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Toward a Systematic Understanding of Photodetectors
Based on Individual Metal Oxide Nanowires
Joan Daniel Prades, Roman Jimenez-Diaz, Francisco Hernandez-Ramirez, Luis
Fernandez-Romero, Teresa Andreu, Albert Cirera, Albert Romano-Rodriguez,
Albert Cornet, Joan Ramon Morante, Sven Barth, and Sanjay Mathur
J. Phys. Chem. C, 2008, 112 (37), 14639-14644• DOI: 10.1021/jp804614q • Publication Date (Web): 22 August 2008
Downloaded from http://pubs.acs.org on April 14, 2009
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Toward a Systematic Understanding of Photodetectors Based on Individual Metal Oxide
Joan Daniel Prades,*,†Roman Jimenez-Diaz,†Francisco Hernandez-Ramirez,
Luis Fernandez-Romero,†Teresa Andreu,†Albert Cirera,†Albert Romano-Rodriguez,†
Albert Cornet,†Joan Ramon Morante,†Sven Barth,‡and Sanjay Mathur*,‡,§
EME/XaRMAE/IN2UB, Departament d’Electronica, UniVersitat de Barcelona, C/ Marti i Franques 1,
Barcelona, E-08028, Spain, Nanocrystalline Materials and Thin Film Systems, Leibniz-Institute of New
Materials, Saarbruecken, D-66123, Germany, and Department of Inorganic Chemistry, UniVersity of Cologne,
Cologne, D-50939, Germany
ReceiVed: May 20, 2008; ReVised Manuscript ReceiVed: July 3, 2008
We present a set of criteria to optimize photodetectors based on n-type metal oxide nanowires and a comparison
methodology capable of overcoming the present lack of systematic studies dealing with such devices. The
response of photoconductors is enhanced following different fabrication strategies, such as diminishing the
distance between the electrical contacts, increasing the width of the photoactive area, or improving the electrical
mobility of the nanomaterials. The validity of the theoretical background is verified by experimental results
obtained with devices based on ZnO nanowires. The performances of our devices show that the normalized
gain of single ZnO nanowire-based photodetectors exceeds those of thin films.
Metal oxide nanowires are gaining growing interest as
photodetectors due to their potential applications in gas sensing
and optoelectronics.1Although preliminary works revealed
promising results,1-11further research is necessary in order to
reach complete control of their photosensing properties. Among
all n-type metal oxide nanomaterials, photoresponses of ZnO
and SnO2nanowires have been widely studied;1-11however,
the lack of well-established fabrication methodologies and
standardized procedures obfuscates a comparison of experi-
mental results. For instance, photoconductive gains (Gph) ranging
from 102to 108 4,9,10and response times (τ) between mil-
liseconds and hours were reported for ZnO nanowires,1,2,4,7,10
which possibly result from different experimental conditions
and device geometry used in all of these studies. Here, we
present some systematic strategies for enhancing the response
of photodetectors based on nanowires and a methodology for
the facile comparison of the measured photoresponses. These
strategies are fundamental to the potential of this technology in
real devices and applications.
ZnO nanowires were fabricated via a vapor-phase carboth-
ermal transport process inside an Atomate’s chemical vapor
deposition (CVD) system. The source material was a 1:1 molar
mixture of commercial ZnO (metal basis, 99.99%) and graphite
powder (crystalline, 300 mesh, 99%) from Alfa Aesar. Gold
nanoparticles were used as catalytic islands on thermally
oxidized Si wafers.12Uniform nanowires were obtained with
mean radius <r> ) 90 ( 15 nm and lengths up to 30 µm.
Some of them were electrically contacted to platinum micro-
electrodes using a FEI Dual-Beam Strata 235 FIB instrument
following a nanolithography process explained elsewhere.13-15
Electrical measurements were performed with the help of a self-
made electronic circuit designed to ensure low current levels
and to avoid undesired fluctuations16inside a homemade
chamber. Photoresponse was excited using both UV LEDs and
a UV lamp. UV LEDs were centered on λ ) 340 ( 10 nm and
λ ) 385 ( 15 nm (Seoul Optodevices T9F34C and Purple-Hi
E1L5M-4P0A2), whereas the UV lamp was a HAMAMATSU
LC8 light source with a type[-01] Hg-Xe lamp enhanced for
the line at λ ) 365 nm. Light intensity impinging on the
nanowires was determined with the help of a thermopile detector
* Corresponding authors: email@example.com, firstname.lastname@example.org.
