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As-grown textured zinc oxide films by ion beam treatment and magnetron sputtering
Thin Solid Films 520 (2012) 4208–4213
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
As-grown textured zinc oxide films by ion beam treatment and magnetron sputtering Wendi Zhang a,⁎, Eerke Bunte a, Florian Ruske b, Dominik Köhl c, Astrid Besmehn d, Janine Worbs a, Hilde Siekmann a, Joachim Kirchhoff a, Aad Gordijn a, Jürgen Hüpkes a a
Institut für Energieforschung 5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany Institute Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany I. Physikalisches Institut IA, Rheinisch-Westfälische Technische Hochschule Aachen, 52074 Aachen, Germany d Zentralabteilung für Chemische Analysen, Forschungszentrum Jülich, 52425 Jülich, Germany b c
a r t i c l e
i n f o
Available online 21 April 2011 Keywords: Ion beam treatment Magnetron sputtering ZnO:Al films TCO Solar cells
a b s t r a c t This work presents as-grown textured ZnO:Al films by rf magnetron sputtering initiated by pre-treatment of glass substrate with mixed argon and oxygen ions. A 650 nm thick of this film exhibits surface texture features with lateral size around 500 nm; the resistivity is below 5 × 10−4 Ω · cm and the transparency in the near-infrared spectral range is high (N 80% at 1000 nm). Microcrystalline silicon thin film solar cells grown on the textured glass exhibit excellent light trapping effect with a short circuit current density of 18.2 mA/cm². © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films with rough surface are widely used as front contact window layers in thin film silicon solar cells. The incident light is scattered at the textured surface. This leads to the so-called light trapping effect, which boosts the short circuit current and consequently the conversion efficiency of microcrystalline silicon solar cells. Nowadays, textured zinc oxide (ZnO) films are prepared by various methods, such as low pressure chemical vapor deposition (LPCVD) [1], metal-organic chemical vapor deposition (MOCVD) [2] or magnetron sputtering and post wet-chemical etching of the asdeposited flat films [3]. Magnetron sputtering of ZnO films is a reliable method, which allows for relative high deposition rates and is available on a large area (3 × 6 m²). Because of the additional wetchemical etching step, deposition methods which produce directly textured zinc oxide films may be advantageous. Thus, some attempts have been undertaken to directly deposit textured zinc oxide films by magnetron sputtering. By adding water-vapor to the reactor chamber during magnetron sputtering [4] or by applying high gas pressures (N2 Pa) ZnO films with a rough surface could be fabricated [5]. However, the resistivity, charge carrier mobility and feature size of these films are limited.
⁎ Corresponding author. E-mail address: [email protected] (W. Zhang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.098
In this work, we report a new method to directly deposit nanotextured, transparent and highly conductive aluminum doped zinc oxide films (ZnO:Al). It is based on the pre-treatment of the glass substrates by low energetic argon and oxygen mixed ion beam. Afterwards the ZnO:Al films are prepared by magnetron sputtering. The film evolution, surface morphology, optical properties and electrical properties of the as-grown textured ZnO:Al films are discussed in this paper. Single junction microcrystalline silicon thin film solar cells based on this type of substrates are prepared and analyzed. 2. Experimental details The argon and oxygen ion beams were generated from a linear anode layer ion source (Type Lion 420, supplied by von Ardenne Anlagentechnik GmbH, Dresden, Germany). The ion source works in collimated mode instead of diffuse mode. Corning glass was used as substrates in the experiments. The working pressure of the chamber was kept at 1 × 10−3 mbar. The discharge voltage of the generator was 1 kV, which means that the average ion energy was around 500 eV [6]. The discharge current of the generator was kept at 80 mA by adjusting the source gas flow rate. The samples were exposed to the ion beam for 5 min to around 2 h. The distance between the ion source and the substrate is around 16.2 cm. To investigate the relation between the ZnO:Al film growth and the species of the ion beam treatment, we used pure Ar, pure O2 and mixed Ar/O2 gas as source gases. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the glass surface before and after ion beam treatment.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213
After the ion beam pre-treatment, the glass substrate was taken out from the ion source chamber. Then the ZnO:Al films with different thicknesses were deposited on the treated glass substrates by rf magnetron sputtering in an in-line sputtering system for a substrate size up to 30 × 30 cm² (VISS 300, supplied by von Ardenne Anlagentechnik GmbH, Dresden, Germany). ZnO:Al films deposited on untreated Corning glass were used as reference. The film was sputtered from a ceramic ZnO:Al2O3 (1 wt.%) target. The power of the generator was 1.5 kW and the substrate heater temperature is 430 °C. Total and diffuse transmissions of the ZnO:Al coated glass were measured with a dual beam spectrometer (Perkin Elmer, Lambda 19). Haze is defined as a ratio of diffuse transmitted light to the total transmitted light. The angle distribution of scattered light was measured by an angular resolved scattering setup with a green laser (wavelength: 550 nm). The resistivity, carrier concentration and mobility were measured by a Hall Effect machine at room temperature. The top surface and cross section of the film were characterized by scanning electron microscopy (SEM) (LEO, Gemini). The evolution of the ZnO:Al films were characterized by atomic force microscopy (AFM). X-ray diffraction (XRD) spectra were recorded for the thickness series. The X-ray was generated from a copper kα source. Microcrystalline silicon p-i-n solar cells were deposited on the ZnO:Al films by PECVD in a 30 × 30 cm² system at 13 MHz. The silicon i-layer is 1.1 μm and the area of the test cells is 1 cm². Solar cell characterization was performed with a Wacom solar simulator under standard test conditions (AM1.5, 100 mW/cm² and 25 °C).
