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The versatile range of applications for two-dimensional (2D) materials hasencouraged scientists to further engineer the properties of these materials.This is often accomplished by stacking layered materials into more complex vander Waals heterostructures. A much less popular but technologically promisingapproach is the alloying of 2D materials with different element compositions.In this work, we demonstrate a first step in manipulating the hBN bandgap interms of its width and indirect/direct character of the optical transitions. Wepresent a set of aluminum alloyed hexagonal boron nitride (hBAlN) samples thatwere grown by metal organic vapor phase epitaxy (MOVPE) on 2-inch sapphiresubstrates with different aluminum concentration. Importantly, the obtainedsamples revealed a sp$^2$-bonded crystal structure. Optical absorptionexperiments disclosed two strong peaks in the excitonic spectral range withabsorption coefficient $\alpha \sim 10^6$ cm$^{-1}$. Their energies correspondvery well with the energies of indirect and direct bandgap transitions in hBN.However, they are slightly redshifted. This observation is in agreement withpredictions that alloying with Al leads to a decrease of the bandgap energy.The observation of two absorption peaks can be explained in terms of mixingelectronic states in the K and M conduction band valleys, which leads to asignificant enhancement of the absorption coefficient for indirect transitions.

Bandgap manipulation of hBN by alloying with aluminum: absorption properties of hexagonal BAlN