Method for thin-film technology of Si with doping

Article abstract of DOI:10.1149/1.1575742

Layer-by-layer growth of single crystalline Si was examined with a doping method that applied a low-temperature process, Si₂H₆ (99.99%) and dopant precursor, PH₃ (or B₂H₆), were injected intermittently on the Si(100) surface, and an atomically smooth surface was obtained with atomic-scale layer controllability of the growth thickness. At a process temperature of 525°C, an n-type layer doped with phosphorus was obtained up to the carrier concentration of 2.8 × 10¹⁹ cm^-3, and a smooth surface of the film was obtained under 2 × 10¹⁹ cm^-3. The carrier concentration of the p-type layer doped with boron reached 5 × 10²⁰ cm^-3 while maintaining an atomically smooth surface at a growth temperature of 510°C. Nanometer-scale multilayer structures using doping were fabricated, and the doping profile was sufficiently sharp and on the order of nanometers.

Full text of DOI:10.1149/1.1575742

Journal of The Electrochemical Society, 150 ͑7͒ G371-G375 ͑2003͒
G371
0
013-4651/2003/150͑7͒/G371/5/$7.00 © The Electrochemical Society, Inc.
Method for Thin-Film Technology of Si with Doping
,z
Jun-ichi Nishizawa,* Toru Kurabayashi, Toru Oizumi, Hideyuki Kikuchi,
and Takashi Yoshida
Semiconductor Research Institute of Semiconductor Research Foundation and Telecommunications
Advancement Organization, Sendai Research Center, Sendai 980-0868, Japan
Layer-by-layer growth of single crystalline Si was examined with a doping method that applied a low-temperature process. Si H
2
6
͑
99.99%͒ and dopant precursor, PH3 ͑or B H ), were injected intermittently on the Si͑100͒ surface, and an atomically smooth
2 6
surface was obtained with atomic-scale layer controllability of the growth thickness. At a process temperature of 525°C, an n-type
19
Ϫ3
layer doped with phosphorus was obtained up to the carrier concentration of 2.8 ϫ 10 cm , and a smooth surface of the film
1
9
Ϫ3
20
Ϫ3
was obtained under 2 ϫ 10 cm . The carrier concentration of the p-type layer doped with boron reached 5 ϫ 10 cm while
maintaining an atomically smooth surface at a growth temperature of 510°C. Nanometer-scale multilayer structures using doping
were fabricated, and the doping profile was sufficiently sharp and on the order of nanometers.
©
2003 The Electrochemical Society. ͓DOI: 10.1149/1.1575742͔ All rights reserved.
Manuscript submitted August 6, 2001; revised manuscript received January 10, 2003. Available electronically May 12, 2003.
Optical communication network systems with the capacity of
terabit/s transmission rather than the conventional gigabit/s trans-
mission are required urgently. To satisfy this demand, high-speed
devices operating in the terahertz frequency range should be devel-
oped. Technological advances in the field of high-quality growth of
a Si layer with atomically accurate thickness control are required to
be made for the fabrication of such high-speed devices with nanom-
eter scale structures. Low-temperature and low-damage processing
techniques are suitable for the fabrication of nanostructures, because
changes of the doping profile and defect densities can be decreased.
Atomic layer epitaxy ͑ALE͒ is a suitable growth technology for
large-scale integrated circuit ͑LSI͒ fabrication due to the availability
of precise thickness control with atomic-order accuracy at low pro-
cessing temperatures. Nishizawa et al. developed the ALE technique
Czochralski method ͑approximately 10 ⍀ cm͒ were used as
substrates. For determining the Si epitaxial layer thickness, the
wafers were partially masked with SiO by thermal oxidation and
2
photolithography. After surface treatment by H SO :H O,
2
4
2
HCl:H O :H O, and diluted HF, the substrate was placed into a
2
2
2
load-lock chamber and transferred into the growth chamber passing
through a transfer chamber. Prior to epitaxial growth, the substrate
was heated in the growth chamber at a temperature of approximately
600°C for 30 s to remove adsorbed impurities.
