Schottky layers

A crucial element in MTR is the profile of the localized state density as a function of eneigy, the so-called density of states (DOS). Unfortunately, a direct derivation of the DOS from the variation of the mobility is not straightforward. In two papers published in 1972 and 1976 [116, 117], Spear and Le Comber developed a method based on a simplified description of the accumulation layer, which was assumed to behave like a depletion (Schottky) layer, with a constant density of carrier up to a given thickness L This method has been more recently analyzed by Powell [118], who concluded that is was only able to give a rough estimate of the DOS. Nevertheless, we have used this method to estimate the DOS in 6T and DH6T [115] and found an exponential distribution of the form... 

Figure 43. Temperature dependence of total and partial conductivities of (a) bulk and (b) grain boundary111 in a 0.016 wt% Fe-doped SrTi03 bicrystal at about 2.0 Pa oxygen. The lower electronic and the much lower ionic grain boundary values are due to Mott-Schottky layer (see Part I, Section V).233 Reprinted from X. Guo, J. Fleig and J. Maier, J. Electrochem. Soc., 148 (2001) J50-J53. Copyright 2001 with permission from The Electrochemical Society, Inc.

In general, however, even at equilibrium, there may also be a double layer, called space charge, next to the surface extending into the oxide interior (Mott-Schottky layer). The width of the space charge layer will be of the order of the... 

Four different types of junctions can be used to separate the charge carriers in solar cebs (/) a homojunction joins semiconductor materials of the same substance, eg, the homojunction of a p—n sibcon solar ceb separates two oppositely doped layers of sibcon 2) a heterojunction is formed between two dissimbar semiconductor substances, eg, copper sulfide, Cu S, and cadmium sulfide, CdS, in Cu S—CdS solar cebs (J) a Schottky junction is formed when a metal and semiconductor material are joined and (4) in a metal—insulator—semiconductor junction (MIS), a thin insulator layer, generaby less than 0.003-p.m thick, is sandwiched between a metal and semiconductor material. 

The most common Schottky contacts for compound semiconductors are gold-based metallizations deposited by thermal or electron beam evaporation. The metal may include a thin titanium layer in direct contact with the semiconductor which acts as an adhesion layer. AdditionaHy, a thin layer... 

Schematic energy level diagrams of a metal/polymer/metal structure before and after the layers are in contact are shown in the top two drawings of Figure 11-6. Before contact, the metals and the polymer have relative energies determined by the metal work functions and the electron affinity and ionization potential of the polymer. After contact there is a built-in electric field in the structure due to the different Schottky energy barriers of the asymmetric metal contacts. Capacitance-voltage measurements demonstrate that the metal/polymer/metal structures are fully depleted and therefore the electric field is constant throughout the bulk of the structure [31, 35]. The built-in potential, Vhh i.e. the product of the constant built-in electric field and the layer thickness may be written...

MIM or SIM [82-84] diodes to the PPV/A1 interface provides a good qualitative understanding of the device operation in terms of Schottky diodes for high impurity densities (typically 2> 1017 cm-3) and rigid band diodes for low impurity densities (typically<1017 cm-3). Figure 15-14a and b schematically show the two models for the different impurity concentrations. However, these models do not allow a quantitative description of the open circuit voltage or the spectral resolved photocurrent spectrum. The transport properties of single-layer polymer diodes with asymmetric metal electrodes are well described by the double-carrier current flow equation (Eq. (15.4)) where the holes show a field dependent mobility and the electrons of the holes show a temperature-dependent trap distribution. 

A polymer layer al a contact can enhance current How by serving as a transport layer. The transport layer could have an increased carrier mobility or a reduced Schottky barrier. For example, consider an electron-only device made from the two-polymer-layer structure in the top panel of Figure 11-13 but using an electron contact on the left with a 0.5 eV injection barrier and a hole contact on the right with a 1.2 cV injection barrier. For this case the electron current is contact limited and thermionic emission is the dominant injection mechanism for a bias less than about 20 V. The electron density near the electron injecting contact is therefore given by... 

In a MESFET, a Schottky gate contact is used to modulate the source-drain current. As shown in Figure 14-6b, in an //-channel MESFET, two n+ source and drain regions are connected to an //-type channel. The width of the depletion layer, and hence that of the channel, is modulated by the voltage applied to the Schottky gate. In a normally off device (Fig. 14-9 a), the channel is totally depleted at zero gate bias, whereas it is only partially depleted in a normally on device (Fig. 14-9 b). 

In Eq. (4.5.5), describing an n-type semiconductor strongly doped with electron donors, the first and third terms in brackets can be neglected for the depletion layer (Af0 kT/e). Thus, the Mott-Schottky equation is obtained for the depletion layer,... 

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