Semiconducting doping

In practice both natural and synthetic crystals are oxygen deficient, leading to donor levels approximately 0.1 eY below the bottom of the conduction band and consequently to n-type semiconductivity. Doping the crystal with group V elements also induces n-type semiconductivity the usual dopant is antimony. The ground state electronic configuration of the Sb atom is 5s2p3, and when it... 

Like semiconducting doped metal oxides such as Sn02 or ZnO, doped polymer films also vary their conductivity when exposed to some gases, but polymer based sensors work at ambient temperature. 

Plenary 14. A K Ramdas et al, e-mail address akr phYsics.purdue.edu (RS). Electronic RS studies of doped diamond as potential semiconducting materials. A Raman active Is (p3 2) ls (Pi/2 transition of a hole... 

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

Semiconductors may also be made from a maferial which is normally an insulator by infroducing an impurify, a process known as doping. Figure 9.9 shows fwo ways in which an impurify may promote semiconducting properties. In Figure 9.9(a) fhe dopanf has one more valence election per atom fhan fhe hosf and confribufes a band of filled impurify levels 1 close to fhe conduction band of fhe hosf. This characterizes an n-fype semiconductor. An example is silicon (KL3s 3p ) doped wifh phosphoms (KL3s 3p ), which reduces fhe band gap to abouf 0.05 eY Since kT af room femperafure is abouf 0.025 eY the phosphoms... 

Polymers. The Tt-conjugated polymers used in semiconducting appHcations are usually insulating, with semiconducting or metallic properties induced by doping (see Flectrically conductive polymers). Most of the polymers of this type can be prepared by standard methods. The increasing use of polymers in devices in the last decade has led to a great deal of study to improve the processabiUty of thin films of commonly used polymers. 

Polyaniline (PANI) can be formed by electrochemical oxidation of aniline in aqueous acid, or by polymerization of aniline using an aqueous solution of ammonium thiosulfate and hydrochloric acid. This polymer is finding increasing use as a "transparent electrode" in semiconducting devices. To improve processibiHty, a large number of substituted polyanilines have been prepared. The sulfonated form of PANI is water soluble, and can be prepared by treatment of PANI with fuming sulfuric acid (31). A variety of other soluble substituted AJ-alkylsulfonic acid self-doped derivatives have been synthesized that possess moderate conductivity and allow facile preparation of spincoated thin films (32). 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

Silane, pure or doped, is used to prepare semiconducting siUcon by thermal decomposition at >600° C. Gaseous dopants such as germane, arsine, or diborane maybe added to the silane at very low concentrations in the epitaxial growing of semiconducting siUcon for the electronics industry. Higher silanes, eg, Si H and Si Hg, are known but are less stable than SiH. These are analogues of lower saturated hydrocarbons. 

The polysdanes are normally electrical insulators, but on doping with AsF or SbF they exhibit electrical conductivity up to the levels of good semiconductors (qv) (98,124). Conductivities up to 0.5 (H-cm) have been measured. However, the doped polymers are sensitive to air and moisture thereby making them unattractive for practical use. In addition to semiconducting behavior, polysilanes exhibit photoconductivity and appear suitable for electrophotography (qv) (125—127). Polysdanes have also been found to exhibit nonlinear optical properties (94,128). 

Table 3 summarizes some of the present state-of-the-art parameters obtained for undoped and doped i -SiH(F) material thus produced. The device-quahty material exhibits semiconductivity because In G vs 10 /Texhibits a straight line with a conductivity activation energy of eV, which is... 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

The PTC materials already mentioned depend directly on the ferroelectric phase transition in solid solutions based on BaTi03, suitably doped to render them semiconducting. This is a typical example of the interrelations between different electrical phenomena in ceramics. 

Modifications of the conduction properties of semiconducting carbon nanotubes by B (p-type) and N ( -type) substitutional doping has also been dis-cussed[3l] and, in addition, electronic modifications by filling the capillaries of the tubes have also been proposed[32]. Exohedral doping of the space between nanotubes in a tubule bundle could provide yet an-... 

This restriction, however, could be circumvented by the doped CNT with either Lewis acid or base [32-36], since such doping, even to semiconductive CNT could enhance the density of states at the Fermi level as well as bring about the metallic property. Appearance of metallic conductivity in helical CNT by such doping process would be of interest in that it could make molecular solenoid of nanometer size [37]. 

Electronic structures of SWCNT have been reviewed. It has been shown that armchair-structural tubes (a, a) could probably remain metallic after energetical stabilisation in connection with the metal-insulator transition but that zigzag (3a, 0) and helical-structural tubes (a, b) would change into semiconductive even if the condition 2a + b = 3N s satisfied. There would not be so much difference in the electronic structures between MWCNT and SWCNT and these can be regarded electronically similar at least in the zeroth order approximation. Doping to CNT with either Lewis acid or base would newly cause intriguing electronic properties including superconductivity. 

Polyfarylene vinylene)s form an important class of conducting polymers. Two representative examples of this class of materials will be discussed in some detail here. There are poly(l,4-phenylene vinylcne) (PPV) 1, poly(l,4-thienylene viny-lenc) (PTV) 2 and their derivatives. The polymers are conceptually similar PTV may be considered as a heterocyclic analog of PPV, but has a considerably lowci band gap and exhibits higher conductivities in both its doped and undoped stales. The semiconducting properties of PPV have been shown to be useful in the manufacture of electroluminescent devices, whereas the potential utility of PTV has yet to be fully exploited. This account will provide a review of synthetic approaches to arylene vinylene derivatives and will give details an how the structure of the materials relate to their performance in real devices. 

---