As a source gas, SiH ͑99.9995%͒ or Si H ͑99.99%͒ was intro-
4
2
6
duced from the gas nozzle to the Si substrate. The flow of the gas
was controlled using a mass flow controller ͑MFC͒ to within a 0.01
standard cubic centimeter per minute ͑sccm͒ accuracy. Si layer-by-
layer growth was performed by an intermittent supply of Si H ͑or
1
2
2
6
for growth of single crystalline GaAs and Si, and referred to the
method as molecular layer epitaxy ͑MLE͒. In Si MLE, growth was
performed by alternating the supply of SiH Cl and H at the opti-
SiH ) by the control of pneumatic valves. As dopant precursors,
4
PH ͑4.2% PH in N ) and B H ͑5% B H in N ) were used for
3
3
2
2
6
2
6
2
2
2
2
n-type and p-type epitaxial growth, respectively. The dopant precur-
sors were also introduced on the grown film intermittently as shown
in Fig. 1. This periodic gas injection process was repeated until the
2
mal temperature of 825°C. Lowering of the growth temperature to
5
40-650°C was achieved by the alternate supply of SiH Cl and
2
2
3
atomic hydrogen. However, the temperature is still too high to fab-
ricate fine structures of doping profiles of sub-0.1 ␮m order. In Si
nanostructure device fabrication processes, high-quality doped lay-
ers are desired for the fabrication of heavily doped abrupt structures.
Hence, a low-temperature growth technique with atomic- and
molecular-level controllability using the well-known hydrides,
4
-7
SiH , Si H , and Si H , is considered to be important.
4
2
6
3
8
Si MLE growth by intermittent injection of Si H gas at a low
2
6
process temperature has been studied. In addition a doping technol-
ogy using an intermittent supply of Si H and a dopant precursor,
2
6
PH3 ͑or B H ) for n-type ͑or p-type͒, epitaxial growth has been
2
6
applied to achieve heavily doped abrupt structures to satisfy the
demand for Si nanostructure devices.
Experimental
Figure 1 shows the system employed and the gas injection se-
quence for Si MLE along with the doping method. The system con-
sists of a growth chamber evacuated by a turbomolecular pump
͑
TMP͒, a dry pump ͑DRP͒, and a load-lock chamber. The base pres-
Ϫ8
sure of the growth chamber is about 1 ϫ 10 Pa. The substrates
were heated using a halogen lamp for infrared heating, and the tem-
perature was monitored and controlled by an optical pyrometer
whose output signal was coupled to the control unit of the heating
lamp system. The temperature monitored by the pyrometer was cali-
brated using a Pt-Pt Rh ͑13%͒ thermocouple. Si͑100͒ boron-doped
p-type wafers and phosphorus-doped n-type wafers fabricated by the
*
Electrochemical Society Fellow.
E-mail: nishizawa@hanken.jp
Figure 1. Schematic illustration of Si MLE system and the gas injection
sequence of growth along with the doping method.
z
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G372
Journal of The Electrochemical Society, 150 ͑7͒ G371-G375 ͑2003͒
Figure 3. Growth rate by a continuous supply of SiH and Si H as a func-
4
2
6
tion of reciprocal temperature of growth.
Figure 2. Surface morphology of the film observed using a high-resolution
was introduced continuously for 120 min at a pressure of 1.3
ϫ 10 Pa ͑0.4 sccm͒ at 700, 650, and 500°C. In the FESEM im-
FESEM under various growth temperatures. SiH was introduced continu-
4
Ϫ2
Ϫ2
ously for 120 min at 1.3 ϫ 10 Pa ͑a͒ and ͑b͒ at 700°C, ͑c͒ and ͑d͒ at
ages in Fig. 2a, c, e, and g, the electron-beam was irradiated per-
pendicularly. However, in the images of Fig. 2b, d, f, and h, the
electron-beam was tilted 10° above the horizontal direction. In the
growth at 650 and 700°C, island growth occurred and the size of the
islands increased with increasing growth temperature. Significant
facet formation was observed when the temperature was higher than
6
50°C, and ͑e͒ and ͑f͒ at 500°C. Si H was introduced continuously for 120
2 6
Ϫ2
min at 1.3 ϫ 10 Pa in ͑g͒ and ͑h͒ at 500°C.
