Influence of point defects on the near edge structure of hexagonal boron nitride

Hexagonal boron nitride (hBN) is a wide-band-gap semiconductor with applications including gate insulation layers in graphene transistors, far-ultraviolet light emitting devices and as hydrogen storage media. Due to its complex microstructure, defects in hBN are challenging to identify. Here, we combine x-ray absorption near edge structure (XANES) spectroscopy with ab initio theoretical modeling to identify energetically favorable defects. Following annealing of hBN samples in vacuum and oxygen, the B and N $K$ edges exhibited angular-dependent peak modifications consistent with in-plane defects. Theoretical calculations showed that the energetically favorable defects all produce signature features in XANES. Comparing these calculations with experiments, the principle defects were attributed to substitutional oxygen at the nitrogen site, substitutional carbon at the boron site, and hydrogen passivated boron vacancies. Hydrogen passivation of defects was found to significantly affect the formation energies, electronic states, and XANES. In the B $K$ edge, multiple peaks above the major $1s$ to ${\pi }^{*}$ peak occur as a result of these defects and the hydrogen passivated boron vacancy produces the frequently observed doublet in the $1s$ to ${\sigma }^{*}$ transition. While the N $K$ edge is less sensitive to defects, features attributable to substitutional C at the B site were observed. This defect was also calculated to have mid-gap states in its band structure that may be responsible for the 4.1-eV ultraviolet emission frequently observed from this material.

Figure 1

TEM images and inset diffraction patterns of (a) hBN powder showing overlapping platelets with some folding of the layers and (b) a cross-sectioned turbostratic hBN thin film with a preferred crystallographic orientation normal to the substrate (bottom right of image).

Figure 2

XANES spectra for the (a) B $K$ edge and (b) N $K$ edge from hBN single crystal, hBN powder, $\mathrm{A}{\mathrm{r}}^{+}$ irradiated hBN powder, and hBN thin film grown at $200{}^{\circ }\mathrm{C}$.

Figure 3

(a) B $K$ edge and (b) N $K$ edge XANES of as-deposited, vacuum annealed and oxygen annealed hBN thin films grown at RT and $200{}^{\circ }\mathrm{C}$. The spectra are normalized to the first ${\pi }^{*}$ peak (labeled A in the B $K$ edge).

Figure 4

Angular dependence of B $K$ edge XANES for the $200{}^{\circ }\mathrm{C}$ thin-film sample. Spectra are normalized to the first ${\pi }^{*}$ peak (labeled A). Inset shows peaks A–D over a reduced energy range following background subtraction.

Figure 5

Simplified band-structure diagrams of hBN defect structures at the $\mathrm{\Gamma }$ point. Energy levels are shown relative to the valence-band maximum (VBM) and conduction-band minimum (CBM) of the pristine calculation. Spin up/down states are shown in blue/red. Occupied states are represented by solid rectangles and arrows. Unoccupied states are represented by hollow rectangles and dashed arrows. The Fermi energy levels calculated by castep are shown as dotted lines.

Figure 6

Structural models of the (a) ${\mathrm{V}}_{\mathrm{B}}$, (b) ${\mathrm{V}}_{\text{B-H3}}$, (c) ${\mathrm{V}}_{\mathrm{N}}$, and (d) ${\mathrm{V}}_{\text{N-H3}}$ in hBN. The calculated (e) B $K$ edges and (f) N $K$ edges from these defects are compared with pristine hBN and experimental data from hBN powder.

Figure 7

Structural models of the (a) ${\mathrm{C}}_{\mathrm{B}}$, (b) ${\mathrm{C}}_{\text{B-H}}$, (c) ${\mathrm{C}}_{\mathrm{N}}$, and (d) ${\mathrm{C}}_{\text{N-H}}$ in hBN. In (b) and (d), the H atoms are above the plane with bond lengths of 1.10 and $1.12\AA$ to the underlying C. (e) The B $K$ edges and (f) N $K$ edges calculated from these defects are compared with the pristine hBN calculations and experimental data of hBN powder.

Figure 8

Structural models of the (a) ${\mathrm{O}}_{\mathrm{B}}$, (b) ${\mathrm{O}}_{\text{B-H}}$, (c) ${\mathrm{O}}_{\mathrm{N}}$, and (d) ${\mathrm{O}}_{\text{N-H}}$ in hBN. (e) B $K$ edges and (f) N $K$ edges calculated from these defect compared to pristine hBN and experimental data of hBN powder.

Figure 9

Structural models of the (a) $2{\mathrm{O}}_{\mathrm{N}}$ and (b) $3{\mathrm{O}}_{\mathrm{N}}$ defects. (c) B $K$ edges and (d) N $K$ edges calculated from multiple ${\mathrm{O}}_{\mathrm{N}}$ substitution defects compared with pristine hBN and experimental data of hBN powder.

Figure 10

(a) Calculated B $K$ edges and (b) N $K$ edges from energetically favorable defects compared with experimental edges from hBN powder and the $200{}^{\circ }\mathrm{C}$ thin-film sample.