For the piezoelectric applications ZnO is considered as insulator; its relative dielectric permittivity is eps33=9.03. But in photovoltaic applications it is considered a semiconductor with Eg=3.4 EV. In which circumstances is the ZnO considered an insulator, semiconductor, or conductor?
Stoichimetric ZnO is an insulator with very high resistivity. mostly seen when ZnO is in the form of single crystal, and a well sintered ceramic, or deposited as a stoichiometric film prepared by any technique (chemical , or physical). Stoichiometric means Zn/O ratio.
It can be become a semiconductor or a conduction when it becomes non stoichiometric, which means oxygen deficient, and the presence of other defects.
It is not a very good semiconductor, because its mobility is very low (max 10), and the carrier concentration depends on the non stoichiometry.
Reduced ZnO, which is highly deficient in oxygen, and also alongwith the presence several kinds of defects can pick up high carrier concentration and slowly become a degenerate semiconductor, and then it becomes a conductor, like the transparent conducting ZnO films. But once again it is not not give a high conductivity close to the normal metals, and will limited to the range of 10-4 or slightly better.
controlling the carrier concentration is the most critical part while making ZnO as a semiconductor, and a good conductor.
K. Sreenivas
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