Direct MOVPE Growth of High-Quality h-BN on the Wafer-Scale Sapphire: The Role of Substrate Off-Cut

Hexagonal boron nitride (h-BN) is an attractive wide bandgap 2D material for possible applications in electronic and optoelectronic devices based on van der Waals heterostructures [1], but direct growth of high-quality h-BN on the wafer-scale is still the bottleneck for future successful implementation of h-BN in industry. Although the synthesis of h-BN by metal-organic vapor phase epitaxy (MOVPE) has already been reported [1-3], there is a fundamental lack in understanding of many basic aspects of this process affecting obtaining a good structural quality and a smooth surface. In particular the role of the substrate off-cut, although well known as being of major importance for the growth of other materials, has not yet been fully addressed so far.<br/>In this communication, we present a study that addresses the influence of the sapphire substrate off-cut angle on the final quality of h-BN obtained in a two-step growth procedure [3]. The main process starts with a self-limiting continuous flow growth (CFG) of a BN buffer followed by flow-modulated epitaxy (FME) in the second step of one MOVPE process and is used to study substrates with five different off-cuts angles for three different CFG times.<br/>Based on results obtained by x-ray diffraction and x-ray reflectometry, Raman and Fourier-transform spectroscopy, atomic force microscopy and scanning electron microscopy, we present a comprehensive study that shows that the quality of h-BN epilayers dramatically depends on the off-cut angle of the c-plane sapphire substrate [4]. We demonstrate that the highest quality of h-BN layers is obtained for a sapphire substrate off-cut angle of 1 degree. Samples with this off-cut have the lowest amount of debris, 3D grains consisting of mostly h-BN material [5], on the surface. They also have most intense x-ray diffraction signal, minimal Raman phonon linewidth and thinnest amorphous BN part. We are able to explain these results by following the evolution of the whole interface, which includes a thin AlN layer coming from initial nitridation step at the beginning of MOVPE process. This AlN layer is shown to smooth out the surface forming (with sapphire) a kind of “effective substrate” which leads to the observed increase in quality.<br/>A detailed analysis of the crystallographic structure of the h-BN obtained layers and mechanisms responsible for the existence of an optimal off-cut angle will be discussed.<br/><br/><b>Acknowledgments</b>: This work was supported by the National Science Centre, Poland, under decisions 2019/33/B/ST5/02766 and 2020/39/D/ST7/02811.<br/><br/>[1] A. E. Naclerio and P. R. Kidambi, Adv. Mater. <b>35</b>, 2207374 (2023).<br/>[2] Y. Kobayashi, T. Akasaka, J. Cryst. Growth <b>310</b>, 5044 (2008).<br/>[3] A. K. Dabrowska, et al. 2D Materials <b>8</b>, 015017 (2020).<br/>[4] M. Tokarczyk, et al. 2D Materials <b>10</b>, 025010 (2023).<br/>[5] D. Chugh, et al. 2D Materials <b>5</b>, 045018 (2018).