Optimization of MBE Growth Conditions of In_0.52Al_0.48As Waveguide Layers for InGaAs/InAlAs/InP Quantum Cascade Lasers

We investigate molecular beam epitaxy (MBE) growth conditions of micrometers-thick In_0.52Al_0.48As designed for waveguide of InGaAs/InAlAs/InP quantum cascade lasers. The effects of growth temperature and V/III ratio on the surface morphology and defect structure were studied. The growth conditions which were developed for the growth of cascaded In_0.53Ga_0.47As/In_0.52Al_0.48As active region, e.g., growth temperature of T_g = 520 °C and V/III ratio of 12, turned out to be not optimum for the growth of thick In_0.52Al_0.48As waveguide layers. It has been observed that, after exceeding ~1 µm thickness, the quality of In_0.52Al_0.48As layers deteriorates. The in-situ optical reflectometry showed increasing surface roughness caused by defect forming, which was further confirmed by high resolution X-ray reciprocal space mapping, optical microscopy and atomic force microscopy. The presented optimization of growth conditions of In_0.52Al_0.48As waveguide layer led to the growth of defect free material, with good optical quality. This has been achieved by decreasing the growth temperature to T_g = 480 °C with appropriate increasing V/III ratio. At the same time, the growth conditions of the cascade active region of the laser were left unchanged. The lasers grown using new recipes have shown lower threshold currents and improved slope efficiency. We relate this performance improvement to reduction of the electron scattering on the interface roughness and decreased waveguide absorption losses.