Advances in Materials Physics and Chemistry, 2011, 1, 70-77
doi:10.4236/ampc.2011.13012 Published Online December 2011 (
Copyright © 2011 SciRes. AMPC
Fabrication and Characteristics of Fast Photo Response
ZnO/Porous Silicon UV Photoconductive Detector
Hanan A. Thjeel1,2, Abdulla. M. Suhail1, Asama N. Naji1, Qahtan G. Al-Zaidi1,
Ghaida S. Muhammed1, Faten A. Naum1
1Department of Physics, College of Science, University of Baghdad, Baghdad, Iraq
2Department of Physics, College of Science, University of Wassit, Wassit, Iraq
E-mail: {Abdulla_shl, hananabd2004},,
Received September 28, 2011; revised October 28, 2011; accepted November 9, 2011
Fast response time UV photoconductive detector was fabricated based on ZnO film prepared by thermal
chemical spray pyrolysis technique. The ZnO nanofilms are grown on the porous silicon (PS) nanosurface
which has drastically reduced the response time of the ZnO UV detector from few seconds to few hundreds
of microseconds. The surface functionalization of the ZnO film deposited on porous silicon (PS) layer by
polyamide nylon has highly improved the photoresponsivity of the detector to 0.8 A/W. The normalized de-
tectivity (D*) of the fabricated ZnO UV detector at wavelength of 385 nm is found to be about 2.12 × 1011
cm·Hz1/2·W–1. The ZnO film grown on the porous silicon layer was oriented in the c-axis and it is found to be
a p-type semiconductor, which is referred to the compensation of the excess charge carriers in the ZnO film
by the nanospikes silicon layer.
Keywords: Porous Silicon, P-ZnO/PSi Junction, Nanostructure Materials, Photoconductive Detectors,
Recombination and Trapping
1. Introduction
The ultraviolet (UV) photoconductive detectors have
attracted a great interest due to its wide range of applica-
tions. Most of the applications are concentrated in the
environmental monitoring, missile warning system, and
solar astronomy [1,2]. In the last decade, the silicon ma-
terial was covered the UV detection region. The silicon
ultraviolet photodetectors have many dramatic limita-
tions, like the low quantum efficiency in the deep UV
range due to the passivation layer. The other limiting
factor is the age reduction of the Si photodiode exposed
to radiation of much higher energy than the Si band gap
The ZnO is a wide and direct band gap semiconductor
material, which has a potential application in the UV
detection [4]. The UV detector based on polycrystalline
ZnO thin film shows low responsivity and long response
time which is of the order of few minutes [5,6]. Since the
one-dimension ZnO nanostructures are characterized by
presence of deep level surface trap states, the ZnO de-
tector exhibits long lifetime of the photo carriers [7].
Despite a great deal of research on ZnO UV detector,
most of the research concentrated on the improvements
of the micro mask electrodes, in order to enhance the
performance of the ZnO photoconductive detectors [8-
The improvement of the photoresposivity of the ZnO
UV detectors was carried out by the surface treatment of
the ZnO thin film. The covering of the ZnO film surface
with nanosheet of different types of polymers has im-
proved the detector performance [11,12]. Coating the
ZnO film surface with polyamide nylon has improved
the photoresponsivity of the photoconductive detector to
about 2.24 A/W, but the response time still in few sec-
onds [13]. Most of published works concerning the en-
hancement of the response time are concentrated on fab-
rication of photodiodes [14]. In this work, a simple and
highly reliable technique is used to fabricate high speed
photoconductive ZnO UV detector of reasonable photo-
responsivity by depositing the ZnO nanofilm on nano-
spikes silicon layer.
2. Experimental Work
N-type Si wafer of 0.05 ·cm resistivity was used as a
starting material in the photochemical etching. The sam-
ples of 2 × 2 cm2 dimensions were cut from the wafer
and rinsed with acetone and methanol to remove dirt. In
order to remove the native oxide layer on the samples,
they were etched in diluted (10%) HF acid. After clean-
ing the samples they were immersed in HF acid of 50%
concentration in a Teflon beaker. The samples were
mounted in the beaker on two Teflon tablets in such a
way that the current required for the etching process
could complete the circuit between the irradiated surface
and the bottom surface of the Si sample.
Tungsten halogen lamp of 100 Watts integrated with
diacnamic ellipsoidal mirror was used as the photon
beam source. The photoetching irradiation time was
chosen to be 30 minutes.
At the end of the photochemical etching process, the
samples were rinsed with ethanol and stored in a glass
containers filled with methanol to avoid the formation of
oxide layer above the nanospikes film.
