Dihydride phase in Si(1 1 0) surface

Solid State Communications, Vol. 56, No. 10, pp. 877-880, 1985. Printed in Great Britain.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

DIHYDRIDE PHASE ON Si(1 1 0) SURFACE ~. Katircio~lu and S. Ciraci Department of Physics, Middle East Technical University, Ankara, Turkey (Received 10 July 1985 by P.H. Dederichs) A bonding model is proposed for the dihydride phase forming on the hydrogen covered Si(1 1 0)-(1 x 1) surface. The state densities calculated for this model is able to explain the room temperature ultraviolet photoemission spectrum without invoking the unconventionally bonded hydrogen.

BECAUSE OF TECHNOLOGICAL importance the interaction of hydrogen with silicon surfaces have been studied extensively during the last decade. Common to these studies was the chemisorption bond between the surface Si and the atomic H terminating the surface dangling bond. The monohydride phase (i.e. Sill-one H atom is attached to a single surface Si) was identified on the Si(1 1 1) surface by the strongly localized states occurring at -- 4.8 and -- 7.0 eV below the maximum of the valence band (VBM). In addition to that, Pandey et al. [1] pointed out the corrosive modification of a H-covered Si(1 1 1)-(1 x 1) surface leading to the trihydride phase (i.e. Sill3: three H atoms bonded to a single surface Si). In spite of the fact that the stability of Sill3 was seriously questioned [2], theoretically it is distinguished from the monohydride phase by the chemisorption states occurring at --6 and --10eV. Apart from Sill and Sill3 configuration, Sacurai and Hagstrum [3] reported an interesting observation regarding the H-covered Si(1 0 0)-(2 x 1)surface: hydrogen adsorbed on this surface at high temperature (~ 250°C) gives rise to a monohydride-like ultraviolet photoemission spectrum (UPS). Upon lowering the temperature under continuing H exposure the (2 x 1) surface superstructure is destroyed, and concommitantly the photoemission intensity undergoes a change. They associated this change of the surface spectrum with the formation of a new phase (i.e. Sill2 - dihydride phase: two hydrogen bonded to a surface Si) that is unstable at high temperature. The exposure of the clean Si(1 1 0) surface to atomic H exhibited also a similar transition depending upon the surface temperature. While a UPS spectrum identical to that of the monohydride phase develops at high temperature (~ 300°C), the lowering of the surface temperature causes the photoelectron emission to enhance in the energy range from --3 eV to -- 7 eV, but caused the high binding energy peak to become less pronounced [4]. This time however, the change in the electronic structure was interpreted to

imply the formation of a completely different binding structure, i.e. an unconventionally (or weakly)bonded hydrogen. This interpretation by Sacurai et al. [4, 5] was reinforced by the self.consistent calculations resulting in a bonding configuration of the H atom above the Si(100) surface, but forming non-directional bonds with the second layer Si atoms [6]. Recently, Ciraci et al. [7] carried out the local density-of-states calculations (LDOS) of the clean and H-covered Si(1 00) surfaces, and were able to explain the evaluation of the low temperature UPS spectrum from the spectrum of the monohydride. Furthermore, by drawing attention to the similarity between the UPS spectra obtained from the H-covered Si(1 00)(1 x 1) and Si(1 1 0)-(1 x 1) surfaces, they pointed out Sill2 (rather than unconventionally bonded I-I) to be a possible origin of a different binding energy structure at room (or low) temperature. Interestingly, their argument was justified by the high resolution electron energy loss spectrum (EELS) taken from the H-covered Si(1 1 0) surface at room temperature: Butz et al. [8] were able to identify a vibrational mode at 900 cm -1 related to the scissor mode of Sill2 in addition to the stretching and binding modes of the Si-H bond. This is considered a strong evidence demonstrating that the dihydride phase forms on the Si(1 1 0) surface. In order to understand the bonding configuration connected with high and low temperature spectra, here we study the electronic structure of the clean and H-covered Si(1 1 0) surface. For the low temperature phase we propose a binding configuration, whereby one of the surface backbonds is broken to allow a Sill2 to form. The main features of the state distribution calculated for the configuration are in agreement with the experimental spectra. In addition, our estimation for the desorption energy from the low temperature phase lies in the range of the experimental data [4]. The state densities are calculated by using the empirical tight binding method with the energy parameters used

