Substitutional carbon reduction in SiGeC alloys grown by rapid thermal chemical vapor deposition

Article abstract of DOI:10.1063/1.124987

The substitutional carbon reduction in Si_{1 - x - y}Ge_xC_y strained layers, annealed at high temperatures, increases the compressive strain in the originally strain-compensated alloys. From the rocking curve simulation, the maximum amount of carbon reduction was below 0.9% for the various samples which were annealed below 1000°C in the nitrogen flow. The interstitial silicon injection by thermal oxidation of the Si cap on the Si_{1 - x - y}Ge_xC_y layer enhances the reduction of substitutional carbon to a concentration of 1.3%. Oxidation of Si_{1 - x - y}G_xC_y alloys yields a Ge-enriched Si_1-xGe_x layer with the Ge concentration larger than the initial content, and the formation of 3C silicon carbide precipitate is observed by the Fourier transform infrared spectroscopy.

Full text of DOI:10.1063/1.124987

Substitutional carbon reduction in SiGeC alloys grown by rapid thermal chemical vapor
deposition
C. W. Liu, Y. D. Tseng, and Y. S. Huang
Citation: Applied Physics Letters 75, 2271 (1999); doi: 10.1063/1.124987
View online: http://dx.doi.org/10.1063/1.124987
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APPLIED PHYSICS LETTERS
VOLUME 75, NUMBER 15
11 OCTOBER 1999
Substitutional carbon reduction in SiGeC alloys grown by rapid thermal
chemical vapor deposition
C. W. Liu,a) Y. D. Tseng, and Y. S. Huang
Department of Electrical Engineering, National Taiwan University, 1, Roosevelt Road, Sec. 4, Taipei,
Taiwan, Republic of China
͑Received 20 May 1999; accepted for publication 16 August 1999͒
The substitutional carbon reduction in Si1ϪxϪyGexCy strained layers, annealed at high temperatures,
increases the compressive strain in the originally strain-compensated alloys. From the rocking curve
simulation, the maximum amount of carbon reduction was below 0.9% for the various samples
which were annealed below 1000 °C in the nitrogen flow. The interstitial silicon injection by thermal
oxidation of the Si cap on the Si1ϪxϪyGexCy layer enhances the reduction of substitutional carbon
to a concentration of 1.3%. Oxidation of Si1ϪxϪyGexCy alloys yields a Ge-enriched Si1ϪxGex layer
with the Ge concentration larger than the initial content, and the formation of 3C silicon carbide
precipitate is observed by the Fourier transform infrared spectroscopy. © 1999 American Institute
of Physics. ͓S0003-6951͑99͒04641-0͔
Recently, there has been impressive progress in the
growth1,2 and characterization3–5 of Si1ϪxϪyGexCy alloys,
which offer great flexibility to tailor the strain and the elec-
tronic properties of group IV heterostructures.6,7 The substi-
tutional incorporation of C can compensate the compressive
strain of Si1ϪxGex layers grown on Si substrates and sup-
press the boron outdiffusion in the base of
Si/Si1ϪxϪyGexCy /Si heterojunction bipolar transistors.7
However, the reduction of substitutional carbon in
Si1ϪxϪyGexCy alloys during high temperature processing
limits the applications of this material. The study8 on thick
80–160 nm Si1ϪxϪyGexCy samples with xϭ0 or 0.11 and
yϭ0.005 and 0.01 grown by rapid thermal chemical vapor
deposition ͑RTCVD͒ showed that the reduction of substi-
tional carbon at 1000–1130 °C was caused by 3C silicon
carbide precipitates. The study9 on thick 230 nm
Si0.9855C0.0145 grown on molecular beam epitaxy also showed
that the carbon related defects or silicon carbide precipitates
were responsible for the strain-relieving process. Both stud-
ies conclude that the width of carbon profiles does not
change during the annealing process. The study10 on thin
Si1ϪxϪyGexCy ͑ϳ20 nm͒ samples showed the reduction of
substitutional carbon increased the compressive strain in the
epilayer and thus yields misfit dislocation relaxation, while
the film thickness is below the critical thickness. In this let-
ter, we report the quantitative analysis of the substitutional
carbon reduction in the Si1ϪxϪyGexCy alloys with various
sample structures. The substitutional carbon reduction can be
enhanced by the interstitial silicon injection.
for a hydrogen carrier, 26 sccm for dichlorosilane, and 0.8
sccm for germane. The Si cap layers for less-carbon-content
samples (yр0.12) were grown at 700 °C, using a 26 sccm
dichlorosilane flow and a 3 slpm hydrogen flow. The Si caps
of high-carbon content samples (yϭ0.022) were grown at
550 °C, using a 4 sccm silane flow and a 3 slpm hydrogen
flow. The Ge fraction and thickness of Si1ϪxGex was ex-
tracted by fitting x-ray rocking curves. The details of the
Si1ϪxϪyGexCy growth can be found in Ref. 2. As small
amounts of methylsilane were added to the source gases, we
observed a shift in the ͑400͒ x-ray diffraction peak of the
resulting Si1ϪxϪyGexCy layers away from that of a similar
layer without addition of methylsilane. Vegard’s law with a
compensation ratio of 1% C to 8.3% Ge was used1,2 to esti-
mate the carbon concentration in alloys. The carbon content
was obtained from the shift of the ͑400͒ peak by assuming
the Ge concentration was unchanged as methylsilane was
added. The thickness of the as-grown Si1ϪxϪyGexCy quan-
tum well was also extracted by fitting the rocking curves,
which has been calibrated by Auger electron spectroscopy
depth profiles on thick samples. Secondary ion mass spec-
troscopy ͑SIMS͒ also has confirmed flat C profiles, and that
Ge concentration was unchanged by adding the methylsilane
on calibration samples.
Quantum well structures were investigated in this study.
The nominal thickness of the Si caps was about 60 and 30
nm for 625 °C-grown Si1ϪxϪyGexCy layers and 550 °C-
grown Si1ϪxϪyGexCy layers, respectively. All as-grown
samples are pseudomorphic and fully strained due to the low
