Growth of Single-Crystalline and Layer-Controllable Hexagonal Boron Nitride

Abstract

Two-dimensional (2D) materials provide great potential for their applications in electronics and photonics because they can offer opportunity for extending Moore's law in beyond-CMOS (complementary metal-oxide-semiconductor) devices. Among 2D materials, hexagonal boron nitride (hBN) is a representative 2D insulting material with bandgap (~6 eV). Owing to atomically flat surface without dangling bonds yet with excellent thermal and chemical stabilities, hBN has been introduced as a promising material for an excellent dielectric layer to efficiently reduce charge scattering and a screening layer from surroundings. A key technological challenge is the scalable manufacture of single-crystal 2D hBN film to avoid a lack of durability and a poor performance influenced by inhomogeneities and grain boundaries. In addition, the controllability of the number of layers is also highly required due to the electron tunneling properties depending on the thickness of hBN. Even though several approaches to achieve large-scale single-crystal hBN and control the number of layers have been demonstrated, a growth method for few-layer single-crystalline hBN and precise control of the number of layers is still unknown. In this thesis, I demonstrate an approach to grow large-scale single-crystal hBN by chemical vapor deposition (CVD) method. First, I show the epitaxial growth of single-crystal trilayer hBN on Ni (111) foil of 2 x 5 cm at 100 oC higher temperature than normal growth temperature for Ni substrate. The trilayer hBN grains show unidirectional alignment and seamless stitching to form single-crystal film on Ni23B6/Ni (111) where a Ni23B6 layer is formed between hBN and Ni (111) during cooling. Microscopic investigations reveal epitaxial relationship between hBN, Ni23B6, and Ni (111) and enable to understand the hBN growth mechanism, the surface-mediated growth. Furthermore, single-crystal trilayer hBN on Ni23B6/Ni (111) plays a role of a catalytic-transparent protection layer for enhanced long-term stability of hydrogen evolution reaction catalyst and a dielectric layer to prevent electron doping from SiO2 substrate in MoS2 transistors. Our results suggest that few-layer single-crystal hBN allows wide applications for 2D devices and catalytic-transparent protection layer of (electro)catalysts. Next, I demonstrate a method for controlling the number of layers of 2-inch wafer-scale single-crystal hBN film on sapphire substrate by remote inductively coupled plasma CVD, which is a temperature-dependent growth method for mono-, bi-, and trilayer hBN. The x-ray photoelectron spectroscopic and transmission electron microscopic investigations show the formation of a Al-N buffer layer between sapphire substrate and the first layer and the reduction of the interlayer spacing of hBN by the Al-N bond. However, the transferred hBN onto SiO2/Si substrate shows a typical interlayer spacing of hBN. This work takes a step towards the layer-controlled growth of wafer-scale uniform hBN films.