3. Results and discussion 3.1. Film morphology Fig. 1 shows the top and cross sectional SEM images of the ZnO:Al films deposited on untreated reference glass (a) and Ar/O2 ion beam treated glass (b). The reference film has small crater like shape, and the average size of the craters is smaller than 50 nm, so it is flat. To apply in solar cells, the flat film has to be etched in diluted HCl solution to get textured morphology. The film sputtered on the ion treated glass has large pyramidal shape and the size of the pyramid is around 500 nm.
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Comparing the two cross sectional SEM images, we see that the reference film has columnar structure, while the other one has conical grains originating from the beginning of the film. The pyramids on the film surface are formed by the connection of the conical grains. In addition, though the two films were co-sputtered, their growth rates are different. The as-grown textured film is around 150 nm thinner than the reference. The differences in surface morphology and growth rate indicate that the growth of the ZnO:Al films on ion treated glass is quite different from the reference film. XPS was used to analyze the chemical composition of the glass surface after pure Ar, pure O2 and mixed Ar/O2 ion beam treatment. The penetration depth of the XPS measurement is around 5 nm, which means that only the surface materials are examined. The results are given in Table 1 (values are given only when larger than 0.1 at.%). One untreated glass surface is added as reference. Elements like C, Al, Cl, and Ca are also detected by XPS, however here we only list the important ones. The table shows that the percentage of Si and O are decreased on the surface after ion beam treatments, while Zn and Fe were found on the ion beam treated surface. On argon ions treated glass surface, Fe contamination is severe (3.8 at.%), but on oxygen ions treated glass surface, there is almost no Fe contamination (b0.1 at.%). This is a common phenomenon for this type of ion source: Fe is sputtered from the steel cathode because of the strike of argon ions, whereas the oxygen ions lightly passivate the surface of the steel cathode resulting in a thin oxide film on the cathode. Since the etch rate for FeO is lower than Fe, less cathode erosion is enabled by the use of oxygen gas [7]. The Zn contamination is supposed to be from the process chamber, since the ion source is installed in a same chamber with a rotatable ZnO:Al2O3 target. The difference between argon and oxygen treatment is that argon is physically milling off materials from glass surface while oxygen ions can also react with the substrate atoms or contaminations as well as etching off the substrate materials. More FeO, Fe2O3 or ZnO might be formed on the glass surface by oxygen ion beam treatment. This could be an explanation of the higher Fe and Zn concentration on the glass surface after mixed argon and oxygen ion beam treatment. ZnO:Al films were deposited on the above ion beam treated glass substrates. The average feature sizes estimated from SEM images are listed in the last row of Table 1. We find that on mixed ion beam treated glass, the ZnO:Al film has the largest feature size. Therefore,
Fig. 1. Top view and cross sectional SEM images of the ZnO:Al films sputtered on (a): reference glass, and (b): argon and oxygen ion beam treated glass.
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W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213
Table 1 XPS analysis of the corning glass surface after different ion beam treatments. The last row gives the average feature size of the sputtered ZnO:Al films on those different treated glass.
Fe (at.%) Zn (at.%) Si (at.%) Ar (at.%) O (at.%) Feature size of ZnO:Al films
Untreated
Ar
O2
Ar/O2
– – 18.7 – 56 b 50 nm
3.8 0.1 8.1 1.2 44 100– 200 nm
– 0.5 14.6 – 46.7 200– 400 nm
5.0 1.2 7.5 1.3 49.9 400– 500 nm
we suppose that the conical growth of the as-grown textured ZnO:Al surface is related to the contaminations on the glass surface. However, in later experiments, we find that there is no direct relation between the contamination level and rms roughness of the ZnO:Al films.
3.2. Film evolution Four as-grown textured ZnO:Al films were prepared with different thicknesses, and AFM was used to characterize their surface morphology. The film topographies are shown in Fig. 2. The scan sizes for all the films are 4 × 4 μm². From (a) to (d), the film thicknesses are 95 nm, 225 nm, 350 nm and 650 nm, respectively. For the first three films, the surface is composed of two types of grains: large grains and small flat grains. The large grains here are the conical grains in SEM images. The large grains which start from the nucleation stage distribute on the surface randomly. Because they grow faster than the surrounding flat grains, finally they grow to the surface of the asgrown textured ZnO:Al film. We count the number, the coverage and the mean diameter of the large grains by the SPIP program. The results are listed in Table 2. The number of the large grains decreases,
