IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 65, NO. 3, MARCH 2018 895
Back-Gate Modulation in UTB GeOI pMOSFETs
With Advanced Substrate Fabrication Technique
Zejie Zheng ,XiaoYu ,
Member, IEEE
, Yanyan Zhang, Min Xie, Ran Cheng ,
Member, IEEE
,
and Yi Zhao,
Senior Member, IEEE
Abstract
— In this paper, the back-gate modulation
in ultrathin body germanium-on-insulator (UTB GeOI)
p-channel metal–oxide–semiconductor field-effect transis-
tors (pMOSFETs) has been investigated.Based on the direct
wafer boding and polishing technique, high-performance
20-nm-thick GeOI pMOSFETs have been fabricated. Peak
hole mobility of 158 cm
2
/V · s and high-field mobility (
N
s
=
1 × 10
13
cm
−2
) of 121 cm
2
/V · s are obtained, respec-
tively. The back-gate modulation in UTB GeOI pMOSFETs
has been systematically investigated by experiments and
simulations. For the case that the back channel does not
form, the carrier distributions in the front channel become
wider with larger negative back-gate voltage
V
bg
applied.
As a result, the mobility increases because of the reduced
scattering in the whole
N
s
range. When
V
bg
is negative
enough, the back channel forms. In this case, the modula-
tion effect wouldbestrongly affectedbythe carriertransport
properties at the back channel.
Index Terms
— Back-gate modulation, metal–oxide–
semiconductor field-effect transistors (MOSFETs), ultrathin
body germanium on insulator (UTB GeOI).
I. INTRODUCTION
A
S THE feature size of Si-based metal–oxide–
semiconductor field-effect transistors (MOSFETs)
is scaling down to its fundamental limit, alternative channel
materials and device structures are extensively explored to
further improve the performance of complementary metal–
oxide–semiconductor (CMOS) transistors. Germanium is
considered as one of the most promising channel materials
for future CMOS technology, due to its higher mobilities for
both electron (3900 cm
2
/V · s) and hole (1900 cm
2
/V · s) than
those in Si [1]–[4]. In the past decade, high-performance
Ge p- and n-MOSFETs have been demonstrated by several
Manuscript received November 29, 2017; revised January 22, 2018;
accepted January 23, 2018. Date of publication February 9, 2018;
date of current version February 22, 2018. This work was supported
in part by the National Natural Science Foundation of China under
Grant 61504120, Grant 61604129, and Grant 61604130 and in part
by the Zhejiang Provincial Natural Science Foundation of China under
Grant LR14F040001. The review of this paper was arranged by Editor
R. Wang.
(Corresponding author: Yi Zhao.)
Z. Zheng, X. Yu, Y. Zhang, M. Xie, and R. Cheng are with the College
of Electronic Engineering and Information Science, Zhejiang University,
Hangzhou 310027, China.
Y. Zhao is with the College of Electronic Engineering and Informa-
tion Science, Zhejiang University, Hangzhou 310027, China, and also
with the State Key Laboratory of Silicon Materials, Zhejiang University,
Hangzhou 310027, China (e-mail: yizhao@zju.edu.cn).
Color versions of one or more of the figures in this paper are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2018.2798407
research groups [5]–[9]. To further improve the CMOS
performance, ultrathin body germanium-on-insulator (UTB
GeOI) MOSFETs are of great interests due to its process
simplicity and technical advantages such as back-gate
modulation, suppression of short-channel effects, reduction
of junction leakage, and capacitances. Recently, UTB GeOI
substrates have been realized with different fabrication
techniques [10]–[15], e.g., SmartCut, flipped GeOI, and
condensation techniques. Recent studies of the carrier
transport properties suggest that the performances of the UTB
GeOI MOSFETs are affected by two main factors: Ge crystal
quality and the MOS interfaces [16]–[20]. In addition, strain
engineering has been introduced into UTB GeOI MOSFETs
to further improve the device performance [21], [22]. On the
other hand, it is also known that the characteristics of the
-OI devices, such as carrier mobility, can be modulated
by back-gate voltage (V
bg
) [23]–[26]. However, the V
bg
modulation to the UTB GeOI channel is still not systematically
investigated in the past.
Recently, we have proposed a direct wafer bonding and
polishing (DWP) technique to realize high crystal quality UTB
GeOI substrates. Bulk Ge and bulk handle Si are directly
bonded and polished by combining mechanical polishing and
chemical mechanical polishing (CMP) methods [27]. Without
any further trimming process, high crystal quality is main-
tained similar with that of the bulk Ge wafer.
In this paper, based on the UTB GeOI substrates with
high crystal quality fabricated by DWP technique, 20-nm-thick
GeOI pMOSFETs have been fabricated and characterized.
Owing to the superior crystal quality of the GeOI sub-
strates, the peak and the high-field effective mobilities reach
158 and 121 cm
2
/V · s, respectively. In addition, the back-
gate modulation of UTB GeOI pMOSFETs are systematically
investigated to study its effect on the Ge hole mobility.
II. E
XPERIMENTS AND SIMULATIONS
To realize high crystal quality UTB GeOI substrates, DWP
technique was carried out as we described in [27]. The
(100) n-Ge substrate with a resistivity of 1–10 · cm and
heavily doped (100) p-Si substrate were used for bonding after
40-nm Al
2
O
3
deposition on both wafers. Then, the bulk Ge
layer was mechanically polished to ∼2 μm followed by a
CMP procedure to trim the Ge layer down to 30 nm with a flat
surface. Furthermore, thermal oxidation process was adopted
to thin the Ge layer to 20 nm without sacrificing the surface
roughness (root mean square, rms =∼0.2 nm).
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