Morphology of basal plane dislocations in 4 H - Si C homoepitaxial layers grown by
chemical vapor deposition
X. Zhang, S. Ha, Y. Hanlumnyang, C. H. Chou, V. Rodriguez, M. Skowronski, J. J. Sumakeris, M. J. Paisley, and
M. J. O’Loughlin
Citation: Journal of Applied Physics 101, 053517 (2007); doi: 10.1063/1.2437585
View online: http://dx.doi.org/10.1063/1.2437585
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/101/5?ver=pdfcov
Published by the AIP Publishing
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JOURNAL OF APPLIED PHYSICS 101, 053517 共2007兲
Morphology of basal plane dislocations in 4H-SiC homoepitaxial layers
grown by chemical vapor deposition
X. Zhang, S. Ha, Y. Hanlumnyang, C. H. Chou, V. Rodriguez, and M. Skowronskia兲
Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213
J. J. Sumakeris, M. J. Paisley, and M. J. O’Loughlin
Cree Inc., 4600 Silicon Drive, Durham, North Carolina 27703
共Received 3 October 2006; accepted 19 December 2006; published online 9 March 2007兲
The morphology of basal plane dislocations 共BPDs兲 in 4H-SiC homoepitaxial layers has been
investigated by plan-view transmission x-ray topography and molten KOH etching. Three types of
BPDs are distinguished based on their morphologies. These include interfacial dislocations, curved
dislocations, and circular loop dislocations around micropipes. Their characteristics are studied in
detail and possible sources of their formation during epitaxy are discussed. © 2007 American
Institute of Physics. 关DOI: 10.1063/1.2437585兴
I. INTRODUCTION
The origin and characteristics of basal plane dislocations
共BPDs兲 in 4H-SiC homoepitaxial layers are of great interest
from the fundamental understanding point of view as well as
due to their effects on the reliability of SiC bipolar devices. It
was reported that under forward bias, the voltage drop across
SiC p-i-n diodes changes over time.1–3 Extensive research
effort worldwide has resulted in good understanding of how
and why the degradation phenomenon occurs.4–14 It is attributed to the formation of single layer Shockley-type stacking
faults produced by dissociation of basal plane dislocations in
the epilayers inherited from the substrates.5,6,15 The stacking
faults expand by the motion of mobile silicon-core partial
dislocations activated by electron-hole recombination under
forward bias.14,16,17
4H-SiC epilayers are usually grown by step-controlled
epitaxy, a variant of chemical vapor deposition 共CVD兲. Layers are grown on vicinal substrates with several degrees of
off-cut angle with respect to 兵0001其 basal plane. The high
step density on the substrate surface allows adatoms to attach
at the step edges and prevents two dimensional nucleation
which will lead to growth of cubic 3C-SiC polytype. Typical
densities of basal plane dislocations in 4H-SiC substrates are
in the range of 104 – 105 cm−2.18,19 BPD densities in epilayers
are typically significantly lower than those in substrates due
to conversion of BPDs in substrates to threading edge dislocations in epilayers.3,18 The conversion is approximately
90% efficient in nonoptimized epitaxy.18,20 The remaining
10% of the BPDs propagate from the substrates into the epilayers along the basal plane.21,22 The influence of growth
conditions on the conversion efficiency was investigated including the factors such as C / Si ratio, growth temperature,
growth rate, surface preparation process prior to growth, and
surface polarity.20,23 Recently developed low BPD growth
techniques utilize KOH-etched or patterned substrate surface
to enhance BPD conversion. BPD densities as low as
10 cm−2 have been reported.24,25
a兲
Electronic mail: [email protected]
0021-8979/2007/101共5兲/053517/8/$23.00
A comprehensive understanding of the source and morphology of basal plane dislocations remaining in SiC epilayers is needed. In this study, transmission x-ray topography
was used in plan-view geometry to image the BPDs and to
investigate their characteristics. Complementary data were
obtained by molten KOH etching. Based on these results,
possible sources of BPDs in SiC homoepitaxial layers are
discussed.
