Van der Waals heterobilayers based on 2D transition metal dichalcogenides have been recently shown to support robust and long-lived valley polarization for potential valleytronic applications. However, the role of the band structure and alignment of the constituent layers in the underlying dynamics remains largely unexplored. Here we study spin--valley relaxation dynamics in heterobilayers with different band structures engineered via the use of alloyed monolayer semiconductors. Through a combination of time-resolved Kerr rotation spectroscopic measurements and theoretical modelling for Mo$_{1-x}$W$_{x}$Se$_2$/WSe$_2$ samples with different chemical compositions and stacking angles, we uncover the roles of interlayer exciton recombination and charge carrier spin depolarization in the overall valley dynamics. Our results provide insights into the microscopic spin--valley polarization mechanisms in van der Waals heterostructures for the development of future 2D valleytronic devices.
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