PHYSICAL REVIEW B
VOLUME 59, NUMBER 19
15 MAY 1999-I
„A33 A3… R30° reconstruction of the 6H-SiC „0001… surface: A simple T4 Si adatom structure
solved by grazing-incidence x-ray diffraction
A. Coati
LURE, CNRS-MENRS-CEA, Bâtiment 209d, Centre Universitaire Paris-Sud, BP34, 91898 Orsay Cedex, France
and INFM Dipartimento di Fisica, Università di Padova, via Marzolo 8, 33100 Padova, Italy
M. Sauvage-Simkin*
LURE, CNRS-MENRS-CEA, Bâtiment 209d, Centre Universitaire Paris-Sud, BP34, 91898 Orsay Cedex, France
and Laboratoire de Minéralogie-Cristallographie, CNRS, Universités Paris 6 et 7, 4 Place Jussieu, 75252 Paris Cedex 05, France
Y. Garreau
LURE, CNRS-MENRS-CEA, Bâtiment 209d, Centre Universitaire Paris-Sud, BP34, 91898 Orsay Cedex, France
R. Pinchaux
LURE, CNRS-MENRS-CEA, Bâtiment 209d, Centre Universitaire Paris-Sud, BP34, 91898 Orsay Cedex, France
and Université Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
T. Argunova
Ioffe Physico-Technical Institute of the Russian Academy of Sciences, Polytekhnicheskaya Street 26, 194021 Saint Petersburg, Russia
K. Aı̈d
LURE, CNRS-MENRS-CEA, Bâtiment 209d, Centre Universitaire Paris-Sud, BP34, 91898 Orsay Cedex, France
~Received 1 February 1999!
The atomic structure of the ( A33 A3) R30° reconstructed surface of 6H-SiC ~0001! silicon terminated, is
solved by grazing-incidence x-ray diffraction in ultrahigh vacuum. The simple adatom structure with one
silicon atom per reconstructed unit cell sitting in a T4 site over the top SiC bilayer is the only one compatible
with the data, thus ruling our other models involving Si adatoms in H3 site, Si trimers, C adatoms, or Si
vacancies, and supporting the most recent theoretical predictions. The predominance on the surface of three
bilayer steps with an A-type termination of the bulk stacking is fully confirmed. In addition, it is shown that
varying the preparation conditions changes the density of faulted boundaries in the reconstructed surface but
preserves a unique atomic structure within the ( A33 A3) R30° ordered domains. @S0163-1829~99!00519-6#
Silicon carbide is one of the most promising materials for
high-temperature, high-power electronics, and although superseded by gallium nitride in the blue laser-diode production challenge, it still occupies a key position in that field as
a possible substrate for gallium-nitride epitaxial growth.
Among the various polytypes the cubic b SiC and the two
hexagonal forms 4H and 6H are the most studied. Homoepitaxial growth, interface formation, and heteroepitaxial
growth are mandatory steps in device elaboration, which require a perfect knowledge and control of the substrate surface. Accordingly, the surface structures of SiC templates
have been the subject of many investigations. Since the pioneering paper of van Bommel, Crombeen, and van Tooren1
on the hexagonal polytypes ~0001! surface, numerous studies
have been performed and reviews of the recent progresses
are available.2,3 In the particular example of the Siterminated 6H SiC ~0001! ( A33 A3) R30° reconstructed
surface, object of the present paper, a simple adatom model
with a single adsorption site was inferred from highresolution scanning tunneling microscopy ~STM!
