PESALD oxide semiconductor TFTs

The aim of this work is the development of atmospheric pressure, inexpensive, deposition methods for the fabrication of thin film transistors (TFTs). Plasma-enhanced spatial atomic layer deposition (PESALD) combines the superior reactivity of plasma precursors, which allows operating faster and at relative low temperature, that is below 150 °C, and the advantages of spatial ALD with respect to the conventional technique. The result is an attractive alternative to sputtering and solution process in the fabrication, among other devices, of active layers for thin film transistors. The deposition rate of this technique can reach almost 1 nm/s. In this project, plasma enhanced processes is considered for SALD deposition of binary oxide films. Special attention is given to zinc oxide, indium oxide and zinc oxide/indium oxide layered structure deposition: the idea behind the third system is to confine the motion of the charge carriers at the interface between the two materials. The behavior of these three films in a thin film transistor, in which they constitute the active channel layer, is then investigated. In the first part the process parameters are tuned in order to optimize the growth of zinc oxide and indium oxide thin films, and the same study is carried out for the multiple layered structures. Structural and optical properties of the deposits are characterized with several techniques. The results are a polycrystalline nature of the deposits, with a transition to an amorphous structure with the stacking of very thin layers, a good compositional control along the thickness, and a good level of transparency. These films are then integrated in TFTs. The single oxide layers show a linear field effect mobility of about 0.1 cm2/(Vs), while the same figure of merit is ten times higher for multilayered structures.