High rate reactive magnetron sputtering of ZnO:Al films from rotating metallic targets

Transcription

1 High rate reactive magnetron sputtering of ZnO:Al films from rotating metallic targets H. Zhu 1, 2, *, J. Hüpkes 1, E. Bunte 1 1 IEF5-Photovoltaik, Forschungszentrum Jülich GmbH, D Jülich, Germany 2 Engineering Research Center for Nanophotonics and Advanced Instrument, Ministry of Education, East China Normal University, , Shanghai, P. R. China Abstract Aluminum doped zinc oxide (ZnO:Al) films were reactively sputtered at high discharge power from dual rotating metallic targets (Zn : Al = 99.5 : 0.5 wt%). Deposition conditions like substrate temperature and working points were varied in order to prepare high quality ZnO:Al films. The influences on electrical and optical ZnO:Al thin film properties and surface texture before and after chemical etching in diluted HCl were studied in order to achieve light scattering films as front contact for solar cells. High dynamic deposition rate close to 90 nm m/min and high Hall mobility of up to 47 cm 2 /Vs were obtained. Transmission of more than 85% in the visible spectral range is obtained for all ZnO:Al films in this study. In addition, the absorption in near infrared region is low due to low doping. Surface texture after etching is usually much rougher than before. However, some films reveal after etching small surface features that are similar to initial surface features. We propose a relationship between initial and post-etched surface textures. Key words: ZnO:Al films, Reactive sputtering, Chemical etching, Surface texture

2 1. Introduction Magnetron sputtered aluminum doped zinc oxide (ZnO:Al) films with high optical transmission and low resistivity as well as suitable surface structures for efficient light-trapping after a wet-chemical etching step, were successfully applied into thin film silicon solar cell [1, 2]. Wet-chemically etched rough ZnO:Al films could enhance the short circuit current as compared to as-deposited films and thus, increase the conversion efficiency. Moreover, they could effectively decrease the absorber layer thickness while making sure that most of light could be absorbed inside the solar cells. High rate ZnO:Al films for thin film silicon solar cells, which were prepared from dual rotatable ceramic targets, have been reported [3, 4]. The use of rotatable tube targets effectively enhances the utilization of target material. Additionally, the deposition rate could be increased by increasing discharge power while keeping good electrical and optical properties of the sputtered ZnO films. However, ceramic targets are expensive. A more cost-effective sputtering technology is reactive sputtering from metallic targets. It was already successfully employed by different research groups [5-11]. Szyszka et al. [5, 6] have achieved high quality ZnO:Al films by sputtering in the non-stable transition mode. To achieve stable deposition conditions and thus, constant film growth, a closed loop control system to adjust discharge power according to the oxygen partial pressure measured by a λ- sensor is used. Moreover, high target utilization is achieved by moving the magnetron behind the targets. Hüpkes et al. [9] studied reactive magnetron sputtering of ZnO:Al films with focus on the surface structures after etching. The working point was found to be the crucial

3 parameter. Recently, on-line monitoring of film properties was developed to control the working point of the reactive sputtering process which shall guarantee reproducible sputtering conditions and thus, film properties [11] In this study, we address reactive magnetron sputtering of high rate ZnO:Al films prepared from dual rotatable metallic targets under high discharge power and different substrate temperatures. We focus on electrical and optical properties as well as the etching behaviors and resulting surface structures of ZnO:Al films. 2. Experimental ZnO:Al films were reactively sputtered on Corning Eagle XG glass substrates in a vertical in-line rotatable dual magnetron sputtering system (VISS 300, von Ardenne Anlagentechnik, Dresden, Germany). Two rotatable magnetron (RDM) cathodes with metallic Zn:Al tube targets (99.5:0.5 wt%) were operated under an AC discharge power of 10 kw in total and a mid-frequency (MF) of 40 khz. The substrate temperature was measured before the process without plasma. 200 sccm argon was inputted during sputtering deposition. Oxygen gas flow was controlled by plasma emission monitoring (PEM) system. The intensity of the 307 nm zinc emission line was used to characterize the process for different working points [7]. Fig. 1 shows the relationship between oxygen gas flow and PEM intensity at a discharge power of 10 kw. At low oxygen flow in the so-called metallic mode a high PEM intensity is observed due to high sputter rate of the metallic target. At high oxygen flow in the oxide mode the targets get oxidized and the sputtering rate drops down leading to a