while the coverage and mean diameter increase when the film gets thicker. XRD measurements were done for the thickness series samples to characterize the film structure properties. All ZnO:Al films were found to have c axis, [002] preferential orientation (Fig. 3). Except the very thin film, all other films also have [004] preferential orientation. The intensity of the [002] peak increases with increased thickness. The [002] peak position of the as-grown textured films is around 34.27°, compared to the 34.4° of reference films. It indicates that the as-grown textured films have more stress in the films. For the as-grown textured films at 650 nm thick, except the [002] and [004] peaks, additional [100], [101], [102], [103] and [112] peaks appear, which means that after the large grains connect to each other, some crystallites with different orientations become pronounced. 3.3. Electrical properties As-grown textured ZnO:Al films and reference ZnO:Al films were sputtered with different thicknesses, and the resistivity, mobility and carrier concentration were determined at room temperature by Hall effect measurements. As shown in Fig. 4, the gradient in the resistivity, mobility and carrier concentration of the as-grown textured ZnO:Al films is much lower than the reference film. There is a crossover, i.e. thick as-grown textured films have higher resistivity and lower mobility, whereas in the thin film region an opposite trend can be observed. In bulk polycrystalline ZnO:Al films, the electron mobility can be limited mainly by two mechanisms: the electron scattering within the grains (e.g. the scattering by the ionized impurities, lattice defects, or phonons) or the electron scattering that occurs at the grain boundaries, due to the potential barrier formed at these locations [1]. If the mobility is dominated by the latter, the mobility should increases with increased grain size, because the grain boundary density decreases, which is the case for the reference films, their mobility increases with increased thickness. However, for the bulk
a
b
c
d
Fig. 2. AFM images of the ZnO:Al films grown on argon and oxygen ion beam treated glass with different thicknesses.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213 Table 2 The analysis of the large grains for the thickness series in Fig. 2. Film
Thickness
Number of large grains
nm (a) (b) (c) (d)
95 225 350 650
220 172 163 128
Coverage
Mean diameter
%
nm
16 22 42 67
118 157 220 298
as-grown textured films, the mobility gradient is lower. For the films from 95 nm to 350 nm, i.e. before the large conical grains connect to each other, the mobility is constant at 23 cm²/(V · s). This indicates that during the growth of as-grown textured films, two possible mechanisms exist. One is that the electron scattering in the grains contribute more to the mobility, which means that there are more defects in the grains. The other one is that the grain boundary density keeps similar during the thin film growth, i.e. the average grain size doesn't increase accordingly with film thickness. However, from the AFM results, we know that the large grains size increases during the film growth. The latter possible mechanism can be almost excluded. The two possible reasons result in a lower mobility for as-grown textured film, 27 cm²/(V · s) at 650 nm, compared to 48 cm²/(V · s) for the reference film at 800 nm. The carrier concentration, which is 5 × 1020 cm−3, is roughly constant for the two films after nucleation stage. The resistivity has similar trends like the mobility because ρ = 1 / (q · n · μ). For the as-grown textured ZnO:Al film at 650 nm the resistivity is 4.2 × 10−4 Ω · cm, even though it is not as low as the reference ZnO:Al films, it is as already low enough to be considered as the front contact for thin films solar cells.
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reference ZnO:Al film is the highest, and the other two are much lower. That is in accordance to scalar scattering theory [8]. The lateral feature size of the HCl etched reference ZnO:Al film is around 1–2 μm, which is larger than as-grown textured ZnO:Al film and AsahiU SnO2:F film. The haze and ARS of the as-grown textured film and AsahiU SnO2:F film are very similar because they have very similar texture morphology. The ARS curves of the as-grown textured ZnO:Al film and AsahiU SnO2:F film are lower according to the lower haze value at 550 nm, but have a more Lambertian-like distribution. The highest intensity of the scattered light for HCl etched reference ZnO:Al film occurs at 12°, while for the other two films, the highest intensities are shown around 40°. It means that for the as-grown textured ZnO:Al films and AsahiU SnO2:F film, the transmitted light tends to be scattered to large angles. 3.5. Growth model of as-grown textured films We build a model to explain the growth mechanism of the ZnO:Al on ion treated glass. The thin film deposition is very sensitive to the substrate conditions. The impurities or defects on the substrate surface can significantly influence the nucleation of the deposited materials. In our case, Zn–O, Fe–O bonds and even implanted O2 are present on the Ar/O2 ion beam treated glass surface, while on untreated glass substrate, the surface is covered only by Si–O bonds. The open O dangling bonds tend to develop O–terminated [00–1] ZnO:Al grains which has the lowest growth rate compared to other crystallographic structures such as [001], [100] and [101] grains [9]. At other surface area where no O bonds are covered, the ZnO:Al films tend to have random orientation, partly are Zn-terminated [001] grains. They have faster growth rate than the O-terminated [00–1] grains and therefore overgrow [001] grains and dominate the surface.
3.4. Optical properties
3.6. Application of as-grown textured ZnO:Al films
The total transmission, haze versus light wavelength from 300 nm to 1300 nm and ARS at 550 nm of the as-grown textured ZnO:Al film are shown in Fig. 5. For comparison, the HCl etched reference ZnO:Al film and one AsahiU SnO2:F film are added. The total transmission of the as-grown textured ZnO:Al film is higher than 80% in the wavelength range of 600–1000 nm. The haze of the HCl etched
The electrical properties and optical properties of the as-grown textured ZnO:Al films show that they are suitable as substrate for thin film solar cells. Therefore, μc-Si cells were deposited by PECVD on HCl etched reference ZnO:Al film and as-grown textured ZnO:Al film in the same PECVD deposition run. Silver was used as back contacts. The solar cell results are shown in Table 3 and the quantum efficiency
Fig. 3. XRD measurements of the thickness series samples, in which (a) is the reference ZnO:Al films and (b) is the as-grown textured ZnO:Al films. The thickness of each sample is noted in the figure.
