Competition between surface modification and abrasive polishing: a method of controlling the surface atomic structure of 4H-SiC (0001)

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Received
19 November 2014

Accepted
11 February 2015

Published
10 March 2015

Correspondence and

requests for materials

should be addressed to

[K.Y. (yamamura@upst.](mailto:yamamura@upst.eng.osaka-u.ac.jp)

[eng.osaka-u.ac.jp)](mailto:yamamura@upst.eng.osaka-u.ac.jp)

## Competition between surface modification and abrasive polishing: a method of controlling the surface atomic structure of 4H-SiC (0001)

Hui Deng, Katsuyoshi Endo & Kazuya Yamamura

Research Center for Ultra-precision Science and Technology, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka,
Suita, Osaka 565-0871, Japan.

The surface atomic step-terrace structure of 4H-SiC greatly affects its performance in power device
applications. On the basis of the crystal structure of 4H-SiC, we propose the generation mechanism of the
a-b-a*-b* type, a-b type and a-a type step-terrace structures. We demonstrate that the step-terrace structure
of SiC can be controlled by adjusting the balance between chemical modification and physical removal in
CeO2 slurry polishing. When chemical modification plays the main role in the polishing of SiC, the
a-b-a*-b* type step-terrace structure can be generated. When the roles of physical removal and chemical
modification have similar importance, the a-b-a*-b* type step-terrace structure changes to the a-b type.
When physical removal is dominant, the uniform a-a type step-terrace structure can be generated.

wing to the excellent electrical and mechanical properties of single-crystal silicon carbide (4H-SiC), it is
considered to be one of the most attractive materials for high-power, high-frequency and hightemperature applications. However, because of its high hardness and chemical inertness, the flattening
# O
of 4H-SiC substrates is very difficult, which greatly limits the applicability of 4H-SiC. To fully utilize the excellent
properties of 4H-SiC, such as its wide bandgap, high thermal conductivity and high breakdown electric field, the
preparation of a SiC substrate with a ‘‘perfect’’ surface is essential. On a perfect SiC surface, no scratches or
subsurface damaged layers should exist, and the surface should be atomically flat. In recent years, polishing
techniques combining surface modification and mechanical polishing, such as chemical mechanical polishing 1–3,
catalyst-referred etching 4–7, plasma-assisted polishing 8–11 and so forth 12,13, have been proposed for the damagefree and atomic-scale flattening of the Si (0001) face of 4H-SiC substrates. In all these techniques, the surface of the
4H-SiC substrate is modified by chemical reactions, and a modified layer that is easy to remove is generated. The
modified layer is removed by abrasive polishing or etching to realize surface flattening. With the application of
these polishing techniques, the generation of atomically flat SiC substrate surfaces with a well-ordered stepterrace structure has been reported.

Although the step-terrace structure obtained by these polishing techniques was well-ordered, a periodic stepterrace structure was usually formed. In plasma-assisted polishing, a step-terrace structure with four types of
terrace that appear alternately is generated (a narrow terrace, a wide terrace and two terraces of intermediate
width) 9 . This type of step-terrace structure is called the a-b-a*-b* type. In the case of catalyst-referred etching, a
step-terrace structure with alternating narrow and wide terrace pairs has been reported 5 . We call this type of stepterrace structure the a-b type. In the case of SiC surfaces processed by chemical mechanical polishing, which is the
most widely used method nowadays, an a-b type step-terrace structure or a step-terrace structure with a uniform
terrace width, which is called the a-a type, can be observed on different substrates 14 .

Although the generation of the a-b-a*-b* type, a-b type and a-a type step-terrace structures on 4H-SiC has
been reported in our previous research 11, their generation mechanism has not been thoroughly clarified. These
three types of step-terrace structure have occasionally been obtained in different polishing techniques 5,9,14, and the
process conditions under which they are generated are unclear. Therefore, the control of the step-terrace structure
of 4H-SiC has not been realized. It is expected that control of the surface atomic structure of 4H-SiC will be highly
advantageous for improving its performance in power device applications. In this study, the generation mechanism of these different types of step-terrace structure is proposed and experimentally clarified. Furthermore,

control of the step-terrace structure of 4H-SiC was realized by adjusting the balance between chemical modification and physical removal
in polishing.