4 low PEM intensity. Due to the hysteresis and self-accelerating transition from metallic to oxide mode and vice versa, the transition mode is not accessible in constant flow mode. It can be stabilized by active process control via lambda sensor and discharge power control [6] or via PEM and gas flow control as in our case, leading to an S-shape of the transition curve. More details can be found elsewhere [7, 9]. The rotating speed for both targets was 10 rounds per minute (rpm). The deposition rates are calculated from the measured thicknesses. The electrical properties were investigated by Hall effect measurement using van der Pauw method (Keithley 926 Hall set-up). ZnO:Al films were etched by a chemical etching step in diluted HCl (0.5%). The morphologies of as-deposited and etched ZnO:Al films were evaluated by high resolution scanning electron microscope (HR-SEM). Optical transmission and reflection of surface-textured thin films were carried out with a double beam spectrometer equipped with an integrating sphere (Perkin Elmer Lambda 19). An index matching fluid (CH 2 I 2 ) was used to avoid systematic measurement errors due to light scattering of the rough films for transmission and reflction measurement [12]. Haze was determined without the index matching fluid. 3. Results 3.1. Deposition rate and electrical properties Fig. 2 (a, b, c and d) show the deposition rates and electrical properties of ZnO:Al films reactively deposited at different substrate temperatures as a function of PEM intensity. As shown in Fig. 2 (a), the deposition rate decreases with higher PEM

5 intensity (towards the metallic mode), which is due to low sticking coefficient of non-oxidized zinc at hot substrates [5, 9]. Even though this effect should be more pronounced at high temperatures, high dynamic rates close to 90 nm m/min have been achieved for ZnO:Al films with resistivities below Ω cm (see Fig. 2(b)). These films are deposited with PEM intensities between 20% and 50% (in the transition mode). The deposition rate at low PEM intensity and 250 C is higher than that of samples sputtered at 300 C and 350 C. This could be due to less zinc evaporation and re-sputtering effects at 250 C. Due to the calibration process for each set of deposition parameters PEM intensities at different substrate temperature cannot directly be compared. In addition, the deposition rate at 250 C decreases when the PEM intensity value of 20% approaches oxide mode. This is caused by the lower sputtering yield of oxide when the sputtering closes to oxide mode further [5]. Low resistivity down to Ω cm is achieved at PEM intensity of 50% for all employed temperatures (Fig. 2(b)). However, resistivity increases towards low PEM intensity. The films resistivity also shows the trend to increase with lower substrate temperature. This is especially pronounced for depositions at low PEM intensities. The carrier concentration almost increases linearly with PEM intensity from cm -3 to cm -3 for samples deposited at 250 C while it only increases from about cm -3 to cm -3 for samples deposited at 300 C and 350 C (see Fig. 2(c)). In Fig. 2(d) the Hall mobility is plotted. In general, it increases with substrate temperature and PEM intensity. At 350 C, however, mobility shows a

6 maximum at PEM of 35%. The highest Hall mobility obtained is 47 cm 2 /Vs. The observations are similar to that reported by Hüpkes et al. for planar targets [9]. Electrical properties of the reactively sputtered ZnO:Al films strongly depend on the working point. Different working points are represented by different oxygen partial pressures [7]. Fig. 3 shows that the correlation between oxygen partial pressure and PEM intensity is almost linear. The working pressure in the investigated deposition regime varies by less than 10% between PEM intensities of 20% to 50%. This will not affect film growth significantly [10]. The different film properties mainly originate from the variation of the oxygen partial pressure with PEM intensity which covers a range of mpa. The oxygen partial pressure affects the film formation chemistry and high energetic oxygen ion bombardment. In more oxidic mode, pronounced high energetic oxygen ion bombardment leads to the increase of stresses and defects inside the ZnO:Al [6, 13, 14]. Together with the tendency to Al 2 O 3 formation Hall mobility decreases. Additionally, surplus oxygen might incorporate in the grain boundaries and hinder charge carrier transport [15]. Close to metallic mode, high energetic oxygen ion bombardment plays only a minor role. However, the tendency of metallic zinc atoms to adhere to the substrate is extremely low. Thus deposition rate is reduced and at the same time Al doping concentration increases [9]. At the higher doping level one can explain reduced mobility as was observed at high temperature and high PEM intensity by ionized impurity scattering [15]. Higher substrate temperatures are beneficial for highly compact ZnO:Al films with excellent electrical properties [2, 9, 16]. The high rate reactively sputtered

