The improvement in the electrical properties of nanospherical ZnO:Al thin film exposed to irradiation using a Co-60 radioisotope N. Baydogan a,n , O. Ozdemir b , H. Cimenoglu b a Istanbul Technical University, Energy Institute, Nuclear Researches Division, Ayazaga Campus, 34469 Maslak-Istanbul, Turkey b Istanbul Technical University, Metallurgical & Materials Engineering Department, Ayazaga Campus, 34469 Maslak-Istanbul, Turkey HIGHLIGHTS  Gamma radiation affected the nanospherical structure of film depending on the cumulative dose.  Resistivity, carrier density and carrier mobility were improved by Co-60 radioisotope.  There is a relation between improvement of electrical and optical properties of irradiated film. article info Article history: Received 8 October 2010 Accepted 8 February 2013 Available online 9 April 2013 Keywords: Absorbed dose Radiation effect Sol–gel Thin film ZnO:Al. abstract Al doped ZnO (ZnO:Al) thin films were prepared using a sol–gel dip coating technique and deposited on borosilicate glass substrates. The irradiation treatment, was conducted using Co-60 radioisotope, played an important role in enhancing electrical properties. The absorbed dose was a key parameter to decrease the electrical resistivity and to increase carrier density and carrier mobility of nanospherical ZnO:Al thin film. ZnO:Al thin film with the doping of Al at 0.8 at% had the lowest electrical resistivity and the highest optical transmittance after the irradiation treatment. Optical properties, such as transmittance and reflectance, were affected at an absorbed dose of 0.2 Gy. The curves of optical density were improved at ∼380, 420, and 520 nm in visible range after the irradiation process. Besides, another characteristic optical density band between∼900 and 1100 nm was enhanced by gamma irradiation. It has been suggested that the mechanism of absorption is related to an allowed direct transition at the irradiated ZnO:Al thin film on borosilicate glass. The optical band gap of the ZnO:Al thin films broadened with increasing doping concentration. However, there is a decrease in optical energy gap of ZnO:Al thin film along with the absorbed dose of the film. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction The researches of transparent conducting films have focused on the optical thin films. ZnO thin films have been used in many applications due to their advantages, such as cost effectiveness and non-toxicity (Gordillo and Calderon, 2001; Nunes et al., 2000; Hyun et al., 1996; Young-Sung Kim and Weon-Pil Tai, 2007). The doping process greatly influences the electronic and optical properties of ZnO. The fact that doped ZnO thin films have great potential for various applications such as transparent conducting electrodes makes them important in technological aspect. For the high con- ductivity and good optical transmittance, Al doped ZnO (ZnO:Al) films have drawn considerable attention in terms of transparent conducting electrodes (Hiramatsu et al., 1998). They are suitable for fabrication of transparent electrode of solar cells (Xue et al., 2006; Nunes et al., 2000). Transparent and conductive materials are broadly used in the terrestrial and the space applications. The ionising radiation is available especially as gamma radiation, and determina- tion of its effect is important for the efficient usage of devices on terrestrial or space missions. Devices consisting of thin films are exposed to ionising radiation, as the cumulative dose, during their missions. Thus the changes that occur in structural properties, electrical properties, and optical behaviour, such as transmittance and reflectance, with the absorbed dose are important. The results of radiation damage in transparent materials are classified in three categories: atomic displacement by momentum and energy transfer; ionisation and charge trapping; and radiolytic or photochemical effects. These effects are associated with the energy of radiation, as well as the total dose (Badhwar et al., 1995). Trapping an electron in transition elements of optical materials prevents discoloration. These electrons contribute to new induced colours with irradiation. The ionising radiation creates bound electron–hole pairs (excitons). Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/radphyschem Radiation Physics and Chemistry 0969-806X/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radphyschem.2013.02.042 n Corresponding author. Tel.: +90 212 285 34 92; fax: +90 212 285 38 84. E-mail address: dogannil@itu.edu.tr (N. Baydogan). Radiation Physics and Chemistry 89 (2013) 20–27