Effect of surface polarity on gallium adsorption on 6H-SiC surfaces
L. Li, C. Tindall, Y. Hasegawa, and T. Sakurai
Institute for Materials Research, Tohoku University, Sendai 980-77, Japan
~Received 8 July 1997; accepted for publication 10 September 1997!
We have performed the first scanning tunneling microscopy ~STM! study of gallium adsorption on
both the Si-terminated 6H-SiC~0001! ()3)) and C-terminated 6H-SiC~0001̄! (2)32))
surfaces. The structure of the Ga terminated 6H-SiC surface showed strong polarity dependence. On
the Si-terminated )3) surface, parallel rows of Ga atoms arranged in three different domains
oriented at 120° with respect to each other at 1 ML coverage were observed. On the C-terminated
2)32) surface, sets of two concentric rings formed an overall 4)34) reconstruction at 1 ML
coverage. We propose a structural model for the 4)34) structure which explains the STM
image. © 1997 American Institute of Physics. @S0003-6951~97!00545-7#
Because of their favorable electronic and chemical
properties,1 the group III nitrides ~Al, Ga, and In! are excellent materials for a variety of high power micro- and optoelectronic devices, including field effect transistors2 and blue
laser diodes.3 However, a number of obstacles must still be
overcome in order to optimize the epitaxial growth processes. These include a lack of a lattice-matched substrate,
high growth temperatures, and detrimental predeposition reactions between commonly used metalorganic chemical vapor deposition precursors. Because of its relatively low cost
and high crystalline quality, sapphire is currently the most
popular substrate for the growth of single crystal group III
nitride thin films.4 However, because 6H-SiC has a much
smaller mismatch in both lattice parameter and thermal expansion coefficient,5 the difficulty in producing single crystal
epitaxial layers is correspondingly reduced.6 Nonetheless,
when used as a substrate material for GaN growth, 6H-SiC
displays a strong polarity dependence. On the ~0001! silicon
terminated surface, two-dimensional GaN growth results in a
smooth film. In contrast, three-dimensional growth occurs on
the ~0001̄! carbon terminated surface, producing hexagonal
pyramids of GaN on the SiC surface.7 It has been suggested
that this marked difference in film morphology arises from
differences in the lattice mismatch due to interfacial charge.8
Furthermore, GaN growth studies have shown the predeposition of gallium on the SiC surface prior to growth leads to
marked improvement in the quality of the GaN epitaxial
layer.9 In order to further elucidate the initial stages of
growth of GaN on 6H-SiC, we have performed the first scanning tunneling microscope ~STM! study of the adsorption of
Ga on the 6H-SiC surfaces. In a separate paper we will describe our study of N2H4 adsorption on 6H-SiC.10 N2H4 is an
active nitrogen precursor which can be used to lower the
growth temperature, and also to avoid unfavorable predeposition chemistry during the growth process.11
The experiments were carried out in an ultrahigh
vacuum ~UHV! chamber containing a STM equipped with a
low energy electron diffraction ~LEED! and a field-ion microscope ~FIM!, which is used to monitor and fabricate the
tip. The base pressure was in the 10211 Torr range. The
samples were n-type (n5231018 cm23) 6H-SiC~0001! and
6H-SiC~0001̄! single crystals supplied by Cree Research. Elemental Ga ~99.999%! was evaporated onto the surface from
a tungsten basket. The surface was cleaned in situ by Si
molecule beam etching at 950 °C. The Si beam was produced by heating a small Si slab to 1100–1150 °C placed 10
cm away from the sample. The removal of SiO2 by Si beam
etching has been successfully applied to clean Si and SiC
surfaces.12–14 Typically, a 5 min exposure to the Si beam at
900–950 °C was sufficient to remove the SiO2 films to obtain the clean SiC surface.
The clean 6H-SiC~0001! and ~0001̄! surfaces exhibit a
number of surface reconstructions which have been described in detail in other publications.15–17 As the stoichiometry of the surface changes, the surface reconstruction also
changes. On the Si terminated side, under very C rich conditions, an overall 636 structure, with local 232 ordering
exists. As the amount of Si present at the surface is increased, the structure first converts to a )3) structure.
Again, when the Si/C ratio is increased, the structure converts to 2)36). This is followed by a 333 structure.
Finally, when several layers of Si are evaporated onto the
surface, the Si(111)2737 structure is obtained. On the carbon terminated side, with increasing Si content, the surface
phases progress as follows: 131, 2)32), 232, 333,
and 737. The two clean surface structures which were used
in this study were the relatively silicon deficient )3)
structure on the silicon terminated side, and the 2)32)
structure on the carbon terminated side.
We consider first the C-terminated 2)32) surface.
When submonolayer coverages of Ga are adsorbed on this
surface, an overlayer with local 232 structure is formed.
However, there is no long range order. In this letter we focus
on the more ordered structures obtained at approximately 1
ML coverage. At 1 ML coverage, the overlayer consists of
sets of two concentric interlocking rings of Ga atoms as
shown in Fig. 1. The outer rings consist of 12 distinct spots
in the STM image. Thus the original threefold symmetry of
the substrate is maintained. In addition, there is an inner ring
of six atoms, with a vacancy in the center. Overall, the Ga
forms a 4)34) structure. Some striking features are observed in the image. First, there is a contrast difference between the inner and outer rings, with the atoms in the inner
ring appearing lower or darker than those of the outer ring.
The exact amount of the contrast depends on the sample bias.