experiments.4 However, STM does not allow to discriminate
between the various possible sites ~T1, T4, or H3!; neither
does this technique provide a chemical identification of the
0163-1829/99/59~19!/12224~4!/$15.00
PRB 59
adsorbed species. Theoretical calculations5,6 by ab initio
methods selected the Si adatom in the T4 site as the lowest
energy structure compared to several other models including
the Si adatom in the H3 site, the C adatom in the T4 site,
and the Si or C trimers centered on a T4 site. The calculated
electronic structure for the T4 adatom model within the oneelectron local-density approximation leads to a half-filled
dangling-bond-derived band corresponding to a metallic surface, whereas both direct7 and inverse8 photoemission experiments have given evidence for fully occupied and totally
empty electron states characteristic of a semiconducting surface, thus questioning the validity of the model. However,
Northrup and Neugebauer9 have recently shown that when
electronic correlation effects are taken into account on the
basis of a Hubbard model, a splitting of the dangling-bondderived states can be obtained in agreement with the experimental band structure.7,8 Nevertheless, a satisfactory explanation is still missing for the two surface-shifted components
on the C1s peak revealed by high-resolution core-level
photoemission,10 which cannot be readily accounted for with
an Si adatom sitting in a T4 site on a complete SiC bilayer.
The need for accurate data on the surface and near-surface
atomic arrangement is thus patent. The present experiment
12 224
©1999 The American Physical Society
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BRIEF REPORTS
brings the first full determination of the atomic structure for
the ( A33 A3) R30° reconstructed surface, and definitely assesses the validity of the simple Si adatom in the T4 site
model by use of grazing incidence x-ray diffraction. Being
sensitive to long-range order at and near the surface, this
method is the most appropriate tool to complement the STM
information.
The sample was a thin wafer cut out of a 6H SiC single
crystal grown at the Ioffe PhysicoTechnical Institute ~St. Petersburg, Russia! by a modified Lely method. After a series
of alternate mechanical and chemical polishing, the sample
was submitted to hydrogen etching at 1600 °C in order to
remove the polishing damage. After introduction into the
UHV chamber, a rather sharp 131 low-energy electron diffraction ~LEED! pattern was observed, although Auger spectra and x-ray diffraction rod analysis confirmed the presence
of a thin residual-oxide layer. The sample was further deoxidized by vacuum annealing at 1000 °C and exposed at room
temperature to the silicon flux from a Knudsen cell prior to a
final annealing at about 1000– 1050 °C. At this step, a sharp
( A33 A3) R30° diffraction pattern was observed by LEED,
without tracks of any other reconstruction, and a ratio of
about 2.3 between Si LVV and C KLL Auger lines was
measured in agreement with previous LEED Auger experiments on this surface.11 The sample was transferred in situ
onto the diffraction stage of the W12 surface diffraction
beamline ~LURE-DCI, Orsay-France! and laser aligned with
the surface normal parallel to the diffractometer rotation axis
v. Diffraction data were collected at an energy of 15 keV, at
the critical angle for total external reflexion ~0.15°!. The data
set comprises 44 in-plane fractional intensities resulting in
13 independent experimental structure-factor values together
with 43 fractional and 42 integer out-of-plane independent
rod data. As expected, the in-plane measurements showed a
sixfold symmetry, but fractional rods that should be inequivalent due to the actual threefold symmetry of the
( A33 A3) R30° structure, namely rods related by 60 or 180°
rotations such as 31 43 l and 13 43 l, were measured identical.
Such apparent sixfold symmetry, already observed in LEED
I(V) measurements,12,13 has been assigned to the presence of
steps on the surface. The bulk structure14 of 6H SiC can be
described as a periodic stacking along the c axis of sequences
of six bilayers such as . . . AbBcCaAcCbBa . . . with reference to the standard description of closed-packed structures
and where capital ~respectively low case! letters denote silicon ~respectively, carbon! layers. For an Si-terminated bulk,
six different terminations are thus possible and ought to be
present in the assumption of single bilayer steps. However,
recent STM data have shown that most of the steps were of
three-bilayer height.13 The bulk termination could be either
couples of AbBcCaAcCbBa . . . and AcCbBaAbBcCa . . .
terraces ~referred to as the AA termination! or BcCaAcCbBaAb . . . and CbBaAbBcCaAc . . . terraces ~BC termination! or CaAcCbBaAbBc . . . and BaAbBcCaAcCb . . . terraces ~CB termination!. The LEED quantitative data12 were
in favor of the AA termination, which corresponds to fcclike stacking within the first three bilayers. It should be remarked that as far as the surface structure can be described
by one adlayer on a single SiC bilayer, which is the case for
most feasible models, either pair of terraces ~AA, BC, or CB!
