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«ABSTRACT Transparent conducting Al-doped ZnO (AZO) thin films were deposited on soda-lime glass substrates by DC magnetron sputtering with a novel ...»

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The Hall mobility (µ) of the films increases monotonously with Ts while for the carrier concentration (N), there is discrepancy between these two values. At first, N dropped to its lowest value of 1.00 × 1021 cm−3 at Ts = 200◦ C then increased gradually to its maximum value of 1.86 × 1021 cm−3 at Ts = 400◦ C. The effects of Ts on the electrical resistivity (ρ) of the AZO films are, at least, twofold. Firstly, the crystallinity was effectively improved at higher Ts as shown in Fig. 1b whereas the crystal size of the films increased considerably. Minami [12] had showed that owing to the improvement in the crystallinity, the µ increased with Ts up to 250◦ C for AZO films deposited by RF magnetron sputtering. Secondary, the substrate heating is detrimental to the surface morphology as shown in Fig. 2. One of the main factors affecting ρ seems to be the roughness of the film since higher surface roughness leads to a non uniform morphology of the film [25]. The resultant films were embedded with more defects, which subsequently causes stronger boundary scattering and results in shrinkage of carrier lifetime. Therefore, the film deposited at Ts = 200◦ C with largest RMS roughness has lowest carrier concentration. In contrast, a minimum resistivity of 4.62 × 10−4 Ω·cm was achieved for film deposited at Ts = 400◦ C which has compact surface structure, good crystallinity, larger crystal grains and small RMS roughness, simultaneously.

3.3. Optical Properties Figure 7a shows the optical transmittance of the films. The variations in transmittance are dependent on the films thickness which is ascribed to the interference phenomena [20]. It is noteworthy that the transmittance for the AZO films in the NIR region improved considerably as compared to those using alloy targets by reactive magnetron sputtering [7, 26] and even slightly higher than those using Al-doped ZnO (1 wt.%) ceramic targets by RF sputtering [27]. The average transmission as function of Ts was shown in the insert of Fig. 7a. It is obvious that both the average transmissions in the visible region (Tvis ) and those down to NIR region (Ttotal ) were enhanced consistently by substrate heating. Nevertheless, as Ts ≥ 300◦ C, the Tvis exceeded Ttotal. The absorption coefficients (α) near the absorption edge of the films were determined by [28] 1 IO α = ln, (1) d I where d is the film thickness; Io and I are the intensities of the incident and transmitted beams, respectively.

Being a direct-gap semiconductor, the optical band gap (Eopt ) of the films can be described by the following

relationship [28]:

Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 Fig. 7. (a) Optical transmittance of AZO films deposited at various Ts. Insert: the average transmittance of the deposited AZO films; (b) plot of (αhv)2 versus photon energy for AZO films deposited at various Ts. Insert: optical band gap as function of carrier concentration.

–  –  –

where N is the carrier concentration; h is the Plank constant; m∗ is the conduction band effective mass. A vc plot of Eopt versus N 2/3 gave a linear relationship as shown in the insert of Fig. 7. Besides, the Hall effect measurements indicate that the carrier concentrations in this study are ranged in ∼ 1021 cm−3 which are far higher than the Mott density (1018 cm−3 ) [29]. Therefore, the degenerated electrons appear to enter the conduction band, causing a shift of the quasi-Fermi level of electrons in the conduction band.

4. CONCLUSIONS Transparent conducting Al-doped ZnO thin films with 500 ± 10 nm in thickness had been deposited by DC magnetron sputtering using a novel sintered ceramic AZOY® target on soda-lime glass substrates. The as-deposited AZO film has resistivity of 1.68 × 10−3 Ω·cm and average visible transmittance of 75.4%.

