S. Christiansen

Strained state of Ge(Si) islands on Si: Finite element calculations and comparison to convergent beam electron‐diffraction measurements

In this letter we present calculations by three‐dimensional finite element method and measurements by convergent beam electron diffraction of the displacement field resulting from misfitting Ge0.85Si0.25 islands on Si(001). A good agreement between the results of both methods indicates that the three‐dimensional finite element method is a reliable tool to calculate the strain, and thus the stress field, in such nanostructures. As a result both methods show that the substrate substantially takes part in the elastic relaxation process in such heteroepitaxial systems.

Phonons as probes in self-organized SiGe islands

We show how optical phonons can be used as efficient probes in self-organized Si1−xGex islands grown on Si(001). Both the alloy composition and residual strain in the islands were originally determined from the phonon frequencies and Raman intensities. The experimental results are in good agreement with the strain relaxation simulated by means of the finite element method.

Pinholes, Dislocations and Strain Relaxation in InGaN

We analyse by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) the strain relaxation mechanisms in InGaN layers on GaN as dependent on the In content. At the experimentally given thickness of 100 nm, the layers remain coherently strained, up to an In concentration of 14 %. We show that part of the strain is reduced elastically by formation of hexagonally facetted pinholes. First misfit dislocations are observed to form at pinholes that reach the InGaN/GaN interface. We discuss these results in the framework of the Matthews-Blakeslee model for the critical thickness considering the Peierls force for glide of threading dislocations in the different slip systems of the wurtzite lattice.

Surface ripples, crosshatch pattern, and dislocation formation : Cooperating mechanisms in lattice mismatch relaxation

We study the interplay of elastic and plastic strain relaxation of SiGe/Si(001). We show that the formation of crosshatch patterns is the result of a strain relaxation process that essentially consists of four subsequent stages: (i) elastic strain relaxation by surface ripple formation; (ii) nucleation of dislocations at the rim of the substrate followed by dislocation glide and deposition of a misfit dislocation at the interface; (iii) a locally enhanced growth rate at the strain relaxed surface above the misfit dislocations that results in ridge formation. These ridges then form a crosshatch pattern that relax strain elastically. (iv) Preferred nucleation and multiplication of dislocations in the troughs of the crosshatch pattern due to strain concentration. The preferred formation of dislocations again results in locally enhanced growth rates in the trough and thus leads to smoothing of the growth surface.

Reduced effective misfit in laterally limited structures such as epitaxial islands

Numerical finite element calculations have been reported to determine a correction function Φ that describes the reduction of the misfit that occurs when laterally limited structures such as faceted islands or mesa structures are grown on a substrate. The reduction of the average strain energy density is calculated in these three‐dimensional islands and compared to the constant strain energy density in a continuous layer. Ratios Φ are obtained from the calculation of different island geometries, i.e., different facet angles γ and different aspect ratios island width l to island height h. These discrete values are fitted by a function which can easily be applied to the full range of aspect ratios (l/h≳0) and facet angles (0°<γ<90°). Faceted Ge(Si) islands on Si(001) substrate, grown from the solution in the Stranski–Krastanov growth mode, serve as an example for the calculation. Experimental and theoretical values for the critical thickness of these islands agree well. This result demonstrates the drastic ...

Composition of self-assembled Ge/Si islands in single and multiple layers

The degree of Si alloying in vertically aligned self-assembled Ge islands increases with the number of stacked layers. We find that the Si–Ge interdiffusion coefficient increases by more than two orders of magnitude for stacked hut clusters. Furthermore, we determine the composition profiles through the center of dome-shaped islands, capped with Si. These profiles exhibit a plateau near the base and a Ge enrichment near the apex of the islands. In this case, too, the upper dome island experiences a state of increased alloying with Si.

Evolution and properties of nanodiamond films deposited by direct current glow discharge

Nanocrystalline carbon films possessing a prevailing diamond character are deposited by a direct current glow discharge chemical vapor deposition method using a 9:91 volu200a% methane to hydrogen gas mixture. In the present work the evolution and properties of nanodiamond films deposited by this method onto silicon substrates as a function of time were studied by various complementary techniques. Our analysis showed that prior to formation and growth of continuous films of a predominantly nanodiamond character, a graphitic phase is formed. After the nanodiamond phase is stabilized, near edge x-ray adsorption fine structure measurements proved the predominant diamond character of the film to be about 80%. By electron energy loss spectroscopy analysis the sp2-like character of the nanodiamond grain boundaries has been determined. The nanodiamond films were found to be thermally stable up to temperatures of ∼950u200a°C as established by vacuum heating. By scanning electron microscopy and atomic force microscopy the ...

Nature of grain boundaries in laser crystallized polycrystalline silicon thin films

The grain boundary populations in laser crystallized polycrystalline silicon thin films are determined by electron microscope analysis, using electron backscattering contrast in the scanning electron microscope, and convergent beam electron diffraction in the transmission electron microscope. The grain boundary populations of the grains larger than 0.5 μm are dominated by first and second order twin boundaries. This result is found to be a general feature of laser crystallization independent of the experimental details of the laser crystallization process. Texture analysis of the laser crystallized poly-Si films shows that under certain experimental conditions a {111}-preferential orientation of the grains perpendicular to the substrate can be obtained.

Microstructure of laser-crystallized silicon thin films on glass substrate

We investigate the microstructure of polycrystalline silicon films (grain size, orientation distribution, and grain boundary population). These films are produced by laser crystallization of amorphous silicon on glass substrates by a frequency doubled Nd:YVO4 laser operating at a wavelength of 532 nm. Transmission electron microscopy reveals that the grains have an average width between 0.25 and 5 μm depending on the crystallization parameters and a length of several 10 μm. Electron backscattering diffraction experiments show that the grain orientation of the poly-Si films is textured. Type and extent of texture depend in a complex way on the thickness of the crystallized amorphous silicon layer, on the repetition rate of the laser pulses, and on whether or not an additional buffer layer is present on the glass substrate. In any case, the grain boundary population is dominated by first and second order twin boundaries.

Strain and composition in self-assembled SiGe islands by Raman spectroscopy

We have investigated self-assembled Si1−xGex islands grown on Si (001). We show that the average composition and both the exx and ezz average strain components can be derived from Raman scattering spectra. Both nm-sized and μm-sized islands are investigated. The experimental results are compared successfully with finite-element strain simulations. Raman scattering is shown to be a versatile and reliable tool for investigating capped and uncapped islands. It is shown that strain profiles in μm-sized islands can be obtained by means of micro-Raman.