Semiconductors nonstoichiometric oxides

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. ... 

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. 

The fraction of electrons of the valence band that are raised to the conduction band by thermal energy corresponds to the Boltzmann factor exp( — e/2kT). The /-type semiconductors play only a minor role in catalysis the n- and p-type semiconductors are far more important. Nonstoichiometric oxides and sulfides are of industrial importance. The conductivity of these materials is low but can be considerably increased by doping with foreign atoms. 

An n-type cation interstitial metal-excess oxide semiconductor has an excess of interstitial cations or oxygen anion vacancies in the crystal lattice. An excess of electrons maintains the neutrality and electrical conductivity. The oxygen vacancies are created from single-point defects. The nonstoichiometric oxide MO2-X with large cations is oxygen deficient and creates anion vacancies. 

A typical example of the application of EIS is the investigation of passive films on Zn, Zn-Co, and Zn-Ni (Fig. 7-18), which were carried out to explain the difference in the corrosion behavior of pure and low-alloyed zinc by the possible formation of electron traps through the incorporation of cobalt or nickel into the oxide film (Vilche et al., 1989). Passive films of zinc in alkaline solutions are known to be n-type semiconductors with a band gap Eg = 3.2 eV (Vilche et al., 1989). The n-type character arises from an excess of zinc atoms in the nonstoichiometric oxide. The impedance measurements in 1 N NaOH solution were carried out at potentials at which Faraday reactions like transpassive dissolution and oxygen evolution do not interfere. The passive layer was formed for 2 h at positive potential before the potential was swept in the negative direction for the impedance meas-... 

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 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. 

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. 

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. 

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. 

Gallium arsenide s native oxide is found to be a mixture of nonstoichiometric galhum and arsenic oxides and elemental arsenic. Thus, the electronic band structure is found to be severely disrupted, causing a breakdown in normal semiconductor behavior on the GaAs surface. As a consequence, the GaAs MISFET (metal insulator semiconductor field-effect transistor) equivalent to the technologically important Si-based MOSFET (metal-oxide semiconductor field-effect transistor) is, therefore, presently unavailable. 

Semiconductors are classified as p type if they tend to attract electrons from the chemisorbed species, or as n type if they donate electrons to this species. The p type are normally the compounds, such as NiO. The n type are substances which contain small amounts of impurities, or the oxide is present in nonstoichiometric amounts (as, for example, when some of the zinc in ZnO has been reduced). Reviews of semiconductors as catalysts are given by P. H. Emmett ( New Approaches to the Study of Catalysis, 36th Annual Priestly Lectures, Pennsylvania State University, April 9-13, 1962) and by P. G. Ashmore ( Catalysis and Inhibition of Chemical Reactions, Butterworths Co. (Publishers), London, 1963. 

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... 

Many of the oxides, carbides, and nitrides with the NaCl structure tend to be nonstoichiometric. Titanium monoxide exists over the range Tio.ssO to TiO, while FeO never occurs it is always nonstoichiometric with a composition ranging from Feo.goO to Feo.geO. As a consequence of these vacancies, the transition metal exists in two valence states, causing the oxide to exhibit semiconductor properties (as for NiO). 

Most oxide semiconductors are either doped to create extrinsic defects or are aimealed under conditions in which they become nonstoichiometric. 

On the substrate side, the same process occurs for the holes, but on a different energy level. The holes are injected with a high work fimction metal or semiconductor like the transparent indium-tin-oxide ITO, which consists of a nonstoichiometric composite of 10-20% Sn02 and 80-90% ln203. The work function of ITO depends strongly on the surface treatment and lies in the range of 4.4-S.2 eV [43,44]. As in the case of the cathode, hole injection can be improved by an additional layer of, e.g., copper phthalocyanine [45] or polyethylenedioxythiophene (FEDOT), doped with polystyrenesulfonic acid (PSS) [46]. The holes are injected into the hole transport layer and proceed... 

The number of charge carriers in an insulating compound or a semiconductor is related to the concentration of the impurities, which can be aliovalent ions or vacancies if the compound is nonstoichiometric. Binary nonstoichiometric compounds (many transition metal oxides are berthollides) can be n-type or p-type semiconductors (Table 4.7) even if they are not doped with other ions. The excess of electrons or holes is the result of a higher or lower valency of the lattice ions owing to oxidation or reduction that is caused by an excess or a depletion of oxygen ions. Whether a given oxide is n-type or p-type depends on the valency of the metal ion and not primarily on the structure of the lattice. 

Epitaphial effects of a scale can influence diffusivity as does any defect such as porosity, grain boundaries, cracks, dislocation substructures, etc. Impurity cations can have a great effect on diffusivity in the oxide depending on the valence of the impurity ion and the semiconducting properties of the scale. Common scales formed from oxides, sulfides, and nitrides can be classified as p-type, n-type, or amphoteric semiconductors. The p-type, metal-deficit scales are nonstoichiometric with cation vacancies present. Impurity ions with valencies greater than the p-type semiconductor will tend to increase the concentration of cation vacancies and, hence, diffusivity. Lower vacancy ions will have the opposite effect. Impurity ions with the same valence should have little effect on diffusion. The n-type semiconductors... 

Tungsten-based materials as n-type semiconductor with nonstoichiometric compositions are extremely stable under electrochemical oxidation conditions and could be used as a non-carbon support for catalyst. The interest in their use as catalyst support is due to the possible synergetic effect between metal catalyst and support. 

Most metal oxides are nonstoichiometric., such as Feo. O instead of the ideal molecular formula FeO. This characteristic may be due to the different concentration of cations (Cc) and anions (Go). If Gc > Go, the metal oxide is an n-type semiconductor since there is metal-excess. On the other hand, if Gc < Go, then a p-type semiconductor occurs due to metal-deficit condition. 

In these models, gases are or not dissociated, but react with the surface oxygen ion of the nonstoichiometric structure. In a semiconductor p-type (Eq. 4.42), the CO chemisorbs first on the cation, where it reacts with an oxide ion, forms CO2, and, consequently, leads to the reduction of the metal oxide in the metal. For... 

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

---