†Universitat de Barcelona.
‡Leibniz-Institute of New Materials.
§University of Cologne.
Figure 1. Diagram of a metal-oxide structure with an arbitrary volume
of length L, width W, and thickness T, under a photon flux Φph.
Photocarrier generation is induced in the upper layer of the structure
until a depth R-1and leads to photoconduction (jph) under polarization.
J. Phys. Chem. C 2008, 112, 14639–14644
10.1021/jp804614q CCC: $40.75
2008 American Chemical Society
Published on Web 08/22/2008
(FieldMax-TOP). Up to 10 devices based on single ZnO
nanowires were fabricated, and their photoresponses were
studied as a function of different experimental parameters to
validate the theoretical discussion. Two-probe I-V measure-
ments revealed symmetric responses in great accordance with
results published elsewhere.17The effective voltage drop along
the nanowires was determined by decoupling the rectifying
contact from the purely resistive contribution of the nanowires
following the procedure described elsewhere.17To evaluate the
role of surface contribution on the photoresesponse of these
devices, some samples were coated with a 475 ( 50 nm thick
layer of PMMA (2 wt % poly (methyl methacrylate) in
dichloromethane) by spin coating. All experiments were per-
formed in real air atmosphere.
Results and Discussion
1. Theoretical Background. Photodetectors based on single
n-type metal oxide nanowires, modeled as an arbitrary volume
of length L, width W, and thickness T (Figure 1), can be studied
using the fundamental principles ruling light carrier generation
on semiconductors.18Thus, current density jphis given by the
where q is the elemental charge, ∆nphis the concentration of
generated carriers, and V is their velocity. ∆nph can be also
written as shown in eq 2,
where η is the quantum efficiency of carrier generation by one
photon, F is the absorption rate of photons, τ is the carrier
lifetime, and Vphis the photogeneration volume (Figure 1).
The approximation in eq 2 assumes a constant carrier
generation profile until the depth R-1, where R is the absorption
coefficient of the metal oxide at one fixed wavelength (see the
Supporting Information). ∆nphand τ are also related through
the following continuity equation,18
where gph is the generation rate of charge carriers under
illumination. Assuming that the concentration of electrons ∆nph
is independent of τ, the photocurrent dynamics at the rising (time
constant τr) and falling (time constant τd) edges are given by
(see Supporting Information),
where Iphis the photocurrent at the steady-state. However, if
an external field E is applied parallel to the longitudinal axis of
ohmic nanowires, then the velocity of carriers (V) can be
expressed in terms of the applied voltage (V) using the following
where µ* is the effective carrier mobility. According to the
Matthiessen’s rule, µ* can be divided into the bulk (µB) and
surface (µS) contribution according to eq 7.18
The absorption rate of photons F when metal oxide nanowires
are exposed to a flux of Φphphotons can be also expressed by
where ? is the fraction of photons not reflected by the surface,
and WL is the effective area of one nanowire (see Figure 1).
Therefore, using eqs 2, 6, and 8, current density jph can be
rewritten as eq 9
To evaluate the total photogenerated current Iph, which is the
experimental response of real devices, we assume that nanowires
are thick enough to absorb all the incident photons. That is to
Therefore, it can be deduced that thinner nanowires (T <
R-1) will lead to lower photoresponses. On the contrary, the
use of thicker nanowires (T . R-1) will not imply a further
signal enhancement. For instance, the penetration depth R-1of
near-UV photons (wavelength from 400 to 250 nm) in ZnO is
almost constant at 50 nm.19Thus, ZnO nanowires with radii
slightly above r ≈ 25 nm should be used to maximize photo-
response to UV-light in this wavelength range.