II. EXPERIMENT
The samples examined in this study were grown by a
silane-based CVD method on Si face of 4H-SiC n-type conducting substrates 共n = 8 ⫻ 1018 cm−3兲. The substrates were
off cut by 8° from 关0001兴 towards either 关11− 20兴 or 关21
− 30兴 direction. The off-cut direction defined here is opposite
to the step-flow direction during epitaxial growth. Most of
the epitaxial structures contained 5 ␮m n+-doped 共n = 5
⫻ 1017 cm−3兲 buffer layers and the top epilayers were doped
with nitrogen in 1014 – 1015 cm−3 range. One epitaxial structure had a very thick buffer layer of 50 ␮m 共n = 1
⫻ 1018 cm−3兲 and a 50 ␮m n−-doped 共n = 2 ⫻ 1015 cm−3兲 epilayer on its top.
Plan-view transmission x-ray topographs were obtained
using the Mo K␣ radiation. The epiwafers were diced into
1 ⫻ 1 cm2 samples. Either most or all of the substrate thickness was removed by mechanical polishing in order to obtain
clear images of BPDs in the epilayers without interference of
contrast from the substrates. After polishing, the samples
were etched in molten KOH to remove the damage caused
by polishing and to map the defect distribution on the surface
of epilayers. Some samples were gradually thinned from the
substrate side 共including removal of epitaxial buffer layer
and part of the epilayer兲 with x-ray topographs taken after
each polishing step. This procedure allowed the determination of the locations of dislocations within the epitaxial structure. The thickness uniformity across 1 cm2 area after mechanical polishing was in the range of ±10 ␮m.
101, 053517-1
© 2007 American Institute of Physics
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FIG. 1. Transmission x-ray plan-view 共11− 20兲 topographs showing the interfacial dislocations in epiwafers with 共a兲关11− 20兴 off-cut direction and 共b兲
关21− 30兴 off-cut direction.
III. RESULTS AND DISCUSSION
Basal plane dislocations in 4H-SiC epilayers have been
imaged and reported in numerous publications.4–6,15,26–29
Electroluminescence imaging in a forward-biased p-i-n diode is a straightforward way to map the BPDs that serve as
nucleation sites for stacking faults. Straight and curved basal
plane dislocations have been observed in the diode
structures.4–6,15 However, many of their characteristics, such
as the Burgers vectors or locations in the structure, have not
always been determined. In addition, some dislocations dissociate and move under very low current density which
makes it difficult to image the original line directions of the
dislocations. On the other hand, x-ray topography is widely
used to provide the structural information of the basal plane
dislocations.26–29 In this study, three types of BPDs in
4H-SiC epilayers were distinguished according to their morphologies. The summary of the relevant literature data is
included in each of the following sections.
A. Interfacial dislocations
Long basal plane dislocations 共with lengths of hundred
microns to several millimeters兲 perpendicular to the off-cut
directions were revealed by plan-view x-ray topography in
4H-SiC epitaxial layers 共marked with triangles in Fig. 1兲.
Their positions were determined to be at or close to the
epilayer/substrate interface, and therefore they will be referred to as interfacial dislocations. Many of the characteristics of interfacial dislocations are similar to those described
by Jacobson et al.,26,27 who referred to this type of BPDs as
misfit dislocations.
J. Appl. Phys. 101, 053517 共2007兲
Both x-ray topographs in Fig. 1 were obtained using
共11− 20兲 reflection. The g vector is labeled in the figure.
Some of the interfacial dislocations are marked with triangles. The image in Fig. 1共a兲 was obtained on a sample
consisting of 99 ␮m n− epilayer, 5 ␮m n+ buffer layer, and
12 ␮m of the substrate remaining. The off cut is along the
关11− 20兴 direction, parallel to the g vector. The image in Fig.
