Dangling bond

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In condensed matter physics, a dangling bond occurs when an atom is missing a neighbor to which it would be able to bind. Such dangling bonds are defects that disrupt the flow of electrons and that are able to collect the electrons.

A dangling bond is a broken covalent bond. Dangling bonds are found on the surface of most crystalline materials due to the absence of lattice atoms above them. For example, the Si (001) surface would have two dangling bonds per surface lattice atom. Because these dangling bonds do not constitute the lowest energy configuration, the surface tends to reconstruct or adsorb atoms to reduce the surface energy.

When a metal is deposited on silicon, these dangling bonds give rise to interface states within the energy band gap of silicon. The interface Fermi energy is pinned by these interface states, making the Schottky barrier height independent of the metal work function and primarily controlled by the interface states. This is the most commonly encountered difficulty in fabricating a semiconductor-metal ohmic contact. To make such a junction ohmic we will have to resort to doping the semiconductor, which would result in the thinning down of the energy barrier at the interface and consequently allowing for tunneling. It should be however noted that the resistance of such a junction is fairly high because electrons have to tunnel through the thinned Schottky barrier.

Intrinsic defects at Si-SiO₂ interfaces are important in the operation of metal-oxide-semiconductor devices. Unsaturated dangling bonds, generically referred to as Pb-type centers, occur at the interface between the Si substrate and the oxide and are detected by ESR techniques. The proper Pb center is found at (111) interfaces, while two distinct defects, referred to as Pb0 and Pb1, are distinguished at (100) interfaces. The Pb center has a clear microscopic characterization as an isolated sp3 dangling bond of the substrate pointing into the (111) direction, orthogonal to the interface.

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Stirling, Dangling Bond Defects at Si-SiO2 Interfaces: Atomic Structure of the Pb1 Center . Physical Review Letters, Vol 85, No 13, pp 2773, Sep'2000.

Meng Tao, D. Udeshi, S. Agarwal, E. Maldonado and Wiley P. Kirk, “Negative schottky barrier between titanium and n-type Si (001) for low-resistance ohmic contacts.”, Solid-state Electronics 48 (2004), 335-338.