Two dimensional (2D) metallic conductivity has been studied for more than a decade because the unusual surface metallic states of ultrathin films may result in applications for 2D superconductors, spintronics and optoelectronic devices. Surface metallic states were predicted on ZnO polar surfaces, which is caused by charge transfer from O-terminated (000-1) surface to Zn-terminated (0001) surface to stabilize the polar instability.¹ This charge transfer in polar thin films was also found to cause abrupt phase transition (i.e. ferromagnetism,² and semiconductor to semimetal transition³), opening a new way to design functional devices by engineering surface or interface of ultrathin films.

2D metallic conductivity on ZnO has not been previously demonstrated in experiments. The charge transport properties of 2D surfaces have proved to be difficult to measure because of the dominant effect of bottom, bulk conduction channels. In our study, we reduce the ZnO thickness to several nanometer to suppress the bulk conduction channels and first experimentally show 2D metallic states on polar ZnO surfaces through surface conductivity measurements on SiO₂ substrates. Then, introduction of a high density of oxygen vacancies to the polar ZnO surfaces can cause metal to semiconductor transition. These experimental results are also confirmed by the first principle calculations.

[1] Wander, A.; Schedin, F.; Steadman, P.; et al., Phys. Rev. Lett. 2001, 86, (17), 3811-3814.

[2] Wang, X. R.; Li, C. J.; Lu, W. M.; et al., Science 2015, 349, (6249), 716-719.

[3] Kim, J.; Baik, S. S.; Ryu, S. H.; et al., Science 2015, 349, (6249), 723-726.