Time-resolved observation of CVD-growth of silicon on Si(111) with STM

Scanning tunneling microscopy is used to study the epitaxial growth of silicon on Si(111)-(7×7) by Chemical Vapour Deposition (CVD) of disilane (Si₂H₆) at elevated substrate temperatures directly during the growth process. Different kinetic processes, as island nucleation, growth and coarsening and step flow have directly been imaged as a function of temperature and Si₂H₆ flow. On a substrate with a low defect concentration several growth modes depending on the flux and the total coverage are distinguished: the formation of multi-level islands as a transient mode leaving the substrate partially uncovered up to 20 bilayers, a transition to layer-by-layer growth when the multi-level islands initially formed coalesce and the formation of three-dimensional islands with tetrahedral shape at higher growth rates which are only metastable due to the presence of hydrogen at the surface. The equilibrium shape of two-dimensional islands is a hexagon whereas the kinetically influenced shape during growth is triangular.     

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Monolayers of the organic molecules perylene-3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and diimide (PTCDI) on graphite and MoS₂ have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations.     

Surfactant-mediated molecular beam epitaxy of high-quality (111)B-GaAs

We present a novel approach to the molecular beam epitaxy of [111]-oriented GaAs. Surface-segregating In employed as an isoelectronic surfactant allows us to achieve mirror-like (111) GaAs surfaces within a wide range of growth conditions. Scanning electron and atomic force microscopy confirm the excellent morphology of the resulting samples. High-resolution X-ray diffraction shows the incorporation of In into the films to be negligible. Finally, we demonstrate a 10 Å-In_0.2Ga_0.8As/300 Å-GaAs superlattice based on surfactant-grown GaAs with a photoluminescence linewidth as narrow as 4.2 meV.     

STM investigation of the adsorption and temperature dependent reactions of ethylene on Pt(111)

The adsorption and reactions of ethylene adsorbed in UHV on Pt(111) have been studied as a function of temperature by STM. The STM images taken at 160K show an ordered structure of adsorbed ethylene. Annealing to 300 K produces ethylidyne (C-CH₃) irreversibly, as has been demonstrated by a wide variety of surface science techniques. The ethylidyne on Pt(111) is not visible to the STM at room temperature. Cooling the sample allows direct observation of the ethylidyne ordered structure by STM. Annealing above 430 K results in further dehydrogenation, eventually leaving only carbon on the surface. The decomposition products appear as small clusters which are localized and uniformly distributed over the surface. Further annealing to temperatures >800 K results in the growth of graphite islands on the Pt(111) surface. The annealed graphite islands exhibit several supersturctures with lattice parameters of up to 22 Å, which are thought to result from the higher order commensurability with the Pt(111) substrate at different relative rotations.     

Surfactants: Perfect heteroepitaxy of Ge on Si(111)

The hetero growth of Ge on Si results in formation of 3D clusters with an uncontrolled defect structure. Introduction of a monolayer of a surfactant completely changes the growth mode to a 2D-layer growth (Frankvan der Merwe) with a continuous and smooth Ge film on Si(111). The surfactant is not incorporated but segregates and floates on the growing Ge film. The saturation of the dangling bonds of the semiconductor reduces the surface free energy and drives the strong segregation. The effect on the growth process is the selective change of activation energies which are important for diffusion and the mobility of the Ge. Up to a thickness of 8 MLs (MonoLayers) the misfit-related strain of the pseudomorphic Ge film is relaxed by formation of a micro rough surface. This allows a partial relaxation of the Ge towards its bulk lattice constant which would not be possible for a flat and continuous film. For thicker Ge films the misfit of 4.2% is relieved by a periodic dislocation network, which is confined to the Si-Ge interface. Ge-films thicker than 20 MLs are free of defects and completely relaxed to the Ge bulk lattice constant: a model system for perfect heteroepitaxial growth.     

Direct observation of strain relaxation in iron layers on W(110) by time-resolved STM

Time-resolved,in-situ-applied STM has been used to study the epitaxial growth of iron on W(110) at room temperature. By this way, sequences of STM images show directly the atomistics of the growth process on the surface. The first layer of iron on W(110) grows pseudomorphically without a preferred growth direction. Beginning with the second layer, the islands grow anisotropically with preferred growth in the [001]-direction. The generation of an ordered two-dimensional dislocation network starts at a coverage of 1.4 pseudomorphic monolayers to relax the misfit of 9.4%. A direct correlation of the creation of misfit dislocations in the second layer and the nucleation of the third-layer islands was found. Together with the onset of strain relaxation, the growth mode abruptly changes from layer-by-layer to statistical growth. A quantitative statistical analysis of the data allows to exactly determine the onset of relaxation, the vertical location of the dislocation lines, and the lateral extension of an island that is necessary to induce the formation of dislocations.     

