In this study of 4H-SiC and 6H-SiC epitaxial films we found that film morphology was strongly dependent on the tilt angle of the substrate. Large surface steps (up to 25 nm high) due to step bunching were more prevalent at smaller tilt angles. Also, 4H films were more susceptible than 6H to 3C-SiC inclusions during growth. The lateral growth of steps from screw dislocations in low-tilt-angle substrates demonstrated that step bunching on the atomic scale was anisotropic with respect to growth direction for both 4H-SiC and 6H-SiC. A model explaining this behavior is presented. We observed and directly measured the Burgers vector of a 'super' screw dislocation in a 6H-SiC epilayer

Abel, P. B.

Larkin, D. J.

Pirouz, P.

Powell, J. A.

Zhou, L.

English

NASA Technical Reports Server

Institute of Physics Conf. Series 142: Silicon Carbide and Related Materials 1995, pp. 77-80
Paper presented at Silicon Carbide and Related Materials 1995 Conference, Kyoto, Japan
Effect of tilt angle on the morphology of SiC epitaxial
films grown on vicinal (0001)SiC substrates
J. A. Powell, D. J. Larkin, and P. B. Abel
NASA Lewis Research Center, M.S. 77-1, 21000 Brookpark Road, Cleveland, OH 44135, U.S.A.
L. Zhou and P. Pirouz
Case Western Reserve University, Department of Materials Science and Engineering, Cleveland, OH
44106, U.S.A.
Abstract. In this study of 4H-SiC and 6H-SiC epitaxial films we found that film morphology was
strongly dependent on the tilt angle of the substrate.  Large surface steps (up to 25 nm high) due to step
bunching were more prevalent at smaller tilt angles.  Also, 4H films were more susceptible than 6H to
3C-SiC inclusions during growth.  The lateral growth of steps from screw dislocations in low-tilt-angle
substrates demonstrated that step bunching on the atomic scale was anisotropic with respect to growth
direction for both 4H-SiC and 6H-SiC. A model explaining this behavior is presented.  We observed and
directly measured the Burgers vector of a "super" screw dislocation in a 6H-SiC epilayer.  
1.  Introduction
Nearly all current SiC epitaxial growth is carried out on vicinal (0001)SiC substrates with the
polished growth surface tilted 3° to 5° from the (0001) basal plane[1]. Typical commercial
wafers have tilt angles of 3.5°±0.5°[2].  This tilt angle ensures that Òstep-flowÓ epitaxial growth
occurs (i.e. homoepitaxial growth of the same SiC polytype as the substrate).  With regard to
epitaxial growth, the tendency for 3C-SiC to nucleate on a-SiC substrates increases as the tilt
angle becomes smaller.  However, we have shown previously[3] that 6H-SiC can be grown on
6H-SiC at tilt angles as small as 0.1° with proper growth conditions and proper substrate
preparation.   With regard to morphology, SiC epilayers exhibit a variety of defects and surface
features.  The dominant features are steps due to macro and atomic-scale step bunching, growth
pits, and hillocks[4,5].   This paper presents the results of an investigation of surface
morphology of 4H-SiC and 6H-SiC epilayers grown on substrates with tilt angles in the range
0.1° to 3.5°. 
2.  Experimental
Epilayers of 4H and 6H-SiC were grown by chemical vapor deposition (CVD) by a process
reported previously[3].  The SiC substrates were Si-face vicinal (0001) boule- grown wafers
obtained from Cree Research[2] with tilt angles ranging from 0.1° to 3.5°.  Epilayer thicknesses
were typically about 5 mm.  The surface morphology of the epilayers was characterized by
optical microscopy (OM) and atomic force microscopy (AFM).  The AFM (Park Scientific Auto
Probe LS) allowed measurement of features down to the atomic scale.  The vertical noise level
of the AFM was usually less than 0.1 nm which was sufficient to distinguish between atomic
step heights that were single or multiples of the unit bilayer SiC step height (0.25 nm) in the
c-axis stacking direction .  The crystal structure was characterized by transmission electron
microscopy (TEM).
https://ntrs.nasa.gov/search.jsp?R=19970020726 2020-06-16T01:57:31+00:00Z
3.  Results
Macro scale step bunching (steps as high as 25 nm) was observed in some epilayers grown on
3.5°-tilt substrates .  Over a given 3.5°-tilt epilayer, the occurrence of large steps was quite
variable; large steps were evident over some millimeter-sized regions, but were absent over
other millimeter-sized regions.  Large steps were always present over the whole area of
epilayers with 1.2° and 0.4° tilt angles.  An example of large steps on a 0.4° tilt 6H-SiC epilayer
is shown in Fig. 1.
Fig. 1.  AFM image of macro step bunching on the surface of
a 6H-SiC epilayer with a tilt angle of 0.4°.
Epilayers of 4H were more susceptible than 6H to 3C inclusions.  For example,
triangular patterns frequently seen[4] in 4H epilayers, but seldom in 6H, were characterized by
oxidation[6] and found to contain surface layers of 3C-SiC within some of the triangular areas.
Cross section TEM demonstrated the presence of thin planar 3C sections parallel to the (0001)
plane in 4H epilayers containing a high
density of the triangles. Also, we
attempted to grow homoepitaxial films
of 4H and 6H on 0.4° tilt 4H and 6H
substrates, respectively, in the same
growth runs.  In every case, no
continuous films of 4H were achieved,
only 3C films with a high density of
double positioning boundaries
(DPBÕs)[3] and isolated 4H hillocks
were obtained.  In contrast, continuous
6H films on 6H substrates were
achieved in most cases. 
Isolated hexagonal hillocks were
observed in both 4H and 6H epilayers grown on  substrates with tilt angles less than 0.5°. Step
bunching on hillocks was quite different from the typical behavior shown in Fig. 1.  Multiple
spiral patterns of growth steps, assumed to be caused by screw dislocations,  propagated from
the peak of each hillock.  An example is illustrated 
   
