U. Köhler

Determination of the local electronic structure of atomic‐sized defects on Si(001) by tunneling spectroscopy

Tunneling spectroscopy and voltage‐dependent scanning tunneling microscopy have been used to study the geometry and electronic properties of atomic‐sized defects on the Si(001) surface. Individual dimer vacancies are shown to be semiconducting, consistent with the π‐bonded defect model of Pandey. Another type of characteristic defect is found which gives rise to strongly metallic tunneling I–V characteristics, demonstrating that it has a high density of states at the Fermi level and is likely active in Fermi level pinning on Si(001). Spatially dependent I–V measurements and tunneling barrier height measurements also directly reveal the spatial extent of this metallic character and provide direct measures of the ‘‘size’’ of the defects.

Scanning tunneling microscopy study of low‐temperature epitaxial growth of silicon on Si(111)‐(7×7)

Scanning tunneling microscopy is used to investigate nucleation and growth phenomena in the molecular‐beam epitaxial (MBE) growth of silicon on Si(111)‐(7×7) from the submonolayer range up to a few monolayers. At room temperature small amorphous clusters form which grow in locally ordered arrays on the (7×7) lattice. Deposition at a higher substrate temperature produces triangular islands of epitaxial silicon which have preferred step propagation in the [112] direction. Preferred nucleation of Si islands is found to occur along boundaries between (7×7) superstructure translational domains of the substrate. The preferred nucleation which arises from defects in the epilayer accounts for the formation of a second epitaxial layer long before the first layer is completed. A variety of metastable reconstructions which differ from (7×7) are also found in the epitaxial islands and are discussed.

Strained-layer growth and islanding of germanium on Si(111)-(7 × 7) studied with STM

Abstract The growth of in situ prepared germanium layers on Si(111)-(7 × 7) has been studied as a function of substrate temperature and coverage. At room temperature, Ge grows in irregular clusters arranged in an ordered array on the substrate and the (7 × 7) reconstruction is preserved. At elevated temperature, in the submonolayer range triangular islands form with preferred growth in [ 1 1 2] direction. The islands show Si-like (7 × 7) and (5 × 5) DAS reconstruction. Ge nucleates preferentially at step edges and at (7 × 7) domain boundaries. Coverages over 2 ML result in a completely (5 × 5) reconstructed layer. On substrate with a ( 3 × 3 ) R 30° adatom arrangement after boron segregation, the Ge epilayer also exhibits DAS reconstructions of the same kind found on the pure Si substrates. Above 4 ML the formation of 3D islands is observed, which show mainly (113) and (111) facets. The islands are relaxed and show a mixture of c(2 × 8), c(2 × 4), and (2 × 2) reconstructions known for bulk Ge(111), when they are grown below 450° C. At a higher deposition temperature a (7 × 7) reappears on top of the 3D islands. Defects emerging from the bulk have been imaged.

Epitaxial growth of silicon on Si(001) by scanning tunneling microscopy

The epitaxial growth of silicon on Si(001)‐(2×1) substrates at temperatures between 580 and 850 K is studied using scanning tunneling microscopy (STM). The growth is strongly anisotropic, forming long narrow structures only a few dimers wide but more than 100 A long. Models are proposed for the two types of antiphase boundaries that are observed on the epitaxially grown surfaces.

Nucleation and growth of epitaxial silicon on Si(001) and Si(111) surfaces by scanning tunneling microscopy

Abstract The epitaxial growth of silicon on Si(111)-(7×7) and Si(001)-(2×2) substrates at temperatures between 300 and 700 K is studied using scanning tunneling microscopy. On Si(111)-(7×7), the epitaxial islands are triangular and exhibit (7×7)-like reconstructions even at low coverage. STM images show that multilayer growth initiates at boundaries between different (7×7) domains and between (5×5) and (7×7) phases. On Si(001), the epitaxial islands are highly anisotropic, forming long narrow rows only a few dimers wide. Multilayer growth initiates at (2×1) anti-phase boundaries. A model is proposed for the structure at these anti-phase boundaries.

