- Email: [email protected]
RF sputtered indium-tin-oxide as antireflective coating for GaAs solar cells
SoUw~ and Solar ELSEVIER
Ma~
Solar Energy Materials and Solar Cells 45 ( 1997) 161- 168
RF sputtered indium-tin-oxide as antireflective coating for GaAs solar cells E. Aperathitis a,*, Z. Hatzopoulos a, M. Androulidaki a, V. Foukaraki a, A. Kondilis a, C.G. Scott b, D. Sands b, P. Panayotatos a,c a Microelectronics Research Group, Institute of Electronic Structure and Laser, Foundation For Research and Technology - Hellas, P.O. Box 1527, Heraklion 71110, Crete, Greece b Department of Applied Physics, Hull University, Hull HU67RX, United Kingdom c Department of Electrical and Computer Engineering, Rutgers, The State University of New Jersey, P.O. Box 909, Pitcataway, NJ 08855-0909, USA
Received 5 February 1996; revised 24 June 1996; accepted 3 July 1996
Abstract Conductive and antireflective indium-tin-oxide (ITO) has been prepared by RF sputtering in Ar atmosphere, without introducing oxygen into the plasma and on room temperature substrates in order to be used as antireflective coating on GaAs solar cells. The electrical resistivity of the n-type, degenerate ITO films exhibited a reduction with deposition rate and an increase with total pressure, while it was independent of the film thickness in the range of 20 nm to 130 nm. Further reduction of resistivity, up to 4 x l0 -4 [),cm, was obtained by annealing at 400°C. This is the lowest resistivity that has been reported for ITO films prepared under similar conditions. The transmittance of 90 nm thick ITO film was 85% and the reflectance of p / n GaAs solar cell was reduced from 35% to 2% after the ITO layer application. Keywords: Antireflective coating; Solar cell materials; Thin films; ITO; Solar cells; GaAs
* Corresponding author. Email: [email protected] 0927-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 6 ) 0 0 0 6 7 - 0
162
E. Apercahilis el al./ Solar l;ner,v~ Materiah am/S~Jlar ('ells 45 (1997) 161--168
1. Introduction Indium-tin-oxide (ITO) is generally used as conductive and antireflective layer on flat panel displays, solar cells and other optoelectronic devices. This is due to the fact that ITO is a degenerate semiconductor with high transmissivity in the visible solar spectrum and high conductivity. The oxygen ~acancies (along with the Sn donors), which are responsible for the high conductivity, can also lead to non-stoichiometric ITO and, thus, non-optimum optical properties, if the layer is to be used as antireflective coating. As a result, most of the reports in the literature have been focused on attempts to fabricate ITO with the optimum properties, by adding oxygen during deposition a n d / o r heated substrates and under controllable conditions by using various deposition techniques such as spray pyrolysis [1], evaporation [2-4], dc or rf sputtering [5-9] or laser ablation [10]. Furthermore and particularly li3r the case of solar cells, the front contact metallization not only introduces shadowing effects to the device but also requires rather expensive and complicated photolithographic techniques to be employed in the processing procedure for the fabrication of such devices. The potential of ITO as a candidate for transparent top contact for solar ceils, towards reducing processing costs and eliminating shadowing losses, has also been considered [10]. In this paper we report on the properties of ITO films that have been fabricated by RF sputtering in such a way that it will enable the fabrication procedure to be incorporated in the processing procedure of GaAs solar cell devices since the ITO layer will be used as antireflective coating [1 I]. For this purpose, the ITO films were deposited on unintentionally heated substrates without introduction of oxygen gas in the plasma.
2. Experimental procedure The ITO samples were deposited by RF sputtering from an ITO (90%In203 -F 10%SnO 2) target. The target-substrate distance was II cm. The ITO was deposited on Coming 7059 glass, on semi-insulating GaAs substrates or on n + GaAs substrates on which a typical p / n GaAs solar cell structure had been grown in a VG-80H MBE system. The substrates were cleaned in an ultrasonic bath containing organic solvents for 15 minutes and thoroughly dried before being placed in the sputtering chamber. Two different Ar (99.999% pure) pressures were used, 5 mTorr and 10 mTon'. The RF power was 300, 400 or 500 W. The sputtering system was pumped down to better than 1 X 10 ~ mTorr and cleaning/sputtering of the target for 10 minutes always took place prior to opening the shutter for the deposition. The electrical properties of the ITO films were measured using the four probe van der Pauw technique. The optical properties were monitored by transmittance and reflectance measurements, using double grating monochromator and standard lock-in techniques. The thickness, refractive index and the optical energy gap of the films were deduced from the transmittance and reflectance spectra [12,13]. The thickness of the films was also determined independently after deposition, by measuring the height of the step created during the deposition with a stylus alpha-step technique. The post-fabrication annealing was performed in a Rapid Thermal Annealing System.
