Wannier Exciton in an Electric Field. II. Electroabsorption in Direct-Band-Gap Solids

The electric field and temperature dependence of excitonic electroabsorption is presented for direct-band-gap semiconductors and insulators. Each excitonic electroabsorption spectrum is characterized in terms of three energy parameters: (i) R, the binding energy of the exciton; (ii) Γ, the thermal broadening parameter; and (iii) θ, the electro-optical energy. The relative magnitude of these three quantities determines which, if any, of the competing forces dominate. In comparison with the single-particle electroabsorption theory, the excitonic electroabsorption theory gives several new results. The electric field and temperature dependence of ε₂, Δε₂, and Δε₁ are displayed. Fitting of the excitonic electroabsorption theory to lead iodide data gives the values of 2.58 eV for the direct gap in lead iodide and 0.21 electron masses for the reduced mass of the electron-hole pair associated with the extrema in the valence and conduction bands, as compared with previous values of 2.55 eV and 0.24 electron masses.