Necessity of Quantifying Urbach Energy through Scanning Tunneling Spectroscopy

In this Letter, we introduce scanning tunneling spectroscopy (STS) to quantify the Urbach energy (E_U) in disordered semiconductors. The technique enabled us to gain precise information on the extending component of conduction and valence band-edges responsible for Urbach tailing, individually; such information has been obtained from the width of band-energy-histograms drawn from STS studies at many different points. STS, as a probing method at the microscopic scale to derive E_U, is in contrast to commonly employed optical spectroscopy studies which provide information at the macroscopic scale. A comparison between Urbach energy values from optical studies and distribution of band-edges obtained from STS revealed the inherent inaccuracies involved in the optical characterization process. We have considered copper oxide (CuO) thin films in this regard; we show that through STS and the associated density of state (DOS) spectra, we can derive accurate information on the band-edges' distribution leading to E_U in different phases of the binary oxide thin films.