E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
163
3. Results and discussion
3.1. Electrical properties The resistivity of ITO samples prepared under different RF powers and at two different Ar pressures, before and after post-fabrication annealing, is shown in Fig. 1. The deposition rate for the samples deposited under 10 mTorr Ar pressure was 1.5, 2.5 and 3.2 n m / m i n at 300, 400 and 500 W RF power, respectively, whereas for the 5 mTorr Ar pressure deposited samples the rate was 2.0, 2.8 and 3.6 n m / m i n at 300, 400 and 500 W, respectively. Hall measurements of the as-deposited ITO films revealed that they were n-type and, as it will be seen later, degenerate oxides. All electrical measurements reported here concern ITO films deposited on semi-insulating GaAs substrates. The respective values for the ITO films deposited on glass substrates exhibited some differences from the films deposited on GaAs substrates. The electrical resistivity, sheet resistance and charge carrier mobility was lower by a factor of two, whereas the carrier concentration was higher by the same factor. These differences must be attributed to the different structural properties and surface nature of the two substrates and also to thicker ITO layers, up to 8%, when deposited on glass substrates under the same deposition parameters. The latter is probably due to the different sticking coefficients of the impinging atoms onto the two different substrates. By observing the behaviour of the samples immediately after deposition, it can be seen that higher deposition rates, due to higher RF power delivered to the target, resulted in films with lower electrical resistivity. It is possible that at higher deposition rates oxygen was removed from the forming oxide on the substrate and consequently the deposited films contained more oxygen vacancies which reduced the resistivity of the samples [6,8,14]. This is also supported from the fact that the samples prepared at 400 W and 500 W RF power, at the same total pressure, exhibited almost the same resistivity (Fig. 1), whereas at 300 W RF power a higher increase in resistivity was observed. This phenomenon was more profound for samples deposited at the lower pressure (5 mTorr),
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RF POWER (WATT) Fig. 1. Resistivity of ITO samples, as grown and after 400°C/2 min annealing, as a function of RF power, for two different Ar pressures.
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E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
where, as can be seen in Fig. 1. the difference in the resistivity of samples deposited at different RF powers is much smaller. The lower the pressure, the more energetic the species in the plasma causing re-sputtering of the forming oxide on the substrate. As expected, similar behaviour to the resistivity results was observed for the sheet resistance of the ITO samples. The sheet resistivity of samples deposited at 10 mTorr Ar pressure decreased with RF power from 104 l ) / s q to 103 l~/sq, whereas, at 5 mTorr Ar pressure the decrease was from 400 l ] / s q to 60 l ] / s q . The change of carrier concentration for samples with different deposition conditions followed inversely the respective changes of resistivity. Thus, with increasing RF power, the carrier concentration, ND, varied from 6 X 10 ~9 cm s to 3.5 X 102o cm -3, at 5 mTorr Ar pressure and, from 1 X 10 t~ cm s to 5 X 10 ~9 cm 3 at 10 mTorr Ar pressure. The charge carrier mobility, /z, increased with RF power from 3 to 13 cm2/Vs, regardless of Ar pressure used. Annealing of the ITO films resulted in an improvement of their electrical properties. They became increasingly degenerate and more conductive, as shown in Fig. 1. The improvement of ITO resistivity seemed to saturate when the annealing was performed at 400°C for 2 minutes and saturation could be reached regardless if the annealing was performed first at 200°C for 2 minutes and then at 400°C or straight to the temperature of 400°C. No attempts were made to anneal the samples at higher temperature or for longer annealing time because such annealing conditions are incompatible with solar cells processing technology. The improvement in the electrical properties must be attributed to further release of oxygen during annealing which, as expected, was more drastic for the 10 mTorr Ar pressure deposited films. However, the best values were exhibited for the ITO films deposited at 5 reTort Ar pressure, for which the resistivity was reduced to 3 x 10 -4 f~cm, the carrier concentration increased to 1.5 X 10 21 c m 3 the charge carrier mobility increased to 18 cm2/Vs and the sheet resistance reduced to 15 l ) / s q . These low values of the electrical properties of the ITO samples are the best that have been reported [5,7,14-17] for samples fabricated under similar conditions, that is without introducing oxygen in the plasma and with the substrate kept at room temperature. These conditions are preferable for solar cell processing technology for volume production. ITO films with lower resistivity and sheet resistance values (6.8 X 10 5 f~cm and 4.5 f~/sq, respectively) have also been fabricated by RF sputtering but the substrate was kept at 370°C [15]. The change in electrical resistivity for the ITO samples versus thickness, before and after annealing, is shown in Fig. 2. The deposition conditions were 300 W RF power and 5 mTorr Ar pressure. An increase in resistivity for thicknesses higher than 130 nm can be observed, which is associated with the plasma becoming gradually oxygen-rich. This is in accordance with the observation made in Fig. 1, where lower deposition rates resulted in films with higher resistivity. There was improvement in resistivity with annealing, as expected (Fig. 2), to values of p = 4 x 10 4 ~ c m , N D = 1.5 X 102~ cm -, /z = 10 cm~/Vs and R~h = 20 l~/sq. Samples prepared at 10 mTorr Ar pressure having thicknesses lower than 130 nm exhibited a scattering in resistivity values. It is believed that such high pressure, and consequently low deposition rate, for the fabrication of quite thin samples must have resulted in an unstable plasma species resulting in non-reproducible layers. Such
E. Aperathitis et al. /Solar Energy Materials and Solar Cells 45 (1997) 161-168 l 0 -2
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fabrication conditions might require a longer period of plasma stabilization prior to opening the shutter before commencing of the actual deposition. The RF sputtering conditions shown in Fig. 2, resulted in ITO films with reproducible and optimum electrical properties. Furthermore, such low RF power used for these results ensures the elimination of any damage on the surface of the deposited substrates [18]. Thus, the optical properties that are presented below concern the optical properties of ITO films prepared under the above mentioned optimal fabrication conditions (300 W RF power and 5 mTorr Ar pressure).
3.2. Optical properties Transmittance measurements of as-deposited ITO films, on glass substrates, have been found to exhibit an increase in transmittance from 80%, for a 90 nm thick ITO film, to 92% (peak value), for a 640 nm thick film [19]. A typical transmittance spectrum of a thick (640 nm) ITO film is shown in Fig. 3. The sum of transmittance and reflectance for photon energies less than 2.0 eV was found to be 0.99, suggesting that, in 1.0
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Fig. 3. Transmittance spectrum of a 640 nm thick ITO film.
166
If. Aperathiti,~" el al. ,/.Solar I;I ergy Materia/v attd Solar ('ells 45 (1997) 161-168
this photon energy range, the ITO films were highly transparent, within experimental error (1%). Fig. 4 shows the absorption data (fundamental absorption region) taken from the near UV transmittance spectrum of Fig. 3, where the absorption coefficient, o~, and the square of the absorption coefficient, oe-. have been plotted against hu. The direct optical band gap, which was obtained by extrapolating the linear portion of o~-' versus hu at c~-"= 0, was 3.01 eV. An indirect transition at 2.92 eV was also revealed from the plot at ( f J e versus h~'. Even though such a tow value for the optical direct energy gap of ITO fihns has been reported in the literature [16], larger values, 3.2-4.4 eV, have also been observed [7,14,17], which is a clear indication of the strong dependence of ITO films properties on fabrication conditions. The low energy band gap value lk)r the ITO layers prepared in this work would not affect solar cell performance when such a layer is used as an antireflective coating, since both correspond to the UV region of the solar spectrum. The index of refraction for the ITO fihns, which was determined from the position of the interference maxima and minima of the transmittance spectrum of Fig. 3, was 2.1 [14]. There was no dependence of index of retraction on thickness for films with thicknesses higher than 130 nm. Values of the index of retraction from transmittance data of ITO films with thicknesses lower than 130 nm could not be obtained due to the presence of only one interference peak in the monitored transmittance spectrum. The thickness of the highly transmittive ITO layer which is required if the layer is to be used as antireflective coating on GaAs solar cell (which has high index of refraction, 4.8 at 0.45 ~ m and 3.46 at 1.1 l-tm) was calculated to be around 90 nm [18,20]. Such a thin 1TO layer was deposited on a p / n GaAs solar cell (with p-type emitter) and glass substrates, in the same deposition run. The effect of the ITO deposition on the reflectance of the p / n GaAs solar cell can be seen in Fig. 5. The change in the reflectance of the GaAs solar cell was monitored before and alter the application of the ITO layer, as well as after annealing of the I T O / G a A s sample. The reflectance, immediately after deposition, as seen in Fig. 5, was reduced from 35% to 2% for photon energies between 1.42 eV and 1.8 eV and to up to 15% f~r the longer and shorter photon
