Probing Charge Dynamics\nin Amorphous Oxide Semiconductors\nby Time-of-Flight Microwave Impedance Microscopy

The unique electronic properties of amorphous indium\ngallium zinc\noxide (a-IGZO) thin films are closely associated with the complex\ncharge dynamics of the materials. Conventional studies of charge transport\nin a-IGZO usually involve steady-state or transient measurements on\nfield-effect transistors. Here, we employed microwave impedance microscopy\nto carry out position-dependent time-of-flight (TOF) experiments on\na-IGZO devices, which offer spatial and temporal information on the\nunderlying transport dynamics. The drift mobility calculated from\nthe delay time between carrier injection and onset of TOF response\nis 2–3 cm<sup>2</sup>/(V s), consistent with the field-effect\nmobility from device measurements. The spatiotemporal conductivity\ndata can be nicely fitted to a two-step function, corresponding to\ntwo coexisting mechanisms with a typical time scale of milliseconds.\nThe competition between multiple-trap-and-release conduction through\nband-tail states and hopping conduction through deep trap states is\nevident from the fitting parameters. The underlying length scale and\ntime scale of charge dynamics in a-IGZO are of fundamental importance\nfor transparent and flexible nanoelectronics and optoelectronics,\nas well as emerging back-end-of-line applications.