Investigation of persistent photoconductance and related electron mobility in thin IGZO layers with the PDL Hall technique

Indium gallium zinc oxide (IGZO) is a popular material to fabricate thin-film transistors (TFT). The light induced long-term conductance change in the thin IGZO layers (known as persistent photoconductance, PPC) is a significant phenomenon that should be deeply understood and controlled. We used the Parallel dipole line (PDL) AC Hall measurement technique to study the light induced change of the conductance and mobility of charge carriers in IGZO layers and its relaxation in dark. In the PDL systems rotating permanent magnets are applied to realize the AC Hall measurement at large magnetic field with harmonic modulation in a very compact design. Several samples with varying layer thickness, composition, and passivation conditions were prepared to investigate the PPC phenomenon. The magnitude and relaxation of PPC show a very strong dependence on IGZO preparation conditions. In some samples over tenfold conductance increase was observed at room temperature. Relaxations times varied from a few hours up to a few days. An interesting observation was that the mobility (in the range of 1 cm2/Vs-8.9 cm2/Vs) also increases with the conductance as the result of illumination.