Atom Layers and Semiconductor Materials

- Van der Waals (vdW) Materials

A Van der Waals (vdW) molecule is a weakly bound complex of atoms or molecules held together by intermolecular attractions such as Van der Waals forces or by hydrogen bonds. The name originated in the beginning of the 1970s when stable molecular clusters were regularly observed in molecular beam microwave spectroscopy. Examples of well-studied vdW molecules are Ar2, H2-Ar, H2O-Ar, benzene-Ar, (H2O)2, and (HF)2. Others include the largest diatomic molecule: He2 and LiHe. 

van der Waals crystals are extremely thin crystals, made up of atomically thin layers. These layers are not bound by ordinary covalent bonds, like in traditional semiconductors, but by much weaker van der Waals bonds. That makes it much easier to make extremely thin layers, on the scale of a few atomic layers", Professor Patanè says. "This creates a plethora of opportunities for science and technology. New quantum properties emerge at the atomic scale; the layers can be bent; they can be combined together without the limitations that apply to conventional semiconductors. Also, they can be rotated relative to each other, and by twisting them it is possible to create new crystal structures. This means that these materials may hold the key to "post-silicon" electronics.

 Semiconductor physics is on the cusp of a revolution. Ultra-thin crystals may take the place of today’s ever-present silicon semiconductors and may transform many applications. Van der Waals (vdW) materials are made up of strongly bonded two-dimensional (2D) layers that are bound in the third dimension through weaker dispersion forces. Graphite is a vdW material broadly used in industry in electrodes, lubricants, fibers, heat exchangers, and batteries.

- Semiconducting Materials

Semiconductors are built on semiconducting materials – that is, material with electrical conductivity between metals (good conductors) and insulators (poor conductors). Importantly, the conductivity can be controlled by adding small amounts of other elements.

Semiconductors have the unique ability to act as either insulator or conductor, depending on environmental factors. Temperature, light, electric currents, or even electric fields can affect a semiconductor’s properties. In chemistry and physics, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bond contribute one valence electron in order to form a shared pair.

- The Most Commonly Used Semiconductor Materials: Silicon, Germanium and Gallium arsenide

The most commonly used semiconductor materials are silicon, germanium and gallium arsenide. Among these three materials, germanium was one of the earliest semiconductor materials used. Germanium has four valence electrons, which are electrons located on the outer shell of the atom.

The number of valence electrons in a semiconductor material determines its conductivity. While germanium was an important step in the development of semiconductor materials, it has largely been abandoned in favor of silicon, the current king of semiconductor materials.  

Silicon has been widely used as a semiconductor material since the 1950s. Silicon is the most abundant element on Earth after carbon, has four valence electrons, and has a higher melting temperature than germanium (1,414 degrees Celsius compared to 938.3 degrees Celsius for germanium).

Quartzite is rich in silicon. Silicon extraction, purification and crystallization processes are efficient and economical. The element crystallizes in the form of diamond, forming a relatively strong bond that gives silicon crystals their strong mechanical properties.

Gallium arsenide is the second most commonly used semiconductor today. Unlike silicon and germanium, gallium arsenide is a compound, not an element, made of gallium, which has three valence electrons, combined with arsenic, which has five valence electrons.

Octavalent electrons allow gallium arsenide devices to respond quickly to electrical signals, making the compound ideal for amplifying the high-frequency signals seen in television satellites. However, gallium arsenide has some limitations: the compound is more difficult to manufacture on a large scale than silicon, and the chemicals used in gallium arsenide production are highly toxic.