Schottky barrier interface

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. 

Friend et at. studied the influence of electrodes with different work-functions on the performance of PPV photodiodes 143). For ITO/PPV/Mg devices the fully saturated open circuit voltage was 1.2 V and 1.7 V for an ITO/PPV/Ca device. These values for the V c are almost equal to the difference in the work-function of Mg and Ca with respect to 1TO. The open circuit voltage of the ITO/PPV/A1 device observed at 1.2 V, however, is considerably higher than the difference of the work-function between ITO and Al. The Cambridge group references its PPV with a very low dark carrier concentration and consequently the formation of Schottky barriers at the PPV/Al interface is not expected. The mobility of the holes was measured at KT4 cm2 V-1 s l [62] and that for the electrons is expected to be clearly lower. 

An important step toward the understanding and theoretical description of microwave conductivity was made between 1989 and 1993, during the doctoral work of G. Schlichthorl, who used silicon wafers in contact with solutions containing different concentrations of ammonium fluoride.9 The analytical formula obtained for potential-dependent, photoin-duced microwave conductivity (PMC) could explain the experimental results. The still puzzling and controversial observation of dammed-up charge carriers in semiconductor surfaces motivated the collaboration with a researcher (L. Elstner) on silicon devices. A sophisticated computation program was used to calculate microwave conductivity from basic transport equations for a Schottky barrier. The experimental curves could be matched and it was confirmed for silicon interfaces that the analytically derived formulas for potential-dependent microwave conductivity were identical with the numerically derived nonsimplified functions within 10%.10... 

The numerical calculation of the potential-dependent microwave conductivity clearly describes this decay of the microwave signal toward higher potentials (Fig. 13). The simplified analytical calculation describes the phenomenon within 10% accuracy, at least for the case of silicon Schottky barriers, which serve as a good approximation for semiconduc-tor/electrolyte interfaces. The fact that the analytical expression derived for the potential-dependent microwave conductivity describes this phenomenon means that analysis of the mathematical formalism should... 

This result was interpreted by the formation of a Schottky barrier at the CdS/ Ru02-interface as already discussed in the previous section. The H2-production at CdS/Ru02-suspensions could be considerably increased by addition of sulfite because the latter rved as a sink for sulfur produced via reaction (40)... 

Fig. 5.23. Band diagram of the Schottky barrier at the gold - zinc oxide interface...

The interfaces between a semiconductor and another semiconductor (e.g. the very important pin junction, the interface between p- and ft-type semiconductors), between a semiconductor and a metal (the Schottky barrier) and between a semiconductor and an electrolyte are the subject of solid-state physics, using a nomenclature different from electrochemical terminology. 

The term photovoltaic effect is further used to denote non-electrochemical photoprocesses in solid-state metal/semiconductor interfaces (Schottky barrier contacts) and semiconductor/semiconductor pin) junctions. Analogously, the term photogalvanic effect is used more generally to denote any photoexcitation of the d.c. current in a material (e.g. in solid ferroelectrics). Although confusion is not usual, electrochemical reactions initiated by light absorption in electrolyte solutions should be termed electrochemical photogalvanic effect , and reactions at photoexcited semiconductor electodes electrochemical photovoltaic effect . 

The Schottky-Mott theory predicts a current / = (4 7t e m kB2/h3) T2 exp (—e A/kB 7) exp (e n V/kB T)— 1], where e is the electronic charge, m is the effective mass of the carrier, kB is Boltzmann s constant, T is the absolute temperature, n is a filling factor, A is the Schottky barrier height (see Fig. 1), and V is the applied voltage [31]. In Schottky-Mott theory, A should be the difference between the Fermi level of the metal and the conduction band minimum (for an n-type semiconductor-to-metal interface) or the valence band maximum (for a p-type semiconductor-metal interface) [32, 33]. Certain experimentally observed variations of A were for decades ascribed to pinning of states, but can now be attributed to local inhomogeneities of the interface, so the Schottky-Mott theory is secure. The opposite of a Schottky barrier is an ohmic contact, where there is only an added electrical resistance at the junction, typically between two metals. 

