High-energy X-ray photoelectron spectroscopy study of MBE grown (Eu, Gd) Te layers

Abstract

The substitution of Gd3+ for Eu2+ ions introduces conducting electrons and qualitatively changes the model of description of magnetic properties of these semiconductor alloys: antiferromagnetic order observed in insulating layers of EuTe is replaced by ferromagnetic (RKKY interaction driven) state in metallic n-type (Eu,Gd)Te. The high-energy photoemission spectroscopy (hν=3000eV) was used to study the electronic structure and valence of magnetic ions in about one micron thick monocrystalline, ferromagnetic n-type (Eu,Gd)Te layers grown by molecular beam epitaxy on BaF2 (111) substrate with EuTe buffer layer.The photoemission spectra of Eu (3d,4d,4f), Gd (3d,4d,4f), Te (3d,4d) electrons as well as valence band electrons were measured after sequential Ar ion etching. For surface of the sample exposed to the air the oxidation of surface takes place and measured spectra showed the dominant Eu3+ contribution in comparison to Eu2+. The chemical shift of Te3d and Te4d peaks was observed for Te atoms oxidized on the surface. Surface sputtering by Ar ions removes the top layer with oxygen adsorbed at the surface and oxygen diffused in the surface region and clean (Eu,Gd)Te surface of the layer was measured. For clean surface the Eu2+ dominate in the spectra and the peaks of oxidized Te atoms disappear. The contribution of Eu2+ 4f electrons to the electronic structure of the (Eu,Gd)Te was found at the top of the valence band.