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E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168 WAVELENGTH 1.2
0.5 rx~ 0.4
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i
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energies. The peaks that can be seen below 1.42 eV (the energy gap of GaAs above which absorption of photons occurs) are interferences arising from reflections of the incident photons on the surface of the solar cell and the interface between the buffer layer and the n + GaAs substrate on which the p / n GaAs solar cell structure was grown. The reflectance was further decreased by annealing for energies above 1.8 eV but increased for energies below 1.42 eV. The interference minima, at around 1.53 eV and 2.05 eV, seen in Fig. 5 and generally observed for p / n GaAs solar cells, are characteristic reflection features from these particular solar cell structures, despite the high absorption at these photon energies. These interference features disappeared after the application of the ITO layer due to the change of the optical path of the light in the I T O / G a A s structure. Measurement of the transmittance of the ITO film, which had been deposited on glass substrates, revealed that there was an increase in transmittance from 80% to 85% with annealing. The optical properties that have been obtained for the ITO samples show clearly that such films can make a major contribution to the economy of the incident photons on GaAs solar cells since they exhibit excellent antireflective properties. Most importantly, the whole ITO fabrication procedure is compatible with GaAs solar cell processing procedure and technology.
4. C o n c l u s i o n s
Conductive and antireflective ITO films have been prepared by RF sputtering, for antireflective coatings on GaAs solar cells, without the introduction of oxygen in the plasma and with the substrates kept at room temperature, in order to conform with solar cell processing technology. The electrical resistivity of the ITO films was as low as 4 × 10 - 4 f / c m and the carrier concentration 1.5 X 10 21 c m - 3 , after post-fabrication annealing at 400°C, values
168
E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
which are satisfactory lor solar cell applications. The transparency of a 90 nm thick annealed ITO film was 85% and the application of this layer on the surface of p / n GaAs solar cell reduced the reflectivity to 2%. These are the optimum fabrication conditions for preparing ITO antireflective coating films with optimal optical properties in an ITO fabrication procedure capable of incorporation in the processing technology of GaAs solar cells. The conductive properties of the ITO films can be further exploited by careful design of the top grid.
Acknowledgements The authors wish to thank the British Council and the Greek Ministry for Industry, Energy and Technology for financial support.
References [1] J.C. Manifacier, L. Szepersy, J.F. Bresse, M. Perotin and R. Stuck, Mater. Res. Bull. 13 (t978) 109. [2] Y. Shigesato, D.C, Paine and T.E. Haynes, J. Appl. Phys. 73 (1993) 3805. [3] I. Elfallal. R.D. Pilkington, A.E. Hill, R.D. Hill, R.D. Tomlinson, R. Diaz. M. Leon, L. Galan and F. Rueda, Proc. l lth European Photovoltaic Solar Energy Conf.. Switzerland, 1992, p. 925. [4] N. Balasubramanian and A. Subrahmanyam, Thin Solid Films 193 (1990) 528. [5] W.K. Lee, T. Machino and T. Sugihara, Thin Solid Films 224 (1993) 105. [6] M.A. Martinez, J. Herrero and M.T. Gutierrez. Sol. Energy Mater. Sol. Cells 26 (1992) 309: 13th European Photovoltaic Solar Energy Conf. and Exhibition, Nice, France, 23-27 October 1995. [7] A. Mansingh and C.V.R.V. Kumar. J. Appl. Phys. 22 (1989) 455. [8] H. Hoffmann, J. Pickl, M. Schmidt and D. Krause, Appl, Phys. 16 (1978) 239 and 381. [9] J-W. Seo. A.A. Ketterson, D.G. Ballegeev, K-Y. Cheng. I. Adesida, X. Li and T. Gessert, IEEE Photonics Technology Lett. 4 (1992) 888. [10] S.J. Taylor, A. Leycuras, B. Beaumont, C. Coutal, J.