Acceptor surface state

D. J. Leary, J. O. Barnes, and A. G. Jordan, Calculation of carrier concentration in polycrystalline films as a function of surface acceptor state density Application for ZnO gas sensors, J. Electrochem. Soc. 129 (1982) 1382-1386. 

In the case of the interaction with surface acceptor states, not related to oxygen adsorption, one can proceed as in the case of the first mechanism proposed by Kohl. In the case of an interaction with oxygen adsorbates, one can consider that the dissociation of oxygen ions is increased and examine the implications. 

Figure A3.10.23 Schematic diagram of molecular CO chemisorption on a metal surface. The model is based on a donor-acceptor scheme where the CO 5 a FIOMO donates charge to surface unoccupied states and the surface back-donates charge to the CO 2 71 LUMO [58].

Fig. 2-30. Surface dangling states and surface ion-induced states (a) surface dangling donor (DL-B) and acceptor (DL-AB) leveb on covalent bonding semiconductors, (b) surface cation-induced acceptor (SCL) and surface anion-induced donor (SAL) levels on ionic bonding semiconductors.

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

From Fig. 4.22 pd = 2Vs e where Ns is the surface density of acceptor states near the grain boundary and e is the magnitude of the electronic charge. Therefore the height [Pg.169]

With the chemisorption of, for example, oxygen a surface density of electron acceptor states leads to the establishment of a Schottky barrier. The process is essentially the same as that which occurs in the case of the PTC thermistor (see Figs 4.21 and 4.10). The electron potential barrier height (p) is given by... 

The presence of this depletion layer has profound consequences if light of energy exceeding the bandgap is incident on the semiconductor, then the electron excited to the conduction band and the - hole left behind in the valence band can separate under the influence of the internal - electric field, with the electron drawn into the interior of the semiconductor and the hole driven to the surface, where, as can be seen from Fig. 3, it can be captured by an acceptor state in solution, driving an electrochemical reaction. The electron can pass round an external circuit to the counter electrode, and two types of electrochemical reaction are possible either a second different electrochemical couple... 

An alternate mechanism for spectral sensitization, not involving direct electron injection into the semiconductor, is energy transfer from the excited, adsorber dye to some unidentified acceptor state near the surface of the silver halide, followed by promotion of an electron into the conduction band. In an elegant series of... 

In semiconductors of high-dopant density and correspondingly thin depletion layers, tunneling may occur directly between the electrons in the conduction band and the surface of the electrode provided acceptor states or redox species in solution are available. The tunneling contribution to the total current has been considered by a number of workers [118-121] and the total anodic current can be written, with some generality, as... 

An ideal tool for the investigation of the electronic structure of metal clusters is offered by scanning probe methods which have been used in three pioneering studies of nanometer-size clusters of Au on GaAs(llO) [229], Fe clusters on the same surface [230], and of size-selected Siio clusters on a reconstructed Au(OOl) surface [231], which all have been published already in 1989. In the first investigation [229], a characteristic spectrum of band-gap states was observed for the Au particles grown on GaAs. Both donor and acceptor states... 

Should the charge carrier become trapped at a surface-state, then the problem becomes virtually identical to the homogenous two-state problem, i.e., the donor and acceptor states are both spatially localised over dimensions comparable to typical molecular dimensions. 

Density of states at the lower edge of the conduction band Density of states at the upper edge of the valence band Density of donor states in the semiconductor Density of acceptor states Density of surface states... 

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