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

TL;DR

This study systematically investigates lanthanide defects in GaN using hybrid density-functional calculations, revealing their stable charge states, defect levels, and optical properties relevant for optoelectronic and quantum applications.

Contribution

It provides a comprehensive first-principles analysis of lanthanide dopants in GaN, detailing their charge states, defect levels, and optical transitions, which was previously not fully understood.

Findings

All lanthanides are stable as trivalent ions in GaN.

Eu and Yb can also be stabilized as divalent; Ce, Pr, and Tb as tetravalent.

Defects can act as carrier traps and enable intra-4f luminescence.

Abstract

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.