('. Roustan and A. Azema, 1st World Conf. on Energy Conversion, Hawai, 5 9 December 1994. [1 I] S.P. Tobin, S.M. Vernon, C. Bajgar, I,.M. Geofl?oy. (7.J, Keavney. M.M. Sanfacon and V.E. Haven, Solar Cells 24 (1988) 103. [12] J. Pankove, Optical Processes in Semiconductors (Dover Publications, NY, 1971) Ch. 4, p. 87, [I 3] W.L. Wolfe, Properties of optical materials, in: W.G. Driscoll (Ed.), Handbook of Optics (McGraw-Hill, NY, 1978) Ch. 7. [14] C.V.R.V. Kumar and A. Mansingh, J. Appl. Phys. 05 (1989) 1270. [15] S. Ray, R. Banerjee, N. Basu, A.K. Batabyal and A.K. Barua. J. Appl. Phys. 54 (1983) 3497. [16] W.G. Haines and R.H. Bube, J. Appl. Phys. 49 (1978) 304. [17] A.J. Steckl and G. Mohammed, J. Appl. Phys. 51 (1980) 3890, [18] V. Foukaraki. Project, Physics Department, 1995, (Trete University, Greece. [19] K. Susuki. N. Hashimoto, T. Oyama, J. Szimizu, Y, Akao and H. Kojima, Thin Solid Films 226 (1993) 104. [20] H.J. Hovel, Solar cells, in: R.K. Willardson and A.C. Beer (Ed.), Semiconductors and Semimetals, Vol. I 1 (Academic Press, 1975) p. 203.
Ma~
Solar Energy Materials and Solar Cells 45 ( 1997) 161- 168
RF sputtered indium-tin-oxide as antireflective coating for GaAs solar cells E. Aperathitis a,*, Z. Hatzopoulos a, M. Androulidaki a, V. Foukaraki a, A. Kondilis a, C.G. Scott b, D. Sands b, P. Panayotatos a,c a Microelectronics Research Group, Institute of Electronic Structure and Laser, Foundation For Research and Technology - Hellas, P.O. Box 1527, Heraklion 71110, Crete, Greece b Department of Applied Physics, Hull University, Hull HU67RX, United Kingdom c Department of Electrical and Computer Engineering, Rutgers, The State University of New Jersey, P.O. Box 909, Pitcataway, NJ 08855-0909, USA
Received 5 February 1996; revised 24 June 1996; accepted 3 July 1996
Abstract Conductive and antireflective indium-tin-oxide (ITO) has been prepared by RF sputtering in Ar atmosphere, without introducing oxygen into the plasma and on room temperature substrates in order to be used as antireflective coating on GaAs solar cells. The electrical resistivity of the n-type, degenerate ITO films exhibited a reduction with deposition rate and an increase with total pressure, while it was independent of the film thickness in the range of 20 nm to 130 nm. Further reduction of resistivity, up to 4 x l0 -4 [),cm, was obtained by annealing at 400°C. This is the lowest resistivity that has been reported for ITO films prepared under similar conditions. The transmittance of 90 nm thick ITO film was 85% and the reflectance of p / n GaAs solar cell was reduced from 35% to 2% after the ITO layer application. Keywords: Antireflective coating; Solar cell materials; Thin films; ITO; Solar cells; GaAs
* Corresponding author. Email: [email protected] 0927-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 7 - 0 2 4 8 ( 9 6 ) 0 0 0 6 7 - 0
162
E. Apercahilis el al./ Solar l;ner,v~ Materiah am/S~Jlar ('ells 45 (1997) 161--168
1. Introduction Indium-tin-oxide (ITO) is generally used as conductive and antireflective layer on flat panel displays, solar cells and other optoelectronic devices. This is due to the fact that ITO is a degenerate semiconductor with high transmissivity in the visible solar spectrum and high conductivity. The oxygen ~acancies (along with the Sn donors), which are responsible for the high conductivity, can also lead to non-stoichiometric ITO and, thus, non-optimum optical properties, if the layer is to be used as antireflective coating. As a result, most of the reports in the literature have been focused on attempts to fabricate ITO with the optimum properties, by adding oxygen during deposition a n d / o r heated substrates and under controllable conditions by using various deposition techniques such as spray pyrolysis [1], evaporation [2-4], dc or rf sputtering [5-9] or laser ablation [10]. Furthermore and particularly li3r the case of solar cells, the front contact metallization not only introduces shadowing effects to the device but also requires rather expensive and complicated photolithographic techniques to be employed in the processing procedure for the fabrication of such devices. The potential of ITO as a candidate for transparent top contact for solar ceils, towards reducing processing costs and eliminating shadowing losses, has also been considered [10]. In this paper we report on the properties of ITO films that have been fabricated by RF sputtering in such a way that it will enable the fabrication procedure to be incorporated in the processing procedure of GaAs solar cell devices since the ITO layer will be used as antireflective coating [1 I]. For this purpose, the ITO films were deposited on unintentionally heated substrates without introduction of oxygen gas in the plasma.
