Diffusion of donor elements in mono-crystalline zinc oxide: the interplay with zinc vacancies
Understanding donor diffusion in transparent and semiconducting oxides, like zinc oxide (ZnO), is crucial for device processing and has revealed important limitations to the electrical conductivity of the oxides. To be able to predict dopant distributions and concentrations, and to obtain atomistic information about the donor-defect interactions involved, we have conducted an extensive diffusion study of the group III elements boron (B), aluminium (Al), gallium (Ga) and indium (In) in ZnO.
Secondary ion mass spectrometry (SIMS) is used to investigate the diffusion behavior of the donor dopants B, Al, Ga and In in ZnO. A thin film of B-, Al-, Ga-, or In-doped ZnO was deposited onto hydrothermally grown single crystal bulk ZnO by the use of magnetron sputtering. To ensure dopant diffusion from the film into the bulk ZnO the samples were sequentially (isochronal) heat treated in controlled atmospheric conditions within the temperature range 700-1300 oC. The diffusion of Al, Ga and In all reveal abrupt diffusion fronts which can be modelled using the Fair model [1], including only the contribution of a double negatively charged defect. Further, to obtain detailed information about the dopant-defect interplay we use a recently developed reaction-diffusion type model [2]. This suggests that the diffusion of B, Al, Ga and In are mediated by the double negatively charged zinc vacancy (VZn2-), where the migration is strongly dependent on the local Fermi level position.
[1] R. B. Fair and J. C. C. Tsai, J. Electrochem. Soc. 124, 1107 (1977).
[2] K. M. Johansen et al. Phys. Rev. Applied 3, 024003 (2015).
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