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

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