Density donor

Donor Density Receptor density Thickness of barrier, mm Delay in microsecs... 

Doping can be divided into two parts native doping (e.g., S vacancies) and extrinsic doping by foreign elements. This section deals with the latter, not because it is more important but because there is httle in the literature to link native doping with the electrical properties of CD films. It will be enough to note that the few measurements of No (donor density) carried out tend to give values typically... 

We have studied the photoelectrochemical behavior of Chi a and Chi b on an n-type Sn02 (60) optically transparent electrode (OTE thickness of the Sn02 layer on the glass substrate, ca. 2000 A donor density, 1020-21cm-3). Chi monolayer assemblies, deposited by means of the Langmuir-Blodgett technique (20,21), were employed. The use of such monolayer assemblies as interfacial dye layers... 

The situation is different when donor and acceptor molecules are located at different interfaces, that are separated by a fatty acid monolayer of well defined thickness. There are no longer close pairs of donor and acceptor with a high probability of electron transfer as in the "contact" case. Consequently, no change in relative fluorescence intensity with increasing donor density is expected, contrary to the former case. Indeed, in systems with a spacer monolayer of... 

Figure 7. Photoinduced electron transfer in monolayer systems with the cyanine dye CY as donor and the viologen derivative SV as acceptor. Relative fluorescence intensity of the donor monolayer vs. donor density at constant acceptor density. Bars Donor and acceptor at the same interface, density of A, o(A) = 0.01 nm". Circles donor and acceptor at different interfaces, distance 2.3 nm, o(A) = 0.43 nm-. ...

The literature abounds with reports of thermal activation energies for shallow donors in GaN, obtained from Hall effect measurements over a range of temperatures, above and below room temperature, though their interpretation is rendered problematic by a number of complicating factors. At low temperatures there is clear evidence for impurity band conduction (see, for example, [31]) which severely limits the temperature range over which data may usefully be fitted to the standard equation for free carrier density n in terms of the donor density ND and compensating acceptor density NA ... 

Here e is the relative permittivity of the solid and Nd is the donor density. W0 ranges from 10-4 to 10-6 cm for doping densities from 1015 to 1019... 

Fig. 8.14. Theoretical plots showing competition between recombination and current doubling. Calculated for a surface state density of 5 x 10,2cm "2. The surface state is located 0.3eV below the bulk Fermi level. Donor density 1.5 x I0,ftcm. Arinj 5 x 104s" rccn = 2 x 10 7 s. Band bending values (a) 0.40 eV, (b) 0.35 eV, (c) 0.30 eV, (d) 0.2 eV. Note the transition in the IMPS response from current from current doubling control at 0.4 eV to...

Figure 9. A schematic representation of the situation (F = 0 K) for P doped Si at both high and low donor densities. Also shown are two scenarios for the composition dependence of the electrical conductivity, showing the metal—nonmetal transition.

As shown in Fig. 5, this pattern of behaviour becomes less clear-cut as the donor density and/or the dielectric constant within the semiconductor increase as Escnh and selCN0 become comparable, the depletion layer shrinks and an increasing proportion of the potential drop is accommodated within the Helmholtz layer. 

An example of the type of behaviour encountered for exponential surface-state distributions is provided by n-Fe203 in 1M NaOH [69]. The equivalent conductance and susceptance of the circuit comprising Zss in parallel with C8c clearly show a power law dependence on values obtained from this model again obeyed the Mott-Schottky relationship, although the donor density of 8 x 1018 cm 3 and dielectric constant of 25 suggest that the true flat-band potential may lie rather positive of the value given. 

Fig. 58. Potential dependence of break-down currents at ZnO electrodes with different donor densities.

Figure 58 shows the current density for n-ZnO at positive bias potentials [130]. It can be seen that, above a certain potential, which depends upon the donor density, there is a sharp increase in the current. For the most highly doped material, this rise occurs just above the potential at which the conduction band in the bulk has the same energy as the valence band at the surface. Higher potentials are needed with the lower iVD values and the obvious model is that holes are generated in the VB at the surface by electron tunneling from the VB to the CB. The classical tunneling distance is easily seen to be... 

The solution to Poisson s equation for the depletion layer is discussed further in Chapter 9. The hatched region in Fig. 4.15 represents the gap states which change their charge state in depletion and so contribute to p(x). When there is a continuous distribution of gap states, p(x) is a spatially varying quantity. For the simpler case of a shallow donor-like level, the space charge equals the donor density N-a and the solution for the dependence of capacitance on applied bias, is (see Section 9.1.1)... 

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