Results
Crystal structure of 4H-SiC. In all the above-mentioned polishing
techniques, chemical modification, such as oxidation or etching, and
physical removal, such as abrasive polishing, are combined 5,9,14 .
Chemical modification softens the SiC substrate surface and
abrasive polishing removes the modified layer to flatten the
surface. Consistent with its name, there are four Si-C bilayers in
one unit cell of 4H-SiC single crystal as shown in Figure 1. It is
considered that there may be a relationship between the generation
of these periodic step-terrace structures with different terrace widths
and the four Si-C bilayers in a unit cell of 4H-SiC. In fact, it has been
reported that there are two types of Si-C terrace, 4H1 and 4H2
terraces, existing in 4H-SiC depending on the physical relationship
with the bilayers below 15,16 . According to a previous calculation
result, the extra energy required to deposit a new layer on a 4H1
terrace is much higher than that for a 4H2 terrace 17–19 . In other
words, a 4H1 terrace is much more stable than a 4H2 terrace. On
the basis of this calculation result, Arima et al. proposed that the
etching rates of 4H1 and 4H2 terraces are different, resulting in the
generation of an a-b type step-terrace structure in a catalyst-referred
etching process 20 . However, the difference between 4H1 and 4H2
terraces cannot explain the generation of a-b-a*-b* type and a-a
type step-terrace structures. On the other hand, in the calculation
of the extra energy required for deposition, the number of dangling
bonds (DBs) at the step edge was not taken into consideration. When
4H-SiC is oxidized or polished, chemical reactions or physical
removal starts from the step edge since the atoms at the step edge
are the most unstable. Therefore, the number of DBs at the step edge
strongly affects the oxidation rate (roxi) of the terrace. As shown in
Figure 1, taking the number of DBs of C atoms at the step edge into
consideration, it is found that there are two terrace pairs in a unit cell
of 4H-SiC. In one terrace pair, there is only one DB for each C atom at
the step edge (4H1* and 4H2*), while in the neighboring terrace pair
there are two DBs for each C atom at the step edge (4H1 and 4H2) as
shown in Figure 1. This means that roxi for the 4H1-4H2 terrace pair
is higher than that for the 4H1*-4H2* terrace pair. Also, in the
terrace pairs 4H1-4H2 and 4H1*-4H2*, roxi for 4H2 or 4H2* is
higher than that for 4H1 or 4H1* according to the results of
previous first-principles calculations 15,16 . Correspondingly, it is
concluded that these four types of Si-C terrace in a unit cell of 4HSiC (4H1, 4H2, 4H1* and 4H2*) have different values of roxi.

Generation mechanism of different step-terrace structures. When
4H-SiC is oxidized, oxidation starts from the step edges, which are
the most unstable areas on the substrate surface. Since there are four
values of roxi in a unit cell of 4H-SiC, the widths of the corresponding

Figure 1 | Crystal structure of 4H-SiC. Bond configuration of step-terrace
structure on a 4H SiC (0001) surface viewed from the [11 20] direction

oxidized terraces are different. Therefore, it is reasonable to consider
that the generation of the a-b-a*-b* type step-terrace structure
results from the four types of Si-C terrace, with different values of
roxi. However, the a-b type and a-a type step-terrace structures can
also be observed in other polishing techniques. Obviously the
generation of these two types of step-terrace structure cannot be
explained only by the oxidation process.

