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Atomic force microscopy and photoluminescence spectroscopy were used to study the effects of molecular hydrogen on the structural and optical characteristics of self assembled Ge dots grown on Si(001).

The Si capping of small Ge dots nucleated on a C-precovered Si (001) surface has been investigated by means of reflection high-energy electron diffraction and atomic force microscopy. The growth at 500° C of three Ge monolayers on the C-induced c (4× 4) reconstruction leads to the nucleation of isolated dots, without any wetting layer. Up to a nominal thickness of 5 nm, the surface evolution during the Si cap layer deposition reveals island preservation. Contrary to the Si capping situation of Ge islands on bare Si surfaces (without C seeding), where a surface smoothing is observed after few monolayers, the modified C-induced surface strain promotes a final Si adatom adsorption on the top of the Ge islands. A rough morphology is therefore maintained during the cap layer growth by this selective Si coverage. The general surface smoothing and the in-plane lattice parameter recovery toward Si bulk values are also strongly delayed. [ABSTRACT FROM AUTHOR]

Si surfaces manipulated by a carbon (C) pre-deposition have been used to modify the growth morphology of Ge islands. In situ reflection high-energy electron diffraction and x-ray photoelectron diffraction and ex situ atomic force microscopy studies have been conducted for constant C seeding and varying growth temperatures and Ge coverages, with the aim of deepening the understanding of the relevant Ge quantum dot formation. With temperatures ranging from 400 to 600 °C, well structured Ge islands grow in a Volmer–Weber mode as soon as 0.4 ML of C and 1 ML of Ge are deposited. Strongly modified behaviors are nevertheless observed by changing the Ge growth temperature from 500 to 600 °C. By increasing the Ge coverage from 1 to 6 ML at 500 °C, the island height increases at constant density, whereas, at 600 °C, a strong reduction of the density is observed, with a three-dimensional-two-dimensional transition probably due to a partial Ge intermixing in the Si matrix. These different nucleation schemes are connected with varying evolutions of the initial C-related c(4×4) reconstruction. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Research describing the growth kinetics of germanium deposited on silicon is presented. In particular the characteristics of germanium island density and morphology are investigated.

The Ge growth on SiC(0 0 0 1) follows a Stranski–Krastanov mode for Si-rich (3×3) and (√ of 3×√ of 3)R30° reconstructed surfaces. For Ge deposit in particular temperature conditions, a new (4×4) superstructure takes place and the reflection high energy electron diffraction (RHEED) specular spot intensity presents one oscillation proving a wetting layer formation. An island nucleation is then ascertained by the oscillation vanishing and by the appearance of a k⊥-modulated RHEED pattern. On the other hand, on a (6√ of 3×6√ of 3)R30° C-rich surface, a direct Ge island nucleation is observed from the first growth stage. Indeed, for 1 ML Ge, the RHEED diagram consists in spots and rings, and the atomic force microscopy analysis indicates a high density (∼8×1010 cm-2) of small islands (∅∼30 nm, h∼3 nm). The RHEED spot analysis shows a preferential epitaxial relationship with the substrate Ge(1 1 1)//SiC(0 0 0 1). The Ge–C bonding being energetically unfavourable, Ge tends to form islands immediately rather than wetting the graphite-terminated surface. The Ge growth mode on C-rich surface is thus of Volmer–Weber type. [Copyright &y& Elsevier]

Several cross-sectional transmission electron microscopy images have shed light on the intermixing and island-smoothing mechanisms which occur during Si capping of roughened Si[sub 0.5]Ge[sub 0.5] hut cluster morphologies, strained on Si(001). By modifying either Si cap layer growth rates or temperatures upon identically corrugated alloys, more or less Si and Ge surface migrations are allowed, accordingly affecting the capped final morphology. In these images a Ge marker technique is used to take snapshots of the interfacial region in the first growing stages. They clearly demonstrate that, at 500°C and using low Si growth rates (a few monolayers/min), the segregation does not mainly operate by vertical site exchange but by Ge depletion from the island crests. Ge is able to diffuse laterally onto Si over distances comparable to the island rippling period (a few hundreds of Å) during Si heteregeneous growth in the morphology troughs. The initial island structure, subjected to strain gradients, thus acts as a Ge reservoir for noninterrupted Si surface energy reduction by lateral Ge segregation. The latter operates as long as Si is strain driven to accommodate heterogeneously in the island troughs where it minimizes its elastic energy. This finally results in a vanishing undulated morphology and in a smoothed Si/SiGe interface, at the cost of an alloyed region enlarged by the lateral segregation. As a consequence, the thickness of the smeared region is dominantly determined in this case by the initial hut cluster roughness and not by the usual vertical segregation length. [ABSTRACT FROM AUTHOR]

PtSi/p-strained-Si1-xGex (x=0.19 and x=0.29) Schottky barrier diodes have been fabricated and characterized for the determination of barrier height, ideality factor and the interface state density distribution with energy. Diodes having an epitaxial layer thickness of 20 and 52 nm were modeled with emphasis on comparison with experiment. To achieve better agreement with experimental data, an interfacial layer and associated series resistance were included in the model. The capacitance–voltage (C–V) technique has been used for the determination of the energy distribution of interface state density. The interface state density distribution for the Si0.81Ge0.19 and Si0.71Ge0.29 diodes is found to decrease with increasing energy from the valence band edge. [ABSTRACT FROM AUTHOR]

