Demonstration of tunneling FETs based on highly scalable vertical silicon nanowires

Abstract

This letter demonstrates a vertical silicon-nanowire (SiNW)-based tunneling field-effect transistor (TFET) using CMOS-compatible technology. With a Si p + −i−n + tunneling junction, the TFET with a gate length of ∼200 nm exhibits good subthreshold swing of ∼70 mV/dec, superior drain-induced-barrier-lowering of ∼17 mV/V, and excellent I on −I off ratio of ∼10 7 with a low I off (∼7 pA/μm). The obtained 53 μA/μm I on can be further enhanced with heterostructures at the tunneling in-terface. The vertical SiNW-based TFET is proposed to be an excel-lent candidate for ultralow power and high-density applications. Index Terms—Gate-all-around (GAA), top-down, tunneling field-effect transistor (TFET), vertical silicon nanowire (SiNW).

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754IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 7, JULY 2009
Demonstration of Tunneling FETs Based on Highly
Scalable Vertical Silicon Nanowires
Z. X. Chen, H. Y. Yu, N. Singh, N. S. Shen, R. D. Sayanthan, G. Q. Lo, and D.-L. Kwong
Abstract—This letter demonstrates a vertical silicon-nanowire
(SiNW)-based tunneling field-effect transistor (TFET) using
CMOS-compatible technology. With a Si p+−i−n+tunneling
junction, the TFET with a gate length of ∼200 nm exhibits good
subthreshold swing of ∼70 mV/dec, superior drain-induced-
barrier-lowering of ∼17 mV/V, and excellent Ion−Ioff ratio of
∼107with a low Ioff(∼7 pA/μm). The obtained 53 μA/μm Ion
can be further enhanced with heterostructures at the tunneling in-
terface. The vertical SiNW-based TFET is proposed to be an excel-
lent candidate for ultralow power and high-density applications.
Index Terms—Gate-all-around (GAA), top-down, tunneling
field-effect transistor (TFET), vertical silicon nanowire (SiNW).
I. INTRODUCTION
T
power applications [1]–[16], as it can offer a low leakage
current (Ioff), weak temperature dependence, and, more impor-
tantly, a subthreshold swing (SS) not limited to kT/q. On the
other hand, silicon nanowires (SiNWs) with a gate-all-around
(GAA) structure provide volume inversion of the thin silicon
body and excellent gate electrostatic control, resulting in im-
munity to short-channel effects and high transconductance [17].
In addition, vertical GAA SiNW MOSFETs, where the SiNW
was fabricated by either chemical synthesis [18] or using fully
CMOS-compatible technology [19], have been demonstrated.
In this letter, by marrying these two technologies, a highly
scalable vertical SiNW TFET is fabricated for low-power and
high-device-density applications. The vertical SiNW platform
is ideal for TFET fabrication, as source and drain implants can
be independently controlled without lithography. It is interest-
ing to note that the carbon-nanotube TFET with GAA has also
been reported mainly by simulation to provide high Ionand
low SS [14], [15]. In this letter, with Si p+−i−n+tunneling
junction, the TFET with a gate length of ∼200 nm exhibits
a good SS of ∼70 mV/dec, superior drain-induced barrier-
lowering (DIBL) of ∼17 mV/V, and excellent Ion−Ioff ratio
HE TUNNELING field-effect transistor (TFET) has
emerged as a promising device candidate for ultralow
Manuscript received February 3, 2009; revised March 23, 2009. First pub-
lished May 27, 2009; current version published June 26, 2009. The review of
this letter was arranged by Editor B.-G. Park.
Z. X. Chen and H. Y. Yu are with the School of Electrical and Electronic
Engineering, Nanyang Technological University, Singapore 639798, and also
with the Institute of Microelectronics, A∗STAR, Singapore 117685 (e-mail:
hyyu@ntu.edu.sg).
