0 o ); (b) RT NH 3 exposure (8 = 0 o ); (c) RT NH 3 exposure (8 = 60 o ); (d)-(h) NH 3 exposures at various substrate temperatures (0=60~ (B) (a') clean surface spectrum at 8 = 60 o; (c')-(g') difference curves between exposed (c)-(g) and clean (a') spectra, (C) Insert giving S~ and S2 surface state evolution before (upper curve) and after (lower curve) NH 3 exposure with a dilated BE scale. In all cases saturation exposure (10 L) was achieved.

(S) a n d bulk (B) states of the clean S i ( l l l ) - ( 7 x 7) surface are exhibited (fig. 2a). After initial N H 3 exposure, He I p h o t o e m i s s i o n reveals a r a p i d disapp e a r a n c e of the S2 surface state at 0.8 eV (fig. 2b). The S] c o n t r i b u t i o n ( - 0.2 eV) which is better resolved after the Sz vanishing, as well as the S3 one, decrease only slightly after exposure (insert i n fig. 2). W e also observe strong a n g u l a r d e p e n d e n c e for the level at 4.9 eV (figs. 2b a n d 2c) which is very weak at n o r m a l emission angle, particularly with He I excitation. T h u s m a n y spectra are given at high emission angle (O = 60 o) (figs. 2 c - 2 h ) . T h e b e h a v i o u r of the 4.9 eV peak either as a f u n c t i o n of N H 3 exposure at a given polar emission angle 8 or as a f u n c t i o n of 8 at a given N H 3 coverage suggests the presence of two c o m p o n e n t s . Besides the d o m i n a n t c o m p o n e n t at 4.9 eV, the presence of a smaller feature at higher BE values can be inferred f r o m spectra at low 8 or low exposures ( < 10 L) where the 4.9 eV peak i n t e n s i t y becomes quite small. A c t u a l l y the c o n t r i b u t i o n from that feature

L. Kubler et aL / UPS o f N H 3 a d s o r b e d on Si(l l l)

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results in a polar angle a n d / o r N H 3 exposure dependent asymmetric peak shape and location. Furthermore it is noteworthy that initial ammonia saturation of the surface prevents further oxygen adsorption from residual gases. Thus the oxygen build up on the sample is strongly reduced compared to the usual contamination of a clean surface staying in UHV. Obviously N H 3 adsorption inhibits further adsorption strongly. Interesting observations can be made from the Ts dependent adsorption above RT. With increasing T~ from RT to 310 ~ the strong peak at 10.6 eV becomes broader, decreases and finally disappears almost completely (figs. l d - l g ) . The similar decrease of the peak at 4.9 eV results in an apparent BE shift and allows the observation in the higher BE tail of a contribution at 5.4 eV, particularly obvious if we examine (figs. 1 and 2B) the differences between exposed and clean spectra. In this case only simple subtraction was achieved excluding any other data treatment. This component at 5.4 eV can be easily overlooked if the spectra are not analyzed carefully (i.e. in drawing difference spectra), since it is just located in a valley of the clean Si surface spectrum (figs. lc, 2a and 2a'). Nevertheless it was seen not only on the spectra presented here but also under many other observation conditions, particularly at low exposure and low emission angle where the contribution at 4.9 eV is already minimized at RT as mentioned above. The same shift from 4.9 to 5.4 eV is also observed by annealing at increasing temperature (RT < TA < 400 o C) the Si substrates exposed to N H 3 at RT. A bump in the 7-7.5 eV region is

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L. Kubler et al. / UPS of N H 3 adsorbed on Si(lll)

509

also present after each exposure. Exposures on heated substrates above 300 ~ C result in the appearance of new peaks in the 4.2 eV (figs. 2g, 2g' and 2h) and 7.1 eV regions, for He I and He II, respectively. 3.3. X P S results on S i ( l l l ) - ( 7 •

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Saturation coverage estimates after 10 L exposures at RT using XPS N ls core level intensity give scattered values in the range 0.15-0.30 ML. This scattering seems to be the consequence of subtle residual impurity changes, particularly a few hundredths of ML of Ni before exposure. The corresponding BE is 398.4 eV, a value which is to be compared to the much higher one for molecular adsorption at low T (400.0 eV). With increasing Ts, for constant exposures at 10 L, this coverage passes through a smooth minimum where the line becomes very weak and broad, increasing then again rapidly above 300 ~ C (fig. 3). These coverage variations with T~ are correlated with BE changes reflecting two different surfaces nitridation processes on either side of the minimum near 300~ Above 3 0 0 ~ the N ls BE increases from 397.6 to 398.1 eV with the thickness of the nitride layer.