If the constant absorption profile approximation is maintained,
photocurrent Iphin nanowires that satisfy eq10 is given by eq
where three different contributions are clearly identified. The
first one is related to geometric parameters of the device (W/
L), the second one to the intrinsic properties of nanowires
(?ητµ*) and the third one only depends on the experimental
conditions (VΦph). The performance of these devices can be
also analyzed in terms of their photoconductive gain Gph, which
is defined in eq 12,18
where identical types of contributions are involved.
2. Experimental Validation and Discussion. 2.1. Geo-
metrical Aspects. Concerning the geometry of photodetectors,
eq 11 implies enhanced Iphwith increasing width (W) of the
photoactive area. To verify this, the responses of single ZnO
nanowires and lamellae were compared.20ZnO lamellae were
randomly obtained during the synthesis process of ZnO nanow-
ires when the density of catalytic gold nanoparticles is very high
(Figure 2). Other authors have postulated that the origin of these
structures could be related to variations of the oxygen concen-
tration during synthesis.20ZnO lamellae presented thicknesses
(T) comparable to the diameter of the nanowires (2r ≈ 220 nm),
but their widths (W) were up to 10 times larger, allowing us to
confirm the enhancement of Iphwith increasing photoactive areas
(Figure 2a). To compare the UV photosensing performances of
individual ZnO nanowires with ZnO lamellae, identical condi-
tions were used in all experiments (Φph) 3.3 × 1018ph m-2
J. Phys. Chem. C, Vol. 112, No. 37, 2008
Prades et al.
s-1; λ ) 340 ( 10 nm) and a comparable photocurrent (I˜ph)
value is obtained by normalization to the interelectrode distances
The photoresponse [Figure 2] obtained with a nanowire was
4.7 times lower than the one measured with the lamella shown
in Figure 2a. The average width of this lamellar structure was
WL≈ 1000 nm, whereas the nanowire showed WL≈ 220 nm
in diameter (WL/WNW≈ 4.5). In accordance to eq 11, the higher
active area of ZnO lamellae caused enhanced photocurrent
values I˜ph as predicted by geometrical factors and shown by
the presented values (WL/WNW≈ I˜ph(L)I˜ph(NW)).
Another convenient way to increase the width of the
photoactive area is using multi-nanowire-based configurations.
These devices can be realized by self-assembly techniques, such
as dielectrophoresis, to electrically contact large amounts of
nanowires in parallel.21,22This fabrication methodology prevents
parasitic effects arising from uncontrolled nanowire-nanowire
contacts, and the resulting devices admit higher currents (with
simpler conditioning electronics) without damaging the nanow-
ires16(see the Supporting Information). It is noteworthy that,
according to eq 12, the photoconductive gain Gphobtained with
these multi-nanowire configurations is equivalent to the gain
provided by one single nanowire, if all of them are identical.
The distance between contacts (L) also determines the
response of the photodetector since both Iphand Gphincrease
inversely with this parameter. L not only influences the
photocapture area (WL) but also determines the effective electric
field (E) inside the nanowire due to the bias voltage (V) applied
externally. Indeed, this second aspect dominates the overall
contribution of L to the photoresponse (see eqs 11 and 12). The
dependence of Iphand Gphon L was experimentally confirmed
by fabricating five electrical contacts separated at different
distances on an individual ZnO nanowire (Figure 3a), and
measuring the UV photoresponse between different pairs of
electrodes. It was experimentally found that Iphand Gphincrease
inversely with the distance between contacts L, in accordance
with eqs 11 and 12 (Figure 3b). Therefore, it can be concluded
that higher-gain photodetectors are obtained by diminishing this
fabrication-related parameter. The lower limit for L will strictly
depend on the precision of the nanolithography technique and
other size-associated phenomena such as diffraction, if L
approaches the wavelength of photons, or uncontrolled degrada-
tion effects produced when the rupture electrical field of the
metal oxide is overcome (see the Supporting Information). To
exemplify the later point, it can be roughly estimated that
nanowires contacted between two electrodes with a separation
of only 50 nm23and polarized at 5 V1,10will be subjected to
electrical fields as high as 1 MV/cm.
