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SURFACE PASSIVATION EFFECT IN SGOI NANOWIRE
BIOSENSOR WITH HIGH GE FRACTION
Kow-Ming Chang
1,2
, Chu-Feng Chen
1
, Chiung-Hui Lai
3
, Cheng-Ting Hsieh
1
, Chin-Ning Wu
1
,
Yu-Bin Wang
1
, Chung-Hsien Liu
1
and Kuo Chin Chang
4
1
Dept. of Elec. Engineering and Inst. of Elec., National Chiao-Tung University, No. 1001
University Rd., Hsinchu 300, Taiwan, R.O.C.
2
Department of Electronic Engineering, I-Shou University, No.1, Sec. 1, Syuecheng Rd., Kaohsiung 840, Taiwan, R.O.C.
3
Dept. of Electronics Engineering, Chung Hua University, No. 707, Sec.2, WuFu Rd., Hsinchu, 300, Taiwan, R.O.C.
4
Dept. MD Tse-An Clinic 319, Chung Cheng Rd., Tse Kuan District, Kaohsiung City, Taiwan, R.O.C.
Keywords: SiGe-on-insulator, Biosensor, Passivation, Sensitivity.
Abstract: The increase of surface to volume ratio results in the enhancement of the sensitivity of the nanowires. Our
previous studies have shown that the higher Ge fraction of Si
1-x
Ge
x
nano-wire improves the sensitivity of
the nanowire biosensor as a result of carrier mobility enhancement in strain-Si. Increasing the fraction of Ge
in SiGe-on-Insulator (SGOI) using Ge condensation by oxidation has obtained a significant enhancement in
hole mobility, further improving the sensitivity of SGOI nanowire. However, the sensitivity of SGOI
nanowire was degraded for exceeding a Ge fraction of 20% (i.e., high Ge fraction), resulting from the
unstable surface state. In this work, a top surface passivation SiO
2
layer was deposited on Si
0.8
Ge
0.2
nanowire and the sensitivity was about 1.3 times greater than nanowire sample without the top passivation
layer.
1 INTRODUCTION
The sensitivity of SGOI can be enhanced by
increasing the surface to volume ratio to condense
carriers in very thin conductivity layer. The
conduction is modified by the surface charges that
surround the nanowire surface (Li, 2005). In our
previous studies, the higher surface to volume ratio
was achieved by utilizing SiGe/a-Si stacking
structure (Chang, 2011). Moreover, an increase of a
Ge fraction of Si
1-x
Ge
x
improves the nanowire
biosensor sensitivity due to higher carrier mobility
(Chang, 2008). Ultrathin SGOI with high Ge
fraction was fabricated by utilizing Ge condensation
and Ge piling up at the SiO
2
/SiGe interface by
oxidation. However, the higher interface trap density
at the SiO
2
/SiGe interfaces was about 10
-12
cm
-2
after
oxidation (LeGoues, 1989). Besides, an unstable
surface state of semiconductor with free surface
which caused by a lot of dangling bond at free
surface and higher Ge fraction of Si
1-x
Ge has higher
surface state and results in fast oxidation rate
(Tanaka, 2008). Fast oxidation rate can create more
unstable surface state (Yang, 2008).To reduce
unstable surface state, a SiO
2
passivation layer is
introduced to suppress surface state less than 10
11
cm
-2
eV
-1
.The surface state of free surface is around
10
15
cm
-2
eV
-1
. In this work, SiO
2
layer is selected as
passivation layer to improve interface state and O
2
gas buffer layer to reduce oxidation rate for 20% Ge
fraction of SiGe nanowire sensors.
2 EXPERIMENT
An a-Si/Si
1-x
Ge
x
was deposited on the patterned 300-
nm-hight bottom oxide. The deposition thickness of
a-Si is 200Å, and Ge fraction of Si
1-x
Ge
x
splits in
this experiment is 7, 14, and 20 %, respectively. To
clarify the influence of passivation layer on the
nanowire sensitivity, nanowire sensors with and
without the passivation SiO
2
layer were fabricated.
The passivation SiO
2
was split in two thicknesses,
100 and 200 Å. The poly-Si nanowire was also
fabricated as control group to verify oxidation rate.
After the a-Si/SiGe layer formation, the samples
384
Chang K., Chen C., Lai C., Hsieh C., Wu C., Wang Y., Liu C. and Chang K..
SURFACE PASSIVATION EFFECT IN SGOI NANOWIRE BIOSENSOR WITH HIGH GE FRACTION.
DOI: 10.5220/0003875403840387
In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 384-387
ISBN: 978-989-8425-91-1
Copyright
c
2012 SCITEPRESS (Science and Technology Publications, Lda.)