A novel method for preparing thin films with selective doping in a single evaporation step

Abstract

Thin films exhibiting high optical transmission, low emissivity for thermal radiations and good electrical conductivity have important applications, such as transparent electrodes for electrochemical studies, infrared reflectors, antistatic coatings, thin-film resistors, antireflection coatings, resistance-reducing top layers on various solar cells and for a variety of display devices and imaging tubes [1-5]. Tin-doped indium oxide (ITO) is the best and most extensively studied material of this type. The required quality of the film increases with increasing sophistication of application. Achievement of the lowest possible resistivity is of practical significance, as it provides some freedom over the choice of film thickness to achieve high optical transmission while still maintaining low sheet resistance. Efforts to improve the resistivity while maintaining high optical transparency have included: postdeposition heat treatment [6], doping at both cation and anion sites [7], and production of multilayered indium oxide-tin oxide films [8]. Two methods which could reduce the resistivity and have not yet been tried are: growth of single-crystal indium oxide films followed by epitaxial growth of a thin film of Sn203, so that the single-crystal epilayer of S n 2 0 3 provides free electrons which could then be mobile in the indium oxide layers; and doping of the top few layers of a single-crystal indium oxide film with tin. Both of these methods require very expensive equipment and impose limitations on the choice of the substrate. Hence, they are impractical for commercial production. In addition to the achievement of the lowest possible resistivities, the other main objectives of the present research were to achieve the highest possible electron mobility and to provide an answer to the long-debated [9-12] question of the dominant scattering mechanism in tin-doped indium oxide films. This letter reports a very simple method for preparing selectively doped thin films. The objective was to devise a simple cost-effective method to produce films containing "zones" of heavily doped material sandwiches between the "zones" or relatively high-purity material. The heavily doped "zones" would act as free electron generators and the high-purity "zones" would provide a path for the electrons. The result would be high electron mobility and charge carrier density, and hence lower electri-