Doping technology for silicon thin films grown by temperature-modulation molecular layer epitaxy

Article abstract of DOI:10.1149/1.1481531

Layer-by-layer growth of single-crystalline Si with doping was achieved by a temperature-modulation (TM) method combined with an intermittent supply of Si₂H₆ and dopant precursor B₂H₆ for p-type growth on Si(100). B₂H₆ was introduced at a predetermined timing of supply in several doping modes. This TM Si molecular layer epitaxy enabled the growth of a flat surface with layer-by-layer growth in a self-limiting manner on Si(100) and with an outstanding high carrier concentration of over 5 × 10²⁰ cm^-3 while maintaining selective epitaxy at the maximum temperature of 470°C. Based on the results of doping in several modes of growth, the incorporation mechanism of dopant on the growing surface is discussed with respect to the TM process.

Full text of DOI:10.1149/1.1481531

Journal of The Electrochemical Society, 149 ͑7͒ G399-G402 ͑2002͒
G399
0
013-4651/2002/149͑7͒/G399/4/$7.00 © The Electrochemical Society, Inc.
Doping Technology for Silicon Thin Films Grown by
Temperature-Modulation Molecular Layer Epitaxy
z
Jun-ichi Nishizawa,* Toru Kurabayashi, Toru Oizumi, Akihiko Murai,
and Takashi Yoshida
Semiconductor Research Institute of Semiconductor Research Foundation, Aoba-ku, Sendai 980-0862, Japan
Telecommunications Advancement Organization, Sendai Research Center, Aramaki-Aza Aoba-ku,
Sendai 980-0868, Japan
Layer-by-layer growth of single-crystalline Si with doping was achieved by a temperature-modulation ͑TM͒ method combined
with an intermittent supply of Si H and dopant precursor B H6 for p-type growth on Si͑100͒. B H6 was introduced at a
2
6
2
2
predetermined timing of supply in several doping modes. This TM Si molecular layer epitaxy enabled the growth of a flat surface
with layer-by-layer growth in a self-limiting manner on Si͑100͒ and with an outstanding high carrier concentration of over 5
2
0
Ϫ3
ϫ 10 cm while maintaining selective epitaxy at the maximum temperature of 470°C. Based on the results of doping in several
modes of growth, the incorporation mechanism of dopant on the growing surface is discussed with respect to the TM process.
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1481531͔ All rights reserved.
Manuscript submitted September 4, 2001; revised manuscript received January 22, 2002. Available electronically May 16, 2002.
growth chamber is about 1 ϫ 10Ϫ8 Pa. The substrates were heated
using a heating lamp, and the temperature was monitored and con-
trolled using the pyrometer, the output signal of which was coupled
to the control unit of the heating lamp system. The pyrometer was
calibrated by the temperature measured directly using a Pt-Pt•Rh
Recent progress in silicon devices has increased the importance
of small-scale fabrication techniques. Technological advances in the
growth of high-quality silicon layers with atomically accurate thick-
ness control are required before such high-speed devices with
nanometer-scale structures can be fabricated. Low-temperature and
low-damage processing techniques are suitable for the fabrication of
nanostructures, because the amounts of change of the doping profile
and defect density can be decreased. To achieve this, Nishizawa
et al. developed the atomic layer epitaxy ͑ALE͒ technique for the
͑
13%͒ thermocouple. Si͑100͒ phosphorus-doped n-type wafers fab-
ricated by the Czochralski method ͑approximately 10 ⍀ cm͒ were
used as substrates. In order to determine the Si epitaxial layer thick-
ness, the wafers were partially masked with SiO by thermal oxida-
2
1
growth of single-crystalline Si and referred to the method as mo-
tion and photolithography. After surface treatment by H SO :H O ,
2
2
4
2
2
lecular layer epitaxy ͑MLE͒. In Si MLE, growth is performed by
HCl:H O :H O, and diluted HF, the substrate was placed into a
2
2
2
alternating the supply of SiH Cl and H at the optimal temperature
2
2
2
load-lock chamber and transferred into the growth chamber. The
etched depth of the Si surface as a result of the diluted HF treatment
of 825°C.1 Lowering the growth temperature to 540-650°C was
3
achieved by alternately supplying SiH Cl and atomic hydrogen.
2
2
However, this temperature is still too high for the fabrication of fine
structures with doping profiles of sub-0.1 ␮m order. In the case of 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 SiH ,
4
4-6
Si H , and Si H is considered essential. Recently, we have re-
2
6
3
8
ported the growth method called temperature-modulation ͑TM͒ Si
7
-9
MLE using Si H6 in which the growth thickness per cycle satu-
2
rated at 0.3 monolayers when Si H supply was sufficient. Accord-
2
6
ing to an in situ observation of the Si surface reaction by real-time
7
mass spectroscopy, Si submonolayer growth is dominated by the
self-limiting adsorption of Si-H compounds at 380-480°C and the
9
desorption of hydrogen at 530°C. However, surface roughness due
to three-dimensional growth was considerable at the modulation
temperature of 530°C. Surface morphology was improved drasti-
cally by inducing two-dimensional growth by lowering the modula-
tion temperature to 470°C.8
In this work, TM Si MLE growth with doping has been studied
using the intermittent supply of Si H6 and a dopant precursor,
2
B H , for p-type growth on Si͑100͒.
