Probing Charge Dynamics in Amorphous Oxide Semiconductors by Time-of-Flight Microwave Impedance Microscopy

The unique electronic properties of amorphous indium gallium zinc oxide (a-IGZO) thin films are closely associated with the complex charge dynamics of the materials. Conventional studies of charge transport in a-IGZO usually involve steady-state or transient measurements on field-effect transistors. Here, we employed microwave impedance microscopy to carry out position-dependent time-of-flight (TOF) experiments on a-IGZO devices, which offer spatial and temporal information on the underlying transport dynamics. The drift mobility calculated from the delay time between carrier injection and onset of TOF response is 2–3 cm²/(V s), consistent with the field-effect mobility from device measurements. The spatiotemporal conductivity data can be nicely fitted to a two-step function, corresponding to two coexisting mechanisms with a typical time scale of milliseconds. The competition between multiple-trap-and-release conduction through band-tail states and hopping conduction through deep trap states is evident from the fitting parameters. The underlying length scale and time scale of charge dynamics in a-IGZO are of fundamental importance for transparent and flexible nanoelectronics and optoelectronics, as well as emerging back-end-of-line applications.