JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 11 (2000) 383±387 Conductive and transparent ZnO:Al thin ®lms obtained by chemical spray M. DE LA L. OLVERA, A. MALDONADO, R. ASOMOZA Depto. de Ingenieri  a Ele Âctrica, CINVESTAV-IPN, Apdo. Postal 14-740, Me Âxico, D. F. 07000, Me Âxico R. CASTANEDO-PE  REZ, G. TORRES-DELGADO Lab. de Investigacio Ân en Materiales, CINVESTAV-IPN, Unidad Quere Âtaro, Apdo Postal 1-798, Quere Âtaro, Qro. 76001, Me Âxico J. CAN Ä ETAS-ORTEGA Instituto de Fisica, Universidad Nacional Auto Ânoma de Me Âxico, Apdo. Postal 20-364, Me Âxico 01000, D. F. Me Âxico E-mail: molvera@mail.cinvestav.mx Electrical, structural, morphological and optical characteristics of ZnO:Al thin ®lms obtained by chemical spray are presented in this paper. The dependence of the resistivity on the substrate temperature and the ®lm thickness is reported. For the optimized conditions with no post-annealing the lowest resistivity values obtained for ZnO:Al thin ®lms were 3610  3 and 1:4610  2 O cm for ®lms with thicknesses of 1500 and 600 nm, respectively. Preferential growth in the (0 0 2) direction was observed in all cases. The surface morphology was analyzed by using atomic force and scanning electron microscopy (AFM and SEM) techniques. High transmittance, 85%, was obtained in all cases. The band gap was of the order of 3.35eV. 1. Introduction Aluminum-doped zinc oxide thin ®lms, ZnO:Al, show simultaneously low resistivity and high transmittance in the visible spectrum [1, 2]. In recent years ZnO textured surfaces have been used in photovoltaic applications due to their light trapping properties as well as the chemical stability of ZnO in a hydrogen plasma during the fabrication of amorphous silicon solar cells by plasma- enhanced chemical vapor deposition [3]. Many other applications of ZnO thin ®lms, such as gas sensors [4], anti-re¯ection coatings [5], compact disk memories [6] and pressure sensors [7], among others, are common. ZnO:Al thin ®lms can be deposited by several techniques, such as: sputtering [8], reactive evaporation [9], chemical vapor deposition [10], spray pyrolysis [11] and sol-gel [12]. Physical deposition techniques offer high quality ®lms in a straightforward way. However, in the case of ®lms deposited by chemical techniques a post-deposition annealing treatment in hydrogen or vacuum is required to obtain low resistivity. It is dif®cult to obtain as-grown ZnO:Al thin ®lms with low resistivity by chemical spray [13±15]. In fact, as-deposited ®lms by spray pyrolysis show a resistivity of 0.2 O cm; after an annealing treatment in vacuum, the resistivity reduces to 10  3 O cm [6]. Hence, it is desirable to ®nd a direct way to obtain ®lms with low resistivity without additional post-annealing processes. In order to achieve this, the effects of substrate temperature, molarity and dopant content on the electrical properties of ZnO:Al ®lms have been already studied. The lowest resistivity obtained for ®lms deposited on glass substrates is of the order of 10  1 O cm [16]. To overcome this resistivity limit, other deposition factors should be examined thoroughly. In this respect, there is a lack of information on the role of the starting solution acidity and dopant sources, other than aluminum chloride and aluminum nitrate, in ZnO:Al thin ®lms deposited by chemical spray. In this paper, we report some characteristics of ZnO:Al thin ®lms deposited by chemical spray, obtained from zinc acetate and aluminum pentanedionate, with a high acetic acid content in the starting solution. The effects of the substrate temperature and the ®lm thickness dependence on the electrical, optical, structural and morphological properties are examined. 2. Experimental ZnO:Al thin ®lms were obtained from a 0.2 M solution of zinc acetate ([CH 3 COO] 2 Zn ? 2H 2 O) diluted in a mixture of methanol, deionized water and acetic acid (CH 3 CO 2 H) (30:10:1 in volume, respectively). Aluminum pentanedionate, Al(C 5 H7O 2 ) 3 , dissolved in a mixture of deionized water and CH 3 CO 2 H (50:50 in volume) for a 0.2 M concentration was used as dopant. 0957±4522 # 2000 Kluwer Academic Publishers 383