By employing substrate heating during deposition, both the electrical conductivity and the optical transparency of the films had improved considerably. At Ts = 400◦ C, the resistivity of the film has its minimum value of 4.62 × 10−4 Ω·cm under which the carrier concentration and mobility was 1.86 × 1021 cm−3 and

7.29 cm2 V−1 s−1, respectively. It is noteworthy that the transmittance in the NIR region was also improved considerably as compared to those employing traditional alloy targets by reactive magnetron sputtering and even slightly higher than those using ceramic targets (Al-doped ZnO, 1 wt.%) by RF sputtering. The optical 310 Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013 band gaps lies between 3.23–3.56 eV, which are apparently wider than that of pristine ZnO owing to the Burstein–Moss effect.

REFERENCES

1. Kelly, P.J. and Arnell, R.D., “Magnetron sputtering: A review of recent developments and applications”, Vacuum, Vol. 56, pp. 159–172, 2000.

2. Minami, T., “Present status of transparent conducting oxide thin-film development for indium-tin- oxide (ITO) substitutes”, Thin Solid Films, Vol. 516, pp. 5822–5828, 2008.

3. Ellmer, K., Klein, A. and Rech, B., Transparent Conductive Zinc Oxide, 1st. ed., Springer, 2007.

4. Lin, Y.C., Le, Q.K., Lai, L.W., Liao, R.M., Jeng, M.S. and Liu, D.S., “Optimizing the organic/inorganic barrier structure for flexible plastic substrate encapsulation”, International Journal of Engineering and Technology Innovation, Vol. 2, pp. 184–194, 2012.





5. Minami, T., Sato, H., Nanto, H. and Takata, S., “Heat treatment in hydrogen gas and plasma for transparent conducting oxide films such as ZnO, SnO2, and indium tin oxide”, Thin Solid Films, Vol. 176, pp. 277–282, 1989.

6. Minami, T., Oohashi, K. and Takata, S., “Preparations of ZnO:A1 transparent conducting films by d.c. magnetron sputtering”, Thin Solid Films, Vol. 194, pp. 721–729, 1990.

7. Szyszka, B., “Transparent and conductive aluminum doped zinc oxide films prepared by mid-frequency reactive magnetron sputtering”, Thin Solid Films, Vol. 351, pp. 164–169, 1999.

8. Ruske, F., Sittinger V., Werner, W., Szyszka B., Osten, K.-U., Dietrich, K. and Rix, R. “Hydrogen doping of DC sputtered ZnO:Al films from novel target material”, Surface and Coating Technology, Vol. 200, pp. 236–240, 2005.

9. Minami, T., Yamamoto, T. and Miyata, T. “Highly transparent and conductive rare earth-doped ZnO thin films prepared by magnetron sputtering”, Thin Solid Films, Vol. 366, pp. 63–68, 2000.

10. Kaur, R., Singh, A.V. and Mehra, R.M., “Sol-gel derived highly transparent and conducting yttrium doped ZnO films”, Journal of Non-Crystal Solids, Vol. 352, pp. 2335–2338, 2006.

11. Yao, P.C., Hang, S.T., Lin, Y.S., Yen, W.T. and Lin, Y.C., “Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition”, Applied Surface Science, Vol. 257, pp. 1441–1448, 2010.

12. Minami, T., Sato, H., Sonoda, T., Nanto, H. and Takata, S., “Influence of substrate and target temperatures on properties of transparent and conductive doped ZnO thin films prepared by r.f. magnetron sputtering”, Thin Solid Films, Vol. 171, pp. 307–311, 1989.

13. Kluth, O., Schope, G., Hupkes, J., Agashe, C., Muller, J. and Rech, B., “Modified Thornton model for magnetron sputtered zinc oxide: Film structure and etching behaviour”, Thin Solid Films, Vol. 442, pp. 80–85, 2003.

14. Chang, J.F. and Hon, M.H., “The effect of deposition temperature on the properties of Al-doped zinc oxide thin films”, Thin Solid Films, Vol. 386, pp. 79–86, 2001.

15. Chen, M., Pei, Z.L., Wang, X., Sun, C. and Wen, L.S., “Structural, electrical, and optical properties of transparent conductive oxide ZnO:Al films prepared by dc magnetron reactive sputtering”, Journal of Vacuum Science and Technology A, Vol. 19, No. 3, pp. 963–970, 2001.

