Study On Electrical Instability Of Amorphous Indium-gallium-zinc Oxide Thin Film Transistors And Schottky Barrier Diodes

Recently, amorphous oxide semiconductors (AOSs) represented by amorphous indium-gallium-zinc-oxide (a-IGZO) have received great interest as a potential material for various applications such as next generation flat-panel displays and transparent flexible electronics due to their superior electrical and optical characteristics. A significant amount of work has been devoted to the development of a-IGZO based thin film transistors (TFTs) and Schottky barrier diodes (SBDs), both of which are basic building blocks in circuits.In particular, a-IGZO TFTs are intensively investigated as a replacement for traditional silicon-based TFTs in active matrix displays as they could simultaneously offer high channel carrier mobility, high optical transparency, low off-state leakage and low process cost. Although the potential advantages of a-IGZO TFTs are manifold, an issue that is yet to be extensively explored is the electrical stability of TFTs, which is very important for practical applications of TFTs for many purposes. In this thesis, one of our focuses is to investigate the electrical stability of a-IGZO TFTs.As another basic building block in circuits, research on a-IGZO SBDs is much limited compared with a-IGZO TFTs. however, a-IGZO SBDs are no less worthy of investigation as it can be used not only for oxide-based flexible electronic circuits but also for other devices such as metal-semiconductor field-effect transistors (MESFETs) and photosensors. So far, the limited research on a-IGZO SBDs has been focusing on improving basic device performance, aiming to achieve higher Schottky barrier height, smaller ideality factor, larger rectifying ratio, and higher breakdown voltage. Meanwhile, for practical applications, the SBD has to endure continuous voltage stress without suffering from serious electrical instability. Thus, anther focus of this thesis is to investigate the reliability issue of a-IGZO SBDs.The main results in this thesis are highlighted as below:1. The time and temperature dependence of threshold voltage shift under positive bias stress (PBS) and the following recovery process are investigated in a-IGZO thin-film-transistors. It is found that the time dependence of threshold voltage shift can be well described by a stretched exponential equation in which the time constant τ is found to be temperature dependent. Based on Arrhenius plots, an average effective energy barrier Eτ,stress= 0.72 eV for the PBS process and an average effective energy barrier Eτ,recovery= 0.58 eV for the recovery process are extracted respectively. A charge trapping/detrapping model is used to explain the threshold voltage shift in both the PBS and the recovery process.2. Vertical SBDs based on a-IGZO with either top or bottom Schottky contact are fabricated by simply controlling the oxygen partial pressure during a-IGZO deposition. The effect of negative bias stress on device performance is studied. The Schottky barrier height and series resistance of the a-IGZO SBD are found to be increased upon negative bias stress, which is correlated with reduction of trap state and background carrier concentration within the a-IGZO film. A physical model based on subgap state transitions from ionized Vo2+ states to neutralized VO states is purposed to explain the observed electrical instability behavior.