In the above-mentioned polishing techniques, chemical reactions
and abrasive polishing simultaneously occur 5,9,14 . Since the values of
roxi for the four types of Si-C terrace in a unit cell are different, the
oxidation process preferentially leads to different step widths in a
unit cell of 4H-SiC, resulting an a-b-a*-b* type step-terrace structure. On the other hand, in the abrasive polishing process, the modified layer is first removed. Then, there is abrasive contact with SiC at
the step edge and strain is introduced. With the repetition of this
process, Si-C terraces are removed from the step edge. Since abrasive
polishing is a physical removal process, the removal rates by polishing (rpol) of the four types of Si-C terrace in 4H-SiC are the same. This
means that abrasive polishing preferentially leads to a uniform step
width, resulting in an a-a type step-terrace structure. In the above
polishing techniques, both chemical modification and physical
removal occur. It is considered that under different polishing conditions, the balance between chemical modification and physical
removal is different, which leads to different types of step-terrace
structure.

On the basis of the above analysis, we consider that the generation
of the three types of step-terrace structures, a-a type, a-b type, and ab-a*-b* type, can be controlled by changing the balance between
chemical modification and physical removal in the polishing of
4H-SiC. The generation mechanism of the three types of step-terrace
structure of 4H-SiC is proposed as shown in Figure 2. This mechanism is based on the balance between roxi and rpol. As shown in

Figure 2 | Generation mechanism of step-terrace structure of 4H-SiC.
(a) Surface modification was dominant, resulting in the generation of the
a-b-a*-b* type step-terrace structure. (b) Physical removal was
comparable with surface modification, resulting in the generation of the ab type step-terrace structure. (c) Physical removal was dominant, resulting
in the generation of the a-a type step-terrace structure

Figure 2a, in the case that chemical modification plays the main role
in slurry polishing, roxi is higher than rpol and abrasive polishing only
removes the modified oxide layer. As previously introduced, the four
types of Si-C terrace in a unit cell of 4H-SiC, 4H1, 4H2, 4H1* and
4H2*, have different values of roxi. The oxidation of SiC starts from
the step edge. For these four types of Si-C terrace, the widths of the
corresponding oxidized terraces are different. After a modified layer
is generated, it is removed by abrasive polishing. Upon the removal of
the modified layers, the a-b-a*-b* type step-terrace structure is
generated.

If the value of rpol for abrasive polishing increases and become
comparable with roxi for surface modification, as shown in Figure 2b,
contact between the abrasive and the SiC terraces occurs. The diameter of the abrasive particles is very large compared with the height
of the SiC steps of 0.25 nm. Thus, after the a-b-a*-b* type stepterrace structure is generated, the step edge of the wide terraces
preferentially comes in contact with the abrasive particles. If rpol is
low, the amount of physical removal is very limited and the modified
layer is mainly removed. However, contact between the abrasive and
the wide terraces frequently occurs when the value of rpol for abrasive
polishing increases. Owing to the contact between the abrasive and
SiC, strain is applied to the contact area. The area subjected to strain
is rapidly oxidized and immediately removed by abrasive polishing,
i.e., the wide terraces are preferentially removed. Thus, the a-b-a*-b*
type step-terrace structure is changed to the a-b type. In this case, the
physical removal factor is comparable to the chemical oxidation
factor in the polishing of SiC.

Finally, if the value of rpol for abrasive polishing is greatly increased
to higher than the values of roxi for Si-C terraces, physical removal
plays the main role as shown in Figure 2c. In the case of a high rpol for
abrasive polishing, the modified layer is removed rapidly and physical removal of the Si-C terraces occurs. Owing to the high rpol, all the
terraces in SiC are in uniform contact with the abrasive particles after
the oxide layer is removed. Therefore, all the terraces are removed at
the same rate and the uniform a-a type step-terrace structure is
generated.

