Huang Shengxiang

Synthesis of multilayer hexagonal boron nitride on Cu-Ni alloy by chemical vapor deposition

Hexagonal boron nitride (h-BN) is isoelectronic to graphite with an equal number of boron (B) and nitrogen (N) atoms. For bulk h-BN, B and N atoms are bonded together by strong covalent bonds in plane while weak van der Waals force dominates the interaction in-between the layers. h-BN has attractive properties including high mechanical strength, high thermal conductivity, chemical inertness, and excellent electrical insulation. The unique properties of h-BN provide a potential for a wide range of applications as both a structural and electronic material. As h-BN is of a wide band gap, a ultra-smooth surface and a low density of surface charge states, it is always regarded as an ideal substrate for other two dimensional crystals, such as graphene and transition metal dichalcogenides, in electronic applications. Although monolayer single crystal of h-BN has been synthesized on Cu and Cu-Ni alloy foil by chemical vapor deposition (CVD), multilayer h-BN single-crystal has yet not been synthesized successfully. Normally the growth of h-BN by CVD method on metal surface shows an obvious self-limited behavior, and thus h-BN monolayer is always obtained. As a dielectric material for two dimensional semiconductors, the screening effect of monolayer h-BN is limited for electronics. As such, multilayer crystals of h-BN are required to overcome the inextricable difficulty. In this work, we demonstrate an approach to synthesize multilayer single crystal of h-BN on Cu-Ni alloy by using growth-etching-regrowth strategy. In the strategy, growth of the first layer of h-BN was suppressed when H2 flow is increased and the supply of B-N precursors is reduced. The exposed Cu-Ni alloy significantly improves the growth rate of multilayer h-BN in the next growth process. The morphology of multilayer h-BN samples was characterized by field-emission scanning electron microscope. It is found that multilayer domains of h-BN finally obtained are in triangle with each side up to ~20 μ m. X-ray photoemission spectroscopy (XPS) measurement was also done on the h-BN/Cu-Ni sample, the results show that the spectra of B 1s and N 1s were at 190.4 and 397.8 eV, respectively. Raman spectroscopy is used to understand the lattice structure of the h-BN. In the Raman spectrum, only the E 2g Raman peak at 1365 cm - 1 was observed, it indicates that the configuration for B and N atoms is B-N bonding, implying that the hexagonal phase exists in our BN films. The full width at half maximum (FWHM) of Raman peak is 18 cm - 1, which is less than the values in earlier literatures and comparable to the single-crystalline bulk h-BN fabricated by high temperature high pressure method. Transmission electron microscope (TEM) and selected area electron diffraction (SAED) were further conducted to characterize the microstructure of the multilayer h-BN domains. Cross-sectional TEM image in a high magnification shows that the interlayer distance of multilayer h-BN is 0.33 nm, and the SAED results indicates that h-BN domains is well-stacked with and AA′ stacking order and each layer of h-BN has the same lattice orientation. All the results show that the multilayer h-BN domains we synthesized are in single-crystalline with high quality.