temperature growth. No defect was observed in these as-
grown films after defect etching, using four parts of 49% HF
and five parts of 0.3M CrO3.
To study the substitutional carbon reduction of the
Si/Si1ϪxϪyGexCy /Si quantum wells, the x-ray rocking
curves from as-grown and annealed samples were measured.
The samples were annealed at temperature from 800 to
1000 °C in nitrogen. To avoid the nonuniformity on 100 mm
wafer, a cumulative annealing scheme is adopted. In other
words, the annealing treatment at different temperatures was
The Si1ϪxϪyGexCy single quantum wells were grown on
Si ͗100͘ substrates by RTCVD. The Si1ϪxϪyGexCy layers
with carbon content less than 0.012 were grown at 625 °C
using methylsilane as the C source. For higher carbon con-
tent samples (yϭ0.022), the growth temperature was
550 °C. The higher substitutional carbon incorporation at
lower temperature is consistent with the previous report.11
The growth pressure was 6 Torr. The gas flows were 3 slpm
a͒
Electronic mail: chee@cc.ee.ntu.edu.tw
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0003-6951/99/75(15)/2271/3/$15.00 2271 © 1999 American Institute of Physics
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2272
Appl. Phys. Lett., Vol. 75, No. 15, 11 October 1999
Liu, Tseng, and Huang
FIG. 3. The concentration of the substitutional carbon reduction for various
Si1ϪxϪyGexCy samples processed at different temperature. The empty
square indicates the oxidation.
FIG. 1. The high-resolution x-ray diffraction spectra of
Si/Si0.701Ge0.277C0.022 /Si quantum wells. The annealing time is 2 h for each
temperature.
a 29 nm
complicated behavior of Si1ϪxϪyGexCy relaxation is most
likely due to the combined effects of Ge out-diffusion, misfit
dislocation formation, and the reduction of substitutional car-
bon. The Ge out-diffusion and misfit dislocation formation
increase the vertical lattice constant, but the reduction of
substitutional carbon decreases the vertical lattice constant.
For thin samples, the Ge out-diffusion is important to change
the vertical lattice constant, since the diffusion can lower the
average Ge concentration more significantly, as compared to
thick samples. However, the effect of substitutional carbon
reduction on the vertical lattice is more pronounced in thick
samples. To obtain the amount of substitutional carbon, the
box profile of substitutional C distribution is assumed to fit
the ͑400͒ diffraction rocking curves. From the secondary ion
mass spectroscopy ͑SIMS͒, the width of carbon profile is
unchanged.8,9 The Ge profile is taken from the solution of
one-dimensional interdiffusion equation.12 The relaxation of
misfit dislocation is determined by the ͑422͒ asymmetrical
reflex. The substitutional carbon content in Si1ϪxϪyGexCy
epilayers can be obtained. The details can be found in Refs.
10 and 13.
Figure 3 shows the amount of substitutional carbon re-
duction for various samples at different annealing tempera-
tures. It is clear that the substitutional carbon reduction in-
creases as the temperature increases for all the samples. The
reduction of substitutional carbon reaches a maximum
amount of 0.009 for all these samples. A 40 nm
Si0.701Ge0.277C0.022 sample was continuously annealed from
800 to 900 °C for 2 h in the nitrogen flow. The vertical lattice
constant ͑Fig. 2͒ and the substitutional carbon reduction ͑Fig.
3͒ of this sample increases for each annealing step. For the
annealing at 850 °C, this sample was annealed two times.
The vertical lattice constant of the 40 nm Si0.701Ge0.277C0.022
sample continues to increase after the second annealing step
at 850 °C. This indicated substitutional carbon reduction
does not reach thermal equilibrium at the annealing time.
Similar results have been reported.9 To further investigate
the substitutional carbon reduction behavior, we performed a
performed on the same piece of samples. The peaks of
the ͑400͒ x-ray rocking curves for
a
29 nm
Si/Si0.701Ge0.277C0.022 /Si quantum well sample shifts away
from Si peak continuously ͑Fig. 1͒, when the sample was
annealed for 2 h from 800 to 1000 °C. The slight shift of
800 °C annealing process indicates the structure starts to re-
lax. The low growth temperature of this sample may be re-
sponsible for this, since 625 °C-grown samples are stable at
800 °C annealing for 2 h.10 From the positions of the ͑400͒
diffraction peaks, the vertical lattice constant can be obtained
for various samples ͑Fig. 2͒. The 37 nm Si0.723Ge0.277 and
20 nm Si0.77Ge0.23 control samples reveal a continuous de-
crease of vertical lattice constant for the 2 h annealing from
800 to 1000 °C. This is due to the Ge out-diffusion and misfit
dislocation formation.10 The annealing is performed in the
ultrapure nitrogen flow. For the 18 nm Si0.762Ge0.23C0.008
sample, the vertical lattice constant drops slightly for the 2 h
annealing from 800 to 1000 °C. For the 18 nm
Si0.758Ge0.23C0.012 sample, the vertical lattice constant de-
creases at 900 °C annealing for 2 h, but increases for the 2 h
annealing from 900 to 1000 °C. For the thick 29 nm
Si0.701Ge0.277C0.022 sample, the vertical lattice constant in-
creases for the 2 h annealing from 800 to 1000 °C. This
2
h oxidation process at 950 °C on the 40 nm
FIG. 2. The difference between vertical lattice constant of Si1ϪxϪyGexCy
and Si substrate normalized with respect to Si lattice constant at different
annealing temperature. Solid symbols indicate the annealing in nitrogen.
Si0.701Ge0.277C0.022 sample. This oxidizes the 3 nm Si cap and
injects interstitial Si into the Si0.701Ge0.277C0.022 alloys, the
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The empty square indicates the oxidation.
vertical lattice constant continues to increase as determined
130.216.129.208 On: Sat, 06 Dec 2014 08:17:03