Control of the step-terrace structure. To prove the validity of the
proposed mechanism, 4H-SiC (0001) substrates were polished using
CeO2 slurry with different rotation speeds of the polishing pad. CeO2
slurry was used because it has been widely reported that some Sibased materials, such as Si, SiO2, Si3N4 and SiC, can be polished by
CeO2 abrasive owing to its tribocatalytic properties 21–25 . In the
polishing of 4H-SiC substrates with CeO2 slurry, modification of
the SiC substrate surface and abrasive polishing to remove the
modified layer are combined. The surface of SiC is modified to
silicon oxycarbide (Si-C-O) and further modified to SiO2 owing to
the tribocatalytic effect of the CeO2 abrasive 26 . Also, this
modification process is promoted by the chemicals in the slurry
such as hydroxide (OH) as well as the lattice strain caused by the
pressure applied by the polishing pad 27 . After the surface is modified,
the modified layer is immediately removed by the CeO2 abrasive. The
composition of the slurry and the load during polishing affect the
value of roxi for SiC in CeO2 slurry polishing. Therefore, SiC is
polished under a constant load with the same slurry, which means
that the factors affecting the chemical modification of SiC do not
change in the polishing experiments. The pad rotation speed is
increased in three stages from 500 rpm to 2500 rpm. According to
Preston’s law, the value of rpol in abrasive polishing increases linearly
with the relative speed between the specimen and the polishing pad.
This means that the value of rpol in abrasive polishing increases with
the pad rotation speed 28 . In this way, the balance between the
chemical modification (roxi) and physical removal (rpol) of SiC in
CeO2 slurry polishing can be controlled.

A SiC specimen was first polished by CeO2 slurry with a pad
rotation speed of 500 rpm Then the same specimen was polished

by CeO2 slurry with the pad rotation speed increased to 1500 rpm
followed by polishing with a pad rotation speed of 2500 rpm. To
confirm the change in the step-terrace structure with the increase
in the pad rotation speed, it is very helpful to observe the same area of
the same specimen. However, owing to the small observation area in
atomic force microscopy (AFM), it is very difficult to find the same
place on the same specimen after each polishing experiment.
Therefore, a nanoindenter was used to form some indents on the
surface to be polished, which were helpful for finding the same area
in AFM observation. Figure 3a shows an image of the indents
obtained by the charge-coupled device (CCD) camera of the microscope. Using these indents as a reference, the AFM observation after
each polishing experiment was localized to the area in the lower-left
corner, which included four indents as shown in Figure 3b. In this
way, the change in the step-terrace structure could be observed at the
same area of the same specimen.

The change in the step-terrace structure with the increase in the
pad rotation speed is shown in Figure 4. When the surface was
polished with a pad rotation speed of 500 rpm, the a-b-a*-b* type
step-terrace structure was generated as shown in Figure 4a. In such a
step-terrace structure, four types of terrace, a narrow terrace, a wide
terrace and two terraces with intermediate width, alternately
appeared on the polished surface. After this specimen was polished
with a pad rotation speed of 1500 rpm, the step-terrace structure
changed from the a-b-a*-b* type to the a-b type as shown in
Figure 4b. In the a-b type step-terrace structure, terrace pairs with
narrow and wide terraces alternately appeared on the polished surface. Finally, after the specimen was polished with a pad rotation
speed of 2500 rpm, the step-terrace structure changed from the a-b
type to the uniform a-a type, as shown in Figure 4c, in which all the
terraces had the same width.

It was concluded that the step-terrace structure on the polished
SiC substrate surface could be changed by changing the pad rotation
speed. When the pad rotation speed increased from 500 rpm to
1500 rpm and 2500 rpm, the step-terrace structure changed from
the a-b-a*-b* type to the a-b type and uniform a-a type, respectively.
Consistent with the mechanism proposed in Figure 2, increasing the
pad rotation speed increased the value of rpol for abrasive polishing,

Figure 3 | Indents on the SiC surface. (a) CCD image of the indents
formed on the surface of 4H-SiC to be polished. (b) AFM image of the
lower-left corner of the indented area. The change in the step-terrace
structure on the polished surface after each polishing experiment was
observed in the same area indicated by the dashed square

Figure 4 | AFM observation of polished SiC surfaces. Change in the step-terrace structure with increasing pad rotation speed observed by AFM.
(a) 500 rpm. (b) 1500 rpm. (c) 2500 rpm.

resulting in a change in the balance between surface modification and
abrasive polishing in slurry polishing.

