John Lundsgaard Hansen

Fibronectin adsorption, cell adhesion, and proliferation on nanostructured tantalum surfaces

The interaction between dental pulp derived mesenchymal stem cells (DP-MSCs) and three different tantalum nanotopographies with and without a fibronectin coating is examined: sputter-coated tantalum surfaces with low surface roughness <0.2 nm, hut-nanostructured surfaces with a height of 2.9 +/- 0.6 nm and a width of 35 +/- 8 nm, and dome structures with a height of 13 +/- 2 nm and a width of 52 +/- 14 nm. Using ellipsometry, the adsorption and the availability of fibronectin cell-binding domains on the tantalum surfaces were examined, as well as cellular attachment, proliferation, and vinculin focal adhesion spot assembly on the respective surfaces. The results showed the highest fibronectin mass uptake on the hut structures, with a slightly higher availability of cell-binding domains and the most pronounced formation of vinculin focal adhesion spots as compared to the other surfaces. The proliferation of DP-MSCs was found to be significantly higher on dome and hut surfaces coated with fibronectin compared to the uncoated flat tantalum surfaces. Consequently, the results presented in this study indicate that fibronectin-coated nanotopographies with a vertical dimension of less than 5 nm influence cell adhesion. This rather interesting behavior is argued to originate from the more available fibronectin cell-binding domains observed on the hut structures.

Nanostructure of Ge deposited on Si(001): a study by XPS peak shape analysis and AFM

Abstract The morphology of Ge overlayers with different thicknesses deposited on Si(001) at 560°C, has been investigated with XPS peak shape analysis and AFM. From the peak shape analysis of the Ge 2p and the Si KLL lines we find island growth for all evaporations and no wetting layer. In the analysis, the structural parameters of the islands (coverage and height) are determined after each evaporation. The AFM images confirm island formation, and the magnitude of the structural parameters are in qualitative agreement with the XPS results. Additionally the amount of evaporated Ge is measured with RBS and compared with the amount determined from the structural parameters of the islands. There is a systematic discrepancy which might be due to inaccuracy in the inelastic mean free path or to forward focusing effects. Our study demonstrates that combining the complementary techniques, XPS peak shape analysis and AFM, gives a much more detailed picture of the surface nanostructure than with any of the techniques alone.

Structural and electrical characteristics of Ge and Se implanted InP after rapid thermal annealing

The electrical activation of Ge and Se implanted into <100≳ InP at elevated temperatures and the annealing of the implantation‐induced disorder have been investigated by differential Hall/resistivity measurements, Rutherford backscattering spectrometry, and transmission electron microscopy. At implantation temperatures below 170u2009°C, an amorphous layer is created by the implantation process. After recrystallization by rapid thermal annealing of the amorphized layer, a localized defect band was found, which gives rise to a dip (M shape) in the carrier density profile. This band is believed to be caused by a stoichiometric imbalance of In and P. For implantation temperatures above 170u2009°C, the samples remain crystalline during implantation and as a result, no localized defect band is observed.

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.

Porosity as a function of stoichiometry and implantation temperature in Ge/Si1−xGex alloys

The development of porosity in single-crystal germanium and silicon-germanium alloys (c-Si1−x Gex) of (100) orientation was studied under bombardment with 140u2009keV Ge− ions over a wide range of temperatures (−180 to 400u2009°C) and ion fluences up to 1u2009×u20091018 ions/cm2. The surface swelling and morphology were investigated using multi-characterization techniques including optical profilometry, transmission electron microscopy, and scanning electron microscopy. The initiation of porosity and the evolution of the near-surface microstructure strongly depend on the ion fluence, the irradiation temperature, and the stoichiometry of the substrate. Significant results and new findings include: (i) the fact that, over the entire temperature and stoichiometry range, porosity is only developed once the substrate is rendered amorphous; (ii) with increasing Si content in the alloy, the onset of porosity is pushed to higher fluences; (iii) porosity is observed for Si contents in the alloy up to 23% but not higher under the ...

