Effects of postgate dielectric treatment on germanium-based metal-oxide-semiconductor device by supercritical fluid technology

Article abstract of DOI:10.1063/1.3365177

Supercritical fluid (SCF) technology is employed at low temperature as a postgate dielectric treatment to improve gate SiO₂ /germanium (Ge) interface in a Ge-based metal-oxide-semiconductor (Ge-MOS) device. The SCF can transport the oxidant and penetrate the gate oxide layer for the oxidation of SiO₂ /Ge interface at 150 °C. A smooth interfacial GeO ₂ layer between gate SiO₂ and Ge is thereby formed after SCF treatment, and the frequency dispersion of capacitance-voltage characteristics is also effectively alleviated. Furthermore, the electrical degradation of Ge-MOS after a postgate dielectric annealing at 450 °C can be restored to a extent similar to the initial state.

Full text of DOI:10.1063/1.3365177

Effects of postgate dielectric treatment on germanium-based metal-oxide-
semiconductor device by supercritical fluid technology
Po-Tsun Liu, Chen-Shuo Huang, Yi-Ling Huang, Jing-Ru Lin, Szu-Lin Cheng, Yoshio Nishi, and S. M. Sze
Citation: Applied Physics Letters 96, 112902 (2010); doi: 10.1063/1.3365177
View online: http://dx.doi.org/10.1063/1.3365177
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APPLIED PHYSICS LETTERS 96, 112902 ͑2010͒
Effects of postgate dielectric treatment on germanium-based metal-oxide-
semiconductor device by supercritical fluid technology
Po-Tsun Liu,1,a͒ Chen-Shuo Huang,2 Yi-Ling Huang,2 Jing-Ru Lin,2 Szu-Lin Cheng,3
Yoshio Nishi,4 and S. M. Sze5
1Department of Photonics and Display Institute, National Chiao Tung University, 1001 Ta-Hsueh Rd.,
HsinChu 30010, Taiwan
2Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University,
HsinChu 30010, Taiwan
3Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
4Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
5Department of Electronic Engineering, National Chiao Tung University, HsinChu 30010, Taiwan
͑Received 25 October 2009; accepted 25 February 2010; published online 17 March 2010͒
Supercritical fluid ͑SCF͒ technology is employed at low temperature as a postgate dielectric
treatment to improve gate SiO2/germanium ͑Ge͒ interface in a Ge-based metal-oxide-semiconductor
͑Ge-MOS͒ device. The SCF can transport the oxidant and penetrate the gate oxide layer for the
oxidation of SiO2/Ge interface at 150 °C. A smooth interfacial GeO2 layer between gate SiO2 and
Ge is thereby formed after SCF treatment, and the frequency dispersion of capacitance-voltage
characteristics is also effectively alleviated. Furthermore, the electrical degradation of Ge-MOS
after a postgate dielectric annealing at 450 °C can be restored to a extent similar to the initial
state. © 2010 American Institute of Physics. ͓doi:10.1063/1.3365177͔
Germanium ͑Ge͒ semiconductor has been considered
as an alternative channel material in place of silicon ͑Si͒
for future high-performance complementary metal-oxide-
semiconductor technology, because its higher carrier mobil-
ity for both electrons and holes, lower dopant thermal acti-
vation energies for shallower junction formation and
compatible fabrication processes with existing silicon manu-
facturing infrastructure. However, the Ge-metal-oxide-
semiconductor ͑MOS͒ technology still has many challenges
and not been widely deployed. The most critical issue hin-
dering the application of Ge is lack of high-quality and stable
Ge insulation oxide comparable to silicon dioxide ͑SiO2͒ for
silicon.1,2 The poor native Ge oxide layer would be thermally
decomposed at low temperature ͑about 420 °C͒ and Ge dif-
fuses into gate dielectric layer during the thermal deposition
or postdielectric annealing ͑PDA͒ processes. Sequentially,
poor interface properties and high gate leakage current will
be exhibited in the Ge-MOS device.3–7 Various pre-gate sur-
face modification techniques, such as surface nitridation or
Si passivation, have been developed to improve the quality
of gate insulator/Ge interface.13 It was also reported that
high-performance Ge MOSFET could be realized by careful
control of interfacial GeO2 formation.2 In this study, a low-
temperature supercritical CO2 ͑SCCO2͒ fluid technology is
proposed as a postgate dielectric treatment at 150 °C to im-
prove the SiO2/Ge interface after high-temperature PDA pro-
cess. The SCCO2, which exists above its critical pressure
͑1170 psi͒ and temperature ͑30 °C͒, provides good liquidlike
solvency and high gaslike diffusivity, giving it excellent
transport capacity.8 The oxidant is also easily dissolved in
SCCO2 fluid with specific surfactants. It is thereby allowed
for SCCO2 fluid to transport the oxidant and penetrates the
dielectric layer for trap passivation and interface oxidation at
low temperature.9–11 The SCCO2 fluid mixed with oxidant is
proposed in this work for the formation of interfacial GeO2
layer after gate dielectric deposition to prevent interfacial
decomposition from subsequent thermal processes.
