Abstract
Electronic structures and carrier transport mechanisms in disordered oxide
semiconductors, crystalline InGaO<sub>3</sub>(ZnO)<sub><i>m</i></sub> (<i>m</i> = 1,5) (c-IGZO)and amorphous InGaZnO<sub>4</sub> (a-IGZO), are examined based on a percolation conduction model.
Donor levels (E<sub>d</sub>) and densities (<i>N</i><sub>D</sub>) are estimated by numerical calculations of free electron
densities (<i>n</i><sub>e</sub>) obtained by Hall measurements. It shows that the donor levels are
rather deep, ~0.15 eV for c-IGZO and ~0.11 eV for a-IGZO. This analysis indicates that use of a simple
analytical relation of <i>n</i><sub>e</sub> exp(-<i>E<sub>d</sub></i>/2<i>kT</i> can not always be used to estimate <i>E<sub>d</sub></i> and <i>N<sub>D</sub></i> even for a low <i>n<sub>e</sub></i> film because the film can be in the saturation regime at room
temperature if <i>E<sub>d</sub></i> and <i>N<sub>D</sub></i> are small, which is actually the case for a-IGZO. The temperature
dependences of electron mobilities are analyzed using an analytical equation
of the percolation conduction model, which reveals that distributed
potential barriers exist above mobility edges in IGZO with average heights
30–100 meV and distribution widths 5–20 meV, which
depend on atomic structure and deposition condition of IGZO films.
High-quality a-IGZO films have the lowest potential barriers among the IGZO
films examined, in spite that a-IGZO has a more disordered amorphous
structure than c-IGZO have. It is explained by the partly disordered
structure of c-IGZO.
© 2009 IEEE
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