ZnO based transparent conducting oxides (TCOs) offer earth abundant alternatives to conventional indium tin oxide (ITO) films. At the same time, ALD enables discretized doping of substitutional cations at a monolayer level, providing fine and exquisite control over dopant concentration and resultant properties. In this work, we present the ALD of tantalum (Ta) doped ZnO (TZO) films. Compared to dopants such as Al³⁺ and Ti⁴⁺, a Ta cation exists in +5 oxidation state and thus, can ideally transfer 3 electrons by substitutionally doping a Zn²⁺ site. We show that 30 nm TZO films with 2 at% Ta provides a minimum resistivity of 4 mW.cm at room temperature and optical transmissivity of > 86% in the visible range.

The precursors used for Zn, O and Ta are diethyl Zn (DEZ), H₂O and pentakis-dimethylamido Ta (PDMAT). The targeted % Ta is varied from 2 to 20% by introduction of PDMAT pulse between DEZ and H₂O pulses. The electron concentration and the mobility are measured using Hall measurements. A minimum resistivity of 4 mW.cm with carrier density of 9.0x10¹⁹ cm^-3 and a mobility of 17.7 cm²/V-sec is observed at 2 at% Ta at room temperature. UV-Vis spectroscopy indicates a monotonic increase in bandgap of TZO from 3.2 to 3.4 eV upon addition of Ta. Photoluminescence (PL) shows a quenching of the green band emission of ZnO associated with oxygen defects upon the addition of 2 at% Ta. Upon further Ta addition, a red shifted defect peak appears which is attributed to formation of new Zn vacancies. This data is supported by X-ray photoelectron spectroscopy (XPS), showing removal of O vacancies and a clear Ta-O-Zn bond formation for higher Ta doping. The conductivity of ZnO and 2% Ta doping ZnO behave differently as a function of temperature. TZO have different activation energy compare to pure ZnO. X-ray Diffraction (XRD) and Transmission Electron Microscope (TEM) indicate the crystallinity change with the Ta doping. The results provide a clear understanding on the mechanism of Ta doping in ZnO and its direct impact on vacancies and resulting electronic and optical properties.

Finally, ALD TZO is deposited on flexible glass substrates and inside nanoporous anodic alumina templates to demonstrate formation of transparent conducting electrodes, conformally on a variety of platforms of relevance to electrochemical research.