Work function of a semiconductor

In the preceding chapters the conditions determining chemisorption and the thermodynamic treatment of surface equilibria have been discussed. We shall now derive a general formula for the dependence of the work function of a semiconductor (e.g. of an oxide) upon the surface concentration of the chemisorbed oxygen ions. 

Thus, the work function of a semiconductor is not a well-defined quantity but depends on the nature of surface states, (intrinsic, adsorbate, or defect induced), the type and density of doping, and the electron affinity as an intrinsic property of the semiconductor. 

In general, the peculiarities of the surface effects in thin semiconductors, for which application of semi-infinite geometry becomes incorrect were examined in numerous papers. As it has been shown in studies [101, 113, 121 - 123] the thickness of semiconductor adsorbent becomes one of important parameters in this case. Thus, in paper [121] the relationship was deduced for the change in conductivity and work function of a thin semiconductor with weakly ionized dopes when the surface charge was available. Paper [122] examined the effect of the charge on the temperature dependence of the work function and conductivity of substantially thin adsorbents. Papers [101, 123] focused on the dependence of the surface conductivity and value of the surface charge as functions of the thickness of semiconductor and value of the surface band bending caused by adsorption and application of external field. 

Excrements show that all the alkyl, hydroxyl, and amine radicals which we have studied considerably reduce the conductivity and increase the work function of oxide semiconductors like ZnO, Ti02, CdO, WO2, M0O3, etc. during chemisorbtion. It should be noted that the revealed effects are rather profound especially if we are dealing with the effect of chemisorbtion of active particles on conductivity of a thin (less than 1 pm) sintered polycrystal semiconductor films. Thus, conductivity of such films in the presence of free CH3-radicals with the concentration of even 10 cm and less may change from initial value by dozens or hundreds percent depending on experimental conditions. 

The first effect is a change in the work function of the semiconductor. In Fig. 22 the work function is denoted by distance between the Fermi level and the level corresponding to the value of the potential outside the crystal. (We have in mind the thermoelectron work function which figures in Richardson s formula.) We have... 

In the literatme, the work function of a metal, p (in eV), is often used to estimate the degree of charge transfer at semiconductor/metal junctions. The work function of a metal is defined as the minimum potential experienced by an electron as it is removed from the metal into a vacuum. The work function ip is often nsed in lieu of the electrochemical potential of a metal, because the electrochemical potential of a metal is difficult to determine experimentally, whereas tp is readily accessible from vacuum photoemission data. Additionally, the original model of semiconductor/metal contacts, advanced by Schottky, utilized differences in work functions, as opposed to differences in electrochemical potentials, to describe the electrical properties of semiconductor/metal interfaces. A more positive work function for a metal (or more rigorously, a more positive Fermi level for a metal) would therefore be expected to produce a greater amount of charge transfer for an n-type semiconductor/metal contact. Therefore, use of metals with a range of tp (or fip.m) values should, in principle, allow control over the electrical properties of semiconductor/metal contacts. 

A mathematical model of metal-semiconductor contacts has been employed to estimate the quantity of charge transferred through the interface, based on parameter values that pertain to the M/Ti02 system [88]. The direction of electron flux in a metal-semiconductor contact depends on the relative values of the work function of the two materials. The work function of the semiconductor is a function of the kind (valence) and concentration of the dopant and of temperature. Doping of... 

It should be mentioned that for p-type materials, usually, a negative space charge is formed because the work function of the semiconductor is below that of the metal. Again assuming an ideal contact, the energy barrier is given by... 

Schottky effect A reduction in the work function of a substance when an external accelerating electric field is applied to its surface in a vacuum. The field reduces the potential energy of electrons outside the substance, distorting the potential barrier at the surface and causing field emisslorL A similar effect occurs when a metal surface is in contact with a semiconductor rather than a vacuum, when It Is known as a Schottky barrier. The effect was discovered by the German physicist Walter Schottky (1886-1976). 

The work function of a solid is a fundamental physical property of the solid which is related to its electronic structure. It is defined as the potential that an electron at the Fermi level must overcome to reach the level of zero kinetic energy in the vacuum. In semiconductors and... 

Two dipole layers with large electric dipole moments have been introduced at the surfaces [578-586] via a binding group (X—) and a terminal group (—Y). The molecular control of work functions of other semiconductors, such as CdTe, CdSe, GaAs, and InP, than ITO were also reported using benzoic acid [587] and dicarboxylic acid derivatives [451]. 

Fig. 15 Variations of the work function of a typical polymeric semiconductor as a function of the work function of the underneath electrode. The three regimes tire illustrated by the corresponding energy diagram, (a) Fermi level pinning near the HOMO level, (b) Vacuum level alignment, (c) Fermi level pinning near the LUMO level...

Fiq.4.85a-c. (a) Work functions of a metal and g of a semiconductor and electron affinity x- Ec is the energy at the bottom of the conduction band and Ep is the Fermi-energy. (b) Schottky barrier at the contact layer between metal and n-type semiconductor, (c) Generation of a photocurrent... 

Trigonal selenium is a -type semiconductor with an energy gap of 1.85 eV (104) and a work function of about 6 eV (105), which is the largest value reported for all the elements. Accordingly, a Schottky barrier should be created at the contact of selenium with any metal. This is consistent with the... 

In a light-emitting MSM structure the two metal electrodes selected such that the work functions of the electrodes are near the edge of the valence band (VB) and the conducting band (CB) of the semiconductor, respectively, so that oppositely charge carriers are injected from the opposite electrodes. An ohmic and a rectifying contact is therefore formed in the MSM structure (see Fig. 9-22). 

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