Tunnelling Schottky barrier

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

The electrical contact with the bulk of the doped crystal is made through a very heavily doped layer, to reduce the height of the Schottky barrier between the bulk and the metal of the external contact (Au). The charge carriers cross this layer by tunnel effect. 

Perello DJ, Chulim S, Chae SJ et al (2010) Anomalous Schottky barriers and contact band-to-band tunneling in carbon nanotube transistors. ACS Nano 4 3103-3108... 

Fig. 10.16. Energy band diagrams for (a) Schottky barrier situation and (b) tunneling situation. CB, conduction-band energy VB, valence-band energy F, Fermi energy level. (Reprinted from A. Gonzalez-Martin, thesis, Texas A M University, 1993.)...

However, it is important to note that, since the seminal publication of Aviram and Rattner [75], there have been attempts to demonstrate that suitably designed organic semiconductors deposited in a layer between two electrodes would give current voltage behavior analogous to the behavior of a p-n junction, even when Schottky barrier or tunnelling effects due to the metal electrodes are not important. There is a class of small molecular... 

Because the spacing between pores is always less than the width of the depletion layer and PS has a very high resistivity, Beale et al. proposed that the material in the PS is depleted of carriers and the presence of a depletion layer is responsible for current localization at pore tips where the field is intensified. This intensification of field is attributed to the small radius of curvature at the pore tips. For lowly doped p-Si the charge transfer is by thermionic emission and the small radius of curvature reduces the height of the Schottky barrier and thus increases the current density at the pore tips. For heavily doped materials the current flow inside the semiconductor is by a tunneling process and depends on the width of the depletion layer. In this case the small radius of curvature results in a decrease of the width of the depletion layer and increases the current density at pore tips. The initiation was considered to be associated with the surface inhomogeneities, which provide the initial localized high current density at small surface depressions. 

The electron temperature was measured by superconductor-semiconductor-superconductor (S-Sm-S) junctions with Schottky barrier [7], In the S-Sm-S structure the quasiparticle tunneling across the junction is very sensitive to the electron temperature in the normal electrode and it can be used as an electron temperature probe with negligible heat leak. Bias current used for electron temperature measurements was few orders of magnitude smaller than the current used for the electron heating in Si film and therefore the possible heating by the bias current can be neglected. The S-Sm-S thermometers used in experiments were calibrated against the ruthenium oxide thermometer (see inset in Fig. 1). 

Several possible current transport mechanisms are illustrated in Fig. 3. The schematic represents a Schottky barrier on an undoped sample under forward bias. The three arrows for electron transport are drawn for comparison with crystalline semiconductors in which thermionic emission, tunneling via thermionic field emission, or field emission represent the usual mechanisms. 

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