16644037

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Growth of pure ZnO thin ﬁlms prepared by chemical spray

pyrolysis on silicon

R. Ayouchi

a

, F. Martin

b

, D. Leinen

a

, J.R. Ramos-Barrado

a,

*

a

Laboratorio de Materiales y Superficie (Unidad asociada al CSIC) and Departamento de F!ısica Aplicada Facultad de Ciencias, Universidad de Malaga, E29071 M! alaga, Spain!

b

Laboratorio de Materiales y Superficie (Unidad asociada al CSIC) and Departamento de Ingenier!ıa Qu!ımica, Facultad de Ciencias, Universidad de Malaga, E29071 M! alaga, Spain!

Accepted 17 September 2002 Communicated by D.P. Norton

Abstract

Structural, morphological, optical and electrical properties of ZnO thin films prepared by chemical spray pyrolysis fromzinc acetate (Zn(CH3COO)22H2O) aqueous solutions, on polished Si(1 0 0), and fused silica substrates for optical characterization, have been studied in terms of deposition time and substrate temperature. The growth of the films present three regimes depending on the substrate temperature, with increasing, constant and decreasing growth rates at lower, middle, and higher-temperature ranges, respectively. Growth rate higher than 15 nm min1can be achieved at Ts¼543 K. ZnO film morphological and electrical properties have been related to these growth regimes. The films have been characterized by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy.

PACS: 81.15.Rs

Keywords: A1. Growth models; A1. Surface processes; A1. Surface structure; B2. Semiconducting materials; B3. Solar cells

1. Introduction

Zinc oxide thin ﬁlms attract much interest due to their valuable properties such as chemical stability in hydrogen plasma [1], high optical transparency in the visible and near-infrared region of the electromagnetic spectrum [2] and high refractive index [3]. Due to these properties,

ZnO is a promising material for solar cell applications such as antireflection coating [4] and transparent conducting material [5]. Various growth techniques, including RF/DC magnetron sputtering [6], chemical vapour deposition [7], sol– gel method [8], pulsed laser deposition [9], and spray pyrolysis, have been employed to obtain pure ZnO thin films on various substrates. How-ever, the spray pyrolysis technique is particularly attractive because of its simplicity, efficient and noncontaminating production [10]. In this work, we report on the growth and characterization of

*Corresponding author. Tel.: +34-52131922; fax: +34-952-132000.

E-mail address:barrado@uma.es (J.R. Ramos-Barrado).

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ZnO thin ﬁlms deposited on Si (1 0 0) and fused silica substrate in the temperature range between 473 and 673 K. The substrate temperature inﬂu-ence on the growth mechanisms is also discussed.

2. Experimental methods

In the present work, ZnO thin filmwere prepared by spraying 4103M zinc acetate dissolved in ultra-pure water, using compressed air as carrier gas. To improve the acetate solubility, the precursor dissolution temperature was kept at 333 K during the deposition process. All the films were deposited onto polished Si (1 0 0) and fused silica substrates at a constant tempera-ture in the range 473–673 K with an accuracy of 72 K. The solution flow rate and gas pressure was kept constant at 50 ml h1and 3 bars, respectively. The nozzle to substrate distance was 10 cmand the spraying time was varied between 5 and 30 min. The thickness of the films was measured ex situ by means of XRF (Siemens, SRS 3000) from the attenuation of Si Ka signal in films deposited on

Si, with an accuracy of 75 nm .

Filmstructure was characterized by X-ray diffraction (XRD), using Cu Ka radiation. Two

types of goniometers were used: (i) a conventional goniometer with y22y scanning mechanism and (ii) a ganiometer with a ﬁxed small X-ray incidence (21incidence), and 2y scanning mechanism.

Surface, in-depth composition, and electronic states of the ions of ZnO ﬁlms were studied by X-ray photoelectron spectroscopy (XPS) in a Physi-cal Electronic model PHI 5700 X-ray photoelec-tron spectrometer with Mg Ka (1253.6 eV)

radiation and Al Ka (1486.6 eV) radiation as

excitation sources. The energy scale of the spectro-meter was calibrated using Cu 2p3/2, Ag 3d5/2and Au 4f7/2 photoelectron lines at 932.7, 368.3 and 84.0 eV, respectively. A PHI ACCESS ESCA-V6.0 F package was used for data acquisition and analysis. Atomic concentrations were determined fromthe photoelectron peak areas using Shirley background subtraction and sensitivity factors provided by the instrument manufacturer. All sample spectra were referred to the C1s line of the residual carbon set at 284.6 eV. Depth proﬁling

was carried out using 4 keV Ar+bombardment at a current density ofB3mA cm2.

