H. Mai

Interaction of Sn atoms with the intrinsic dangling-bond states of Si(111)-(7 × 7)

Abstract At the initial stages of room-temperature deposition we have observed the preferential adsorption of the individual Sn atoms on the Si center-adatom (T 1 ) sites on Si(111)-(7 × 7) using scanning tunneling microscopy. A spectroscopic analysis of changes in the energy spectrum of surface states around the Fermi level ( E f ) suggests that covalent bonding occurs between individual Sn and Si adatoms. This results in a surface metal-insulator transition due to the removal of Si adatom surface states at E f while preserving the (7 × 7) sub-lattice. An energy-level scheme is proposed to explain this behavior.

Voltage-dependent scanning tunneling microscopy images of the Ge(111)-c(2×8) surface

We present scanning tunneling microscopy images of the Ge(111)-c(2×8) surface where both clean and impurity-containing portions exhibit significant voltage-dependent variations. While only adatoms are observed in empty state images, both adatoms and rest atoms simultaneously appear in filled state images but changing contrast with varying bias voltage. The filled state image taken with a voltage as low as 0.5 V confirms that a surface state exists between rest-atom bands and the Fermi level and it is not dangling-bond-like, i.e., its associated charge is found to be distributed over both adatoms and rest atoms. The voltage-dependent images of the impurity-containing surface show delocalized features around the impurity. These delocalized features are observed for the first time on the surface other than III–V semiconductor surfaces. It is attributed to the charged impurity relative to the clean part, causing the band bending near the impurity.

Charged defects on Ge(111)-c(2×8): characterization using STM

Abstract We have studied various defects present on the Ge(111)- c (2×8) surface using scanning tunneling microscopy (STM). Images at different bias-voltages reveal defects that appear as voltage-dependent variations in brightness. Empty-state images, in particular, taken with low bias voltages show characteristic delocalized brightness variation around some defects. These particular defects have a net charge relative to the clean, unperturbed Ge(111)- c (2×8) surface. We identify various types of defects and describe their charge states. This unique observation of a delocalized variation in the images of Ge(111)- c (2×8) is attributed to the various charged defects allied to poor surface screening of this semiconducting surface.

Scanning tunneling spectroscopy examination of surface electronic structures of Si(111)(23×23)30°-Sn surface

Abstract Scanning tunneling spectroscopy (STS) measurements have been performed on Si (111)(2 3 ×2 3 )30°- Sn surface. Tunneling spectral analysis of the energy of the surface states around the Fermi-level (Ef) indicates two filled and two empty states straddling Ef, showing a ∼ 1.6 eV surface bandgap. As part of the identification of these surface states, STS spectra were also taken on a coexisting well-known 3 × 3 surface, and the intrisic nature of these surface states is discussed.

Stabilizing influence of double bonds on the Si(111)-(3◊1) and Ge(111)-(3◊1) surface reconstructions: new experimental evidence

Abstract We have obtained new real-space images of the filled dangling-bond states of the alkali-metal-induced (3×1) reconstruction of the Si(111) surface associated with recently proposed SiSi double-bond surface structures. Our new experimental evidence reveals significant subtle differences between this local bonding on Na/Si(111)-(3×1) and Na/Ge(111)-(3×1) which argues a strong case for the proposed ‘honeycomb chain-channel model’ but with a relaxation of a strict double-bond requirement, particularly on the germanium surface.

Na- and Li-induced Ge(111)3×1 reconstruction: Different electronic configurations revealed by scanning tunneling microscopy images

We present scanning tunneling microscopy (STM) observation of the 3×1 reconstruction of the Ge(111) surface induced by the adsorption of Na and Li. The STM images of the Na- and Li-induced Ge(111)3×1 surfaces reveal substantial differences not only from each other but also from those of Si(111)3×1 induced by the same metal atoms. We suggest plausible interpretations of these STM images within the structural models proposed for the alkali-metal-induced Si(111)3 ×1 reconstruction having similar atomic structures but different electron bonding configurations.

STM study of the charged defects on the Ge(111)-c(2×8) surface and the effect of density of states on defect-induced perturbation

Abstract Defects present on the Ge(111)-c(2×8) surface and an effect of surface electronic states on the defect-induced perturbation have been studied by scanning tunneling microscopy (STM). The defects exhibit voltage-dependent characteristics in the image. In particular, the empty-state images with low bias voltages exhibit delocalized brightness variation around the defects, suggesting that these defects are charged relative to the clean, unperturbed surface. The voltage-dependent but nonmonotonic amplitude of the delocalized brightness in the STM image is explained in relation to the surface electronic structure of Ge(111)-c(2×8) and the tunneling probabilities.