2.19.3 Electronic vs Ionic Compensation of Solutes

Here we will discuss which of the electronic or ionic compensation of solute incorporation in oxides is favoured and what are the conditions determining this.

In oxide semiconductors, the effectiveness of a donor or an acceptor is not only governed by their ionization energies, it is also governed by the extent of oxidation and reduction, even in case of shallow dopants with smaller ionization energies. This is due to the fact that an aliovalent impurity in an ionic compound can be charge compensated by ionic defects (ionically compensated ) or by electrons or holes (electronically compensated ) or by a combination of the two. Variables governing the extent of these are pO₂, dopant  concentration and temperature.

We will take the example of Nb₂O₅  doping in TiO₂.

The defect reactions are written as

Ionic compensation		(1)
Electronic compensation		(2)

Combination of the two reactions i.e. ((1) – (2)) leads to

(3)

Equation (3) shows that as pO₂ increases, oxidation is favored and hence formation of titanium vacancies is more likely. Similarly, as the temperature reduces, oxidation is again favored.

Thus Nb doping of TiO₂ tends to  be compensated by V_Ti''''  if Nb₂O₅ concentration is large, pO₂  is high and the temperature is low ,whereas the inverse conditions favour the electronic compensation.

In any case, the electrical neutrality condition requires that

(4)

Similar effects are observed in care of titanates such as BaTiO₃.