In situ Si Doping in GaAs using Low-energy Focused Si Ion
Beam/Molecular Beam Epitaxy Combined System
Kuniyuki Kubo*, Junichi Yanagisawa*,**, Fujio Wakaya*,**, Yoshihiko Yuba*,**,
and Kenji Gamo*,**
*
Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyamacho, Toyonaka, Osaka 560-8531, Japan
**
Research Center for Extreme Materials at Extreme Conditions, Osaka University,
1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531, Japan
Abstract. Using 200 eV focused Si ion beam (Si FIB) combined with molecular beam epitaxy (MBE) system, highdose Si implantation in GaAs at laterally selected area was performed masklessly and the doped layer was buried by
successive overlayer regrowth. From a depth profile of carriers using a capacitance-voltage (C-V) measurement of this
layer, it was found that although the doping efficiency was low, carriers were still observed at the implanted region at a
dose as large as 3.5x1014 cm-2 without post-annealing. The doping efficiency was improved when the temperature of the
sample during the FIB implantation was high as 150oC. The present results indicate the advantage of using low-energy
ion implantation for nano-scale doping in semiconductors.
Using low-energy ion implantation, straggling of
implanted ions in the direction of depth, as well as
projected range, becomes very short. Therefore, peak
carrier density exceeding 10 % can easily be obtained
which is advantageous for formation of non-alloyed
contacts, two-dimensional arrangement of thin alloyed
layer such as GaMnAs ferromagnetic layers2 and nano
particles, and so on. For high-dose implantation in
GaAs reported so far3-6, because implantation was
performed at several hundred keV, post-annealing at
about 900 oC was essential to activate dopants.
INTRODUCTION
Formation of nanometer-sized doping regions in
semiconductors is a fundamental of three-dimensional
nano-devices which are very important for future
nano-electronics. Focused ion beam (FIB) can be used
for doping at laterally selected region masklessly, and
the minimum feature size for fabrication is mainly
determined by the beam size. Kubena et al. reported a
beam size as small as 8 nm in diameter1. On the other
hand, very narrow doping profile in the direction of
depth, as well as in the lateral direction, is also
required strongly for fabrication of nano-devices. This
can be realized by lowering the implantation energy.
In addition, irradiation damage can be reduced
drastically by using low-energy implantation. However,
the depth of doping became very shallow. To form a
patterned doped layer at desired depth, successive
overlayer regrowth of a cap layer with clean interface
should be needed after the low-energy implantation.
FIB combined by molecular beam epitaxy (MBE)
system via an ultra-high vacuum (VHV) tunnel has a
potential for such fabrication procedures.
We have been investigating the maskless formation
of patterned Si doping layer in GaAs using a Si FIB
and MBE combined system. We found that very
narrow (as small as 16 nm of FWHM) distribution of
carriers were observed in the depth profile of a C-V
measurement in the low-energy (200 eV) Si implanted
region even without post-annealing and showed that
low-energy implantation is promising to reduce
damage in implanted layers7,8.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
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FIGURE 1. Schematic (a)top and (b)cross-sectional view of
the sample structure.
FIGURE 2. Carrier distribution for 7x1012 cm-2 Si
implanted region after post-annealing observed at 120, 210
and 300 K and calculated results. Those for 120 and 210 K
have been offset by 4x and 2x1017 cm-3, respectively.
EXPERIMENTAL
The peak depth roughly
annealed at 840oC.
corresponds to the implanted surface and regrown
layer interface. The carrier profiles were observed at
120, 210 and 300 K. At these temperature range, the
carrier concentration was almost independent of
temperature although the peak carrier density
increased and the distribution width decreased slightly.
This suggests that no significant amount of carrier
trapping centers exist in the implanted region. The
solid lines are calculated carrier profiles using onedimensional Poisson equation. In the calculation, Si
dopant profiles were assumed to be Gaussian with a
straggling of 5 nm though TRIM calculation predicts
the straggling to be 0.7 nm. The broader carrier
profiles are mainly caused by diffusion of carriers. The
discrepancy between the calculated and observed
profiles may be caused by channeling. A preliminary
estimation suggests that about 18 % of implanted Si
caused channeling.
After 1000-nm-thick Si-doped GaAs was grown on
n-type GaAs substrate by MBE at 580oC, the sample
was cooled down below 200 oC and transferred to the
FIB chamber via the UHV tunnel. 200 eV Si FIB was
implanted in 1.4x1.4 mm2 area at doses of 3.5x1012 3.5x1014 cm-2 at room temperature or at 150oC and
successive overlayer regrowth of the 250-nm-thick Sidoped GaAs cap layer was performed by MBE at
530oC. Schematic sample structure was shown in Fig.
1. Doping density of the MBE-grown Si-doped GaAs
was nominally 5x1016 cm-3. The growth interruption
for the FIB implantation was taken 3.5 - 7 h and the
background pressure in the sample-transfer and FIB
chambers during the growth-interruption were lower
than 1x10-8 Torr. Some samples were post-annealed at
840oC for 10 s in hydrogen and argon mixed gas
ambient using another GaAs chip covered on the
surface by rapid thermal annealing. Depth profile of
carrier density was obtained by C-V measurement at
room temperature. Schottky contacts for C-V
measurements were formed by Al evaporation on the
sample surface at an area of about 1 mm2 using an
UV-lithography.