7 ZnO:Al films exhibit low resistivity of Ω cm and high Hall mobility up to 47 cm 2 /Vs. The excellent electrical quality of the ZnO:Al films is very similar to that of samples deposited from rotatable ceramic targets at high rate [3] and even to ZnO:Al films from laboratory developments sputtered with radio frequency (rf) excitation at rather low rate [2, 4] Etching behavior and surface morphologies As-grown and texture-etched ZnO:Al films show different topographies. Fig. 4 shows the morphologies of ZnO:Al films deposited at 300 C and PEM intensities of 20%, 30%, 40% and 50%, respectively, in as-grown (left) and post-etched states (right). As seen from Fig. 4 (a), the initial morphology of ZnO:Al film deposited at PEM intensity of 20% shows small crater structures with the feature size of 100 nm in diameter. The crater walls are formed by grains that have pyramidal shape or form ridges. This film represents a transition between fully pyramid-shaped to crater-like surfaces as both feature types are present. For a PEM intensity of 30% the features on the un-etched sample have crater-shape with typical diameters around 200 nm (Fig. 4 (b)). The surface of as grown ZnO:Al film deposited at 40% shows some wider but more shallow craters compared to films deposited at lower PEM intensity (Fig. 4 (c)). ZnO:Al films deposited at 50% show a rather flat surface with flat grains as shown in Fig. 4 (d). These ZnO:Al films also show different surface structures after a wet chemical etching step in dilute HCl as shown in Fig. 4 (e, f, g and h). The ZnO:Al film deposited at PEM intensity of 20% shows flat plateaus, 1 µm wide moon craters

8 and sharp spikes as shown in Fig. 4 (e). The samples deposited at PEM intensity of 30% and 40% show large conical craters of about 1 µm which are much more shallow for 40 % PEM intensity (Fig. 4 (f) and (g)). The ZnO:Al film deposited at PEM intensity of 50% (Fig. 4 (h)) shows similar craters to that of the PEM 40% film (Fig. 4 (g)), but the craters are smaller. Additionally, one can find small features which look similar to that on the initial as-grown surface even inside the craters shown in Fig. 4 (h). The surfaces of the samples deposited at different substrate temperatures and a constant PEM intensity of 35% are shown in Fig. 5 for the as-grown and etched states. Fig. 5 (a, b, and c) show the morphologies of as-grown films deposited at 250 C, 300 C and 350 C, respectively. For the samples deposited at 250 C, grains with pyramidal shape with nm bases are visible on the surface as shown in Fig. 5 (a). At 300 C and 350 C the grains become smaller and form distinct craters of various diameters on the surfaces (Fig. 5 (b and c)). Fig. 5 (d, e and f) show the corresponding ZnO:Al surfaces after etching. Fig. 5 (d) shows cliffy surface with feature sizes similar to the as-grown surface while for sample deposited at 300 C and 350 C many small shallow craters together with a few larger craters distribute on the surface (see Fig. 5 (e and f)). Surfaces of samples deposited above 300 C in metallic mode at high PEM intensities seem much flatter and more etch resistant compared to that of ZnO:Al films deposited at 250 C or close to the oxide mode. This is supported by the etching rates for the three series already discussed before (see Fig. 6). The etching rate

9 decreases with the increase of PEM intensity as well as with the increase of substrate temperature. Note the remarkable increase of etch rate by a factor of four by varying PEM intensity from 50% to 20% at 250 C. The high etch rate and corresponding surfaces after etching (Fig. 4(e) and Fig. 5 (d)) are attributed to much lower film compactness at low temperature or close to the oxide mode [9, 10, 16] Optical properties Fig. 7 shows the transmission and absorption after etching of ZnO:Al thin films deposited at 300 C and PEM intensities between 20-50%. All samples show a high optical transmission of more than 85% and a low absorption in the visible spectral range. The transmission in the near infrared (NIR) spectral range is affected by the free carrier absorption. Here it increases with higher PEM intensity, which is in good agreement with the trend for the carrier concentration extracted from Hall measurements (compare Fig. 2(c)) [10]. The corresponding transmission data vary conversely in this spectral range. The thicknesses of samples (given in the legend of Fig. 7) play a minor role in light absorption or transmission compared to the effect of carrier concentration. The transmission and absorption of samples prepared at 250 C and 350 C show a similar dependency on the PEM intensity (not shown). The absorption edge (as shown in Fig. 7) shifts towards shorter wavelengths with increasing PEM intensity. Based on the optical measurements, we extracted the optical energy gap of the 300 C series as plotted in Fig. 8. The band gap increases from 3.43 ev to 3.59 ev with increase of PEM intensity. In Fig. 9 the extracted