In addition, higher contrast is observed in the empty-state
image. Second, the atoms of the inner ring appear alternately
darker and brighter relative to one another. This is seen in
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Appl. Phys. Lett. 71 (19), 10 November 1997
0003-6951/97/71(19)/2776/3/$10.00
© 1997 American Institute of Physics
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FIG. 2. Proposed model for the structure of 1 ML of Ga adsorbed on the
2)32) surface.
FIG. 1. ~a! Filled state image of 1 ML of Ga on the 2)32) C-terminated
6H-SiC~0001̄! surface. Sample bias521.6 V. Image5165 Å3165 Å. ~b!
Empty state image of 1 ML of Ga on the 2)32) C-terminated 6HSiC~0001̄! surface. Sample bias511.6 V. Image5165 Å3165 Å.
both the empty and filled state images, but is more pronounced in the empty-state image. This observed contrast
can be explained based on the structural model which we
propose below ~see Fig. 2!.
In constructing the proposed model, we start first with
the unreconstructed 6H-SiC~0001̄! surface. The silicon atoms
in the top SiC bilayer are illustrated as small gray circles in
the figure. To construct a structure which is consistent with
the STM image, the first step is to replace the top bilayer C
atoms with Ga atoms. This is indicated by the medium size
white and purple circles. The substitution of Ga for the C
atoms in the first layer is analogous to the behavior of Ga
adsorbed on Si~111! and Ge~111! surfaces. On those surfaces, the Ga atoms substitute for Si ~Ge!, respectively, forming a domain wall structure. The driving force for those
structures is the relaxation of strain induced by the large
lattice mismatch between Ga and Si ~Ge!. We believe this is
a reasonable first step given the large lattice mismatch between SiC and Ga. There are two distinct sites for the adsorbed gallium: hcp and fcc. The type of site is determined
by the stacking sequence. The fcc and hcp sites are analogous to the ‘‘unfaulted’’ and ‘‘faulted’’ sites formed on the
Si(111)2737 surface. The purple circles in the figure show
the faulted sites, while the white circles show the unfaulted
sites. The large red and yellow circles in the figure show the
second layer Ga atoms. These atoms form the ‘‘ring’’ structure which is observed in the STM image. As indicated in the
legend, the red circles show the atoms in the outer ring,
while the yellow circles show those of the inner ring. Note
that all of the Ga atoms of the outer ring lay on unfaulted
sites, while those of the inner ring alternate between unfaulted and faulted sites. There are two additional similarities
to the Si(111)2737 structure. First, there are corner holes
in which the silicon atom in the top bilayer is missing. Second, the silicon atoms in the top bilayer are dimerized.
The contrast observed in the STM image can be explained by the differing electronic structure at the different
bonding sites. These types of contrast are also seen in the
classic Si(111)2737 structure. On that surface, due to the
Appl. Phys. Lett., Vol. 71, No. 19, 10 November 1997
Li et al.
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FIG. 3. Empty state image of the Ga row structure obtained on the )
3) Si-terminated 6H-SiC~0001! surface. Sample bias511.0 V. Image
size5260 Å3260 Å.
differing local electronic structure, the corner adatoms appear slightly higher than the central adatoms in the filled
state image.18 In the Ga/SiC case, the atoms of the inner and
outer rings are analogous to the corner and center adatoms,
respectively, and thus it is reasonable to believe that one
could observe a contrast difference between the two rings.
Furthermore, three of the inner ring Ga atoms sit in faulted
sites while the other three sit in unfaulted sites. Thus the
electronic structure of these two types of Ga atoms should be
different, plausibly explaining the fact that three of the atoms
of the inner ring appear relatively bright, while the other
three look relatively dark. The geometric height of all of the
atoms in the second layer is the same in this model. However, the apparent height difference seen in the images is due
to the different local electronic structure of the faulted and
unfaulted sites.
Turning now to the Si-terminated )3) surface, 1 ML
coverage of Ga results in an ordered structure. The adsorbed
gallium forms a row structure consisting of domains oriented
120° with respect to each other. The image shown in Fig. 3
illustrates the three possible domains of the row structure.
The direction of the rows is oriented 30° away from the
^101̄0& direction of the clean surface. Some meandering of
the rows is also visible, but the changes in direction of the
rows is always 120°. The spacing between the rows was
measured to be 9.6 Å. In addition to the primary order, i.e.,
formation of rows on the surface, some fine structure can be
seen in the image. The rows are not perfectly straight, but
rather ‘‘zig-zag’’ slightly back and forth regularly, so that the
average direction is still a straight line. In addition, the position of the row does not depend on the polarity of the bias
voltage. This indicates that the position of the maxima seen
in the STM images corresponds to the physical position of
the Ga atoms. However, since we were not able to resolve
the internal structure of the rows, we have not yet proposed a
detailed structural model for this surface.
In conclusion, adsorption of Ga on both the Cterminated and Si-terminated 6H-SiC ~0001! and ~0001̄! surfaces was studied using STM. Dramatically different surface
structures were observed at approximately 1 ML coverage.
An ordered row structure was observed on the Si-terminated
face, while on the C-terminated face a ring structure reminiscent of the Si(111)2(737) surface was seen. A detailed
model of the ring structure was proposed. The model explains the contrast between the different Ga atoms observed
in the STM image. The different structures observed at 1 ML
coverage provides new insight into the behavior of these two
surfaces as substrates for GaN growth.
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