Optical gain of InGaAsSb/AlGaAsSb type-I long wavelength quantum-well lasers

We present a theoretical study of the optical gain of InGaAsSb/AlGaAsSb type-I quantum-well lasers, whose lasing wavelength is designed to be 2.7 mum. A self-consistent solution, which solves the Schrodinger equations and Poisson equation simultaneously, is used to calculate the band structure. Gain spectra of the quantum wells are calculated by the density-matrix theory. By studying the influence of strain and width of well material, we find that the main factor influencing the optical gain is not the optical matrix element, but the population inversion, especially the probability to find a hole in the first valence subband. Increasing the compressive strain or (and) decreasing the well width will enlarge the optical gain. The former lowers the in-plane effective mass of holes. Although the latter slightly increases the in-plane effective mass of holes, it does enlarge the energy separation of the valence subbands. Both effects lower the total state density near the valence band edge, and finally enlarge the optical gain. Our theoretical results can explain qualitatively the reported experiment results, and are useful for the design of InGaAsSb/AlGaAsSb long wavelength quantum-well lasers.