2.2. Intrinsic Properties of the Semiconductor. In addition
to geometrical factors, the dependence of Iphon intrinsic material
properties, such as η , τ, µ* and ? , have to be considered (see
eq 11). The spectral response of photodetectors is determined
by the quantum efficiency η, which was observed to increase
by up to 3 orders of magnitude when photons with energies
above the bandgap interact with these devices, compared to
typical responses obtained with sub-bandgap photons.24The
variation of quantum efficiency η as a function of incident light
wavelength (λ) was investigated. In Figure 7, it is demonstrated
how Iphscaled up when above-bandgap photons (λ ) 340 (
10 nm) interacted with a single ZnO nanowire-based device,
compared to low Iph response obtained with sub-bandgap
photons (λ ) 385 ( 15 nm). This result shows the importance
of tuning the bandgap of such photodetectors to select their
Figure 2. (a) ZnO lamella of length LL) 3.9 µm and average width
around WL≈ 1000 nm contacted with FIB nanolithography techniques.
(Inset) SEM image of the mixture of ZnO nanowires and ZnO lamellae
obtained after the synthesis process. (b) Normalized photoresponse I˜ph
to the distance between electrical contacts L of one ZnO lamella and
one ZnO nanowire (LNW) 13 µm) under identical conditions (Φph)
3.3 × 1018ph m-2s-1; λ ) 340 ( 10 nm). Iphis higher with the lamella
due to a larger value of W.
Figure 3. (a) ZnO nanowire with radius r ) 90 ( 10 nm contacted to
five contacts fabricated with FIB nanolithography techniques. (b)
Photoresponse Iph and photoconductive gain Gph (inset) of the ZnO
nanowire as a function of the distance L between electrical contacts
obtained with Φph) 3.3 × 1018ph m-2s-1, λ ) 340 ( 10 nm, and V
) 1 V.
Photodetectors Based on Individual Metal Oxide Nanowires
J. Phys. Chem. C, Vol. 112, No. 37, 2008 14641
active/blind spectral regions.11,18,24It is noteworthy that the
bandgap edge of nanowires depends not only on the material
but also on their dimensions.11,18Thus, controlling the radii of
nanowires is critical to tune the spectral sensitivity of the final
The photogenerated carrier lifetime (τ) is the second param-
eter directly related to the intrinsic properties of nanowires,
which is known to be higher in nanomaterials compared to bulk
due to the large surface-to-volume ratio and the formation of
deep level surface states.18,25For metal oxide nanowires, it is
generally accepted that photocarrier relaxation dynamics consists
of an initial decay process in the nanosecond range, explained
by the fast carrier thermalization and hole-trapping by surface
states, followed by a slow decay dependent on the surrounding
atmosphere and the nanowire surface coating.3,4,26,27This second
process, with characteristic time constants in seconds, dominates
the final response of nanowire-based photodetectors. For this
reason, the carrier lifetime contribution τ to the photoresponse
Iph (see eq 11) can be modified by controlling the surface
interactions of this type of nanowires. Time-resolved measure-
ments allowed us to estimate the rising time constant τrand
falling time constant τdexhibited by our devices (Figure 4). The
evolution in time (iph(t)) was in accordance with the theoretical
model (eqs 4 and 5), whereby values close to a few minutes
The third parameter related to the intrinsic properties of
nanowires is the electrical mobility µ/, which is known to be
dependent on their radii. In the case of ZnO, mobility values
ranging from 2 to 30 cm2/(V s) were reported for nanowires
with radii below r ≈ 100 nm.28-31The diminished mobility
increases up to the bulk value (∼200 cm2/(V s)) in thicker
nanowires.10This behavior is attributed to scattering and
trapping of the electrons by surface defect states and becomes