4. Discussion 4.1. L o w T condensation: molecular spectra

By comparison with the literature [1,3-6,16] our two main peaks concerning NH3, reported in fig. lb, can be assigned to the 3al and le molecular orbitals. It is generally admitted that the 3al orbital is composed mainly of the nitrogen lone pair electrons and the l e orbital of the N - H bonding electrons. Without charging effects, for low coverages (fig. lb), the related UPS features are found at 5.85 and 11.1 eV, respectively. In the case of our multilayer adsorption experiments (solid NH3) the observed resulting energy separation between 3a I and le (5.4 eV) well agrees with the values obtained from gas phase spectra by Weiss and Lawrence (5.4 eV) [17], Potts and Price (5.1 eV) [18], Brion et al. (5.3 eV) [19], from physisorption measurements on various substrates by Campbell et al. (5.4 eV) [16], Jacobi et al. (5.1 eV) [6] and from orbital energy level calculations by Kaplan (5.3 eV) [20]. For the molecules physisorbed on the first layer, in contact with Si, our value is slightly lower (5.25 eV). For metals, under analogous circumstances, the energy separation 3 a l - l e is reported to decrease more strongly: Egawa et al. give 4.1 eV for W [7], Seabury et al. 4.3 eV for N i ( l l l ) [1], Jacobi et al. 3.9 eV [6], Grunze and coworkers 4.5 eV for N i ( l l 0 ) [3], and 4.4 eV for Fe [5], and Purtell et al. 3.6 eV for Ir [21].

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L. Kubler et al. / UPS of NH 3 adsorbed on Si(lll)

Moreover, available data of physisorbed and chemisorbed forms on silicon substrates for another gas, i.e. H20, also show such a decrease: Schmeisser and Demuth [22] obtain for H 2 0 5.8 eV in the physisorbed (or condensed) form at 80 K, in good agreement with our own experiment, and - 4.7 eV for the chemisorbed one at 300 K. These decreases seem to be a general trend when the interaction between the substrate and the molecular lone pair (3a 1 for NH3, b 1 for H 2 0 ) becomes stronger. 4.2. Adsorption above R T

In the T~ range between R T and 3 0 0 ~ we have to recall two main observations with increasing T~ (figs. 1 and 2): - The decrease and disappearance of the two peaks at 4.9 and 10.6 eV which, owing to the apparent similarities between the R T and low T spectra (fig. 1) still reflect the molecular 3al lone pair and the le N - H orbital, respectively. Decreasing the 4.9 eV peak intensity either in increasing T~ or, at RT, by low exposures or with some angular recording conditions allows the observation of another contribution at 5.4 eV, certainly weak, but always present. This is attributed to the well known S i - H monohydride feature [23,24]. The 4.9 and 10.6 eV features reflect the presence of S i - N H x bonds at R T and their enhanced desorption and eventual more complete dissociation with increasing T~. We will discuss later (section 4.3) the exact nature of these up to now unknown species labeled N H x ( X = 1, 2 or 3). The precedent interpretation is in agreement with the XPS results (fig. 3) where the smooth intensity minimum below 300 ~ and the BE changes from 398.4 to 397.6 eV are explained by progressive transformation from S i - N H x to (Si)3=N nitride environments. The same conclusions can be drawn from the UPS difference spectra in fig. 2. With raising T~, the drop in the S i - N H x component at 4.9 eV is associated with the appearance of a broad peak in the 4 eV region attributed to N adsorption from completely dissociated molecules and S i - N bond formation. Only above 300 ~ C, when the N H x adsorption is negfigible, we can observe the strong increase of the XPS N ls signal (fig. 3) and of the UPS features at 4.2 eV (figs. 2g and 2h) marking the start of true thermal nitridation. For higher Ts the S i - N broad component, near 4 eV for the early nitridation stages at 300 ~ is reinforced (fig. 2h) and progressively shifted to higher BE values up to - 4.8-5 eV below E v, when a Si3N 4 layer develops. K~ircher et al. [25] find 4.9 eV for the N 2p~ lone pair band and 7.5 eV for the N 2pxy states in bulk Si3N 4. Similarly the 7.1 eV peak observed in our work with H e l l excitation in the first nitridation stages shifts progressively towards 7.5 eV with increasing T~ and thus accounts for the N 2pxy contribution. These shifts, together with the N ls one from 397.6 to 398.1 eV in the same conditions, can be explained by a relaxation energy decrease when the matrix -

