Strain- and kinetically induced suppression of phase separation in MBE-grown metastable and unstable GaInAsSb quaternary alloys for mid-infrared optoelectronics

The authors have examined the dependence of phase separation in thick layers (1 μm) of molecular beam epitaxially grown, thermodynamically metastable Ga(0.25)In(0.75)As(0.22)Sb(0.78) and unstable Ga(0.50)In(0.50)As(0.44)Sb(0.56) alloys on growth kinetics and strain. For the metastable alloy, which emits at 2.8 μms, they found that phase separation does not occur for any growth temperature, and the alloy grows stoichiometrically, with step flow growth and with high optical output at around 400 °C and 440–480 °C. Moreover, optical quality is robust for alloys grown up to 30 times the critical thickness, with evidence of some improvement for compressive strain. Remarkably, no relaxation of layers occurs, as evidenced in atomic force microscopy or reciprocal space maps up to 12 times the critical thickness. The unstrained unstable alloy, which emits at 3.9 μms, can be grown with optimal optical output and a low degree of phase separation by limiting adatom diffusion length with lower temperature growth (400–440 °C) than metastable alloy. At 450 °C and hotter, severe spinoidal decomposition occurs; however, the authors show that compressive strain may be employed to dramatically suppress phase separation.