Charge transport in nanoparticular thin films of
zinc oxide and aluminum-doped zinc oxide†
Thomas Lenz,‡
a
Moses Richter,
a
Gebhard J. Matt,
*
a
Norman A. Luechinger,
b
Samuel C. Halim,
b
Wolfgang Heiss
c
and Christoph J. Brabec
a
In this work, we report on the electrical characterization of nano-
particular thin films of zinc oxide (ZnO) and aluminum-doped ZnO
(AZO). Temperature-dependent current–voltage measurements
revealed that charge transport for both, ZnO and AZO, is well
described by the Poole–Frenkel model and excellent agreement
between the experimental data and the theoretical predictions is
demonstrated. For the first time it is shown that the nature of the
charge-transport is not affected by the doping of the nanoparticles
and it is proposed that the Poole–Frenkel effect is an intrinsic and
universally limiting mechanism for the charge transport in nano-
particular thin films with defect states within the bandgap.
Zinc oxide (ZnO) – aII–VI compound semiconductor with a
large band gap (3.37 eV)
1
– exhibits an excitingly widespread
range of potential applications,
2
such as lasers,
1
eld-effect
transistors,
3–5
transducers,
6
varistors,
7
sensors,
8
UV-detectors
9
and also thin-lm solar cells, where it can serve as an active
10,11
or interfacial
12,13
or electrode material.
14
Moreover, ZnO is a promising material in the eld of trans-
parent
3,4,15
and exible electronics,
15,16
because large-area solu-
tion processing on exible substrates using techniques like ink-
jet printing appears feasible.
17,18
The solutions for lm deposi-
tion are either based on the sol–gel route or on nanoparticle
synthesis. The latter has the advantage that the synthesis and
the lm deposition can be separated from each other. As
pointed out previously,
19,20
this allows cheap and high-
throughput synthesis at elevated temperatures, while the lm
deposition is achieved at lower temperatures, which is a
prerequisite for the use of exible (oen polymer-based)
substrates.
The problem of nanoparticulate ZnO thin lms deposited at
low temperatures is that they hardly reach the electric perfor-
mance of zinc oxide lms based on sputtering
21
or spray
pyrolysis.
22
Therefore, a better understanding of the electric
conduction in these lms is desired.
So far, most of the charge transport studies on ZnO nano-
particle lms used the transistor device structure: Meu-
lenkamp
23
and Roest et al.
24,25
studied ZnO nanoparticles with
small sizes (z5 nm) permeated in electrolyte solutions and it
was demonstrated that transport occurs via tunnelling between
discrete electronic states with or without additional thermal
activation depending on the characteristics of the electrolyte.
25
Besides, space-charge limited conduction (SCLC) was shown for
ZnO by Bubel et al.
19
and by Caglar et al.
26
(sandwich device
geometry). In these two cases,
19,26
the sizes of the nano-
structures were larger (above 25 nm) compared to the above-
mentioned reports. This might partially explain the different
ndings.
Discrepancies between various transport investigations were
also revealed for other nanoparticular materials, e.g. for nano-
structured lms of silicon. Besides hopping,
27,28
SCLC,
29
the
Poole–Frenkel effect,
30,31
and tunnelling
32,33
were reported for
nano-Si (porous silicon or nanoparticles). These inconsistent
results further emphasize the need for a detailed description of
charge transport in nanoparticular lms.