12 225
FIG. 1. Three models for the reconstructed surface used in the data
refinement. Left panel: top view; right panel: side view. The Si adatoms in
the top view are represented as large empty circles, silicon and carbon atoms
in the successive bilayers appear, respectively, as empty and full small
circles. The notation for the distances are those used in Refs. 5 and 6.
should lead to the same conclusions, and in a first step it was
decided to analyze the x-ray data within the AA hypothesis.
A major drawback of the presence of such steps lies in the
fact that it is no longer possible to discriminate between the
T4 and H3 sites for the simple adatom structure by direct
inspection of the fractional rods. Indeed, calculations made
for the theoretical6 values of the position parameters in the
T4 and H3 Si adatom cases have shown that a fractional rod
hkl (H3) is almost identical to the fractional rod h̄k̄l (T4),
averaging hkl and h̄k̄l values to account for the apparent
sixfold symmetry should thus lead to the same rod profile in
either case. The minute differences induced by the rumpling
Dz in the carbon layer, allowed in the T4 adatom case ~Fig.
1!, is far beyond the accuracy of the surface x-ray measurements. The answer should then be derived from integer rods,
which are both sensitive to the bulk termination and to the
adsorption site.
The in-plane fractional data were of high quality with full
width at half maximum Dq;1022 Å 21 , leading to a reconstructed domain size (2 p /Dq) of 800–600 Å in the successive preparations of the surface. Although Si and C are rather
light elements, the signal over background in the fractional
diffraction peaks could reach 200 counts per second. An error of 10% was assigned to the in-plane data to take into
account the dispersion of equivalent reflexions. In-plane data
were first refined to determine the projected positions of the
atoms in the surface unit cell. The three models shown in
Fig. 1 have been tested, Si adatoms in T4 or H3 sites and an
Si trimer, plus a C adatom in a T4 site and the vacancy
model proposed by Li et al.15 where 1/3 ML Si atoms is
removed from the last bilayer.
Three layers were introduced in the surface cell: the adlayer and the terminal SiC bilayer. Since data were collected
with a small nonzero l component (l50.2), the average between the two possible terraces was considered and Fig. 2
shows the two surface cells used in the case of the T4 sites:
surface cell 1: adatom in B, silicon in the top bilayer in A,
carbon in the top bilayer in b
bulk cell 1: BcCaAcCbBaAb
12 226
BRIEF REPORTS
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FIG. 2. The surface cells with an adatom in a T4 site used on each
terrace in the case of a surface with three bilayer steps exposing an A-type Si
bulk-termination layer. Arrows denote the silicon displacements entered in
the refinement process.
surface cell 2: adatom in C, silicon in the top bilayer in A
carbon in the top bilayer in c
bulk cell 2: CbBaAbBcCaAc
The bulk cells 1 and 2 have been used to treat the integer
rod data, which will be discussed below. They start, respectively, by B and C silicon layers since the actual top layers,
both of A type on each terrace, have been introduced in the
surface cells to allow for atomic displacements.
The indices are given with respect to the bulk derived
131 hexagonal basis with
u a u 5 u b u 53.073 Å
u c u 515.08 Å
The basis vectors for the ( A33 A3) R30° reconstructed
surface are then given by:
as 5a2b
and bs 5a12b
cs 5c.
