RESEARCH RESULTS - MICROSCOPIC TECHNIQUES AND APPLICATIONS
Microscopy: Surface states and origin of fermi level pinning on non-polar GaN surfaces
Group-III nitrides raised considerable attraction for green, blue, and ultraviolet laser and light emitting devices. Therefore, intensive efforts have been invested to improve the quality of the epitaxial growth. One particular challenge is the position of the Fermi level at the growth surface. For the presently used polar GaN(0001) growth surface, surface states were identified as 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 the 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 the light that the growth along nonpolar GaN directions is very appealing, due to the absence of electric fields caused by piezoelectricity and spontaneous polarization. Therefore, we investigated non-polar n-type GaN(1-100) cleavage surfaces by scanning tunnelling microscopy and spectroscopy. We 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, also 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, only represents a sine qua non for the imaging of object details of the very same length scale. Additionally, an unaltered transfer of information through the microscope’s lens system constitutes a mandatory requirement in the direct interpretability of recorded micrographs. Recent experience with theIFF-8's state-of-the-art instruments, however, shows that the time-dependent variation of environmental-induced parasitic higher-order wave aberrations aij together with a system-inherent residual image delocalisation still present at instruments equipped with 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 wavefunction from through focus series of micrographs is illustrated by highlighting their combined use for the atomic-scale measurement of lattice displacements coming along 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 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 sub-Ångström information limit the ultimate structure resolution, the power to resolve adjacent atom columns in a crystalline object, depends on both the instrumental pointspread and an object pointspread due to 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 presently available first generation of sub-Ångström transmission electron microscopes yields high-resolution images with unprecedented quality. At the same time such microscopes impose new efforts in controlling their imaging properties. The smaller the resolved object details are, the stronger becomes the influence of unwanted lens imperfections. Both, measurement and control of such imperfections, which are called lens aberrations, represent substantial challenges on the way to quantitative sub-Ångström work. 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, and which is 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 theoretical considerations according to which the Young’s-fringe method does not reveal the information limit.
Detailed analysis revels 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 to the object, to the microscope’s optical transfer properties, to the camera, and to 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 find potential applications in electronic and electro-optical devices including, e.g., 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 polarization of the ferroelectric layer. Polarisation switching is realised by 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 a transversal wall and a longitudinal wall are measured, and on this basis the local polarisation has been 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 characterisation of nanostructures down to the atomic scale becomes essential since physical properties are strongly related with it. Properties, like electrical conductivity, depend closely upon 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 like aberration-corrected high resolution transmission electron microscopy.
Iterative refinement of aberration-corrected transmission electron microscopy images with 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 a nanocale. Analysis and engineering of such interfaces demand for appropriate techniques in nanoscale characterisation besides theoretical understanding. The latest
advances in aberration-corrected high-resolution transmission electron microscopy towards 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 be well explained by atomic agglomeration processes taking place during sputter deposition.