Effect of deposition rate on the structural, optical and electrical properties of Zinc oxide (ZnO) thin films prepared by spray pyrolysis technique

Accepted Manuscript Title: Effect of deposition rate on the structural, optical and electrical properties of Zinc oxide (ZnO) thin films prepared by spray pyrolysis technique Author: Yacine Aoun Boubaker Benhaoua Said Benramache Brahim Gasmi PII: DOI: Reference:

S0030-4026(15)00497-0 http://dx.doi.org/doi:10.1016/j.ijleo.2015.06.025 IJLEO 55660

To appear in: Received date: Accepted date:

28-4-2014 5-6-2015

Please cite this article as: Y. Aoun, B. Benhaoua, S. Benramache, B. Gasmi, Effect of deposition rate on the structural, optical and electrical properties of Zinc oxide (ZnO) thin films prepared by spray pyrolysis technique, Optik - International Journal for Light and Electron Optics (2015), http://dx.doi.org/10.1016/j.ijleo.2015.06.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of deposition rate on the structural, optical and electrical properties

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of Zinc oxide (ZnO) thin films prepared by spray pyrolysis technique

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Yacine Aoun a, Boubaker Benhaoua b, Said Benramache c,*, Brahim Gasmid

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a

Mechanical Department, Faculty of Technology, University of Biskra, Biskra 07000, Algeria

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b

VTRS Laboratory, Institute of Technology, University of El-Oued, El-Oued 39000, Algeria

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c

Material Sciences Department, Faculty of Science, University of Biskra, Biskra 07000,

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Algeria

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Laboratoire de Physique des Couches Minces et Application, Biskra University, Algeria

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Key words: ZnO; Thin films; deposition rate; Spray pyrolysis technique.

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Abstract

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Zinc oxide (ZnO) thin films were deposited on glass substrates by spray pyrolysis technique

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decomposition of Zinc acetate dihydrate in an ethanol solution with various deposition rates,

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the ZnO thin films were deposited at 350 °C, the substrates were heated by using the solar

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cells method. The substrate was R217102 glass in a size of 30 cm × 7.5 cm × 0.1 cm.

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Nanocrystalline films with a hexagonal wurtzite structure with a strong (002) preferred

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orientation were observed at all sprayed films. The maximum value of grain size (21.91 nm)

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is attained of sprayed films with 30 ml. The decrease of the strain of ZnO films is probably

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due to an improvement of the crystallinity of the films. The average transmittance of all films

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is about 90 –95 % measured by UV-vis analyzer. The band gap energy varies from 3.265 to

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3.286 eV was affected by deposition rates was lying between 10 to 35 ml. The electrical

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resistivity of the films decreased from 0.394 to 0.266 (Ω.cm). The best results are achieved in

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sprayed films between 25 and 30 ml.

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* Corresponding author: [email protected] +213779276135; E-mail address:

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(S. Benramache) Tel.:

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1. Introduction Zinc oxide (ZnO), a II–IV semiconductor, has a wide direct gap of 3.37 eV at room

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temperature and large exciton binding energyof 60 meV [1–3], which has attracted much

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attention for its wide prospects optoelectronic devices such as solar cells, light emitting diodes

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(LED), laser diodes and acoustic–optical devices [4–8]. In solar cells, ZnO thin films are used

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as an anti-reflective coating (ARC) and transparent conductive oxide (TCO) due to its high

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optical transmittance in the visible light region, high band gap energy (Eg∼ 3.3 eV), optimum

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refractive index (n ∼ 2.0) and natural n-type electrical conductivity [9–11].ZnO can be used

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as a heat mirrors, piezoelectric devices [12], thin films [13] and chemical and gas sensing

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[14].

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ZnO thin films have been prepared using various methods such as molecular beam

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epitaxy (MBE) [15], chemical vapor deposition [16], electrochemical deposition [17], pulsed

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laser deposition (PLD) [18], sol-gel process [19], reactive evaporation [20], magnetron

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sputtering technique [21] and spray pyrolysis [22], have been reported to prepare thin films of

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ZnO. The spray pyrolysis technique is one of these techniques to prepare large-scale

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production for technological applications. It is possible to alter the mechanical, electrical,

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optical and magnetic properties of ZnO nanostructures.

