Direct and indirect optical absorptions of cubic BAs and BSb

Recently, boron arsenide (BAs) has been measured with high thermal conductivity in the experiments, great encouragement for low-power photoelectric devices. Hence we systematically investigate the direct and indirect optical absorptions of BAs and BSb by using first-principles calculations. We obtain the absorption onset corresponding to the value of indirect bandgap by considering the phonon-assisted second-order indirect optical absorption. The temperature-dependent calculations also capture the redshift of absorption onset, enhancement, and smoothness of optical absorption spectra. Moreover, in order to introduce the first-order absorption in the visible range, the doping effect of congeners is studied without the assist of phonon. It is found that the decrease of local direct bandgap derives from either the decrease of bonding-antibonding repulsion of p orbital states by the heavier III group elements or the similar influence of lighter V group elements on the s orbital states. Thus, the doping of congeners can improve the visible optical absorptions.