In this work, the charge transport of nanoparticulate ZnO
and AZO thin lms was investigated. A dispersion of ligand-
stabilized ZnO or AZO nanoparticles was deposited on top of an
ITO substrate via multiple doctor blading
34,35
steps. The result-
ing lm thickness is 0.7–1 mm. Aerwards the sample was
transferred to a vacuum chamber equipped with a physical
vapor deposition (PVD) system for deposition of a 100 nm thick
Ag top contact (for further details see ESI†).
a
Institute Materials for Electronic and Energy Technology (i-MEET), Department of
Materials Science, Friedrich-Alexander-Universit
¨
at (FAU), Martensstraße 7, 91058,
Germany. E-mail: gebhard.matt@ww.uni-erlangen.de; Fax: +49 9131 8528495; Tel:
+49 9131 8527726
b
Nanograde Ltd, Staefa, Laubisr
¨
utistr. 50, CH-8712, Switzerland
c
Institute for Solid-State and Semiconductor Physics, Johannes Kepler University, Linz,
Austria
† Electronic supplementary information (ESI) available: The experimental
procedure, the symmetry of the J–V data of AZO and ZnO, the tting of the AZO
J–V plot and tables containing the data related to Fig. 2b and 3b. See DOI:
10.1039/c4tc01969e
‡ Current address: Max-Planck-Institute for Polymer Research, Ackermannweg 10,
55128 Mainz, Germany.
Cite this: J. Mater. Chem. C,2015,3,
1468
Received 2nd September 2014
Accepted 22nd December 2014
DOI: 10.1039/c4tc01969e
www.rsc.org/MaterialsC
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The commercially available ZnO/AZO dispersions from
Nanograde Ltd were selected for the investigations. These
nanoparticles were produced by ame spray synthesis
36–38
and
are expected to have high crystallinity and no adverse surface
ligands. In a ame spray synthesis a precursor solution of
Zn-acetate is fed (5 ml min
1
) to a spray nozzle, dispersed by
oxygen (7 l min
1
) and ignited by a premixed methane–oxygen
ame (CH
4
: 1.2 l min
1
,O
2
: 2.2 l min
1
). For the synthesis of
AZO nanoparticles with a nominal composition of 2 wt% Al
2
O
3
in ZnO, the same Zn-acetate precursor with additional Al-ace-
tylacetonate 2-ethylhexanoic acid under the same process
conditions was used. The nanoparticles were collected by air-
ltration of the off-gas. In order to prepare stable suspensions
of ZnO/AZO, 5 wt% of nanoparticles were dispersed in ethanol
by the use of a phosphonate-based ligand.
The ZnO/AZO nanoparticles are crystalline with a diameter
of 10–15 nm (see Fig. 1). Due to the high temperatures within
the ame spray synthesis and the precisely controlled process
conditions, the nanoparticles exhibit high purity and controlled
stoichiometric correlation between the dopant material and the
oxides.
Fig. 2a and 3a depict the current density–voltage (J–V) char-
acteristics for AZO (thickness of 950 nm) and ZnO (740 nm),
respectively, at various temperatures. The J–V characteristics
appear to be symmetric (within 5%) with respect to the bias (see
Fig. S1†). We tested different models to t the J–V data of both
materials. Consistent and precise results could only be obtained
with a combination of Ohm's law for low voltages and the
Poole–Frenkel (PF) effect for higher voltages (see Fig. S2†). The
former is not unexpected, as Ohm's law is usually valid for the
low-voltage – intrinsic – regime of a low-conductivity material.
39
In contrast, the occurrence of the PF effect
40–42
is rather
surprising, as it was originally derived for band-like insulators,
such as Si
3
O
4
(ref. 43) or Ta
2
O
5
,
44
which contain traps limiting
conductivity. The PF effect describes the lowering of the trap
barrier height F due to an external electric eld E and
conductivity s as follows:
40–42
s ¼ s
const
exp
b
ffiffiffiffi
E
p
F
kT
: (1)
Here, s
const
is the conductivity constant, k is the Boltzmann
constant and T is the absolute temperature. The factor b
deserves special attention, because it only depends on funda-
mental physical constants and the dielectric constant 3
r
of the
material
b ¼
ffiffiffiffiffiffiffiffiffiffiffi
e
3
3
0
p3
r
s
; (2)
Fig. 1 Transmission electron microscopy images for AZO and ZnO
nanoparticles.
