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J Sol-Gel Sci Technol (2010) 55:335–342
Indium and aluminium-doped ZnO thin films
deposited onto FTO substrates: nanostructure,
optical, photoluminescence and electrical properties
M. Benhalilibaa • C. E. Benouisa • M. S. Aidab •F. Yakuphanoglu c• A. Sanchez Juarezd
a Physics
Department, Sciences Faculty, USTOMB University, BP1505 Oran, Algeria
Films & Plasma Lab, Physics Department, Ment. University, 25000 Constantine, Algeria
c Physics Department, Faculty of Sciences and Arts, Firat University, 23119 Elazig, Turkey
d Centro de Investigacion en Energia -UNAM, Temixco,Morelos 62580, Mexico
b Thin
指導教授：林克默 博士
報告學生：郭俊廷
報告日期：99/8/30
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Outline
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Introduction
Experimental detail
Results and discussions
Conclusions
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Introduction
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Zinc oxide (ZnO) is one of the most multifunctional
semiconductor material used in different areas for the
fabrication of optoelectronic devices operating in the blue and
ultraviolet (UV) region, owing to its direct wide band gap
(3.37 eV) at room temperature and large exciton binding
energy (60 meV)[1].
The conditions of deposition and the choice of the substrate
are important for the growth of the films. The substrate chosen
must present a difference in matching lattice less than 3% to
have good growth of the crystal on the substrate[6, 7].
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Various depositions methods have been employed for the
growth including, laser deposition[11], sol–gel[12, 13],
electrodeposition[14].
In the present study, undoped, Al- and In-doped ZnO thin
films were prepared by ultrasonic spray pyrolysis onto FTO.
The structural, electrical and optical characterization of ZnO
films were investigated for a better understanding of its
physical properties.
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Experimental detail
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The precursor compound used was zinc acetate dihydrated, Zn
(CH3COO)2(H2O)2 salt (0.1 M)[10]. Doping sources were
aluminum nitrate nonahydrated, (Al (NO3)3‧9H2O), and
indium (III) chloride, InCl3. Both, precursor and doping
compounds were dissolved in methanol.
The weight of the added dopant source was calculated as
function of the desired Al/Zn, In/Zn ratios, which were taken
equal to 2%.
The substrates temperature was fixed equal to 300℃ . The
substrate temperature was controlled by digital thermometer
connected to the heater.
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Susbtrate-tonozzle distance was fixed at 5 cm. Solution flow
rate was kept at 1 L min-1 and deposition time was 5 min.
The thicknesses of the films were determined using a Talystep
Dektat 3st profilometer and were found to be 150 nm for
undoped ZnO and 1 μm for AZO and IZO, respectively.
X-ray measurements were done by a Rigaku X-ray
diffractometer model DMAX 2,200, using a copper
anticathode (Cu Ka, 1.54 A ).
The optical parameters were measured using a Shimadzu UV3101PC double beam spectrophotometer.
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Photoluminescence (PL) experiments were performed using
an Ar-ion laser which has an excitation wavelength of 351 nm
and a power of 100 mW. all spectra were taken at room
temperature by grating spectrometer and photomultiplier
detector.
The morphology of samples was carried out in a field
emission scanning electron microscope Jeol JSM 700 1F.
The electrical conductivity of the films was measured using a
Keithley 6517A electrometer.
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Results and discussions
Fig. 1 XRD spectra for
undoped (150 nm), IZO and
AZO (1 μm) films in range
2θ = 20-70°(left);
the evolution of mean peaks
with doping in the range of
2θ = 30-37°(right)
The diffraction peaks
related to FTO coated glass
are signed by* (star) as
sketched
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The (100) and (101) crystal planes intensities increase
strongly with doping, either indium or aluminum.
The IZO films grew preferentially in the [100] direction and
the dopant increases the grain size for this preferential
direction.
Al doping induced a decrease in average grain size. Many
authors have reported that Al doping decrease the grain size
due to the difference between Zn (0.074 nm) and Al ionic
radius(rAl = 0.054 nm)[4, 25, 26].
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The films grow with a hexagonal wurtzite type structure and
the calculated lattice parameters (a and c) are given in Table 1.
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The preferred orientation of ZnO films were quantitatively
evaluated by texture coefficient (TC), which was calculated,
for the mains peaks from the X rays diffractograms. It is
expressed as follows for a plane (hkl),
I(hkl)/I 0 (hkl)
TC 
1/N N I(hkl)/I 0 (hkl)
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The coefficient TC along [002] direction was decreased by
doping. The change in growth orientation can be attributed to
dopant rather than thickness[26].
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By comparing the values of the parameters found with the
theoretical values, we deduce that these films are under
constraints.
