Fabrication and characterization of thin-film transistor materials and devices

Abstract

A class of inorganic thin-film transistor (TFT) semiconductor materials has emerged involving oxides composed of post-transitional cations with (n-1)d¹⁰ns⁰ (n≥4) electronic configurations. This thesis is devoted to the pursuit of topics involving the development of these materials for TFT applications: Deposition of zinc oxide and zinc tin oxide semiconductor layers via reactive sputtering from a metal target, and the characterization of indium gallium zinc oxide (IGZO)-based TFTs utilizing various insulator materials as the gate dielectric.

The first topic involves the deposition of oxide semiconductor layers via reactive sputtering from a metal target. Two oxide semiconductors are utilized for fabricating TFTs via reactive sputtering from a metal target: zinc oxide and zinc tin oxide. With optimized processing parameters, zinc oxide and zinc tin oxide via this deposition method exhibit similar characteristics to TFTs fabricated via sputtering from a ceramic target.

Additionally the effects of gate capacitance density and gate dielectric material are explored utilizing TFTs with IGZO as the semiconductor layers. IGZO-based TFTs exhibit ideal behavior with improved TFT performance such as higher current drive at a given overvoltage, a decrease in the subthreshold swing, and a decrease in the magnitude of the turn-on voltage. Additionally it is shown that silicon dioxide is the preferred dielectric material, with silicon nitride a poor choice for oxide-based TFTs.

Finally a simple method to characterize the band tail state distribution near the conduction band minimum of a semiconductor by analyzing two-terminal current-voltage characteristics of a TFT with a floating gate is presented. The characteristics trap energy (E[subscript T]) as a function of post-deposition annealing temperature is shown to correlate very well with IGZO TFT performance, with a lower value of E[subscript T], corresponding to a more abrupt distribution of band tail states, correlating with improved TFT mobility. It is shown that as the post-deposition anneal temperature increases, the total number of band tail states does not change significantly, however the energy distribution of these states approaches that of a crystalline material.