Along the absorption coefficient curve and near the optical band edge there is an exponential part called Urbach tail. This exponential tail appears in the low crystalline, poor crystalline, the disordered and amorphous materials because these materials have localized states which extended in the band gap. In the low photon energy range, the spectral dependence of the absorption coefficient (α) and photon energy (hν) is known as Urbach empirical rule, which is given by the following equation: α = αo exp (hν/EU) where αo is a constant and EU denotes the energy of the band tail or sometimes called Urbach energy, which is weakly dependent upon temperature and is often interpreted as the width of the band tail due to localized states in the normally band gap that is associated with the disordered or low crystalline materials. Taking the logarithm of the two sides of the last equation, hence one can get a straight line equation. It is given as follows: ln α = ln αo + (hν/EU) Therefore, the band tail energy or Urbach energy (EU) can be obtained from the slope of the straight line of plotting ln (α) against the incident photon energy (hν).