N. Singh, N. S. Shen, R. D. Sayanthan, G. Q. Lo, and D.-L. Kwong are
with the Institute of Microelectronics, A*STAR, Singapore 117685 (e-mail:
navab@ime.a-star.edu.sg).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2009.2021079
Fig. 1.
p-substrate. (b) Nanowire etch using deep RIE. (c) Vertical implant to define
n+region. (d) HDP oxide nonconformal deposition and (e) DHF etch-back.
(f) Gate oxide growth, poly-Si gate deposition, and patterning. (g) HDP oxide
deposition and DHF etch-back. (h) Isotropic etch of poly-Si tip. (i) Nanowire-
tip implantation. (j) Aluminum contact formation.
Vertical SiNW TFET process flow. (a) Nitride hard-mask definition on
Fig. 2.
showing a gate length of 200 nm and a bottom HDP oxide thickness of 80 nm.
The inset shows a SEM image of a vertical SiNW TFET after nanowire etching
(diameter of ∼70 nm).
of ∼107with a low Ioff(∼7 pA/μm). Given the smaller band
gap of Ge, the Ionof 53 μA/μm can be further enhanced with
heterostructures(e.g.,SiGe)atthetunnelinginterface,asshown
in simulation [7]–[12] and experimental [9] data.
TEM image of a vertical SiNW TFET with large diameter (∼750 nm)
II. DEVICE FABRICATION
The fabrication process is schematically shown in Fig. 1. A
nitride hard mask was first patterned on 8-in bulk Si wafers
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CHEN et al.: DEMONSTRATION OF TUNNELING FETs755
Fig. 3.
junction. (b) Id–Vgand (c) Id–Vn+characteristics of a vertical SiNW TFET with diameter of 70 nm, gate length of 200 nm, and gate oxide thickness of 4.5 nm.
(d) n-TFET transfer characteristics with Vn+= 0 and Vp+< 0, showing a Vtshift with Vp+.
(a) Device characteristics obtained from MEDICI simulations with varying diffusion characteristic length, Y.CHAR, of the p+doping profile at the p+−i
(p-type, ∼1015cm−3) by nanodots defined lithographically
using a 248-nm KrF scanner. This was followed by SiN etch
[Fig. 1(a)] and 400-nm Si etch using deep RIE [Fig. 1(b)].
Thermal oxidation at 1000◦C and DHF etch-back was done
to smooth the wire surface and to reduce the nanowire diam-
eter. Vertical nanowires with diameters of 30–800 nm were
obtained. A vertical As implant and activation was done to
dope the substrate and the bottom 80 nm of the nanowire n+
[Fig. 1(c)]. Nonconformal high-density plasma (HDP) oxide
deposition [Fig. 1(d)] followed by DHF etch-back [Fig. 1(e)]
was performed to cover only the bottom 80 nm of the wire (up
to the n+−p junction) before a 4.5-nm gate oxide was grown
and poly-Si was deposited, implanted with BF2(∼1019cm−3),
and patterned to form a gate extension pad [Fig. 1(f)]. HDP
oxide deposition and DHF etch-back was once again used to
expose only the top of the wire covered with poly-Si [Fig. 1(g)].
The oxide thickness would determine the resulting gate length
as the exposed poly-Si was then isotropically etched using RIE
to expose the nanowire tip [Fig. 1(h)]. A BF2implant at a tilt
of 45◦was done from four orthogonal directions to form the
p+region [Fig. 1(i)]. To prevent counterdoping of the poly-Si
gate, the same dopant type is used for the gate and top region.
Finally, a layer of HDP oxide is added before contact and
metal formation using aluminum [Fig. 1(j)]. The TEM image
of the fabricated device shown in Fig. 2 was taken on a wider
nanowire (diameter of ∼750 nm) for ease of sample prepara-
tion, showing the well-defined gate length of 200 nm. The inset
of Fig. 2 shows a vertical SiNW with diameter of ∼70 nm.