A 0.5 ⍀ cm p-type ͑100͒ Si wafer was cleaned with
standard RCA clean process and immediately loaded into the
Applied Materials reduced-pressure chemical vapor deposi-
tion reactor. The initial 600 nm thick Ge film was grown at
400 °C with a GeH4 partial pressure of 8 Pa. Annealing
under H2 ambient was then performed at 825 °C for 40 min.
The growth temperature was ramped to 600 °C for the depo-
sition of another 1.4 ␮m-thick Ge layer at 8 Pa, followed by
a 15 min H2 bake at 750 °C. This epitaxial Ge ͑epi-Ge͒ layer
is p-type with an electrically activated concentration of
4ϫ1015 cm−3. The wafer was immediately loaded into a
low-pressure chemical vapor deposition ͑LPCVD͒ furnace
with 300 mTorr and a thin silicon dioxide ͑SiO2͒ layer was
deposited at 300 °C on top of the epi-Ge layer, as the gate
insulator of the following Ge-MOS device. It was followed
that the samples were divided into two groups for study in
this work. In the first group, the supercritical fluid ͑SCF͒
treatment was performed right after the gate SiO2 deposition
to enhance the Ge-MOS device performance. The sample
was placed in a SCF system at 150 °C for 1 h, where was
injected with 2000–3000 psi of SCCO2 fluid that were mixed
with 5 vol % of propyl alcohol and 5 vol % of pure H2O.
The propyl alcohol acts as
a
surfactant between
nonpolar-SCCO2 fluid and polar-H2O molecules, such that
the H2O molecules are uniformly distributed in SCCO2 fluid
and delivered into the gate SiO2 film to passivate defect
states. In the second group, the influence of PDA on the Ge
MOS device characteristics was studied further. The sample
after the gate SiO2 deposition was subjected to a PDA pro-
cess at 450 °C for 30 min in a vacuum furnace with 1
ϫ10−7 torr, and then the SCF post-treatment was imple-
mented with the same conditions as mentioned above. Fi-
nally, aluminum electrodes were thermally evaporated on the
a͒
Author to whom correspondence should be addressed. Electronic mail:
ptliu@mail.nctu.edu.tw. Tel.: 886-3-5712121 ext. 52994. FAX: 886-3-
5735601.
0003-6951/2010/96͑11͒/112902/3/$30.00
96, 112902-1
© 2010 American Institute of Physics
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112902-2
Liu et al.
Appl. Phys. Lett. 96, 112902 ͑2010͒
FIG. 1. The cross-sectional HRTEM images of LPCVD-SiO2 on epi-Ge
substrate ͑a͒ before and ͑b͒ after the treatment of 3000 psi SCCO2 fluids
mixed with 5 vol % of propyl alcohol and 5 vol % of pure H2O at 150 °C.
top surface of SiO2 film with an electrode area of 7.07
ϫ10−4 cm2 and the back side of silicon wafer to fabricate
Ge-MOS capacitors. The material analysis of x-ray photo-
electron spectroscopy ͑XPS͒ on epi-Ge channel layer was
also performed to examine the evolution of chemical bond-
ing before and after SCCO2 treatment. In order to clearly
distinguish the gate insulator/epi-Ge interface for signal col-
lection, the SCCO2 process was applied to a stack structure
of 13 nm thick HfO2/epi-Ge layers. It is noted that the HfO2
layer was in situ removed by Ar+ sputtering process before
XPS spectra collection. Therefore, the information of chemi-
cal bonding at the epi-Ge surface can be obtained after SCF
treatment.
Figures 1͑a͒ and 1͑b͒ show the cross-sectional high-
resolution transmission electron microscopy ͑HRTEM͒ im-
ages for the LPCVD-SiO2 film deposited on epi-Ge layer,
before and after the SCCO2 postgate dielectric treatment,
respectively. In Fig. 1͑a͒, the thickness of as-deposited SiO2
film is observed to be about 13.5 nm. After immersion of
SCCO2 fluids with oxidant ͑H2O molecule͒ at 150 °C for
1 h, the dielectric thickness above the Ge layer is increased
to about 16.6 nm in total, and a clear and even interface is
exhibited, as shown in Fig. 1͑b͒. It is inferred that the in-
crease of dielectric thickness and the even interface formed
are originated from the formation of interfacial germanium
oxide ͑GeOx͒ during the SCF treatment with excellent per-
meability. The following XPS analysis results will support
the inference.
