Bernhard Schaffer

Energy-filtering TEM at high magnification: spatial resolution and detection limits.

Energy-filtering TEM (EFTEM) has turned out to be a very efficient and rapid tool for the chemical characterization of a specimen on a nanometer and even subnanometer length scale. Especially, the detection and measurement of very thin layers has become a great application of this technique in many materials science fields, e.g. semiconductors and hard disk technology. There, the reliability of compositional profiles is an important issue. However, the experimentally obtainable spatial resolution strongly influences the appearance of a thin layer in an EFTEM image, when dimensions reach subnanometer levels, which mainly leads to a broadening of the layer in the image. This fact has to be taken into account, when measuring the thickness of such a thin layer. Additionally, the convolution decreases contrast which makes the layer less visible in the image and finally determines the detection limit. In this work we present a systematic study on specifically designed Mn/PdMn multilayer test specimens to explore the practical aspects of spatial resolution and detection limits in EFTEM. Although specific to the ionization edges used, we will present general conclusions about the practical limitations in terms of EFTEM spatial resolution. Additionally, work will be shown about low energy-loss imaging of thin oxide layers, where delocalization is the main factor responsible for broadening.

Monochromated, spatially resolved electron energy-loss spectroscopic measurements of gold nanoparticles in the plasmon range.

Gold nanoparticles show optical properties different from bulk material due to resonance phenomena which depend on local structure and geometry. Electron energy-loss spectrometry (EELS) in scanning transmission electron microscopy (STEM) allows the spatially resolved measurement of these properties at a resolution of few nanometers. In this work, the first monochromated measurements of gold nanoparticles (spheres, rods and triangles) are presented. Due to the improved energy resolution of about 0.2 eV, surface plasmon excitations at energies below 1 eV could be accurately measured from raw experimental data.

Width determination of SiO2-films in Si-based devices using low-loss EFTEM: image contrast as a function of sample thickness.

Energy filtering transmission electron microscopy (EFTEM) has become one of the most efficient tools for specimen characterization at nanometer length scales. EFTEM imaging is most often carried out in the core-loss region but image intensity becomes more and more a limiting factor with decreasing feature size. Alternatively, it is possible to record EFTEM images in the low-loss region, where intensities are essentially higher and where in many cases the images contain material specific contrasts. In this paper we investigate the influence of the important parameters on the material contrast between silicon and silicon dioxide, e.g. specimen thickness, specimen orientation, energy-loss and energy selecting slit width. We show that sample thickness plays an important role and present two methods to calculate material contrast as a function of energy-loss and sample thicknesses. The first method uses spectra taken from both materials at different sample thickness by electron energy-loss spectroscopy, the second calculates contrast directly from a series of energy filtered images. From the results we determine the ideal acquisition parameters for the Si/SiO(2) system and demonstrate imaging at sufficient resolution below 2nm with a test sample of thin SiO(2) layers on Si.

Absence of phase separation in nano-chessboard super-lattices in A-site deficient Ca-stabilized Nd 2 /3TiO 3

A-site deficient perovskites form a class of functional oxides of particular interest because of their attractive properties, such as ionic conductivity [1], dielectric behaviour [2] and transport properties [3]. A number of these ceramics show ‘cross-type’ satellite reflections in their [001] diffraction patterns indicating the presence of a two-dimensional superstructure, that has been attributed to a micro domain model comprising of a system of periodically tilted oxygen octahedra [4]. Intriguingly several such compounds exhibit a peculiar contrast in their [001] High Resolution Transmission Electron Microscopy images (HRTEM), resembling a ‘nano-chessboard’. The origin of this contrast has been the object of debate, with two main models put forward: the first is based on a chemical phase separation into ‘chessboard’ domains [5] and the second one attributes the origin of observed contrast to strain arising from a network of incommensurately titled oxygen octahedra [6].

Interfaces and Extended Structural Defects in Chalcopyrite Thin-Film Solar Cells Studied by Transmission Electron Microscopy

Electronic defects at interfaces between different materials or at extended structural defects such as grain boundaries (GB) or dislocations can deteriorate the performance of a semiconductor device considerably. Cu(In,Ga)Se2 (CIGS) thin-film solar cells consist of several interfaces between individual layers of different materials. In addition, the main light-absorbing layer, the CIGS absorber, exhibits a high density of GBs (i.e., the average grain size is typically smaller than the layer thickness) as well as of dislocations (up to 10 10 cm -2 in high-efficiency solar cells [1]). Still, CIGS solar cells exhibit highest power-conversion efficiencies of up to 20.8 % [2]. Today, the roles of interfaces and extended structural defects in the solar-cell device are not yet fully understood, although extensive research efforts have been made. We applied various methods in transmission electron microscopy (TEM), such as inline electron holography, electron energy-loss spectroscopy (EELS), as well as energy-dispersive X-ray spectroscopy (EDS) in order to obtain more information about these features.