10 optical band gaps of PEM series for the investigated substrate temperatures are shown. All series show a similar trend of increasing band gap with PEM intensity. This is caused by the Burstein-Moss effect [17, 18] that enlarges the optical energy gap for higher carrier density. At high temperatures the slope and range of energy gaps decreases as was already observed for carrier density (see Fig. 2(c)). 4. Discussion ZnO:Al films with excellent electrical and optical properties were prepared. High optical transmittance of more than 85 % coincides with low resistivity of less than Ω cm. The low doping level together with high substrate temperature during sputter deposition lead to high charge carrier mobility at low carrier density to limit free carrier absorption in the NIR. These properties of ZnO:Al are very close to other publications [2, 9, 11 13] and will be appropriate for most applications. For solar cell application the surface texture after etching plays an important role. Thus we concentrate our discussion on the ZnO:Al surface properties. Taking a close look at the SEM images we propose a relationship between as-grown and post-etch surfaces. Some post-etched surfaces exhibit features similar to the initial surface structures. The small craters inside the larger ones of Fig. 4 (f) are of similar size and shape as the craters formed by adjacent grains on the as-grown surface. Similarly, the etched surface of the ZnO:Al sputtered at PEM intensity of 50% (Fig. 4 (h) is covered by small granular roughness, that might stem from the initial as-grown surface. Furthermore, the overall feature size of the ZnO:Al film sputtered at 250 C is similar

11 for initial and etched surface (Fig. 5 (a and d)). However, the shape is modified by the etching step. In contrast, the films prepared at high temperatures in Fig. 5 (e and f) do not clearly reveal features of the initial surface. Furthermore, the granular structure of the initial ZnO:Al film has vanished after etching on the plateaus between moon craters (Fig. 4 (e)) rather completely, even though the ground of the moon craters shows the features comparable to the as-grown surface. In conclusion, parts of the post-etched surface seem to be residues of the as-grown surface; however, the initial thickness is strongly reduced and thus the initial surface features must survive the etch process. Owen et al. investigated the etch attack in the initial stage and also found interrelation between crater formation during etching and characteristic features on the as-grown surface [19]. However, it is not yet possible to predict the etching behavior and crater evolution from as-grown ZnO:Al film properties. Most samples of this study show a relatively flat surface structure after chemical wet etching in dilute HCl and consequently, their Haze (Fig. 10) is low. The highest haze is achieved for the film deposited at 30% PEM intensity and 300 C substrate temperature. The corresponding surface structure is shown in Fig. 4 (f). Even though a high haze is no guarantee for good for light trapping in thin-film solar cells [20], we consider this surface structure together with the reasonable optical and electrical properties as most promising selection of our ZnO:Al films as transparent front contact in silicon based thin film solar cells. However, in order to successfully apply these high rate reactively sputtered ZnO:Al films into silicon based thin film solar cells, further improvement of the

12 surface structure is needed for the sake of a good light trapping effect inside solar cells [21]. 5. Conclusions ZnO:Al films were prepared by reactive sputter deposition from rotatable metallic targets. At high growth rate of close to 90 nm m/min low resistivity down to Ω cm and high Hall mobility up to 47 cm 2 /Vs were achieved. The excellent electrical properties are comparable to those of ZnO:Al deposited at high rate from rotatable ceramic targets and even to ZnO:Al films prepared at low rate. High transmission of more than 85% in the visible spectral range is obtained for all ZnO:Al films in this study. In addition, the absorption in NIR region is low due to low doping, high substrate temperature and sputtering in transition mode. The good optical and electrical properties, high deposition rate from cost effective metallic targets and high target utilization due to the use of tube targets make this process applicable for ZnO:Al production. A relationship between initial and post-etched surface topography is proposed. A prediction of the post-etched surface by investigating as-grown films would mark a great progress for quality control of sputtered ZnO:Al films in a production line. Acknowledgements The authors would like to thank our colleagues J. Worbs, H. Siekamnn and H. P. Bochem for great help in experimental support. Fruitful discussions with our project

13 partners W. Dewald, V. Sittinger, B. Szyszka, D. Köhl and F. Ruske are gratefully acknowledged. This study was financially supported by the German ministry BMU under contract No A.

