In this Letter, we introduce scanning tunneling spectroscopy (STS) to quantify the Urbach energy () 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, ; 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 , 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 in different phases of the binary oxide thin films.
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