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Many researchers have studied the effects of microstructure and processing on electrical

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conduction in ZnO nanostructures [23–26]. It is known that ZnO films prepared by the spray

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pyrolysis technique can have a wide band gap between 3 and 3.37 eV,

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El Sayed et al. [27] had controlled the effect of cadmium content on the film structure

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and optical absorption for this ZnO microstructured.

In present study, nanostructure ZnO based thin films can be deposited by spray pyrolysis

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technique on glass substrate where substrate temperatures are maintained at 350 °C for all

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experimentation. The thin films were deposited at different rates, the aim of this work to study

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the effect of deposition rate on crystalline structure, optical gap energy and electrical

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conductivity.

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2. Experimental

ZnO solution were prepared by dissolving 0.1M (Zn(CH3COO)2, 2H2O) in the solvent

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containing equal volume absolute methanol solution (99.995%) purity, then we have added a

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few drops of concentrated HCl solution as a stabilizer, the mixture solution was stirred at 60

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°C for 120 min to yield a clear and transparent solution. The substrate was R217102 glass in a

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size of 30 cm × 7.5 cm × 0.1 cm, prior to pumping, the substrate (R217102 glass) were

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cleaned with alcohol in an ultrasonic bath and blow-dried with dry nitrogen gas.

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The resulting solutions were sprayed on the heated glass substrates by spray pyrolysis

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technique, the substrates were heated by using the solar cells method, and this letter was

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prepared in our laboratory. The thin films were deposited at different rates varies from 10 to

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35 ml started by 5 ml at 350 °C (3 min of deposition time), which transforms the liquid to a

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stream formed with uniform and fine droplets of 35 µm average diameter (given by the

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manufacturer).

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Crystallographic and phase structures of the thin films were determined by X-ray

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diffraction (XRD, Bruker AXS-8D) with CuKα radiation (λ = 0.15406 nm) in the scanning

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range of (2θ) was between 20° and 60°. The optical transmittance of the deposited films was

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measured in the range of 300–900 nm by using an ultraviolet-visible spectrophotometer

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(SHUMATZU 1800), whereas the electrical conductivity  of the films and Urbach energy

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( Eu ), which is related to the disorder in the film network, the electrical resistivity  was measured

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in a coplanar structure obtained with evaporation of four golden stripes on the deposited film

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surface; the measurements were performed with keithley Model 2400 Low Voltage Source

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Meter instrument. All X-ray diffraction, transmittance spectra T(λ) and electrical

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measurements are carry out at room temperature (RT).

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3. Results and discussion

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3.1. The crystalline structure of ZnO thin films

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The X-ray diffraction (XRD) spectrum of the ZnO thin films is shown in Fig. 1. The

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obtained XRD spectra matched well with the space group P63mc (186) (No. 36-1451) [28].

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As it can be seen the only diffraction peak was observed at 2 = 34.5°, which is related to

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the plan of (0 0 2). The peak at position 34.5° corresponding to the (0 0 2) plans is very sharp,

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the film obtain with 30 ml has higher and sharper diffraction peak indicating an improvement

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in (0 0 2) peak intensity compared to other films, revealing that the films are nanocrystalline

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and a preferred orientation with the c-axis perpendicular to substrate. The crystalline quality

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of thin films enhanced at a deposition rate of 30 ml. Similar observations have been found by

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other researchers [29–31].

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The lattice constant c and diffraction peak angles of ZnO thin films (see Table 1) are calculated using the following equation [22]:

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dhk l  (

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4 h 2  hk  k 2 l 2  2  2) 3 a2 c

(1)

where a , c are the lattice parameters, ( h, k , l ) is the Miller indices of the planes and d hkl is the

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interplanar spacing.

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The strain ε values in our films were estimated from the observed shift, in the (0 0 2)

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where ε is the mean strain in ZnO thin films (Table 1), c the lattice constant of ZnO thin films and c0 the lattice constant of bulk (standard c0 = 0, 5206 nm).

