Boron carbides formed by coevaporation of B and C atoms: Vapor reactivity, ${\mathrm{B}}_x{\mathrm{C}}_{1-x}$ composition, and bonding structure

Boron carbides $\left({\mathrm{B}}_x{\mathrm{C}}_{1-x}\right)$ in thin film form have been synthesized in a high vacuum by coevaporation of B and C atoms from independent sources, allowing a study of the whole composition range from pure B films to pure C films. The relationship between the impinging $\mathrm{B}∕\mathrm{C}$ atomic fluxes and the film composition has been studied, providing information on the chemical reactivity between the B and C vapors. The composition was determined with x-ray emission energy dispersion spectroscopy and x-ray absorption near edge spectroscopy (XANES). Finally, the bonding structure of the films has been determined by XANES, showing a change from structures based on ${\mathrm{B}}_{12}$-icosahedral units for the B-rich samples to hexagonal-like structures for the C-rich samples. The study shows that the structural transition takes place for $x\sim 0.5$.

Figure 1

Incident flux of evaporated boron and carbon atoms as a function of the electron beam intensity of the evaporators for a fixed electron acceleration voltage of $7\mathrm{kV}$.

Figure 2

Dependence of the film composition with the ratio of impinging atomic fluxes.

Figure 3

Dependence with the film composition of (a) normalized growth rate relative to the evaporation rate $\gamma$ and (b) sticking coefficients $\alpha$ of B and C atoms.

Figure 4

$\mathrm{B}\left(1s\right)$ and $\mathrm{C}\left(1s\right)$ core level photoabsorption spectra of ${\mathrm{B}}_x{\mathrm{C}}_{1-x}$ compounds. The curves are labeled with the names of the reference compounds and the carbon at. % in each ${\mathrm{B}}_x{\mathrm{C}}_{1-x}$ sample. Up to five different bonding environments are distinguished in the $\mathrm{B}\left(1s\right)$ edge.