The present invention relates generally to an optical device, and more particularly, to an optical device utilizing a superlattice of one or more indirect bandgap materials.
The strengths of optical transitions between conduction band states and valence band states in semiconductors are greatly influenced by whether or not these transitions require phonon participation. Direct transitions (i.e. transitions between states with the same wavevector) can occur without phonon participation, and can have relatively large transition strengths. However, indirect transitions (i.e. transitions between states with unequal wave vectors) require phonon emission or phonon absorption in addition to the photon emmision or absorption process, and have relatively weak transition strengths.
The efficiency of optical devices can be significantly improved by the use of active region materials with direct transitions at the photon energies of interest. In addition, certain optical devices such as laser diodes essentially require the use of materials with direct band-edge transitions; i.e., electron-hole recombination events which occur at energies close to the band gap.
Many materials having direct transitions are used in optical devices. For instance, bulk GaAs and InAs are examples of conventional direct transition materials. However, some applications require the use of materials having indirect bandgaps. For example, all of the known technologically relevant compound semiconductors with appropriate bandgaps for green LEDs and green laser diodes have indirect band gaps. The emission efficiencies of sub-bandgap transitions can be enhanced somewhat in certain materials by the incorporation of selected impurities. However, the efficiencies obtained in this manner do not approach those obtained from direct gap materials.