Doping Mechanism in Transparent Conductive ZnO Films Grown by Atomic Layer Deposition

Transparent conducting oxides (TCOs) have been one of the essential elements of various optoelectronic devices, photovoltaics, and photoelectrochemical energy-harvesting cells which require an electrically conducting layer (s ~10³-10⁴ S/cm) with high optical transparency (>80%). The TCOs are typically doped with foreign elements, as in the cases of Sn-doped In₂O₃ (ITO) and Al-doped ZnO (AZO), and become degenerate semiconductors with an extremely high electron concentration of 10²⁰-10²¹ cm^-3. While TCOs have been deposited by various methods including sputtering, pulsed laser deposition, chemical vapor deposition, and spray pyrolysis, recent emergence of nanostructured solar cells and organic light emitting diodes has accelerated necessity of ultrathin TCO coating onto complex or large surface area substrates at a low deposition temperature which are quite challengeable for the conventional deposition methods. In order to meet those emerging requirements, atomic layer deposition (ALD) has gathered great interest. Since ALD uses an alternative supply of source gases, conformal coating of desired films can be readily elucidated onto high-aspect-ratio structures and film thickness can be exactly controlled by repeating discrete ALD cycles. Therefore, there have been increasing demands of integrating ALD-grown TCOs into the emerging applications addressed above. 

In this presentation, we report structural and electrical properties of ‘atomic-layer-doped’ ZnO films.^[1,2] Firstly, Al-doped ZnO films were deposited by ALD via alternate stacking of a ZnO matrix  and AlO_x dopant layers. Scanning TEM-HAADF analysis revealed that the ALD-AZO films have a unique layer-by-layer structure as those were grown. Upon correlation of the structural and electrical properties, effects of the layer-by-layer doping manner were interpreted in order to investigate a carrier generation mechanism of the ALD-AZO films. In the latter part, efforts of optimizing electrical properties of ALD-ZnO films will be given by controlling surface coverage of dopant ions incorporated in each atomic-doping layer. These efforts include two approaches; i) control of the amount of Al ions deposited during 1 ALD cycle of AlO_x via process-parameter adjustment, and ii) introduction of other dopant oxides (group III: In, B, transition metals: Hf, Zr, Ti) which have different surface coverage. Among the tested dopants, it is remarkable that Ti exhibited the highest electrical conductivity (950 S/cm) and doping efficiency (51%). This finding is extraordinary compared to the conventional homogeneous compounds where Al has long been considered as one of the best dopants for the ZnO.

[1] D.-J. Lee, K.-B. Kim et al., Adv. Funct. Mater. 21, 448 (2011).

[2] D.-J. Lee, K.-B. Kim et al., J. Electrochem. Soc. 158, D277 (2011).