An overview of the formation of reconstructed facets on vicinal surfaces of Si(111), Si(100) and Si(110) is given, which have been observed by using scanning tunneling microscopy (STM). Most of the samples have been prepared by mechanical grinding, subsequent chemical etching leading to concave-shaped surfaces and by in situ treatment of high-index Si wafers. The orientation of the experimentally observed facets are represented in a stereographic projection, which allows the recognition of more general trends in facet formation. Moreover, it provides information on the availability of unit cells in a wide range of sizes and may therefore become important for the growth of self-organized surface structures.

Vicinal silicon surfaces auto-organise in a very systematic way, creating stepped surfaces and facets. The driving forces for these morphological changes are the intrinsic material properties governed by the need to minimise the surface free energy. Depending on their initial crystallographic orientation (hkl) and the individual misorientation, very different final morphologies have been observed. Systematic changes in the surface morphology and the associated atomic surface structures can be studied in a very straightforward way on concave-shaped silicon surfaces. These samples contain a wide range of polar misorientation angles and all azimuthal directions. In the present paper, we summarise previous results obtained with scanning tunnelling microscopy on concave silicon samples with an initial (111) and (100) orientation and additional measurements on Si(211) and (322). Some remarks on the more open Si(110) surface will also be given.

An overview is given on scanning tunneling microscopy (STM) studies of the interaction of O-2, NO and NO2 with Si(111)7x7 and Si(100)2x1. The initial stages of adsorption and reaction have been measured in real-time which allows the observation of atomic processes in detail. The results show a large variety of individual effects which in some cases follow characteristic reaction paths. The assignment of the individual adsorption and reaction events to individual atomic effects, i.e., to adsorption of atomic or molecular species on specific adsorption sites is difficult, since the resulting local changes of the electronic structure are not known apriori. For O and O-2 a number of possible adsorption sites have been treated theoretically which may be used for their identification in the STM images. For Si(100)2x1 already from the beginning of the gas exposure different adsorption sites will be occupied. The N containing gases show similar effects as pure O-2 except for the reactivity and the distribution of specific adsorption events. Adsorption at higher temperatures gives rise to macroscopic changes of the surface which appear as Si etching and for longer exposure as oxide film formation. In contrast to results from the literature for Si(lll) no indication for an island growth mode in the initial stage was found. In case of NO on Si(111)7x7 subsequent heating produces ordered Si nitride structures.

Using variable temperature scanning tunneling microscopy vicinal surfaces of Ge(100) have been studied in a temperature range between 80 and 300 K (room temperature). Annealing the sample gives rise to a nonuniform terrace and step distribution. Terraces with dimer rows perpendicular to the step edges cover approximately 3/4 of the entire surface. At room temperature about 1/2 of the surface is covered by buckled dimers. Dimers of straight SA steps are always pinned, i.e., they appear in their buckled form. On top of terraces buckling is also found, associated either with kinks in the step edges or in the formation of a single antisite dimer. At 80 K all dimers appear asymmetric in c(2×4) or p(2×2) geometry. A transition between both structures may occur due to sample–tip interaction which indicates that the energies of both reconstructions are rather similar.

By using scanning tunneling microscopy, low-energy electron diffraction, and Auger electron spectroscopy adsorption and reaction of NO on Si(111)7×7 at 860 K and the effect of subsequent annealing to 1230 K have been studied. The latter results are compared with those obtained on a surface which has been heated to 1260 K during NO exposure. In both cases the previously described ‘‘8×8’’ structure has been obtained. At 860 K etching of Si and formation of oxynitride islands are found at the same time. In scanning tunneling microscopy the 8×8 structure appears as a rather regular array of protrusions in a mean distance of 8/3 aSi. No indications of the previously proposed coincidence lattice structure [A. G. Schrott and S. C. Fain, Jr., Surf. Sci. 123, 204 (1982)] are seen in the scanning tunneling microscopy results.

The surface structure, step- and terrace-width distribution of mechano-chemically prepared, concave-shaped Si(100) substrates, have been determined by scanning tunneling microscopy (STM). The as-prepared surfaces present a continuous range of local polar miscut angles Θ = 0° – 12° and the full range of azimuthal directions. Under UHV treatment the surface breaks up into stepped surfaces with 2× 1 and 1 × 2 reconstructed terraces. Depending on the local miscut angle Θ, three topographically different regions can be distinguished. A double-domain structure with single-layer steps is observed for Θ around 1°. In [001]-like directions scaly arrangements of small terraces with both reconstructions were found. In [011]-like directions and for Θ exceeding 6°, this phase is transformed into a single-domain structure where terraces are connected by double-layer steps. For intermediate Θ, a mixture of SL and DL steps was observed confirming a gradual and not abrupt transition between these phases. Well ordered high-index faces like (911) and (711) were observed for Θ exceeding 9°.