The morphology of the nanospikes surface produced
by photochemical etching on Si wafer is studied using
Scanning Probe Microscope. The nanospikes silicon lay-
ers were used as a substrate for the ZnO photoconductive
detector elements.
The ZnO nanofilms were prepared by chemical spray
pyrolysis technique. The films were deposited on porous
silicon layer heated to (400˚C). A 0.1 M Spray solution
is prepared by dissolving Zinc Chloride (ZnCl2) of 98%
purity in 100 ml distilled water. The above mixture solu-
tion was placed in the flask of the atomizer and spread by
controllable pressurized nitrogen gas flow on the heated
substrats. The spraying time was 4 seconds, which is
controlled by adjustable solenoid valve. The heated sub-
strate was left for 12 sec after each spraying run to give
time for the deposited (ZnO) layer to be dry. The opti-
mum experimental conditions for obtaining homogene-
ous ZnO thin film at (400˚C) were determined by the
spraying time, the drying time and the flashing gas pres-
The thickness of the prepared films was measured by
laser interferometer technique. The thickness of the films
was found to be in the range between (800 - 1000 μm).
The morphology of the film was scanned using Scanning
probe Microscope (type AA3000) from Angstrom Ad-
vanced Inc. working in tapping mode. The micro mask of
(0.4 mm) electrode spacing was used to deposit the gold
electrical electrodes on the film surface. The variation of
photoresponsivity of ZnO Photoconductive UV detector
with the bias voltage was carried out under the illumina-
tion with UV diode of 2.5 m Watt power and of 385 nm
wavelength. The operation circuit diagram of ZnO
photoconductive detector and the schematic cross section
of the fabricated ZnO porous silicon detector are shown
in Figures 1(a)-(b).
The response time of the prepared detector was tested
through illuminating the fabricated detector with a nitro-
gen laser type (LN 120 C frm) from laser photonics
company. The laser output pulses of 337.1 nm wave-
length, has energy of 10 μJ and 0.3 ns pulse duration.
The ZnO photocondutive detector output signal was dis-
played by digital oscilloscope of 200 MHz model TDS
202413 from Tektronix.
0.1 μF
Detector R
porous silicon
800 nm ZnO
electr o d e s
(a) (b)
Figure 1. (a) The operation circuit diagram of ZnO photoconductive detector where; Rd is the detector element, RL is the load
resistance and VC is the bias voltage
; (b) Schematic cross section of the fabricated ZnO porous silicon detector.
Copyright © 2011 SciRes. AMPC
3. Result and Discussion
3.1. The Surface Morphology Studies
The morphology and the line scanning of the porous
silicon layer etched in 30 minutes is illustrated through
the micrograph of the scanning probe Microscope, as
shown in Figure 2.
The figure shows that the nanospikes distribution for
the 30 minutes etching time is uniform and it is of about
2 nm heights, and around 1.5 nm dimensions. The for-
mation of the nanospikes layer increased the resistivity
of the silicon porous layer to the order of 105 ·cm. This
can be attributed to several reasons; the capturing of the
charge carriers by the traps at the nanospikes, the diffu-
sion of the impurity atoms to the electrolyte, or to the
wall of the pores and may be due to the passivation of
the impurity atoms with hydrogen [15,16]. The surface
morphology of the ZnO film deposited on (PS) is shown
in Figure 3. It can be noticed from the figure that the
nanostructure formed on the surface of the ZnO sample
deposited on PS is very clear. The size and distribution
of the nanocrystalline structure of the ZnO nanofilms,
deposited on the silicon nanolayer, are affected by the
Figure 2. (a) Scanning probe Microscope image of porous
silicon layer of 30 sec; (b) The histogram of the PS layer.
Figure 3. 2D and 3D Scanning Probe Microscope images of
ZnO thin film deposited on PS.
silicon nanospikes substrate in size and distribution.
3.2. Structural Characteristics
The X-ray diffraction (XRD) pattern of the 800 nm thick
ZnO nanofilm deposited on nanospike layer of n-type
silicon substrate is illustrated in Figure 4.
The figure shows the (100), (002), and (101) peaks
occurred at 2θ values of 33°, 34.4° and 36.25° respectively,
with full width at half maximum (FWHM) of (002) peak
of about 0.15°. The broadening peak around 69.8° is at-
tributed to the silicon nanospikes layer which has been
observed by other authors [17].
3.3. Optical and Electrical Properties
The room temperature photoluminescence (PL) spectrum
Copyright © 2011 SciRes. AMPC
of the prepared ZnO film on porous silicon layer is
shown in Figure 5.
