Atomic and electronic structures of thin NaCl films grown on a Ge(0 0 1) surface

Density functional theory (DFT) with LDA and GGA have been employed to study the interface and thin film properties of NaCl on a Ge(0 0 1) surface. The atomic and electronic structures of thin NaCl films from one to ten monolayers were analyzed. The layer adsorption energies show that a quasi-crystalline (0 0 1) fcc NaCl film is built up via a layer-by-layer growth mode with NaCl thickness above 2 ML. Simulated STM images show a well-resolved (1 � 1) NaCl atomic structure for sample bias voltage Vs < � 2.5 V and the bright protrusions should be assigned to the Cl � ions of the NaCl film. The Ge substrate dimer is reserved and buckled like a clean Ge(0 0 1)-p(2 � 2) surface as the result of weak interface interaction between the dangling bonds coming from valence p states of the Ge substrate and the 3p states of the interfacial Cl � ion. These results are consistent with the experiments of STM, LEED and EELS.