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W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213 Table 3 The solar cell results based on two different substrates. Substrates
As-grown textured ZnO:Al film HCl etched reference ZnO:Al film
Fig. 4. Electrical properties of the as-grown textured ZnO:Al films and reference ZnO: Al films with different thicknesses.
together with the 1-cell reflection (R) are shown in Fig. 6. The HCl etched reference ZnO:Al film provides remarkable light trapping effect, that the cell current densities is 18.1 mA/cm². The as-grown textured ZnO:Al can also provide excellent light trapping, which gives comparable current density for solar cells, even though its haze is
Cell results Eta
FF
Voc
Jsc
%
%
mV
mA/cm²
6.18 6.93
69.3 73.5
491 521
18.2 18.1
much lower than the HCl etched reference sample (Fig. 5). Similarly results were found on cells deposited on AsahiU SnO2:F films [10]. It proves again that the haze of the substrates is an inadequate parameter to assess the light trapping effect in thin film silicon solar cells [11]. ARS which describes the distribution of the scattered light is expected to be an additional criterion to correlate the optical quality of the substrate. The Jsc is higher when the light intensity scattered into larger angles. The efficiency of the cell grown on as-grown textured ZnO:Al film is not as high as on the HCl etched reference sample, because of the lower fill factor (FF) and open circuit voltage (Voc), even though the Raman measurements show that the crystallinity fraction of the silicon i-layer is very similar for both substrates. The morphology of the as-grown textured ZnO:Al films might be less suitable for the μc-Si growth compared to the etched ZnO. Dropped FF and Voc also happened for cells grown on LPCVD deposited ZnO:B films and AsahiU SnO2:F films [12,13]. The reasons are generally considered to be more grain boundary defects or voids in silicon materials grown on the v-shaped valleys of the substrates [14]. Therefore, the morphology of the as-grown textured ZnO:Al films needs further optimization to improve the electrical performance of the μc-Si thin film solar cells. Nevertheless, the high current density and QE prove that the asgrown textured ZnO:Al films as the front contact can provide excellent light trapping effect for the thin film solar cells. 4. Conclusions In this paper, the as-grown textured ZnO:Al films were deposited on glass substrates by magnetron sputtering with a pre-treatment of argon and oxygen mixed ion beam. It has been shown that the films at 650 nm thick have pyramidal shape, with lateral feature size around 500 nm. The haze and ARS curves are similar to AsahiU SnO2:F film. The pyramids on the film surface originate from small cones at nucleation stage. The residual O bonds left by the ion beam pre-treatment are considered as the reason of the rough growth. For the as-grown textured ZnO:Al film at 650 nm thick, the resistivity is below 5 × 10−4 Ω · cm and the transparency in the near-infrared spectral range is high (N80% at
Fig. 5. Optical properties of the as-grown textured ZnO film, HCl etched reference ZnO: Al film and AsahiU SnO2:F film.
Fig. 6. Quantum efficiency of the μc-Si cells prepared on different ZnO:Al films.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213
1000 nm). Therefore, the as-grown textured ZnO:Al films are considered as efficient TCO films. Finally, μc-Si solar cell with short circuit current density of 18.2 mA/cm² was obtained. Acknowledgements The authors would like to thank W. Reetz, H.P. Bochem and M. Hülsbeck for extensive technical support. This study was financially supported by the German ministry BMU under contract no. 0327693A. References [1] S. Faÿ, J. Steinhauser, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Thin Solid Films 515 (2007) 8558. [2] M. Creatore Volintiru, B.J. Kniknie, C.I.M.A. Spee, M.C.M. van de Sanden, J. Appl. Phys. 102 (2007) 043709. [3] J. Hüpkes, J. Müller and B. Rech, Transparent conductive zinc oxide—basics and applications in thin film solar cells, ISBN 978-3-540-73611-0, 2008, p. 359–413.
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[4] T. Nakada, Y. Ohkubo, A. Kunioka, Jpn. J. Appl. Phys. 30 (12A) (1991) 3344. [5] T. Minami, H. Sato, S. Takata, N. Ogawa, T. Mouri, Jpn. J. Appl. Phys. 31 (1922) 1106. [6] A. Shabalin, M. Amann, M. Kishinevsky, C. Quinn, N. Capps, Characterization of the Low-Voltage High-Current Single-Cell Ion Source, 43rd Annual Technical Conference of the Society of Vacuum Coaters, 2000, p. 283. [7] S.V. Thomsen, R.H. Petrmichl, V.S. Veerasamy, A.V. Longobardo, H.A. Luten, D.R. Hall, US patent No.: US 6808606 B2, 2004. [8] J. Krč, M. Zeman, O. Kluth, F. Smole, M. Topič, Thin Solid Films 426 (2003) 296. [9] Z. Lu, H. Xu, M. Xin, K. Li, H. Wang, J. Phys. Chem. C 114 (2010) 820. [10] O. Kluth, C. Zahren, H. Stiebig, B. Rech, H. Schade, Proc. 19th European Photovoltaic Solar Energy Conference, Paris, France, 2004, p. 1587. [11] H. Stiebig, M. Schulte, C. Zahren, C. Hasse, B. Rech, P. Lechner, Proc. SPIE 6197 (2006) 619701. [12] Y. Nasuno, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 74 (2002) 497. [13] L. Feitknecht, J. Steinhauser, R. Schlüchter, S. Faÿ, D. Dominé, E. Vallat-Sauvin, F. Meillaud, C. Ballif, A. Shah, Technical Digest of the 15th International Photovoltaic Solar Energy Conference, vol. 1, 2005, p. 473. [14] M. Python, E. Vallat-Sauvain, J. Bailat, D. Dominé, L. Fesquet, A. Shah, C. Ballif, J. Non-Cryst. Solids 354 (2008) 2258.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
As-grown textured zinc oxide films by ion beam treatment and magnetron sputtering Wendi Zhang a,⁎, Eerke Bunte a, Florian Ruske b, Dominik Köhl c, Astrid Besmehn d, Janine Worbs a, Hilde Siekmann a, Joachim Kirchhoff a, Aad Gordijn a, Jürgen Hüpkes a a