These constraints are two types: a thermal component relating
to the differences between the coefficients of thermal
expansion and an intrinsic component which depends on
various parameters.
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Although FTO used as substrate shows a white appearance
and has an absorption edge at 327 nm (3.79 eV). This is a
higher energy than the expected band gap energy for the zinc
oxide thin films, which is usually reported in the range
between 3.1 and 3.4 eV[14–16].
Therefore, it can be assumed that FTO is transparent in the
region of interest for determining ZnO band gap energy.
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A maximum transmittance
(substrate absorption
included) of 80% is
obtained for undoped ZnO
films at around 600 nm,
whereas both the AZO
and IZO thin films exhibit
a maximum of ~65% at
the same wavelength.
The transmittance was decreased sharply from 1,340 nm. This
behavior was also obtained by Fortunato[5] and the decrease is
due to the thickness effect.
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Fig. 3 Transmittance and first
derivative dT/dλ dependence of
wavelength (UV range): a undoped
ZnO, b In-doped ZnO and c Aldoped ZnO; the insets show (ahn)2
versus photon energy
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The two methods, (ahn)2 plot and transmittance derivative,
used for determined Eg are in agreement with literature[8, 14, 29,
30]. Furthermore, the refractive index n of zinc oxide can be
expressed as follows[8] ,
Eg
n 2 -1
 12
n 1
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The values deduced of n were respectively for ZnO 2.3304,
2.3267 for IZO and 2.3377 for AZO. The optical band gaps of
the ZnO, IZO and AZO samples change with doping. This
effect is known as Burnstein–Moss effect.
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Fig. 4 Photoluminescence
spectra of undoped ZnO,
AZO and IZO films (left),
the dominant PL peaks are
shown; others peaks near
band edge emission occur
at 2.66, 2.69 and 2.72 eV
(right)
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The undoped films contain a strong UV emission and a very
weak broad in the visible region. It is dominated by a strong
ultraviolet feature corresponding to near band edge emission.
The increase in the photoluminescence intensity in the UV
range and the deconvolution of the spectra in the visible range
show a weak broad in the visible range and shift to green
emission for indium doping and to the green blue emission for
aluminium.
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The peaks positions are given in Table 2. The band around
3.122 eV has been attributed to free exciton emission.
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We remark a weak intensity for the green emission. So, the
effect of trapped emission on the photoluminescence is not
important and this may be due to the presence of impurities or
excess of oxygen[1].
For aluminium doping, we have a shift emission to 386 nm
which corresponds to 3.215 eV. It is originated from the
recombination of free exciton through an exciton-exciton
collision, corresponding to the near band edge (NBE)[34, 35].
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A higher intensity is obtained in the visible range for
aluminium. This implies that high concentration of defaults
exists due to the difference in the ionic radius between Al and
Zn. Also, a large broad is observed in the UV range.
For indium doping, the shift is about 391.5 nm which
corresponds to strong peak and UV photoluminescence, also
the peaks, 2.67, 2.69 and 2.72 eV which are less important
compared with peak of position 3.169 eV .
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The conductivity of the films
increases with the
temperature. This suggests
that the samples show a
semiconducting behavior.
This variation in conductivity
can be analyzed by the
following relation[39],
   0 exp(-
E
)
kT
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The conductivity plots of undoped ZnO and IZO samples
indicate two slopes representing two different conduction
mechanisms, whereas AZO samples show a linear behavior.
For undoped and IZO films the conductivity mechanism can
be controlled by a two different thermally activated
conduction process taken place in different energy levels.
The activation energy values, EI for region ‘‘I’’ (low
temperature) and EII for region ‘‘II’’ (high temperature),
correspond to the energy difference between the donor level
and the conduction level.
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The EI values at low temperatures for undoped ZnO and IZO
samples were found to be 51.1 and 53.7 meV, respectively.
The obtained EI values results from deep levels below the
conduction band.
The EII values at high temperatures ZnO, IZO and AZO
samples were found to be 0.295, 0.242 and 0.146 eV,
respectively. This indicates that EII values correspond to the
shallow donor level.
The conductivity plot of Al doped ZnO indicates a linear
region and electrical conductivity of ZnO sample changes
with Al and In dopants.
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Conclusions
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從SEM圖得知，AZO與IZO的奈米纖維結構直徑分別為
260與400nm。
螢光光譜顯示摻雜銦後，其發光光譜偏綠光，而摻雜鋁，
則偏綠藍光。
另外，藉由不同的摻雜其結構、織構、光學和電性都會有
不同的結果。
從XRD數據得知，藉由摻雜可以提高薄膜的結晶性。
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Thanks for your attention
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