1共b兲 was obtained on a sample with 39 ␮m n− epilayer and
30 ␮m of the remaining substrate. The off cut is along the
关21− 30兴 direction, which is 15° counterclockwise away from
the g vector. It is apparent that the interfacial dislocations are
not confined to one specific crystallographic direction but
instead their average direction is perpendicular to the off cut.
Locally, however, the interfacial dislocation line can form
angles other than 90° with respect to the off-cut direction and
can frequently form a zigzag line. An example of such dislocation is marked with an arrow in Fig. 1共a兲. This characteristic is more evident in the case of interfacial dislocations
observed in the epiwafer with a thick buffer layer, as the two
marked with arrows in Fig. 2共a兲. The image was obtained on
a sample with 50 ␮m n− epilayer, 50 ␮m n+ buffer, and
10 ␮m of the remaining substrate using 共11− 20兲 reflection.
The zigzag segments are about 70° clockwise and counterclockwise away from the off-cut direction, showing a tendency to be aligned along 关−2110兴 and 关1 − 210兴 directions.
The apparent differences in dislocation densities in Figs.
2共a兲–2共c兲 are explained later in this section. Most of the short
curved line contrasts shown in Fig. 1共b兲 are probably produced by the basal plane dislocations in the substrate, since
the sample had 30 ␮m of substrate remaining after polishing.
Line contrasts other than interfacial dislocations were also
observed in Fig. 2共a兲, as the one marked with a solid triangle.
They are produced by another type of basal plane dislocations observed in 4H-SiC homoepitaxial layers, referred to as
“curved dislocations.” They will be described in the next
section.
The location of the interfacial dislocations in the
epilayer/buffer/substrate structure was determined by gradual
removal of material from the substrate side and repeated
x-ray imaging. For the epiwafers with 5 ␮m of thin buffer
layers, the interfacial dislocations were determined to be
within ±15 ␮m from the buffer/substrate interface. Still it
was unclear in which part of the epilayer/buffer/substrate
structure the dislocations stayed, and 5 ␮m of buffer layer is
too thin for mechanical polishing to resolve it. Thus the
samples with 50 ␮m thick buffer layer were used to clarify
the position of interfacial dislocations. The images shown in
Figs. 2共b兲 and 2共c兲 are 共11− 20兲 topographs obtained from
the same area as shown in Fig. 2共a兲, with all the substrate
removed and buffer layer thinned down to 25 and 5 ␮m,
respectively. The long dislocation lines perpendicular to the
off-cut direction are clearly visible in Fig. 2共b兲, but are gone
in Fig. 2共c兲. This indicates that the interfacial dislocations are
present in the top half of the buffer layer. Several very short
line contrasts are still visible in Fig. 2共c兲, two of which are
labeled with hollow triangles. Compared with Figs. 2共a兲 and
2共b兲, both of these were determined to be from one interfacial dislocation. The left one was produced by a short segment of basal plane dislocation connecting to one end of the
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FIG. 3. Transmission x-ray plan-view topographs of the same area using 共a兲
共11− 20兲, 共b兲共01− 10兲, 共c兲共10− 10兲, and 共d兲共1 − 100兲 reflections. The
sample was from the epiwafer with a 50 ␮m thick buffer layer.
FIG. 2. Transmission x-ray plan-view 共11− 20兲 topographs of the same area
of the epiwafer with 共a兲 50 ␮m epilayer, 50 ␮m buffer, and 10 ␮m substrate: 共b兲 50 ␮m epilayer and thinned buffer with only 25 ␮m of remaining
thickness: and 共c兲 50 ␮m epilayer and 5 ␮m remaining buffer. The two
arrows in 共a兲 mark two interfacial dislocations. The solid triangles in 共a兲 and
共b兲 point out one curved dislocation. The two hollow triangles in 共a兲 and 共c兲
point out one end and tip of the zigzag line of one interfacial dislocation.
