Photosensitivity of nanocrystalline ZnO films grown by PLD R. Ayouchi a , L. Bentes a , C. Casteleiro a , O. Conde b , C.P. Marques c , E. Alves c , A.M.C. Moutinho d , H.P. Marques d , O. Teodoro d , R. Schwarz a, * a Departamento de Fı´sica, Instituto Superior Te ´cnico, Av. Rovisco Pais 1, P-1049-001 Lisboa, Portugal b Departamento de Fı´sica, Faculdade de Cieˆncias da Universidade de Lisboa, P-1749-016 Lisboa, Portugal c Instituto Tecnolo ´gico e Nuclear, ITN, P-2686-953 Sacave ´m, Portugal d CeFiTec, Departamento de Fı´sica, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal 1. Introduction Due to their wide direct band gap of 3.3 eV and their high electrical conductivity, thin films of zinc oxide (ZnO) are widely used in practical applications such as transparent conducting oxides for flat panel displays and solar cells, ultraviolet (UV) lasers and thin films transistors [1]. Zinc oxide shows n-type conduction, when deposited without intentional doping. Recent reports have suggested that p-type conductivity in ZnO can be obtained by doping with group V elements. However, the stability of level and even type of conduction is still a concern [2]. Various deposition techniques have been developed for depositing ZnO thin films, including chemical vapor deposition, radio frequency sputtering, magnetron sputtering, sol–gel, ion- beam-assisted or molecular-beam epitaxy, and pulsed laser deposition (PLD). Amongst them, the pulsed laser deposition technique is interesting because it offers an easy way to add other elements for alloying or for doping purposes. An important point is that PLD gives the advantage of carrying out the growth in a high-O partial pressure for better control of possible oxygen deficiency [3]. In the present work we report on the morphological, composi- tional, structural and photoelectrical properties of ZnO thin films prepared by PLD at different substrate temperatures and 0.25 mbar oxygen background pressure. In particular, we study the char- acteristic decay times in the persistent photoconductivity effect (PPC) which seems to be correlated with the crystallite size. 2. Experimental All the ZnO films studied in this work were grown by PLD on c-plane sapphire. A Q-switched Nd:YAG laser, operating at 1064 nm wavelength with variable pulse energy of up to 100 mJ and pulse duration of 5 ns, is used to ablate the target at a frequency of 5 Hz. ZnO targets were made from zinc oxide powder (purity 99.99%). The target-to-substrate distance was 5 cm. Substrate temperature is controlled by an electric heater. After deposition, the films were allowed to cool down to room temperature in oxygen ambient, and then removed from the reactor for further characterization. To examine the films structure, X-ray diffraction (XRD) studies were carried out on a Siemens D5000 diffractometer (Cu K a1,2 ) Applied Surface Science 255 (2009) 5917–5921 ARTICLE INFO Article history: Received 21 April 2008 Received in revised form 12 January 2009 Accepted 12 January 2009 Available online 21 January 2009 PACS: 68.35.Ct 81.15.Fg 72.20.Jv 72.40.+w Keywords: ZnO thin films Pulsed laser deposition Persistent photoconductivity Polycrystalline films ABSTRACT We have studied the properties of ZnO thin films grown by laser ablation of ZnO targets on (0 0 0 1) sapphire (Al 2 O 3 ), under substrate temperatures around 400 8C. The films were characterized by different methods including X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and atomic force microscopy (AFM). XPS analysis revealed that the films are oxygen deficient, and XRD analysis with u–2u scans and rocking curves indicate that the ZnO thin films are highly c-axis oriented. All the films are ultraviolet (UV) sensitive. Sensitivity is maximum for the films deposited at lower temperature. The films deposited at higher temperatures show crystallite sizes of typically 500 nm, a high dark current and minimum photoresponse. In all films we observe persistent photoconductivity decay. More densely packed crystallites and a faster decay in photocurrent is observed for films deposited at lower temperature. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +351 21 841 9152; fax: +351 21 841 9118. E-mail address: rschwarz@fisica.ist.utl.pt (R. Schwarz). Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.033