The Exponential Absorption Edge in Amorphous Ge–Se Compounds

The compositional dependence of the Urbach tail of Ge x Se 1- x compounds was measured in the range 0≤ x ≤0.4. The slope of the Urbach tail of Ge x Se 1- x compounds varies greatly with Ge content. It has a maximum at about x =0.3 and a minimum at the stoichiometric composition GeSe 2 . This specific dependence of the slope of the Urbach tail on composition is related not only to the molecular nature of these compounds, but also to structural distortions. A convenient method is proposed for measuring absorption coefficients at the Urbach tail. When the absorption edge is an exponential function of photon energy, the absorption coefficient can be determined easily by this method, which uses the wavelength modulation technique.