Dependence of glow discharge Si:H:Cl film morphology on diluent gas and SiCl4 partial pressure

TEM, THEED and field-assisted silver ion exchange have been employed to study the glow discharge (GD) SiHCl films deposited from SiH₄-SiCl₄ gas mixtures. The character of the THEED patterns shows that the films are rather amorphous, and that their structure does not alter with the change in the gas mixture. The honeycomb-like morphology of the films is strongly affected by the type of gas in which SiH₄ is diluted (Ar or H₂). An increase in the SiCl₄ partial pressure leads to the uniformity and to the decrease of the island dimension only for the films deposited from SiH₄(H₂)-SiCl₄. A possible correlation between the film morphology and the micropore density is proposed.     

Atomic scale characterization of oxygen adsorbates on Al(111) by scanning tunneling microscopy

We present atomic scale STM pictures of clean and oxygen containing Al(111) surfaces. Little influence of the surface oxygen on the topography of the surfaces is found. Three different oxygen species can be distinguished. One of them is associated with adsorbed oxygen and found to grow in small islands upon adsorption at 300 K. Characteristic hexagonal nuclei, created upon annealing of a dilute oxygen adlayer, represent the second one. By comparison with existing spectroscopic data these are assigned to nuclei of a surface oxide.     

Nano-scale metal cluster deposition using STM

Received: 27 March 1998     

A scanning tunneling microscopy study of the adsorption of Xe on Pt(111) up to one monolayer

The adsorption of Xe on Pt(111) has been investigated from the arrival of the very first atoms up to completion of the monolayer using a variable-temperature Scanning Tunneling Microscope (STM). Surprisingly, in the initial stages of the adsorption Xe preferentially binds to a low coordination site, theupper edge of the platinum steps. The strong binding to these sites leads to a local repulsive interaction with further Xe atoms. Therefore, the Xe atoms located at the upper edge of the steps do not serve as nuclei for 2D Xe islands, which, instead, form on the terraces and at thelower edges of the platinum steps. Only during completion of the monolayer do these islands make contact with the atoms adsorbed at the beginning in the upper-edge positions. The full monolayer exhibits the Hexagonal Incommensurate Rotated (HIR) phase already known from earlier helium-diffraction experiments.     

Trench formation in surfactant mediated epitaxial film growth of Ge on Si(100)

We have used STM to study the surface morphology of thin epitaxial Ge films grown on Si(001) in the presence of the surfactant As. The surfactant forces layer-by-layer growth up to 12 ML Ge coverage which could partly be explained by the geometrical surface arrangement of the growing film. Beyond 12 ML coverage we observed a network of trenches which decorate the earlier described V-shaped defects inside the film. Overgrowth of such defects is studied and a mechanism discussed.     

Laser-induced oxidation of the Si(111) surface

Pulsed laser induced oxidation of clean Si(111) surfaces has been studied by Auger electron spectroscopy and electron energy loss spectroscopy. The short duration time of the pulse has allowed a precise investigation of the first stages of the oxidation. About 1–2 oxide monolayers first grow in less than 10 s. Their stoichiometry evolves from SiO_x towards SiO₂ with increasing beam energy densities. Once this superficial layer has formed, no evolution is seen with further irradiation, suggesting that oxygen diffusion during the pulse duration cannot sustain the oxide growth.     