more significant in thin nanowires possessing higher surface-
to-volume ratios. Thus, thicker nanowires are convenient to
obtain optimal devices for µ* optimization due to minimized
surface contributions µS(eq 7). The limitation introduced by
the dependence of µ* with radius can be also circumvented by
passivating the nanowire surface, which is reported to dramati-
cally increase the mobility of ZnO nanowires (up to 1.000 cm2/
It was demonstrated that the mobility of ZnO nanowires
dramatically increased (up to ∼1.000 cm2/(V s)) with Si3N4/
SiO2,31polyimide,32poly(methyl methacrylate) (PMMA),33and
polyacrylonitrile34coatings. Therefore, we covered some devices
with PMMA by spin coating after eliminating the surface
contamination with oxygen plasma. The passivation layers were
approximately 500 nm thick, and presented high UV transpar-
ency (optical transmittance above 92% from 300 to 800 nm)
and extremely low conductance (below 0.1 nS) independent of
the illumination. The photoresponse of these devices was ∼9.5
times higher after coating (see Figure 5). To elucidate this point,
the mobility of the nanowires15before and after passivation were
estitmated to be µ* ≈ 3-5 cm2/(V s) and µ*(PMMA)≈ 40-53
cm2/(V s), respectively. This improvement in the nanowire
mobility (factor of ∼10) and the high transmittance of the
PMMA layer explain the increased photoresponse according to
Figure 4. Dynamic behavior of the photoresponse Iphmeasured with
an individual ZnO nanowire when a UV pulse is applied (dashed line)
(Φph ) 3.3 × 1018ph m-2s-1; λ ) 340 ( 10 nm; V ) 1 V). The
response fits with theoretically deduced exponential laws and exhibits
time constants of τr) 170 ( 3 s and τd) 300 ( 8 s.
Figure 5. Photoresponse (I˜ph≡ IL) normalized to the distance between
electrical contacts L of one single ZnO nanowire (LNW) 13 µm) before
and after coating with PMMA under identical illumination (Φph) 3.3
× 1018ph m-2s-1; λ ) 340 ( 10 nm). I˜phis higher after coating due
to surface passivation induced higher µ/.
Figure 6. Linear dependency of photoresponses Iph on the applied
voltage V of an individual ZnO nanowire (under two different photon
Figure 7. Photoresponse Iph of an individual ZnO nanowire under
different photon fluxes Φphat constant polarization voltage (V ) 1 V).
A linear dependence between these two parameters is experimentally
observed. Remarkably, photoresponse Iph is highly dependent on the
energy of the incident photons.
J. Phys. Chem. C, Vol. 112, No. 37, 2008
Prades et al.
The last intrinsic parameter of nanowires to be considered in
this work is the fraction of photons (?) not reflected by the
surface of the metal oxide, which was recently demonstrated
to be lower in photodetectors based on aligned nanowires
compared to thin films.35
2.3. Working Conditions. As far as the experimental condi-
tions are concerned, it can be expected from eqs 11 and 12 that
photoresponse raises linearly with applied voltage V and flux
of photons Φph. Figures 6 and 7 demonstrate the linear increase
in device response in dependence to the two parameters and
UV-light according to theory. This feature complicates compar-
ing most of the reported results, since different experimental
conditions were used in these experiments.1-10For this reason
we propose a more generic way to express eq 12,
which is a normalized photoconductive gain, independent of
the device geometry and the experimental conditions.
Apart from the photoconductive gain, the dynamic behavior
of these devices is also important for optimizing real devices.
Their low-pass bandwidth can be defined as BW ≈ (2πτ)-1.
Thus, the normalized gain per bandwidth becomes
From equation 15, it can be observed that the quantum
efficiency η and mobility µ/are the key parameters to evaluate
the overall performance of these photodetectors.
