Interplays between plastic relaxation, surface morphology and composition modulation in InAlAs graded buffer layers under various growth conditions

The InAlAs/InGaAs heterostructure is highly interesting for microwave devices like HEMTs or HBTs due to the high electron mobility in the InGaAs layer when the In content is increased from 30% to 50%. To avoid the use of InP subtrates which are fragile and expensive, metamorphic structures on GaAs have been demonstrated. Since the optimal electrical properties are obtained with fully relaxed active layers without threading dislocations, different buffer layers have been proposed to compensate the large mismatch between the substrate and the heterostructure. In this work, InAlAs graded buffers have been investigated. The InAlAs alloy has been chosen to take advantages of the better isolating behavior compared to a InGaAs buffer and of the growth in one step of the InAlAs confining layers. Nevertheless this alloy exhibits a high tendency for decomposition, a poorer surface morphology and a more anisotropic relaxation compared to InGaAs buffer layer. As these characteristics are key parameters for the devices performance, the growth of InAlAs graded buffer layer requires an optimization concerning growth temperature and arsenic fluxes.