Besides an overall scale factor, a single in-plane displacement parameter was introduced in the least-square
refinement:16 either the radial motion of the Si atom in the
top bilayer towards the adatom site ~Fig. 2! labeled d r, or the
Si trimer atom coordinate. The simple Si adatom structure
lead to a x 2 value of 2.2, whereas the other models gave
much larger values, x 2 512 for the Si trimer, x 2 55.5 for the
C adatom, and 5.6 for the vacancy model, and were thus not
further considered. Adding the out-of-plane data and fixing
the z coordinates at the values proposed by Sabisch, Krüger,
and Pollmann for the Si T4 adatom case, brought down the
x 2 value to 1.3. It should be remarked that the out-of-plane
measurements were assigned a larger error than the in-plane
data ~15% compared to 10%! since they were more impaired
by the terrace structure, especially a high noise level is observed for those rods that would be markedly different on a
perfectly flat surface ~for example 31 43 l and 13 43 l). Due to the
large error bars on the out-of-plane data, the refinement of
the z positions was not efficiently guided and was thus not
considered at this stage. The comparison between calculated
and measured fractional-structure factors is displayed in Fig.
3.
The in-plane displacement d r of the Si atoms towards the
adatom amounts to 0.160.01 Å, compared to 0.06 and 0.05
Å given by Northrup and Neugebauer5 and to 0.075 and 0.05
Å derived from the results of Sabisch, Krüger, and
Pollmann6 for the T4 and H3 sites, respectively. The experimental displacement is thus of the expected order of magnitude and closer to the one predicted for the T4 adatom than
for the H3 case, although the comparison with the theoretical
results depends on the lattice parameter selected for the bulk
unit cell ~here a is taken at 3.073 Å!. In order to evaluate the
bond lengths, the theoretical z coordinates given by Sabisch,
Krüger, and Pollmann6 were used including the buckling in
FIG. 3. Comparison between measured and calculated structure factors.
~a! in-plane results in the asymmetric unit; shaded ~respectively, empty!
semicircles correspond to measured ~respectively, calculated! values. The
rhombic grid subtends the 131 reciprocal lattice whose unit cell is outlined.
The ( A33 A3) R30° unit cell is marked by a dashed line. ~b! selected
out-of-plane fractional rods.
the carbon layer, since they were fully compatible with the
fractional rod data. For the T4 case, this procedure yielded a
Si-Si bondlength of 2.37 Å, close to the bulk silicon value
~2.35 Å! and the lengths of the Si-C back bonds were found,
respectively, at 1.84 Å for the C below the Si adatom and
1.91 Å for the other bonds in the bilayer. In the assumption
of an H3 site, no buckling is allowed but in-plane displacements are permitted in the carbon layer in order to follow the
silicon atom motion. When introducing this additional parameter in the fit, the minimum x 2 is obtained for a carbon
displacement of 0.02560.015 Å, which brings the Si-C
bonds in the bilayer at 1.85 Å and 1.96 Å, implying a somewhat larger strain in the surface. This stage of the analysis is
then slightly in favor of the T4 site, however a full confirmation has been obtained by analyzing the integer rod (12l):
both T4 and H3 sites have been probed on AA, BC, and CB
bulk terminations, and for the sake of completeness, a mixture of the six terminations in equal proportions has also
been compared to the experimental data. An excellent agreement is observed for the T4 Si adatom adsorbed on an AAterminated bulk as can be seen in Fig. 4, where are also
displayed the calculated curves for the H3 adatom on an
AA-terminated bulk and the T4 adatom on the six terminations. Since large discrepancies are observed in the minima
of the rod, where the surface sensitivity is enhanced, for any
other configuration, this measurement confirms simultaneously the T4 Si adatom nature of the reconstructed surface
cell and the AA-type of the bulk termination. A refinement
of the adatom z parameter brought it slightly closer to the
surface, although the theoretical value stays within the experimental accuracy. Owing to the range of the out-of-plane
reciprocal space exploration ~maximum q' 52.5 Å 21 ! restricted by the high background level always present as soon
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BRIEF REPORTS
12 227
TABLE I. Distances and angle determined in the present experiment,
compared to the values given in or calculated from Refs. 5 and 6.
d r(Å)
d 1 (Å)
d 2 (Å)
d 3 (Å)
d 4 (Å)
a~°!
a
Si(T4)exp.
Si(T4)th.
