Ultraviolet photoemission spectroscopy of NH3 and NO on Si(111) surfaces

Solid State Communications, Vol. 42, No. 6, pp. 477--479, 1982. Printed in Great Britain.

0038-1098/82/180477-03 $03.00/0 Pergamon Press Ltd.

ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY OF NH3 AND NO ON Si(1 1 1) SURFACES T. Isu and K. Fujiwara Central Research Laboratory, Mitsubishi Electric Corporation, Amagasaki, Hyogo 661, Japan

(Received 14August 1981 by A.A. Maradudin) Ultraviolet photoemission spectroscopy has been applied for the study of Si(1 1 1) surfaces covered with NH3 and NO. Different photoemission spectral features were observed for the chemisorption states, depending on substrate temperatures at 3 0 0 - 1 5 0 0 K. The results for the NH3 chemisorption are consistent with the model that NHa molecules are adsorbed in a molecular state at 300 K, being distinct from the atomic N and H chemisorption states. THERE HAS BEEN a considerable interest about chemisorption states of various gases on silicon surface. This is partly because of the technological importance in the fabrication of semiconductor devices. The initial stage of oxidation as well as nitridation of low index silicon surfaces is an important subject to make clear the chemical bonding natures in relation to elucidating the Si-SiO2 or Si-Si3N4 interface electronics. In the past decade extensive investigations have been made to understand the physics and chemistry behind the elemental processes of the oxidation [1 ]. However, in spite of increasing technological importance, very few works have been reported so far for the study of the nitride, especially the interactions of simple molecules such as NHa with silicon surfaces [2, 3]. In this paper, chemisorption states of NHa and NO on thermally cleaned Si(l I 1) surfaces have been studied by ultraviolet photoemission spectroscopy (UPS). Changes of the photoemission spectral features induced by adsorbates are examined in detail at 3 0 0 - 1 5 0 0 K. It is found that UPS features for the room temperature NH3 chemisorption state are distinct from those expected for the dissociative chemisorption states. The results are consistent with our previous proposal [4] that NH3 molecules are nondissociatively adsorbed in a single state at room temperature. From the study of annealing effects on the NH3 and NO chemisorption states, it is shown that thermal treatment above 1000 K resulted in an atomic nitrogen chemisorption state on Si(1 1 1) surfaces. UPS measurements were performed in an ultrahigh vacuum (UHV) chamber with the base pressure of 3 x 10 -s Pa (2 x 10-1° torr). The light source for UPS was a differentially pumped glow-discharge He resonance lamp operated at a photon energy hco = 21.2 eV (He I) with the main chamber near 8 x 10 -7 Pa. Kinetic energy distribution curves of the photoelectrons were obtained 477

in an angle averaged form with a spherical sector type analyzer and standard pulse counting electronics. All energies in the UPS spectra were measured relative to the vacuum level Evae, and the binding energy E B was determined assuming that the work function of the clean Si(1 1 1) surface is 4.7 eV [5]. Further details concerning the experimental apparatus will be described elsewhere [6]. An atomically clean Si(1 1 1) surface was prepared by indirect heating of the sample around 1500 K in UHV with the same method as described previously [5, 6]. Cleanness of the initial surface was checked in situ with Auger electron spectroscopy (AES) and UPS. The coverage of the adsorbates was monitored with AES. Figure 1 shows the UPS spectra for the clean Si(1 1 1) surface and the surfaces after NH 3 exposures at room temperature. For the clean surface, two characteristic bulk peaks were observed at 2.9 eV 0(4) and 7.4 eV (L 1) below Fermi level EF in the UPS spectrum in agreement with the previous observation [6]. In addition, two shoulders were also at 0.8 and 2.0 eV below EF in the spectrum. These are attributed to the intrinsic surface states which are the characteristics of the Si(1 1 1) 7 x 7 surface [7, 8]. When the clean Si(1 11) surface was exposed to NH3 gas, the coverage of the adsorbate was determined by monitoring the intensity of N-KLL Auger peak at 379 eV. Saturation was reached at roughly 102 L (1 L = 1 0 - 6 torr" sec) exposures. After NH3 exposures at room temperature, the photoemission due to the surface states decreased, while new peaks appeared at 3.2, 4.9, 7.2 and 9.4 eV below E F in the UPS spectra. Beyond the 102 L exposures we observed no further changes of the UPS spectral features. So the UPS spectrum is assumed to correspond to the NH3-saturated Si(11 1) surface in agreement with the AES result. The intensity of the peak at -- 3.2 eV saturated at the early stage of the

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OF NHs AND NO ON Si(l11)

S~(lll)-NH, UPS

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Fig. 1. Family of He I UPS spectra N(E) for the Si(ll1) surface before and after successive exposures to NHa molecules at room temperature: (a) 0 L; (b) 1 L; (c) 10 L; (d) 10’ L.

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Fig. 3. He I UPS spectra N(E) for the NO chemisorbed surface after isochronal annealing at each temperature: (a)300K;(b) lOOOK; 1150K.