I~ Kubler et al. / UPS of N H 3 adsorbed on Si(111)

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surrounding the local (Si)3=-N environment changes from a semiconducting one (at the S i ( l l l ) surface for the first nitridation stages) to an insulating one (in bulk nitride). Isu and Fujiwara [13] have also obtained UPS features near 4.2 and 7.2 eV for initial nitridation at high temperature with N H 3. Coming now back to the S i - H bond formation deduced from the UPS feature at 5.4 eV, further arguments can be given in order to corroborate this assignment: - Actually, by preliminary A R U P S measurements we are able to confirm unambiguously the presence of S i - H bonds and therefore dissociation, already after RT N H 3 exposure. Indeed comparing ammonia and atomic hydrogen (obtained by dissociation of H 2 with a W filament) adsorption experiments, not only the same BE values at 5.4 eV and a larger structure around 7.2 eV, but also the same angular signature of the S i - H bonds was obtained. By reactive evaporation of Si under N H 3 ambient we have prepared S i N x : H films in the same low T~ range as for our adsorption study. Similar interactions seem to occur during the film growths: introduction of N - H and S i - H bonding groups is evidenced by the I R associated vibration bands near 3300 and 2000 cm -1, respectively, and very similar XPS N I s shifts are observed as a function of T~ [26-28]. During the preparation of this paper Bozso and Avouris [14] have reported on N H 3 adsorption on clean Si(100)-(2 • 1) surfaces. At 90 K they find two N ls XPS core line locations at 399.7 and 397.7 eV attributed to the adsorption of molecular N H 3 and nitrogen coming from wholly dissociated molecules, respectively. These BE values are consistent with ours on S i ( l l l ) in similar circumstances. The other dissociation product, i.e., hydrogen is supposed to passivate the dangling bond and to prevent further dissociation. These authors infer the presence of S i - H bonds from thermal desorption measurements. In this respect our data clearly establish the formation of S i - H bonds upon ammonia adsorption on Si(lll), but also N H x adsorption, not notified in Bozso's work. All these S i - H bond observations after an N H 3 exposure on clean Si surfaces prove the ammonia dissociation - at least partial - on some Si surface sites. These results bring us back to the question of the nature of the N H x ( X = ?) complexes simultaneously adsorbed in the low T regime, e.g. at RT. -

-

4.3. Nature o f the N H x species

One of the unsettled issues concerns the strong structures (4.9 and 10.6 eV) observed at RT by UPS and attributed above to unknown N H x species ( X = 1, 2 or 3). Owing to the apparent similarities between the spectra taken either at 100 K or at R T (figs. l b and ld) molecular N H 3 R T chemisorption might be suggested. In their letter Isu and Fujiwara [13] came indeed to that