The spectrum displays two luminescence peaks around
370 nm and 430 nm. The first peak (near-band edge) is
due to the intrinsic band to band transition which corre-
sponds to 3.35 eV and it is originated from the recombi-
nation of the free exciton. The second peak is due to do-
nor-acceptor pair emission at 2.88 eV with a relatively
high intensity ratio with respect to the first peak. The
deep level broadening luminescence was observed at 540
nm with low intensity compared with the band to band
transition peak and the donor-acceptor emission. The
quenching of the broad band intensity around 540 nm
may be attributed to the improvement of the crystalliza-
tion structure of the ZnO film deposited on the nanospikes
silicon layer. This result gives a good evidence of the re-
duction in the surface state formation when ZnO depos-
ited on porous silicon.
The absorption spectrum of ZnO nanofilm deposited
on glass substrate is shown in Figure 6. The figure
shows high absorption coefficient in the UV region,
whereas it is transparent in the visible region.
The variation of the photoconductive response of the
fabricated photoconductive detector as a function of the
bias voltage at dark and under illumination with UV
source of 2.5 mWatt radiation power is illustrated in
Figure 7.
It was found from the I-V measurements of the fabri
cated detector that the dark current was about 10 µA at
10 V bias whereas the photoconductive current was 2100
µA. This result reflects a good UV radiation sensitivity
with photoconductive gain (G) of more than 200.
The measured gain was calculated from the ratio of
carrier life time to carrier transit time. The correction
2030 4050 6070 8090
Theta- 2t h eta
(100) (101)
Figure 4. The XRD pattern of ZnO thin film on PS.
365 415 465 515 565
wavelength (nm )
photoluimence ( a.u )
Figure 5. The Photoluminescence (PL) spectrum of ZnO thin film deposited on PS.
Copyright © 2011 SciRes. AMPC
Figure 6. The absorption spectrum of ZnO thin film
glass substrate.
Figure 7. The variation of the photocurrent of the fabricated ZnO UV detector on porous silicon layer as a function of the
bias voltage.
factor include the trap saturation and the carrier bimol-
ecular recombination at the high light intensity which is
usually shortening the carrier life time was ignored .this
is because the gain was measured at low intensity which
is in order of (I - 10–4 W/cm2) [18].
The photoconductive gain (G) which is calculated
from the ratio between the photocurrent to the dark cur-
rent at the same bias voltage is given by; GT
where τ = is the charge carries life time, and T is the
transient time between the detector electrodes. The tran-
sient time is related to the electrode spacing and the car-
rier mobility by the relation; 2
, where L is the
electrodes spacing, µ is the carrier mobility and V is the
bias voltage. Using the values of G = 200, µ = 43.89
cm2/V·s as found from Hall measurements, L = 0.04 cm
and v = 10 V the carries life time (τ) was found to be
about 730 µs. This value is very close to the experimen-
tal value obtained in this work from the transient meas-
urements of the pulsed N2 laser using the fabricated ZnO
UV detector element. The specific detectivity D* which
is some time called the normalized detectivity, is the
reciprocal of the Noise Equivalent Power NEP normal-
ized to the detector area of 1 cm2 and a noise electrical
band width Δf of 1 Hz, and it can be written as:
Copyright © 2011 SciRes. AMPC
 (1)
where Rλ is the photoresponsivity of the photoconductive
detector in (A/W), A is the detector sensitive area and In
is the noise current which is estimated from the dark
current by the following relation:
eI f (2)
where Id is the dark current, e is the electronic charge and
Δf is the noise bandwidth. The value of dark current of
about 10 µA at the bias voltage of 10 V, lead to noise
current of about 1.8 × 10–12 A, at Δf = 1 Hz. Using the
value of photoresponsivity R = 0.8 A/W, A = 0.23 cm2
and In = 1.8 × 10–12 A, the specific detectivity of the fab-
ricated ZnO UV detector deposited on porous silicon
layer is found to be 2.12 × 1011 cm·Hz1/2·W–1. The value
of photoresponsivity (R = 0.8 A/W) was found with the
ZnO film coated by nano layer of polyamide nylon. This
polymer layer is working as antireflecting coating and
highly improves the ZnO photoconductive detector re-
sponse to the UV radiation [13].
The response time of the fabricated ZnO UV detector
on PS layer was tested with nitrogen laser of 0.3 ns pulse
duration and 10 μJ energy.
The trace of the output pulse on the digital oscillo-
scope of 200 MHz band width is illustrated in Figure 8.
