Evidence for energy band tailing and localization of states in highly disordered crystalline GeTe

Substantial absorption coefficient ‘tails’ at energies below the absorption edge have been observed in the crystalline Ge_1?x Te_x system. For part of this ‘tail’ region the absorption coefficient is found to vary as the photon energy squared, and the extent of this behavior to depend on the stoichiometry of the sample, while near the band edge a more suitable variation is αE = B(E ? E₀)². At very low energies an exponential variation α = const · exp (E/Δ), with Δ ≈ 0.1 eV (3.8 kT), is more appropriate. This behavior can be understood only if localized states (α = kE²) and band edge tailing (α = const · exp (E/Δ), features normally associated with the Mott model of the amorphous state, are present in the band structure of the crystalline material. The variation of the extent of the α = kE² region with stoichiometry supports this model, as the depth of localization, being comparable to the strength of the fluctuation potential due to the disorder, depends on the amount of disorder present in the lattice.