APPLIED PHYSICS LETTERS
VOLUME 76, NUMBER 20
15 MAY 2000
Ion beam synthesis of graphite and diamond in silicon carbide
V. Heeraa) and W. Skorupa
Forschungszentrum Rossendorf, D-01314 Dresden, Germany
B. Pécz and L. Dobos
Research Institute for Technical Physics and Materials Science, H-1525 Budapest, Hungary
共Received 28 January 2000; accepted for publication 21 March 2000兲
A high dose of 1⫻1018 cm⫺2, 60 keV carbon ions was implanted into single crystalline 6H silicon
carbide 共SiC兲 at elevated temperatures. The formation of carbon phases in the crystalline SiC lattice
was investigated by cross sectional transmission electron microscopy. An amorphous, carbon rich
phase was produced at 300 °C. Precipitates of graphite were obtained at 600 °C, whereas at 900 °C
small diamond grains were produced. These grains are in perfect epitaxial relation with the
surrounding SiC lattice. © 2000 American Institute of Physics. 关S0003-6951共00兲02820-5兴
900 °C. The atomic and damage energy distribution of the
implanted carbon in SiC calculated by SRIM14 are shown in
Fig. 1. This is only a rough estimation of the expected profiles because high dose effects like sputtering, dynamic
changes of the target composition, and density are neglected.
The amount of implanted carbon corresponds to a deposited
graphitic layer of 81 nm. According to the calculation the
composition of the compound is about Si0.20C0.80 in the carbon maximum at a depth of 130 nm. The sputtered depth is
estimated to be about 20 nm. Therefore the ion energy
should be high enough to produce a buried layer of carbon
phase completely covered by SiC. The temperature range
was chosen because 300 °C is the critical temperature necessary to avoid amorphization of SiC,11 whereas 900 °C is well
above the temperature where graphitization of diamond is
obtained by ion implantation.15 Cross sectional specimens
for transmission electron microscopy 共TEM兲 were prepared
by ion milling. The specimens were investigated in a Philips
CM20 electron microscope.
The surface of the implanted SiC pieces was inspected
with an optical microscope. Neither surface cracks nor inhomogeneities were observed. The phase formation was investigated by cross-section TEM 共XTEM兲. Figure 2 shows the
XTEM micrograph of the sample implanted with 1
⫻1018 C⫹/cm2 at 300 °C. Although the damage energy in the
near-surface region is more than 200 times higher than the
High dose ion implantation in combination with thermal
annealing is a promising technique to produce precipitates or
complete layers of new phases with desired properties below
the surface of arbitrary materials. This process—also called
ion beam synthesis 共IBS兲—was mainly employed to modify
the electronic properties of silicon substrates.1 One prominent example is the process of separation by implanted oxygen 共SIMOX兲,2 where a buried, insulating SiO2 layer in Si is
formed. Further interesting applications of IBS are the formation of metal silicides3 or SiC layers in Si.4,5
In recent years the semiconductor related materials research has been focused more and more on wide band gap
semiconductors like SiC, diamond, and GaN because of its
outstanding properties for electronic applications.6,7 Among
the wide band gap semiconductors SiC is the one with the
most mature device technology. However, little is known
about the IBS of new phases in SiC. Only a few attempts
have been made to produce insulating8,9 or conductive10 layers in SiC by IBS. There are some special problems of IBS in
SiC. SiC is an extremely dense material with very tight
bonds. Therefore it is difficult to eliminate the implantation
damage or to redistribute the implanted ions by thermal
annealing.11 In addition the implanted ions produce a huge
strain in the lattice. One way to overcome this problem is to
perform the implantation at temperatures high enough for
dynamic annealing and strain relaxation. Recently, it has
been shown that crystalline precipitates of Al4C3 in perfect
epitaxial orientation with the surrounding SiC lattice can be
synthesized by high dose Al implantation into 6H–SiC at
500 °C.12 However, the implanted Al ions react only with the
C atoms leaving remainders of crystalline Si in the SiC substrate.
In view of the recent results it is an interesting question
whose phase is formed by excess carbon in the SiC lattice
under the nonequilibrium conditions of IBS. Several carbon
phases are known: cubic and hexagonal diamond, graphite,
and amorphous carbon.13
Pieces of n-type 6H–SiC wafers were implanted with 60
keV, 1⫻1018 C⫹/cm2 at temperatures of 300, 600, and
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0003-6951/2000/76(20)/2847/3/$17.00
FIG. 1. Calculated profiles of damage and excess carbon.