Appl. Phys. Lett., Vol. 75, No. 15, 11 October 1999
Liu, Tseng, and Huang
2273
FIG. 5. The Fourier transform infrared spectrum of
Si0.701Ge0.277C0.022 sample, oxidized at 1000 °C for 2 h. The spectrum of 3C
silicon carbide is also shown for reference.
a
40 nm
FIG. 4. The high-resolution x-ray diffraction spectra of
Si/Si0.701Ge0.277C0.022 /Si quantum well.
a 40 nm
various samples for annealing below 1000 °C in nitrogen, but
kinetics of the substitutional carbon reduction is not clear.
The interstitial Si injection can enhance the substitutional
carbon reduction to 0.013. The oxidation of Si1ϪxϪyGexCy
can inject the Ge into the unoxidized layer and can form a
Ge-enriched layer.
from the ͑400͒ rocking curves ͑Fig. 4͒ and the substitutional
carbon reduction increases from 0.009 to 0.013. This indi-
cated that the interstitial silicon injection can kick out sub-
stitutional carbon and reduce the concentration of the substi-
tutional carbon. Please note that the concentration of
interstitial silicon at thermal equilibrium is very low
(1013 cmϪ3 at 1000 °C͒.14 The interstitial silicon cannot be
the sole source of the substitutional carbon reduction at ther-
mal equilibrium. The substitutional carbon may jump out of
the lattice site with its own kinetics, but it is clear that the
interstitial silicon can enhance this process.
The assistance of x-ray diffraction measurement by Pro-
fessor M. Y. Chern, National Taiwan University is highly
appreciated. The authors would also like to thank the group
of Professor James Sturm at Princeton University for supply-
ing the samples. This work is supported by National Science
Council ͑NSC 88-2218-E-002-004͒.
After further oxidation of the Si0.701Ge0.277C0.022 sample
at 1000 °C for
2
h, the oxidation reaches the
1 J. L. Regolini, F. Gisbert, G. Dolino, and P. Boucaud, Mater. Lett. 18, 57
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Si0.701Ge0.277C0.022 layer, and the layer thickness decreases to
14 from 40 nm estimated from the ͑400͒ rocking curves ͑Fig.
4͒. The Ge content in this layer becomes approximately 0.66,
assuming the negligible substitutional carbon content, and
the layer was completely relaxed, obtained from the ͑422͒
reflex. The Ge content will be even higher, if there is some
residual substitutional carbon. This severe Ge enrichment ef-
fect was not observed in the pure Si1ϪxGex control samples.
The oxidation of Si1ϪxGex alloys results in the Ge pile-up at
the oxide/Si1ϪxGex interface.15 The Ge diffusion into
Si1ϪxϪyGex underneath may be enhanced by the interstitial-
carbon-related defect10 and yields a Ge-enriched layer. The
Fourier transform infrared spectrum of the 1000 °C-oxidized
Si0.701Ge0.277C0.022 sample with reference to the as-grown
sample reveals a decrease of 600 cmϪ1 substitutional carbon
vibration peak, and an increases of silicon-carbide-like ab-
sorption peak, similar to the 3C silicon carbide absorption
spectrum ͑Fig. 5͒.16
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