Imaging ultra thin layers with helium ion microscopy: Utilizing the channeling contrast mechanism

Background: Helium ion microscopy is a new high-performance alternative to classical scanning electron microscopy. It provides superior resolution and high surface sensitivity by using secondary electrons.\ud \ud Results: We report on a new contrast mechanism that extends the high surface sensitivity that is usually achieved in secondary electron images, to backscattered helium images. We demonstrate how thin organic and inorganic layers as well as self-assembled monolayers can be visualized on heavier element substrates by changes in the backscatter yield. Thin layers of light elements on heavy substrates should have a negligible direct influence on backscatter yields. However, using simple geometric calculations of the opaque crystal fraction, the contrast that is observed in the images can be interpreted in terms of changes in the channeling probability.\ud \ud Conclusion: The suppression of ion channeling into crystalline matter by adsorbed thin films provides a new contrast mechanism for HIM. This dechanneling contrast is particularly well suited for the visualization of ultrathin layers of light elements on heavier substrates. Our results also highlight the importance of proper vacuum conditions for channeling-based experimental methods\u

Molecular bridges

The adsorption of Cu-phthalocyanine (CuPc) molecules on Au-modified Ge(001) surfaces has been studied with low-temperature scanning tunneling microscopy. The Au-modified Ge(001) surface consists of well-ordered arrays of perfectly straight nanowires, which are separated by 1.6 nm wide and about 0.6 nm deep trenches. Six different adsorption configurations for CuPc are identified. Four of these configurations are “molecular bridge” configurations where the molecule bridges two adjacent nanowires. The core of the CuPc molecule, i.e., the Cu atom, is fully decoupled from the underlying substrate. For sufficiently high sample biases (>1 V), rotation and diffusion events of the CuPc molecules are observed

Properties of 1D metal-induced structures on semiconductor surfaces

In 1965 Gordon Moore stated that every 12 months a doubling of the number\ud density of transistors placed on an integrated circuit will take place [1].\ud Up to now this law still holds, however, a limit to this law will be reached\ud soon. When using the conventional ‘top-down’ approach for fabrication of\ud electronic circuits and devices we are limited in scaling down the lateral size\ud of electronic devices. With optical lithography the resolution is limited by\ud the wavelength of the light. This thesis deals mainly with the creation of\ud 1D (but also 2D and 3D) nanostructures, produced via the ‘bottum-up’ approach.\ud With the term ‘bottom-up’ approach we mean a self-organization\ud approach to nanofabrication using chemical or physical forces. Nature is\ud capable of harnessing chemical forces for the creation of all the structures\ud that are needed for life. And we like to take this example of nature’s ability\ud and let atoms and small clusters of atoms self-organize into nanosized structures.\ud Three different metals deposited on semiconductor surfaces form the\ud bulk of this thesis. Firstly, the gold (Au) on germanium (Ge)(001) system,\ud where Au atoms together with Ge atoms self-organize into very narrow and\ud long nanowires. These Au-induced nanowires have the potential to serve as\ud a model system for a 1D electron system. Secondly, cobalt- (Co) induced\ud nanostuctures on Ge(001) and Ge(111) are studied. Nanosized crystals and\ud islands are observed of which some show metallic behavior. Finally, iridium\ud (Ir) nanowires are studied, showing a growth behavior that is stabilized by\ud quantum confinement of Friedel oscillations.\u

Co induced nanocrystals on Ge(001)

The deposition of several monolayers of cobalt on germanium (001) substrates results in the formation of two types of clusters: flat-topped and peaked nanocrystals. Scanning tunneling spectroscopy and helium ion microscopy measurements reveal that these nanocrystals contain cobalt. The shape evolution of the flat-topped and peaked nanocrystals as a function of their size is investigated with scanning tunneling microscopy. For small sizes the nanocrystals are compact. Beyond a critical size, however, the peaked nanocrystals exhibit an elongated shape, whilst the flat-topped nanocrystals remain compact. The shape transition of the peaked nanocrystals is driven by a competition between boundary and strain energies. For small sizes the boundary energy is the dominant term leading to a minimization of the peaked nanocrystal's perimeter, whereas at larger sizes the strain energy wins resulting in a maximization of the perimeter. On the top facet of the flat-topped nanocrystals one-dimensional structures are observed that are comprised of small square shaped units of about 1 nm2. Time-resolved scanning tunneling microscopy measurements reveal that these square shaped units are dynamic at room temperatur

Electronically stabilized nanowire growth

Metallic nanowires show unique physical properties owing to their one-dimensional nature. Many of these unique properties are intimately related to electron–electron interactions, which have a much more prominent role in one dimension than in two or three dimensions. Here we report the direct visualization of quantum size effects responsible for preferred lengths of self-assembled metallic iridium nanowires grown on a germanium (001) surface. The nanowire length distribution shows a strong preference for nanowire lengths that are an integer multiple of 4.8 nm. Spatially resolved scanning tunneling spectroscopic measurements reveal the presence of electron standing waves patterns in the nanowires. These standing waves are caused by conduction electrons, that is the electrons near the Fermi level, which are scattered at the ends of the nanowire