Ultramicroscopy 35 (1991) 19-26 19 North-Holland A comparative study of microstructure (in ITO films) and techniques (CTEM and STM) I.A. Rauf and M.G. Walls MP Group, Caoendish Laboratory, University of Cambridge, Madingley Road, Cambridge, CB3 0HE, UK Received 13 September 1990 Understanding the quantitative dependence of carrier mobility on the microstructural characteristics is a major problem in optimizing the performance of ITO films. The microstructure of ITO films is studied here by STM and TEM. Images from the two techniques are compared. Generally the images from these complementary techniques are consistent. The grain size calculated from STM images is larger than that calculated from TEM images. A possible interpretation for this difference is reported. The possibility of developing a method to give a quantitative estimate of grain boundary scattering is presented. 1. Introduction Tin-doped indium oxide (ITO) and some other doped and undoped oxide semiconductors show an interesting and technologically important com- bination of properties. They have high luminous transparency, good electrical conductivity and high infrared reflectivity. The dopant concentration is a crucial factor in optimizing the performance of such films, since it affects the carder concentra- tion and mobility and causes variations in micro- structure which further affect the carder mobility. Ideally Sn is a substitutional dopant at random In sites. The difference in valence when Sn 4+ re- places In ~÷ requires that an extra electron is donated to the lattice and thus contributes to the free carrier density. Although Sn is a neighbour of In in the periodic table, the difference in the ionic radius (In3+: 0.218 nm; Sn4÷: 0.205 nm) will intro- duce a strain as well as a charge centre in the In20 3 lattice. Free carrier electron waves will no longer see a perfect periodic lattice and will be scattered, resulting in the reduction of free carrier mobility, which will lower the electrical conductiv- ity (an undesirable effect for most applications). In practice, there are some other inevitable conse- quences of doping: (i) High dopant concentrations may result in a segregation of dopant atoms towards the grain boundaries as suggested on the basis of EXAFS data of Gaskell and Geere [1]. (ii) Dopant atoms may occupy neighbouring In sites and form clusters that locally have the com- position Sn20 3. The appearance of clusters at the surface of grains has been suggested by Rauf [2]. (iii) Excessive disorder can appear as a result of an enhanced dopant concentration in the form of an intergranular amorphous phase. Rauf et al. [3] have reported the occurrence of an intergranular disordered phase even in lightly-doped In20 3 thin films. It is clear that the free electron mobility criti- cally depends on the dopant concentration and, through this dependence, on the variations in the microstructure caused by enhanced doping. Grain boundary scattering is generally believed [4,5] to be the dominant process affecting the mobility. It is, as yet, unclear how the mobility is quantita- tively correlated to the microstructural character- istics of the film. In this paper we have employed 0304-3991/91/$03.50 © 1991 - Elsevier Science Publishers B.V. (North-Holland)