14 Reference [1] B. Rech. H. Wagner. Appl. Phys. A 69 (1999) 155 [2] M. Berginski, J. Hüpkes, M. Schlute, G.. Schöpe, H. Stiebig, M. Wuttig. J. Appl. Phys. 101 (2007) [3] H. Zhu, E. Bunte, J. Hüpkes, H. Siekmann, S. M. Huang. Thin Solid Films 517 (2008) [4] E. Bunte, H. B. Zhu, J. Hüpkes. Proceeding of 23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain, p2105. [5] B. Szyszka. Thin Solid Films 351 (1999) 164. [6] B. Szyszka, V.Sittinger, X. Jiang, R.J. Hong, w. Werner, A. Pfug, M. Ruske, A. Lopp. Thin Solid Films 442(2003)179. [7] C. May, R. Menner, J. Strümpfel, M. Oertel, B. Specher. Surf. Coat. Technol (2003) 512. [8] J. Müller, G. Schöpe, O. Kluth, V. Sittinger, B. Szyszka, R. Geyer, P. Lechner, H. Schade, M. Ruske, G. Dittmar, H.-P. Bochem, Thin Solid Films 442 (2003) 158. [9] J. Hüpkes, B, Rech, S. Calnan, O. Kluth, U. Zastrow, H. Siekmann, M. Wuttig. Thin Solid Films 502(2006)286 [10] J. Hüpkes, B. Rech, O. Kluth, T. Repmann, B. Zwaygardt, J. Müller, R. Drese, M. Wuttig. Sol. Energy. Mater. Sol. Cells 90 (2006) [11] V. Sittinger, F. Ruske, A. Pflug, W. Dewald, B. Szyszka and G. Dittmar, Thin Solid Films 518 (2010) 3115.

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16 Figure captions Fig. 1 Relationship between oxygen gas flow and plasma emission monitoring (PEM) intensity, which is characterized by intensity of 307 nm zinc emission line during the reactive sputtering. Fig. 2 Dependence of deposition rate (a) and electrical properties: resistivity (b), carrier concentration (c) and Hall mobility (d) of ZnO:Al films on substrate temperature and PEM intensity. Fig. 3 Working pressure and oxygen partial pressure as a function of PEM intensity. Fig. 4 SEM images of as-grown ZnO:Al films deposited at 300 C and different PEM intensity: (a) 20%, (b) 30%, (c) 40%, (d) 50%. The SEM images of corresponding after-etched ZnO:Al films are shown in (e) 20%, (f) 30%, (g) 40% and (h) 50% Fig. 5 SEM images of as-grown ZnO:Al films deposited at PEM intensity of 35% and different substrate temperatures: (a) 250 C, (b) 300 C, (c) 350 C. The SEM images of corresponding after-etched ZnO:Al films are shown in (d) 250 C, (e) 300 C, (f) 350 C as well. Fig. 6 Etching rate of ZnO:Al films reactively sputtered at different substrate temperatures as a function of PEM intensity. Fig. 7 Light transmission and absorption of ZnO:Al films reactively sputtered at 300 C and different PEM intensities. Fig. 8 (α) 2 as a function of (hν) for ZnO:Al films sputtered at 300 C and different PEM intensities, where α is absorption coefficient and (hν) is the incident photon energy.

17 Fig. 9 Optical band gaps of ZnO:Al films sputtered at different substrate temperatures and different PEM intensities. Fig. 10 Haze of ZnO:Al films reactively sputtered at 300 C and different PEM intensities.

18 Zhu_ Fig. 1. tif Metallic mode Oxygen gas flow control mode PEM intensity control mode PEM intensity (%) Oxide mode to Metallic mode Metallic mode to Oxide mode Oxide mode Transition mode Oxygen gas flow (sccm)

19 Zhu_ Fig. 2. tif Depostion rate (nm m/min) C 300 C 250 C a 350 C 300 C 250 C b Resistivity (10 4 Ω cm) Carrier Concentration (10 20 cm -3 ) PEM intensity (%) 350 C 300 C 250 C c PEM intensity (%) 350 C 300 C 250 C d Mobility ( cm 2 / Vs) PEM intensity (%) PEM intensity (%)

20 Zhu_Fig. 3. tif Working pressure (Pa) C 300 C 250 C Oxygen partial presssure (mpa) PEM intensity (%)

21 Zhu_Fig. 4. tif Surface and Coatings Technology 205 (2010),

22 Zhu_ Fig. 5. tif Surface and Coatings Technology 205 (2010),

23 Zhu_ Fig. 6. tif 20 Etching rate (nm/s) C 300 C 250 C PEM intensiy (%)

24 Zhu_ Fig. 7. tif Transmission and Absorption (%) PEM intensity Thickness 20% 779 nm 30% 677 nm 40% 656 nm 50% 561 nm Absorption Wavelength (nm) Transmission

25 Zhu_ Fig. 8. tif (α) 2 (10 10 cm -2 ) PEM intensity 20% 25% 30% 35% 40% 50% 3.54 ev 3.59 ev ev 3.50 ev 3.47eV 3.43eV hν (ev)

26 Zhu_ Fig. 9. tif 3.72 Optical band gap (ev) C 300 C 250 C PEM intensity (%)

27 Zhu_ Fig. 10. tif Haze PEM intensity 20% 25% 30% 35% 40% 50% Wavelength (nm)