AZO films prepared by r.f. magnetron sputtering: structural, electrical and optical properties Maria Luisa Grilli a *, Anna Krasilnikova Sytchkovaa , Sylvia Boychevab, Angela Piegaria Optical Coating Group, ENEA Research Center, via Anguillarese 301, 00123 Rome, Italy b Dept. Thermal and Nuclear Engineering, Technical University of Sofia, 8 Kl. Ohridsky Blvd., Sofia 1000, Bulgaria a
ABSTRACT Aluminium-doped zinc oxide films with 91% transmittance in the visible range and electrical resistivity of the order of 10-3 Ω⋅cm were fabricated by radio frequency magnetron sputtering in Ar atmosphere starting from a target of ZnO mixed with 2% wt Al2O3. A systematic study of the deposition conditions such as substrate temperature, working gas pressure, radio frequency power, magnetron strength, target to substrate distance, etc., was performed when searching for improved electrical and optical performances of the films. Several deposition conditions govern the film characteristics, so that films with same good optical and electrical properties can be obtained by opportunely combining different deposition parameters. Keywords: Transparent conductive oxides, Al-doped ZnO, r.f. sputtering, optical properties, electrical properties, structural properties.
1. INTRODUCTION Transparent conductive oxide (TCO) films are widely investigated because of their application in solar cells, optoelectronic devices, panel displays, etc [1-4]. Moreover, their high reflectivity in the IR can be used in heat reflecting filters for various applications [5, 6]. Recently, the authors of this paper have investigated the possibility to fabricate optical filters for art-protection employing ITO films with high reflectivity in the near IR [7-9]. Among TCOs, often indium tin oxide (ITO) remains the material of choice. However, since two decades, great interest has been dedicated to ZnO which is an abundant material, much cheaper and less toxic than ITO, with a wide band gap, and in particular to the doping of ZnO [10-12]. The undoped ZnO has an n-type conductivity, due both to oxygen vacancies and Zn interstitials, but doping is essential to achieve the performances required by different optoelectronic applications. In this study Al doped ZnO (AZO) films on glass substrates were prepared by radio frequency (r.f.) magnetron sputtering. Optical and electrical properties of the films were studied as a function of the deposition parameters. A correlation between the electrical properties and the structure and microstructure of the films was found. Similarly to results previously reported on ITO films [13], it was found that the films characteristics depend on the energy efficiency of the deposition process, and not only on the single deposition parameter, in such a way that films with same good properties can be obtained in different conditions.
2. EXPERIMENTAL DETAILS AZO films were prepared by r. f. magnetron sputtering on B270 glass substrates starting from a 6 inch diameter target made of ZnO with 2% wt Al2O3. A base pressure of about 1x10-4 Pa was ensured in the deposition chamber by a cryogenic pump. The deposition conditions were varied systematically in a search of the best performances of the films, i.e. a high VIS transmittance and a high NIR reflectance. Even very low addition of oxygen to the Ar working gas, resulted in films completely transparent in the studied spectral range (300-2500 nm), hence all further experiments have been performed in pure Ar atmosphere, varying the pressure in the range of 0.2-1 Pa. On the other hand, all the films deposited without magnetron were either NIR transparent or their absorptance and reflectance in the NIR were very low. The values of r.f. power were chosen in the range of 70-350 W, and two series of sample were prepared: with single and double magnetron strength. The target to substrate distance was kept to the value of 64 mm for the unheated samples, and to the value of *
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48 mm for the samples heated during the deposition. The substrate temperature, measured by a K-type thermocouple, raised to 100-120 °C due to the plasma heating for the non intentionally heated samples, while the temperature of the heated samples varied from 200 °C to 240 °C. This last variation was produced by different Ar pressures and r.f. powers in various deposition runs. Transmittance and reflectance spectra were measured in the range of 300-2500 nm using a Perkin Elmer Lambda 900. Film thickness and optical constants were estimated from the spectrophotometric data. The sheet resistance of the films was measured by the four point probe method and the mobility and carrier concentration by the Hall measurements, using a HP Profiler Biorad HL 5900. Structural characterization of the films was done by X-ray diffraction using a Philips X-Pert Pro 500 Diffractometer with Cu Kα radiation in the conventional θ−2θ geometry. The microstructure of the films was observed by a field emission scanning electron microscope (Leo Supra 35).
3. RESULTS AND DISCUSSION 3.1 Optical and electrical properties
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Films of good optical and electrical properties were grown at different deposition conditions. In order to compare better their characteristics, films with comparable thickness in the range of 700-900 nm were deposited. Indeed, it was found that the optical constants and the electrical resistivity of TCO films may be thickness dependent in the NIR [13-15].
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Figure 1. Transmittance values vs Ar pressure in the VIS range (full squares) and at 2500 nm (open circles): a) at r.f. power of 100 W, b) at r.f. power of 300 W.