Observation of surface morphology was per-formed using a scanning electron microscope (SEM JEOL model JSM5300). Optical measure-ment data were obtained with an UV–Visible Shimadzu 3101 PC double beam spectrometer and a UV/Vis/NIR Perkin Elmer Spectrometer Lamb-da 19. The absorption coefﬁcient was estimated fromthe formula ð1=dÞ1nð1=TÞ; with ﬁlmthick-nessd and optical transmissionT:

3. Results and discussion

3.1. Growth regimes

Fig. 1 shows the plot of filmthickness against deposition time for various substrate tempera-tures. As expected, the filmthickness increases with deposition time. The film thickness growth rate is defined as the slope of the curve corre-sponding to plot filmthickness vs. deposition time. The lineal appearance points out the existence of a nearly constant growth rate. On the other hand, the intersections of the lineal fits with the time axis do not cross at the origin. The initial nonlineal behaviour could be explained by considering two stages during the growth process: (1) nucleation and coalescence, (2) thin filmgrowth [11].

0 10 20 30

0 100 200 300

400 473K

553K 593K

Film Thickness (nm)

Deposition time (min)

Fig. 1. Evolution of ﬁlmthickness vs. deposition time atTsof

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In Fig. 2, we can see, in a logarithmic plot, the growth rate as a function of the inverse substrate temperature. Three growth temperature regions are present. For low temperatures (below 523 K), the ﬁlmgrowth rate increases exponentially with substrate temperature according to an Arrhenius behaviour in which the deposition rate is con-trolled by an activated process such as adsorption,

surface diffusion, chemical reaction and deso-rption. The growth rate is thus controlled by mass transfer and reaction kinetics. The net activation energy for the ZnO thin ﬁlms deposition in this region is calculated to be 0.1470.01 eV.

In the intermediate zone (523–553 K), the growth rate reaches its maximum and remains constant, indicating that the growth rate is diffusion limited, which leads to very smooth surfaces [12].

At higher temperatures (above 553 K), the logarithmic plot shows a negative slope of about (0.1570.04) eV. At these temperatures, the pre-cursor solvent vaporizes away fromthe substrate and the precursor chemical reaction is carried out in the vapour phase. The growth mechanism is thus controlled by the formation of precursor microcrystallites in the vapour space and the deposited ﬁlms show a rough aspect [13].

4. Chemical composition, morphology and structure

4.1. Chemical composition

Fig. 3 shows the wide scan XPS spectrumof a deposited ZnO thin ﬁlmprepared at 553 K, in the

Fig. 2. Evolution of the growth rateVcvs. inverse of substrate

temperatureTs:

1200 1000 800 600 400 200 0

Binding Energy (eV) 3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

Intensity (A.U.)

Zn2p1

-Zn LMM

-Zn3s _-Zn3p3

-Zn3d

-Zn2p3

-O KLL

-C KLL

-O1s

-C1s

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binding energy range of 0–1200 eV; only the expected Zn, O and C contributions can be found. After 20 s Ar+etching no carbon was detected on the ﬁlmsurface suggesting that its superﬁcial presence was due to atmospheric contamination or specimen handling.

High-resolution spectra of O1s and Zn2p3/2 photoelectron lines for sputter-etched ZnO thin filmsurface, not shown, were recorded. It can be seen that O1s peak consists of two contributions separated by approximately 1.63 eV. The first one located at a lower energy (530.19 eV) is attributed to O–Zn bonds. While the second one, located at a higher energy (531.82 eV), which disappears com-pletely after a few minutes of Ar+ sputtering, corresponds to O–H bonds fromabsorbed water molecules, as reported by Futsuhara [14]. For the Zn2p3/2 line, only one peak that corresponds to Zn–O bonds can be observed. The atomic ratio of Zn/O is reported in Table 1. At lower tempera-tures, the Zn/O ratio is greater than 1, indicating that the films were O deficient; this fact is probably due to an incomplete oxidation process [15]. As the substrate temperature was increased, the ratio decreased and went close to 1 at temperatures around 553 K. At higher temperatures, Zn/O ration increases again due the desorption of O or atomic zinc incorporation [16].

