View source for Metal-induced gap states

In bulk [[semiconductor]] [[band structure]] calculations, it is assumed that the [[crystal lattice]] (which features a periodic potential due to the atomic structure) of the material is infinite.  When the finite size of a crystal is taken into account, the [[wavefunctions]] of [[electrons]] are altered and states that are forbidden within the bulk semiconductor gap are allowed at the surface.  Similarly, when a [[metal]] is deposited onto a semiconductor (by thermal [[evaporation (deposition)|evaporation]], for example), the wavefunction of an electron in the semiconductor must match that of an electron in the metal at the interface.  Since the [[Fermi level]]s of the two materials must match at the interface, there exists gap states that decay deeper into the semiconductor.

==Band-bending at the metal-semiconductor interface==
[[File:Diagram of band-bending interfaces between two different metals and two different semiconductors.jpg|thumb|right|250px|[[Band diagram]] of the [[band-bending]] at the interface of (a) a low [[work function]] metal and n-type [[semiconductor]], (b) a low work function metal and a p-type semi conductor, (c) a high work function metal and an n-type semi conductor, (d) a high work function metal and a p-type semi conductor. (Figure adapted from H. Luth's ''Solid Surfaces, Interfaces, and Thin Films,'' p. 384.<ref>H. Luth, ''Solid Surfaces, Interfaces, and Films,'' Springer-Verlag Berlin Heidelberg, New York, NY, 2001.</ref>)]]

As mentioned above, when a [[metal]] is deposited onto a [[semiconductor]], even when the metal film is smaller than a single atomic layer, the Fermi levels of the metal and semiconductor must match. This [[Fermi level pinning|pins the Fermi level]] in the semiconductor to a position in the bulk gap.  Shown to the right is a diagram of band-bending interfaces between two different metals (high and low [[work function]]s) and two different semiconductors (n-type and p-type).

[[Volker Heine]] was one of the first to estimate the length of the tail end of metal [[electron]] states extending into the semiconductor's energy gap.  He calculated the variation in surface state energy by matching wavefunctions of a free-electron metal to gapped states in an undoped semiconductor, showing that in most cases the position of the surface state energy is quite stable regardless of the metal used.<ref>V. Heine, "Theory of Surface States," Phys. Rev., '''138''', A 1689 (1965).</ref>

==Branching point==
It is somewhat crude to suggest that the metal-induced gap states (MIGS) are tail ends of [[metal]] states that leak into the [[semiconductor]].  Since the mid-gap states do exist within some depth of the semiconductor, they must be a mixture (a [[Fourier series]]) of [[valence (chemistry)|valence]] and [[Electrical conduction|conduction]] band states from the bulk.  The resulting positions of these states, as calculated by [[C. Tejedor, F. Flores and E. Louis]],<ref>	C. Tejedor, F. Flores and E. Louis,"The metal-semiconductor interface: Si(111) and zincblende junctions", J. Phys. C: Solid State Phys. '''10''', 2163 (1977).</ref> and [[J. Tersoff]],<ref>J. Tersoff, "Theory of semiconductor heterojunctions: The role of quantum dipoles," Phys. Rev. B '''30''', 4874 (1984).</ref><ref>J. Tersoff, "Schottky barriers and semiconductor band structures," Phys. Rev. B '''32''', 6968 (1985).</ref> must be closer to either the valence- or conduction- band thus acting as acceptor or donor [[dopants]], respectively.  The point that divides these two types of MIGS is called the branching point, E_B.  Tersoff argued
:<math> E_B = \frac{1}{2}[\bar{E_V} + \bar{E_C}] </math>
:<math> \bar{E_V} = E_V - \frac{1}{3} \Delta_{so} </math>, where <math>\Delta_{so} </math> is the spin orbit splitting of <math> E_V</math>  at the <math> \Gamma </math> point.
:<math> \bar{E_C} </math> is the indirect conduction band minimum.

