Doping p-type

A number of theoretical studies have addressed the fundamental microscopic aspects of doping in wide-bandgap semiconductors. The majority of these studies 

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Tyndalization Tyndall scattering Type 4340 alloy steel n-Type dopants p-Type doping... 

An alternative approach to stabilizing the metallic state involves p-type doping. For example, partial oxidation of neutral dithiadiazolyl radicals with iodine or bromine will remove some electrons from the half-filled level. Consistently, doping of biradical systems with halogens can lead to remarkable increases in conductivity and several iodine charge transfer salts exhibiting metallic behaviour at room temperature have been reported. However, these doped materials become semiconductors or even insulators at low temperatures. 

Commonly used II-VI compounds include zinc sulfide, zinc selenide, zinc telluride, cadmium sulfide, cadmium telluride, and mercury cadmium telluride. These materials are not as widely used as the III-V compounds, one reason being that it is difficult to achieve p-type doping. Mercury cadmium telluride is used extensively in military night sights, which detect in the 8-13 im spectral band (a similar material, platinum silicide, is being developed for that purpose). The major applications ofCVD II-VI compounds are found in photovoltaic and electroluminescent displays. 

Poly(2-methoxy, 5-(2 -ethylhexyloxy)-1,4-phenylene vinylene) MEH-PPV Emission peak = 605 nm p-type doping by sulfuric acid (H2SO4) -type doping by sodium (electron donor) Iodine (I2) = electron acceptor = > oxidizing agent... 

Regular jc-stack Peierls distorted 7E-stack p-type doping (a) (b) (c)... 

Figure 4 Schematic band structures for (a) a regular n-stack (b) a Peierls distorted n-stack and (c) a Peierls distorted n-stack after p-type doping...

Recent studies of doped a-Si H have found that the background density of localized states, that is, the electrically active dopants and dangling bond defects, are metastable (Ast and Brodsky, 1979 Street et al., 1986, 1987a Muller et al., 1986). After annealing above 150°C in the dark, the dark conductivity at room temperature of n- and p-type doped a-Si H decreases by nearly a factor of two over a time scale of several weeks for n-type and several hours for p-type a-Si H. As shown in Fig. 9 (Street et al., 1987a), the relaxation rate of the occupied band tail density nBT is a sensitive function of temperature, so that the time to reach... 

Fig. 14. Time dependence of the hydrogen diffusion coefficient for a p-type doped a-Si H sample annealed at 200°C (Street et al., 1987b).

Fig. 23. Hydrogen evolution rate against temperature for p-type doped a-Si H films for increasing diborane gas phase doping level, as indicated (Beyer and Wagner, 1981). 

In comparison to the research in n-type oxide semiconductors, little work has been done on the development of p-type TCOs. The effective p-type doping in TCOs is often compensated due to their intrinsic oxide structural tolerance to oxygen vacancies and metal interstitials. Recently, significant developments have been reported about ZnO, CuA102, and Cu2Sr02 as true p-type oxide semiconductors. The ZnO exhibits unipolarity or asymmetry in its ability to be doped n-type or p-type. ZnO is naturally an n-type oxide semiconductor because of a deviation from stoichiometry due to the presence of intrinsic defects such as Zn interstitials and oxygen vacancies. A p-type ZnO, doped with As or N as a shallow acceptor and codoped with Ga or Zn as a donor, has been recently reported. However, the origin of the p-type conductivity and the effect of structural defects on n-type to p-type conversion in ZnO films are not completely understood. 

Electrical cells based on semiconductors that produce electricity from sunlight and deliver the electrical energy to an external load are known as photovoltaic cells. At present most commercial solar cells consist of silicon doped with small levels of controlled impurity elements, which increase the conductivity because either the CB is partly filled with electrons (n-type doping) or the VB is partly filled with holes (p-type doping). The electrons have, on average, a potential energy known as the Fermi level, which is just below that of the CB in n-type semiconductors and just above that of the VB in p-type semiconductors (Figure 11.2). 

Highly p-doped layers can also be used as masking layers. If the p-type doping level of silicon substrates is high enough to cause degeneracy (NA > 1019 cm-3), a decrease in etch rate with doping density is observed in all alkaline solutions inde-... 

Fig. 3.3 The I—V curves, as recorded and compensated for ohmic losses (iRcor.), of Si electrodes in aqueous HF (1M HF, 0.5M NH4CI) are found to shift cathodically with increasing p-type doping density. In a V versus log(i) plot (inset) a p-type electrode (1 2 cm,...

It has been speculated that there is a common origin of the reduced chemical etch rate for (111) oriented silicon substrates and for highly p-type doped substrates. But the electrochemical investigations discussed above indicate that the passivation of highly doped p-type Si can be ascribed to an oxide film already present at OCP, while no such oxide film is observed on (111) silicon below PP. This supports models that ascribe the reduced chemical etch rate on (111) planes to a retarded kinetic for Si surface atoms with three backbonds, present at (111) interfaces [Gil, A12], as discussed in Section 4.1. 

For p-type electrodes with doping densities below 1018 cm-3 diffusion and thermionic emission of charge carriers across the SCR is dominant. For p-type doping densities below 1016 cm4 this charge transfer is associated with the formation of macropores, as discussed in Chapter 9. 

Fig. 8.3 SEM micrographs of the interface between bulk and meso PS for p-type doped (100) silicon electrodes anodized galvanostatically in ethanoic HF. After [Le23].

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