Meiken Falke

Highly phosphorescent organic mixed films: The effect of aggregation on triplet-triplet annihilation

The efficiency roll-off at high brightness levels is a key factor limiting the application of organic light emitting diodes. We investigate triplet-triplet annihilation in an archetype phosphorescent host-guest system. We show that the currently used host-guest systems are not at the physical limit set by intrinsic annihilation, but have an increased roll-off due to aggregate formation. The existence of these aggregates is directly proven by transmission electron microscopy.

An approach to the systematic distortion correction in aberration corrected HAADF images

Systematic distortion has been analysed in high‐angle annular dark‐field (HAADF) images which may be caused by electrical interference. Strain mapping techniques have been applied to a strain‐free GaAs substrate in order to provide a broad analysis of the influence of this distortion on the determination of local strain in the heterostructure. We have developed a methodology for estimating the systematic distortion, and we correct the original images by using an algorithm that removes this systematic distortion.

Real structure of the CoSi2∕Si(001) interface studied by dedicated aberration-corrected scanning transmission electron microscopy

The interface structure of epitaxial cobalt disilicide thin films buried in (001) silicon was studied by dedicated aberration-corrected scanning transmission electron microscopy. Two different CoSi2∕Si interface structures, one representing a (2×1) reconstruction containing sevenfold coordinated Co and the other, a (1×1) structure containing eightfold coordinated Co, were unequivocally identified. The reconstructed sevenfold interface structure was observed more frequently than the (1×1) eightfold interface, which confirms first-principles total-energy calculations. Further, the atomic arrangement found in the eightfold interface reveals an atomic displacement, which is due to relaxation and has been predicted theoretically as well. Complex defect structures at interface domain boundaries are described.

Preparation and properties of MnSi 1.7 on Si(001)

Abstract The most rich silicon silicides of manganese, the group of higher manganese silicides (HMS), have the composition MnSix with x in the range from 1.67 to 1.75. This material group with a tetragonal crystal structure shows semiconducting electronic properties, with promising application in thermoelectric devices. This paper reports the structural and morphological properties of HMS prepared by a reactive deposition process on Si(001) under UHV conditions. X-ray diffraction shows, for all samples grown at substrate temperatures ranging from 400 to 750°C, the growth of HMS only. Scanning electron microscopy and Rutherford backscattering spectrometry show a transition from film growth to island growth with increasing substrate temperature. A detailed analysis of the XRD spectra shows a change of the texture of HMS at the transition of the sample morphology. The results are discussed on the basis of anisotropic growth rates for differently oriented grains of HMS.

Quantitative strain mapping applied to aberration--corrected HAADF images.

A systematic distortion in high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) images, which may be caused by residual electrical interference, has been evaluated. Strain mapping, using the geometric phase methodology, has been applied to images acquired in an aberration-corrected STEM. This allows this distortion to be removed and so quantitative analysis of HAADF-STEM images was enabled. The distortion is quantified by applying this technique to structurally perfect and strain-free material. As an example, the correction is used to analyse an InAs/GaAs dot-in-quantum well heterostructure grown by molecular beam epitaxy. The result is a quantitative measure of internal strain on an atomic scale. The measured internal strain field of the heterostructure can be interpreted as being due to variations of indium concentration in the quantum dot.

Electron microscopic investigation of MnSi 1.7 layers on Si(001)

Semiconducting higher manganese silicides (HMS) with a composition near that of MnSi1.7 are of special interest due to their thermoelectric properties. We report on the growth of HMS layers deposited by MBE using the template technique. In particular the influence of the template thickness on the structure and morphology of MnSi1.7 films on (001)Si substrates was investigated. It was found that there is an optimal template thickness that causes preferred epitaxial growth of the major amount of the silicide. Three different epitaxial orientation relations of the silicide crystals to the substrate were observed. Considering the specific features of the electron diffraction patterns of MnSi1.7 the HMS phase was identified as Mn4Si7.

The growth of an intermediate CoSi phase during the formation of epitaxial CoSi2 by solid phase reaction

Abstract Among the epitaxial transition metal silicides CoSi 2 is of special interest for applications in ULSI technology due to its high thermal stability and low electrical resistivity. The solid phase reaction of metallic bilayers with Si in an N 2 ambient is a well-known process for growing epitaxial CoSi 2 . However, the real function of the so-called diffusion barrier, which is arising during the annealing process, remains unclear. TEM studies revealed the intermediate growth of a CoSi phase with grains of preferred orientation to Si. The growth of this phase raises the temperature of CoSi 2 nucleation, which starts at the CoSi/Si interface. The nucleation temperature determines the quality of the growing epitaxial CoSi 2 , which is better for Ti as a barrier forming material than for Hf, also used successfully. Whether an intermediate oriented growth of CoSi is promoting the epitaxial quality of CoSi 2 otherwise than by shifting the nucleation temperature, remains an open question.

Epitaxial CoSi 2 by solid phase reaction of Co/Ti and Co/Hf bilayers on Si(001)

Abstract TEM and RBS studies of the solid phase reaction of Co/Ti- and Co/Hf/Si(001) layer systems are reported. In addition to conventional annealing procedures a special thermal treatment was applied to investigate the intermediate stages of the reaction during heating up. With rising temperature a complex Co–Si-phase sequence was observed which finally results in the growth of epitaxial CoSi 2 . The emerging silicide phases were proved to grow with preferred orientation relations to the substrate. The observed growth behaviour is compared for both the systems and discussed considering aspects of energy minimization and material supply under the conditions of a thin film reaction.

Surfactant mediated growth of MnSi 1.7 layers on (001) Si

This study reports on transmission electron microscopy investigations of higher manganese silicide (HMS) layers grown by molecular beam epitaxy (MBE) on (001)Si using the surfactant mediated reactive deposition technique. Applying Sb as a surfactant (surface active substances) results in an increased silicide island density as well as in a change in the crystalline orientation of the silicide islands. The silicide islands were found to grow into the Si matrix. The preferential epitaxial relationship was determined to be (100)[010]Mn4Si7 || (001)[100]Si or (010)[??00]Mn4Si7 || (001)[100]Si. In [100]Si cross-sectional view the silicide crystallites of this orientation appear with a smooth Mn4Si7/(010)Si interface and a slightly inclined to (001)Si Mn4Si7/Si interface, which is suggested to split up into two local sections: Mn4Si7/(001)Si and Mn4Si7/(011)Si.

Structure, interface roughness, and growth mechanism of reactive deposition epitaxy of CoSi2 on Si(100) substrates

Thin CoSi2 films have been grown on Si(100) substrates using the relative deposition epitaxy method. The structure of the silicide films have been analyzed using x-ray diffraction, transmission electron microscopy (TEM), and Rutherford backscattering spectrometry and channeling, and the interface roughness of the CoSi2/Si(100) is analyzed using specular x-ray reflectivity and cross-sectional TEM. The structure and interface roughness of CoSi2/Si(100) is found to be dependent on the substrate temperature. Highly epitaxial CoSi2 with minimum interface roughness is obtained when the film is grown at substrate temperatures around 900 K. The observed interface roughness is a parabolic function of temperature. The achievement of the best silicide at a substrate temperature around 900 K is explained on the basis of the instantaneous diffusion of Co through growing CoSi2.