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Charge transport in nanoparticular thin films of zinc oxide and aluminum-doped zinc oxide

05 Feb 2015-Journal of Materials Chemistry C (The Royal Society of Chemistry)-Vol. 3, Iss: 7, pp 1468-1472
TL;DR: In this paper, the authors report on the electrical characterization of nanoparticular thin films of zinc oxide (ZnO) and aluminum-doped ZnO (AZO).
Abstract: In this work, we report on the electrical characterization of nanoparticular 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 nanoparticular thin films with defect states within the bandgap.

Summary (1 min read)

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Journal of

  • These nanoparticles were produced by ame spray synthesis36–38 and are expected to have high crystallinity and no adverse surface ligands.
  • For the synthesis of AZO nanoparticles with a nominal composition of 2 wt% Al2O3 in ZnO, the same Zn-acetate precursor with additional Al-acetylacetonate 2-ethylhexanoic acid under the same process conditions was used.
  • Consequently the thermal activation energy for the charge transport is reduced.

Conclusion

  • 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 elddependent barrier to the next nanoparticle.
  • To the best of their knowledge, this is the rst time that the Poole–Frenkel effect was unambiguously demonstrated as the dominant transport mechanism in a nanoparticulate thin lm.

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Charge transport in nanoparticular thin lms 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 lms of zinc oxide (ZnO) and aluminum-doped ZnO
(AZO). Temperature-dependent currentvoltage measurements
revealed that charge transport for both, ZnO and AZO, is well
described by the PooleFrenkel model and excellent agreement
between the experimental data and the theoretical predictions is
demonstrated. For the rst time it is shown that the nature of the
charge-transport is not aected by the doping of the nanoparticles
and it is proposed that the PooleFrenkel eect is an intrinsic and
universally limiting mechanism for the charge transport in nano-
particular thin lms with defect states within the bandgap.
Zinc oxide (ZnO) aIIVI 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-eect
transistors,
35
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 solgel 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 dierent
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
PooleFrenkel eect,
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 0.71 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 JV data of AZO and ZnO, the tting of the AZO
JV 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
3638
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 methaneoxygen
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 o-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 1015 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 densityvoltage (JV) char-
acteristics for AZO (thickness of 950 nm) and ZnO (740 nm),
respectively, at various temperatures. The JV characteristics
appear to be symmetric (within 5%) with respect to the bias (see
Fig. S1). We tested dierent models to t the JV data of both
materials. Consistent and precise results could only be obtained
with a combination of Ohm's law for low voltages and the
PooleFrenkel (PF) eect 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 eect
4042
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 eect describes the lowering of the trap
barrier height F due to an external electric eld E and
conductivity s as follows:
4042
s ¼ s
const
exp
b
ffiffiffi
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
ffiffiffiffiffiffiffiffiffiffi
e
3
3
0
p3
r
s
; (2)
Fig. 1 Transmission electron microscopy images for AZO and ZnO
nanoparticles.
Fig. 2 (a) Current densityvoltage (JV) characteristics of AZO (950
nm) recorded at dierent temperatures. The inset shows the device
setup. (b) Natural logarithm of conductivity determined for the JV
data of AZO versus the square root of the applied electric eld. This
plot corresponds to eqn (3). (c) The y-intercepts of the linear t
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 JV data of ZnO
and AZO, we apply the natural logarithm to both sides of eqn (1)
leading to
lnðsÞ¼
b
kT
ffiffiffi
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 eect in
nanostructured silicon.
30,31
In those reports, the experimentally
and theoretically derived values of 3
r
, which directly correlate
with b, were completely dierent. It was argued that the theory
behind the PF eect 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 eect. 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
ffiffi
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 RichardsonSchottky emission: it is the
attenuation of a metalinsulator barrier arising from electrode
imageforce interaction with the eld at the metalinsulator
interface.
47
The RichardsonSchottky (RS) eect and the PF
eect are easily mixed up, as the JV characteristics follow the
same functional dependency on the electric eld but the expo-
nent for the RS eect 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 JV 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 (PooleFrenkel), and not electrode-limited
(RichardsonSchottky).
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 densityvoltage (JV) characteristics of ZnO (740
nm) recorded at dierent temperatures. The inset shows the device
setup. (b) Natural logarithm of conductivity determined for the JV
data of ZnO versus the square root of the applied electric eld. This
plot corresponds to eqn (3). (c) The y-intercepts of the linear t
functions (lns
0
) in (b) are plotted versus the inverse of temperature to
determine the trap barrier height.
1470
| J. Mater. Chem. C,2015,3, 14681472 This journal is © The Royal Society of Chemistry 2015
Journal of Materials Chemistry C Communication
Published on 22 December 2014. Downloaded by Universitat Erlangen Nurnberg on 23/01/2017 12:38:20.
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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 PooleFrenkel-like
charge transport was found in nanoparticulate thin lms.
In order to experience the PF eect, 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.
5256
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
5258
and that a direct
comparison with the literature needs to be done with caution,
as the defect density is sensitively aected 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 unaected 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 JV characteristics at
low voltage obey Ohm's law, transport in the high voltage data
regime is dominated by the PooleFrenkel eect. It is outlined
that the occurrence of the PooleFrenkel eect 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 PooleFrenkel
eect 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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1472 | J. Mater. Chem. C,2015,3, 14681472 This journal is © The Royal Society of Chemistry 2015
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References
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Journal ArticleDOI
TL;DR: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature.
Abstract: The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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08 Jun 2001-Science
TL;DR: Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated and self-organized, <0001> oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process.
Abstract: Room-temperature ultraviolet lasing in semiconductor nanowire arrays has been demonstrated The self-organized, oriented zinc oxide nanowires grown on sapphire substrates were synthesized with a simple vapor transport and condensation process These wide band-gap semiconductor nanowires form natural laser cavities with diameters varying from 20 to 150 nanometers and lengths up to 10 micrometers Under optical excitation, surface-emitting lasing action was observed at 385 nanometers, with an emission linewidth less than 03 nanometer The chemical flexibility and the one-dimensionality of the nanowires make them ideal miniaturized laser light sources These short-wavelength nanolasers could have myriad applications, including optical computing, information storage, and microanalysis

8,592 citations

Book
01 Jan 1970

3,145 citations

Frequently Asked Questions (9)
Q1. How was the dispersion of a phosphonate-based ligand prepared?

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. 

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. 

These nanoparticles were produced by ame spray synthesis36–38 and are expected to have high crystallinity and no adverse surface ligands. 

For the synthesis of AZO nanoparticles with a nominal composition of 2 wt% Al2O3 in ZnO, the same Zn-acetate precursor with additional Al-acetylacetonate 2-ethylhexanoic acid under the same process conditions was used. 

The problem of nanoparticulate ZnO thin lms deposited at low temperatures is that they hardly reach the electric performance of zinc oxide lms based on sputtering21 or spray pyrolysis. 

It is outlined that the occurrence of the Poole–Frenkel effect is related to coulombically bound electrons, which have to overcome a elddependent barrier to the next nanoparticle. 

Al is a shallow donor in ZnO (in ref. 59, it was shown that Al is even substitutionally shallow) and by the increased free charge-carrier concentration the Fermi-energy is closer to the “transportband”. 

A dispersion of ligandstabilized ZnO or AZO nanoparticles was deposited on top of an ITO substrate via multiple doctor blading34,35 steps. 

This plot corresponds to eqn (3). (c) The y-intercepts of the linear fit functions (lns0) in (b) are plotted versus the inverse of temperature to determine the trap barrier height.