desired film thickness was obtained. The epitaxial layer was selec-
tively grown on the Si patterned by a SiO mask. To determine the
2
700°C. The surface obtained by this sequence at a growth tempera-
film thickness, the steps formed by epitaxial growth were measured
using a microstylus profilometer after the selective etching of SiO2
with HF. In addition, the cross-sectional view by a high-resolution
field-emission scanning electron microscope ͑FESEM͒ ͑Hitachi,
S-5000͒ enabled not only measurement of the growth thickness of
the film, but also observation of the surface morphologies of the Si
epitaxial layers grown under various conditions.
The n-type layer growth doped with phosphorus was formed on
p-type wafers, and the p-type layer growth doped with boron was
formed on n-type wafers. The carrier concentrations of the grown
layers were set significantly higher than that of the wafers. Accord-
ingly, the carrier concentration of the grown films was measured by
the van der Pauw method directly with no correction.
ture of 500°C was atomically flat as observed using FESEM as
shown in Fig. 2e and f. When Si H was introduced continuously
2
6
Ϫ2
for 120 min at 1.3 ϫ 10 Pa ͑0.7 sccm͒ at 500°C in Fig. 2g and h,
the surface was atomically flat and the growth rate was tenfold that
when SiH4 was used. A higher growth temperature enhances the
decomposition of adsorbed silicon-hydride and the surface migra-
tion of the adsorbate, accordingly the island growth was significant.
The deposition rates obtained under each growth condition are
shown in Fig. 3. The growth rate using a continuous supply of SiH4
shows an exponential increase in growth rate with temperature. The
activation energy was 0.95 eV ͑22 kcal/mol͒, which is about half the
value reported for the desorption reaction of H2 from SiH4 on a
8
The atomic concentration and the doping profile of boron in a
multilayered structure were measured by a time-of-flight ͑TOF͒
secondary-ion mass spectrometer TOF-SIMS IV ͑ION-TOF, Cam-
eka͒.
Si͑100͒ surface. Therefore, the energy of 0.95 eV is assumed to be
due to the surface reaction of SiH (x ϭ 1,2) with surface
x
9
migration. These experiments revealed the possibility of low-
temperature growth at a temperature close to 500°C with flat surface
morphology; a growth rate of less than 0.1 nm/min using SiH as a
4
Results and Discussion
source material was too low, however the growth rate using Si H6
2
To determine the surface reaction of material gases, Si growth
was performed by a continuous supply of SiH ͑or Si H ) by con-
trolling the supply pressure. The surface morphology of the film was
observed using a high-resolution FESEM as shown in Fig. 2. SiH4
was sufficiently high for its application to thin-film technology.
When the dopant precursors were introduced alternately with the
source gas supply ͑MLE mode͒, the growth rate per cycle and the
carrier concentration were determined by the injection pressure of
4
2
6
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Journal of The Electrochemical Society, 150 ͑7͒ G371-G375 ͑2003͒
G373
Figure 5. Dependence of the growth rate, the carrier concentration, and the
Hall mobility of phosphorus-doped films grown by MLE on PH3 supply at
the growth temperature of 510 and 525°C. The films were grown by 350
cycles of 30Љ(Si H )-2Љ-10Љ(PH )-2Љ for various PH supply amounts.
