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 linewidth 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.

Flow Modulation Epitaxy of Thick Boron Nitride Epilayers and Wafer-Level Exfoliation

“…2,25 But limited by the heating power, the current growth temperature is mainly located at 1050−1350 °C, which restricts the further improvement of sp 2 -BN quality. 26,27 The thermal expansion coefficient of sp 2 -BN along the a axis is negative, while the thermal expansion coefficient of sapphire is positive. 28,29 Relatively large compressive stress is accumulated in sp 2 -BN during the cooling process, and the wrinkled morphology is formed on the surface.…”

“…However, the relationship between sp 2 -BN with a flat surface morphology and the proportion of NH 3 and TEB in a pulse is still unclear. Moreover, the desired growth temperature of sp 2 -BN grown by MOCVD is predicted to be 1500–1600 °C. , But limited by the heating power, the current growth temperature is mainly located at 1050–1350 °C, which restricts the further improvement of sp 2 -BN quality. , The thermal expansion coefficient of sp 2 -BN along the a axis is negative, while the thermal expansion coefficient of sapphire is positive. , Relatively large compressive stress is accumulated in sp 2 -BN during the cooling process, and the wrinkled morphology is formed on the surface. The exfoliation of sp 2 -BN will be affected by the increased strain as its thickness increases, which remains relatively unexplored.…”

Strain modulation of epitaxial h-BN on sapphire: the role of wrinkle formation for large-area two-dimensional materials

“…Apart from the quality of epitaxial h-BN, another important issue is its mechanical interaction with the substrate which determines the properties of the grown material. Extensive research is conducted to find appropriate substrates for the growth of h-BN to obtain the highest qualities and the largest possible areas, [20,21] also by taking into account the substrate modification caused by the growth process itself [22]. MOVPE h-BN is grown at high temperatures often exceeding 1000 °C.…”

Homoepitaxy of Boron Nitride on Exfoliated Hexagonal Boron Nitride Flakes

“…Recently, mechanically polished pyrolytic BN was used as a substrate, resulting in the growth of polycrystalline sp 2 -BN . Various technologies like Molecular Beam Epitaxy (MBE), , Chemical Vapor Deposition (CVD) ,,,, and Metal–Organic Vapor Phase Epitaxy (MOVPE) − were used for epitaxial growth of hBN.…”