2
6
Experimental
Figure 1 shows the growth sequence of TM Si MLE without
doping and the experimental setup used in this study for growth with
doping. The system consists of a growth chamber, gas supply con-
trol system for Si H , B H , and PH , a heating lamp system, a
2
6
2
6
3
temperature-control system with an optical pyrometer, a load-lock
chamber, and a vacuum pump system. The base pressure of the
*
Electrochemical Society Fellow.
E-mail: kurabayashi@hanken.jp
Figure 1. Schematic illustration of a growth sequence and TM Si MLE
system with doping.
z
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Journal of The Electrochemical Society, 149 ͑7͒ G399-G402 ͑2002͒
Figure 3. Carrier concentration and growth rate per cycle of boron-doped
film grown in each mode. The supply duration of Si H and B H was 4 s at
2
6
2
6
Figure 2. Growth modes for boron-doped films. (Si ϩ B)-TM: Si H6 and
2
1
.8 and 0.5 sccm flow, respectively, and the pauses were constant at 1 s in all
modes. In the TM process, heating time from TL ͑400°C͒ to TH ͑470°C͒ was
s, and cooling time was 5 s.
B H are supplied at the same time intermittently followed by TM. B-Si-TM:
2
6
intermittent B H supply is followed by intermittent Si H supply and TM.
2
6
2
6
6
Si-B-TM: intermittent Si H supply is followed by intermittent B H supply
2
6
2
6
and TM. B-TM-Si-TM: intermittent B H supply is followed by TM, which
2
6
in turn is followed by intermittent Si H supply with subsequent TM.
2
6
decreased with decreasing T , and 0.2 Å/cycle at 470°C, which
H
means 15% of the monolayer value for Si. On the basis of undoped
film growth, boron-doped p-type Si growth was examined with four
modes of gas supply and TM, as shown in Fig. 2. In the first mode,
as indicated by mode (Si ϩ B)-TM, Si H and B H are supplied
was 1 nm, by which the thickness of the epitaxial layer was cali-
brated. Prior to epitaxial growth, the substrate was heated in the
growth chamber to a temperature of approximately 600°C for 30 s to
remove adsorbed impurities. Undoped growth by Si MLE was per-
formed cyclically, as shown in Fig. 1. The Si H ͑99.99%͒ source
2
6
2
6
at the same time intermittently at T , and then the TM process is
L
2
6
performed after stopping the supply of gases. In the second mode,
was introduced under a controlled flow rate for a constant duration
B H is supplied intermittently followed by an intermittent Si H
2
6
2
6
at temperature T , then the TM process was performed. In the TM
L
supply at T , and the TM process is performed: mode B-Si-TM. As
L
process, the substrate temperature was raised from T to the modu-
L
lated temperature, T , and then cooled to T again. This periodic
H
L
gas supply and TM process were repeated until the desired layer
thickness was obtained. To obtain an atomically flat surface mor-
phology and to achieve selective epitaxy, TH was fixed at 470°C.
The epitaxial layer was selectively grown on the Si surface and no
growth or deposition occurred on the patterned SiO mask at T of
2
H
470°C. To determine the growth thickness, the steps formed by ep-
itaxial growth were measured using a microstylus profilometer after
the removal of SiO2 .
In this study, B H ͑5% B H in N ͒ was used as the dopant
2
6
2
6
2
precursor for p-type epitaxial growth. The dopant precursor was also
introduced intermittently on the surface in the TM process. This
periodic gas injection process was repeated until the desired film
thickness was obtained. The p-type layers doped with boron were
grown on n-type wafers, and 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 at room temperature without any
correction.
Results and Discussion
Thin, undoped Si epitaxial layers were fabricated by the TM
Si MLE method using Si H gas at T of 400°C and TH of 470°C.
2
6
L
With the decrease of TH from 530 to 470°C, the surface morphology
was improved drastically from a three-dimensional island feature to
a two-dimensional atomically flat surface. The growth rate per cycle
Figure 4. Dependences of the growth rate and the carrier concentration of
boron-doped p-type layer on B H supply in each mode.
2
6
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Journal of The Electrochemical Society, 149 ͑7͒ G399-G402 ͑2002͒
the third mode, Si-B-TM, an intermittent supply of Si H followed
G401
2
6
by B H6 and the TM process is also examined. Finally, in mode
2
B-TM-Si-TM, B H is supplied intermittently followed by TM, and
2
6
Si H is supplied intermittently followed by TM. These modes were
2
6
repeated for 600 cycles, and the characteristics of the grown layer
were measured.
The duration of Si H and B H supply was 4 s, and the pauses
2
6
2
6
were constant at 1 s in all modes. In the TM process, the heating
time from TL to TH was 6 s at the rate of 11.6°/s, and the cooling
time was 5 s at 14.0°/s with 0 s of retention.