Fig. 2 (a) Current density–voltage (J–V) characteristics of AZO (950
nm) recorded at different temperatures. The inset shows the device
setup. (b) Natural logarithm of conductivity determined for the J–V
data of AZO versus the square root of the applied electric field. This
plot corresponds to eqn (3). (c) The y-intercepts of the linear fit
functions (lns
0
) in (b) are plotted versus the inverse of temperature to
determine the trap barrier height.
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where e is the electronic charge and 3
0
is the permittivity of
vacuum.
For the determination of b
exp
based on the J–V data of ZnO
and AZO, we apply the natural logarithm to both sides of eqn (1)
leading to
lnðsÞ¼
b
kT
ffiffiffiffi
E
p
þ
lnðs
const
Þ
F
kT
: (3)
Following eqn (3), Fig. 2b and 3b depict the natural loga-
rithm of conductivity versus the square root of the electric eld
for AZO and ZnO, respectively. Both graphs show broad linear
regimes for the datasets for all considered temperatures. The
slopes and the y-intercepts of these linear regimes are listed in
Tables 1 and 2 in the ESI† together with the resulting values of b.
The experimentally derived values of b
exp
are not temperature
dependent with mean values of b
exp
¼ 2.64 0.06 10
5
eV m
0.5
V
0.5
for ZnO and b
exp
¼ 2.42 0.11 10
5
eV m
0.5
V
0.5
for
AZO. This is in extraordinarily good accordance with the theo-
retical values of b
theo
¼ 2.57 10
5
eV m
0.5
V
0.5
based on 3
r
¼
8.72 for ZnO and of b
theo
¼ 2.43 10
5
eV m
0.5
V
0.5
based on 3
r
¼ 9.72 for AZO. The respective dielectric constants were
measured with an independent impedance measurement and
are conrmed by results from the literature.
45
We emphasize that our results are in stark contrast to
comparable transport studies investigating the PF effect in
nanostructured silicon.
30,31
In those reports, the experimentally
and theoretically derived values of 3
r
, which directly correlate
with b, were completely different. It was argued that the theory
behind the PF effect might not be adaptable to nanostructured
lms, because these lms would not provide a straight-forward
physical meaning of 3
r
. Here, we show that b
exp
and b
theo
can
indeed match remarkably and thus unambiguously prove the
PF effect. We note that the values of b
exp
at lower temperatures
show a larger deviation from b
theo
for AZO, but the plot of ln(s)
versus
ffiffiffi
E
p
still provides broad linear regimes particularly for
higher voltages. This rules out other transport mechanisms,
such as tunneling or SCLC.
46
Another possibility which should
also be considered is Richardson–Schottky emission: it is the
attenuation of a metal–insulator barrier arising from electrode
image–force interaction with the eld at the metal–insulator
interface.
47
The Richardson–Schottky (RS) effect and the PF
effect are easily mixed up, as the J–V characteristics follow the
same functional dependency on the electric eld but the expo-
nent for the RS effect is by a factor of 2 smaller. Therefore, our
results clearly underline that transport in nanoparticular ZnO
and AZO follows the PF theory and not the RS theory. Moreover,
the symmetry of the J–V characteristics despite the asymmetric
contacts and the lm thickness in the micrometre range clearly
demonstrate that the conduction through the sandwich devices
is bulk-limited (Poole–Frenkel), and not electrode-limited
(Richardson–Schottky).
We sought to determine the barrier height F for AZO and
ZnO. From eqn (1) it is evident that the conductivity at zero
electric eld (s
0
) is thermally activated. Fig. 2c and 3c show an
Arrhenius plot of (s
0
) for ZnO and AZO. Note that ln(s
0
) equals
the y-intercept of the linear regimes in Fig. 2b and 3b. The
activation energies determined from the slopes in Fig. 2c and 3c
are F
AZO
¼ 135 meV and F
ZnO
¼ 337 meV, respectively.
We emphasize that the activation energy of ZnO is by more
than a factor of 2 higher than that of AZO but b
exp
is practically
the same for AZO and ZnO. This suggests that the nature of the
charge transport mechanism is equivalent for intrinsic ZnO
and AZO.