Self- and Dopant Diffusion in Extrinsic Boron Doped Isotopically Controlled Silicon Multilayer Structures

Isotopically controlled silicon multilayer structures were used to measure the enhancement of self- and dopant diffusion in extrinsic boron doped silicon. {sup 30}Si was used as a tracer through a multilayer structure of alternating natural Si and enriched {sup 28}Si layers. Low energy, high resolution secondary ion mass spectrometry (SIMS) allowed for simultaneous measurement of self- and dopant diffusion profiles of samples annealed at temperatures between 850 C and 1100 C. A specially designed ion- implanted amorphous Si surface layer was used as a dopant source to suppress excess defects in the multilayer structure, thereby eliminating transient enhanced diffusion (TED) behavior. Self- and dopant diffusion coefficients, diffusion mechanisms, and native defect charge states were determined from computer-aided modeling, based on differential equations describing the diffusion processes. We present a quantitative description of B diffusion enhanced self-diffusion in silicon and conclude that the diffusion of both B and Si is mainly mediated by neutral and singly positively charged self-interstitials under p-type doping. No significant contribution of vacancies to either B or Si diffusion is observed.

Thermal stability of highly Sb‐doped molecular beam epitaxy silicon grown at low temperatures: Structural and electrical characterization

The structural and electrical properties of highly Sb‐doped molecular beam epitaxy grown silicon have been investigated as function of rapid thermal annealing (RTA) temperature. Doping levels of 3×1020 cm−3 were obtained using low temperature epitaxy (LTE) performed at a growth temperature of 300u2009°C. Ion channeling and transmission electron microscopy (TEM) measurements showed that the as‐grown samples were of very high quality. The combination of Hall‐effect profiling and Rutherford backscattering spectroscopy revealed an electrically active Sb fraction of 0.8. Short time RTA processing improved the electron mobility and the activation: RTA at 600u2009°C for 10 s yielded unity activation and RTA at 800u2009°C gave mobilities matching phosphorus doped bulk values, thus significantly exceeding previously reported values for highly doped LTE material. A degradation of the crystalline quality was observed for higher RTA temperatures: RTA at 1000u2009°C for 10 s reduced both the Sb‐substitutional fraction and electrical ...

Light emission from silicon with tin-containing nanocrystals

Tin-containing nanocrystals, embedded in silicon, have been fabricated by growing an epitaxial layer of Si1−x−ySnxCy, where x = 1.6 % and y = 0.04 % on a silicon substrate, followed by annealing at various temperatures ranging from 650u2009∘C to 900u2009∘C. The nanocrystal density and average diameters are determined by scanning transmission-electron microscopy to ≈1017u2009cm−3 and ≈5 nm, respectively. Photoluminescence spectroscopy demonstrates that the light emission is very pronounced for samples annealed at 725u2009∘C, and Rutherford back-scattering spectrometry shows that the nanocrystals are predominantly in the diamond-structured phase at this particular annealing temperature. The origin of the light emission is discussed.

Micro- and macro-structure of implantation-induced disorder in Ge

Abstract The structure of ion implantation-induced damage in Ge substrates has been investigated with a combination of ion- and photon-based techniques including Rutherford backscattering spectrometry (RBS), perturbed angular correlation (PAC) and extended X-ray absorption fine structure (EXAFS) spectroscopy. For MeV Ge ion implantation at −196°C, the dose dependence of the decrease in local atomic order, determined from EXAFS and PAC, was compared to the number of displaced atoms determined from RBS measurements. An EXAFS determined damage fraction was shown to be a better estimate of amorphous fraction than the number of displaced atoms. PAC was used to elucidate the evolution of defective configurations, and was compared to the RBS and EXAFS results. A fit to the Overlap model with the overlap of two ion cascades for complete amorphization best described the experimental results.

Trimodal island distribution of Ge nanodots on (001)Si

Molecular beam epitaxy (MBE) grown Ge nanodots are found to come in a clear trimodal island distribution of huts, pyramids, and domes when grown on (001)Si at 550°C. The island types appear in this order as Ge coverage increases and for a certain coverage all three types are found to coexist at this growth temperature. Previously Ge nanodots have mostly been divided into huts and domes at growth temperatures below 600°C, or pyramids and domes above 600°C. The {105} faceted pyramidal and elongated huts and the multifaceted domes are well known, but a distinction has not previously been seen between huts and a separate size distribution of similarly {105}-faceted pyramidal nanodots twice the size of huts, at temperatures below 600°C. The 20–25nm wide huts also appear to be the smallest obtainable self-assembled Ge dots on (001)Si, in accordance with predictions based on Si1−xGex nanodots on (001)Si. They are about a factor of two too large for quantum dot applications.