Effective substrate for the growth of multilayer h-BN on sapphire—substrate off-cut, pre-growth, and post-growth conditions in metal-organic vapor phase epitaxy

Abstract

The substrate is one of the key components that determines the quality of the epitaxial layers. However, the implications of growing two-dimensional layers on three-dimensional bulk substrates have not yet been fully understood, and these implications need to be studied for different combinations of materials and substrates. Here, we present a study that addresses the influence of the sapphire substrate off-cut angle on the final growth of two-dimensional layers of hexagonal boron nitride (h-BN) by metal-organic vapor phase epitaxy (MOVPE). A two-step wafer-scale process was used in one epitaxial MOVPE procedure. The main process starts with a self-limiting continuous growth of a BN buffer followed by flow-modulated epitaxy in the second step, and is used to study substrates with different off-cuts angles, pre-growth nitridation steps, and post-growth annealing. An initial nitridation step at the growth temperature allowed for the growth of an AlN sublayer. This layer is shown to smooth out the underlying sapphire and establishes an ‘effective’ sapphire/AlN substrate. This step is also responsible for enforcing a specific growth of the BN layer in a crystallographic orientation, which is shown to strongly deviate from the substrate for off-cut angles larger than 0.3°. A substrate with off-cut angle of 1° clearly yields the highest quality of h-BN layers as evidenced by the lowest amount of debris on the surface, most intense x-ray diffraction signal, minimal Raman phonon line width and thinnest amorphous BN (a-BN) at the interface with the effective substrate. Our study shows that the off-cut angles of sapphire substrates strongly influence the final epitaxial h-BN, clearly indicating the importance of optimal substrate preparation for the growth of two-dimensional BN layers. Post-growth annealing in N2 atmosphere at 800 °C improves the top surface morphology of the final stack, as well as suppresses further the presence of a-BN.