Sub-gap absorption study of amorphous InGaZnO4 films by photothermal deflection spectroscopy

Sub-gap absorption spectra of amorphous InGaZnO4 films have been investigated using extended photothermal deflection spectroscopy from 0.31 to 3.4 eV. DC sputtered amorphous InGaZnO4 films show optical bandgap of ~ 3.6 eV, Urbach energy of ~ 0.25 eV, weak absorption tail around 2.37 eV and strong absorption peaks around 0.38 eV. The intensity of absorption tail around 2.37 eV increased and the intensity of the absorption peaks around 0.38 eV decreased with annealing temperature at 400 °C. We speculate that reduction of oxygen and hydrogen is caused by thermal treatment at 400 °C, and then oxygen vacancies form deep levels in the bandgap.