P-type semiconducting metal oxides

FIGURE 22.38 Electronic energy diagrams for a metal electrode in contact with a p-type semiconducting metal oxide (a) prior to contact, (b) posterior to contact, and (c) with photoexcitation neF = quasi-Fermi level for photoexcited electrons in oxide, eM = Fermi level of metal dissolution reaction, and eHY = Fermi level of hydrogen reaction. 

Barsan, N., Simion, C., Heine,T., Pokhrel, S. and Weimar, U. (2010) Modehng of sensing and transduction for p-type semiconducting metal oxide based gas sensors . Journal of Electroceramics, 25(1), 11-19, DOI 10.1007/sl0832-009-9583-x... 

Sensing and transduction for p- and n-type semiconducting metal oxides... 

At the other end of the conduction spectrum, many oxides have conductivities dominated by electron and positive hole contributions to the extent that some, such as Re03, SnOa and tire perovskite LaCrOs have conductivities at the level of metallic conduction. High levels of p-type semiconduction are found in some transition metal perovskites especially those containing alio-valent ions. Thus the lanthairum-based perovskites containing transition metal ions, e.g. LaMOs (M-Cr, Mn, Fe, Co, Ni) have eirlranced p-type semiconduction due to the dependence of the transition metal ion valencies on the ambient... 

The procedure is best illustrated by an example. Suppose that a nonstoichiometric phase of composition MA can have an existence range, which spans both sides of the stoichiometric composition, MX, oo. Assume that in this phase only vacancies are of importance, so that the stoichiometric composition will occur when the number of vacancies on the cation sublattice is exactly equal to the number of vacancies on the anion sublattice, which is, therefore, due to a population of Schottky defects. At other compositions, electrical neutrality is adjusted via mobile electrons or holes, leading to n-type or p-type semiconductivity. Thus there are four defects to consider, electrons, e, holes, h, vacancies on metal sites, Vm, and vacancies on anion sites, Vx. Finally, assume that the most important gaseous component is X2 as is the case in most oxides, halides, and sulphides. 

Fermi level is a kind of measiue of equilibrium electronic energy of a solid material. It is thought that Fermi level is located just below the CB bottom and above the VB top for n-type and p-type semiconducting materials 13), respectively. Most metal oxides are categorized as n-type semiconductors with Fermi levels more cathodic (higher) than the standard electrode potential of electrolyte in contact with the metal oxide and thereby electrons in donor levels a little below the CB are injected into the electrol5rte to form a space charge (depletion) layer with an electric field, that is, Schottky barrier. In the... 

E3.41 Low oxidation number d-metal oxides can lose electrons through a process equivalent to the oxidation of the metal atoms, with the result that holes appear in the predominately metal band. The positive charge carriers result in their p-type semiconductor classification. NiO is an example of this p-type semiconduction. Early transitional metal oxides with low oxidation number such as TiO and VO have metallic properties owing to the extended overlap of the d orbitals of the cations. See Section 24.6b for more details. 

Metal sulfides and several important oxides display n-type or p-type semiconducting or nieialiic properties. As a result of their electronic conductivity, cermin minerals can participate in coupled charge tmasfer processes aanlogous to a metal corroding in an electrolye, and the kinetics of leaching can be related 1o the potential of the solid in contact with the aqueous electrolyte. 

Key words semiconducting metal oxide (SMOX), sensing and transduction of SMOXs, p- and n-type MOXs, conduction mechanism, modelling, measurements taken in working conditions. 

The operation of a metal-oxide gas sensor relies on the change in resistance of an n- or p-type semiconducting layer - mainly Sn02 - when exposed to reducing or oxidizing gases. 

On the other hand, p-type semiconducting oxides like Cu2-.,0, Ni jO, and CO., 0 have a deficit X of metal relative to ideal stoichiometry. This deficit increases with increasing oxygen partial pressure. Ionic lattice defects occurring in oxides of this type act as electron acceptors. In this case the concentration of the defects, and therefore the conductance, increases with increasing oxygen partial pressure according to the expression ... 

Normally, metal oxides such as SnO, TiO, WO3, ZnO, and Inj03 usually used for gas sensor design are n-type semiconductors (see Fig. 2.27). However, there are few metal oxides such as CrO, CuO, and NiO that also exhibit p-type conductivity (Krilov and Kisilev 1981 Stamataki et al. 2008). Recently p-type conductivity was reported in ZnO (Basu and Hazra 2005 Hazra and Basu 2006). Thus, the fabrication of semiconducting metal oxide p-n homojunctions could also be achieved with ZnO. It was shown that p-n ZnO homojunction is sensitive to H as there is a substantial shift in the forward bias I-V characteristics on exposnre to the reducing gases (Basu and Hazra 2005 Hazra and Basu 2006). 

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