It can be noticed from the traced signal that the rise time
(10% - 90%) was of the order of 180 μs and the fall time
(1 - 1/e) was about 750 μs. The slow decay time is due to
slow escape of holes from tarps. Hole captured into the
hole traps may be emitted back into the valance band
Figure 8. The photoresponse time of fabricated ZnO UV
detector to the nitrogen laser. The time base on x-axis is 250
according to a time constant depends on energy separa-
tion between the corresponding hole traps and the edge
of the valance band. Traps in a wide band semiconductor
such as ZnO are extremely deep which are centered
around 540 nm in abroad peak as shown in Figure 5 il-
lustrated in this work. The deep traps have slow response
and this may explain the slow fall time of the output
pulse as shown in Figure 8.
The long tail accompanied with the pulse is due to
sample heating by the high repetitions rate N2 laser illu-
minate the ZnO photoconductive detector. Comparing
this result with the response time measurements for the
ZnO photoconductive detector deposited on glass sub-
strate, prepared by the same technique, and working in
the same conditions, it is found that the speed of re-
sponse of the new detector is four orders of magnitude
faster than the sample prepared on glass substrate [13].
The huge observed reduction in the response time for
the ZnO photoconductive detector deposited on porous
silicon compared to the one deposited on glass substrate
can be attributed to the improvement in the electrical
properties of the ZnO film deposited on porous silicon.
The improvement of the electrical properties of the
ZnO film deposited on porous silicon is highly influ-
enced the performance of the ZnO UV detector. This
improvement can be noticed from the Hall measurement
of this film compared to same measurement for the film
deposited on glass substrate as in the Table 1.
The table shows that the ZnO film deposited on glass
substrate is n-type semiconductor, whereas the film de-
posited on PS layer has shown p-type behavior with Hall
coefficient of 6.63 × 104 m
2/C and carrier mobility of
43.89 cm2/V·s.
This behavior indicates that the porous silicon sub-
strate is beneficial to improve the crystalline quality of
ZnO film in lattice mismatch heteroepitaxy due to its
sponge-like structure [19]. The above result was checked
by depositing ZnO films on porous silicon layers formed
by different photochemical etching time and the results
are show that all the ZnO films deposited on nanospikes
silicon layers are p-type with different carriers mobility
depending on the etching time.
Table 1. The Hall measurements of ZnO film deposited on
porous silicon and glass.
Parameter ZnO on Porous silicon ZnO on glass
Bulk concentration cm–3 9.405E+13 –5.222E+18
Resisitivity (ρ) ·cm 1.512E+3 6.368
Average Hall coefficient
(RH) m2/c 6.63E+4 –1.195
Mobility(μ) cm2/V·sec 43.89 0.1877
Copyright © 2011 SciRes. AMPC
The silicon nanospikes tips may be working as a com-
pensator reducing the carrier concentration in the ZnO
deposited film which changes its polarity from n-type to
p-type. The same result was found by Vanmaekelbergh
and Liljeroth for ZnO film deposited on silicon substrate
[20]. The capturing of the excess charge carriers in the
ZnO film by the silicon nanospikes layer reduced the
surface charge density leading to increase the surface resis-
tivity of the deposited ZnO film. The ZnO nanocrystals
grow around the silicon nanospikes having a size similar
to the nanospikes dimension. Thus the size of ZnO
nanocrystals are reduced when the film is deposited on
porous silicon compare to its size when the film depos-
ited on glass. The nanocrystalline size reduction may
help in improvement of the speed of response of the ZnO
UV detector. The reduction in nanocrystalline dimension
helped in maximizing the surface to volume ratio. The
increase of the surface to volume ratio led to the increase
in the overlap of the electron and hole wave functions.
Since the increasing of the overlap functions account for
the reduction in the carriers recombination life time, the
speed of response of the ZnO UV detector is improved
for the ZnO nanofilm deposited on nanospikes silicon
layer. The charge carriers recombination mechanisms in
semiconductor nanocrystals were intensively studied by
many authors [21-23].
4. Conclusions
The fast response ZnO UV detector prepared by chemi-
cal spray pyrolysis technique was fabricated. The ZnO
films deposited on chemically etched silicon substrate
show a p-type behavior rather than n-type when they are
deposited on glass substrates .The p-ZnO film show high
carrier mobility leading to high speed UV detector. The
functionalization of the ZnO film surface by polyamide
nylon highly improved the photoconductive gain of the
detector. The fabricated detector was tested to detect fast
nitrogen laser pulses. The output signal was character-
ized by 180 µs rise time and 750 µs fall time. These re-
sults indicate that the deposition of ZnO nanofilm on
porous silicon is recommended for improving the re-
sponse time of the fabricated ZnO UV Photoconductive
5. Acknowledgements
The author would like to thank Prof. Dr. R. A. Radhi,
chairman of Physics Department, College of Science of
Baghdad University for his interest and useful discussion
during the work. Also, we would like to appreciate the
contribution of Dr. M. T. Hussein for his help in using
the nitrogen laser.
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