FIG. 2. ͑Color online͒ ͑a͒ The C-V characteristics of Ge-MOS devices with
various SCF treatments. Electrical measurement with measuring frequencies
of 10, 200, and 500 KHz was conducted at 25 °C, and ͑b͒ XPS spectra of
Ge 3d signal on the epi-Ge channel layer with and without SCF treatment.
The circles and the lines present the experimental data and peak fitting
results, respectively. The inset indicates numerical intensity of chemical
bonds by calculating the XPS peak area. It shows that the signal intensity of
GeO2 bonding increases after SCF treatment.
The frequency dependence of capacitance-voltage ͑C-V͒
curves for the Ge-MOS device with various post-treatments
is studied at 300 K, as depicted in Fig. 2͑a͒. It is observed
that the inversion capacitance which occurs at positive gate
bias for p-type Ge exhibits frequency dispersion in different
levels. The frequency dispersion behavior is attributed to the
response of minority carrier generation from interface defect
states to measuring frequencies. The fast minority-carrier re-
sponse can be achieved at low frequency.12 Compared with
the case of lower interface state densities, the Ge-MOS ca-
pacitor with higher interface state densities also will present
a larger inversion capacitance, and the gap of the inversion
capacitances between both cases shrinks as the increase of
measuring frequencies. In this work, the inversion capaci-
tance of Ge-MOS device with SCF treatment declines fastest
and approaches to an ideal minimum capacitance as com-
pared to the one without SCF treatment, especially in the
high measuring frequency of 500 KHz. In addition, it was
creasing the SCF pressure. It is reasonably believed that the
solubility of oxidant ͑H2O͒ and surfactant ͑propyl alcohol͒ is
increased with increasing the CO2 pressure.14 The increased
pressure can also enhance the penetration of SCF to the in-
terface between gate dielectric film and epi-Ge layer. Figure
2͑b͒ shows XPS spectra of Ge 3d signal on the interface
between gate dielectric layers and epi-Ge before and after
SCF treatment. The detected signal of Ge 3d spectra prima-
rily comes from the surface of epi-Ge channel layer, since
the gate dielectric layer was in situ removed previously by
Ar+ sputtering before XPS spectra collection. The signals of
GeOx and GeO2 bonding were observed for both samples
from the XPS analysis. For the sample without SCF process,
it is inferred that the species of oxygen will oxidize the Ge
surface to form loose native oxide layer during the early
shown that the C-V frequency dispersion decreased with in-
stage of gate dielectric film deposition. After SCF treatment,
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112902-3
Liu et al.
Appl. Phys. Lett. 96, 112902 ͑2010͒
again that oxidant ͑H2O molecule͒ is effectively transported
into SiO2 film by the high-pressure SCF and passivates the
defect states generated in the Ge-MOS device during high-
temperature thermal PDA process.
In summary, a low-temperature SCCO2 process at
150 °C has been proposed to treat the gate oxide/epi-Ge
interface and restore Ge-MOS device degradation after a
high-temperature PDA process. It is observed that the uneven
and poor interface was easily formed during thermal deposi-
tion processes on epi-Ge layer. After the SCF treatment, a
smooth GeO2 interface layer is formed and the frequency
dispersion of inversion capacitance is alleviated. Further-
more, electrical degradation of Ge-MOS device after 450 °C
PDA process leads to the reduction of accumulation capaci-
tance and the increase of gate leakage current. The SCF
treatment also can transport the oxidant into the gate dielec-
tric layer and passivate the Ge-related defect states generated
by PDA process. Electrical characteristics of Ge-MOS device
are effectively recovered to an extent similar to the one be-
fore PDA process.
FIG. 3. The C-V characteristics of 450 °C PDA-treated Ge-MOS devices
before and after SCF post-treatments. The initial Ge-MOS device without
PDA process was referred to as the control sample. The inset shows the gate
leakage current density of PDA-treated Ge-MOS devices before and after
SCF post-treatment.