interfacial dislocation and leading to the epilayer/buffer interface. The right one was produced by one tip of the zigzag
line of the interfacial dislocation. Their corresponding contrasts in the topograph before the buffer layer being thinned
down were labeled with two hollow triangles in Fig. 2共a兲. It
should be noted that the length of the curved dislocations 共as
the one marked with a solid triangle兲 was reduced as the
material was removed from the substrate side. All of them
are invisible in Fig. 2共c兲 as most of the buffer layer was
removed, indicating that they thread through the entire buffer
layer but are not present in the n− epilayer. This could be due
to basal plane dislocations converting into threading ones at
the epilayer/buffer interface. However, clear contrasts of the
ends and tips of zigzag lines remain in Fig. 2共c兲 for some
interfacial dislocations. This indicates that those segments of
the interfacial dislocations may extend into the epilayer.
Based on the observation described above, the position of
interfacial dislocations is determined to be in the top half
portion of the buffer layer. They are closer to the epilayer/
buffer interface than the buffer/substrate interface. The randomly oriented straight lines visible in Figs. 2共b兲 and 2共c兲 are
contrasts due to polishing scratches.
The set of six prismatic reflections of 兵11− 20其 and 兵1
− 100其 was used to determine the Burgers vector of interfacial dislocations. Figures 3共a兲–3共d兲 show four topographs of
the same area using 共11− 20兲, 共01− 10兲, 共10− 10兲, and 共1
− 100兲 reflections, respectively. The sample consisted of
50 ␮m n− epilayer, 50 ␮m n+ buffer layer, and 10 ␮m substrate. The interfacial dislocation lines are out of contrast in
the 共1 − 100兲 reflection. Thus their Burgers vector is determined to be along the 关11− 20兴 direction 共parallel to the offcut direction兲. The same extinction condition applied to the
interfacial dislocations in the epiwafers with thin buffer layers.
Jacobson et al.27 proposed that the interfacial dislocations are formed due to the misfit strain between SiC substrate and epilayer induced by different nitrogen doping concentrations. These authors considered the different radii of
substitutional nitrogen atoms and the host carbon atoms and
calculated the lattice parameter change with nitrogen doping
concentration. Increasing the nitrogen concentration tends to
reduce the lattice parameters of the host SiC crystal. In their
back-reflection x-ray topography experiments, edge-type dislocation lines perpendicular to the off cut were detected in
30 ␮m thick 4H-SiC epilayer doped with 3 ⫻ 1015 cm−3 nitrogen grown on conducting substrate with a nitrogen doping
concentration of 3 ⫻ 1019 cm−3. However, such features were
not found in the epilayers with the same growth conditions
and nitrogen doping concentration grown on substrates
doped with 4 ⫻ 1018 or 1.3⫻ 1019 cm−3 nitrogen. These experimental results indicated that the formation of interfacial
dislocations was related to the misfit strain between substrate
and epilayer induced by different nitrogen doping concentrations. There are two widely accepted models for nucleation
of misfit dislocations.30 One is through the elongation of a
grown-in dislocation to form a segment of misfit dislocation
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FIG. 4. Transmission x-ray plan-view 共11− 20兲 topograph showing two interfacial dislocations. Triangles point out one pinning point on each interfacial dislocation.
line at the interface. The other model argues that the dislocation half-loops nucleate on the surface of the growing epilayer and glide into the crystal, finally forming misfit dislocation lines at the interface. In either case, the dislocations
start gliding only after a relatively thick epilayer has been
deposited and enough strain energy has accumulated. The
minimum energy of the relaxed system is achieved if the
misfit segment is located at the interface. In the epilayer/
buffer/substrate structure with three doping concentrations,
the interfacial dislocations should be located close to the
buffer/substrate interface because both the epilayer and
buffer layer are compressively strained with respect to the
substrate, and the nitrogen doping concentration difference is
larger between substrate and buffer than that between buffer
and epilayer. However, the interfacial dislocations were determined to be located in the top half of the thick buffer
layer, closer to the epilayer/buffer interface than the buffer/
substrate interface. It should also be noted that some of the
interfacial dislocations show the characteristic of a moving
dislocation pinned by an obstacle, as shown in Fig. 4. The
image was obtained using 共11− 20兲 reflection on a sample
with 50 ␮m epilayer, 50 ␮m buffer layer, and 10 ␮m substrate. Two interfacial dislocations are imaged in the topograph, while other line contrasts are produced by curved dislocations. One pinning point on each interfacial dislocation is
marked with one triangle. This morphology indicates that the
dislocations are moving towards the substrate side in the
buffer layer. Both interfacial dislocations are connected at
their ends to basal plane dislocations extending towards the
epilayer side and the substrate side approximately along the
off-cut direction. By measuring their lengths projected to the
off-cut direction, their positions could be determined. The
two basal plane dislocations extend towards the epilayer side
end at the epilayer/buffer interface. They may have converted to threading dislocations in the n− epilayer.