2
6
3
3
When PH3 was supplied with Si H continuously such as in
2
6
gas-source molecular beam epitaxy ͑GSMBE͒ in the MLE system,
growth was scarcely observed because of the competitive adsorption
of supplied gases. Phosphorus-doped films were grown after 350
cycles of growth according to 30Љ(Si H )-2Љ-10Љ(PH )-2Љ, that is
2
6
3
Si H was injected for 30 s, the injection was paused for 2 s, PH
2
6
3
was injected for 10 s, and the injection was paused for 2 s at the PH3
supply of 0.05 and 0.01 sccm. The electron concentrations obtained
Figure 4. FESEM images of a Si thin film grown by intermittent injection of
Si H under various conditions of doping, ͑a͒ undoped growth at growth
temperature of 510°C, ͑b͒ and ͑c͒ phosphorus-doped n-type growth at the
PH3 supply of 0.05 and 0.01 sccm at the growth temperature of 525°C,
respectively, and ͑d͒ boron-doped p-type growth at the growth temperature of
2
6
19
Ϫ3
by van der Pauw Hall effect measurement were 2.2 ϫ 10 cm
1
9
Ϫ3
͑
Fig. 4b͒ and 1.2 ϫ 10 cm ͑Fig. 4c͒. On the Si surface, rough-
ness was observed as shown in Fig. 4b, however, an atomically
smooth surface was obtained in Fig. 4c. The surface of Fig. 4b is
composed of hemispherical bumps which indicate three-dimensional
growth. A highly smooth surface was obtained when the doping
510°C. The electron-beam was tilted 10° above the horizontal direction.
1
9
Ϫ3
level was under 2 ϫ 10 cm . This means that an excess of PH3
Si H and that of dopant precursors, respectively. Figure 4 shows
supply leads to a carrier concentration of the film of over 2
ϫ 10 cm which leads to surface roughening. On decreasing the
2
6
1
9
Ϫ3
the FESEM images of a Si thin film grown by Si MLE under various
conditions of doping, undoped growth, phosphorus-doped n-type
growth, and boron-doped p-type growth. The electron-beam was
tilted 10° above the horizontal direction. Growth temperature was
PH supply, the onset of two-dimensional film growth takes place,
3
which is characterized by a marked decrease of the three-
dimensional bumps. When PH3 is supplied at a temperature of
525°C, the resultant surface is expected to be phosphorus-covered
fixed at 510°C, and the supply pressure of Si H was 4
2
6
Ϫ2
8
ϫ 10 Pa for 30 s in a cycle of gas injection in each case.
Si͑100͒. The coverage of phosphorus influences the surface migra-
In the undoped film growth Si H was injected for 30 s, and the
tion of Si-H species, namely, a higher concentration of phosphorus
on Si͑100͒ prevents surface migration for the two-dimensional
growth. Single crystalline film growth even on the surface shown in
Fig. 4b was confirmed by RHEED measurement.
2
6
injection was repeated every 4 s as represented by 30Љ(Si H )-4Љ.
2
6
For undoped Si, an atomically smooth surface was obtained with a
growth rate of 0.18 nm/cycle, however the deposition of polycrys-
talline Si occurred on the SiO mask. Single crystalline film growth
on the Si͑100͒ surface was confirmed by reflection high-energy elec-
tron diffraction ͑RHEED͒ measurement.
Figure 4d shows the FESEM image of a boron-doped film
which was grown at 510°C, by 240 cycles of
30Љ(Si H )-2Љ-10Љ(B H )-2Љ at a B H supply of 0.07 sccm. The
2
2
6
2
6
2
6
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G374
Journal of The Electrochemical Society, 150 ͑7͒ G371-G375 ͑2003͒
Figure 6. Dependence of the atomic concentration of boron, the carrier con-
centration, and the Hall mobility of boron-doped films grown by MLE on
B H supply at the growth temperature of 510°C. The films were grown by
2
6
2
40 cycles of 30Љ(Si H )-2Љ-10Љ(B H )-2Љ for various B H6 supply
2 6 2 6 2
amounts.
hole concentration was obtained at 1.1 ϫ 1020 cmϪ3. The carrier
2
0
Ϫ3
concentration of the p-type layer reached almost 5 ϫ 10 cm
with increasing the B H6 supply amount while maintaining an
2
atomically flat surface. Growth rate reduction as observed in phos-
phorus doping did not occur in boron-doped growth. The growth of
the single crystalline film doped with boron shown in Fig. 4d on a
Si͑100͒ surface was confirmed by RHEED measurement.