The authors acknowledge the financial support of the
National Science Council ͑NSC͒ under Contract Nos. NSC
96-2221-E-009-130-MY3, NSC 97-2218-E-009-003, and 97-
2918-I-009-003.
higher signal intensity of GeO2 bonding at the epi-Ge surface
is observed obviously. The results reasonably explain that the
oxidation at the epi-Ge/gate dielectric interface has occurred
by adding oxidant ͑H2O molecules͒ to the SCCO2 fluid with
excellent transport capacity. The formation of interfacial
GeO2 layer can smoothen the epi-Ge surface and alleviate
frequency dispersion of inversion capacitance.
The thermal stability and the effects of SCF treatment
on PDA-treated Ge-MOS device are investigated further for
realistic Ge-MOSFET fabrication consideration. Figure 3
shows C-V characteristics of 450 °C PDA-treated Ge-MOS
devices before and after SCCO2 post-treatment. The inset of
Fig. 3 also depicts the leakage current characteristics of the
PDA-treated Ge-MOS devices before and after SCCO2 post-
treatment. The least accumulation capacitance is observed in
the PDA-treated Ge-MOS device, about a 66% reduction
compared with the control sample ͑without PDA process͒.
The significant reduction of the accumulation capacitance
due to poor charge holding capability can be attributed to the
large leakage current of PDA-treated Ge MOS device, as
shown in the inset of Fig. 3. It was reported that thermal
process induces Ge decomposition and desorption into gate
dielectric layer. The incorporation of Ge in dielectric insula-
tor is believed to act as defect traps and thereby causes an
1S. Takagi, T. Maeda, N. Taoka, M. Nishizawa, Y. Morita, K. Ikeda, Y.
Yamashita, M. Nishikawa, H. Kumagai, R. Nakane, S. Sugahara, and N.
Sugiyama, Microelectron. Eng. 84, 2314 ͑2007͒.
2H. Matsubara, T. Sasada, M. Takenaka, and S. Takagi, Appl. Phys. Lett.
93, 032104 ͑2008͒.
3K. Prabhakaran, F. Maeda, Y. Watanabe, and T. Ogino, Appl. Phys. Lett.
76, 2244 ͑2000͒.
4J. Oh and J. C. Campbell, J. Electron. Mater. 33, 364 ͑2004͒.
5S. Van Elshocht, B. Brijs, M. Caymax, T. Conard, T. Chiarella, S. De
Gendt, B. De Jaeger, S. Kubicek, M. Meuris, B. Onsia, O. Richard, I.
Teerlinck, J. Van Steenbergen, C. Zhao, and M. Heyns, Appl. Phys. Lett.
85, 3824 ͑2004͒.
6N. Lu, W. Bai, A. Ramirez, C. Mouli, A. Ritenour, M. L. Lee, and D.
Antoniadis, Appl. Phys. Lett. 87, 051922 ͑2005͒.
7Q. Zhang, N. Wu, D. M. Y. Lai, Y. Nikolai, L. K. Bera, and C. Zhu, J.
Electrochem. Soc. 153, G207 ͑2006͒.
8C. W. Wang, R. T. Chang, W. K. Lin, R. D. Lin, M. T. Liang, J. F. Yang,
and J. B. Wang, J. Electrochem. Soc. 146, 3485 ͑1999͒.
9P. T. Liu, C. T. Tsai, T. C. Chang, K. T. Kim, P. L. Chang, C. M. Chen, and
H. F. Cheng, Electrochem. Solid-State Lett. 9, G124 ͑2006͒.
10P. T. Liu, C. T. Tsai, T. C. Chang, K. T. Kin, P. L. Chang, C. M. Chen, and
Y. C. Chen, IEEE Trans. Nanotechnol. 6, 29 ͑2007͒.
11P. T. Liu, C. T. Tsai, and P. Y. Yang, Appl. Phys. Lett. 90, 223101 ͑2007͒.
12P. Batude, X. Garros, L. Clavelier, C. Le Royer, J. M. Hartmann, V. Loup,
P. Besson, L. Vandroux, S. Deleonibus, and F. Boulanger, Microelectron.
Eng. 84, 2320 ͑2007͒.
increased gate leakage current.2,5,13 In this study, the imple-
mentation of SCF treatment after PDA process significantly
reduces leakage current of gate insulator and recovers the
C-V characteristic to a similar state as the initial Ge-MOS
device without PDA process ͑control sample͒. This indicates
13Y. Kamata, Mater. Today 11, 30 ͑2008͒.
14C. T. Tsai, T. C. Chang, K. T. Kin, P. T. Liu, P. Y. Yang, C. F. Weng, and
F. S. Huang, J. Appl. Phys. 103, 074108 ͑2008͒.
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