N-type behavior

Simple electroless techniques have been used for the formation of CdTe layers following an anodic or a cathodic route of deposition. For instance, spontaneous cathodic formation of CdTe was observed on Ti or glass electrodes short circuited with a corroding A1 contact (electron source) in a solution of Cd " " and HTe02 ions [96]. After thermal treatment and subsequent growth of an a-Pb02 layer on them, the as-obtained CdTe thin films were found to exhibit n-type behavior in alkaline polysulfide PEC cells. 

The microstructure of commercial varistors is extremely complex, and commercial preparations also contain other dopants, mainly oxides of cobalt, manganese, chromium, and antimony, that are used to fine tune the varistor characteristics. The transition-metal dopants are chemically similar to Zn2+ and mainly form substitutional defects within the ZnO grains, such as CoZn, that modify the n-type behavior of the grain interior. (See also Chapter 8 for further discussion of the electronic... 

On the other hand, n-type behavior was observed at more negative potentials, where excess Cd may have been present. Type conversion, from n-to p-, was also frequently achieved by annealing deposits near 350°C [218],... 

ZnS-CdS (bandgap = 2.3-2.4 eV) composite semiconductor photoelectrodes show a broad spectral response and n-type behavior, with saturation of the anodic photocurrent upon increasing anodic potential making the system suitable for use as a photoelectrochemical cell photoanode [72], Nanostructured ZnS-CdS thin film electrodes show that anodic photocurrent saturation can be attained with the application of a small, 0.1 V, bias [73], while hydrogen evolution is observed at the Pt cathode. The performance of the ZnS-CdS photoanodes appear strongly dependent upon the method of film preparation [72,73], with Zn rich films demonstrating superior photocurrent generation, and stability, in comparison to Cd rich films. 

HOMO of DBTTF and the LUMO of TCNQ, respectively (Fig. 4b). Source and drain electrodes are several organic metals of the TTF TCNQ type having different chemical potentials predicted using Fig. 4c which is the same as Fig. 2a. For the electrodes whose chemical potentials are set within the conduction band of the channel material, FET exhibited n-type behavior (A in Fig. 4d). When the chemical potentials of organic metals are allocated within or near the valence band of the channel, p-type behaviors were observed (E, F in Fig. 4d). When the chemical potentials of the electrodes are within the gap of the channel, FET exhibited ambipolar-type behavior (B-D in Fig. 4d). Since the channel material is the alternating CT solid, the drain current is not excellent and a Mott type insulator of DA type or almost neutral CT solid having segregated stacks is much preferable in this context. 

For nonstoichiometric compounds, the general rule is that when there is an excess of cations or a deficiency of anions, the compound is an n-type semiconductor. Conversely, an excess of anions or deficiency of cations creates a / -type semiconductor. There are some compounds that may exhibit either p- or n-type behavior, depending on what kind of ions are in excess. Lead sulfide, PbS, is an example. An excess of Pb + ions creates an n-type semiconductor, whereas an excess of ion creates a /7-type semiconductor. Similarly, many binary oxide ceramics owe their electronic conductivity to deviations from stoichiometric compositions. For example, CU2O is a well-known / -type semiconductor, whereas ZnO with an excess of cations as interstitial atoms is an n-type semiconductor. A partial list of some impurity-controlled compound semiconductors is given in Table 6.9. 

Alkali-Metal-Free Solutions. Films of CD PbS are usually p-type as deposited. One early suggestion to explain this was that the alkali metal ions used in the deposition solution (as NaOH or KOH) act as a p-type dopant [33]. Based on this supposition, Bloem deposited PbS from a solution of PbAci, hydrazine hydrate, and thiourea (free of Na or K). The as-deposited films were initially n-type but changed to p-type on exposure to air. Attempts to dope the films permanently n-type by adding trivalent ions to the deposition solution were unsuccessful. However, by depositing the films on a substrate coated with trivalent ions (such as Al, In, Ga), n-type behavior could be maintained for a considerable time. PbS p-n junctions were fabricated using this approach (see Chap. 9). 

It was found that 368 exhibits ambipolar activity with appreciable electron (0.12 cm2 V 1 s ) and hole (0.008 cm2 V 1 s-1) mobilities at the substrate growth temperature of 70°C, and 371 shows monopolar n-type activity with a high mobility of 0.32 cm2 V-1 s-1 for semiconducting films deposited at a substrate temperature of 25°C. 373 exhibits stable n-type activity even in the air although the observed electron mobility in the air (0.01 cm2 V-1 s ) is somewhat lower than that under vacuum (0.08 cm2 V-1 s-1). In the case of 368 films grown at 70°C, both n- and p-type mobilities as well as current on-off ratios are enhanced by orders of magnitude compared to films grown at 25°C. 369 exhibits only p-type activity, with no detectable n-type behavior. Hole mobility extracted from the transfer plot is 5 x 10-14 cm2 V-1 s 1 with... 

Regarding the electrical properties of other dodecaborides, interestingly, Hamada et al. have reported that ScBi2 has p-type conduction (Hamada et al., 1993) in contrast to the n-type behavior observed for the other trivalent RB12 compounds and which is expected for these metals with excess electrons in the bonding as noted above. 

As noted in Section 9, the structures of the R-B-C(N) compounds (Figure 21) are homologous to that of boron carbide which exhibits typical p-type characteristics. Boron carbide is the limit where the number of boron icosahedra and C-B-C chain layers separating the metal layers reaches infinity (i.e. no rare earth layers). It has been speculated that the 2 dimensional metal layers of these rare earth R-B-C(N) compounds are playing a role for the unusual n-type behavior, but the mechanism is not yet clear. 

Recent investigations have revealed that the intrinsic behavior of RB28.5C4 is also n-type (Mori et al., 2008a). Very small inclusions of boron carbon "B4C" can cause the p-type behavior previously observed in some samples (Mori and Nishimura, 2006). The origin of the striking n-type behavior observed in the homologous R-B-C(N) compounds is not completely resolved yet but indicated to pertain to the two-dimensional rare earth layers (Mori et al., 2008a). 

The most dramatic example of the use of functionalization to tune the properties of an organic semiconductor is found in perfluoropentacene 41 (Scheme 3.9) [53]. Perfluoropentacene has a strongly shifted LUMO, evidenced by lowering of the reduction potential by more than 700 mV. The fluorinated compound adopts a herringbone packing in the crystal that is very similar to that of the parent acene. Top-contact FET devices made from evaporated 41 had an electron mobility of 0.11 cm2 V-1 s 1, demonstrating the change from p-type to n-type behavior. 

Applications of CNTs based on their electrical properties strongly depend on the diameter and helicity as well as parity. Doping of CNTs by boron and nitrogen renders them p-type and n-type, respectively. MWNTs and SWNTs doped with nitrogen " and boron have been reported. Boron-doped carbon nanotubes appear to exhibit enhanced electron field emission due to the presence of the boron atom at the nanotube edges. - N-doped CNTs show n-type behavior regardless of tube chirality. ... 

Taking this into account, the IMPS plots at FiePcZn electrodes were discussed in a model shown in Figure 10.10 - . As already seen in the photocurrent transient measurements, the n-type behavior of FiePcZn is compensated by oxygen after storage in air. Both electrons and holes are photogenerated in significant amounts compared to their concentration in the dark. At +400 mV vs. 

The ab initio calculations were successfixl in describing the surface electronic structure of the uppermost few Angstroms. But, because the samples showed n-type behavior, it is likely that the redistribution of electrons in response to the surface involved... 

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