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Rare-earth defects in GaN: A systematic investigation of the lanthanide series

Rare-earth (RE) doped GaN is of interest for optoelectronics and spintronics and potentially for quantum applications. A fundamental understanding of the interaction between RE dopants and the semiconductor host is key to realizing the material's full potential. This work reports an investigation of lanthanide ($Ln$) defects in GaN using hybrid density-functional defect calculations. We find that all the $Ln$ dopants incorporated at the Ga lattice site, $Ln_{\rm Ga}$ ($Ln$ = La--Lu), are stable as trivalent ions, but Eu and Yb can also be stabilized as divalent and Ce, Pr, and Tb as tetravalent. The location of $Ln$-related defect levels and the $Ln$ $4f$ states in the energy spectrum of the host material is determined from first principles. We elucidate the interplay between defect formation and electronic structure, including the $Ln$--N interaction, and the effect of doping on the local lattice environment. Optical properties are investigated by considering possible defect-to-band and band-to-defect transitions involving $Ln_{\rm Ga}$ defects with in-gap energy levels, including broad "charge-transfer" transitions. These defects can also act as carrier traps and mediate energy transfer from the host into the $4f$-electron core of the $Ln$ ion which leads to sharp intra-$f$ luminescence.