[1010]
   
[1120]
   
[0110]    
1 
   
m
   
m
Fig. 2.  AFM image of spiral steps propagating
from the peak of a 4H-SiC hillock.
in Fig. 2, which shows two spirals of 0.5
nm high steps propagating from the peak of a hillock grown on a 4H substrate.  Each 0.5 nm
step is a doublet step of the basic 0.25 nm high SiC bilayer step.   The spiral evolves into a
hexagonal pattern as shown.  Since the total height of the two steps is 2 x 0.5 = 1.0 nm, the
repeat height (Burgers vector) for this screw dislocation is equal to the repeat height of the 4H
polytype.  Thus, each 0.5 nm step is one of the two doublet steps of the 4H stacking sequence.
As can be seen from Fig. 2, the  doublet steps
bunch to form a wide step, narrow step
combination step with a height of 1.0 nm in the
1 1 00  directions (e.g. 10 1 0  and 
01 1 0 ). As the growth direction rotates to the
next 1 1 00  direction, the step pattern
undergoes a transition at the 11 2 0  direction.
At this transition direction, the step heights
revert back to a doublet step height, i.e. 0.5 nm.
An unusual hillock grown on a 6H
substrate is shown in Fig. 3. This hillock
consists of 16 growth steps propagating from
the center of the spiral.  The number 16 was
determined by following a specific step around
one complete revolution and counting the
number of steps along the transition region back
to the starting point. Each of these 16 steps is a
triplet step, 3 x 0.25 nm = 0.75 nm high. These
triplet steps bunch into 8 triplet-step pairs in the
1 1 00  directions.  The bunching forms
single large steps rather than wide step, narrow
step combination steps as was observed for 4H. 
Each triplet-step pair has a total
height of  2 x 0.75 nm = 1.5 nm,
the repeat height of the 6H
polytype.  The Burgers vector
for this screw dislocation is
equal to 8 x the repeat height of
the 6H polytype (total height: 12
nm).  Screw dislocations with
such large Burgers vectors have
been called super screw
dislocations[7]. The step
bunching seen for this hillock is
similar to that for the 4H hillock
of Fig. 2 except that triplet-step
pairs form in the 1 1 00
di rec t ions ,  ins tead  o f
doublet-step pairs as was
observed for 4H.
4.  Discussion
In order to appreciate the significance of the hillock observations, a few aspects of SiC
polytypism must be recalled.  The 6H structure consists of the stacking of alternate triplet-step
layers of the 3C polytype and these are usually designated as ABC and ACB.  The difference in
structure between these two triplet-step layers is a rotation of 60° about the stacking axis (the
c-axis).  Similarly, the 4H structure consists of alternate doublet-step layers of  the 3C polytype
and are designated AB 
Fig. 3.  AFM image of spiral steps propagating from the peak of a
6H-SiC hillock.  Steps are produced by a super screw dislocation.
and AC.   Sakamoto et al.[8] pointed out that for steps on a Si(111)
surface, there are either two dangling bonds per atom or one dangling bond per atom on the step
riser depending on the crystallographic direction of the step.  Because of threefold symmetry of
Si(111), the number of bonds per atom alternates every 60° between one and two as the
direction rotates about the surface normal.  They suggested that faces (steps) with two dangling
bonds per atom grow faster than faces with one dangling bond per atom.  We apply these ideas
to SiC with the following result.  For a given 71 1 00 ? face of 6H-SiC, there is a particular
pattern of one and two dangling bonds per atom;  if we assume, for example, that the ABC
triplet-step layers in a stacking sequence have one dangling bond per atom on this given
71 1 00 ? face, then the other triplet-step layers in the sequence (the ACB layers) on this same
71 1 00 ? face will have two dangling bonds per atom.  If one examines the pattern of bonds
per atom on adjacent 71 1 00 ? faces, the pattern is reversed: the ABC triplet-step layers have
two bonds per atom, and the ACB triplet-step layers have one bond per atom.
We propose the following model, based on ideas of the previous paragraph, to explain
the anisotropic step bunching demonstrated in Figs. 2 and 3.  We will use the  4H hillock
depicted in Fig. 2 to describe the model.  If, in the case of Fig. 2, we designate the top (wider)