Institut für Energieforschung 5-Photovoltaik, Forschungszentrum Jülich, 52425 Jülich, Germany Institute Silicon Photovoltaics, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, 12489 Berlin, Germany I. Physikalisches Institut IA, Rheinisch-Westfälische Technische Hochschule Aachen, 52074 Aachen, Germany d Zentralabteilung für Chemische Analysen, Forschungszentrum Jülich, 52425 Jülich, Germany b c
a r t i c l e
i n f o
Available online 21 April 2011 Keywords: Ion beam treatment Magnetron sputtering ZnO:Al films TCO Solar cells
a b s t r a c t This work presents as-grown textured ZnO:Al films by rf magnetron sputtering initiated by pre-treatment of glass substrate with mixed argon and oxygen ions. A 650 nm thick of this film exhibits surface texture features with lateral size around 500 nm; the resistivity is below 5 × 10−4 Ω · cm and the transparency in the near-infrared spectral range is high (N 80% at 1000 nm). Microcrystalline silicon thin film solar cells grown on the textured glass exhibit excellent light trapping effect with a short circuit current density of 18.2 mA/cm². © 2011 Elsevier B.V. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) films with rough surface are widely used as front contact window layers in thin film silicon solar cells. The incident light is scattered at the textured surface. This leads to the so-called light trapping effect, which boosts the short circuit current and consequently the conversion efficiency of microcrystalline silicon solar cells. Nowadays, textured zinc oxide (ZnO) films are prepared by various methods, such as low pressure chemical vapor deposition (LPCVD) [1], metal-organic chemical vapor deposition (MOCVD) [2] or magnetron sputtering and post wet-chemical etching of the asdeposited flat films [3]. Magnetron sputtering of ZnO films is a reliable method, which allows for relative high deposition rates and is available on a large area (3 × 6 m²). Because of the additional wetchemical etching step, deposition methods which produce directly textured zinc oxide films may be advantageous. Thus, some attempts have been undertaken to directly deposit textured zinc oxide films by magnetron sputtering. By adding water-vapor to the reactor chamber during magnetron sputtering [4] or by applying high gas pressures (N2 Pa) ZnO films with a rough surface could be fabricated [5]. However, the resistivity, charge carrier mobility and feature size of these films are limited.
⁎ Corresponding author. E-mail address: [email protected] (W. Zhang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.098
In this work, we report a new method to directly deposit nanotextured, transparent and highly conductive aluminum doped zinc oxide films (ZnO:Al). It is based on the pre-treatment of the glass substrates by low energetic argon and oxygen mixed ion beam. Afterwards the ZnO:Al films are prepared by magnetron sputtering. The film evolution, surface morphology, optical properties and electrical properties of the as-grown textured ZnO:Al films are discussed in this paper. Single junction microcrystalline silicon thin film solar cells based on this type of substrates are prepared and analyzed. 2. Experimental details The argon and oxygen ion beams were generated from a linear anode layer ion source (Type Lion 420, supplied by von Ardenne Anlagentechnik GmbH, Dresden, Germany). The ion source works in collimated mode instead of diffuse mode. Corning glass was used as substrates in the experiments. The working pressure of the chamber was kept at 1 × 10−3 mbar. The discharge voltage of the generator was 1 kV, which means that the average ion energy was around 500 eV [6]. The discharge current of the generator was kept at 80 mA by adjusting the source gas flow rate. The samples were exposed to the ion beam for 5 min to around 2 h. The distance between the ion source and the substrate is around 16.2 cm. To investigate the relation between the ZnO:Al film growth and the species of the ion beam treatment, we used pure Ar, pure O2 and mixed Ar/O2 gas as source gases. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical composition of the glass surface before and after ion beam treatment.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213
After the ion beam pre-treatment, the glass substrate was taken out from the ion source chamber. Then the ZnO:Al films with different thicknesses were deposited on the treated glass substrates by rf magnetron sputtering in an in-line sputtering system for a substrate size up to 30 × 30 cm² (VISS 300, supplied by von Ardenne Anlagentechnik GmbH, Dresden, Germany). ZnO:Al films deposited on untreated Corning glass were used as reference. The film was sputtered from a ceramic ZnO:Al2O3 (1 wt.%) target. The power of the generator was 1.5 kW and the substrate heater temperature is 430 °C. Total and diffuse transmissions of the ZnO:Al coated glass were measured with a dual beam spectrometer (Perkin Elmer, Lambda 19). Haze is defined as a ratio of diffuse transmitted light to the total transmitted light. The angle distribution of scattered light was measured by an angular resolved scattering setup with a green laser (wavelength: 550 nm). The resistivity, carrier concentration and mobility were measured by a Hall Effect machine at room temperature. The top surface and cross section of the film were characterized by scanning electron microscopy (SEM) (LEO, Gemini). The evolution of the ZnO:Al films were characterized by atomic force microscopy (AFM). X-ray diffraction (XRD) spectra were recorded for the thickness series. The X-ray was generated from a copper kα source. Microcrystalline silicon p-i-n solar cells were deposited on the ZnO:Al films by PECVD in a 30 × 30 cm² system at 13 MHz. The silicon i-layer is 1.1 μm and the area of the test cells is 1 cm². Solar cell characterization was performed with a Wacom solar simulator under standard test conditions (AM1.5, 100 mW/cm² and 25 °C).