A carrier density profile obtained for high dose
implantation is shown in Fig. 3. The carrier generation
was observed even in the unannealed sample at the
high dose. This suggests that the low-energy
implantation is advantageous to reduce damage. It took
about 5.5 h to implant to the dose of 3.5x1014 cm-2. It
is likely that surface may be degraded and
compensation centers may be induced during such
long growth interruption. In fact, samples implanted at
a dose of 3.5x1012 cm-2 and growth-interrupted for the
same period shows no carriers as shown in Fig. 3(a)
due to carrier compensation.
RESULTS AND DISCUSSION
Fig. 2 shows the carrier density profiles in GaAs
implanted with 200 eV Si at a dose of 7x1012 cm-2 and
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FIGURE 4. Relation between dose and sheet carrier density.
FIGURE 3.
Carrier distribution for (a)un-implanted,
3.5x1012 cm-2 and 3.5x1014 cm-2 Si implanted regions and
(b)1.4x1014 cm-2 Si implanted region observed before and
after post-annealing. Data for annealed sample have been
offset by 2x1017 cm-3.
The doping efficiency was improved by postannealing, as shown in Fig. 3(b) for the samples at a
dose of 1.4x1014 cm-2. In the figure, data for annealed
ones have been offset by 2x1017 cm-3. The peak
density became 60 % larger and the doping efficiency
was improved from 0.7 to 1.1 %.
FIGURE 5. Carrier distribution width (FWHM) as a
function of sheet carrier density for the samples fabricated
by various conditions (annealed or unannealed, dose,
regrowth temperature of the cap layer). Calculated results are
also shown.
The relation of sheet carrier density and distribution
width (FWHM) is shown in Fig. 5 observed for
samples fabricated by various conditions. The dotted
and the solid lines show the calculated width for Si
dopant profiles with straggling of 2 and 5 nm,
respectively. We can say that a narrow doped layers
with a width >10 nm can be formed without significant
broadening by diffusion and/or channeling.
The observed sheet carrier density was summarized
as a function of dose in Fig. 4. As the dose became
large, the sheet carrier density was decreased. The
decrease of carrier density with increasing Si dopant
concentration was also observed for MBE δ-doping at
a dose higher than 1013 cm-2 9.
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ACKNOWLEDGMENTS
The authors would like to thank Mr. K. Kawasaki
for his valuable technical assistance. One of the
authors (J. Y.) is grateful to the Murata Science
Foundation for financial support.
REFERENCES
1. Kubena, R. L., Ward, J. W., Stratton, F. P., Joyce, R. J.,
and Atkinson, G. M., J. Vac. Sci. Technol. B 9, 30793083 (1991).
2. Itou, M., Kasai, M., Kimura, T., Yanagisawa, J., Wakaya,
F., Yuba, Y., and Gamo, K., submitted to J. Vac. Sci. and
Technol. B.
FIGURE 6. Carrier distribution for Si implanted region at
room temperature and 150oC at a dose of 1.4x1014 cm-2. Data
for 150oC have been offset by 5x1017 cm-3.
3. Gamo, K., Inada, T., Krekeler, S., Mayer, J. W., Eisen, F.
H., and Welch, B. M., Solid State Electron. 20, 213-217
(1977).
This width is about 2 times wider than that observed
by MBE9.
4. Inada, T., Tokunaga, K., and Taka, S., Appl. Phys. Lett.
35, 546-548 (1979).
It was found that implantation at an elevated
temperature produce narrower profiles with higher
peak carrier concentration. Figure 6 shows the carrier
distribution profiles observed for implantation at a
dose of 1.4x1014 cm-2 and temperature of 150oC. The
peak carrier concentration was 1.7x1018 cm-3 which
was about 3 times higher than that achieved by
implantation at room temperature and the width was
only 7 nm. This result suggests that higher activation
of implanted dopant take place selectively at the peak
concentration
region
for
high
temperature
implantation.
5. Tandon, J. L., Nicolet, M.-A., and Eisen, F. H., Appl.
Phys. Lett. 34, 165-167 (1979).
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7. Yanagisawa, J., Goto, T., Hada, T., Nakai, M., Wakaya,
F., Yuba, Y., and Gamo, K., J. Vac. Sci. Technol. B 17,
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(2000).
9. Schubert, E. F., ``Capacitance-Voltage Profiling,’’ in
Delta-Doping of Semiconductors, edited by E. F.
Schubert, Cambridge: Cambridge University Press, 1996,
pp. 224-237.
CONCLUSION
High-dose Si implantation in GaAs was performed
using low-energy FIB-MBE combined system. It was
observed that activation of the dopants and very
narrow carrier distribution profiles were observed
without post-annealing. The narrowest width of 7 nm
was observed for an elevated temperature implantation
at 150oC. In order to improve the doping efficiency
more, it might be necessary to optimize the
temperature of the sample during the implantation and
confirm the effect of the post-annealing in this hotimplantation.
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