Mark Turiansky Graduate Student Researcher, Physics Dept., Van de Walle’s Group
Understanding boron nitride with computational techniques
We are all familiar with the conventional nitrides, InN, GaN, and AlN; they’ve given us numerous electronic and optoelectronic devices that enhance our everyday lives. In this talk, I’d like to focus on a relative newcomer to the nitrides, boron nitride. In contrast to the conventional nitrides, BN stabilizes in the hexagonal phase, which is a two-dimensional layered phase bonded by van der Waals forces. BN has a large band gap around 6 eV, as well as excellent chemical and thermal stability, making it interesting for applications in high-power electronics and as a host for quantum defects. Computational modeling, based on the framework of hybrid density functional theory, can provide insight into the underlying physics of this material and allows us to identify promising routes of exploration.
First, we will talk about wurtzite alloys of GaN [1,2] and AlN  with BN. While BN is not stable in the wurtzite phase, computational modeling allows us to probe its behavior. We will discuss the electronic structure of such alloys, specifically the band gap bowing as a function of boron concentration and the crossover from direct to indirect. The use of a hybrid functional enables a quantitative description of the band gap. We will also discuss the thermodynamics of boron incorporation in GaN.
In the hexagonal phase, BN has been found to host single-photon emitting defects in the visible spectrum. Computational modeling provides a powerful tool to identify and characterize the defects responsible for such emission. We have identified boron dangling bonds as the likely origin of the single-photon emission in hexagonal BN . It has an optical transition at 2.06 eV with minimal coupling to phonons, consistent with experiment. We also predict the existence of a metastable state that gives rise to spin-dependent transitions, which could be useful for spin-based sensing.
 M. E. Turiansky, J.-X. Shen, D. Wickramaratne, and C. G. Van de Walle, J. Appl. Phys. 126, 095706 (2019).
 J.-X. Shen, M. E. Turiansky, D. Wickramaratne, and C. G. Van de Walle, submitted.
 J.-X. Shen, D. Wickramaratne, and C. G. Van de Walle, Phys. Rev. Materials 1, 065001 (2017).
 M. E. Turiansky, A. Alkauskas, L. C. Bassett, and C. G. Van de Walle, Phys. Rev. Lett. 123, 127401 (2019).