X-ray topography in plan-view geometry can be used to
determine the relation between interfacial dislocations and
other basal plane dislocations in the epitaxial layers and/or in
the substrate. One example was shown in Fig. 4 and described in the previous paragraph. The ends of the interfacial
dislocations could be in the form of either a threading or a
basal plane dislocation extending into the epilayer or the
substrate. Since threading dislocations will propagate along
the c axis and they are unlikely to glide in the system with
J. Appl. Phys. 101, 053517 共2007兲
FIG. 5. Schematics of the four configurations of the interfacial dislocations
共IDs兲 in the buffer layer related to other basal plane dislocations 共BPDs兲. A1,
both ends connected to BPDs through buffer and epilayer above the interfacial dislocation; A2, both ends connected to BPDs only in buffer above the
interfacial dislocation, then converting to threading edge dislocations
共TEDs兲 in the n− epilayer; B1, one end connected to a BPD in the substrate,
the other end connected to a BPD through buffer and epilayer; and B2, one
end connected to a BPD in the substrate, the other end connected to a BPD
in the buffer layer, then converting to a TED in the n− layer.
few dislocations and low strain,31 only two configurations
are consistent with the misfit-induced glide model for interfacial dislocations. One is that both ends of the interfacial
dislocation are connected to basal plane dislocations above it
in the epitaxial layer 共configuration A兲. Configuration A corresponds to the formation of interfacial dislocations through
glide of basal plane dislocation half-loops nucleated on the
surface of growing epilayer. The other configuration is that
one end is connected to a basal plane dislocation in the substrate while the other end is connected to a basal plane dislocation above it in the epitaxial layer 共configuration B兲.
Configuration B corresponds to the formation of interfacial
dislocations through the elongation of basal plane dislocations propagating from the substrate at the interface. In the
epilayer/buffer/substrate structure, each configuration will
contain two categories assuming that the basal plane dislocations in the buffer layer could possibly convert to threading edge dislocations 共TEDs兲 in the epilayer at the epilayer/
buffer interface. Thus totally four types are expected for the
relation between interfacial dislocations in the buffer layer
and other basal plane dislocations. They are A1, both ends
connected to BPDs through buffer and epilayer above the
interfacial dislocation; A2, both ends connected to BPDs
only in buffer above the interfacial dislocation, then converting to TEDs in the n− epilayer; B1, one end connected to a
BPD in the substrate, the other end connected to a BPD
through buffer and epilayer; and B2, one end connected to a
BPD in the substrate, the other end connected to a BPD in
the buffer layer, then converting to a TED in the n− layer.
The schematics of the four configurations in a cross-sectional
view are shown in Fig. 5. The off-cut angle was exaggerated
in the schematics. The vertical and tilted lines indicate the
TEDs and BPDs, respectively. The horizontal lines in the top
half of the buffer layer indicate the interfacial dislocations
共IDs兲.
