Surface core-level shifts for clean and halogen-covered Ge(100) and Ge(111)

The intensity and energy shift of the Ge 3d surface core-level photoemission is determined with the use of synchrotron radiation. Cl- and Br-terminated Ge(111)?(hy(1×1) surfaces are used to calibrate the absolute coverage CTHETA and to obtain accurate line shapes. We find binding-energy shifts ΔE=+0.57 eV for Cl/Ge(111)-(1×1) and for Cl/Ge(100)-(2×1), ΔE=+0.47 eV for Br/Ge(111)-(1×1), Δ$E_1$=-0.26 eV and Δ$E_2$=-0.76 eV for clean Ge(111)-c(2×8), and ΔE=-0.43 eV for clean Ge(100)-c(4×2). The coverage of special surface atoms (in monolayers) is ${\mathrm{CTHETA}}_1$=0.93, ${\mathrm{CTHETA}}_2$=0.25 for clean Ge(111)-c(2×8), CTHETA=0.62 for clean Ge(100)-c(4×2), and CTHETA=1.03 for Cl/Ge(100)-(2×1). The electron mean free path is 5.4 Å at a kinetic energy of 25 eV. With regard to structural models we conclude that the asymmetric dimer model fits the Ge(100) data best, with a possible contribution of 24% symmetric dimers. For Ge(111)-c(2×8) two new models are proposed with adatoms and tetramer building blocks, respectively.