Y Peng

Spatial resolution and information transfer in scanning transmission electron microscopy.

The relation between image resolution and information transfer is explored. It is shown that the existence of higher frequency transfer in the image is just a necessary but not sufficient condition for the achievement of higher resolution. Adopting a two-point resolution criterion, we suggest that a 10% contrast level between two features in an image should be used as a practical definition of resolution. In the context of scanning transmission electron microscopy, it is shown that the channeling effect does not have a direct connection with image resolution because sharp channeling peaks do not move with the scanning probe. Through a quantitative comparison between experimental image and simulation, a Fourier-space approach is proposed to estimate defocus and sample thickness. The effective atom size in Z-contrast imaging depends on the annular detectors inner angle. Therefore, an optimum angle exists for the highest resolution as a trade-off between reduced atom size and reduced signal with limited information transfer due to noise.

Chapter 2:Scanning Transmission Electron Microscopy

The Scanning Transmission Electron Microscope1(STEM) is one of the most useful tools in many areas of atomic-scale materials science and nanocharacterisation. A STEM has the ability to generate local maps of the chemical composition and electronic structure at atomic resolution, even in complex or u...

Transmission Electron Microscopy: Overview and Challenges

We review recent advances in aberration‐corrected scanning transmission electron microscopy that allow sub‐Angstrom beams to be used for imaging and spectroscopy, with enormous improvement in sensitivity for single atom detection and the investigation of interfacial electronic structure. Comparison is made between the electronic and structural width of gate oxides, with interpretation through first‐principles theory. Future developments are discussed.

Ultimate Resolution Limit for Z–contrast STEM: Atoms are Smaller in ADF

Achieving higher resolution has been a longstanding goal for electron microscopy. Steady instrumental improvements have enabled experimental observations down to the atomic scale and even to the subÅngstrom level. For example, with an aberration-corrected VG Microscope’s 300 kV HB603U STEM, pairs of Si columns 0.78 Å apart in Si [112] (Fig. 1a) have been directly resolved at ORNL [1]. Comparison to image simulations allows one to extract the sample thickness, probe defocus and source size using a linear regression technique [2] (Fig. 1d). There is excellent agreement between the simulated and experimental image profile (Fig. 1c), which has a dip contrast, (Icolumn-Idip) / Icolumn, of about 20% for the Si dumbbell.

Sub-Ångstrom and 3-dimensional STEM for semiconductor research

Electron microscopy has been one of the foremost tools for analysis of semiconducting materials and, in turn, benefits from the processing power provided by faster computers. As semiconductor devices become ever smaller and faster, the shrinking of components means that even single dopant or impurity atoms can significantly affect device performance. Thus the enhanced resolution, sensitivity and new techniques enabled by aberration correction should ensure that this relationship continues.

Nanostructure Functionality through Aberration-Corrected STEM

The 300 kV VG Microscopes’ HB603U STEM at Oak Ridge National Laboratory with a Nion aberration corrector has achieved the first direct image of a crystal at sub-Angstrom resolution, using the incoherent Z-contrast or high-angle annular dark field (HAADF) mode, as shown in Fig. 1a,b. [1] To validate use of the Fourier transform to measure a resolution limit of 0.61A, Fig. 1c compares Fourier transforms of a simulated image and the probe used for the simulation. Excellent agreement is seen for a thin crystal where ideal incoherent imaging applies. Thicker crystals show reduced high frequency transfer, but no spurious sum or difference frequencies [2]. The sub-Angstrom probe allows Z-contrast imaging of oxygen columns next to heavy columns (Fig. 1d). Furthermore, an efficient, simultaneous, aberration corrected, phase contrast image is available using a small axial detector giving improved oxygen visibility, although spurious features are seen between the Sr columns (Fig. 1e). The STEM has become a viable means of acquiring aberration-corrected phase contrast images, with the advantage of simultaneous Z-contrast imaging and EELS.