UC Riverside

Two dimensional (2D) hexagonal boron nitride (h-BN) continues to attract tremendous research interest across the globe thanks to their unique optical, electronic, and mechanical properties. However, the controllable synthesis of large size single crystal h-BN is still challenging to realize the technological potential. In this thesis, we focus on growth and characterization of h-BN by molecular beam epitaxy (MBE). By exploring the role of interstitial carbon in transition metal substrate, we have achieved a big step forward toward controllable synthesis of large-size single crystal h-BN.

In the first project, i.e., Chapter 2, we carried out a systematic study of hexagonal boron nitride/graphene (h-BN/G) heterostructure growth by introducing high incorporation of carbon (C) source on a heated cobalt (Co) foil substrate followed by boron and nitrogen sources in a molecular beam epitaxy system. With the increase of C incorporation in Co, three distinct regions of h-BN/G heterostructures were observed from region (1) where the C saturation is not attained at the growth temperature (900 °C) and G is grown only by precipitation during cooling process to form a “G network” underneath the h-BN film; to region (2) where the Co substrate is just saturated by C atoms at the growth temperature and a part of G growth occurs isothermally to form G islands and another part by precipitation, resulting in a non-uniform h-BN/G film; and to region (3) where a continuous layered G structure is formed at the growth temperature and precipitated C atoms add additional G layers to the system, leading to a uniform h-BN/G film. We realized that the top h-BN film has the largest effect on the growth of underneath G layers in region 1. It is also found that in all three h-BN/G heterostructure growth regions, a 3-hrs h-BN growth at 900 ºC leads to h-BN film with a thickness of 1~2 nm, regardless of the underneath G layers’ thickness or morphology. Growth time and growth temperature effects have been also studied.

In the second project, i.e., Chapter 3, we demonstrate that the dissolution of carbon

into cobalt (Co) and nickel (Ni) substrates can facilitate the growth of h-BN and attain

large-area 2D homogeneity. The morphology of the h-BN film can be controlled from 2D

layer-plus-3D islands to homogeneous 2D few-layers by tuning the carbon interstitial

concentration in the Co substrate through a carburization process prior to the h-BN growth

step. Comprehensive characterizations were performed to evaluate structural, electrical,

optical, and dielectric properties of these samples. Single-crystal h-BN flakes with an edge

length of ∼600 μm were demonstrated on carburized Ni. An average breakdown electric

field of 9 MV/cm was achieved for an as-grown continuous 3-layer h-BN on carburized Co. Density functional theory calculations reveal that the interstitial carbon atoms can increase the adsorption energy of B and N atoms on the Co(111) surface and decrease the diffusion activation energy and, in turn, promote the nucleation and growth of 2D h-BN.

In the third project, i.e., Chapter 4, we designed a synthetic approach, in which secondary recrystallized Ni (100) substrates underwent a carburization process, followed by the growth of h-BN in a molecular beam epitaxy system. The h-BN growth dynamics were studied by tuning different growth parameters including the substrate temperature, and the boron and nitrogen source flux ratio. With assistance from density functional theory calculations, we rationalized the role of interstitial C atoms in promoting h-BN growth by enhancing the catalytic effect of the transition metal, which lowers the nucleation activation energy barrier. Through the control of the growth parameters, a single-crystal h-BN monolayer domain as large as 1.4 mm in edge length was achieved. In addition, a high-quality, continuous, large-area h-BN single-layer film with a breakdown electric field of 9.75 MV/cm was demonstrated. The high value of the breakdown electric field suggests that single-layer h-BN has extraordinary dielectric strength for high-performance 2D electronics applications.

In the fourth project, i.e., Chapter 5, we present a study of h-BN adlayer growth and provide a strategy towards eliminating these adlayers for the precise control of the number of 2D layers. By varying the growth parameters such as substrate property, nitrogen source composition, and substrate carburization time, we found that the adlayer growth can be controlled by controlling the nucleation and intercalation processes, which is achieved by engineering the defects and impurities on substrate and the activeness of the h-BN edges. While crystallographic defects and impurities stimulate the multilayer nucleation process, activated edge tends to turn off the intercalation process by reducing the probability of precursors penetrating into the interface. We have achieved the growth of a large-area adlayer-free single-layer h-BN film.