Over 20 interfacial dislocations were examined in the
epiwafers with thick and thin buffer layers, both ends of
which could be differentiated without overlap with other features in the topographs. Eighteen of them show configuration
B2, three exhibit configuration B1, two exhibit configuration
A2, and none corresponds to configuration A1. Representative examples are shown in Fig. 6. Both topographs in Fig. 6
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FIG. 6. Transmission x-ray plan-view 共11− 20兲 topographs obtained from two different epiwafers. Interfacial dislocations with configurations A2, B1, and B2
are labeled. Ends of the interfacial dislocations are marked with arrows. Two triangles in 共a兲 mark both ends of one interfacial dislocation, whose configuration
could not be determined by x-ray topography in plan-view geometry.
were obtained using 共11− 20兲 reflection. Figure 6共a兲 was obtained on a sample with 50 ␮m epilayer, 50 ␮m buffer layer,
and 10 ␮m of remaining substrate. Figure 6共b兲 was obtained
on a sample with 99 ␮m epilayer, 5 ␮m buffer layer, and
12 ␮m of remaining substrate. Interfacial dislocations with
A2, B1, and B2 configurations are labeled in the two topographs. Ends of the interfacial dislocations are marked with
arrows. It could be seen that for configuration A2, two short
basal plane dislocations, both in the buffer layer above the
interfacial dislocation, are connected to its ends. For configuration B2, one BPD in the buffer layer and one BPD from the
substrate are connected to each end of the interfacial dislocation. That the BPD segment in the substrate is not visible
in Fig. 6共b兲 might be due to the thickness variance of the
sample after mechanical polishing such that at the region
shown in the topograph the substrate was totally removed.
The corresponding BPDs in the substrate had been detected
in the 共11− 20兲 topograph obtained at the same area with
45 ␮m of remaining substrate 共the image is not shown in this
paper兲. For configuration B1, one BPD continuing through
both the buffer layer and the n− epilayer is connected to the
interfacial dislocation, as the one shown in Fig. 6共b兲. However, configurations other than A2, B1, and B2 could also
exist. For example, the interfacial dislocation marked with
two triangles in Fig. 6共a兲 has no visible BPD connecting to
either of its ends. The BPD close to its left end was detected
as a separate dislocation by polishing experiment. After part
of the buffer layer was removed, the BPD and the end of the
interfacial dislocation were clearly separated. For such an
interfacial dislocation, its end is difficult to be differentiated
between connecting to a threading dislocation and to a BPD
which is too short to be resolved.
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FIG. 7. Transmission x-ray plan-view 共11− 20兲 topograph obtained from a
sample with 50 ␮m epilayer, 50 ␮m buffer, and 10 ␮m substrate. Three
curved basal plane dislocations are marked with triangles. Two arrows point
out two pinning sites on one curved dislocation.
To summarize the observations described above, interfacial dislocations are edge-type dislocations with the average
line direction perpendicular to the off-cut direction. Frequently, they are close to or at the epilayer/substrate interface. In structures with thick buffer layer, the interfacial dislocations are found in the top half of the buffer layer. The
dislocation lines are zigzagged, with zigzag segments about
70° away from the off-cut direction, showing a tendency to
be aligned along the 关−2110兴 and 关1 − 210兴 directions. Pinning points due to point sources or threading dislocations
were detected along the interfacial dislocation lines, which
indicates that they are moving towards the substrate. Most of
the interfacial dislocations are connected to one basal plane
dislocation from the substrate at one end and to another BPD
continuing toward the epilayer surface on the other end.
Based on these observations, the formation of interfacial dislocations may be due to the sideway glide of preexisting
basal plane dislocations in the buffer layer propagating from
the substrate. The long dislocation lines perpendicular to the
off cut can move towards the buffer/substrate interface
driven by the stress in the epitaxial layers. At this stage, it is
not clear whether the stress is due to doping-induced misfit
strain. In addition, a number of unanswered questions remain: where does the sideway glide start, how does the glide
continue for millimeters, why do some dislocations glide for
millimeters while others in the same epilayer do not move
more than 500 ␮m, and why have not the interfacial dislocations in the thick buffer layer reached the buffer/substrate
interface. Further investigation is needed to resolve these issues.