According to the results of AFM observation, the change in the
step-terrace structure in the same area was confirmed. However,
compared with the AFM observation area, the polished area was
much larger. Therefore, it was necessary to evaluate the distribution
of step-terrace structures on the whole polished area after each polishing experiment. For each specimen, 100 points uniformly distributed over the polished area were observed. Figure 5 shows the
distribution of different types of step-terrace structure on 4H-SiC
substrate surfaces polished with different pad rotation speeds. In the
case of a pad rotation speed of 500 rpm, although both the a-b-a*-b*
type and a-b type step-terrace structures were observed, the a-b-a*b* type had a very high frequency of 91% as shown in Figure 5a. Also,
in this case, the a-a type step-terrace structure was not found. On the
surface polished with a pad rotation speed of 1500 rpm, as shown in
Figure 5b, all three types of step-terrace structure were observed and
the a-b type step-terrace structure had a high frequency of 81%.
When the pad rotation speed was increased to 2500 rpm, 94% of
the observed areas showed the a-a type step-terrace structure.

Surface composition after polishing. According the proposed
mechanism, with the change in the balance between chemical
modification and physical removal caused by changing the pad
rotation speed, there should be more oxidation products on a
surface polished with a low pad rotation speed than on a surface
polished with a high pad rotation speed. Therefore, the residual
oxidation products on the polished surfaces were determined by
angle-resolved X-ray photoelectron spectroscopy (ARXPS). To
increase the difference in the amount of residual oxidation
products on the polished surfaces, SiC surfaces polished with the
lower pad rotation speed of 500 rpm and the higher pad rotation
speed of 2500 rpm were used for ARXPS measurements. Figure 6
shows the carbon core-level (C1s) spectra of the SiC substrate
surfaces polished by CeO2 slurry with pad rotation speeds of

500 rpm and 2500 rpm. The strong Si-C peaks were assigned to
the bulk SiC. Other peaks, such as Si-C-O and C5O, were
considered to originate from residual oxidation products on the
polished surfaces 29,30 . On the surface polished with a pad rotation
speed of 500 rpm, it was found that the Si-C-O peak intensity was
even stronger than that of Si-C as shown in Figure 6a; thus, the
thickness of silicon oxycarbide was greater than that of SiC within
the detection depth of XPS. Figure 6b shows the C1s spectra of the
surface polished with a pad rotation speed of 2500 rpm. The amount
of oxidation products on this surface was small since the intensities
of the Si-C-O and C5O peaks were relatively weak compared with
those of the surface polished at 500 rpm. According to our proposed
mechanism, for a pad rotation speed of 500 rpm, chemical reactions
played the main role in CeO2 slurry polishing; thus, there should be
more oxidation products remaining on the polished surface. On the
other hand, for a pad rotation speed of 2500 rpm, rpol was increased
and physical removal played the main role, therefore the amount of
residual oxidation products should be small. The results of XPS
measurement shown in Figure 6 support our proposed mechanism.

Discussion
In our previous research, the a-b-a*-b* type step-terrace structure
was obtained by plasma-assisted polishing 8–10 . In plasma-assisted
polishing, the irradiation of atmospheric-pressure water vapor
plasma was used for surface modification. Plasma irradiation and
abrasive polishing (CeO2) were simultaneously conducted for surface flattening. For the oxidation of 4H-SiC, water vapor plasma has a
very high initial roxi of 0.185 mm/h 31 . The value of rpol for abrasive
polishing was much lower than that of roxi for oxidation in water
vapor plasma. According to the proposed mechanism, the high roxi
was the reason for the generation of the a-b-a*-b* type step-terrace
structure in plasma-assisted polishing.