Studies of interface structures of W films grown on patterned and bare Si(100) by TEM

+ Si(100) and bare Si(100) wafers by low pressure chemical vapour deposition (LPCVD) at 230–280 °C. The films were investigated by transmission electron microscopy (TEM). The cross-sectional TEM samples of W/Si(100) exhibited a fine scale interface roughness, which was attributed to the surface preparation. Irregular W plug structures were observed depending on the predeposition procedures. It was observed that an insufficient deposition of W films on the contact surface leads to the presence of aluminium around and underneath the plugs. This was observed by energy dispersive X-ray spectrometry (EDX). A study, using conventional electron diffraction, confirmed that no silicides formed at the interfaces of W-bare Si(l00) wafers. Received: 16 December 1996/Accepted: 6 May 1997     

Scanning force microscopy on the Si(111)7×7 surface reconstruction

Scanning force microscopy images of the Si(111)7×7 surface reconstruction are presented which are taken in the contact mode in ultrahigh vacuum. Topographic and lateral force data are acquired simultaneously. A special treatment of the sensing tip with PTFE helps to overcome the strong adhesion and wear effects that normally occur on this particular surface.     

Investigation of heteroepitaxial diamond films by atomic force and scanning tunneling microscopy

We report on Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) investigations on chemical vapour deposited heteroepitaxial diamond films. Besides the good macroscopic crystal morphology a statistical tilt up to ±5.2° of the oriented crystallites has been found relative to the silicon substrates. By optimizing the process conditions, however, the crystal tilt of the films can be reduced, resulting in an improved film perfection. On crystallite (001)-surfaces a substructure of growth facets or islands has been found and high resolution STM images have established a 2×1 surface reconstruction on these growth facets. AFM and SEM were applied to study the morphology of diamond nuclei initially grown on the silicon substrate. Strong island like (Volmer-Weber) growth has been found, with a nucleus height to diameter ratio of 1:1. While the islands are growing in size with respect to time of nucleation, its aspect ratio does not change, due to the high surface free energy of the diamond relative to silicon.     

Prevention of micropipes and voids at β-SiC/Si(100) interfaces

Received: 23 April 1997/Accepted: 30 July 1997     

Transfer of Au Cusp-clusters on Si(111) 7×7 surfaces from a pure Au tip of a scanning tunneling microscope

Received: 16 August 1996/Accepted: 6 March 1997     

Hydrogen on Si: Ubiquitous surface termination after wet-chemical processing

The manufacture of microelectronic devices based on silicon technology is largely dominated by wet chemical processes. By ultraclean sample preparation in air and a fast transfer into UltraHigh Vacuum (UHV) we open up a way for the atomic-scale structural and chemical characterization of silicon surfaces immediately after wet-chemical processing. Using Scanning Tunneling Microscopy (STM), ThermoDesorption (TDS) and InfraRed Spectroscopy (IRS), we find that a surface termination predominantly by hydrogen results from all the different wet-chemical treatments investigated (etching with hydrofluoric acid, rinsing with hot water, chemomechanical polishing)-despite the different chemical ambients and process parameters involved. Microscopically, a crystallographically preferential attack of the silicon is observed in all these processes which results, to a different extent, in anisotropic defect structures on the surfaces. This is explained by an interplay of aqueous reaction kinetics and sterical hindrance on the silicon surface. It is pointed out how a UHV surface analysis of the micromorphology of wet-chemically treated silicon surfaces, so far carried out mostly on Si(111) due to its easier preparation and experimental accessability, may help to provide the in-depth understanding of the atomic-scale mechanisms during wet-chemical processing demanded by the progressing miniaturization of microelectronic devices. The atomically smoother and chemically more homogeneous Si(111) obtained after preferential etching with NH₄F suggests that in future applications Si(111) may gain importance over Si(100), which still dominates in today's semiconductor technology, since future devices increasingly rely on tailor-made and ideal properties on an atomic scale. Due to their structural and chemical simplicity and well-controlable characteristics, H-teminated surfaces after wet-chemical preparation also form ideal substrates for conventional UHV surface studies such as absorption and MBE-growth experiments.     

Microstructure of Epitaxial Er2O3 Thin Film on Oxidized Si (111) Substrate

Er₂O₃ thin films are grown on oxidized Si (111) substrates by molecular beam epitaxy. The sample grown under optimized condition is characterized in its microstructure, surface morphology and thickness using grazing incidence x-ray diffraction (GIXRD), atomic force morphology and x-ray reflectivity. GIXRD measurements reveal that the Er₂O₃ thin film is a mosaic of single-crystaldomains. The interplanar spacing d in-plane residual strain tensor ε_| and the strain relaxation degree ξ are calculated. The Poisson ratio μ obtained by conventional x-ray diffraction is in good agreement with that of the bulkEr₂O₃. In-plane strains in three sets of planes, i.e. (440), (404), and (044), are isotropic.