Tail-to-tail Sub-gap Density of States of Metal Oxide Transistors by Ultrabroadband Photoconduction

           Unlike existing silicon technology, semiconductor oxides tend to be highly disordered with large sub-gap trap densities that rigidly control transistor efficiency, turn-on voltage, and reliability. The recently developed Ultrabroadband Photoconductive Density of States (UP-DoS) microscopy method experimentally measures sub-gap defect DoS for thin-film transistors (TFTs) like amorphous InGaZnO_x.  The obtained UP-DoS photoconduction spectra are directly proportional to the integrated trap density, and thus the first derivative is an incisive measure of the sub-gap DoS.  By using highly tunable lasers to extend the spectral range to 0.1-3.6 eV, our resulting sub-gap DoS spectrum now fully extends from the valence to conduction band Urbach tails, offering a new metrology method for TFT defect characterization. Finally, to best identify what defect types are present, the UP-DoS peak energies are mapped against DFT+U simulations.                 

            Recently, we have now extended the UP-DoS method to a variety of emerging metal oxide TFTs, including p-type Cu₂O, SnO and n-type ZTON.  For Cu₂O TFTs, we extracted a 1.4 eV bandgap and measured three distinct peaks in the subgap region.  UP-DoS further identifies a substantial minority phase of CuO within the active TFT channel. By contrast, SnO TFTs can be ambipolar and show a smaller bandgap of 0.68 eV with five defect peaks. The SnO TFT ambipolar character can be rigidly controlled by the amplitude of the sub-gap metal vacancy defect peaks near the conduction band.