Research Results - Microscopic Techniques and Applications
Microscopy: Surface States and Origin of Fermi Level Pinning on Non-polar GaN Surfaces
Group-III nitrides have been shown to demonstrate considerable attraction for green, blue, and ultraviolet laser and light emitting devices. Therefore, intensive efforts have been invested to improve the quality of epitaxial growth. One particular challenge is the position of the Fermi level at the growth surface. For the polar GaN(0001) growth surface used in these experiments, surface states were identified as the origin of the Fermi level pinning. In contrast, for the non-polar GaN surfaces, only little is known about the positions of the surface states and thus their possible influence on Fermi energy. This is due to the lack of experimental data and disagreements between the existing theoretical calculations. This lack of understanding is particularly embarrassing in light of the fact that the growth along non-polar GaN directions is very appealing, due to the absence of electric fields caused by piezoelectricity and spontaneous polarisation. Therefore, we investigated non-polar n-type GaN(1-100) cleavage surfaces using scanning tunnelling microscopy and spectroscopy. We were able to identify that no filled N or empty Ga-derived dangling bond surface states are present within the fundamental band gap. It is found that both the N and Ga-derived intrinsic dangling bond surface states are outside of the fundamental band gap. Their band edges are both located at the gamma point of the surface Brillouin zone. The observed Fermi level pinning 1.0 eV below the conduction band edge is attributed to the high step density, but not to intrinsic surface states.
Microscopy: Advances in Aberration-corrected HRTEM of Solids
With considerable improvements in instrumental resolution well beyond the Ångström barrier accompanied by a simultaneous minimisation of image delocalisation, aberration-corrected HRTEM is presently enjoying increased popularity in the atomic scale imaging of lattice imperfections and heterointerfaces in crystalline solids. The sole availability of ultimate resolution, however, represents a sine qua non for the imaging of object details using exactly the same length scale. Additionally, an unaltered transfer of information through the microscope’s lens system constitutes a mandatory requirement for the direct interpretation of recorded micrographs. Recent experience with PGI-5’s state-of-the-art instruments, however, shows that the time-dependent variation of environmentally-induced parasitic higher-order wave aberrations a_ij together with a system-inherent residual image delocalisation still present in instruments equipped with a spherical aberration-corrector unit, may dramatically deteriorate information transfer and virtually represents the key limitation during experimental analyses at present.
Recent progress in applied spherical aberration-corrected high-resolution transmission electron microscopy performed in troika with the ultra-precise measurement of residual wave aberrations and the numerical retrieval of the exit plane wave function from a series of micrographs is illustrated by highlighting their combined use for the atomic-scale measurement of lattice displacements in conjunction with extrinsic stacking faults in GaAs
Microscopy: Contrast Transfer and Resolution Limits for Sub-ÅNGSTRÖM HRTEM
The optimum imaging of an object structure at the sub-Ångström length scale requires the precise adjustment of the lens aberrations of a high-resolution transmission electron microscope up to the fifth order. A least-squares optimisation of defocus aberration C1 and third-order spherical aberration C3 yields a set of aberration coefficients for strong phase contrast up to the information limit. For instruments with a sub-Ångström information limit, the ultimate structure resolution, the power to resolve adjacent atom columns in a crystalline object, depends on both the instrumental point spread and an object point spread due to the finite width of the atomic column potentials. A simulation study on a simple double-column model yields a range for structure resolutions, dependent on the atomic scattering power, from 0.070 nm down to 0.059 nm, for a hypothetical 300-kV instrument with an information limit of 0.050 nm.
Microscopy: Optical Stability of Ultra-high-resolution Transmission Electron Microscopes
The currently available first generation of sub-Ångström transmission electron microscopes yields high-resolution images with unprecedented quality. At the same time, such microscopes demand the development of new methods of controlling their imaging properties. The smaller the resolved object details are, the stronger the influence of unwanted lens imperfections. Both the measurement and control of such imperfections, known as lens aberrations, represent substantial challenges in acquiring accurate quantitative sub-Ångström measurements. We performed time-stability measurements of lens aberrations with unprecedented accuracy. These measurements support new design concepts for an upcoming generation of high-resolution microscopes characterised by an information limit of about 0.050 nm.
