Electronic and optical properties of boron containing GaN alloys: The role boron atom clustering

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.