Interaction between Sm and GaN––a photoemission study

We report on a comprehensive study of the Sm/GaN(0 0 0 1) interface using synchrotron radiation photoemission. Spectra were taken after stepwise Sm deposition onto a clean GaN(0 0 0 1) surface at room temperature and after annealing. Based on the analysis of resonant photoemission spectra taken for hν=136 eV (divalent samarium resonance) and hν=141 eV (trivalent samarium resonance) we find that both divalent and trivalent samarium states are present in the GaN–Sm interface layer after each step of deposition. The average valence of the system increases from 2.57 for a coverage of 0.6 Å to 2.73 for 3.2 Å. Above this coverage the average valence drops and reaches a final value of 2.63 for a coverage of 10 Å. The Ga3d core level evolution confirms that two valence Sm states are present in the interface region. A large chemical shift of the Ga3d core level after samarium deposition and a moderate decrease in the Ga3d attenuation curve indicate that a reactive interface is created when Sm atoms are deposited on the GaN(0 0 0 1) surface. Annealing of the Sm/GaN system to 500 °C induces a valence change from Sm2+ to Sm3+ and promotes diffusion of Sm from the interface into the bulk.