877

878

DIHYDRIDE PHASE ON Si(1 1 0) SURFACE

Vol. 56, No. 10

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Fig. 1. Bonding configurations for monohydride and dihydride phases. The surface unit cell is shown by dashed lines. The black and open circles indicate the surface and second layer Si atoms. previously to investigate the mono- and trihydride phase [9]. The clean and H-covered Si(110)surface are simulated by slabs having two dimensional periodicity of the ideal surface and consisting of 9 atomic layers. Such a slab is found to be thick enough for a proper treatment of both surface and bulk energy structure. Figure I describes the bonding configurations used for the monohydride and dihydride phases forming on the Si(110) surface. For the monohydride phase one H atom is bonded to each surface Si along the tetrahedral direction, but not along the surface normal. In this respect S i ( 1 1 0 ) + H surface differs from both the Si(100) + H and the Si(1 l l) + Hsurfaces. The bonding model developed for the dihydride phase presumes that the atomic H breaks one of the surface Si-Si bond and forms a Sill2. The activation barrier for this process is smaller than the bulk bond energy, 2.2 eV. Once the atomic H is available on the surface the empirical total energy arguments [7, 10] suggest that the breaking of a backbond to produce two Sill2 units from the existing two Sill is energetically favorable, and should relieve energy of ~ 5eV per broken bond. Two H atoms saturating two dangling bonds originated from the broken bond repel each other, so that two surface Si connected by the other surface backbond raises above the surface. The repulsion between two chemisorbed, and thus negatively charged H atoms at a small internuclear distance was revealed by the self-consistent pseudopotential calculations I l l ] . As a result, the surface layer relaxes upwards, and two backbonds (one in the surface layer, the other between the first and second layer) and two S i - H bonds become connected

to a surface Si. These four bonds emerging from Si lie along the close proximity of the tetrahedral directions, and their length are kept in their ideal values. In addition to bond rotations leading to this bonding model one expects that bond lengths undergo a small change. Certainly, the precise geometry allowing the relaxation of the bonds can be obtained by the minimization of the total energy, which is, we believe, beyond the capability of the present method. It should be noted however that not only Sill2 but the mixture of Sill2 and Sill should exist on the surface at room temperature [8, 12]. We expect that such a mixture does not destroy the (I x 1) LEED pattern. Because of the restrictions imposed by the periodic boundary conditions, we treat Sill and Sill2 seperately so that the surface is assumed to be covered only one of these phases. In what follows the calculated electronic structure based on the Sill and Sill2 bonding models are analyzed, and compared with the experimental data. In Fig. 2 the local densities-of-states calculated for the bulk, ideal, Sill and Sill2 covered surface are illustrated. It is seen that upon the chemisorption of H the high density surface states near the VBM is replaced by two sharp peaks at - 4 . 8 e V and - 7 . 5 eV. These two dominant peaks of curve d are associated with the chemisorption bonds formed mainly by S i ( p ) + H(s) and S i ( s ) + H ( s ) orbitals, respectively. The LDOS corresponding to the monohydride of the Si(1 1 1) and Si(1 1 0) surfaces (shown by curve c and d) are very similar, except that the high binding energy peak shifts by 0.5 eV. This indicates that the electronic structure of the monohydride phase are determined by the highly

Vol. 56, No. 10

DIHYDRIDE PHASE ON Si(1 1 0) SURFACE

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Fig. 2. Local density-of-states at: (a) the middle of the slab, (b) the first layer of the clean and ideal Si(1 1 0) surface, (c)the H-covered Si(1 1 1) surface- Si(1 1 1)+ H, (d) the monohydride of the Si(1 1 0) surface, (e) the dihydride of the Si(1 1 0) surface obtained from the bonding structure described in Fig. 1. All density curves are normalized according to the LDOS per two Si atoms. The zero of energy marks the top of the valence band. localized Si-H bond. The direction of this bond with respect to the surface normal has little effect. Minor differences between curve c and d are attributed to the different bonding structure associated with the Si(1 1 1) + H and Si(1 1 0) + H surfaces. As shown in Fig. 2, two Si-H bonds protrude from both ends of the same Si-Si bond on the Si(1 1 0) surface, that is not the case for the Si(1 1 1 ) + H surface. Such a bonding configuration was shown to have even more pronounced effects on the distribution of the chemisorption states of the Si(1 1 0)-(2 x 1) + H surface [7]. The LDOS of the dihydride phase calculated for the bonding structure described in Fig. 1 is, however, drastically different from the monohydride phase (curve e). First of all, the low binding energy peak becomes pronounced and also shifts to higher binding energies by -- 0.5 eV. In addition, a new peak appears at - - 4 eV. These features are connected with the Si(p) + H(s) bond, and are in comply with the H e - I and N e - I difference spectra [5]. In the high binding energy region we see two peaks located at