4.2. Morphology

SEM observations show that surface morphol-ogy of ZnO ﬁlms depend strongly on substrate temperature. Fig. 4 shows SEM pictures of ZnO

thin films prepared at substrate temperatures (Ts) of 473, 553 and 673 K. At low substrate tempera-tures, since the reaction kinetic would control the filmgrowth rate process, a nonstoichiometric film with rough aspect is obtained. Fig. 4b shows a more homogeneous, dense and crack-free surface, which is in good agreement with the growth mechanism seen in Fig. 2, in the intermediate zone. At higher temperatures, a greater rate of vaporization of the solvent occurs away fromthe substrate, and the precursor condenses as micro-crystallites in the formof small grains, Fig. 4c.

4.3. Structure

Solid bulk zinc-acetate decomposition was ob-served to start around 533 K [17]. However, many authors have reported that in the spray pyrolysis process, the zinc acetate decomposition begins at lower temperatures (about 453–473 K). Studenikin

Table 1

Relative O and Zn content in ZnO thin ﬁlms prepared at different substrates temperatures, as determined from XPS spectrumdisconvolutions

Ts(K) 493 523 553 623

O–Zn (%) 73.40 41.78 46.78 46.79 O1s 53.44 55.29 52.75 50.87 Oads(%) 26.60 13.51 5.97 4.08 Zn2p3/2(%) 46.56 44.71 47.25 49.13 Zn/O ratio 1.18/1 1.07/1 1.01/1 1.05/1

Fig. 4. SEM pictures of ZnO thin ﬁlmprepared at differentTs:

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et al. observed this fact with zinc nitrate as a starting solution, and pointed out that it is due to the lower stability of microcrystals relative to the bulk material [18]. Fig. 5 shows the XRD pattern corresponding to ZnO thin ﬁlms deposited on polished Si(1 0 0), at different substrate tempera-tures and having the same thickness.

At substrate temperatures below 473 K, no evidence of a ZnO crystalline phase was observed. In the temperature range between 493 and 593 K, peaks corresponding to the hexagonal structure of ZnO were detected. At a substrate temperature of 553 K (Fig. 5), the (0 0 2) peak predominates indicating a preferential crystalline orientation along the c-axis perpendicular to the ﬁlmsurface. At temperatures higher than 593 K, XRD patterns show a powder-like pattern with no preferred orientation.

To investigate the influence of the deposition time and film thickness on the structural proper-ties, ZnO films with various deposition times were prepared. Fig. 6 presents the X-ray diffractograms of various ZnO thin films prepared at a constant substrate temperature of 523 K and different deposition times. As can be seen, in the first stages of the films deposition, broad peaks were detected

at 2y¼31:651, 34.451 and 6.51 corresponding to the plane families (1 0 0), (0 0 2) and (1 0 1) in the hexagonal structure of polycrystalline ZnO. When the deposition time is increased (under 15 min), the relative intensity of the (0 0 2) peak increases, indicating that the growth is enhanced along thec -axis normal to the substrate surface. However, when the deposition time is increased to 20 min,

Fig. 5. XRD diffractograms for samples prepared at variousTs:

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the intensity of the (1 0 0) peak reaches the same intensity as the (0 0 2) peak, which indicates that at this substrate temperature, the increase in the film thickness affects the crystallites orientation leading to more randomly oriented films with a powder-like pattern, without preferred orientation. XRD patterns of the films prepared atTsof 553 K show that in all cases, only the (0 0 2) peak can be observed. At this temperature, the films were strongly oriented with the c-axis normal to the surface and the deposition time had no influence on the c-axis preferred orientation growth.

5. Physical properties

5.1. Optical properties

For the purpose of optical characterization, ZnO films were deposited on fused silica sub-strates. The films are highly transparent in the visible range of the electromagnetic spectrum with an average transmittance reaching values up to 90% over 400 nm. A sharp ultraviolet cut-off ranging from l¼370 (for samples prepared at 473 K) to l¼380 nm(for samples prepared at 573 K) were found. This variation can be related to filmstructure and stoichiometry. As we have seen above, the ZnO filmstoichiometry and the crystal-linity is enhanced as the substrate temperature is increased below 593 K.

