Glossary of Terms

(not in alphabetical order)

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STM: The Basics

Evaporation / Substrate / UHV

In surface science and thin film science the evaporation of material on top of a substrate means depositing this material on top of a carrier on which it can be investigated. The material to be deposited is heated (often in a small crucible), and its vapor condenses on top of the substrate. The materials to be investigated are often very reactive, so this process as well as the following investigations have to be carried out in a very good vacuum, called "ultra high vacuum" (UHV). The very complex UHV system is needed to protect the materials from the environment and not the other way round.

Gd / Tb

Chemical elements Gadolinium (Gd) and Terbium (Tb). Group of the Lanthanides or Rare Earth metals. Both are of special interest because of their magnetic properties. Industrial use for magnetic storage media in alloys with iron. The goal of our investigation of these metals was to demonstrate spin polarized tunneling.

Islands / Crystallites

Material deposited on top of the substrate may cover it forming a smooth film or so-called "islands". These islands themselve often represent single crystals as well. Whether a smooth film or islands are formed critically depends on the properties of the substrate, the deposited material and the temperature. Remember water on glass: if the glass is slightly dirty, water forms a film on it; on fresh cleaned glass the formation of drops is favoured.

Reconstruction

Any periodic reorientation of the surface atoms with respect to the bulk crystallographic structure. The motivating force may be foreign atoms (adsorbates) or simply the fact that the atoms are at the surface (the top neighbours are missing). When a surface reconstructs, in most cases it forms a -> superstructure


Bulk atoms that become surface atoms by cleaving the crystal...		... may reconstruct

Superstructure

Periodic arrangement of the atoms of a deposited material on top of a substrate or of the surface atoms themselves. Imagine a chessboard with peas placed on all fields. If you place the peas only on black fields you have arranged them in a 2 x 2 superstructure.

Single Crystal

The distinction if a piece of stuff is amorphous, polycrystalline or singlecrystalline refers to the order in microscopic dimensions. The material itself may consist of atoms or of a chemical compound, SiO₂ for instance.
In the following schematic drawings, a square symbolizes the smallest unit of the material i.e. an atom for pure elements or a molecule for a chemical compound like SiO₂.


Amorphous	Polycrystalline	Single Crystal

In an amorphous material, no long or short range order in between the smallest units exists. They are bound to each other irregularely. For SiO₂, this state corresponds to its best known form: glass, with the main component being SiO₂.
In a polycristalline state, a short range order exists. The material consists of many small crystallites of various size. Within the crystallites, the units are highly ordered, but no inter-crystalline order exists. Most of the materials that surround us are polycrystalline. SiO₂ is found in many rocks and stones in polycrystalline form as the main component.
In a single crystal, there is perfect order over the whole crystal. For SiO₂, this state corresponds to rock crystal. The largest single crystals today are made for the semiconductor industry, with dimensions of several dezimeter being the topical standard.

Crystallographic directions

Cutting and polishing a single crystal defines a certain surface. The orientation of the surface (blue arrow) with respect to the crystallographic structure is given by a number in brackets.For the tungsten (W) single crystal I used and that is therefore the substrate for all the samples presented here it was the "W(110)"-direction.
In the left drawing, the desired direction of the cut is symbolized by the blue line. The actual cut always has a slight error (red line). This results in a surface with monoatomic steps (yellow circle). In some of the images these monoatomic steps of the single crystal surface can be seen.


Single crystal with miscut resulting in monoatomic steps (circle).	monoatomic steps (circle) and Gd island on a W(110) single crystal

Density of States (DOS)

Remember the energetic levels of the hydrogen atom: several distinct states are present. The electron that is bound to the hydrogen atom can only persist within one of these energetic levels. Intermediate states are forbidden. The condensed matter energetic levels are lying so close together that they can not be seperated from each other. They appear to be smeared out to continous bands. Especially in metals there are no forbidden energetic levels, but the relative number of levels per energy interval, the density of states, may vary with energy.


Hydrogen atom energetic levels		Condensed matter density of states DOS		I/V characteristic the STM sees of the former DOS

How will the STM notice the condensed matter density of states? Let us take the example DOS for a surface and look at the I-V characteristics of the tunneling contact at a constant tip sample separation. Every distinct state in the DOS shows up as a small extra rise in the otherwise monotonic rising I-V curve, located at the corresponding sample bias. The STM is capable of probing the sample DOS. This is the simplest form of a Scanning Tunneling Spectroscopy, and it is in principle the way to obtain the presented STM color images.