DPG conference in Dresden 2011
Fabrication and Characterization of WellAligned Zinc Oxide Nanowire Arrays and their
realizations in Schottky-Device Applications
Kin Mun Wong1, Liaoyong Wen1, Yaoguo Fang2, Fabian Grote1, Hui Sun1, and Yong Lei2
1Institute
of Materials Physics and Center for Nanotechnology, University of
Muenster, Wilhelm-Klemm-Str. 10, 48149 Muenster, Germany
2Institute
of Nanochemistry and Nanobiology, Shanghai University, Shanghai
201800, China
Outline
• Fabrication of well aligned ZnO NWs
arrays using UTAM as a substrate
• Device application of the ZnO NWs arrays
as Schottky Diodes
Fabrication of well aligned vertical ZnO
NWs on ultrathin alumina membranes
(a)
Au nanodots
UTAM
0.5 μｍ
(b)
１μｍ
(c)
1 μm
ZnO NWs on the UTAM are grown
by CVD at 550 oC under 400 mbar
at a constant nitrogen flow of
150 cm3/min
Well vertical alignment of the
ZnO NWs due to the well ordered
underlying UTAM pore structure
Growth process of the vertical well
ordered ZnO nanowires arrays
(a)
(b)
(d)
(c)
(a) Thermal Evaporation of Au on the UTAM surface
(b) ZnO + C → Zn + CO (by graphite), negative charges (e.g. C2O42-)
attract Zn atoms to the UTAM for the nucleation process to begin
(c) Mergence of the Zn atoms into (0001)-oriented quasihexagonal ZnO
nuclei upon precipitation and oxidation
(d) Due to the intense (0001)-oriented growth direction, vertical ZnO NWs
are formed after prolonged growth from the nuclei
Process of fabricating horizontal arrays
of ZnO NWs Schottky diodes
The ZnO NWs as
constructed by our
method are densely
packed (with
approximately 4x105 NWs
in an area of 1 cm-2) and
are demonstrated to be
functional as Schottky
diodes.
The overall process of fabricating horizontal arrays of ZnO NWs Schottky diodes
(a)
(b)
Length of the ZnO NWs reach
181 m in 40 minutes
(c)
(d)
Rapid growth of the NWs due
to the plentiful supply of zinc
vapor and oxygen near the
substrate
Temperature : 950 oC
Growth of the ZnO NWs by the CVD process
Dry contact printing process for the realization
of the well aligned arrays of ZnO Schottky diodes
(a)
(b)
Directional sliding of the donor
substrate with respect to the
receiver substrate
NWs were effectively sheared
from the donor substrate
(c)
(d)
90% of the NWs are highly
Aligned in the direction of the
Sliding motion
NWs are aligned along the
[0001] direction (or c-axis), retain
their orginal alignment on the
donor substrate
Separation between the electrodes
 20 m
Ag electrodes on the ZnO NWs
formed by thermal evaporation
I-V Characterization of the Zinc Oxide
Nanowire Arrays
(a)
Movement of the positive and
negative probes along the
neighbouring pairs of electrodes
for the I-V measurements
(b)
Rectifying behavior in the negative
voltage region with some
differences in the forward biased
current for all the 9 electrode pairs
I-V characteristics of horizontally aligned
ZnO NWs arrays of Schottky diodes
(b)
(a)
Zinc and oxygen alternating layers along the
c-axis results in a spontaneous polarization,
P which leads to a potential gradient along
the length of the individual NW
P, which is oriented along the NW direction
(towards the source electrode) leads to a
build in potential decreasing along the NW
c-axis direction
Decrease of the foward biased
current when the number of
sequential electrodes increases
due to the increased number of
Schottky barriers
Increase in the turn on voltage
(b)
20
(a)
16
#
1 electrode
#
2 electrode
#
3 electrode
#
4 electrode
#
5 electrode
#
6 electrode
#
7 electrode
#
8 electrode
#
9 electrode
I (nA)
12
8
4
25 μm
0
-3
-2
-1
0
1
2
3
Vds (voltage)
SEM image of a single ZnO
NW device on the Si substrate
Length of the ZnO NW ~ 100 m
I-V characteristics of
nine different single
ZnO NW Schottky diode
1,0
Single ZnO NW Schottky diodes
Barrier Height, d (eV)
0,9
0,8
The ideality factor and
the barrier height of the
single ZnO NW Schottky
diode is obtained from
the respective I-V
characteristics
0,7
0,6
0,5
0,4
8
10
12
14
16
18
20
Ideality Factor
Weak linear dependency of the barrier height on the ideality factor
indicates lateral inhomogeneity of the ZnO NW Schottky contacts due to
the difference in the inhomogeneity in the Schottky contact
corresponding to the different diode-to-diode interface
The high ideality factor is due to the interface states of the thin
interfacial layer between the Ag electrode and the ZnO NW as an
interfacial layer with surface states forms easily on the surface of the
ZnO NW
Conclusion
•Fabrication of well-aligned arrays of vertical ZnO nanowires using an UTAM
as a substrate
•Fabrication of a large area of repeatable, densely packed and horizontally
well-aligned arrays of ZnO NWs Schottky diodes along their c-axis using a
simple and viable two-step method
•Our technique for the realization of Schottky device applications opens a
possibility for the mass scale production of the ZnO NW based devices for
flexible electronics.
Acknowledgements
Surface Nano-Structuring Group
Group Leader: Prof. Dr. Yong Lei
Group Members:
Stefan Bartels
Fabian Grote
Christian Heckel
Peter Heß
Stefan Ostendorp
Dr. Hui Sun
Nina Winkler
Dr. Kin Mun Wong
Feng Xu
Dr. Shikuan Yang
Dr. Huaping Zhao
Funding
Thank you for your attention