Origin and photoluminescence of a shallow hydrogen-related donor in silicon

Paramagnetic color centers in silicon are being investigated recently for use in quantum technologies. While the origin of most of the prominent photoluminescence (PL) lines are well understood, that is not the case for those connected to hydrogen, even though it is almost always present in silicon, e.g., shallow hydrogen-related donors (SHDs), but their atomic structure remains elusive. Recent experiments suggested connection between the SHDs and the so-called X and the Bᵢ (or Xᵢ) PL-lines. An SHD, termed the MT2 center, was tentatively assigned to a bistable, negative-U defect, arising due to the interaction of hydrogen with the self-interstitial complex I₄. Using high-level density functional theory calculations, we investigate the interaction of hydrogen with self-interstitial complexes In. A shallow effective-mass-theory donor arises when hydrogen attacks a Si-Si bond in Iₙ, inserting itself as a bond-center interstitial (IₙHBC) in an exothermic reaction. We demonstrate this in the case of a single self-interstitial and of a complex of four. The latter I₄HBC complex shows electronic, spin and PL properties remarkably close to MT2. We find that hydrogen in the presence of I₄ follows the same behavior as in pristine silicon, i.e., it is a negative-U bistable defect, only detectable under illumination. In the I₄HBC complex, H can hop via photo-assisted mechanism between equivalent BC sites of the I₄ complex, leading to a motional averaged tetragonal symmetry as experimentally observed for MT2. The zero-phonon line is distinct from the X-line (attributed to the bare I₄ complex), and falls well into the range of the Xᵢ lines. Our results identify the I₄H complex as a prototype for a class of SHDs, inferring that similar SHDs may arise, with the Xᵢ lines as their PL fingerprints, when H attacks Iₙ clusters.