Anisotropic etching versus interaction of atomic steps: Scanning tunneling microscopy observations on HF/NH4F‐treated Si(111)

After ex situ etching with various solutions of hydrofluoric acid (HF) and ammonium fluoride (NH4F) Si(111) samples are transferred into ultrahigh vacuum with an ultrafast load‐lock and characterized by scanning tunneling microscopy (STM): Concentrated HF selectively removes any surface oxide and, thus chemically prepares the initially burried, isotropically rough Si/SiO2 interface while highly buffered HF (i.e., NH4F) attacks bulk silicon anisotropically. After a rapid homogenization of the chemical surface termination (HF: various hydrides, fluorine, ...) towards a perfect, unreconstructed monohydride phase, Si(111)‐(1×1):H, NH4F etching leads to a time‐dependent transformation of isotropic roughness into a pattern of triangular etch defects with monohydride steps perpendicular to <211≳ due to a preferential removal of lower‐coordinated atomic defect sites. A predominant atomic step structure due to sample miscut (vicinal surfaces with azimuth ≠<211≳) can oppose the anisotropic NH4F etching: At low st...

A real space investigation of the dimer defect structure of Si(001)-(2times8)

The atomic arrangement of the Si(001)‐(2 times 8) surface has been directly imaged in real space by scanning tunnelling microscopy. The superstructure is impurity induced and stablilized by as little as 1% Ni. Clean Si(001) surfaces do not form the (2times8) phase. The two‐dimensional Fourier transforms of the STM topographs correspond well to the (2times8) LEED pattern, but the real space images are in fact considerably more complex than previously concluded on the basis of recent diffraction technique measurements. The STM images clearly demonstrate that the (2times8) surface consists of a complex missing dimer structure, namely one and multiple dimer vacancies. Such a dimer defect appears related to the Ni impurity and intimately related to the ordering of the dimer defects probably by third layer dimerization to produce quasi‐ordered ‘channels’.

Misfit-related effects in the epitaxial growth of iron on W(110)

Abstract Time-resolved in situ STM has been used to investigate the growth of iron on W(110). In the submonolayer range, a strong inhibition of island coalescence is present up to 0.6 ML due to the misfit of 9.4%. The onset of coalescence can be correlated with the magnetic percolation in these thin layers. Growth at higher temperature leads to the development of a sequence of dislocation networks which start from the second iron layer on with a wider spaced one-dimensional array. With the third layer a second one-dimensional network in [001]-direction appears, showing relaxation to the bulk Fe lattice constant in only one direction. A further layer is necessary to introduce a two-dimensional network, leading to a fully relaxed Fe-layer. The growth of wedge-shaped 3D-islands on a misoriented W(110)-sample in the Stranski-Krastanov growth regime enables us to study different local layer thicknesses simultaneously. It is thus possible to examine the decrease in influence of the dislocation network on the surface topography.

Layer-by-layer growth of germanium on Si(100)" strain-induced morphology and the influence of surfactants

Abstract Germanium grows on pure Si(100)-(2×1) in the Stranski-Krastanov mode. Layer-by-layer growth is found for coverages below 3 ML before the onset of 3D islanding. In this regime the morphology of the Ge layer is strongly influenced by the misfit of 4.2% between layer and substrate. Around 1 ML aligned missing dimer defects are created which form a semiperiodic (2×12) arrangement. With increasing coverage this periodicity is gradually compressed and reaches a (2×8) reconstruction around 2.3 ML. This behaviour is discussed in terms of partial relaxation of the local strain. When further Ge layers grow on this (2xN) arrangement, only part of the missing-dimer defects of the lower layer are buried and a network of trenches partly reaching down to the substrate remains. Layer-by-layer growth up to higher coverage can be obtained using As as a “surfactant” during growth. Under these conditions no (2×8)-like arrangement is found. Up to 12 ML Ge coverage the layer grows free of defects forming extremely anisotropic Ge islands. At higher coverage a network of trenches arises which decorate an array of V-shaped defects previously found with TEM. The arrangement and the start of the overgrowth of these defects is studied.

CHEMISTRY OF SILICON SURFACES AFTER WET CHEMICAL PREPARATION : A THERMODESORPTION SPECTROSCOPY STUDY

Ultraclean wet chemical preparation in air and a fast new load‐locking technique opens up a way to characterize real Si(111) surfaces after processing for microelectronic device fabrication with the proven surface‐analytical tools available in an ultrahigh vacuum. For the first time thermodesorption spectroscopy can be utilized, without interference from typical artefacts like contamination introduced by the ex situ preparation, to investigate the chemical termination and molecular composition of silicon surfaces after initial wet chemical key processes in semiconductor technology (chemical and UV/ozone‐enhanced cleaning, liquid and gaseous phase etching, rinsing with de‐ionized water). By multiplexing a mass spectrometer and analyzing thermally desorbed molecules over a wide range of masses simultaneously, we can separate quantitatively between the major surface‐terminating molecules that are inherently responsible for the different chemical surface properties (hydrophilic due to –OH groups after wet che...