Local structure determination of NH
2
on Si„ 111… - „ 7 Ã 7 …
S. Bengio
´
,
1
H. Ascolani,
1
N. Franco,
3
J. Avila,
2
M. C. Asensio,
2,3
A. M. Bradshaw,
4
and D. P. Woodruff
4,5
1
CONICET and Centro Ato ´mico Bariloche, Comisio ´n Nacional de Energı ´a Ato ´mica, 8400 Bariloche, Argentina
2
Instituto de Ciencia de Materiales, CSIC, Cantoblanco, 28049 Madrid, Spain
3
LURE, Ba ˆt. 209D, Universite ´ Paris-Sud, F91405 Orsay, France
4
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany
5
Physics Department, University of Warwick, Coventry CV4 7AL, United Kingdom
Received 6 October 2003; revised manuscript received 13 January 2004; published 30 March 2004
N1 s scanned-energy mode photoelectron diffraction has been used to determine the local adsorption ge-
ometry of adsorbed NH
2
species on Si(111)(7 7) resulting from reaction with NH
3
at room temperature. The
results show that NH
2
is adsorbed almost exclusively atop Si surface rest atoms with a Si-N bond length of
1.710.02 Å and very little modification of the geometry of the Si atoms in the layer below. Any coadsorbed
NH on the surface is either of low relative coverage or is also adsorbed in local atop sites. There is evidence
that a small fraction (8 7%) of the NH
x
species may occupy sites atop Si surface adatoms.
DOI: 10.1103/PhysRevB.69.125340 PACS numbers: 68.43.-h, 82.45.Jn, 61.14.Qp
I. INTRODUCTION
The interaction of ammonia with Si surfaces has attracted
quite a lot of attention in the last 10–15 years, motivated in
part by the interest in silicon nitride and the potential utility
of this material in microelectronics. For the purposes of sur-
face nitridation relatively high sample temperatures are used,
but in an attempt to gain a clearer understanding of the un-
derlying surface chemistry, lower temperature investigations
have also been conducted. In particular, the initial stages of
the reaction of ammonia with the reconstructed clean
Si(111)(7 7) surface have been investigated by using a
range of different experimental techniques including ultra-
violet photoelectron spectroscopy UPS,
1
high-resolution
electron energy loss spectroscopy HREELS,
2,3,4
core level
photoelectron spectroscopy using conventional x-ray
sources,
5,6
and soft x-ray synchrotron radiation SXPS,
7,8
scanning tunneling microscopy STM,
9
Auger electron
spectroscopy AES,
10
and temperature programmed
desorption.
3,4
There was an early consensus that at room
temperature NH
3
dissociates to coadsorbed amino (NH
2
)
and atomic hydrogen, and indeed this reaction appears to
occur even at 100 K. At much higher temperatures complete
dissociation occurs, leading to atomic nitrogen being left on
or in the surface. More recent HREELS investigations,
3,4
however, have found evidence for both adsorbed NH
2
and
coadsorbed NH, even at room temperature, and over a range
of surface coverage. High resolution SXPS studies of the N
1 s emission after room temperature adsorption have also
identified two principal states with a photoelectron binding
energy difference of approximately 0.8 eV which have been
attributed to these NH
2
and NH species.
8
Studies of the interaction of NH
3
with the reconstructed
Si(100)(2 1) surface have also shown clear evidence of
dissociation to coadsorbed NH
2
and H at room temperature,
although on this surface there is no evidence of further dis-
sociation at this relatively low temperature. A quantitative
structure determination
11,12
by scanned-energy mode photo-
electron diffraction PhDRef. 13 found that the NH
2
spe-
cies bonds to one end of the Si surface dimers of this surface
such that the N atom occupies an off-atop site with a Si-N
bond length of 1.730.03 Å and a tilt of this bond relative to
the surface normal of 214°. It is generally believed that on
Si100 the H atom removed from the dissociating NH
3
is
bonded to the Si atom at the other end of the surface dimer,
and indeed that it is the interaction of the incoming NH
3
species with the two dangling bonds at either end of a Si
surface dimer which enables the dissociation, a view sup-
ported by theoretical calculations.
14
In the case of the Si(111)(7 7) surface, the only struc-
tural information concerning the adsorption geometry of the
adsorbed NH
x
species is largely based on speculation. In
particular, it has generally been assumed that both NH
2
and
H resulting from the initial dissociation must adopt singly
coordinated atop sites similar to the behavior on Si(100)
(2 1)], while the NH species should adopt a twofold
coordinated bridging site requiring a local modification of
the underlying Si surface structure. Of course, the structure
of the Si(111)(7 7) surface is significantly more compli-
cated than that of the Si(100)(2 1) surface which simply
comprises pairing of atoms of the bulk-terminated structure
each having two dangling bonds to produce surface dimers
of Si atoms having only one dangling bond each. There is a
broad consensus that the structure of the Si(111)(7 7) sur-
face is the dimer-atom-stacking fault DAS model first pro-
posed by Takayanagi et al.
15
on the basis of high energy
electron diffraction data. This structure is shown schemati-
cally in Fig. 1. As implied by the name, there are three key
ingredients in the reconstruction which serve to reduce the
number of Si surface dangling bonds which would, for a
bulk-terminated structure, be one per surface atom and thus
49 dangling bonds per unreconstructed 7 7 surface unit
mesh. The formation of surface dimers by pairing of nearest
neighbor Si surface atoms that lie along the boundaries of the
two halves of the 7 7 unit mesh removes all the dangling
bonds of these atoms. The addition of Si adatoms A in Fig.
4 bonded to groups of three surface Si atoms labeled 1 in
Fig. 4 and hereafter referred to as Si
1
) reduces the number of
dangling bonds in these groups from three one per surface
Si
1
atom below the adatom to one at the adatom. In addi-
PHYSICAL REVIEW B 69, 125340 2004
0163-1829/2004/6912/1253409/$22.50 ©2004 The American Physical Society 69 125340-1