III. RESULTS AND DISCUSSION
TFETs are essentially gated p+−i−n+diodes working under
reverse bias and can operate as n- or p-TFETs. By controlling
the i region with a gate, a tunneling barrier can be created
either at the p+−i (n-TFET, Vg> 0) or n+−i (p-TFET, Vg< 0)
junctions where carriers are able to tunnel through [16]. The
reverse-biased p–i–n diode gives the TFET a low OFF-state
diffusion current.
The TFET fabricated in this letter was found to have poor
p-TFET characteristics, largely due to the more graded junction
at the bottom tunneling (n+−i) interface, compared to the top
interface (p+−i), which is caused by the larger thermal budget
applied after the vertical implant. This results in a much wider
depletion region at the tunneling junction. MEDICI simulations
of the vertical SiNW TFET were performed to study the effects
of junction abruptness on device performance. The characteris-
tic length of the doping profile at the p+−i junction, modeled
as a Gaussian function, is varied. In MEDICI, the characteristic
lengthparameter,orY.CHAR,isdefinedasthelengthwherethe
dopant concentration decreases by 1/e. Based on the MEDICI
simulations [Fig. 3(a)], the device performs poorer, with high
Vt, SS, and low Ion, as the doping profile of the tunneling
junction is more graded, i.e., Y.CHAR is larger. The results
agree with that of [13]. Therefore, the focus in this letter will
be on the n-TFET performance.
Shown inFig. 3(b) and (c)are the n-TFET Id–Vgand Id–Vn+
curves, respectively. These characteristics were obtained from
a vertical SiNW TFET with diameter of ∼70 nm, gate oxide
thickness of 4.5 nm, and gate length of 200 nm. Excellent
Ion−Ioff ratio at Vn+= 1.2 V is observed (∼107), with an
Ioff(at Vg= 0 V) of ∼7 pA/μm and Ion(at Vg= 1.2 V) of
∼53 μA/μm (normalized with the wire circumference). The
resulting high Ionand low DIBL (∼17 mV/V) for this Si TFET
is a result of the excellent gate control of the GAA nanowire
structure. From simulation [7]–[12] and experimental [9] data,
it is believed that having an Si–SiGe interface at the tunneling
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756IEEE ELECTRON DEVICE LETTERS, VOL. 30, NO. 7, JULY 2009
Fig. 4.
with (b) |Vp+| = 1 V and (c) |Vp+| = 1.5 V, illustrating the Vt shift with
|Vp+|. Not to scale.
Band diagrams of a TFET under (a) zero bias, and n-TFET operation
junction can further improve the drive current. The obtained
SS of 70 mV/dec is beyond the limit of kT/q (≈60 mV/dec),
which is likely due to the tunneling junction (p+−i) not being
perfectly abrupt.
The n-TFET Id–Vgcharacteristics withVn+=0and Vp+<0,
as shown in Fig. 3(d), shows a Vtshift with Vp+. This can be
explained using Fig. 4. Since the p+−i−n+diode is always
kept reverse-biased, i.e., Vn+> Vp+, an n-TFET (Vg> 0)
can operate at Vn+> 0 or Vp+< 0, with the other terminal
grounded. An n-TFET operating with a |Vp+| of 1 V, for
example, requires a Vgof 1 V to form a tunneling barrier with a
certain barrier width [Fig. 4(b)]. If |Vp+| is increased to 1.5 V, a
Vgof only 0.5 V is required to form a tunneling barrier with the
same barrier width [Fig. 4(c)], effectively reducing Vtby 0.5 V.
This shift in Vthas been observed in [6].
IV. CONCLUSION
The highly scalable vertical SiNW TFET has been demon-
strated and has shown to have a high Ion−Ioff ratio and a
fairly high Ionof 53 μA/μm. Such a device is ideal for low-
power high-density applications. It is believed that a SiGe
layer can be added at the tunneling interface to further en-
hance Ion.
ACKNOWLEDGMENT
The authors would like to thank all the staff of the Semi-
conductor Process Technology Laboratory, IME, for their help
in wafer processing. H. Y. Yu would like to thank the support
from a Nanyang Assistant Professorship.
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