Nonstoichiometric semiconductors

Figure 7.12 Schematic of energy levels for (a) intrinsic semiconductors, (b) extrinsic semiconductors, and (c) nonstoichiometric semiconductors.

To appreciate the similarities between an extrinsic semiconductor and a nonstoichiometric oxide, compare Fig. 7.13a with Fig. 6.4a or b. In both cases the electron(s) is(are) loosely bound to its(their) mooring(s) and is(are) easily excited into the conduction band. The corresponding energy diagrams for the singly ionized and doubly ionized oxygen vacancies are shown in Fig. 7.12c. In essence, a nonstoichiometric semiconductor is one where the electrons and holes excited in the conduction and valence bands are a result of reduction or oxidation. For example, the reduction of an oxide entails the removal of oxygen atoms, which have to leave their electrons behind to maintain electroneutrality. These electrons, in turn, are responsible for conduction. 

What connection is there between the structure of semiconductors and their properties As already mentioned nonstoichiometric semiconductor oxides play an important role. On heating, their crystal lattices tend to release or take up oxygen. For an n-type semiconductor such as ZnO, the release of oxygen is described by Equations 5-42 and 5-43. 

The general behavior of nonstoichiometric semiconductor oxides is summarized in Table 5-22. Table 5-23 classifies the most important oxides according to their electronic behavior. 

Catalyst Concepts in Heterogeneous Catalysis 1159 Table 5-22 Behavior of nonstoichiometric semiconductor oxides... 

Nonstoichiometric semiconductors are very similar to extrinsic semiconductors, and can really be con-... 

Dopants or impurities may transform oxides in nonstoichiometric semiconductors which can be attributed to ... 

Nonstoichiometric oxide phases are of great importance in semiconductor devices, in heterogeneous catalysis and in understanding photoelectric, thermoelectric, magnetic and diffusional properties of solids. They have been used in thermistors, photoelectric cells, rectifiers, transistors, phosphors, luminescent materials and computer components (ferrites, etc.). They are cmcially implicated in reactions at electrode surfaces, the performance of batteries, the tarnishing and corrosion of metals, and many other reactions of significance in catalysis. ... 

The interest of physicists in the conducting polymers, their properties and applications, has been focused on dry materials 93-94 Most of the discussions center on the conductivity of the polymers and the nature of the carriers. The current knowledge is not clear because the conducting polymers exhibit a number of metallic properties, i.e., temperature-independent behavior of a linear relation between thermopower and temperature, and a free carrier absorption typical of a metal. Nevertheless, the conductivity of these specimens is quite low (about 1 S cm"1), and increases when the temperature rises, as in semiconductors. However, polymers are not semiconductors because in inorganic semiconductors, the dopant substitutes for the host atomic sites. In conducting polymers, the dopants are not substitutional, they are part of a nonstoichiometric compound, the composition of which changes from zero up to 40-50% in... 

The situation with zinc oxide, ZnO, a material that has been investigated for a similar number of years, is comparable. Usually, nonstoichiometric ZnO is an n-type semiconductor. In the past it has been generally accepted that this is due to an excess of Zn in the form of Zn+ interstitials ... 

The high-pressure region is associated with the electroneutrality equation [h ] = 2[V ]. Holes predominate, so that the material is a p-type semiconductor in this regime. In addition, the conductivity will increase as the g power of the partial pressure of the gaseous X2 component increases. The number of metal vacancies (and nonmetal excess) will increase as the partial pressure of the gaseous X2 component increases and the phase will be distinctly nonstoichiometric. There is a high concentration of cation vacancies that would be expected to enhance cation diffusion. 

The discussion of Brouwer diagrams in this and the previous chapter make it clear that nonstoichiometric solids have an ionic and electronic component to the defect structure. In many solids one or the other of these dominates conductivity, so that materials can be loosely classified as insulators and ionic conductors or semiconductors with electronic conductivity. However, from a device point of view, especially for applications in fuel cells, batteries, electrochromic devices, and membranes for gas separation or hydrocarbon oxidation, there is considerable interest in materials in which the ionic and electronic contributions to the total conductivity are roughly equal. 

On the substrate side, the same process occurs for the holes, but on a different energy level. The holes are injected by a high work function metal or semiconductor like the transparent ITO, which consists of a nonstoichiometric composite... 

Kim YS, Ha SC, Kim K et al (2005) Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film. Appl Phys Lett 86(21) 213105-1-213105-3... 

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. 

Nonstoichiometric composition producing impurity levels can arise in two ways, either (1) excess atoms in interstitial positions or (2) holes in the lattice. Both methods [(1) and (2)] are theoretically possible in n- and p-type semiconductors. 

In the extreme class III behaviour,360-362 two types of structures were envisaged clusters and infinite lattices (Table 17). The latter, class IIIB behaviour, has been known for a number of years in the nonstoichiometric sulfides of copper (see ref. 10, p. 1142), and particularly in the double layer structure of K[Cu4S3],382 which exhibits the electrical conductivity and the reflectivity typical of a metal. The former, class IIIA behaviour, was looked for in the polynuclear clusters of copper(I) Cu gX, species, especially where X = sulfur, but no mixed valence copper(I)/(II) clusters with class IIIA behaviour have been identified to date. Mixed valence copper(I)/(II) complexes of class II behaviour (Table 17) have properties intermediate between those of class I and class III. The local copper(I)/(II) stereochemistry is well defined and the same for all Cu atoms present, and the single odd electron is associated with both Cu atoms, i.e. delocalized between them, but will have a normal spin-only magnetic moment. The complexes will be semiconductors and the d-d spectra of the odd electron will involve a near normal copper(II)-type spectrum (see Section 53.4.4.5), but in addition a unique band may be observed associated with an intervalence CuVCu11 charge transfer band (IVTC) (Table 19). While these requirements are fairly clear,360,362 their realization for specific systems is not so clearly established. 

Crystal Self-Diffusion in Nonstoichiometric Materials. Nonstoichiometry of semiconductor oxides can be induced by the material s environment. For example, materials such as FeO (illustrated in Fig. 8.14), NiO, and CoO can be made metal-deficient (or O-rich) in oxidizing environments and Ti02 and Zr02 can be made O-deficient under reducing conditions. These induced stoichiometric variations cause large changes in point-defect concentrations and therefore affect diffusivities and electrical conductivities. 

Photocatalytic decomposition of water on semiconductors is usually conducted under reduced pressure. There are arguments that 02 production is much less than stoichiometric when water photolysis is carried out under atmospheric pressure.20-34,353 Such nonstoichiometric 02 evolution is often ascribed to the photoadsorption of 02 onto semiconductor particles20,34,353 or the formation of peroxides.363 On the other hand, the electrochemical potential of H2 evolution shifts to the positive direction with increasing ambient pressure according to the Nemst equation. Therefore, pressure effect may be negatively significant for water photolysis by Ti02 photocatalysts, since tne flat band potentials of TiO are close to the potential of NHE.23,243... 

Nanoparticles of nonstoichiometric tungsten oxides W03 x are promising material to produce active elements for hydrogen sensors. High work temperature that causes degradation processes is a problem of exploitation of gas sensors based on nanoparticles of semiconductor oxides. 

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