3. Comparison to State-of-the-art Devices. To substantiate
and validate the performances of our nanowire-based devices,
the photoresponse values were critically compared with previ-
ously reported results. Soci et al.10recently reported photocon-
ductive gain values of Gph,lit) 5 × 107in devices with Llit)
1 µm and <r>lit) 110 nm polarized at Vlit) 5 V for equivalent
illumination conditions, which is much higher than the typical
Gphvalues measured in our experiments. For example, Gph) 6
× 103was obtained with V ) 1 V, L ≈ 13 µm, and r ≈ 110
nm. However, an accurate comparison of these two devices can
be only achieved if the photoconductive gain Gphis rewritten
as shown in equation 14. If gphis calculated, we find that gph,lit
) 10-5m2/V and gph) 10-6m2/V, showing that the literature
value is only 1 order of magnitude higher than ours. This
discrepancy can be explained considering the mobility µ/and
lifetime τdof nanowires. On one hand, our ZnO nanowires have
response time constants close to τ ≈ 300s (Figure 4) and
exhibited mobility values of ∼3 cm2/(V s), which was estimated
following a procedure described elsewhere.15On the other hand,
Soci and co-workers reported τlit≈ 33 s and µlit
s), whose product (τlitµlit
(τµ*), which justifies the divergence of values in gph(see eq
14). Fully consistent results were obtained with the rest of
devices under test, demonstrating that gain normalization of
photodetectors to the geometry and polarization conditions is
necessary for adequate comparison. Moreover, comparison of
gphvalues allows the determination of which intrinsic properties
of the photoactive material must be improved to enhance the
performance of future devices. For example, according to the
experimental results, the electron mobility µ/should be im-
proved in our case. From all of these results, it can be concluded
that the experimental responses obtained with photodetectors
based on single nanowires are precisely modeled by the here-
summarized theoretical approach.
*≈ 270 cm2/(V
*) is a factor 10 time higher than ours
Finally, the same methodology was applied to compare the
photoresponse of ZnO nanowires and ZnO thin films.36In
general, higher photocurrent values Iphwere obtained with larger
grains,37-41due to increasing mobility which approaches bulk
behavior (∼200 cm2/(V s)) in high-quality thin films,42with
typical photoconductive gain values of Gph,lit
the polarization and geometric conditions reported by the authors
) 5 V; Lph,lit
) 10 µ), the normalized photoconductive
gain was found to be gph,lit
) 3 × 10-8m2/V, whose value is
clearly lower than the one reported with single ZnO nanowires.
On the contrary, the normalized gain per bandwidth of ZnO
thin films is significantly higher (gph,lit
V) than that obtained with ZnO nanowires (gph,lit
10-8m2Hz/V10), if τlit
eqs 14 and 15, it is concluded that the higher photogain achieved
with individual nanowires is mainly associated to the longer
lifetime of the photocarriers, which increases at the expense of
diminishing dynamic response.
TFBW ) 3 × 10-3m2Hz/
NWBW ) 5 ×
TF≈ 1.5 µs.38Comparing this result with
We presented in detail the principles ruling the response of
UV photodetectors based on metal oxide nanowires. Different
design and fabrication strategies to enhance their performances
were identified and discussed, such as controlling their geometry
or tuning the intrinsic electrical properties of nanowires. Most
of them were validated with experimental results obtained with
photodetectors based on single ZnO nanowires. Finally, a
rigorous methodology to compare different devices was pre-
sented, overcoming the present lack of systematic study in this
field. On the basis of this methodology we conclude that current
photodetectors based on single ZnO nanowires achieve better
normalized gains (up to 3 orders of magnitude) with slower
response compared with thin film devices.
Acknowledgment. This work has been partially supported
by the Spanish Ministry of Education (MEC) through the project
N-MOSEN (MAT2007-66741-C02-01), the UE through the
project NAWACS (NAN2006-28568-E), the Human Potential
ProgramsAccess to Research Infrastructuress, and the project
MAGASENS and CROMINA. J.D.P. and R.J.D. are indebted
to the MEC for the FPU grant. F.H.-R. is indebted to the MEC
for the FPU grant and the support of the Torres Quevedo
program (PTQ05-02-03201). Thanks are due to the German
Science Foundation (DFG) for supporting this work in the frame
of priority on nanomaterialssSonderforschungsbereich 277sat
the Saarland University, Saarbruecken, Germany.
Supporting Information Available: (I) Photogeneration
theory details: nonuniform absorption and transient solution and
(II) self-heating effects on nanowires. This information is
available free of charge via the Internet at http://pubs.acs.org.
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