Phase separation and defect formation in stable, metastable, and unstable GaInSaSb alloys for infrared applications

GaInAsSb is a promising material for mid-infrared devices such as lasers and detectors because it is a direct band gap material with large radiative coefficient and a cut-off wavelength that can be varied across the mid-infrared (from 1.7 to 4.9 μm) while remaining lattice matched to GaSb. On the other hand, the potential of the alloy is hampered by predicted ranges of concentration where the constituents of the alloy become immiscible when the crystal is grown near thermodynamic equilibrium at typical growth temperatures. There have been efforts to extend the wavelength of GaInAsSb alloys through such techniques as digital alloy growth and non-equilibrium growth, but most of the compositional range has for a long time been inaccessible due to immiscibility challenges. Theoretical studies also supported the existence of thermodynamic immiscibility gaps for non-equilibrium growth conditions. Lower growth temperatures lead to shorther adatom diffusion length. While a shorter adatom diffusion length suppresses phase separation, too short an adatom length is associated with increased defect formation and eventually loss of crystallinity. On the other hand, hotter growth temperatures move epitaxial growth closer to thermodynamic equilibrium conditions, and will eventually cause phase separation to occur. In this study thick 2 μm bulk GaInAsSb layers lattice-matched to GaSb substrates were grown across the entire (lattice-matched) compositional range at low growth temperatures (450◦C), including the immiscibility region, when grown under non-equilibrium conditions with MBE. High quality epitaxial layers were grown for all compositions, as evidenced by smooth morphology