2. Experimental procedure The ITO samples were deposited by RF sputtering from an ITO (90%In203 -F 10%SnO 2) target. The target-substrate distance was II cm. The ITO was deposited on Coming 7059 glass, on semi-insulating GaAs substrates or on n + GaAs substrates on which a typical p / n GaAs solar cell structure had been grown in a VG-80H MBE system. The substrates were cleaned in an ultrasonic bath containing organic solvents for 15 minutes and thoroughly dried before being placed in the sputtering chamber. Two different Ar (99.999% pure) pressures were used, 5 mTorr and 10 mTon'. The RF power was 300, 400 or 500 W. The sputtering system was pumped down to better than 1 X 10 ~ mTorr and cleaning/sputtering of the target for 10 minutes always took place prior to opening the shutter for the deposition. The electrical properties of the ITO films were measured using the four probe van der Pauw technique. The optical properties were monitored by transmittance and reflectance measurements, using double grating monochromator and standard lock-in techniques. The thickness, refractive index and the optical energy gap of the films were deduced from the transmittance and reflectance spectra [12,13]. The thickness of the films was also determined independently after deposition, by measuring the height of the step created during the deposition with a stylus alpha-step technique. The post-fabrication annealing was performed in a Rapid Thermal Annealing System.
E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
163
3. Results and discussion
3.1. Electrical properties The resistivity of ITO samples prepared under different RF powers and at two different Ar pressures, before and after post-fabrication annealing, is shown in Fig. 1. The deposition rate for the samples deposited under 10 mTorr Ar pressure was 1.5, 2.5 and 3.2 n m / m i n at 300, 400 and 500 W RF power, respectively, whereas for the 5 mTorr Ar pressure deposited samples the rate was 2.0, 2.8 and 3.6 n m / m i n at 300, 400 and 500 W, respectively. Hall measurements of the as-deposited ITO films revealed that they were n-type and, as it will be seen later, degenerate oxides. All electrical measurements reported here concern ITO films deposited on semi-insulating GaAs substrates. The respective values for the ITO films deposited on glass substrates exhibited some differences from the films deposited on GaAs substrates. The electrical resistivity, sheet resistance and charge carrier mobility was lower by a factor of two, whereas the carrier concentration was higher by the same factor. These differences must be attributed to the different structural properties and surface nature of the two substrates and also to thicker ITO layers, up to 8%, when deposited on glass substrates under the same deposition parameters. The latter is probably due to the different sticking coefficients of the impinging atoms onto the two different substrates. By observing the behaviour of the samples immediately after deposition, it can be seen that higher deposition rates, due to higher RF power delivered to the target, resulted in films with lower electrical resistivity. It is possible that at higher deposition rates oxygen was removed from the forming oxide on the substrate and consequently the deposited films contained more oxygen vacancies which reduced the resistivity of the samples [6,8,14]. This is also supported from the fact that the samples prepared at 400 W and 500 W RF power, at the same total pressure, exhibited almost the same resistivity (Fig. 1), whereas at 300 W RF power a higher increase in resistivity was observed. This phenomenon was more profound for samples deposited at the lower pressure (5 mTorr),
I0 °
i
4,
i
i
'
i
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TO - Thickness = 150 nm ~pen squares: 10 mTorr Ar squares: 5mTorr A r
E o lO-I
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,--V---~°
>- 10 .2
ro ared
•
> 10 -3
• ....
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I
300
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I
[
400
500
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600
RF POWER (WATT) Fig. 1. Resistivity of ITO samples, as grown and after 400°C/2 min annealing, as a function of RF power, for two different Ar pressures.
