Optical and electrical characteristics of aluminum-doped ZnO thin films prepared by solgel technique

Pure and aluminum-doped ZnO thin films of high quality were prepared by the solgel deposition method. ZnO thin films are formed by a three-step decomposition of the chemical complex (Zn(CH3COO)(2). 2H(2)O) to ZnO. The aluminum doping was achieved by the addition of AlCl3. 2H(2)O and Al(NO3)(3). 9H(2)O in the methanol solution of the chemical complex at an atomic ratio of Al/Zn = 10(-3)-10(-2). The films are transparent (90% optical transmittance) in the near UV, VIS and near IR ranges. X-ray diffraction analysis indicates that the crystallites of both ZnO and ZnO : Al thin films are preferentially oriented along the c-axis, [0 0 2] direction of the hexagonal crystal structure. The electrical conductivity of ZnO thin films increases with film thickness, under selected heat treatments and with aluminum doping. The dark conductivity and photoconductivity values of the films are in the range 10-100 [Omega cm](-1), which are considered adequate for certain solar cell and electrochromic applications. Measurements of the activation energy show that pure and aluminum doped ZnO thin films have generally one donor level around 50 meV and another at 120 meV below the conduction band. These results are used to explain the high electrical conductivity of the films. (C) 1998 Published by Elsevier Science B.V. All rights reserved.