Far-infrared Absorption by Excitons in Germanium at High Concentrations

Abstract

<p>The dependence on frenuency of the photo-ionization cross~section of indirect excitons in germanium (at concentrations corresponding to "isolated" excitons) was found to be v⁻¹.⁵. A dependence of v⁻².⁷ had been expected, by comparison with the hydrogen theory previously thought applicable. No adequate replacement theory exists. A probable explanation might lie in the Lucovsky model of impurities in semiconductors, which predicts the correct frequency dependence, assuming an enhanced interparticle potential at small separations. This central cell correction was justified in excitons because the dielectric constant of the material varies with the election- hole separation. The - 1.5 power was found to decrease slowly as concentration increased.</p> <p>The main observation at concentrations of 2x10¹⁴ cm⁻³ and above was a new, strongly absorbing, component of non excitonic character in the absorption spectrum between 1 and 2 meV, peaking at 1.5 meV with a FWHM of ~ 1 meV. The peak grew at the 1.5th power of the exciton peaks, having a fairly sharp onset between 4 and 5.5°K and a very small temperature dependence above that range. This peak was interpreted in terms of an electron-hole plasma analogous to ordinary electron-hole fluid, which appeared in the form of drops in the high density exciton cloud. A double peaked phase diagram was constructed, in qualitative agreement with some theoretical predictions, and experimental results obtained by workers using the recombination luminescence technique, some of whom attribute their results to the formation of biexcitons.</p>