Figure 5 shows the dependence of phosphorus-doped films by
MLE on PH3 supply. The films were grown by 350 cycles of
3
0Љ(Si H )-2Љ-10Љ(PH )-2Љ for various PH supply amounts. The
2 6 3 3
growth temperature of 525°C was suitable for a high level doping in
MLE mode. The electron concentration increased and the mobility
decreased on increasing the supply pressure of PH up to about
3
1
9
Ϫ3
2
.8 ϫ 10 cm at 0.07 sccm, however, the electron concentration
decreased and the mobility increased when the pressure of PH in-
3
creased from 0.07 sccm. The growth rate per cycle decreased with
increase of the supply of PH . This phenomenon seems to depend
3
on the effect that the desorption temperature of hydrogen for
phosphorus-doped film shifts to a higher temperature compared to
intrinsic Si͑100͒.10 The coverage of phosphorus influences not only
the surface migration, but also the surface reaction rate of Si-H
species for Si growth, namely, a higher concentration of phosphorus
on Si͑100͒ acts to prevent hydrogen desorption from Si-H adsorbate.
It is assumed that the Si-H bonding of P-SiH heterodimers is more
stable than that of Si-SiH homodimers. In addition, an excess of
phosphorus on the growing surface leads to the precipitation of
phosphorus in the Si crystal, as observed by the decrease of the
carrier concentration and the increase of the Hall mobility with a
Figure 7. The depth profiles of boron at 11 amu measured by TOF-SIMS in
a planar-doped structure, ͑a͒ i-p -i, and ͑b͒ n -i-p -i-n , fabricated by Si
ϩ
ϩ
ϩ
ϩ
MLE with doping at 510°C. The i layer indicates an intrinsic undoped epi-
ϩ
taxial layer, boron-doped p layer at the carrier concentration of 1.1
ϫ 1020 cm , and phosphorus-doped n layer at the carrier concentration of
Ϫ3
ϩ
1 ϫ 1019 cmϪ3
.
decrease in the growth rate at a PH supply higher than 0.07 sccm.
layer reached the middle of 1020 cmϪ3 with maintaining an atomi-
cally flat surface. The atomic concentration of B in Si increased
linearly with increasing B H supply in a different manner to the
3
The dependence of boron doped p-type films by MLE on B H
2
6
supply was examined. The films were grown by 240 cycles of
2
6
3
0Љ(Si H )-2Љ-10Љ(B H )-2Љ at a growth temperature of 510°C.
carrier concentration. Therefore, the activation rate of boron as an
acceptor decreases with an increase in atomic concentration beyond
2 ϫ 10 cm . In boron doping, the growth rate per cycle was not
2
6
2
6
The carrier concentration, the atomic concentration of boron mea-
sured by a SIMS, and the Hall mobility are shown in Fig. 6. The
Hall mobility decreased, and the carrier concentration increased
with increasing B H supply. The carrier concentration of the p-type
2
0
Ϫ3
influenced by the amount of B H supply, however, the growth rate
2
6
was almost 0.25 nm/cycle which was 30-40% larger than that in
2
6
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Journal of The Electrochemical Society, 150 ͑7͒ G371-G375 ͑2003͒
G375
undoped film growth. This means that the reaction temperature of
hydrogen desorption from Si-H is shifted lower for boron-doped
films than that for undoped films in an opposite manner for the
phosphorus-doped film, as predicted by the result of temperature
programmed desorption ͑TPD͒ of hydrogen from each doped
layer.10 In our study, when B H was supplied along with Si H ,
growth, when the temperature was higher than 700°C. As a result,
the PDB structure for I-SIT was achieved successfully by Si MLE
with doping at 510°C.
Conclusion
2
6
2
6
We achieved Si molecular layer epitaxy using our doping method
at the process temperature of 510°C by intermittent injection of
Si H ͑99.99%͒ and dopant precursor, PH3 or B H , into the
doping efficiency was low; however, when B H was supplied al-
2
6
ternately with Si H , the efficiency increased by about one order of
2
6
2
6
2
6
magnitude. These facts clearly show that the doping method used in
MLE is advantageous for impurity doping technology.
