Potassium-induced charge redistribution on Si(111) surfaces studied by core-level photoemission spectroscopy

Understanding the insulating nature of alkali-metal/Si(111):B interfaces

We have recently revisited the phase diagram of alkali-metal/Si(111):B semiconducting interfaces previously suggested as the possible realization of a Mott-Hubbard insulator on a triangular lattice. The insulating character of the 2√[3] × 2√[3]R30 surface reconstruction observed at the saturation coverage, i.e. 0.5 ML, has been shown to find its origin in a giant alkali-metal-induced vertical distortion. Low energy electron diffraction, photoemission spectroscopy and scanning tunneling microscopy and spectroscopy experiments coupled with linear augmented plane-wave density functional theory calculations allow a full understanding of the k-resolved band structure, explaining both the inhomogeneous charge transfers into an Si-B hybridized surface state and the opening of a band gap larger than 1 eV. Moreover, √[3] × √[3]R30, 3 × 3 and 2√[3] × 2√[3]R30 surface reconstructions observed as a function of coverage may reveal a filling-controlled transition from a half-filled correlated magnetic material to a strongly distorted band insulator at saturation.

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects.

Optical second harmonic spectroscopy of boron-reconstructed Si(001)

Optical second harmonic generation (SHG) spectroscopy is used to probe Si(001) following thermal decomposition of diborane at the surface. Incorporation of boron (B) at second layer substitutional sites at H-free Si(001) intensifies and redshifts the E1 SHG spectral peak, while subsequent H termination further intensifies and blueshifts E1, in sharp contrast to the effect of bulk B doping or nonsubstitutional B. Ab initio pseudopotential and semiempirical tight binding calculations independently reproduce these unique trends, and attribute them to the surface electric field associated with charge transfer to electrically active B acceptors, and rehybridization of atomic bonds.