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L. Kubler et al. / UPS of N H 3 adsorbed on Si(111)

conclusion arguing that the features observed at RT cannot be confused with those of N H 3 dissociation products, i.e., hydrogen or nitrogen adsorbed on Si. Yet they do not consider the possibility of N H 2 or N H adsorption as often suggested by other authors for metals [2-4,7,29] and particularly in the presence of electron beams [1,8,12,30]. At first sight one can still recognize in the RT spectrum the S i - N bond electrons at 4.9 eV and the N - H ones at 10.6 eV reflecting apparently the molecular 3a 1 lone pair and the l e N - H orbitals, respectively. Nevertheless the unusual increase in the 3 a l - l e energy separation must be noticed in the present case (5.25 to 5.7 eV) since it is usually found that the 3 a l - l e molecular orbital energy splitting is lowered in passing from physisorbed to chemisorbed N H 3 species. This is not an argument in favour of molecular adsorption. Moreover, our XPS BE determinations clearly show that the N ls value corresponding to RT adsorbed species (398.4 eV) is intermediate between the molecular one at 400.0 eV and that in the nitride environment at 397.6 eV. Molecular adsorption at RT would be very surprising. Grunze and coworkers observed very similar BE shifts for N H 3 adsorption on Ni [3] and W [4] interpreted as coming from N H 2 and N H fragments. They attribute the peaks at 398.8 and 397.8 eV on W ( l l 0 ) to the N H 2 and N H forms, respectively, the nitride form being at 397.3 eV. All these findings strongly suggest the formation, upon RT N H 3 adsorption, of partly dissociated species NH2 or NH. In order to better characterize these adsorbed forms, detailed UPS measurements between 100 K and RT, as well as other techniques such as electron-energy loss spectroscopy (EELS) or electron stimulated desorption ion angular distribution (ESDIAD) would be helpful. For instance using this latter technique Alvey et al. [30] identified N H 2 species on Ni(ll0). 5. Conclusion

This UPS study, which is the first to address the question of the T~ (RT < T~ < 400~ dependent interaction of S i ( l l l ) with N H 3 below the thermal nitridation regime, supports dissociative chemisorption on some surface sites on the (7"• 7) surface: in the whole investigated T~ range the characteristic UPS features of S i - H bonding are detected after exposure. But other species characterized by the more prominent RT features at 4.9 and 10.6 eV are adsorbed on other sites with a saturation coverage ~< 1 / 3 monolayer. These features by comparison with the molecular spectrum obtained by condensation at 100 K might be interpreted as originating from the molecular nitrogen lone pair and the bonding N - H orbital, respectively. Nevertheless, the question remains open whether these adsorbed species are N H 3 molecules, or possible dissociation products, i.e. intermediate N H 2 or N H units or their mixture. XPS data, however, clearly favours the interpretation in terms of dissociated species chemisorption.

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In a n y case, the a m o u n t of this N H x a d s o r p t i o n d e c r e a s e s b y i n c r e a s i n g d e s o r p t i o n w i t h T~ a n d b e c o m e s u n d e t e c t a b l e n e a r 300 ~ C. A b o v e this t e m p e r a t u r e n e w U P S features, r e l a t e d to a d s o r b e d n i t r o g e n in n i t r i d e l o c a l ( S i 3 = N ) environments, resulting from completely dissociated molecules, become detectable. F u r t h e r m o r e , this w o r k e n a b l e s us to u n d e r s t a n d o r c o n f i r m q u a l i t a t i v e l y m a n y p r e v i o u s results o n t h e S i - N H 3 s y s t e m : - the X P S N l s c o r e level i n t e n s i t y d e c r e a s e f r o m R T to 3 0 0 ~ in r e a c t i v e S i N x : H f i l m g r o w t h e x p e r i m e n t s [26], the S i - H a n d N - H b o n d s d e t e c t e d b y i n f r a r e d a d s o r p t i o n a n d their T~ d e p e n d e n c e in S i N x : H films [26,28], the E S R s p i n d e n s i t y r e d u c t i o n in S i N x : H films [28] r e l a t e d t o the S i - H b o n d i n g a n d the S 2 s u r f a c e state d i s a p p e a r a n c e a f t e r N H 3 a d s o r p t i o n . F i n a l l y , b e s i d e s its f u n d a m e n t a l interest, w e h o p e t h a t this w o r k will s h e d s o m e light u p o n t h e i m p o r t a n t l o w Ts e l a b o r a t i o n o f a m o r p h o u s S i N x : H films.