16. Chang, J.F., Shen, C.C. and Hon, M.H., “Growth characteristics and residual stress of RF magnetron sputtered ZnO:Al films”, Ceramics International, Vol. 29, pp. 245–250, 2003.

17. Jou, J.H., Han, M.Y. and Cheng, D.J., “Substrate dependent internal stress in sputtered zinc oxide thin films”, Journal of Applied Physics, Vol. 71, No. 9, pp. 4333–4336, 1992.

18. Park, K.C., Ma, D.Y. and Kim, K.H., “The physical properties of Al-doped zinc oxide films prepared by rf magnetron sputtering”, Thin Solid Films, Vol. 305, pp. 201–209, 1997.

19. Lin, S.S., Huang, J.L. and Sajgalik, P., “Effects of substrate temperature on the properties of heavily Al-doped ZnO films by simultaneous r.f. and d.c. magnetron sputtering”, Surface and Coating Technology, Vol. 190, pp.

39–47, 2005.

20. Oliveira, C., Rebouta, L., de Lacerda-Arôso, T., Lanceros-Mendez, S., Viseu, T., Tavares, C.J., Tovar, J., Ferdov, S. and Alves, E., “Structural and electrical properties of Al doped ZnO thin films deposited at room temperature on poly(vinilidene fluoride) substrates”, Thin Solid Films, Vol. 517, pp. 6290–6293, 2009.

21. Islam, Md Nurul, Ghosh, T.B., Chopra, K.L. and Acharya, H.N., “XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films”, Thin Solid Films, Vol. 280, pp. 20–25, 1996.

Transactions of the Canadian Society for Mechanical Engineering, Vol. 37, No. 3, 2013

22. Kim, K.H., Park, K.C. and Ma, D.Y., “Structural, electrical and optical properties of aluminum doped zinc oxide films prepared by radio frequency magnetron sputtering”, Journal of Applied Physics, Vol. 81, No. 12, pp.

7764–7772, 1996.

23. Hammer, G.E. and Shemenski, R.M., “The oxidation of zinc in air studied by XPS and AES”, Journal of Vacuum Science and Technology A, Vol. 1, No. 2, pp. 1026–1028, 1983.

24. Tansley, T.L. and Neely, D.F., “Adsorption, desorption and conductivity of sputtered zinc oxide thin films”, Thin Solid Films, Vol. 121, pp. 95–107, 1984.

25. Fortunato E., Goncalves, A., Assuncao, V., Marques, A., Aguas, H., Pereira, L., Ferreira, I. and Martins, R., “Growth of ZnO:Ga thin films at room temperature on polymeric substrates: Thickness dependence”, Thin Solid Films, Vol. 442, pp. 121–126, 2003.

26. Pei, Z.L., Sun, C., Tan, M.H., Xiao, J.Q., Guan, D.H., Huang, R.F. and Wen, L.S., “Optical and electrical properties of direct-current magnetron sputtered ZnO:Al films”, Journal of Applied Physics, Vol. 90, No. 7, pp. 3432–3436, 2001.

27. Beyer, W., Hüpkes, J. and Stiebig, H., “Transparent conducting oxide films for thin film silicon photovoltaics”, Thin Solid Films, Vol. 516, pp. 147–154, 2007.

28. Ogwu, A.A., Bouquerel, E., Ademosu, O., Moh, S., Crossan, E. and Placido, F., “The influence of rf power and oxygen flow rate during deposition on the optical transmittance of copper oxide thin films prepared by reactive magnetron sputtering”, Journal of Physics D: Applied Physics, Vol. 38, pp. 266–271, 2005.

29. Liu, Z.F., Shan, F.K., Sohn, J.Y., Kim, S.C., Kim, G.Y., Li, Y.X. and Yu, Y.S., “Photoluminescence of ZnO:Ga thin films fabricated by pulsed laser deposition technique”, Journal of Electroceramics, Vol. 13, pp. 183–187, 2004.

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