  Boron (B) containing III-nitride materials, such as wurtzite (B,Ga)N alloys,
have recently attracted significant interest to tailor the electronic and
optical properties of optoelectronic devices operating in the visible and
ultraviolet spectral range. However, the growth of high quality samples is
challenging and B atom clustering is often observed in (B,Ga)N alloys. To date,
fundamental understanding of the impact of such clustering on electronic and
optical properties of these alloys is sparse. In this work we employ density
functional theory (DFT) in the framework of the meta generalized gradient
approximation (modified Becke Johnson (mBJ) functional) to provide insight into
this question. We use mBJ DFT calculations, benchmarked against
state-of-the-art hybrid functional DFT, on (B,Ga)N alloys in the experimentally
relevant B content range of up to 7.4%. Our results reveal that B atom
clustering can lead to a strong reduction in the bandgap of such an alloy, in
contrast to alloy configurations where B atoms are not forming clusters, thus
not sharing nitrogen (N) atoms. We find that the reduction in bandgap is linked
mainly to carrier localization effects in the valence band, which stem from
local strain and polarization field effects. However, our study also reveals
that the alloy microstructure of a B atom cluster plays an important role: B
atom chains along the wurtzite c-axis impact the electronic structure far less
strongly when compared to a chain formed within the c-plane. This effect is
again linked to local polarization field effects and the orbital character of
the involved valence states in wurtzite BN and GaN. Overall, our calculations
show that controlling the alloy microstructure of (B,Ga)N alloys is of central
importance when it comes to utilizing these systems in future optoelectronic
devices with improved efficiencies.
