New mechanism of excitonic enhanced optical gain for wide-gap quantum wells

Abstract

New mechanism of optical gain in quantum wells are proposed using excitonic effects. Exciton in wide-gap semiconductors plays an important role in optical phenomena since it has a large binding energy and could be stable at room temperature. However, its bound state is constructed by the electron-hole Coulomb interaction and should be related to the electron and hole distributions when the ground state has many electron and holes. We have evaluated the current-current correlation function, i.e. conductivity, treating the mechanism of optical gain and exciton on equal footing. It is shown that the recombination of the exciton does not yield optical gain directly but that excitonic effects enhance an oscillator strength of the coupled transition. Taking into account a localized level in the energy gap, the optical gain in terms of the population inversion between the localized level and one of the band edge subband states is produced with the very small carrier concentration. Simultaneously, the excitonic absorption occurs due to the band edge electron-hole interaction. It is found that the former optical gain is enhanced extremely by the latter excitonic effect through the coupling between the two transitions. This enhanced optical gain might show a possibility of very low threshold current density for wide-gap laser diodes.