Amorphous versus crystalline GeTe films. II. Optical properties

Abstract

Transmittance and reflectance studies have been made in the 0.83-25 μ range on GeTe films to obtain and compare the optical properties of amorphous and crystalline structures. The refractive index of crystalline GeTe films (typical value 5.5 at 1.5 μ) shows free carrier dispersion yielding optical dielectric constant=36 and plasma wavelength of 3-5.9 μ (for a corresponding carrier concentration range of 1.5-0.18× 10²¹/cm³) from which a corresponding susceptibility mass of 0.35-0.16m is deduced. On the other hand, amorphous GeTe films exhibit no free carrier dispersion. The refractive index (typical value 4.2 at 1.5 μ), however, rises sharply at the onset of the interband transitions. This rise shifts to higher energies at lower temperatures yielding a coefficient of dE_opt/dT∼-4.5×10^-4 eV/deg (E_opt=absorption edge). The absorption coefficient, α, of amorphous films at all wavelengths studied is smaller than that of the crystalline films. da/dhv(hv=photon energy) shows a maximum (defined here as the absorption edge) for amorphous films at ∼0.85 eV and for crystalline films at 0.73-0.95 eV (depending on carrier concentration). The observed values of the absorption edge and the relation α ∝ (hv)^1/2 near the absorption edge for the crystalline state can be understood in terms of direct transitions from the Fermi level (∼0.3-0.5 eV in the valence band) to the corresponding k-vector in the conduction band (Burstein shift) with a forbidden bandgap ∼0.1-0.2 eV. The observed value of the absorption edge and the relation α ∝ (hv)² near the absorption edge in amorphous films are proposed to be due to indirect transitions between the nonlocalized (allowed) levels at and beyond the localization limits in the valence and conduction bands. The observed exponential decay of α (i.e., α ∝ exp (hv/Δ), Δ=0.068=2.6kT) below hv=0.85 eV may be attributed to the exponential distribution of the localized levels within the pseudo-bandgap of the amorphous state.