Electronic Structures Above Mobility Edges in Crystalline and Amorphous In-Ga-Zn-O: Percolation Conduction Examined by Analytical Model

Electronic structures and carrier transport mechanisms in disordered oxide semiconductors, crystalline InGaO3 ZnO)(m) (m = 1, 5) (c-IGZO) and amorphous InGaZnO4 (a-IGZO), are examined based on a percolation conduction model. Donor levels (E-d) and densities (N-D) are estimated by numerical calculations of free electron densities (n(e)) obtained by Hall measurements. It shows that the donor levels are rather deep, similar to 0.15 eV for c-IGZO and similar to 0.11 eV for a-IGZO. This analysis indicates that use of a simple analytical relation of n(e) proportional to exp(-E-d/2kT) can not always be used to estimate E-d and N-D even for a low n(e) film because the film can be in the saturation regime at room temperature if E-d and N-D 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.