Sergey N. Rashkeev

Molecular electronics by the numbers

Molecules are an attractive option to develop nanoscale electronic devices. Experimental measurements of current-voltage characteristics of individual molecules have been possible for several years and have revealed rich structure and diverse behavior. This paper reviews recent theoretical work by the authors in which current-voltage characteristics of individual molecules are computed using a fully quantum-mechanical parameter-free method. The results provide detailed understanding of transport in molecules in the context of data and go further by obtaining information about the role of molecule-electrode contacts and predicting the behavior of three-terminal molecular devices and the factors that control their performance.

From 3D Imaging of Atoms to Macroscopic Device Properties

The aberration-corrected scanning transmission electron microscope provides a new level of sensitivity for analyzing semiconductor device structures. Its sub-Angstrom probe provides not only improved resolution [1], but equally important, greatly increased sensitivity to individual atoms [2,3]. Single Hf atoms are visible within the nanometer thick SiO2 interlayer between a HfO2 high-K dielectric and the Si substrate [4]. Furthermore, the depth of field of the aberration-corrected STEM is just a few nanometers, and Hf atoms can be located in depth to better than 1 nm precision from a through focal series of images (see Fig. 1). Just as with confocal optical microscopy, the series of images can be used to reconstruct the 3D distribution of atoms as shown in Fig. 2. In addition the interface roughness is visible.

Aberration-Corrected STEM for Understanding of the Catalytic Mechanisms and Development of New Catalysts

Aberration-corrected Z-contrast (HAADF) STEM has proven an invaluable tool for catalysis research due to its superior resolution and high sensitivity. ORNL’s 300 kV VG Microscopes’ HB603U STEM with Nion aberration corrector was shown to resolve lattice spacings as small as 0.78 A [1] and routinely allows us to image single atoms inside the materials and on their surfaces as well . The ability to simultaneously collect EELS spectra provides additional possibilities for characterization. All these aspects have proven instrumental for the studies of catalytic activity and degradation in Cr-doped transition aluminas, which are widely used for dehydrogenation of alkanes [2]. Even though structural difference between γ-Al2O3 and η-Al2O3 is minimal and only amounts to different distribution of cation vacancies within spinel framework, Cr-doped η-Al2O3 catalysts can last several years, while Cr/γ-Al2O3 catalysts degrade in weeks [3]. Our STEM studies revealed that Cr dopant is distributed differently in these two systems. On γ-Al2O3, Cr is segregated into extended “patches” of Cr2O3 (Fig. 1). On η-Al2O3, on the other hand, no segregation was observed. Contrast in the STEM images was mostly uniform, with the thinnest areas of the alumina flakes sometimes displaying faint spots (Fig.2a)). The spots have similar intensity (Figs. 2b)-d)) of about the right level for Cr. EELS studies (Fig. 2e)) also indicate uniform distribution of dopant on η-Al2O3 surface. Our first-principles calculations suggest that catalytic processes primarily happen on isolated CrOx species, and preserving a high density of these active sites is necessary to retain activity. Due to a unique surface reconstruction resulting from its specific distribution of cation vacancies, γ-Al2O3 (110C) surface can accommodate growth of extended Cr2O3 “patches’, while on η-Al2O3 Cr remains isolated, resulting in much slower degradation.

Atomic and Electronic Structure Investigations of HfO2/SiO2/Si Gate Stacks Using Aberration‐Corrected STEM

Aberration correction in scanning transmission electron microscopy represents a major breakthrough in transmission electron microscopy, enabling the formation of sub‐Angstrom probe sizes. Thus, electron microscopy achieved single atom sensitivity. Here, we show how this technique with its unique spatial resolution in combination with high‐resolution electron energy‐loss spectroscopy can be used to investigate atomic and electronic structures of semiconductor interfaces with single atom sensitivity. We employ a Si/HfO2/SiO2/Si high‐k dielectric interface to show the presence of single Hf atoms in the SiO2 interlayer. Furthermore, we demonstrate how local dielectric properties and local band structure information can be obtained by electron energy‐loss spectroscopy.