Table 1 presents the crystallite size of ZnO thin films which are estimated using the

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(2)

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c  c0  100 % c0

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diffraction peak between their positions in the XRD spectra via the formula [29]:

well-known Debye-Scherrer formula [32]:

G

0.9  cos 

(3)

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where G is the crystallite size,  is the wavelength of X-ray (  = 1.5406 A°),  is the full

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width at half-maximum (FWHM), and  is the half diffraction angle of the centroid of the

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peak.

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Fig. 2 shows the variation of the crystallite size and mean strain of (0 0 2) diffraction

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peak as a function of deposition rate. It can be seen from Fig. 2 that the crystallite sizes

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increased with 30 ml and then decreased within increasing deposition rate of 30 to 35 ml (see

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Table 1). The increases of the crystallite size could be explained by an improvement in the

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crystallinity of the films. Moreover, the decreases of the crystallite size, this confirms the

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deterioration in the crystallinity of the films. As can be seen, an increase in the film deposition

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rate 15 to 30 ml the mean strain decreases from 0.443 to –0.654 %. Swapna et al. [33], they

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observed that the reduction of mean strain with increase in crystallite can be explained by the

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existence of sufficiently thicker films in less strained (or more relaxed) state. This result in 5 Page 5 of 17

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reduction of mean strain with increase in crystallite size indicates enhancement of crystallinity

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[24,32], or by decrease of the defects in the films.

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3.2. The optical properties of ZnO thin films

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The optical transmission T(λ) measurements as a function of the wavelength are

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depicted in Figure 3. Figures 3a to f represents the ZnO thin films with varying deposition

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rate. As it can be seen, a height transparent spectra T(λ) of the thin films in visible region, the

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average transmission is located between 90 to 95 % and the film exhibit significant

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oscillations in long wavelength; this oscillation may be due to the roughness of the top surface

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of ZnO film which can generate interference phenomenon, even at nuked eye can see that the

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ZnO lattice is very smooth then the transmission increased because of the onset fundamental

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absorption in the region between 370–375 nm. The value of transmission T(λ), deposited at

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30 and 35 ml are higher than others. The region of the absorption edge in the all layers due to

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the transition between the valence band and the conduction band is located between 360–390

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nm as it was shown in the inset of Figure 3; in this region the transmission decreased because

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of the onset fundamental absorption. One can note that the deposition rate effect is clearly

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observed in the layer quality such as in the average between 370–375 nm.

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In order to investigate the effect of deposition rate on ZnO thin films further, the

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absorption edge of ZnO film exhibit is an obvious red shift, which shows that the optical band

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gaps of the films are changes. The absorption coefficient is calculated from the relation [34]:

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A   d   ln T (4) The optical band gap energy E g was measured from the transmission spectra using the

following relations [35]:

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( Ah ) 2  C (h  E g )

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where A is the absorbance, d is the film thickness; T is the transmission spectra of thin films;

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 is the absorption coefficient values; C is a constant, h is the photon energy

(5)

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1240 (eV ) ) and E g the band gap energy of the semiconductor. As it was shown in  (nm)

( h 

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(Figure 4) a typical variation of ( Ah ) 2 as a function of photon energy ( h ) used for

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deducing optical band gap E g , it is determined by extrapolation of the straight line portion to

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zero absorption ( A  0 ) [36] the values of E g are listed in Table 2. Besides, we have used the

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Urbach tail energy ( Eu ), which is related to the disorder in the film network, as it is expressed

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follow [22,35]:

 h   A  A0 exp  Eu 

(6)

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where A0 is a constant h is the photon energy and Eu is the Urbach energy, the latter

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decreased with increasing the band gap is indicating the decrease of defects as it was

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illustrated in (Table 2).

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As clearly seen in the Figure 5, the optical gap energy change with increasing of

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deposition rate from 10 to 35 ml, the decrease in E g value of the nanocrystalline thin films

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can be attributed to quantum confinement effect due to the increase in the particle size in ZnO

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thin films. The value of E g increased at 3.28 and 3.279 eV of deposited films with 25 and 35

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ml, respectively, which may be attributed to the similar ionic radius between O and Zn

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[29,34]. As can be seen in Figure 5, that a minimum Urbach energy were reached with ZnO

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thin film at 35 ml, which means that this amount of deposition was adequate for less disorder

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as it was expressed in the literatures [22, 24, 28, 29, 35]. This can be explained by increasing

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of the crystallite size (see Figure 2).

