Transport mechanisms of RF sputtered Al-doped ZnO films by H-2 process gas dilution

Aluminium-doped ZnO thin films, with a doping level in the range 2-2.8 at.%, were deposited by RF magnetron sputtering. Sputtering processes with pure Ar and Ar/H-2 gas mixtures have been explored. Electrical conductivity and Hall mobility of ZnO:AI films were measured in a wide temperature range. It has been found that the addition of hydrogen to the sputtering gas is an effective method to modify the morphological, structural and electrical properties of the ZnO:Al films. A low hydrogen dilution is able to produce a noticeable improvement of the conductivity by means of a better effectiveness of the Al doping. ZnO:Al films deposited at low hydrogen dilution showed a columnar structure whereas at high hydrogen dilution spherical shaped domains were present, formed by many stacked crystallites. Carrier mobility for the former structure was limited by bulk mechanisms, particularly by acoustical phonon and ionized impurity scattering. Carrier mobility for the latter structure was limited by grain boundary mechanisms, particularly by tunnelling effect between neighbouring spherical macroaggregates. (C) 1999 Elsevier Science S.A. All rights reserved.