doublet steps in the  10 1 0  direction as AB doublet steps and assume that AB steps have two
dangling bonds per atom, they will grow faster in this direction than the AC doublet steps
which have only one dangling bond per atom; hence, step bunching of doublet-step pairs will
occur as shown.  If we examine growth in the adjacent 1 1 00 direction (e.g. the 01 1 0
direction) the AB steps will now be growing slower because they will have only one dangling
bond per atom, and the AC steps will now be growing faster;  hence reversed step bunching
will occur.  In this new direction (i.e. 01 1 0 ), the faster growing AC steps becomes the top
(wider) step , overtaking the slower growing AB steps.  The transition between these two states
causes a localized ÒdebunchingÓ of the paired doublet steps in the 11 2 0  direction between
the two adjacent 1 1 00  directions.  The same process occurs for 6H-SiC triplet steps as
shown in Fig. 3.
Calculations were recently carried out by Heuell et al.[9] on the lateral growth of steps
on a 6H-SiC vicinal (0001) surface tilted in a 1 1 00  direction.  They assumed  (1) single
bilayer steps on the surface and (2) each successive group of steps consists of one dangling
bond per atom on the leading step edge of three successive steps followed by two bonds per
atom on the next successive three steps.  The result of the calculations indicated that the single
bilayer steps structure would evolve into a structure with a step height of six bilayers.  Our
results are consistent with these calculations.  Also, our model explains the occurrence of  0.75
nm height steps when 1.5 nm high steps turn by 60° as observed by Tyc[10] on a sublimation-
grown epilayer on a 6H-SiC Lely crystal.
As can be seen in Figs. 2 and 3, the transition between the two bunched-step
orientations in adjacent 1 1 00   directions occurs over a very small angular change in growth
direction in the vicinity of the 11 2 0  direction between these two 1 1 00  directions.  The
instability in step bunching in this transition region may be a factor in the macro scale step
bunching that is observed in the SiC epilayers.  Any small undulation of the growth surface, or
defect on the surface could aggravate the effect of this instability.  This may be significant since
most SiC epilayer growth is carried out on vicinal (0001) wafers tilted toward a 11 2 0
direction.
 Previously, we have found that dislocations and micropipes propagate from the SiC
substrate into the epilayer[11]. Hence, we believe that all of the observed hillocks are produced
from screw dislocations that have their origin in the substrate.  The observation and direct
measurement of the Burgers vector of the super screw dislocation illustrated in Fig. 3 is
significant because it is believed that the formation of micropipes[12] is associated with strain
caused by the presence of super screw dislocations[13].
 5.  Conclusions
This investigation of epitaxial growth on SiC substrates with small tilt angles has yielded
significant new information on  SiC crystal growth processes.  For example, under the
conditions used in our CVD process, the 4H polytype is more susceptible than 6H to 3C
inclusions, and this tendency became worse at smaller tilt angles.  Also, the tendency for macro
step bunching was shown to be greater at smaller tilt angles.  The investigation of hillocks that
are produced on substrates with  small tilt angles led to the demonstration of anisotropic atomic
scale step bunching in 4H and 6H epilayer growth. The anisotropic behavior is consistent with
a model we proposed that is based on the anisotropic distribution of the number of dangling
bonds per atom in the various 1 1 00  directions.
References
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[2] Cree Research, Inc. (Durham, NC)
[3] Powell J A, Petit J B, Edgar J H, Jenkins I G, Matus L G, Yang J W, Pirouz P, Choyke W J, Clemen L,
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[4] Powell J A, Larkin D J and Abel P B 1995 Journal of Electronic Materials  24 295 -303
[5] Kimoto T, Itoh A and Matsuanmi H 1995 Applied Physics Letters  66 3645-3647
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Effect of Tilt Angle on the Morphology of SiC Epitaxial Films Grown on Vicinal (0001) SiC Substrates

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