The Schottky Barrier

We have carried out an investigation of the electrical and electro-optical properties of a series of Schottky barrier diodes fabricated with polyacetylene sandwiched between two metal contact layers, one to form the Schottky barrier and the other (gold) to provide an ohmic contact [56]. This type of structure is straightforward to fabricate with an extrinsically-doped semiconductor and there have been several reports of such devices which use polyacetylene or other conjugated polymers [57-62]. The details of the device fabrication have been given in section 3.2, and we show in figure 10 the details of the typical structures that we have used for this work. We have worked with relatively thick films of polyacetylene, in the range 500 - 1(XX) nm, so as to avoid the possibility of short-circuits tetween top and bottom electrode, but we have kept the metal contact layers thin so that they are semi-transparent and allow optical transmission measurements. 

In the case of a p-type semiconductor, the ideal Schottky barrier is formed between the semiconductor and a metal with a work-function, , that is lower than that of the semiconductor, s [56]. This is shown schematically for a conventional semiconductor in figure 11. ( )s is given by 

The potential barrier, and its variation with an applied bias, determine the rectifying characteristics of the junction, as we discuss below, and in principle the barrier height is determined simply from the work functions for the metal and semiconductor. In practice it is usually the case for inorganic semiconductors that the barrier height is not well predicted by this simple relation and surface states and surface layers at the interface play an important role. For the case of polyacetylene the work function is estimated to be of the order of 5 eV [57-62], and we can expect an ohmic junction with gold (( )ni = 5.1 eV), but Schottky junctions with chromium = 4.5 eV), aluminium (( in = 4.3 eV) and indium ( l)m = 4.1eV). 

For the case where current is limited by thermionic emission over the barrier, the current density, J, varies as 

The criterion that thermionic emissitxi limits current flow across the barrier is given by 

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The second class of atomic manipulations, the perpendicular processes, involves transfer of an adsorbate atom or molecule from the STM tip to the surface or vice versa. The tip is moved toward the surface until the adsorption potential wells on the tip and the surface coalesce, with the result that the adsorbate, which was previously bound either to the tip or the surface, may now be considered to be bound to both. For successful transfer, one of the adsorbate bonds (either with the tip or with the surface, depending on the desired direction of transfer) must be broken. The fate of the adsorbate depends on the nature of its interaction with the tip and the surface, and the materials of the tip and surface. Directional adatom transfer is possible with the apphcation of suitable junction biases. Also, thermally-activated field evaporation of positive or negative ions over the Schottky barrier formed by lowering the potential energy outside a conductor (either the surface or the tip) by the apphcation of an electric field is possible. FIectromigration, the migration of minority elements (ie, impurities, defects) through the bulk soHd under the influence of current flow, is another process by which an atom may be moved between the surface and the tip of an STM. 

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. 

In this context it should be mentioned that the height of the Schottky barrier depends on the proc iure of metal deposition and also on the pretreatment. Aspnes and Heller have investigated for instance metal-semiconductor contacts produced by depositing Ru, Rh or Pt as 400 A thick films. They found barrier heights for the metal in contact with air, of 0.6 eV for Ru on Ti02, which decreased to zero in the presence of hydrogen. These results are consistent with those of Yamamoto et al. . ... 

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

The rate of electron accumulation at ionized traps in the depletion zone of the Schottky barrier in the Au/ZnO contact is in proportion to the concentration of unoccupied traps, frequency of metal parti-cle/metastable atom interaction events, and to the probability of electron capture per a trap in a single event of interaction between metastable atoms and metal particle. 

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. 

Fig. 11. Schottky diode device used for measurement of chemicurrents. Highly exoergic surface reactions like adsorption of an atom to the surface produce excited electrons and holes. Some of these electrons are able to surmount the Schottky barrier and arrive at the semiconductor conduction band. This results in a detectable chemicurrent. (From Ref. 64.)...

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. 

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. 

The Schottky barriers were excellent diodes for films annealed at 600 °C, with turn on voltages of 0.6-0.8V and minimal reverse bias leakage.48 However, many of the contacts on the as-deposited films gave large reverse bias currents and nearly ohmic responses. This behavior is indicative of degeneracy of the semiconductor because of a high carrier density resulting from native defects. The improvement in the diode behavior of the annealed films is attributed to enhanced crystallinity and reduction of defects. 

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... 

C-V and I-V measurements of Si electrodes of different doping density in electrolytes free of fluoride show that in this case the dark current becomes dominated by thermally activated electron transfer over the Schottky barrier rather than by carrier generation in the depletion region [ChlO]. Note that the dark currents discussed above may eventually initiate the formation of breakdown type meso-pores, which causes a rapid increase of the dark current by local breakdown at the pore tips, as shown in Fig. 8.9. This effect is enhanced for higher values of anodic bias or doping density. 

Other types of photodiodes not discussed here include the Schottky barrier, pn homojunction, and heterojunction. 

The energy barrier of a depletion layer (the potential across a depletion layer I I) is called the Schottky barrier in semiconductor physics. Assuming that all the impurity donors or acceptors are ionized to form a fixed space charge in the depletion layer, we obtain the following approximate equation, Eqn. 5—75, for the thickness of depletion layer, dx, [Memming, 1983] ... 

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... 

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... 

Parenthetically, no clear indication of the presence of MnAs clusters has been observed in the transport results, even in the cases where direct magnetization measurements detect their presence. One of possibilities is that the Schottky barrier formation around the MnAs clusters prevents their interaction with the carriers. 

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 width of the space charge layer depends on the height of the Schottky barrier according to... 

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. 

Tetraphenylporphine (TPP) and other metal porphyrine derivatives coated on platinum (87,88,89) or gold (89,90) electrodes have been investigated in photoelectrochemical modes. Photocurrents reported are cathodic or anodic, depending on the pH as well as the composition of the electrolyte employed. Photocurrent quantum efficiencies of 2% (89) to 7% (87) were reported in systems using water itself or methylviologen as the redox species in aqueous electrolyte. Photocurrent generation at Zn-TPP-coated metal cathodes (89) was interpreted in terms of a rectifying effect of the Schottky barrier formed at a metal-p-type... 

There have been several proposed mechanisms for the operation of these sensors (Gopel, 1985 Franke et al., 2006). They all seem to converge on the existence and modulation of the Schottky barrier heterojunctions formed between the grains of the polycrystalline layer. They are equivalent to a chain of resistive elements connected in series. The density of surface states affects the depth of the Schottky barrier and depends on the interaction with the adsorbate (Fig. 8.8). The size of the grains apparently plays a major role. As the diameter of the grains decreases to below 5 nm, the space charge is smeared and the relative response of the sensor increases (Fig. 8.9). 

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