In-gap states in titanium dioxide and oxynitride atomic layer deposited films

Abstract

Valence band (VB) spectra of titanium dioxide (TiO2) and oxynitride (TiOxNy) films prepared by different atomic layer deposition (ALD) processes are compared and related to electrical characterization [current–voltage (JV) and capacitance–voltage (CV)] results. By increasing the nitrogen amount in the TiO2 film, band-gap narrowing is observed. The band-gap decrease is related to the contribution of the nitrogen density of states, which induces defects within the band-gap and thus reduces its optical band-gap. In-gap states are found in the VB spectra at 1 eV below the Fermi energy in all investigated ALD samples, i.e., in TiO2 as well as in TiOxNy films. An exponential correlation between leakage current density and in-gap state intensity is derived by the combination of JV measurements and VB spectra, whereas the in-gap states seem to have no influence on hysteresis and fixed oxide charges found in the CV data. It is argued that the in-gap states in TiO2 and TiOxNy have an excitonic or polaronic origin. Both, band-gap narrowing and in-gap state intensity can be tuned by the ALD process selection and the variation of its parameters.