Acknowledgment

T h a n k s are d u e to P R O M E C O M E

S.A. f o r s u p p l y i n g the c-Si wafers.

References

[1] C.W. Seabury, T.N. Rhodin, R.J. Purtell and R.P. Merill, Surface Sci. 93 (1980) 117. [2] M. Grunze, P.A. Dowben and C.R. Brundle, Surface Sci. 128 (1983) 311. [3] M. Grunze, M. GoLze, R.K. Driscoll and P.A. Dowben, J. Vacuum Sci. Technol. 18 (1981) 611. [4] M. Grunze, C.R. Brundle and D. Tomanek, Surface Sci. 119 (1982) 133. [5] M. Grunze, F. Bozso, G. Ertl and M. Weiss, Appl. Surface Sci. 1 (1978) 241. [6] K. Jacobi, E.S. Jensen, T.N. Rhodin and R.P Merril, Surface Sci. 108 (1981) 397. [7] C. Egawa, S. Saito and K. Tamaru, Surface Sci. 131 (1983) 49. [8] C. Klauber, M.D. Alvey and J.T. Yates, Jr., Surface Sci. 154 (1985) 139. [9] B.M. Biwer and S.L. Bernasek, Surface Sci. 167 (1986) 207. [10] R. Heckingbottom and P.R. Wood, Surface Sci. 36 (1973) 594. [11] C. Maillot, H. Roulet and G. Dufour, J. Vacuum Sci. Technol. B2 (1984) 316. [12] A. Glachant and D. Saidi, J. Vacuum Sci. Technol. B3 (1985) 985. [13] T. Isu and K. Fujiwara, Solid State Commun. 42 (1982) 477. [14] F. Bozso and P. Avouris, Phys. Rev. Letters 57 (1986) 1185. [15] R.I.G. Uhrberg, G.V. Hansson, J.M. Nichols, P.E.S. Persson and S.A. FlodstrSm, Phys. Rev. B31 (1985) 3805. [16] M.J. Campbell, J. Liesegang, J.D. Riley, R.C.G. Leckey and J.G. Jenkin, J. Electron Spectrosc. Related Phenomena 15 (1979) 83. [17] M.J. Weiss and G.M. Lawrence, J. Chem. Phys. 53 (1970) 214. [18] A.W. Potts and W.C. Price, Proc. Roy. Soc. (London) A326 (1972) 181. [19] C.E. Brion, A. Hammet, G.R. Wight and M.J. van der Wiel, J. Electron Spectrosc. Related Phenomena 12 (1977) 323.

514 [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [301

L~ Kubler et al. / UPS of N H 3 adsorbed on S i ( l l l )

H. Kaplan, J. Chem. Phys. 26 (1957) 1704. R.J. PurteU, R.P. MeriU, G.W. Seabury and T.N Rhodin, Phys. Rev. Letters 44 (1980) 1279. D. Schrneisser and J.E. Demuth, Phys. Rev. B33 (1986) 4233. K. Fujiwara, Phys. Rev. B26 (1982) 2036. R. Butz, E.M. Oellig, H. Ibach and H. Wagner, Surface Sci. 147 (1984) 343. R. K~cher, L. Ley and R.L. Johnson, Phys. Rev. B30 (1984) 1896. L. Kubler, R. Haug, J.J Koulmann, D. Bolmont, E.K. Hlil and A. Jaegle J. Non-Cryst. Solids 77/78 (1985) 945. L. Kubler, E.K. Hhl, D. Bolmont and J.C. Peruchetti, Thin Solid Films (1987), in press. L. Kubler, J.J. Koulmann, R. Hang and A. Jaegle, Sohd State Commun. 48 (1983) 61. M. Weiss, G. Ertl and F. Nitschke, Appl. Surface Sci. 2 (1979) 614. M.D. Alvey, C. Klauber and J.T. Yates, Jr., J. Vacuum Sci. Technol. A3 (1985) 1631.