M. Henzler

Adsorption of atomic hydrogen on clean cleaved silicon (111)

Abstract The combination of different experimental methods shows that the hydrogen affects more than just saturation of dangling bonds. Thermal desorption reveals at least two binding states with different saturations (θ 1 = 1, θ 2 ≈ 0.4). At low coverage the desorption kinetics of state β 1 is of second order with activation energy E d = 2.5eV± 0.1eV , which increases with increasing initial coverages. The superstructures of the clean surfaces are destroyed. At high exposures the surface is roughened by etching the surface via desorption of SiH x complexes. The p-type of the 2 × 1 structure (as found by surface conductivity and field effect mobility) is converted to n-type by adsorption. The 7 × 7 surface is found to be n-type. The final state is the same as starting with the 2 × 1 structure. Besides saturation of dangling bonds each model therefore has to include different binding states, additional surface states and chemical surface reactions.

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.

LEED studies of surface imperfections

Abstract The diffraction of low energy electrons is mostly used either for qualitative information (e.g. existence of superstructures) or for determination of atomic positions within unit mesh via intensity evaluation assuming a strictly periodic surface. A lot of additional information is available as soon as, besides existence and intensity of a spot, its profile on the screen (in general, profile in k-space) is used. In this way all kinds of deviation — from simple periodicity, periodic or non-periodic — may be derived quantitatively and qualitatively without much computation. For a qualitative evaluation, a set of basic structural elements and their representation in reciprocal space is discussed, so that a lot of imperfections like steps, islands, domains, point defects and some elements of their arrangement (like random or regular) are easily identified just by visual inspection of the LEED pattern. For quantitative eveluations, the limitations due to the instrument have to be considered. They are described by the average quantity “transfer width”, the maximum distance of surface atoms for a coherent diffraction. New instruments show transfer widths of more than 200 nm, so that the maximum resolvable defect distance is a multiple of that length. For quantitative evaluation of non-constant island sizes, terrace widths and so on, assumption or information on distributions (random or correlated) is needed. Examples of defects from 0 to 3 dimensions are discussed: point defects due to static or thermal disorder, linear defects as borderlines of islands and domains or as steps between terraces, planar defects as facets and bulk defects due to mosaic structure or strain. A wide field is open for further use of spot profile analysis (SPA-LEED).

Epitaxy of Si(111) as studied with a new high resolving LEED system

Abstract The analysis of the angular distribution of the intensity of a LEED diffraction spot is an efficient method to evaluate quantitatively and qualitatively the defect structure of crystal surfaces as well as the nucleation and growth mechanism of epitaxial overlayers. The maximum resolvable distance for such structures is limited by the instrumental response function of the LEED system and by the accuracy with which the spot profile can be determined. Since commercial LEED optics can resolve only distances of about 30 nm, a new system was designed especially for spot profile analysis (SPA-LEED). The improvements are a transfer width of about 100 nm and an accuracy of better than 1% of the FWHM of a LEED spot corresponding to a maximum resolvable distance of about 700 nm. With this instrument the growth behaviour of silicon was investigated on a Si(111) surface which was free from atomic steps. The results for depositing at room temperature and annealing are discussed. The change of the shape of spot profiles as a function of coverage and energy suggest a doublelayer-by-doublelayer growing mechanism in a temperature range between 600 and 700°C.

The Si–SiO2 interface: Correlation of atomic structure and electrical properties

The roughness at the Si–SiO2 interface has been determined quantitatively on an atomic scale by SPA‐LEED (spot profile analysis of low energy electron diffraction) in ultrahigh vacuum after removal of the oxide. At the Si–SiO2 interface the steps are randomly distributed. With the help of model calculations the measured spot broadening provides the step atom density and therefore the roughness on an atomic scale. The roughness will be decreased by low oxidation rates (thick oxides, dry atmosphere) and appropriate annealing in N2 and will be increased by high oxidation rates (thin oxides and wet atmosphere). Furthermore metal oxide semiconductor (MOS) structures were built on chips which were also suited for LEED measurements. Hall mobilities for Si(111) p‐channel inversion layers MOS‐FETs with varying roughness at the Si–SiO2 interface have been measured at temperatures between 4.2 K and room temperature. It was found that there exists a strong correlation between Hall mobility and atomic roughness at hig...

Quantitative evaluation of random distributed steps at interfaces and surfaces

Abstract LEED spot profiles are evaluated to determine random distributed steps at interfaces and surfaces. It is shown that strictly kinematical evaluations are sufficient to provide data on step atom density and terrace width distribution. Experimental results for samples prepared in different ways show the wide range of applications.

LEED‐investigations and work‐function measurements of the first stages of epitaxy of tungsten on tungsten (110)

The arrangement of tungsten atoms (0.2–4 monolayers) evaporated onto W(110) has been studied with LEED and work‐function measurements in ultrahigh vacuum. For temperatures of T=300–430 K island growth has been derived from rings, which are visible at characteristic energies, around each spot of the LEED pattern. By increasing substrate temperature the islands coalesce in the [001] direction. For T=520–800 K the surface shows microfacets with step edges in the [110] direction. Step‐free epitaxial growth has been observed at T=950 K. The decrease of work function during evaporation confirms the formation of islands and facets. The average dipole moment per edge atom has been determined by combination of LEED and work‐function data. Quantitative evaluation of the LEED pattern yields the distance between islands, size of islands, and number of rows of atoms per terrace. The results show that the LEED pattern provides information on epitaxial growth which is not available with any other technique.

Measurement of surface defects by low-energy electron diffraction

Low-energy electron diffraction is capable of detection of non-periodic arrangements by spot profile analysis (SPA-LEED). All kinds of defects may be identified by a qualitative and nevertheless systematic evaluation of the diffraction pattern with respect to spot shape, background and energy dependance. It is shown that the kinematical approximation provides far reaching results. The importance and achievements of the newly developed high-resolution instrumentation is demonstrated. The quantitative evaluation provides data on defect density and distribution, which are not available otherwise, as demonstrated with examples. A comparison of imaging and diffraction techniques shows the special advandages of diffraction with respect to quantitative analysis.

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...

The interplay of surface morphology and strain relief in surfactant mediated growth of Ge on Si(111)

Abstract The growth of Ge on Si is strongly modified by surface active species called surfactants. While the effectiveness of Sb as a surfactant in forcing layer-by-layer growth of Ge on Si(111) and in creating a misfit adjusting dislocation network confined to the Si Ge-interface has been demonstrated in previous studies, the dynamic growth process on an atomic scale leading to this result is still unknown. The relevance of the stress on surface morphology and the growth mode of Ge on Si(111) is presented in a detailed in situ study by spot profile analysing low energy electron diffraction during the deposition. The change from islanding to layer-by-layer growth is seen in the oscillatory intensity variation of the (00)-spot. To relieve the strain the Ge-film forms a microscopically rough surface of small triangular and defect free pyramids in the pseudomorphic growth regime up to 8 monolayers. As soon as the pyramids are completed and start to coalesce, strain relieving defects are created at their base, finally arranging to the dislocation network. After the overgrowth of the dislocations the surface smoothes again showing a much larger terrace length. The periodic dislocation network at the interface gives rise to an elastic deformation of the surface, which results in a spot splitting in LEED. Thus, for the first time the dynamics of the formation of a dislocation network has been observed in situ during the growth process. Surprisingly, the dislocation network is already completed to 70% immediately after 8 monolayers of coverage, which is attributed to the micro-rough surface morphology, providing innumerous nucleation sites for dislocation.