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E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
where, as can be seen in Fig. 1. the difference in the resistivity of samples deposited at different RF powers is much smaller. The lower the pressure, the more energetic the species in the plasma causing re-sputtering of the forming oxide on the substrate. As expected, similar behaviour to the resistivity results was observed for the sheet resistance of the ITO samples. The sheet resistivity of samples deposited at 10 mTorr Ar pressure decreased with RF power from 104 l ) / s q to 103 l~/sq, whereas, at 5 mTorr Ar pressure the decrease was from 400 l ] / s q to 60 l ] / s q . The change of carrier concentration for samples with different deposition conditions followed inversely the respective changes of resistivity. Thus, with increasing RF power, the carrier concentration, ND, varied from 6 X 10 ~9 cm s to 3.5 X 102o cm -3, at 5 mTorr Ar pressure and, from 1 X 10 t~ cm s to 5 X 10 ~9 cm 3 at 10 mTorr Ar pressure. The charge carrier mobility, /z, increased with RF power from 3 to 13 cm2/Vs, regardless of Ar pressure used. Annealing of the ITO films resulted in an improvement of their electrical properties. They became increasingly degenerate and more conductive, as shown in Fig. 1. The improvement of ITO resistivity seemed to saturate when the annealing was performed at 400°C for 2 minutes and saturation could be reached regardless if the annealing was performed first at 200°C for 2 minutes and then at 400°C or straight to the temperature of 400°C. No attempts were made to anneal the samples at higher temperature or for longer annealing time because such annealing conditions are incompatible with solar cells processing technology. The improvement in the electrical properties must be attributed to further release of oxygen during annealing which, as expected, was more drastic for the 10 mTorr Ar pressure deposited films. However, the best values were exhibited for the ITO films deposited at 5 reTort Ar pressure, for which the resistivity was reduced to 3 x 10 -4 f~cm, the carrier concentration increased to 1.5 X 10 21 c m 3 the charge carrier mobility increased to 18 cm2/Vs and the sheet resistance reduced to 15 l ) / s q . These low values of the electrical properties of the ITO samples are the best that have been reported [5,7,14-17] for samples fabricated under similar conditions, that is without introducing oxygen in the plasma and with the substrate kept at room temperature. These conditions are preferable for solar cell processing technology for volume production. ITO films with lower resistivity and sheet resistance values (6.8 X 10 5 f~cm and 4.5 f~/sq, respectively) have also been fabricated by RF sputtering but the substrate was kept at 370°C [15]. The change in electrical resistivity for the ITO samples versus thickness, before and after annealing, is shown in Fig. 2. The deposition conditions were 300 W RF power and 5 mTorr Ar pressure. An increase in resistivity for thicknesses higher than 130 nm can be observed, which is associated with the plasma becoming gradually oxygen-rich. This is in accordance with the observation made in Fig. 1, where lower deposition rates resulted in films with higher resistivity. There was improvement in resistivity with annealing, as expected (Fig. 2), to values of p = 4 x 10 4 ~ c m , N D = 1.5 X 102~ cm -, /z = 10 cm~/Vs and R~h = 20 l~/sq. Samples prepared at 10 mTorr Ar pressure having thicknesses lower than 130 nm exhibited a scattering in resistivity values. It is believed that such high pressure, and consequently low deposition rate, for the fabrication of quite thin samples must have resulted in an unstable plasma species resulting in non-reproducible layers. Such
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fabrication conditions might require a longer period of plasma stabilization prior to opening the shutter before commencing of the actual deposition. The RF sputtering conditions shown in Fig. 2, resulted in ITO films with reproducible and optimum electrical properties. Furthermore, such low RF power used for these results ensures the elimination of any damage on the surface of the deposited substrates [18]. Thus, the optical properties that are presented below concern the optical properties of ITO films prepared under the above mentioned optimal fabrication conditions (300 W RF power and 5 mTorr Ar pressure).