3. Results and discussion 3.1. Film morphology Fig. 1 shows the top and cross sectional SEM images of the ZnO:Al films deposited on untreated reference glass (a) and Ar/O2 ion beam treated glass (b). The reference film has small crater like shape, and the average size of the craters is smaller than 50 nm, so it is flat. To apply in solar cells, the flat film has to be etched in diluted HCl solution to get textured morphology. The film sputtered on the ion treated glass has large pyramidal shape and the size of the pyramid is around 500 nm.
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Comparing the two cross sectional SEM images, we see that the reference film has columnar structure, while the other one has conical grains originating from the beginning of the film. The pyramids on the film surface are formed by the connection of the conical grains. In addition, though the two films were co-sputtered, their growth rates are different. The as-grown textured film is around 150 nm thinner than the reference. The differences in surface morphology and growth rate indicate that the growth of the ZnO:Al films on ion treated glass is quite different from the reference film. XPS was used to analyze the chemical composition of the glass surface after pure Ar, pure O2 and mixed Ar/O2 ion beam treatment. The penetration depth of the XPS measurement is around 5 nm, which means that only the surface materials are examined. The results are given in Table 1 (values are given only when larger than 0.1 at.%). One untreated glass surface is added as reference. Elements like C, Al, Cl, and Ca are also detected by XPS, however here we only list the important ones. The table shows that the percentage of Si and O are decreased on the surface after ion beam treatments, while Zn and Fe were found on the ion beam treated surface. On argon ions treated glass surface, Fe contamination is severe (3.8 at.%), but on oxygen ions treated glass surface, there is almost no Fe contamination (b0.1 at.%). This is a common phenomenon for this type of ion source: Fe is sputtered from the steel cathode because of the strike of argon ions, whereas the oxygen ions lightly passivate the surface of the steel cathode resulting in a thin oxide film on the cathode. Since the etch rate for FeO is lower than Fe, less cathode erosion is enabled by the use of oxygen gas [7]. The Zn contamination is supposed to be from the process chamber, since the ion source is installed in a same chamber with a rotatable ZnO:Al2O3 target. The difference between argon and oxygen treatment is that argon is physically milling off materials from glass surface while oxygen ions can also react with the substrate atoms or contaminations as well as etching off the substrate materials. More FeO, Fe2O3 or ZnO might be formed on the glass surface by oxygen ion beam treatment. This could be an explanation of the higher Fe and Zn concentration on the glass surface after mixed argon and oxygen ion beam treatment. ZnO:Al films were deposited on the above ion beam treated glass substrates. The average feature sizes estimated from SEM images are listed in the last row of Table 1. We find that on mixed ion beam treated glass, the ZnO:Al film has the largest feature size. Therefore,
Fig. 1. Top view and cross sectional SEM images of the ZnO:Al films sputtered on (a): reference glass, and (b): argon and oxygen ion beam treated glass.
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Table 1 XPS analysis of the corning glass surface after different ion beam treatments. The last row gives the average feature size of the sputtered ZnO:Al films on those different treated glass.
Fe (at.%) Zn (at.%) Si (at.%) Ar (at.%) O (at.%) Feature size of ZnO:Al films
Untreated
Ar
O2
Ar/O2
– – 18.7 – 56 b 50 nm
3.8 0.1 8.1 1.2 44 100– 200 nm
– 0.5 14.6 – 46.7 200– 400 nm
5.0 1.2 7.5 1.3 49.9 400– 500 nm
we suppose that the conical growth of the as-grown textured ZnO:Al surface is related to the contaminations on the glass surface. However, in later experiments, we find that there is no direct relation between the contamination level and rms roughness of the ZnO:Al films.
3.2. Film evolution Four as-grown textured ZnO:Al films were prepared with different thicknesses, and AFM was used to characterize their surface morphology. The film topographies are shown in Fig. 2. The scan sizes for all the films are 4 × 4 μm². From (a) to (d), the film thicknesses are 95 nm, 225 nm, 350 nm and 650 nm, respectively. For the first three films, the surface is composed of two types of grains: large grains and small flat grains. The large grains here are the conical grains in SEM images. The large grains which start from the nucleation stage distribute on the surface randomly. Because they grow faster than the surrounding flat grains, finally they grow to the surface of the asgrown textured ZnO:Al film. We count the number, the coverage and the mean diameter of the large grains by the SPIP program. The results are listed in Table 2. The number of the large grains decreases,
while the coverage and mean diameter increase when the film gets thicker. XRD measurements were done for the thickness series samples to characterize the film structure properties. All ZnO:Al films were found to have c axis, [002] preferential orientation (Fig. 3). Except the very thin film, all other films also have [004] preferential orientation. The intensity of the [002] peak increases with increased thickness. The [002] peak position of the as-grown textured films is around 34.27°, compared to the 34.4° of reference films. It indicates that the as-grown textured films have more stress in the films. For the as-grown textured films at 650 nm thick, except the [002] and [004] peaks, additional [100], [101], [102], [103] and [112] peaks appear, which means that after the large grains connect to each other, some crystallites with different orientations become pronounced. 