To avoid having different-sized Schottky barriers at the two interfaces, the same metal (or metals with almost the same work functions) should be used for both electrodes. For example, the different work functions of Pt and Mg made studies of glass I Pt I molecule I Mg I Ag sandwiches hard to interpret [34]. In that case, Mg probably reacted with the end of the molecule containing the strong acceptor TCNQ to form a TCNQ-salt Schottky barrier that dominated the electrical asymmetry [34], With a different molecule lacking TCNQ, the dominating Schottky barrier effect was eliminated [35, 36],... 

The first process is due to Schottky barriers [30], which are electrical dipole moments that form at the metal I molecule interfaces, as discussed above [34,40]. The second process arises if the electrically-active portion of the molecule is placed asymmetrically within the metal I molecule I metal sandwich. This geometry is common, because a long alkyl tail is often needed to make the molecule amphiphilic so that it will form well-ordered Langmuir-Blodgett monolayers [76-78]. 

Figure 3.35 Formation of a Schottky (potential) barrier (a) isolated semiconductor and metal, (b) on contact a Schottky barrier is formed at the interface, and (c) Schottky barriers can also form between semiconducting grains separated by insulating layers.

The R-X plot shows the most variation in the subthreshold region, while the G-B plot shows the most variation above threshold. One sees from the G-B plot that the high frequency response of the diode is independent of bias (>1 MHz). To fit the data, one models each material phase or interface as a parallel R-C combination. These combinations are then added in series, and an overall series resistance and series inductance are added. For the data in Figure 10.6, three R-C elements are used. One R-C element is associated with the Schottky barrier. Another is associated with the high frequency bias-independent arc, which we believe is associated with the capacitance of the alkoxy-PPV. The thinness of the film... 

Kocha SS, Turner JA (1994) Study of the Schottky barriers and determination of the energetics of the band edges at the n- and p-type gallium indium phosphide electrode electrolyte interface. J Electroanal Chem 367 27-30... 

Rectification phenomena through molecules is attributed to three different effects. The first is due to Schottky barriers because a surface dipole is formed at the organic/metal interface. The second effect occurs when the LUMO... 

Figure 3.14. Schematic representation of experiment detecting hot electrons created by atomic/ molecular adsorption on a thin metal film. is the Schottky barrier created by the metal/ Si interface. From Ref. [86].

In addition to photoconductivity, there are a lot of photovoltaic phenomena observed in polymer photoconductors [14]. The most famous ones are the photo-emf at the Schottky barrier due to the separation of the electron-hole pairs in the electrical field at the photoconductor electrode interface photo-emf at the... 

Figure 4.2(d) shows that an energy barrier forms at the semiconductor/redox electrolyte interface, similar to the Schottky barrier at a metal/semiconductor interface. The most important quantity is the barrier height (q ) or the flat band potential U, which essentially determines the surface band positions of the semiconductor with respect to the energy levels of solution species. The q B is given for an n-type semiconductor by... 

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

In many PEC systems the chemical kinetics for the primary charge transfer process at the interface are not observed at the light intensities of interest for practical devices and the interface can be modeled as a Schottky barrier. This is true because the inherent overpotential, the energy difference between where minority carriers are trapped at the band edge and the location of the appropriate redox potential in the electrolyte, drives the reaction of interest. The Schottky barrier assumption breaks down near zero bias where the effects of interface states or surface recombination become more important.(13)... 

Hole photoemission may also occur in appropriately biased PECs, although this process has not yet been observed. The major problem with the observation of photoemission from semiconducting electrodes is interference from the much larger photocurrents produced by the existence of the Schottky barrier at the interface. 

We have extended the technique of Relaxation Spectrum Analysis to cover the seven orders of magnitude of the experimentally available frequency range. This frequency range is required for a complete description of the equivalent circuit for our CdSe-polysulfide electrolyte cells. The fastest relaxing capacitive element is due to the fully ionized donor states. On the basis of their potential dependence exhibited in the cell data and their indicated absence in the preliminary measurements of the Au Schottky barriers on CdSe single crystals, the slower relaxing capacitive elements are tentatively associated with charge accumulation at the solid-liquid interface. 

Figure 4. Energy diagram of a Schottky barrier formed at an n-type semiconductor-electrolyte interface...

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