Electroreflectance of Heavily Dopedn-Type andp-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 ${\Gamma }_8^{+}\to {\Gamma }_7^{-}$ and ${\Gamma }_7^{+}\to {\Gamma }_7^{-}$ 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 $\Gamma$ and the direct energy gap $E_0$. 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.