==Metal–semiconductor contact point barrier height==

[[Image:Migsbarrier.JPG|thumb|right|250px|[[Band diagram]] of the contact point potential barrier at the interface of a metal and semiconductor.  Shown are <math>e\Phi_{bh}</math>, the energy of the barrier, and <math>eV_{if}</math>, the maximum band bending in the semiconductor. (Figure adapted from H. Luth's ''Solid Surfaces, Interfaces, and Thin Films,'' p. 408 (see Refs.)]]

In order for the [[Fermi levels]] to match at the interface, there must be charge transfer between the [[metal]] and [[semiconductor]].  The amount of charge transfer was formulated by Linus Pauling <ref>L. Pauling, ''The Nature of the Chemical Bond.'' Cornell University Press, Ithaca, 1960.</ref>  and later revised <ref>N.B Hanney and C.P. Smith, "The Dipole Moment of Hydrogen Fluoride and the Ionic Character of Bonds," J. Am. Chem. Soc. '''68''', 171 (1946)</ref> to be:
:<math>\delta q = \frac{0.16}{eV}|X_M - X_{SC}| + \frac{0.035}{eV^2}|X_M - X_{SC}|^2</math>
where <math>X_M</math> and <math>X_{SC}</math> are the [[electronegativities]] of the metal and semiconductor, respectively.  The charge transfer produces a [[dipole]] at the interface and thus a potential barrier called the [[Schottky barrier]] height.  In the same derivation of the branching point mentioned above, Tersoff derives the barrier height to be:
:<math>\Phi_{bh} = \frac{1}{2}[\bar{E_C} - \bar{E_V}] + \delta_m = \frac{1}{2} [\bar{E_C} - E_V - \frac{\Delta_{so}}{3}] + \delta_m</math>
where <math>\delta_m </math> is a parameter adjustable for the specific metal, dependent mostly on its electronegativity, <math>X_M</math>.  Tersoff showed that the experimentally measured <math>\Phi_{bh}</math> fits his theoretical model for [[Gold|Au]] in contact with 10 common semiconductors, including [[Silicon|Si]], [[Germanium|Ge]], [[GaP]], and [[GaAs]].

Another derivation of the contact barrier height in terms of experimentally measurable parameters was worked out by [[Federico Garcia-Moliner and Fernando Flores]] who considered the [[density of states]] and [[dipole]] contributions more rigorously.<ref>Garcia-Moliner, Federico and Flores, Fernando, ''Introduction to the theory of solid surfaces,'' Cambridge University Press, Cambridge, London, 1979.</ref>
:<math>\Phi_{bh} = \frac{1}{1+\alpha N_{vs}} [\Phi_M - X_M + D_J + \alpha N_{vs}(E_g - \Phi_0)] </math>
:<math>\alpha </math> is dependent on the charge densities of the both materials
:<math>N_{vs} =</math> density of surface states
:<math>\phi_M =</math> work function of metal
:<math>D_J =</math> sum of dipole contributions considering dipole corrections to the jellium model
:<math>E_G =</math> semiconductor gap
:<math>\Phi_0 = </math> Ef – Ev in semiconductor

Thus <math>\phi_{bh}</math> can be calculated by theoretically deriving or experimentally measuring each parameter.  Garcia-Moliner and Flores also discuss two limits
:<math> \alpha N_{vs} >> 1</math> (The [[Bardeen]] Limit), where the high density of interface states pins the Fermi level at that of the semiconductor regardless of <math> \Phi_M </math>.
:<math> \alpha N_{vs} << 1</math> (The [[Schottky barrier|Schottky]] Limit)where <math> \Phi_{bh}</math> varies with strongly with the characteristics of the metal, including the particular lattice structure as accounted for in <math>D_J</math>.

==Applications==
When a bias voltage <math>V</math> is applied across the interface of an n-type semiconductor and a metal, the Fermi level in the semiconductor is shifted with respect to the metal's and the band bending decreases.  In effect, the capacitance across the depletion layer in the semiconductor is bias voltage dependent and goes as  <math>(V_{if}-V)^{\frac{1}{2}}</math>.  This makes the metal/semiconductor junction useful in [[varactor]] devices used frequently in electronics.

== References ==
{{reflist}}

[[Category:Electronic band structures]]
[[Category:Semiconductor structures]]