B. Curved dislocations
The second type of basal plane dislocations frequently
revealed by plan-view x-ray topography in 4H-SiC epitaxial
layers is curved dislocations. Representative examples are
shown in Fig. 7, marked with triangles. The image was obtained on a sample with 50 ␮m epilayer, 50 ␮m buffer layer,
and 10 ␮m of remaining substrate using 共11− 20兲 reflection.
Curved dislocations are also visible in Figs. 2, 4, and 6共a兲, as
mentioned in the previous section.
As can be estimated from the length of the dislocation
lines projected to the off-cut direction, some of the curved
J. Appl. Phys. 101, 053517 共2007兲
FIG. 8. Transmission x-ray plan-view 共11− 20兲 topograph showing basal
plane dislocation half-loops nucleated from a point source close to the epilayer surface. The step-flow direction is marked with an arrow at the right
top corner. The positions of the epilayer surface and buffer/substrate interface on one basal plane are labeled with two white lines.
basal plane dislocations continue through the entire epitaxial
layer, while others extend only within the buffer layer, as the
three marked with triangles in Fig. 7. This observation was
also supported by gradual removal experiments. The curved
basal plane dislocations which only stay in the buffer layer
may have converted to threading edge dislocations at the
epilayer/buffer interface. There are several even shorter
BPDs visible in Fig. 7. Their exact locations in the epilayer/
buffer/substrate structure are not clear. Although the morphology of curved dislocations is quite diverse, most of them
have a scalloped shape with lobes bowing out towards the
substrate side. Such morphology is indicative of a moving
dislocation which was pinned at several points. A representative example is the curved dislocation at the right bottom
corner of Fig. 7. Two pinning points along the dislocation
line are marked with arrows. The direction of lobes indicates
the direction of dislocation motion which is towards the substrate. The Burgers vector of the curved basal plane dislocations was determined to be along 关11− 20兴, parallel to the
off-cut direction. All of the above observations are indicative
of the 4H-SiC epitaxial layers being under stress, which is
also consistent with the characteristics of interfacial dislocations described in the previous section.
An additional indication of the presence of significant
stresses in the epitaxial layers was provided by observation
of dislocation pile ups shown in Fig. 8. The topograph was
obtained using 共11− 20兲 reflection on a sample from the epiwafer with thick buffer layer and with 10 ␮m of remaining
substrate. The arrow at the right top corner points the stepflow direction. The corresponding positions of epilayer surface and buffer/substrate interface on one basal plane are
marked by two white lines. The image shows a number of
semicircular dislocation lines terminating on the epilayer surface. The dislocations apparently nucleated at the point
source located at the center of the semicircular loops close to
the surface of the epilayer. The dislocations then moved towards the substrate and piled up at the buffer/substrate interface. The fact that the dislocations did not propagate into the
substrate could be due to the reversal of the stress sign which
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Zhang et al.
J. Appl. Phys. 101, 053517 共2007兲
FIG. 9. 共a兲 Transmission x-ray plan-view 共−2110兲 topograph showing three micropipes with basal plane dislocation 共BPD兲 loops around them. The inset
shows a higher magnification image of the BPD loops around the micropipe labeled with a triangle. 共b兲 Optical image of the same area after molten KOH
etching. The three micropipes were marked with white arrows. 关共c兲–共e兲兴 Optical images of corresponding micropipe open cores on back side 共C face兲 of the
sample.
could be expected if the stress was induced by the misfit
between epitaxial layers and substrate. Curved dislocations
could also be seen in Fig. 8.
Our topography observations of curved dislocations are
consistent with images obtained by electroluminescence on
4H-SiC p-i-n diodes.4–6 Most of the dislocations observed
by electroluminescence were believed to be connected with
basal plane dislocations in the substrates. In addition,
Tsuchida et al.29 observed similar features of basal plane
dislocations by x-ray topography in 4H-SiC epilayers grown
on 共0001兲 and 共000− 1兲 substrates with off-cut direction towards 关1 − 100兴. The dislocations tended to align roughly parallel to the step-flow direction during epitaxial growth and
had a mixed or pure edge character.
