2, Integrated Materials Design Centre, University of New South Wales, Sydney, New South Wales, Australia
ALD-ZnO thin films can achieve conductivities of ≤3 mOhm cm without any intentional doping such as by supercycling ALD-Al2O3 to deposit Al-doped ZnO (AZO). Whereas the latter allows for conductivities down to the 10^-4 Ohm cm range, the mobility of ALD-AZO is significantly reduced by impurity scattering and the free carrier absorption is increased, which induces e.g. optical losses when applied to solar cells. Generally, two different models for the n-type conductivity in ZnO are discussed in the literature: intrinsic ZnO point defects and hydrogen. However, ZnO deposited or grown by methods other than thermal ALD rarely reach the very high conductivity range that is essential for its application as transparent conductive oxide (TCO). Hence, ALD appears to introduce a very efficient intrinsic doping mechanism into the material unlike the other methods. We will show via T-dependent Hall-measurements that the intrinsic doping is so high that it induces a semiconductor-to-metal-transition (Mott transition). By means of isotope studies (deuterium vs. hydrogen) the origin of the ~1 at% H-concentration in the ALD-ZnO thin films is determined. Using a comprehensive range of structural-chemical characterization methods (FTIR, NRA, NMR, c-AFM, XANES at the O and Zn K-edges) the bonding configuration of H in ZnO is studied in detail. Finally, different models for the incorporation of H into the ZnO network and their resulting density of states (DOS) are simulated by density functional theory (DFT) to explain the very high conductivity of ALD-ZnO.