In summary, the generation of the a-b-a*-b* type, a-b type and a-a
type step-terrace structures of 4H-SiC could be controlled by adjust

Figure 5 | Step-terrace structure distribution. Distribution of the a-b-a*-b* type, a-b type and a-a type step-terrace structures on 4H-SiC substrate
surfaces polished with different pad rotation speeds (a) 500 rpm (b) 1500 rpm (c) 2500 rpm

Figure 6 | Residual oxidation products on the polished surfaces. C1s
spectra of the SiC substrate surfaces polished with pad rotation speeds of
(a) 500 rpm and (b) 2500 rpm.

ing the balance between chemical modification and physical removal.
When rpol for abrasive polishing was lower than the rate of surface
modification, the a-b-a*-b* type step-terrace structure was generated.
When the pad rotation speed was increased and rpol for abrasive
polishing became comparable to the rate of surface modification,
the step-terrace structure changed from the a-b-a*-b* type to the
a-b type. When rpol for abrasive polishing was higher than the rate
of surface modification, a uniform a-a type step-terrace structure was
generated. On the basis of this mechanism, the results of polishing
using existing polishing techniques in which chemical reactions and
abrasive polishing are combined can be explained. Furthermore, it is
expected that the control of the step-terrace structure on a 4H-SiC
substrate will be very advantageous for realizing excellent device performance, which will be experimentally confirmed in future research.

Methods
CeO2 slurry polishing. Commercially available single-crystal 4H-SiC substrates (onaxis, n-type) supplied by TanKeBlue Semiconductor Co. Ltd. were used in this work.
All the experiments were conducted on the Si (0001) face, which is the most
commonly used face for power devices. Polishing pads (NP178) with a diameter of
10 mm supplied by FILWEL Co. Ltd. were used. The concentration of CeO2 slurry
was 1 wt% and its pH was 8.53. The average diameter of the CeO2 abrasive particles in
the slurry was 190 nm. Both the polishing pad and the SiC specimen were immersed
in CeO2 slurry. The polishing pad was scanned on the specimen with a scanning speed
of 200 mm/min. The polishing pressure was 3.743 kPa. The pad rotation speed was
increased from 500 rpm to 1500 rpm and finally 2500 rpm. Each specimen was
polished for 3 h. In each polishing experiment, a new polishing pad was used and the
slurry was replaced.

Surface characterization. A nanoindenter (ENT-2100, ELIONIX Inc.) was used to
form some indents on the specimen, which were used as a reference in AFM
observations. The maximum load applied for indentation was 100 mN. A total of 400
indents (20 3 20) separated by a distance of 10 mm were formed. The surface
morphology of the polished SiC substrates was measured by AFM (SPA-400, SII
Nanotechnology) in the tapping mode. For each specimen, 100 points that were
uniformly distributed over the polished area were observed. The surface composition of
the SiC substrates polished with pad rotation speeds of 500 rpm and 2500 rpm was
determined by XPS (Quantum 2000, ULVAC-PHI) with AlKa radiation (1486.6 eV).
To centralize the XPS observation to the top surface, the stage on which the specimen
was located was tilted with a very low takeoff angle of 10u. Before the XPS measurements,
to remove the organic contaminants on the specimen, cleaning in a sulfuric acid (H2SO4)
and hydrogen peroxide (H2O2) mixture (SPM) was conducted for 10 min followed by
cleaning in pure water for 10 min. The concentration of the SPM solution was H2SO4
(97 wt%): H2O2 (30 wt%) 5 451. For each specimen, five points uniformly distributed
over the polished area were observed to exclude any area dependence.

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Acknowledgments
This work was partially supported by a Grant-in-Aid for Scientific Research (A) (25249006)
from the MEXT Japan a research grant from the Adaptable and Seamless Technology

Transfer Program through Target-driven R&D. JST, and a Grant-in-Aid for JSPS research
fellow (25-581 2013).

Author contributions

H.D. and K.Y. conceived this research. H.D. performed the experiments, analyzed the data
and wrote the manuscript. K.Y. supervised H.D. and edited the manuscript. K.E.
participated in discussion through the work.

Additional information
Competing financial interests: The authors declare no competing financial interests.

How to cite this article: Deng, H., Endo, K. & Yamamura, K. Competition between surface
modification and abrasive polishing: a method of controlling the surface atomic structure of
4H-SiC (0001). Sci. Rep. 5, 8947; DOI:10.1038/srep08947 (2015).

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