Multiple-site Ag doping in Bi2Se3: Compositional crossover from substitution to intercalation as revealed by photoelectron diffraction and X-ray fluorescence holography

The local structure around Ag atoms in Ag-doped Bi₂Se${}_3$ (Ag${}_x$Bi${}_{2-y}$Se${}_3$) was investigated by photoelectron diffraction and X-ray fluorescence holography to understand the manner of Ag atom doping. At a low Ag concentration ($x=0.05$), photoelectron diffraction indicated that Ag atoms occupied Bi substitution sites. However, a mere accumulation of holes via Ag substitution for Bi fails to explain the observed variation previously reported in the transport properties of Ag-doped Bi₂Se${}_3$ with different $x$ values. In particular, simple Ag substitution for Bi fails to explain the pinning of the Fermi level at the bottom of conduction band, as suggested by the observed transport properties. In the case of a high Ag concentration ($x=0.2$), photoelectron diffraction suggested that the Ag atoms occupied not only the substitutional Bi site but also multiple interstitial sites, namely, the octahedral site in the van der Waals interlayer and the interstitial site in the Se layer. Ag 3d photoelectron spectra revealed that the Ag atoms had the same oxidation state, $+1$, regardless of the type of occupied site. Furthermore, X-ray fluorescence holography was employed for a model-free local structural analysis that refined the exact locations of Ag atoms in the Bi${}_2$Se${}_3$ crystal lattice. The behavioral crossover documented herein from hole doping at the substitutional site to electron doping at multiple sites reasonably explains the dependence of electronic structures and transport properties of Ag-doped Bi${}_2$Se${}_3$ on dopant concentration.