Chemical Vapor Deposition of Hexagonal Boron Nitride Multilayers

Hexagonal boron nitride (h-BN) is a layered, wide-bandgap semiconductor, which has demonstrated to be an ideal complementary material for various other two-dimensional materials as substrates and encapsulating layers. h-BN is also suggested as a promising material to be used for non-volatile memory devices, barrier layers for anti-corrosion and -oxidation, and for quantum information usage, such as single photon emitters.

However, the synthesis of h-BN films in particular multilayers with high crystallinity, low roughness, low defect density and on large scale is still very challenging. In this project, three methods based on chemical vapor deposition (CVD) were developed to explore the possibility to synthesize h-BN multilayers on large scale with high quality.

The first method relies on the low roughness of sapphire which allows sapphire to work as a template for h-BN films. h-BN multilayers were directly deposited onto sapphire with controllable thickness, and the crystallinity was further optimized by post annealing. A specific method was developed to transfer h-BN films from sapphire with the bottom h-BN surface flipped up, and results show that the bottom h-BN surface partially inherits the low roughness of sapphire, and the bottom film surfaces show roughness which is almost 3 times smaller than that from the top h-BN surface.

Such h-BN films show high potential to be used as substrates. With the second method, growth of h-BN multilayers directly on sapphire with high crystallinity from a gallium based catalyst was explored. Both iron-gallium (Fe-Ga) alloy and pure gallium were used for the growth with different sources. Fe-Ga alloy works at high temperature with bulk h-BN as the source, thick h-BN domains as thick as 80 nm could be obtained, but with low coverage.

Pure gallium works at lower temperature with borazine as the source, and continuous films were synthesized but with lower thickness in the range of 4-7 nm. Detailed studies of Ga-based synthesis showed epitaxial growth, as well as some carbon contamination limited to very small domains. A growth mechanism is also proposed based on the study of the growth process.

An easy, scalable, and low-cost way of synthesizing h-BN multilayers was developed as the third method. Here boron-containing metallic glass was used as the substrates, and by annealing in ammonia, h-BN films on centimeter-scale can be obtained. Results show that the thickness of h-BN films within a broad range of process parameters self-limits to ca. 12 nm.

Some carbon contamination was also detected in the film, but can be removed by pre-oxidation of the substrate before annealing. The resulting film shows possible applications in memory devices. These methods and results presented in this project form the basis for further optimization toward the goal of high quality h-BN films on semiconductor grade, and pave the way for developing various applications of 2D materials as a useful platform.