3.3. Electrical properties As-grown textured ZnO:Al films and reference ZnO:Al films were sputtered with different thicknesses, and the resistivity, mobility and carrier concentration were determined at room temperature by Hall effect measurements. As shown in Fig. 4, the gradient in the resistivity, mobility and carrier concentration of the as-grown textured ZnO:Al films is much lower than the reference film. There is a crossover, i.e. thick as-grown textured films have higher resistivity and lower mobility, whereas in the thin film region an opposite trend can be observed. In bulk polycrystalline ZnO:Al films, the electron mobility can be limited mainly by two mechanisms: the electron scattering within the grains (e.g. the scattering by the ionized impurities, lattice defects, or phonons) or the electron scattering that occurs at the grain boundaries, due to the potential barrier formed at these locations [1]. If the mobility is dominated by the latter, the mobility should increases with increased grain size, because the grain boundary density decreases, which is the case for the reference films, their mobility increases with increased thickness. However, for the bulk
a
b
c
d
Fig. 2. AFM images of the ZnO:Al films grown on argon and oxygen ion beam treated glass with different thicknesses.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213 Table 2 The analysis of the large grains for the thickness series in Fig. 2. Film
Thickness
Number of large grains
nm (a) (b) (c) (d)
95 225 350 650
220 172 163 128
Coverage
Mean diameter
%
nm
16 22 42 67
118 157 220 298
as-grown textured films, the mobility gradient is lower. For the films from 95 nm to 350 nm, i.e. before the large conical grains connect to each other, the mobility is constant at 23 cm²/(V · s). This indicates that during the growth of as-grown textured films, two possible mechanisms exist. One is that the electron scattering in the grains contribute more to the mobility, which means that there are more defects in the grains. The other one is that the grain boundary density keeps similar during the thin film growth, i.e. the average grain size doesn't increase accordingly with film thickness. However, from the AFM results, we know that the large grains size increases during the film growth. The latter possible mechanism can be almost excluded. The two possible reasons result in a lower mobility for as-grown textured film, 27 cm²/(V · s) at 650 nm, compared to 48 cm²/(V · s) for the reference film at 800 nm. The carrier concentration, which is 5 × 1020 cm−3, is roughly constant for the two films after nucleation stage. The resistivity has similar trends like the mobility because ρ = 1 / (q · n · μ). For the as-grown textured ZnO:Al film at 650 nm the resistivity is 4.2 × 10−4 Ω · cm, even though it is not as low as the reference ZnO:Al films, it is as already low enough to be considered as the front contact for thin films solar cells.
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reference ZnO:Al film is the highest, and the other two are much lower. That is in accordance to scalar scattering theory [8]. The lateral feature size of the HCl etched reference ZnO:Al film is around 1–2 μm, which is larger than as-grown textured ZnO:Al film and AsahiU SnO2:F film. The haze and ARS of the as-grown textured film and AsahiU SnO2:F film are very similar because they have very similar texture morphology. The ARS curves of the as-grown textured ZnO:Al film and AsahiU SnO2:F film are lower according to the lower haze value at 550 nm, but have a more Lambertian-like distribution. The highest intensity of the scattered light for HCl etched reference ZnO:Al film occurs at 12°, while for the other two films, the highest intensities are shown around 40°. It means that for the as-grown textured ZnO:Al films and AsahiU SnO2:F film, the transmitted light tends to be scattered to large angles. 3.5. Growth model of as-grown textured films We build a model to explain the growth mechanism of the ZnO:Al on ion treated glass. The thin film deposition is very sensitive to the substrate conditions. The impurities or defects on the substrate surface can significantly influence the nucleation of the deposited materials. In our case, Zn–O, Fe–O bonds and even implanted O2 are present on the Ar/O2 ion beam treated glass surface, while on untreated glass substrate, the surface is covered only by Si–O bonds. The open O dangling bonds tend to develop O–terminated [00–1] ZnO:Al grains which has the lowest growth rate compared to other crystallographic structures such as [001], [100] and [101] grains [9]. At other surface area where no O bonds are covered, the ZnO:Al films tend to have random orientation, partly are Zn-terminated [001] grains. They have faster growth rate than the O-terminated [00–1] grains and therefore overgrow [001] grains and dominate the surface.
3.4. Optical properties
3.6. Application of as-grown textured ZnO:Al films
The total transmission, haze versus light wavelength from 300 nm to 1300 nm and ARS at 550 nm of the as-grown textured ZnO:Al film are shown in Fig. 5. For comparison, the HCl etched reference ZnO:Al film and one AsahiU SnO2:F film are added. The total transmission of the as-grown textured ZnO:Al film is higher than 80% in the wavelength range of 600–1000 nm. The haze of the HCl etched
The electrical properties and optical properties of the as-grown textured ZnO:Al films show that they are suitable as substrate for thin film solar cells. Therefore, μc-Si cells were deposited by PECVD on HCl etched reference ZnO:Al film and as-grown textured ZnO:Al film in the same PECVD deposition run. Silver was used as back contacts. The solar cell results are shown in Table 3 and the quantum efficiency
Fig. 3. XRD measurements of the thickness series samples, in which (a) is the reference ZnO:Al films and (b) is the as-grown textured ZnO:Al films. The thickness of each sample is noted in the figure.