Microscopy: Quantification of the Information Limit of Transmission Electron Microscopes
The resolving power of a high-resolution transmission electron microscope is ultimately limited by the degree of temporal coherence available for the imaging process. A fundamental benchmark parameter, which reflects the effect of the partial temporal coherence, commonly used to characterise the performance of a high-resolution electron microscope, is the information limit which reflects the size of the smallest object detail observable with a particular instrument. We introduce a highly accurate measurement method for the information limit, which is suitable for modern aberration corrected electron microscopes. An experimental comparison with the traditionally applied Young’s-fringe method yields severe discrepancies and confirms the hypothesis that the Young’s-fringe method does not reveal the information limit correctly.
Detailed analysis reveals that complementary information about the resolution limitation of transmission electron microscopes is obtained by the traditional Young’s-fringe method and the new approach presented here. The Young’s-fringe method reveals qualitatively a kind of net resolution limit as a result of a mixing of several accumulating effects related not only to the object,but also to the microscope’s optical transfer properties, the camera, and environmental influences. In contrast, our new approach allows us to isolate the resolution limiting effect caused by the partial temporal coherence and thereby to quantify precisely the information limit according to its theoretical definition
Microscopy: Atomic-scale Study of Domain Walls in Ferroelectric PbZrTiO Films
Ferroelectric thin films have potential applications in electronic and electro-optical devices including, for example., non-volatile and high-density memories, thin-film capacitors, and piezoelectric and pyroelectric devices. The performance of such devices depends strongly on the magnitude and stability of the switchable ferroelectric polarisation of the ferroelectric layer. Polarisation switching is realised by the nucleation and growth of polarisation domains under an external electrical field. The properties of the domain walls, in particular their structure, width and mobility, are important parameters.
Using the negative spherical-aberration imaging technique in an aberration-corrected high-resolution transmission electron microscope, we investigate the cation-oxygen dipoles near 180° domain walls in epitaxial PbZrTiO thin films on the atomic scale. The width and dipole distortion across both a transversal and a longitudinal wall are measured, and on this basis the local polarisation is calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls is observed.
Microscopy: HRTEM Studies of Inorganic Nanotubes and Fullerens
The structural characterisation of nanostructures on the atomic scale is seen as an essential step since electronic properties are strongly related to it. Properties, such as electrical conductivity, rely closely on the interface between different phases or compounds inside the particle, or correlate sensitively with the atomic configuration of the nanoparticle. The characterisation of individual nanostructures is possible today in direct imaging methods such as aberration-corrected high resolution transmission electron microscopy.
The iterative refinement of aberration-corrected transmission electron microscopy images using advanced modelling and image calculation was used to study the atomic structure of inorganic nanotubes and inorganic fullerene-like particles. The atomic arrangement in the nanostructures gives new insights regarding their growth mechanism and physical properties of these nanomaterials, for which imminent commercial applications are unfolding.
Microscopy: Quantitative Aberration-corrected HRTEM of Grain Boundaries in YBaCuO
Structural and electronic reconstructions at interfaces in oxide materials determine their physical properties. They can give rise to fundamentally unique and technologically relevant behaviour on the nanocale. The analysis and engineering of such interfaces demand the use of appropriate techniques in nanoscale characterisation in addition to theoretical understanding.
The latest advances in aberration-corrected high-resolution transmission electron microscopy in quantitative research allow for an accurate determination of the atomic structure of such interfaces. The analysis of the structural reconstruction of
atomic bonds of a 90  grain boundary in YBaCuO demonstrates the new capabilities for the localisation of bond environment changes and their quantification with pm-accuracy and of local disorder and stoichiometry changes on the atomic scale.
Microscopy: Atomic Structure of Beta-phase Tantalum Nanocrystallites
The atomic structure of beta-phase tantalum nanocrystallites has been measured by spherical aberration-corrected high-resolution transmission electron microscopy performed in tandem with the numerical retrieval of the exit-plane wavefunction as obtained from a through-focus series of experimental micrographs. For the first time ever, the existence of grain boundaries of 30° tilt type in beta-phase tantalum was substantiated, whose formation could well be explained by atomic agglomeration processes taking place during sputter deposition.