C. Circular loop dislocations around micropipes
Micropipes are superscrew dislocations with hollow
cores.32,33 Basal plane dislocations around micropipes have
been observed in the bulk material in the form of circular
loops.33,34 Similar features, i.e., circular basal plane dislocation loops, were also observed around micropipes in the epilayers. Figure 9共b兲 shows the optical image of the etch pattern of an area on the epilayer surface with three micropipes
in it 共marked with white arrows兲. The magnified images of
their corresponding open cores on the back side 共C face兲 are
shown in Figs. 9共c兲–9共e兲. The defect located in the lower
right corner of Fig. 9共b兲 was actually composed of an array
of micropipes easily distinguishable in Fig. 9共e兲. Plan-view
x-ray topographs of the same area were obtained using
共−2110兲 reflection, as shown in Fig. 9共a兲. The step-flow direction and the g vector are labeled with two arrows. The
substrate was totally removed, thus only defects in the epilayer were imaged. Circular loops of basal plane dislocations
could be seen around each of the three micropipes. The inset
of Fig. 9共a兲 shows a higher magnification image of the BPD
loops around the micropipe labeled with a triangle. The dislocation density near the center of micropipes is too high to
permit resolving individual dislocations. The BPD loops extend up to 300 ␮m from the micropipes with the number of
loops and their spatial extent correlating with the total Burgers vector of the defects. For example, higher density of
BPD loops and larger spatial extent were observed around
the micropipe array at the lower right corner of Fig. 9共a兲 than
the other two lone micropipes shown in the same figure. All
six prismatic reflections were used to determine Burger’s
vectors of the BPD loops. Different dislocation loops were
out of contrast under different reflections, indicating that
they had different Burgers vectors. Our observation is similar
to that reported on the bulk SiC crystal by Pirouz33 and Vetter and Dudley34 using transmission electron microscopy
共TEM兲. TEM provided a higher resolution than x-ray topography so that partial dislocations separated by stacking faults
were identified. Twigg et al.35 also reported observation of
faulted dislocation loops near micropipe in 4H-SiC p-i-n diodes.
The high density of basal plane dislocations around micropipes in epilayers could not be explained by mobile BPDs
pinned by the superscrew dislocations since the BPD density
in epilayers is generally low. Also the model could not explain why a higher density of BPDs is associated with an
array of micropipes than with a lone micropipe. A more plausible model could be that the basal plane dislocations nucleate at the micropipes. However, if the micropipes are pure
screw dislocations with Burger’s vector along 关0001兴, they
should not produce shear stresses resolved in the basal plane.
This apparent contradiction needs to be resolved.
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053517-8
In addition to the three types of basal plane dislocations
discussed in this paper, we have observed two more types
that bound stacking faults in the epitaxial layers. Their morphologies, structures, and nucleation mechanisms will be discussed in separate publications.
IV. CONCLUSIONS
Three types of basal plane dislocations in 4H-SiC homoepitaxial layers were differentiated based on their morphologies observed in the plan-view x-ray topographs. These
are interfacial dislocations, curved dislocations, and circular
loop dislocations around micropipes. The first two types
have a Burgers vector along the off-cut direction and their
morphologies are largely consistent with both types moving
towards the substrate during epitaxial growth under the influence of shear stress. They are either produced by glide of
the BPDs propagating from the substrate into epitaxial layers
or by BPD nucleation on the surface followed by glide towards the epilayer/substrate interface. Basal plane dislocation loops appear to nucleate around micropipes throughout
the epitaxial growth. They could extend several hundred microns from the nucleation source and have all three possible
具11− 20典 Burgers vectors.
ACKNOWLEDGMENT
This work was supported in part by ONR Grant No.
N00014-02-C-0302, monitored by Dr. Harry Dietrich.
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