3.2. Optical properties Transmittance measurements of as-deposited ITO films, on glass substrates, have been found to exhibit an increase in transmittance from 80%, for a 90 nm thick ITO film, to 92% (peak value), for a 640 nm thick film [19]. A typical transmittance spectrum of a thick (640 nm) ITO film is shown in Fig. 3. The sum of transmittance and reflectance for photon energies less than 2.0 eV was found to be 0.99, suggesting that, in 1.0
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166
If. Aperathiti,~" el al. ,/.Solar I;I ergy Materia/v attd Solar ('ells 45 (1997) 161-168
this photon energy range, the ITO films were highly transparent, within experimental error (1%). Fig. 4 shows the absorption data (fundamental absorption region) taken from the near UV transmittance spectrum of Fig. 3, where the absorption coefficient, o~, and the square of the absorption coefficient, oe-. have been plotted against hu. The direct optical band gap, which was obtained by extrapolating the linear portion of o~-' versus hu at c~-"= 0, was 3.01 eV. An indirect transition at 2.92 eV was also revealed from the plot at ( f J e versus h~'. Even though such a tow value for the optical direct energy gap of ITO fihns has been reported in the literature [16], larger values, 3.2-4.4 eV, have also been observed [7,14,17], which is a clear indication of the strong dependence of ITO films properties on fabrication conditions. The low energy band gap value lk)r the ITO layers prepared in this work would not affect solar cell performance when such a layer is used as an antireflective coating, since both correspond to the UV region of the solar spectrum. The index of refraction for the ITO fihns, which was determined from the position of the interference maxima and minima of the transmittance spectrum of Fig. 3, was 2.1 [14]. There was no dependence of index of retraction on thickness for films with thicknesses higher than 130 nm. Values of the index of retraction from transmittance data of ITO films with thicknesses lower than 130 nm could not be obtained due to the presence of only one interference peak in the monitored transmittance spectrum. The thickness of the highly transmittive ITO layer which is required if the layer is to be used as antireflective coating on GaAs solar cell (which has high index of refraction, 4.8 at 0.45 ~ m and 3.46 at 1.1 l-tm) was calculated to be around 90 nm [18,20]. Such a thin 1TO layer was deposited on a p / n GaAs solar cell (with p-type emitter) and glass substrates, in the same deposition run. The effect of the ITO deposition on the reflectance of the p / n GaAs solar cell can be seen in Fig. 5. The change in the reflectance of the GaAs solar cell was monitored before and alter the application of the ITO layer, as well as after annealing of the I T O / G a A s sample. The reflectance, immediately after deposition, as seen in Fig. 5, was reduced from 35% to 2% for photon energies between 1.42 eV and 1.8 eV and to up to 15% f~r the longer and shorter photon
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E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168 WAVELENGTH 1.2
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i
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1.0 ,
i
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0.5
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i
i
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i
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1.6
i
1.8
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2.4
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energies. The peaks that can be seen below 1.42 eV (the energy gap of GaAs above which absorption of photons occurs) are interferences arising from reflections of the incident photons on the surface of the solar cell and the interface between the buffer layer and the n + GaAs substrate on which the p / n GaAs solar cell structure was grown. The reflectance was further decreased by annealing for energies above 1.8 eV but increased for energies below 1.42 eV. The interference minima, at around 1.53 eV and 2.05 eV, seen in Fig. 5 and generally observed for p / n GaAs solar cells, are characteristic reflection features from these particular solar cell structures, despite the high absorption at these photon energies. These interference features disappeared after the application of the ITO layer due to the change of the optical path of the light in the I T O / G a A s structure. Measurement of the transmittance of the ITO film, which had been deposited on glass substrates, revealed that there was an increase in transmittance from 80% to 85% with annealing. The optical properties that have been obtained for the ITO samples show clearly that such films can make a major contribution to the economy of the incident photons on GaAs solar cells since they exhibit excellent antireflective properties. Most importantly, the whole ITO fabrication procedure is compatible with GaAs solar cell processing procedure and technology.
4. C o n c l u s i o n s
Conductive and antireflective ITO films have been prepared by RF sputtering, for antireflective coatings on GaAs solar cells, without the introduction of oxygen in the plasma and with the substrates kept at room temperature, in order to conform with solar cell processing technology. The electrical resistivity of the ITO films was as low as 4 × 10 - 4 f / c m and the carrier concentration 1.5 X 10 21 c m - 3 , after post-fabrication annealing at 400°C, values
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E. Aperathitis et al. / Solar Energy Materials and Solar Cells 45 (1997) 161-168
which are satisfactory lor solar cell applications. The transparency of a 90 nm thick annealed ITO film was 85% and the application of this layer on the surface of p / n GaAs solar cell reduced the reflectivity to 2%. These are the optimum fabrication conditions for preparing ITO antireflective coating films with optimal optical properties in an ITO fabrication procedure capable of incorporation in the processing technology of GaAs solar cells. The conductive properties of the ITO films can be further exploited by careful design of the top grid.
Acknowledgements The authors wish to thank the British Council and the Greek Ministry for Industry, Energy and Technology for financial support.
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