Influence of impurities on the electronic structure of Y(0001) single-crystal surfaces

Rare-earth single crystals are known to contain naturally occurring impurities despite fabrication with the best refinement techniques. This is why thin films grown on W(110) or Mo(110) are usually preferred for studies of the surface electronic structure, although the surface order of thin films is generally worse than on single crystals. In this paper, we demonstrate the influence of impurities on the surface electronic structure below and above the Fermi level for different cleaning stages of a Y(0001) single-crystal surface. While the unoccupied electronic structure is only slightly affected by the presence of different impurities, the occupied electronic structure shows characteristic changes depending on the specific impurity at the surface. The nature of the so-called surface-order-dependent state, which has been a source of speculation for more than 20 years, is revealed as a result of a strongly temperature-dependent chemical reaction at the yttrium surface that involves carbon and oxygen. Furthermore, we show that the surface state close to the Fermi energy reacts very sensitively to the surface chemistry. It exhibits an energetic shift that is characteristic of a certain impurity at the surface.

Figure 1

Angle-resolved PE and IPE spectra for an as-prepared Y(0001) surface along $\overline{\Gamma }\overline{M}$.

Figure 2

Adsorption experiments with ${\text{CO}}_2$ performed on the Y(0001) single crystal at temperatures of 300, 750, and 900 K. The PE spectra ($\text{h}\nu =21.2\text{eV}$, normal emission) were obtained during ${\text{CO}}_2$ exposure of up to 20 L $\left(1\text{L}=1\times {10}^{-6}\text{Torr}\times \text{s}\right)$.

Figure 3

Photoemission ($\text{h}\nu =21.2\text{eV}$, normal emission) and corresponding Auger-electron spectra of different cleaning stages of a Y(0001) single crystal (see text for details). Hydrogen, oxygen, chlorine, and carbon impurities lead to peaks in the photoemission spectra at energies of $-4$, $-6$, $-7$, and $-10\text{eV}$, respectively. When the yttrium sample is heated to 1100 K, all impurity-induced features (except a residual oxygen peak) are quenched. The corresponding Auger spectrum represents a clean Y surface without contaminations.

Figure 4

IPE spectra ($\text{h}\nu =9.9\text{eV}$, normal incidence) obtained from the Cl-enriched surface (open squares), the O-enriched surface (open dots), and from the Y(0001) surface cooled down immediately after a 1100 K flash (filled dots).

Figure 5

Near Fermi-energy region enlarged from the spectra shown in Fig. 3. The enrichment of different impurities leads to the formation of impurity-induced surface states at characteristic energies of $-0.72$, $-0.43\text{eV}$, and $-0.39\text{eV}$ for oxygen, carbon, and chlorine, respectively. The surface-state emission from clean Y(0001) is observed at $-0.24\text{eV}$.