Generally metals have valence and conduction band overlapping each other as the case in Zn. Can anyone explain, what role does oxygen play in creating the 'Band gap' of ~3.3 eV (at room temp) in ZnO?.
What is the reason of conductivity in ZnO in spite of such large band gap.
I am developing ZnO TCO layer using magnetron sputtering.
Band structure of a solid is determined by the atomic arrangement in the solid and the nature of these atoms. Please refer to the Kronig Penney model you must have studied. If a forbidden gap exists between the last occupied band (containing valence electrons, and called valence band) and the next unoccupied band (called conduction band), then the solid is an insulator or semiconductor. If there is no forbidden gap, the solid is a conductor. The crystal structure of Zn and ZnO are very different and the bonding (sharing of electrons) between Zn-Zn and Zn-O is also very different. This causes changes in band structure.
As regards conductivity of ZnO, it arises due to oxygen vacancies (due to non-stoichiometry) inherently present in ZnO. Each oxygen vacancy provides two free electrons. The number of vacancies created depends on how you prepare ZnO (oxygen deficient environment while processing ZnO (during deposition or post-deposition). Lot of literature and review articles are available to read (1980-1990).
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