Flemming Jensen

ON THE NATURE OF CROSS-HATCH PATTERNS ON COMPOSITIONALLY GRADED SI1-XGEX ALLOY LAYERS

The effect of strain relaxation on the surface morphology of compositionally graded Si1−xGex layers grown at 550 °C has been investigated by a combination of transmission electron and atomic force microscopy. By annealing unrelaxed graded layers, we have found that shear displacements caused by dislocation glide roughen the surface dramatically. This effect is attributed to the formation of a network of dislocation clusters which give rise to the pronounced slip‐band pattern on the surface of the graded layers. It is shown that the surface plastic displacements produced by such a network during growth of the graded layer contribute significantly to the formation of the cross‐hatch patterns.

Dislocation patterning — a new tool for spatial manipulation of Ge islands

Abstract The ability of dislocation patterns to control the lateral distribution of islands on the surface of strained heteroepitaxial systems is elucidated. We have studied the behaviour of Ge deposited on a Si surface modified by misfit dislocations generated in buried compositionally graded Si 1− x Ge x layers. A pronounced ordering of the Ge islands on the dislocation-structured substrates is revealed. It is shown that this striking phenomenon is caused by the inhomogeneous surface-strain patterns associated with the misfit dislocations. The results open a new pathway to spatial manipulation of zero-dimensional structures in heteroepitaxial semiconductor systems.

Growth and characterization of compositionally graded, relaxed Si1-xGex

Compositionally graded, relaxed, n-type, Si1-xGex alloy layers have been grown on (100) Si substrates; the main emphasis has been put on compositions with x = 0.25. It is found that for substrate growth-temperatures higher than similar 750°C and a grading rate of 10% Ge/μm relaxed Si0.75Ge0.25 epitaxial layers of high structural, optical, and electrical quality can be grown. The layers are characterized by channeling parameters close to expected bulk values, a threading dislocation density of similar 5 × 105 cm−2, and strong near-band gap luminescence. Electrical measurements have revealed Hall mobilities similar to published bulk values and concentrations of electrically active deep levels ≤2 × 1011 cm−3. The surface morphology is, however, strongly influenced by the grading procedure which produces a high degree of cross-hatching.

Solid-phase synthesis of small molecule libraries using double combinatorial chemistry

Abstract The first synthesis of a combinatorial library using double combinatorial chemistry is presented. Coupling of unprotected Fmoc-tyrosine to the solid support was followed by Mitsunobu O-alkylation. Introduction of a diacid linker yields a system in which the double combinatorial step can be demonstrated. The resulting library of model compounds was verified by LC-MS analysis.

Fabrication of high aspect ratio TiO2 and Al2O3 nanogratings by atomic layer deposition

The authors report on the fabrication of TiO2 and Al2O3 nanostructured gratings with an aspect ratio of up to 50. The gratings were made by a combination of atomic layer deposition (ALD) and dry etch techniques. The workflow included fabrication of a Si template using deep reactive ion etching followed by ALD of TiO2 or Al2O3. Then, the template was etched away using SF6 in an inductively coupled plasma tool, which resulted in the formation of isolated ALD coatings, thereby achieving high aspect ratio grating structures. SF6 plasma removes silicon selectively without any observable influence on TiO2 or Al2O3, thus revealing high selectivity throughout the fabrication. Scanning electron microscopy was used to analyze every fabrication step. Due to nonreleased stress in the ALD coatings, the top parts of the gratings were observed to bend inward as the Si template was removed, thus resulting in a gradual change in the pitch value of the structures. The pitch on top of the gratings is 400 nm, and it graduall...

Dislocation patterning and nanostructure engineering in compositionally graded layer systems

Abstract Dislocation morphologies in compositionally graded Si 1 − x Ge x epilayers grown on (001) Si substrates at low temperatures were studied by a combination of experimental techniques sensitive to the plastic activity at different length scales. We demonstrate that strain relief in the thick metastable layers is provided by ordered dislocation configurations composed of narrow slip bands. These configurations are consistent with the dislocation morphologies predicted by the model of self-adjustment of misfit dislocations. It is shown that crystallographic slip of misfit dislocations in these layers can be used as a basis for a new and straightforward technique for the spatial patterning of semiconductor material on large areas at the nanometer length scale.

Dislocation patterning and nanostructure engineering in compositionally graded Si1−xGex/Si layer systems

Abstract Dislocation morphologies in compositionally graded Si 1 − x Ge x epilayers grown on (001) Si substrates at low temperatures were studied by a combination of experimental techniques sensitive to the plastic activity at different length scales. We demonstrate that strain relief in the thick metastable layers is provided by ordered dislocation configurations composed of narrow slip bands. These configurations are consistent with the dislocation morphologies predicted by the model of self-adjustment of misfit dislocations. It is shown that crystallographic slip of misfit dislocations in these layers can be used as a basis for a new and straightforward technique for the spatial patterning of semiconductor material on large areas at the nanometer length scale.

Mapping strain fields in ultrathin bonded Si wafers by x-ray scattering

X-ray scattering reveals the atomic displacements arising from rotational misalignment in ultrathin silicon bonded wafers. For a 4.3 nm top wafer, the strain field penetrates from the bonded interface to the surface and produces distinctive finite-size oscillations in x-ray data. Analytical calculations permit the atomic displacements throughout the thin top wafer to be modeled.

Silicon waveguides produced by wafer bonding

X-ray waveguides are successfully produced employing standard silicon technology of UV photolithography and wafer bonding. Contrary to theoretical expectations for similar systems even 100μm broad guides of less than 80nm height do not collapse and can be used as one dimensional waveguides to excite single guided modes at typical x-ray energies.

Influence of Ti and Cr Adhesion Layers on Ultrathin Au Films

Efficient adhesion of gold thin films on dielectric or semiconductor substrates is essential in applications and research within plasmonics, metamaterials, 2D materials, and nanoelectronics. As a consequence of the relentless downscaling in nanoscience and technology, the thicknesses of adhesion layer and overlayer have reached tens of nanometers, and it is unclear if our current understanding is sufficient. In this report, we investigated how Cr and Ti adhesion layers influence the nanostructure of 2-20 nm thin Au films by means of high-resolution electron microscopy, complemented with atomic force microscopy and X-ray photoelectron spectroscopy. Pure Au films were compared to Ti/Au and Cr/Au bilayer systems. Both Ti and Cr had a striking impact on grain size and crystal orientation of the Au overlayer, which we interpret as the adhesion layer-enhanced wetting of Au and the formation of chemical bonds between the layers. Ti formed a uniform layer under the Au overlayer. Cr interdiffused with the Au layer forming a Cr-Au alloy. The crystal orientation of the Au layers was mainly [111] for all thin-film systems. The results showed that both adhesion layers were partially oxidized, and oxidation sources were scrutinized and found. A difference in bilayer electrical resistivity between Ti/Au and Cr/Au systems was measured and compared. On the basis of these results, a revised and more detailed adhesion layer model for both Ti/Au and Cr/Au systems was proposed. Finally, the implications of the results were analyzed, and recommendations for the selection of adhesion layers for nano-optics and nanoelectronics applications are presented.