Electric Field Effects on Indirect Optical Transitions in Silicon

Abstract

The effects of an externally applied electric field on the absorption of light by silicon have been measured under a wide variety of experimental conditions. The measurements on 52 samples of both $n$ and $p$ type and a variety of doping species show conclusively that the electro-absorption spectrum is associated with the pure silicon lattice and not with any impurity or imperfection. The spectral characteristics of the electro-absorption peaks associated with phonon-assisted processes are strongly affected by the magnitude of the applied electric field over the measured range of ${10}^4$ to ${10}^6$ V/cm. The electro-absorption spectrum is unchanged, however, in relative magnitude and peak position by a change in temperature, and the absolute spectral positions follow the band-gap dependence on temperature. Concurrent measurements of the maximum-electro-absorption-peak position and high-resolution, field-free optical absorption establish the final state in the transition process as an exciton state. An interpretation of these measurements is given, based on an optically assisted tunneling process to a final exciton state.