Si͑100͒ surface. At the process temperature of 525°C, an n-type
layer doped with phosphorus was obtained up to the carrier concen-
1
9
Ϫ3
As an application, device structures of nanometer scale were
fabricated by Si MLE using the doping method. Figure 7 shows the
tration of 2.8 ϫ 10 cm with three-dimensional growth, however
a smooth surface by two-dimensional growth was obtained when the
ϩ
19
Ϫ3
depth profiles of boron in a planar doped structure, ͑a͒ i-p -i and ͑b͒
doping level was under 2 ϫ 10 cm . The higher concentration
of phosphorus seems to prevent not only the surface migration for
the two-dimensional growth, but also the surface reaction rate of
Si-H species for Si growth.
ϩ
ϩ
ϩ
n -i-p -i-n , fabricated by Si MLE with doping at 510°C. The i
layer indicates an intrinsic undoped epitaxial layer by SiMLE. The
measurement was performed using TOF-SIMS. The parameters for
ϩ
the Ar primary ion beam for TOF analysis were an energy of 10
The carrier concentration of the p-type layer doped with boron
2
0
Ϫ3
keV and current of 0.85 pA with 10 ϫ 10 ␮m raster scan, flooding
reached almost 5 ϫ 10 cm
while maintaining an atomically
Ϫ4
O at 1 ϫ 10 Pa to emphasize the detection sensitivity of boron
smooth surface by two-dimensional growth at 510°C. As an appli-
cation, nanometer-scale devices with n-i-p-i-n structure, which is the
source-drain structure of an I-SIT were fabricated by Si MLE using
the doping method. The doping profile measured by TOF-SIMS in-
dicated a sufficiently sharp distribution on the nanometer scale.
2
ϩ
in Si. The parameters for sputtering were Ar , 1 keV, 7.5 nA, and
100 ϫ 100 ␮m raster scan with sputtering rate of 0.71 nm/min for
Si. The result in Fig. 7a shows that the designed multilayer structure
ϩ
was nearly achieved by Si MLE, and the boron-doped p layer
designed to be 5 nm thick at the carrier concentration of 1.1
ϫ 1020 cm was measured to be 6.8 nm ͓full width at half maxi-
mum ͑fwhm͔͒.
Semiconductor Research Institute assisted in meeting the publication
costs of this article.
Ϫ3
References
Figure 7b shows the planar doped barrier ͑PDB͒ structure
n -i-p -i-n , which is the basic building block, namely, the source-
ϩ
ϩ
ϩ
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͑
1
1,12
the GaAs process.
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ϩ
the length of i-p -i in the structure. Therefore, the PDB structure in
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ϩ
phosphorus-doped n layer at the carrier concentration of 1
ϫ 1019 cm was inserted at the top and the bottom of the i-p -i
structure. The growth temperature in each layer of the PDB structure
was chosen as 510°C to reduce the deterioration of the doping pro-
file at a higher growth temperature. The result in Fig. 7b shows that
the design multilayer structure was nearly achieved by Si MLE, and
Ϫ3
ϩ
8. M. Suemitsu, Y. Tsukidate, H. Nakazawa, and Y. Enta, J. Vac. Sci. Technol. A, 16,
772 ͑1998͒.
1
ϩ
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the boron-doped p layer designed to be 1.2 nm thick at the carrier
concentration of 1.1 ϫ 1020 cmϪ3 was measured to be 5.7 nm
͑
fwhm͒. In this structure, the most probable diffusion source is bo-
ϩ
ϩ
11. P. Plotka, T. Kurabayashi, Y. Oyama, and J. Nishizawa, Appl. Surf. Sci., 82-83, 91
ron from the p layer by the electric field toward the n region
during a high-temperature growth process. By process simulation,
the fatal diffusion of boron occurred in the entire i layer during
͑
1994͒.
1
2. Y. X. Liu, P. Plotka, K. Suto, Y. Oyama, and J. Nishizawa, IEE Proc.-G: Circuits,
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