Amorphous Versus Crystalline GeTe Films. II. Optical Properties
- 1 November 1969
- journal article
- research article
- Published by AIP Publishing in Journal of Applied Physics
- Vol. 40 (12), 4940-4947
- https://doi.org/10.1063/1.1657318
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 dEopt/dT∼−4.5×10−4 eV/deg (Eopt=absorption edge). The absorption coefficient, α, of amorphous films at all wavelengths studied is smaller than that of the crystalline films. dα/dhν(hν=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 ∝ (hν)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 ∝ (hν)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 (hν/Δ), Δ=0.068=2.6kT) below hν=0.85 eV may be attributed to the exponential distribution of the localized levels within the pseudo‐bandgap of the amorphous state.Keywords
This publication has 18 references indexed in Scilit:
- Evidence for A Sharp Absorption Edge in Amorphous GePhysical Review Letters, 1969
- Relativistic Band Structure and Electronic Properties of SnTe, GeTe, and PbTePhysical Review B, 1969
- Optical and Electrical Properties and Band Structure of GeTe and SnTePhysical Review B, 1968
- Electrical and Optical Properties of Amorphous GermaniumPhysical Review B, 1967
- Electrical and Optical Properties of Vitreous SeleniumPhysical Review B, 1963
- Absence of Diffusion in Certain Random LatticesPhysical Review B, 1958
- Optical Absorption of Pure Silver HalidesPhysical Review B, 1956
- Optical Properties of Cadmium Sulfide and Zinc Sulfide from 06 Micron to 14 MicronsJournal of the Optical Society of America, 1955
- The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of SolidsPhysical Review B, 1953
- Some Electrical and Optical Properties of Synthetic Single Crystals of Zinc SulfidePhysical Review B, 1953