The question arises why electron transport through these
nanoparticular thin lms is so well described by the PF theory.
Due to the low post-deposition temperature, the thin lms of
AZO/ZnO can be considered as 3D assemblies of single nano-
particles (neck formation between neighboring nanoparticles is
not expected at 80
C).
23
Electron conduction across the lm
Fig. 3 (a) Current density–voltage (J–V) characteristics of ZnO (740
nm) recorded at different temperatures. The inset shows the device
setup. (b) Natural logarithm of conductivity determined for the J–V
data of ZnO versus the square root of the applied electric field. This
plot corresponds to eqn (3). (c) The y-intercepts of the linear fit
functions (lns
0
) in (b) are plotted versus the inverse of temperature to
determine the trap barrier height.
1470
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requires that electrons are transferred from one nanoparticle to
its next neighbor and so forth.
Practically such a hopping transport at higher temperatures
is very likely to be thermally activated following sfexp
DE
T
.
It is noted that a more complex temperature dependency is
typically just observed in the limit of low temperatures and/or in
amorphous systems.
48,49
On the other hand, the electric eld oen strongly inuences
charge transport in such systems.
50,51
However, to the best of the
authors' knowledge, this is the rst time that Poole–Frenkel-like
charge transport was found in nanoparticulate thin lms.
In order to experience the PF effect, a trap is required to be
positively charged when empty and uncharged when lled.
41
Consequently, the interaction between the positively charged
trap and the trapped electron gives rise to a coulombic barrier.
The question remains what the origin of the observed activation
energies in AZO (F
AZO
¼135 meV) and ZnO (F
ZnO
¼337 meV) is.
In the literature, defects in ZnO (single crystal or sputtered
lm) with similar energy values have been observed in DLTS
and impedance spectroscopy.
52–56
Ref. 53 suggests that these
defects might relate to the incorporation of oxygen in the crystal
lattice. We emphasize that the origin of defects in ZnO is
generally under debate in the literature
52–58
and that a direct
comparison with the literature needs to be done with caution,
as the defect density is sensitively affected by the growth
method and annealing conditions. In addition, the ame spray
synthesis utilized in this work is a new and novel production
method.
The lower observed activation energy for the AZO thin lms
is due to the higher free electron concentration. Al is a shallow
donor in ZnO (in ref. 59, it was shown that Al is even substitu-
tionally shallow) and by the increased free charge-carrier
concentration the Fermi-energy is closer to the “transport-
band”. Consequently the thermal activation energy for the
charge transport is reduced. In either case the nature of charge
transport remains unaffected by the Al doping and is limited by
the inter-particle charge transport. We suggest that this might
be attributed to surface defects, which always occur due to
unsaturated bonds on the surface, but further investigations are
needed to test this hypothesis.
Conclusion
In this work, the conduction through nanoparticulate ZnO and
AZO thin lms was investigated. While the J–V characteristics at
low voltage obey Ohm's law, transport in the high voltage data
regime is dominated by the Poole–Frenkel effect. It is outlined
that the occurrence of the Poole–Frenkel effect is related to
coulombically bound electrons, which have to overcome a eld-
dependent barrier to the next nanoparticle. It is demonstrated
that the conduction mechanism is equivalent for the AZO
nanoparticles, where Al acts as a shallow donor. To the best of
our knowledge, this is the rst time that the Poole–Frenkel
effect was unambiguously demonstrated as the dominant
transport mechanism in a nanoparticulate thin lm.
Acknowledgements
This work has been partially funded by the Sonderfor-
schungsbereich 953 “Synthetic Carbon Allotropes” (DFG) and
the Gradko 1896 “in situ Microscopy” (DFG). We also thank the
support of Solar Technologies go Hybrid (SolTech) project.
Moreover, we would like to thank Wolfgang Grafeneder and
Ronald Wirth for technical support.
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