**2. Structure**

Attributed to atomic bonding, hexagonal boron nitride (h-BN) (graphite-like) and cubic polymorph (c-BN) (diamond-like) are critical materials for a wide range of material applications with small interface traps. Unlike SiO2 and high-k materials, h-BN materials possess an excellent interface with high mobility on different 2D materials transistors. Interestingly, super-hard BN exhibits a polymorphs phase, such as zincblende BN, hexagonal BN, wurtzite BN (w-BN), BN fullerene, BN nanotubes, and amorphous BN, which can be regarded as counterpart systems of graphite, cubic-diamond (C-diamond), hexagonal-diamond (H-diamond), carbon nanotubes (CNTs), fullerene, and amorphous carbon [39]. Due to their excellent optical and mechanical properties, w-BN and c-BN have attracted massive attention for various applications. However, at different process techniques (pressure and temperature changes), BN always faces state changes, i.e. through cold-compressing h-BN exhibits metastable w-BN instead of stable c-BN. Wen et al. suggest there might be another intermediate state between hexagonal and wurtzite-phase BN and proposed a new BN-phase (bct-BN) with considerable stability and excellent mechanical properties. The resulting bonding changes their electrical and mechanical properties through the process steps, such as superconductivity and hardness [39].

## **2.1 Hexagonal boron nitride (h-BN)**

Hexagonal BN (h-BN) structure is similar to graphite. The h-BN consists of sp2 hybridized alternating B and N atoms, which are held together by a covalent bond in a hexagon (honeycomb) lattice structure, as shown in **Figure 1**. For a fully ordered crystal structure, the lattice constant of the boron nitride structure has lattice parameters: a = 2.504A and c = 6.661A [41]. The h-BN has six-membered rings, with boron atoms in one layer as the nearest neighbors to the N atoms in adjacent layers [40].

## **2.2 Cubic boron nitride (c-BN): Diamond-like**

Cubic-phase BN has significant technological applications. The c-BN is the second hardest material, which is resistant to oxidation. It has a natural protective coating and is sought for its protective properties for infrared and visible spectrum applications. Cubic c-BN is also compatible with high-temperature and high-power applications; unlike diamond, c-BN can be dopped with p and n type to make a high-power photodiode. The c-BN possesses a zincblende structure with ABCABC… stacking

**Figure 1.** *Schematic diagram of the atomic configuration of layered h-BN [40].*

sequence arrangements, consisting of tetrahedrally coordinated boron and nitrogen atoms with {111} plans. The atomic arrangement of B and N atoms in the c-BN lattice is represented in **Figure 2** by the ball and stick model.