Figure 3 shows the carrier concentration and the growth rate per
cycle of boron-doped film grown by each mode. In each mode, the
volumes of Si H and B H ͑5% B H in N ͒ supply are constant at
2
6
2
6
2
6
2
1
.8 and 0.5 sccm, respectively. In spite of the same amount of B H
2 6
supply in one cycle of each mode, the difference among modes was
considerable. The highest carrier concentration in the grown film
was obtained in the B-TM-Si-TM mode, followed by the B-Si-TM
mode, whereas the films grown in the (Si ϩ B)-TM and Si-B-TM
modes showed about one order lower carrier concentration. Since
the growth rates were not greatly influenced in each mode, the Si-H
adsorption process might not be influenced by the coverage of boron
compound. However, the coverage of boron compounds formed af-
ter B H supply is influenced by the mode. The grown surface was
2
6
atomically flat, and the epitaxial layer was selectively grown on the
Figure 5. Relationship between Hall mobility and the carrier concentration
of boron-doped p-type film for each doping mode.
Si surface; no deposition occurred on the patterned SiO mask. This
2
tendency was observed independently of the mode of growth. In
addition, single-crystalline film growth on the Si͑100͒ surface was
confirmed by reflection high-energy electron diffraction ͑RHEED͒
measurement.
heavy doping of above 5 ϫ 1020 cmϪ3 of carrier concentration in
The dependences of the carrier concentration of a boron-doped
mode B-TM-Si-TM with 1 sccm B H supply, the segregation of
p-type layer on B H supply in each mode are shown in Fig. 4. The
2
6
2
6
boron is assumed to occur because the Hall mobility decreases as
carrier concentration in each mode increased with increasing B H
2
6
the B H supply increases. Details of the behavior of segregated
supply in the range up to 0.5 sccm. A high carrier concentration was
achieved by growth in modes B-Si-TM and B-TM-Si-TM, in this
range of B H supply. This result means higher doping efficiency
2
6
boron will be discussed in another paper.
2
6
may be realized by supplying B H to the growing surface just after
Conclusions
2
6
the TM process. The surface after the TM is assumed to be fresh,
because the adsorption species of the materials and by-products, for
example, can release from the surface by higher temperature TM
process. Therefore, boron compound incorporation from B H on
We achieved TM Si MLE growth with doping by intermittently
supplying Si H and dopant precursor B H to induce p-type growth
on Si͑100͒. TM Si MLE under the conditions of an intermittent
2
6
2
6
2
6
Si H supply at 400°C followed by the TM process at 470°C en-
2
6
the surface after TM can be increased in modes B-Si-TM and
abled the growth of an atomically flat surface with layer-by-layer
B-TM-Si-TM. In addition, in these modes, supply of excess B H ,
2
6
growth in a self-limiting manner on Si͑100͒. B H was introduced at
2
6
as occurs at 1 sccm, leads to the decrease of carrier concentration.
This means that the amount of nonactive boron electrically increases
with increasing amount of B H under excess supply.
a predetermined timing of supply in several doping modes in TM Si
MLE. The higher carrier concentration was achieved by supplying
B H to the growing surface just after the TM process, in growth
2
6
2
6
On the other hand, over one order of magnitude lower carrier
concentration was obtained in modes (B ϩ Si)-TM and Si-B-TM
than in modes B-Si-TM and B-TM-Si-TM. This indicates that an
interactive competitive adsorption occurs between a boron com-
pound and a silicon compound in mode (B ϩ Si)-TM, because
B H and Si H are introduced at the same time on a fresh surface
modes B-Si-TM and B-TM-Si-TM. The carrier concentration of
20
Ϫ3
over 5 ϫ 10 cm
was attained while maintaining selective
epitaxy.
The surface after the TM process is assumed to be fresh based on
the desorption at higher temperature, therefore, the amount of boron
compound incorporation into the surface after TM can be increased.
On the other hand, competitive adsorption is dominant in conven-
tional growth methods. The intermittent injection of Si H and B H
2
6
2
6
that is formed by the TM process on Si͑100͒. The competitive ad-
sorption reduces the doping efficiency by over one order of magni-
tude compared with the growth in a high-efficiency mode. This com-
petitive adsorption phenomenon should be dominant for
conventional growth processes, for example, vapor phase epitaxy
and molecular beam epitaxy ͑MBE͒ using gas sources. In other
words, TM Si MLE with doping enables the control of the elemental
process steps of impurity incorporation and is advantageous for
achieving effective doping in the Si process.
Figure 5 shows the relationship between Hall mobility and the
carrier concentration of boron-doped p-type film for each mode of
doping. The mobility of the films grown in modes B-Si-TM and
B-TM-Si-TM showed higher values than those of the films grown in
other modes. In particular, the characteristics of the film grown in
mode B-TM-Si-TM showed the highest mobility, in addition to the
maximum doping efficiency, as shown in Fig. 4. In the region of
2
6
2
6
followed by the TM process in our method will be advantageous in
the future for achieving effective doping at low temperature.
The Semiconductor Research Institute of Semiconductor Research Foun-
dation assisted in meeting the publication costs of this article.
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