Low-strain, quantum-cascade-laser active regions grown on metamorphic buffer layers for emission in the 3.0-4.0 μm wavelength region

We have investigated metamorphic buffer layers (MBLs), as so-called virtual substrates, for accessing a compositional range of In_xGa_1-xAs/Al_yIn_1-yAs superlattice (SL) materials which would otherwise be prohibited due to excessive strain when grown on conventional substrates. Such materials have application in the realisation of high-performance Quantum Cascade Lasers (QCLs) of short emission wavelengths (i.e., ≤4.0 μm). Simulation studies suggest that significant enhancement of performance in terms of reduced device temperature sensitivity and reduced thermal resistance is possible over conventional InP-substrate devices by employing MBL-based QCL designs on a GaAs substrate. Furthermore, such devices would exhibit significantly lower strain compared to conventional QCLs on InP emitting within the 3.0-4.0 μm wavelength region. To improve the planarity of MBL top surfaces, we employ chemical mechanical polishing (CMP) prior to the growth of the QCL SL structures. 20-period In_xGa_1-xAs (wells)/Al_yIn_1-yAs (barriers) SLs are grown by metalorganic vapour phase epitaxy (MOVPE) on an InGaAs step-graded, hydride vapour phase epitaxy (HVPE)-grown MBL. Employing CMP on the top of the MBL, prior to the SL growth, results in significantly improved X-ray-diffraction SL fringes. Electroluminescent devices, incorporating a single stage of QCL-SL active-region material grown on an MBL subjected to CMP, demonstrate intersubband emission near 3.6 μm.