Without magnetron strength and without heating the substrate, transparent AZO films in the whole spectral range were obtained at the different r.f. powers and Ar pressures. Nevertheless, at higher r.f. powers, and hence higher substrate temperatures, a slight increase in the NIR reflectance was observed. With magnetron deposition a clear improvement in the NIR behaviour was achieved and a shift of the fundamental absorption edge towards shorter wavelengths (Burstein Moss effect [16]) was observed. Only at r.f. powers of 300 W on pre-heated substrate the NIR reflectance started to increase significantly. Figure 1 shows the behaviour of the VIS and NIR transmission vs Ar working pressure of samples grown on pre-heated substrates at two different r.f. powers: a) at 100 W, and b) at 300 W. The best films were grown at r.f. power of 300 W, temperature of 240 °C and pressures in the range of 0.70-0.78 Pa. Figure 2 shows the transmittance and reflectance spectra of two AZO films, deposited at the optimized conditions. AZO35 was grown at higher Ar flux and pressure, 25 sccm and 0.78 Pa with respect to AZO34 film, grown at a flux of 22 sccm and at pressure of 0.70 Pa. Transmittance values up to 90.5% were measured in the VIS range for AZO34. A further attempt to improve the properties of the AZO films was done by increasing the magnetron strength. At higher magnetron strength, a systematic study on the deposition parameters was again performed. Also in
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this case the best films were obtained on pre-heated substrates. Figures 3 show the trend in the VIS and NIR transmittance of AZO films vs Ar pressure at 200 W (Figure 3a), and at 300 W (Figure 3b). Figures 4 show the transmittance trend vs r.f. power at two fixed Ar pressures.
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Figure 2. Transmission and reflection spectra of AZO34 and AZO35 films.
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Figure 3. Transmittance values vs Ar pressure in the VIS range (full squares) and at 2500 nm (open circles): a) at r.f. power of 200 W, b) at r.f. power of 300 W.
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Figure 4. Transmittance values vs r.f. power in the VIS range (full squares) and at 2500 nm (open circles) at fixed Ar pressure: a) of 1.0 Pa, b) of 0.55 Pa.
With higher magnetron strength the NIR transmittance starts to decrease at lower r.f. powers. The best film, AZO58, with slightly improved electrical properties with respect to AZO34 was grown on pre-heated substrates (T=210 °C) at Ar pressure of 1.0 Pa and r.f. power of 220 W. VIS transmittance of AZO58 (864 nm thick) is about 91% (Figure 5). 100 T
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Figure 5. Transmission and reflection spectra of AZO58 film.
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Figure 6. a) Refractive index n(λ) and b) extinction coefficient k(λ) of AZO34 (solid line) and AZO58 (dash line).
The optical constants of the AZO films were calculated from the transmittance and reflectance spectra in the whole spectral range. Figure 6 shows the refractive index and the extinction coefficient of AZO34 and AZO58 films, while Table 1 summarizes the results of the electrical measurements performed on these samples. Sample
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776
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1.85 x 1020
10.1
864
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11.8
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Table 1. Electrical properties of AZO films.
The carrier concentration of the two films are practically the same, but AZO58 has a 15% higher carrier mobility and about 15 % lower resistivity value. Carrier concentration and mobility values are lower than the best results obtained on AZO films, hence, further studied are needed to better optimize these properties. Probably, the effect of the oxygen concentration on the films characteristics should be more deeply investigated. 3.2 Structural and microstructural properties XRD patterns of AZO films are shown in Figure 7. All the investigated samples, independently on the deposition conditions, crystallize with the hexagonal wurtzite structure and show a preferred orientation with a strong (002) diffraction peak at 2θ ~34.4° and a weak (004) diffraction peak at 2θ ~72.4°. An additional peak (101) at 2θ~36.3°, which intensity relative to that of (002) peak varies according to deposition conditions, was found in samples with higher NIR reflectivity. The microstructure of the films was investigated by field emission scanning electron microscopy. Also in this case, a correlation between the electrical and the microstructural properties was found for all the investigated samples. The morphology of three AZO films is shown in Figure 8. Films with high electrical resistivity show a homogeneous and porous microstructure made by agglomerates of round-shaped grains of size less than 50 nm (Figure 8a). A less porous and homogeneous distribution of irregularly shaped and higher-sized grains was observed in the case of AZO34 film (Figure 8b), and AZO35 film. Finally, AZO58 film (Figure 8c) shows a more compact microstructure, made of grains of even larger size. These results are in agreement with the results of the literature, which reports a strong influence of the deposition conditions on the surface morphology of the sputtered films [17, 18].
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NIR transparent AZO film
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Figure 7. X-ray diffraction patterns of AZO films: a) NIR transparent coatings, b) NIR reflective coatings.
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c MglOO.OOKXf____
Figure 8. Surface morphology of AZO films: a) NIR transparent AZO film, b) AZO34 film, c) AZO58 film.
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4. CONCLUSIONS ZnO: Al films were grown by r.f. magnetron sputtering. Two different optimized deposition conditions were found which give films with a high VIS transmittance (91 %), carrier concentration N=1.86 x 1020 cm-3 and mobility µ=10-11.8 cm2/V s. All the AZO films showed a preferentially oriented structure with a strong diffraction peak at 2θ=34.4° and a small peak at 2θ=72.4°. Additional diffraction peak at 2θ=36.3° was found only in films with low resistivity values. Electrical properties were found to be dependent on the film microstructure. In agreement with previously reported results on ITO films, AZO properties seem to be dependent on the energy efficiency of the sputtering process, i.e. on the combination of several deposition parameters, in such a way that same good electrical and optical properties can be obtained at different sputtering conditions.
5. ACKNOWLEDGMENTS The authors gratefully acknowledge Dr. Massimo Izzi and Dr. Alberto Mittiga from ENEA Casaccia, Rome, for electrical measurements, and Prof. Enrico Traversa and Dr. Elisabetta Di Bartolomeo, from the Department of Chemical Science and Technology, University of Rome “Tor Vergata”, for helpful discussions.
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