Hut cluster formation during Ge or Si[sub 1-x]Ge[sub x] solid source molecular beam epitaxial growth on Si(001) is a well-known kinetic pathway for partial strain relief. It results in undulated morphologies with {105} facets allowing a[sub ||] lattice parameter relaxation on the island apexes. Here, we show how subsequent Si coverages, grown at 500 °C, avoid being tensile strained and impede further increase of stored elastic strain energy. Dominant inhomogeneous Si surface diffusions take place as proven by a Ge marker technique able to provide transmission electron microscopy or high-resolution transmission electron microscopy images of the initial Si morphology stages and by reflection high-energy electron diffraction examinations. This mechanism prevails for high enough Si growth rates, able to quench lateral Ge diffusion and limit chemical strain relief. Mediated by stress variations on the noncapped island curvatures, Si is depleted from the top of the islands and accumulates in the troughs of the ripples where it accommodates mostly unstrained. By this selective Si coverage, the surface undergoes a rapid smoothing and a[sub ||] recovery toward the Si bulk value. When the Ge containing islands are completely buried, their strain, dictated by the Si buffer and cap layers, ends by being mainly along the growth direction or tetragonal (Δa[sub ||]=0). © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

With the aim of comparing initial Ge adsorption and desorption modes on different surface terminations of 4H–SiC(0 0 0 1) faces, 3 × 3, √3×√3R30° (R3) and 6√3×6√3R30° (6R3) reconstructions, of decreasing Si surface richness, have been prepared by standard surface preparation procedures. They are controlled by reflection high energy electron diffraction (RHEED), low energy electron diffraction and photoemission. One monolayer of Ge has been deposited similarly at room temperature on each of these three surfaces, followed by the same set of isochronal heatings at increasing temperatures up to complete Ge desorption. At each step of heating, the structural and chemical status of the Ge ad-layer has been probed. Marked differences between the Si- (3 × 3 and R3) and C-rich (6R3) terminations have been obtained. Ge wetting layers are only obtained up to 400 °C on 3 × 3 and R3 surfaces in the form of a 4 × 4 reconstruction. The wetting is more complete on the R3 surface, whose atomic structure is the closest to an ideally Si-terminated 1 × 1 SiC surface. At higher temperatures, the wetting layer stage transiets in Ge polycrystallites followed by the unexpected appearance on the 3 × 3 surface of a more ordered Si island structure. It denotes a Si clustering of the initial Si 3 × 3 excess, induced by the presence of Ge. A phase separation mechanism between Si and Ge prevails therefore over alloying by Ge supply onto a such Si-terminated 3 × 3 surface. Conversely, no wetting is obtained on the 6R3 surface and island formation of exclusively pure Ge takes place already at low temperature. These islands exhibit a better epitaxial relationship characterized by Ge(1 1 1)//SiC(0 0 0 1) and Ge〈1 1 −2〉//SiC〈1 −1 0 0〉, ascertained by a clear RHEED spot pattern. The absence of any Ge–C bond signature in the X-ray photoelectron spectroscopy Ge core lines indicates a dominant island nucleation on heterogeneous regions of the surface denuded by the 6R3 graphite pavings. Owing to the used annealing cycles, the deposited Ge amount desorbs on the three surfaces at differentiated temperatures ranging from 950 to 1200 °C. These differences probably reflect the varying morphologies formed at lower temperature on the different surfaces. Considering all these results, the use of imperfect 6R3 surfaces appears to be suited to promote the formation of pure and coherent Ge islands on SiC. [Copyright &y& Elsevier]

Bismuth compounds are of growing interest with regard to potential applications in catalysis, medicine, and electronics, for which their environmentally benign nature is one of the key factors. One thing that currently hampers the further development of bismuth oxido-based materials, however, is the often low solubility of the precursors, which makes targeted immobilisation on substrates challenging. We present an approach towards the solubilisation of bismuth oxido clusters by introducing an amino carboxylate as a functional group. For this purpose, the bismuth oxido cluster [Bi 38 O 45 (NO 3 ) 20 (dmso) 28 ](NO 3 ) 4 ·4dmso (dmso = dimethyl sulfoxide) was reacted with the sodium salt of tert -butyloxycabonyl (Boc)-protected phenylalanine (L-Phe) to obtain the soluble and chiral nanocluster [Bi 38 O 45 (Boc-Phe-O) 24 (dmso) 9 ]. The exchange of the nitrates by the amino carboxylates was proven by nuclear magnetic resonance, Fourier-transform infrared spectroscopy, as well as elemental analysis and X-ray photoemission spectroscopy. The solubility of the bismuth oxido cluster in a protic as well as an aprotic polar organic solvent and the growth mode of the clusters upon spin, dip, and drop coating on gold surfaces were studied by a variety of microscopy, as well as spectroscopic techniques. In all cases, the bismuth oxido clusters form crystalline agglomerations with size, height, and distribution on the substrate that can be controlled by the choice of the solvent and of the deposition method.

A new germanene crystallographic structure is investigated by scanning tunnelling microscopy and density functional theory calculations. We found that germanene can crystallize in two stable but different structures when grown on Al(111) at the same temperature. These structures are evidenced in scanning tunnelling images by a honeycomb contrast and by a hexagonal contrast. These contrasts are relevant of a Ge network with one (hexagonal) or two (honeycomb) Ge atoms per unit cell shifted upward with respect to the other Ge atoms. These structures appear alternatively and can be turned on and off by a tip-induced process.