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W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213 Table 3 The solar cell results based on two different substrates. Substrates
As-grown textured ZnO:Al film HCl etched reference ZnO:Al film
Fig. 4. Electrical properties of the as-grown textured ZnO:Al films and reference ZnO: Al films with different thicknesses.
together with the 1-cell reflection (R) are shown in Fig. 6. The HCl etched reference ZnO:Al film provides remarkable light trapping effect, that the cell current densities is 18.1 mA/cm². The as-grown textured ZnO:Al can also provide excellent light trapping, which gives comparable current density for solar cells, even though its haze is
Cell results Eta
FF
Voc
Jsc
%
%
mV
mA/cm²
6.18 6.93
69.3 73.5
491 521
18.2 18.1
much lower than the HCl etched reference sample (Fig. 5). Similarly results were found on cells deposited on AsahiU SnO2:F films [10]. It proves again that the haze of the substrates is an inadequate parameter to assess the light trapping effect in thin film silicon solar cells [11]. ARS which describes the distribution of the scattered light is expected to be an additional criterion to correlate the optical quality of the substrate. The Jsc is higher when the light intensity scattered into larger angles. The efficiency of the cell grown on as-grown textured ZnO:Al film is not as high as on the HCl etched reference sample, because of the lower fill factor (FF) and open circuit voltage (Voc), even though the Raman measurements show that the crystallinity fraction of the silicon i-layer is very similar for both substrates. The morphology of the as-grown textured ZnO:Al films might be less suitable for the μc-Si growth compared to the etched ZnO. Dropped FF and Voc also happened for cells grown on LPCVD deposited ZnO:B films and AsahiU SnO2:F films [12,13]. The reasons are generally considered to be more grain boundary defects or voids in silicon materials grown on the v-shaped valleys of the substrates [14]. Therefore, the morphology of the as-grown textured ZnO:Al films needs further optimization to improve the electrical performance of the μc-Si thin film solar cells. Nevertheless, the high current density and QE prove that the asgrown textured ZnO:Al films as the front contact can provide excellent light trapping effect for the thin film solar cells. 4. Conclusions In this paper, the as-grown textured ZnO:Al films were deposited on glass substrates by magnetron sputtering with a pre-treatment of argon and oxygen mixed ion beam. It has been shown that the films at 650 nm thick have pyramidal shape, with lateral feature size around 500 nm. The haze and ARS curves are similar to AsahiU SnO2:F film. The pyramids on the film surface originate from small cones at nucleation stage. The residual O bonds left by the ion beam pre-treatment are considered as the reason of the rough growth. For the as-grown textured ZnO:Al film at 650 nm thick, the resistivity is below 5 × 10−4 Ω · cm and the transparency in the near-infrared spectral range is high (N80% at
Fig. 5. Optical properties of the as-grown textured ZnO film, HCl etched reference ZnO: Al film and AsahiU SnO2:F film.
Fig. 6. Quantum efficiency of the μc-Si cells prepared on different ZnO:Al films.
W. Zhang et al. / Thin Solid Films 520 (2012) 4208–4213
1000 nm). Therefore, the as-grown textured ZnO:Al films are considered as efficient TCO films. Finally, μc-Si solar cell with short circuit current density of 18.2 mA/cm² was obtained. Acknowledgements The authors would like to thank W. Reetz, H.P. Bochem and M. Hülsbeck for extensive technical support. This study was financially supported by the German ministry BMU under contract no. 0327693A. References [1] S. Faÿ, J. Steinhauser, N. Oliveira, E. Vallat-Sauvain, C. Ballif, Thin Solid Films 515 (2007) 8558. [2] M. Creatore Volintiru, B.J. Kniknie, C.I.M.A. Spee, M.C.M. van de Sanden, J. Appl. Phys. 102 (2007) 043709. [3] J. Hüpkes, J. Müller and B. Rech, Transparent conductive zinc oxide—basics and applications in thin film solar cells, ISBN 978-3-540-73611-0, 2008, p. 359–413.
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[4] T. Nakada, Y. Ohkubo, A. Kunioka, Jpn. J. Appl. Phys. 30 (12A) (1991) 3344. [5] T. Minami, H. Sato, S. Takata, N. Ogawa, T. Mouri, Jpn. J. Appl. Phys. 31 (1922) 1106. [6] A. Shabalin, M. Amann, M. Kishinevsky, C. Quinn, N. Capps, Characterization of the Low-Voltage High-Current Single-Cell Ion Source, 43rd Annual Technical Conference of the Society of Vacuum Coaters, 2000, p. 283. [7] S.V. Thomsen, R.H. Petrmichl, V.S. Veerasamy, A.V. Longobardo, H.A. Luten, D.R. Hall, US patent No.: US 6808606 B2, 2004. [8] J. Krč, M. Zeman, O. Kluth, F. Smole, M. Topič, Thin Solid Films 426 (2003) 296. [9] Z. Lu, H. Xu, M. Xin, K. Li, H. Wang, J. Phys. Chem. C 114 (2010) 820. [10] O. Kluth, C. Zahren, H. Stiebig, B. Rech, H. Schade, Proc. 19th European Photovoltaic Solar Energy Conference, Paris, France, 2004, p. 1587. [11] H. Stiebig, M. Schulte, C. Zahren, C. Hasse, B. Rech, P. Lechner, Proc. SPIE 6197 (2006) 619701. [12] Y. Nasuno, M. Kondo, A. Matsuda, Sol. Energy Mater. Sol. Cells 74 (2002) 497. [13] L. Feitknecht, J. Steinhauser, R. Schlüchter, S. Faÿ, D. Dominé, E. Vallat-Sauvin, F. Meillaud, C. Ballif, A. Shah, Technical Digest of the 15th International Photovoltaic Solar Energy Conference, vol. 1, 2005, p. 473. [14] M. Python, E. Vallat-Sauvain, J. Bailat, D. Dominé, L. Fesquet, A. Shah, C. Ballif, J. Non-Cryst. Solids 354 (2008) 2258.