Cation Size Effects on the Electronic and Structural Properties of Solution-Processed In-X-O Thin Films

The nature of charge transport and local structure are investigated in amorphous indium oxide-based thin films fabricated by spin-coating. The In-X-O series where X = Sc, Y, or La is investigated to understand the effects of varying both the X cation ionic radius (0.89-1.17 angstrom) and the film processing temperature (250-300 degrees C). Larger cations in particular are found to be very effective amorphosizers and enable the study of high mobility (up to 9.7 cm(2) V-1 s(-1)) amorphous oxide semiconductors without complex processing. Electron mobilities as a function of temperature and gate voltage are measured in thin-film transistors, while X-ray absorption spectroscopy and ab initio molecular dynamics simulations are used to probe local atomic structure. It is found that trap-limited conduction and percolation-type conduction mechanisms convincingly model transport for low-and high-temperature processed films, respectively. Increased cation size leads to increased broadening of the tail states (10-23 meV) and increased percolation barrier heights (24-55 meV) in the two cases. For the first time in the amorphous In-X-O system, such effects can be explained by local structural changes in the films, including decreased In-O and In-M (M = In, X) coordination numbers, increased bond length disorder, and changes in the MOx polyhedra interconnectivity.