NH3

He1

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Fig. 2. He I UPS spectra N(E) for the NH, chemisorbed surface after isochronal annealing at each temperature: (a) 300 K; (b) 900 K; (c) 1000 K. exposure while those of the other three peaks increased monotonously with the exposure. However, the energies of these peaks appeared to be independent of the

coverage. This suggests that NH3 molecules are predominantly chemisorbed on the Si(l1 1) surface in a single state at room temperature. The adsorbate-induced features in the UPS spectrum arise in the initial states and are correlated to the development of the extrinsic surface electronic states. But the peak at - 3.2 eV is not likely to be a real new structure associated with the local density of states @DOS) of the chemisorbed NHs. Since the intensity of the peak is irrelevant to the adsorbate coverage, it is tentatively attributed to the Si-3p states inside the topmost layer caused by the presence of the adsorbate. The interpretation of the other three adsorbate-induced peaks is difficult now because no theoretical calculations of the system, NH, on the Si(ll1) surface, are available to date. However, it is clear that the three features in the surface LDOS are different from those of the Si(1 1 1) surface covered with atomic hydrogen only [8,9], atomic nitrogen only (discussed later), or with a linear combination of them, as compared in Fig. 1. Therefore, it is concluded that NHa molecules are non-dissociatively chemisorbed on the Si( 111) surface. Further evidence for the non-dissociative chemisorption is obtained from the effects of annealing on the NHa-saturated surface at higher temperatures. The UPS spectra for the NH,-saturated surface were observed to change by thermal treatment. Annealing at 900 K, the

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SPECTROSCOPY OF NH3 AND NO ON Si(1 1 1) SURFACES

UPS spectral features were completely changed into the different ones as shown in Fig. 2. After annealing at IO00K, the UPS spectrum exhibited a big peak at 4.2 eV and two shoulders at -- 7.2 and -- 9.8 eV. These results indicate a substantial change of the chemi. sorption state upon annealing. Nitrogen atoms still remained on the surface as evidenced by AES measurements. According to the gasvolumetric measurements by Boonstra [ 10], the room temperature chemisorption state of NH 3 on silicon surfaces are unstable at higher temperature. Above 800 K all hydrogen atoms desorb as H2 molecules, while nitrogen atoms remain on the surface. These facts are in accordance with the present results. Therefore, it is evident that the UPS spectrum after annealing the NHa-saturated surface at 1000 K corresponds to the surface covered with nitrogen atoms only. Further convincing evidence for the existence of the atomic nitrogen chemisorption state is given by investigating the NO chemisorption on Si(1 1 1) surface. In the case where NO molecules were exposed to the clean Si(1 1 1) surface at room temperature, AES results showed that saturation was reached at about 5 L. The UPS spectrum for the surface reveals a single big peak at 6.5 eV below b.~ as shown in Fig. 3. The overall similarity of the UPS spectra for the atomic oxygen [5] and NO chemisorption states is noticeable as is the case for the ELS spectra [4]. After annealing this surface at 1000 K, a weak shoulder increased at -- 4.2 eV. If the NO chemisorption state is molecular at room temperature [4], this change is attributed to the dissociation of NO molecules, resulting in the Si-O and S i - N covalent bond formations. This is because the Si-N bonds enhance the photoemission intensity at -- 4.2 eV. Further annealing at 115OK, the peak at -- 6.5 eV disappeared, and a new peak and two shoulders appeared at --4.4, -- 7.2 and -- 9.8 eV, respectively. This spectrum seems to be very similar to the curve (c) in Fig. 2, and is -

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attributed to the atomic nitrogen chemisorption state since oxygen atoms can desorb above 1150 K from the silicon surface [5]. In fact the surface after annealing at 1150K showed no O-KLL Auger peak at 510eV as measured by AES, while no significant changes were observed for the intensity of the N-KLL Auger peak at 379 eV. This means that the desorption of oxygen atoms resulted in the formation of the atomic nitrogen chemisorption state on the Si(1 1 1) surface. In summary, NHa and NO chemisorptions on thermally cleaned Si(1 11) surfaces and their reactions at 3t,3-1500 K have been studied by UPS. It has been shown that NH 3 molecules are nondissociatively chemisorbed on the Si(l 11) surface in a single state at room temperature. The results for the annealing effects indicate that the room temperature NH3 and NO chemisorption states substantially change into the different ones, resulting in the atomic nitrogen chemisorption state at 1150 K. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

S.T. Pantelides (Editor), Physics of Si02 and Its Interfaces. Pergamon, New York (1978). J.F. Delord, A.G. Schrott & S.C. Fain, Jr., J. Vac. Sci. Technol. 17, 517 (1980) (and references therein). For a general review of silicon nitride, see C.E. Morosanu, Thin Solid Films 65, 171 (1980). M. Nishijima & F. Fujiwara, Solid State Commun. 24, 101 (1977). K. Fujiwara & H. Ogata, Surf. Sci. 86,700 (1979). K. Fujiwara, Solid State Commun. 36,241 (1980); K. Fujiwara, Surf. Sci. 108,124 (1981). D.E. Eastman, F.J. Himpsel & J.F. van der Veen, Solid State Commun. 35,345 (1980). K. Fujiwara, Phys. Rev. B 24, 2240 (198 I). T. Sakurai & H.D. Hagstrum, Phys. Rev. B12, 5349 (1975). A.H. Boonstra, PhilipsRes. Rept. Suppl. No. 3 (1968).