Optimization of type-II heterostructures for the tuning region in tunable laser diodes

Type-II superlattices are very attractive as tuning regions in tunable laser diodes exploiting the free-carrier plasma effect. Due to the confining of the electrons and holes in different layers, the recombination of the electron-hole pairs is drastically suppressed. As a result, the carrier density, and hence the tuning efficiency for type-II superlattices, is enhanced by more than an order of magnitude in the small current regime as compared to bulk tuning regions. However, with increasing carrier density the recombination rate increases because of the stronger penetration of the electron and hole wavefunctions into the corresponding barriers. For high carrier densities, the capability of type-II superlattices depends critically on design parameters such as as layer thickness, band offset and strain. Our calculations show that, for an optimized type-II superlattice, the tuning range can be significantly enhanced by more than a factor of 3 for a given current.