Electroreflectance of Heavily Doped n -Type and p -Type Germanium near the Direct Energy Gap

Electroreflectance (ER) spectra of heavily doped germanium, both n type and p type, have been studied extensively in the energy range of Γ₈⁺→Γ₇^- and Γ₇⁺→Γ₇^- transitions at room temperature and also at 90 ^∘{}K. ER spectra of n-type samples were shown to be of the Franz-Keldysh type. We estimate the values of the damping coefficient Γ and the direct energy gap E₀. The spectra of p-type samples were identified as a combination of the Franz-Keldysh effect and a new one called "electron-distribution" effect. The ER spectra characterizing the last effect correspond to a relative change of the conduction-band minimum (valence-band maximum) with respect to the Fermi level in the n-type (p-type) sample caused by the applied dc and ac electric fields, which cause an outstanding change of the occupation probability of the energy states near the absolute extrema of both bands.