The concave surface of mechanochemically treated Si(111) samples breaks up, under UHV treatment, into a large number of vicinal Si(111) surfaces. The concave surface presents a continuous range of polar angles up to 10° for arbitrary azimuthal direction. Scanning tunneling microscopy (STM) measurements, performed on different parts of the surface, have shown facetting into (111)-oriented, 7 × 7 reconstructed terraces, steps or step bunches. Characteristic step arrangements known from individually prepared vicinal Si(111) surfaces have been identified. On step bunches ordered regions with orientations corresponding to high-index (hkl) values have been observed. One of these could be assigned to a (9 13 7) facet and is decribed in the present work.

We used scanning tunneling microscopy and low-energy electron microscopy to study the etching of the Si(100)-(2 X 1) surface by oxygen at low pressures and elevated temperatures. At 5 X 10(-8) torr partial pressure of oxygen, the transition from random etching of terraces to step etching occurs at approximately 580-degrees-C, while at the higher partial pressure of 5 X 10(-7) torr, the transition temperature is approximately 625-degrees-C. The diffusing species is a dimer vacancy, formed by the desorption of two SiO molecules. Anisotropic diffusion of the vacancies is observed, with a preferred direction along the dimer rows. The reaction probability, defined as the number of desorbed SiO molecules to the number of incident 02 molecules, is estimated to be 0.010+/-0.005.

The morphology and surface structure of mechanochemically treated Si(111) samples, originally prepared for transmission electron microscopy studies, have been determined by scanning tunnelling microscopy (STM). During the preparation method, the Si(111) planes are cut under continuously changing polar angles in all azimuthal directions. The result is a dimple whose concave surface breaks up, under UHV heat treatment, into a large number of vicinal (111) surfaces. STM measurements, performed on different parts of the dimple, show the facetting of this surfaces into (111)-oriented, 7 × 7 reconstructed terraces, steps and step bunches. Characteristic features observed by STM on this type of vicinal substrate correspond to results reported on individually prepared vicinal surfaces.

Versatile Si(111) samples consisting of both, Si bulk material for various surface science studies and thin Si films for transmission electron microscopy (TEM) studies were prepared using a combination of mechanical and chemical treatments. The bulk and surface quality of these samples was controlled by different experimental techniques. In-situ low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM) have been performed under UHV conditions to characterize surface properties of the thick parts as well as of the thin films. Ex-situ X-ray topography and TEM have been used to analyse the quality of the bulk material. The preparation method described below enables us to use the same specimen in its whole, first in surface science experiments, e.g. formation and characterization of a metal/Si interface under UHV, and subsequently in TEM, without further sample preparation in a non-destructive and complementary way.In addition, the samples proved to be an interesting new type of specimen as far as the study of vicinal Si(111) surfaces is concerned. A remarkable morphology was found on the treated rear side. During sample preparation the Si(111) planes are cut under continuously changing polar angles in all azimuthal directions. The result is a dimple consisting of a large number of vicinal (111) surfaces with varying polar and azimuthal misorientations. An UHV heat treatment of these samples leads to the build-up of (111)-oriented terraces separated by step bunches. All terraces are reconstucted in the 7 x 7 periodicity.

The structure of the (113) face of Si has received a large amount of interest recently because of its high stability, despite having a high index. There remains some controversy about the surface reconstruction, various groups having reported observations of both 3 × 1 and 3 × 2 structures. We have obtained atomically resolved STM images of this surface showing areas of 3 × 1 reconstructed surface, the structure of which matches a modified version of a previously proposed model. In addition we have also obtained CITS images showing the complex underlying reconstruction and spectroscopy measurements which are comparable with a recent ARUPS study of the same surface.

Using a fast STM we have recorded constant-current topographies during oxygen adsorption on Si(100)2 X 1 at room temperature. At an oxygen pressure of 2 x 10(-8) mbar the development of the surface was followed up to an exposure of a few tens of langmuirs. Numerous atomic processes were identified. For example, bridges are formed between rows of dimers leading to an antiphase buckling in both rows over a length of 100 angstrom, complete dimers disappeared, protruding features on the scale of a 2 x 1 unit cell appeared atop and between dimers. During adsorption a clustering of the deep positions was obvious. The hitherto unknown complexity of adsorption processes is discussed in terms of the local atomic and electronic structure and of current atomic models of O/Si(100)2 x 1.

After a brief description of the theoretical and experimental basis of scanning tunneling microscopy (STM) typical results from the initial stage of metal condensation on semiconductor surfaces will be presented. As substrates Si(111) and Si(100) are used. Their reconstruction is explained by the current atomic models and compared with STM images of the clean surfaces. While the 7 x 7 reconstruction of Si(111) can be prepared with a defect concentration in the order of 0.03% of a monolayer, Si(100) shows a higher density of imperfections by at least one order of magnitude. STM images from nonreactive systems (Ag/Si(111) and Ag/Si(100)) are described and from a reactive system (Fe/Si(111)), which forms well defined silicides. In case of Ag/Si and room temperature condensation the initial stage is characterized by a two-dimensional interface layer, which does not cover the substrates completely and the growth of rather regular three-dimensional Ag islands, whose shape is consistent with metallic character. After Fe deposition on Si(111) the surface is covered by islands, which due to their irregular size and shape indicate compound formation or intermixing with Si. After annealing Fe/Si(111) shows the development of well ordered Fe silicide layers.