Influence of growth conditions on the structural, optical and electrical quality of MBE grown InAlAs/InGaAs metamorphic HEMTs on GaAs

InAlAs/InGaAs metamorphic HEMTs on GaAs have demonstrated low noise figures and high output powers with obvious advantages over structures grown on InP substrates. Indeed, from a processing viewpoint, the GaAs substrate is less brittle, less expensive, available in size up to 6 inches in diameter and then it is preferred for the production of high performance monolithic integrated circuits. Furthermore, the metamorphic scheme allows one to arbitrary choose the indium content in the InAlAs/InGaAs layers, which is a supplementary degree of freedom for the optimization of the active layers. Various buffer layers have been developed to accommodate the lattice mismatch between the active layers and the substrate. Production tools allows the growth of ternary as well as quaternary graded buffer layers; although the growth of a ternary alloy is more simple, it still requires the optimization of growth parameters like temperatures and arsenic fluxes. The layers presented here are based on thick InAlAs with a graded indium content from 1%-10% to 49% and terminated with an inverse step to obtain a highly relaxed In/sub 0.42/Al/sub 0.58/As/In/sub 0.43/Ga/sub 0.57/As structure. In the present work, the quality of the metamorphic HEMT structures is investigated by varying the growth parameters for the graded buffer layer as well as for the active layers. The structural quality is studied with high resolution X-ray diffraction, transmission electron microscopy (TEM) and atomic force microscopy, while the optical quality and the electrical quality of the HEMTs are studied with photoluminescence and Hall effect measurements respectively.