Structural, thermodynamic, electronic, optical and mechanical properties of amorphous BCxN1-x: A first-principles study

Abstract

High-density amorphous boron carbonitride samples, a-BC_xN_1-x, for x = 0, 0.25, 0.5, 0.75, and 1, were generated using first-principles molecular dynamics simulations to investigate an effect of the substitution of N atoms by C atoms. The sample density varied in the range of 2.546 g/cm³ – 2.770 g/cm³ depending on composition. The structure, chemical bonding, optical (dielectric functions, reflectivity spectra), thermodynamic (vibration spectra, heat capacity at constant volume, Debye temperature) and mechanical (elastic moduli, Vickers hardness, stress-shear strain relations) properties were studied in detail. Threefold coordinated B and N atoms and fourfold coordinated C atoms are prevailed in each sample. All the amorphous samples are semiconductors, and their band gap decreases from 2.6 eV to 0.6 eV on approaching a-BC due to an increase in homopolar bonds. For all the samples, the minimum reflectivity is observed in the range of 1 eV – 11 eV (∼20 %). An increase of x in a-BC_xN_1-x leads to: lowering the frequencies of the optical branch of vibrational spectra; an increase of heat capacity; and a decrease of the Debye temperature at 300 K. The amorphous boron carbonitrides were found to be brittle materials. The correlation between sample density and fourfold coordinated B atoms, shear modulus, and Vickers hardness was revealed. The Vickers hardness varies non-monotonically with composition, and a-BN and a-BC_0.75N_0.25 samples exhibit the highest Vickers hardness and ideal shear strength due to their highest density and fourfold coordination of boron atoms. Considering these results, it is clear that amorphous boron carbonitrides can serve as hard semiconductor materials.