Simultaneous control of growth mode and ferromagnetic ordering in Co-doped ZnO layers with Zn polarity

In this paper, we describe a new growth technique realizing the simultaneous control of growth mode and ferromagnetic ordering in Co-doped ZnO layers. Remarkable changes in growth occurred as a result of doping with Co ions. Exposure to O₂ plasma to generate an oxygen-rich atmosphere was indispensable for the fabrication of undoped ZnO layers given stabilization of the negatively charged growing surface in a two-dimensional (2D) mode. In contrast, the 2D mode of Zn0.94Co0.06O layers was adequately retained in the absence of a plasma source, and was attributable to the covalence provided by the $3d$ character of Co ions compared with the $4s$ character of Zn ions. Spontaneous magnetization of Zn0.94Co0.06O layers was closely correlated with the number of free electrons, which was controlled by the activation energy of donor levels. This was also confirmed for O-polar Zn0.94Co0.06O layers. Ferromagnetic (FM) ordering was observed at nₑ values close to the Mott transition and led to hopping conduction between shallow donor bands. However, levels of p(O₂) below 10^-60.3em0exmbar that yielded FM ordering generated a pitted surface and led to crystalline deterioration. Given our understanding of the formation mechanism of pits, we applied a periodic oxygen pressure-modulated epitaxy that resulted in the coherent growth of Co-doped ZnO layers with a 2D mode. This specific growth was effective in producing pit-free surface uniformities over large areas that maintained FM ordering.