Rational design of point defects with small electron-phonon coupling in 2D materials

Point defects in semiconductors have emerged as an attractive candidate for applications in quantum information science. Due to their ability to create well-localized states within the band gap, point defects can serve as effectively isolated atoms that can be utilized as single photon emitters (SPEs) and qubits.

Small Electron-phonon coupling is a key factor in producing higher photon indistinguishability which determines the suitability of defect systems as SPEs. Huang-Rhys (HR) factors are commonly calculated to measure the degree of electron-phonon coupling in a given system. However, not only is the computation of HR factors a complex task, but once determined, they are often used solely as a numerical value without any effort to establish a meaningful connection between HR factors and the physical defect system. Establishing such correlation would enable the development of a rational design principle for defects, with the goal of minimizing HR factors.

 

We propose that small HR factors are linked to the preservation of bonding character between the initial (occupied) and final (unoccupied) states involved in a specific transition. We demonstrate this principle for realistic SPEs candidates in hBN defect systems. HR factors are first calculated through first-principles employing the one-dimensional configuration coordinate diagram (1DCCD) approximation, then compared with full calculations involving phonon spectra, where we carefully extrapolate spectral functions towards the dilute defect limit.  These extrapolations are carried out through an embedding method that depends on the limited range of interatomic force constants within covalent semiconductors.  Calculated HR factors are then related to the degree of bonding-character similarity between the excited and ground states.