Shallow donors in epitaxial GaN

Photoluminescence (PL) of GaN is commonly dominated by the annihilation of excitons bound to a 34.5 meV deep donor appearing at 3.472 eV (for strain free GaN at 4 K). With PL we were able to resolve two additional donor bound exciton transitions. The excitons have localization energies of 3.7 ± 0.3 and 11.3 ± 05 meV, respectively. Using Haynes rule the respective donors lie 56.5 and 18.5 meV below the conduction band. The applicability of Haynes rule could nicely be confirmed by IR absorption where the 1s-2p transition of the 34.5 and 58 meV donor are observed. The issue of residual donors in GaN will be discussed in this context.