Angelika Hähnel

Formation and structure of reaction layers in Sic/glass and SiC/SiC composites

The structure and composition of interlayers between the fibre and matrix in Nicalon/borosilicate glass and Nicalon/SiC composites depend strongly on their formation mechanism. The applicability of existing models for interface reaction is limited, if the results of electron microscopic characterization are taken into account. The thermodynamic fundamentals illustrated by a volatility diagram of the ES-C-0 system reveal that variations in the pressures of 02, SiO and CO, respectively, are responsible for the occurrence of various phases (carbon, carbon and silica, silica) between the fibre and matrix. The thermodynamic approach yields a four-stage model of layer formation including the active oxidation of Sic.

UV-Raman imaging of the in-plane strain in single ultrathin strained silicon-on-insulator patterned structure

Confocal UV-Raman with glycerin-immersed high numerical aperture objective lens was used to probe the local strain in individual strained Si structures. The investigated structures were fabricated from 15 nm thick strained silicon-on-insulator substrates with a tensile strain of 0.8%. Two-dimensional maps of the postpatterning strain were obtained for single structures with lateral dimension of 500 nm. We found that the strain measured at the center partially relaxes and drops to 0.67% as a result of patterning-induced free surfaces. This relaxation increases toward the edges following nearly a parabolic behavior. A different strain behavior was observed for larger structures.

Nanostructure of interlayers in different Nicalon fibre/glass matrix composites and their effect on mechanical properties

Interlayer phenomena, revealed by high‐voltage electron microscopy (HVEM) and high‐resolution electron microscopy (HREM), are presented as they occur in various SiC(Nicalon) fibre‐reinforced Duran glass composites (differing in the specific sol‐gel supported production processes). Their dependence on the production parameters and their influence on the materials properties are discussed, taking into account the results of scanning electron microscope (SEM) in situ tensile tests.

Nanoscale patterning induced strain redistribution in ultrathin strained Si layers on oxide

We present a comparative study of the influence of the thickness on the strain behavior upon nanoscale patterning of ultrathin strained Si layers directly on oxide. The strained layers were grown on a SiGe virtual substrate and transferred onto a SiO(2)/Si substrate using wafer bonding and hydrogen ion induced exfoliation. The post-patterning strain was evaluated using UV micro-Raman spectroscopy for thin (20 nm) and thick (60 nm) nanostructures with lateral dimensions in the range of 80-400 nm. We found that about 40-50% of the initial strain is maintained in the 20 nm thick nanostructures, whereas this fraction drops significantly to approximately 2-20% for the 60 nm thick ones. This phenomenon of free surface induced relaxation is described using detailed three-dimensional finite element simulations. The simulated strain 3D maps confirm the limited relaxation in thin nanostructures. This result has direct implications for the fabrication and manipulation of strained Si nanodevices.

Recombination at Lomer Dislocations in Multicrystalline Silicon for Solar Cells

Lomer dislocations at small-angle grain boundaries in multicrystalline silicon solar cells have been identified as responsible for the dominating inherent dark current losses. Resulting efficiency losses have been quantified by dark lock-in thermography to be locally up to several percent absolute, reducing the maximum power of the cells. By electron beam induced current measurements and scanning transmission electron microscopy investigations, it is revealed that the strengths of the dark current losses depend on the density of Lomer dislocations at the small-angle grain boundaries.

Multiwavelength micro-Raman analysis of strain in nanopatterned ultrathin strained silicon-on-insulator

We developed a heterostructure to assess accurately the strain evolution upon nanopatterning of 15 nm thick tensile strained silicon-on-insulator (SSOI). Here the long-standing concern of substrate background in micro-Raman analysis was circumvented by the introduction of a Ge layer underneath the buried oxide. Unprecedented insights into the strain behavior in SSOI nanostructures were obtained by combining deep UV and visible micro-Raman probes. We found that the formation of edges results in a strong relaxation near the surface parallel to an increase in the strain at the Si/oxide interface. This disparity in the strain evolution between surface and interface leads to the coexistence of compressive and tensile strained regions within the same structure at a lateral dimension of 50 nm. This heterogeneous distribution of strain should be taken into account in the design and fabrication of SSOI-based nanodevices.

Structure and properties of dislocations in interfaces of bonded silicon wafers

The realization of defined dislocation networks by hydrophobic wafer bonding allows the characterization of electrical properties of individual dislocations. The present paper describes the fabrication and characterization of SOI MOFSETs with various dislocations densities in the Si channel. The aim was to investigate the electrical properties in samples containing only 6 dislocations. A drain current of ID > 1×10−2 A induced by a single dislocation was determined by data extrapolation from current measurements in combination with previously analyzed samples containing a varying dislocation density.

Reaction layers in MMCs and CMCs: structure, composition and mechanical properties

Abstract In fibre-reinforced composites, reactions occurring at the fibre-matrix interface and affecting the structure and composition of the interfacial region are studied to control the mechanical properties of the composites. For both metal matrix composites (MMCs) and ceramic matrix composites (CMCs) the interfacial reactions were investigated in different types of materials with different mechanical behaviour by microstructural (high-voltage and high-resolution electron microscopy, HVEM and HREM) and microchemical (energy-dispersive X-ray spectroscqpy and electron energy loss spectroscopy, EDXS and EELS) studies. Their effect on the mechanical properties was studied by corresponding scanning electron microscope in situ deformation tests.

Nanoprocesses of the Formation of Reaction Layers inSi‐C‐O Systems

The microstructure and nanochemistry of heat treated sandwich structures consisting of borosilicate glass and 6H-SiC single crystals have been studied using high resolution and analytical TEM. In the interface region a complex interlayer system was found, which mainly contains carbon (near the glass) and silica (near the SiC). A distinct difference in thickness of this reaction interlayer was observed between Si- and C-terminated 6H-SiC, which is attributed to different rates of the corresponding passive oxidation reactions at the interface. In general, the reaction kinetics can be described by a four step model, and is influenced by the formation of metastable silicon oxycarbide at the interfaces, as verified by processing the ELNES of the Si-L23 edge recorded with nanometre resolution.

Improving accuracy and precision of strain analysis by energy-filtered nanobeam electron diffraction.

This article deals with uncertainty in the analysis of strain in silicon nanoscale structures and devices using nanobeam electron diffraction (NBED). Specimen and instrument related errors and instabilities and their effects on NBED analysis are addressed using a nanopatterned ultrathin strained silicon layer directly on oxide as a model system. We demonstrate that zero-loss filtering significantly improves the NBED precision by decreasing the diffuse background in the diffraction patterns. To minimize the systematic deviations the acquired data were verified through a reliability test and then calibrated. Furthermore, the effect of strain relaxation by specimen preparation using a FIB is estimated by comparing profiles, which were acquired by analyzing slices of strained structures in a 220-nm-thick region of the sample (invasive preparation) and the entire strained nanostructures, which are embedded in a thicker region of the same sample (noninvasive preparation). Together with the random deviation, the corresponding systematic shift results in a total deviation of ∼1 × 10(-3) for NBED analyses, which is employed to estimate the measurement uncertainty in the thinner sample region. In contrast, the strain in the thick sample region is not affected by the preparation; the systematic shift reduces to a minimum, which improves the total deviation by ∼50%.