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© 2000 American Institute of Physics
2848
Appl. Phys. Lett., Vol. 76, No. 20, 15 May 2000
Heera et al.
FIG. 4. SAD pattern taken from zone 共3兲 of the sample shown Fig. 3.
FIG. 2. XTEM micrograph of the sample implanted at 300 °C.
amorphization energy of SiC at room temperature,11 a crystalline 6H–SiC layer of about 75 nm is left as indicated by
the visible lattice fringes marked as region A in Fig. 2. Selected area diffraction 共SAD兲 confirmed this result. This
means that an effective self-annealing of radiation damage
takes place in SiC at 300 °C. The TEM analysis reveals that
an amorphous phase (a-SiC) is produced in the depth region
between 75 and 210 nm which is followed by a small zone B
of highly defective crystalline 6H–SiC of about 45 nm
width. Because the damage energy in the amorphized zone is
not substantially higher than in the near-surface region it can
be concluded that the amorphous phase is stabilized by the
excess carbon atoms in this region. The center of the amorphous region corresponds well with the calculated maximum
of the implanted carbon profile. However, the amorphous
zone is a little broader than expected from the calculated
carbon distribution. This can be accounted for by layer
growth due to the carbon deposition inside the layer.
Substrate amorphization is completely prevented for implantation at 600 °C as shown in Fig. 3. In the cross section
four crystalline zones can be distinguished. The top surface
region 共1兲, 55–60 nm thick, is almost defect-free 6H–SiC.
The adjacent 6H–SiC layer 共2兲 with a width of about 30 nm
FIG. 3. Cross section of the sample implanted at 600 °C.
contains several lattice defects. The main zone 共3兲 is a 100
nm thick layer with crystalline precipitates generating a
bright contrast in the micrograph. This zone is followed by a
stripe of damaged SiC 共4兲. The SAD pattern taken from zone
共3兲 is shown in Fig. 4. It consists of SiC reflections which are
elongated due to lattice defects. Beside the SiC reflections
two arrowheads mark reflections, which can be identified as
共0002兲 graphite reflections. Obviously, the graphite has a
texture. The formation of graphite precipitates by the excess
carbon inside the SiC crystal is not surprising because all
Si–C bonds are saturated and graphite is the only equilibrium phase of carbon.
Unexpected results were obtained for the carbon implantation at 900 °C. An overview of the specimen is shown in
Fig. 5. Around the depth of 150 nm a buried zone with
precipitates is found in the weakly damaged 6H–SiC matrix.
The SAD pattern 共Fig. 6兲 reveals the coexistence of SiC and
diamond grains, which are in perfect epitaxial relation. The
size of the diamond grains is between 2 and 5 nm as deter-
FIG. 5. XTEM micrograph of the sample implanted at 900 °C. Top surface
is marked by an arrow.
Heera et al.
Appl. Phys. Lett., Vol. 76, No. 20, 15 May 2000
2849
ously, the target temperature plays a crucial role for the diamond formation. Occasionally formed diamond nuclei are
destroyed by the implantation damage below temperatures of
700 °C resulting in growing graphite or amorphous carbon at
temperatures of 600 or 300 °C, respectively.
The authors are indebted to J. Stoemenos for many helpful discussions. One of the authors 共V.H.兲 would like to acknowledge financial support by Deutsche Forschungsgemeinschaft 共Contract No. HE 2604/2兲. B.P. acknowledges
financial support from the OTKA T 030447 project.
1
FIG. 6. SAD pattern taken from the implanted region of the sample shown
in Fig. 5.
mined by dark field imaging with the 共220兲 reflection of
diamond. The width of the diamond zone is about 100 nm.
No graphitic inclusions were detected by the TEM analysis
in the implanted layer.
The mechanism of the diamond formation by IBS in SiC
is not quite clear. It can be assumed that the tetrahedrally
coordinated SiC lattice, which is preserved during the high
temperature implantation, acts as a template for the growth
of diamond. In addition, local nonequilibrium conditions in
the ion cascades 共thermal spikes, shock waves兲1 or temporary
stress on interstitially incorporated carbon atoms by the surrounding SiC lattice could contribute to the diamond nucleation. Weselowski et al.16 demonstrated the transformation
of graphitic carbon onions to diamond by ion irradiation in
the temperature range between 700 and 1100 °C. They explained the transformation process by a compression of the
carbon onions induced by knock-on displacements. Obvi-
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