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3.3. The electrical resistivity of ZnO thin films

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Fig. 6 shows the variation of the electrical resistivity  of ZnO thin films as a function of

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deposition rate. As can be seen, deposited films have good resistivity. In the first, the resistivity 7 Page 7 of 17

increased with 20 ml due to the decrease of band gap energy in this point, then decreased from 0.394

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to 0.266 (Ω.cm) with the increasing of deposition rate from 20 to 25 ml then increase to reach 0.509

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(Ω.cm) with 35 ml. The decrease in the resistivity of the films has been can be explained by

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decreasing of mean strain (see Figure 2). Beyond 25 ml the increases of the electrical resistivity with

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increasing of the deposition rate are explained by increasing of the potential barriers, because the

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introduced atoms are segregated into the grain boundaries, this interpretation is consistent with the

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authors [28, 29,32, 36–39], who obtained similar results. This behavior is due to the aggregation of the

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native grains into the larger clusters which influences the surface morphology of the thin films.

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4. Conclusions

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In conclusion, highly transparent conductive ZnO thin films were deposited on glass

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substrate by spray pyrolysis technique. The ZnO thin films were deposited at 350 °C, the

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substrates were heated by using the solar cells method. The substrate was R217102 glass in a

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size of 30 cm × 7.5 cm × 0.1 cm. The influence of deposition rate on structural, optical and

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electrical properties was investigated. Nanocrystalline films with a hexagonal wurtzite

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structure with a strong (002) preferred orientation were observed at all sprayed films. The

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maximum value of grain size (21.91 nm) is attained of sprayed films with 30 ml. The decrease

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of the strain of ZnO films is probably due to an improvement of the crystallinity of the films.

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The average transmittance of all films is about 90 –95 % measured by UV-vis analyzer. The

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band gap energy varies from 3.265 to 3.286 eV was affected by deposition rates was lying

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between 10 to 35 ml. The electrical resistivity of the films decreased from 0.394 to 0.266

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(Ω.cm). The best results are achieved in sprayed films between 25 and 30 ml.

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Figure 1: X-ray diffraction spectra of ZnO thin films at different deposition rates.

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Figure 2: The variation of crystallite size and lattice parameter c as a function of deposition

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rate in ZnO thin films.

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Figure 3: Transmission spectra T(λ) of ZnO thin films as a function of deposition rate: (a) 10

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ml, (b) 15 ml, (c) 20 ml, (d) 25 ml, (e) 30 ml, (f) 35 ml.

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Figure 4: The typical variation of ( Ah ) 2 vs. photon energy all deposited ZnO thin film as a

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function of deposition rate.

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Figure 5: The variation of optical band gap E g and Urbach energy Eu of ZnO thin films

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with deposition rate.

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Figure 6: Electrical resistivity of ZnO thin films at different deposition rate.

311

Table 1: Recapitulating measured values of Bragg angle (2θ), the inter planar spacing (d), the

312

full width at half-maximum (FWHM), the crystallite size (G) and lattice parameters (c and a)

313

for ZnO thin films as a function of deposition rate.

Ac ce p

te

310

deposition rate (ml)

hkl

2

d

(deg)

(A°)

FWHM

G

c

a

(deg)

(nm)

(A°)

(A°)

15

002

34.57

2.59251

0.67

12.41

5.185024

3.240640

25

002

34.66

2.58599

0.41

20.29

5.171972

3.232482

30

002

34.54

2.59469

0.38

21.91

5.189391

3.243369

35

002

34.27

2.61452

0.52

16.01

5.229035

3.268147

314

16 Page 16 of 17

315

Table 2: Recapitulating measured values of band gap energy ( E g ), Urbach energy ( Eu ), and

316

electrical resistivity (ρ) for ZnO thin films as a function of deposition rate. deposition rate (ml)

E g a (eV)

10

3.286

112.3

15

3.277

066.4

20

3.271

061.4

25

3.280

096.6

30

3.265

083.4

35

3.279

056.1

 ( Ω.cm)

Eu (meV

ip t

– 0.059

us

cr

0.394 0.373 0.509

Ac ce p

te

d

M

an

317

0.266

17 Page 17 of 17