20.160.01
2.3160.01
1.6160.01
2.4060.01
1.84
69.5
20.06a
2.42a
1.75a
20.075b
2.41b
1.71b
2.50b
1.88b
70b
Reference 5.
Reference 6.
b
FIG. 4. Comparison between the experimental structure factors ~full
squares! along the integer rod (2,1,l) and calculations performed with a T4
Si adatom on an AA terminated bulk ~continuous bold line!, an H3 Si
adatom on an AA bulk ~dotted line!, and a T4 Si adatom on a mixture of the
six possible bulk terminations of 6H SiC ~0001! ~dashed line!.
as the detector was above grazing exit conditions, the potential accuracy for the z coordinate determination was poor ~0.1
Å!. Nevertheless, the fact that the theoretical value proposed
for the Si T4 site remained within the error bar of the best fit
output brought confirmation of the validity of the model. The
final structural parameters are listed in Table I together with
the corresponding theoretical ones.
Since the previously calculated T4 model did not account
for presence of two surface-shifted components on the C 1s
core-level photoemission peak,10 it could be worth it to perform a new calculation with the present larger in-plane displacements of the Si atoms and possibly a larger carbon layer
buckling in order to reduce the strain in the SiC surface
bonds, for instance a buckling of 0.43 Å instead of the 0.22–
0.25 Å proposed in Refs. 5 and 6 which would bring the
length of the Si-C backbonds at their bulk value 1.88 Å. In
order to explain these two carbon core-level shifts, it has also
been suggested that different phases with the same symmetry
may occur as a function of the preparation conditions. Addressing this assumption, x-ray diffraction measurements
have been made on a surface that had been annealed at
1000 °C without exposure to the Si flux, and equally presented the ( A33 A3) R30° diffraction pattern although the
Auger peak ratio ~Si LVV/C KLL51.2! showed an excess of
carbon with respect to the previous sample. The experimental structure factors were equally compatible with the T4
model, implying that the excess carbon atoms were not a
major component of a new surface cell but more likely disordered. Indeed, the surface diffraction peaks in this second
sample showed a double Lorentzian shape revealing the
presence of two domain sizes, a large one ~800 Å! and a
small one ~200 Å!, both corresponding to the same T4 adatom structure. The main difference between the two samples
is the amount of faulted boundaries that could be favorable
sites for carbon segregation.
By sorting out the signal from atoms responsible for the
selected long-range order at the surface, which is not the case
for photoelectron spectroscopy, the present grazing x-ray diffraction experiment enables us to conclude that the atomic
structure revealed by the ~A33 A3! R30° reconstruction of
the 6H SiC ~0001! surface is unique and due to the adsorption of silicon atoms in T4 sites saturating the dangling
bonds of the silicon top layer. The in-plane displacement of
the Si atoms in the top bilayer is accurately determined and
may stimulate further calculations. In addition, the dominant
termination of the 6H SiC ~0001! bulk is clearly identified as
made of three bilayer steps with an A-type top layer.
The authors are greatly indebted to Y. A. Vodakov, E. N.
Mokhov, and A. D. Roenkov from the Ioffe Institute ~St.
Petersburg, Russia!, for making available the processed
single-crystal samples. The participation of T.A. was made
possible through Grant No. 97-02-18331 from the Russian
Fund of Basic Research. This paper was part of the Italian
‘‘Tesi di laurea’’ prepared by A.C. and supervised by Professor A. Drigo ~University of Padova!. A.C. acknowledges
the financial support of the Italian INFM. Upgrading of the
LURE surface diffraction station was achieved under EC
Contract No. CI1.CT93.0034.
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15
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16
The refinement process is guided by minimization of the x 2 value
calc 2
2
defined as x 2 5 @ 1/(N2 p) # ( (F obs
hkl 2 u F hkl u ) / s hkl where N is
the number of independent data, p the number of free parameters, and s the experimental error taken at 10% for the in-plane
data.
*Author to whom correspondence should be addressed.
10
Electronic address: [email protected]
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11