For materials with a direct band gap, like ZnO, the absorption coefﬁcient (a) is related to the incident energy by

ðahnÞ2¼AðhnEgÞ;

whereAis the edge width parameter, andEgis the optical band gap. The optical band gap values are obtained fromthe plotðahnÞ2vs.hnby extrapolat-ing the linear portion of the plot to ðahnÞ2¼0: Fig. 7 shows the optical band gap calculated by this method for ZnO thin ﬁlms prepared at greater substrate temperatures. As can be seen, Eg decreases from3.33 to 3.31 as the substrate temperature is increased. In all the cases, the gap energies obtained are bigger than the value in stoichiomteric bulk ZnO (3.24 eV) [20], which is probably due to the reduced grain size of ZnO thin

films prepared by this method and to oxygen deficiency confirmed by XPS studies. The same blue shift band edge was observed for ZnO films prepared by photo-chemical vapour deposition [14] and is attributed to excess zinc present in the samples prepared at relatively low growth tem-peratures and is in agreement with our results.

The mean refractive index of ﬁlms calculated in the visible range from transmittance data, for ﬁlms deposited at 553 and 573 K were 1.87 and 1.86, respectively. This result is in good agreement with other workers [19].

Fig. 8 shows the near-normal incidence total hemispherical reflectance spectras of ZnO thin films obtained on polished silicon at different temperatures. Fig. 8a does not show an interfer-ence pattern because of its dusty nature, Fig. 4a. The filmobtained at 593 K with a lower time of deposition, Fig. 8b, shows a blue colour and a reflectance spectra similar to a commercial solar cell with an antireflective TiO2thin film. The thin filmobtained at 673 K, Fig. 8d, shows an inter-ference pattern, which is the proof of a smooth surface in spite of its polycrystalline morphology, Fig. 4c.

5.2. Electrical properties

Fig. 9 represents the evolution of the sheet resistivity of ZnO thin ﬁlms vs. substrate tempera-ture. When the substrate temperature increases,

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the resistivity increases, reaching a maximum atTs of 553 K; at higher temperatures the sheet resis-tance decreases again. It is known that the conductivity of polycrystalline ZnO is due to

oxygen vacancies or interstitial zinc atoms [21], since stoichiometric ZnO is a highly resistive material. XPS studies have revealed that film stoichiometry is governed by substrate tempera-tures and films deposition temperatempera-tures. Between 543 and 593 K, films were nearly stoichiometrics. Deviations fromstoichiometry achieved at low and high temperatures produce an increase in free carriers leading to a decrease in the resistivity.

6. Conclusions

Pure ZnO thin ﬁlms prepared on both Si (1 0 0) and fused silica substrates by chemical spray pyrolysis using aqueous solutions of zinc acetate, at different substrate temperatures present a growth behaviour characterized by three different regimes:

1. at low temperature, the growth rate is con-trolled by an activated process, deposits consists of randomly oriented ﬁne-grained polycrystals leading to ﬁlms with a smooth morphology. 2. in the intermediate zone, the growth rate

reaches its maximum (15.01 nm min1) at a substrate temperature of about 543 K. Very smooth ﬁlms with preferred c-axis orientation were obtained.

3. at higher temperatures, the deposition rate is limited by the precursor reaction in the vapour phase, leading to deposition effectiveness loss and rough surfaces.

XPs studies showed that the stoichiometry of the ﬁlmdepends strongly on substrate temperature and is close to 1 in the intermediate temperature zone.

All the films prepared to substrate temperatures above 473 K were polycrystallines with the hex-agonal wurtzite structure. Finally, the optical band gap of the films decreased from 3.33 to 3.31 eV with the increase of the substrate temperature from473 to 573 K. The ZnO films prepared in the conditions of the maximum growth rate, present high electrical resistivity, a direct optical band gap 3.31 eV, an optical transmittance of 90% and an antireflective characteristic similar to the TiO2thin film in commercial solar cells.

200 220 240 260 280 300 100

150 200 250 300 350 400

Sheet resistance (

Ω

* cm)

Substrate temperature (°C)

Fig. 9. Evolution of sheet resistance vs. substrate temperature. Fig. 8. Near-normal reﬂection spectra (a) 473 K, (b) 593 K, lower time of deposition, (c) 593 K, (d) 673 K ﬁlms.

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Acknowledgements

The authors are grateful to European Union and CICYT (Spain) (grant 1FD97-0839-CO3-O) for ﬁnancial support and to ISOFOTON S.A. for its collaboration. The bursary holder, R. Ayuochi, wishes to thank the Junta de Andaluc!ıa (FQM192).

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