Bonding characteristics of the √3°√3 Ag/Si interface identified by the energy dependence of the photoionization cross section

Synchrotron-radiation photoemission of the metal-semiconductor interface has a unique advantage in identifying the bonding characteristics of the rehybridized orbitals between the metal and the semiconductor. By varying the photon energy and measuring the magnitude of the photoionization cross section, the bonding peak at the metal-semiconductor interface can be identified as being formed with or without hybridization with the metal’s d orbitals. The latter case is clearly illustrated by the annealed Ag/Si(111) interface. A sharp structure at 3.8 eV below the Fermi level grows as a result of annealing of the Ag/Si(111) interface. This structure is also accompanied by the formation of the √3 × √3 (R30°) low-energy electron diffraction pattern and the narrowing of the Ag 4d band signal. Since the so-called Cooper-minimum effect is not observed for this annealing-induced peak, it is concluded that the structure does not have 4d-orbital characteristics within the regime of the theory of linear combination of atomic orbitals. This is the direct evidence to show that the Ag 4d band does not hybridize with the substrate Si ${\mathrm{sp}}^3$ orbitals in forming Ag—Si bonds after high-temperature annealing. Judging from the photoionization cross-section variation, it can instead be inferred that the sharp structure has mostly Ag 5s or 5p and Si sp characteristics. The methodology shown here can be extended to other metal-semiconductor interfaces and can provide information on the bonding formation of the metal atoms and the semiconductor substrate. This is important in discussing metal-semiconductor interfaces but difficult to obtain unambiguously with other techniques and in theoretical predictions.