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Fig. 3. (a) The UPS spectrum of the H-covered Si(l 1 0)(1 x 1) surface at high temperature is reproduced from reference [4]. (b) The LDOS at H and surface Si of the Si(1 1 0)-(1 x 1 ) + H monohydride phase. (c) The UPS spectrum of the H-covered Si(1 1 0)-(1 x 1) surface at low temperature is reproduced from reference [4]. (d) and (e) are H e - I and He-II UPS spectra obtained from Sill2 forming on the Si(1 00)-(1 x 1) surface at room temperature, respectively (references [3, 7]). (f) The LDOS at H and Si atoms of the S i ( l l 0 ) (1 x 1) + 2H-dihydride phase. (g) The LDOS projected to the s-orbitals of surface Si. (h) The contribution of H in the LDOS shown by curve f. (j) The LDOS projected to the p-orbitals of surface Si. (k) The LDOS of Sill2 forming on the Si(1 0 0) surface [7]. 8.5 and -- 11 eV which are associated with the Si(s) + H(s) orbitals. In F i g . 3 the calculated state densities are compared

--

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DIHYDRIDE PHASE ON Si(11 0) SURFACE

with the UPS spectra. As previously noted the LDOS of the monohydride phase is in good agreement with the high temperature spectra [4]. In the low binding energy region, the LDOS of the dihydride phase shown by curve f is compared to the UPS spectrum obtained from the H-covered surface at low temperature (curve c). The H e - I spectra obtained from two different surfaces at low temperature (i.e. Si(1 10)+2H-curve c and S i ( 1 0 0 ) + 2H-curve d) are similar, but due to the secondary emission both are not able to resolve the high binding energy features. However, the He-II spectrum obtained from the Si(10 0) + 2H surface [7] exhibits a high binding energy feature corresponding to the Si(s) + H(s) states. The LDOS of the dihydride phase is projected to the various orbitals contributing to these states. In view of the Si(s)- and Si(p)-orbital contribution the overall spectrum pertaining to Sill2 can be considered in two parts. Finally, the LDOS of Sill2 on the Si(1 00) surface (curve k) was calculated by using the extended Htickel Method [7], and is reminescent of the LDOS corresponding to Sill2 on Si(l 1 0). In conclusion, the bonding configuration proposed for the dihydride phase - a phase unstable at high temperature - is able to reproduce main aspects of the low temperature UPS spectrum. The change from the Sill to Sill2 configuration requires the breaking of the surface backbonds. Two H atoms attached to a surface Si produce additional bonding states and increase the emission near --5 eV. The interaction between Si-H bonds via backbortd causes the chemisorption states to become broader and shift to high binding energies. In this respect, the high energy photoemission spectrum is expected to convey additional information regarding the lower part of the state distribution. If the possibility that Sill and Sill2 coexist is taken into account, the LDOS corresponding to these phases should be averaged. Then, the averaged LDOS contains high density-of-states at about --7.5 eV and become even closer to the low temperature photoemission spectrum. The filling of the

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valley at --6.5 eV, which is predicted by neither Sill nor Sill2 states, is possibly connected with a further erosion of the surface to yield a partial and low coverage of SiHa in equilibrium with Sill2. If SiHa is stable it has strongly localized states at ~ - 6 eV to account for the valley filling. The dihydride phase being unstable at high temperature should give rise to very low energy peak in the desorption spectrum. As pointed out previously [7], the breaking of the Si-H bonds of two neighbouring Sill2 leads to the evaluation of H2 molecule, and subsequently results in the reconstuction of the broken Si-Si bond. Using the bond energy of Sill2 and the dissociation energy of H2 we estimate the energy related to the desorption of H from the dihydride phase to be ~ 1.5 eV.

REFERENCES 1.

K.C. Pandey, T. Sacurai & H.D. Hagstrum, Phys.

Rev. Lett. 35, 25 (1975). 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

R. Butz, R. Memeo & H. Wagner, Phys. Rev. B2S, 4327 (1982). T. Sacurai & H.D. Hagstrum, Phys. Rev. BI4, 1593 (1976). T. Sacurai, K.C. Pandey & H.D. Hagstrum, Phys. Lett. 56A, 204 (1976). T. Sacurai & H.D. Hagstrum, J. Vac. Sci. Technol. 13,807 (1976). J.A. Appelbaum, D.R. Hamann & K.H. Tasso, Phys. Rev. Lett. 39, 1487 (1977). S. Ciraci, R. Butz, E.M. Oelling & H. Wagner, Phys. Rev. B30, 711 (1984). R. Butz, E.M. Oelling, H. Ibach & H. Wagner, Surf Sci. 147, 343 (1984). K.C. Pandey, Phys. Rev. B14, 1557 (1976). D.C. Allan, J.D. Joannopoulos & W.B. Pollard, Phys. Rev. B25, 1065 (1982). K.M. Ho, M.L. Cohen & M. Schltiter, Phys. Rev. B1S, 3888 (1977). H. Wagner, R. Butz, U. Backes & D. Bruchmann, SolidState Commun. 38, 1155 (1981).