Nb (V, Nb/V)-Doped TiO2 Films as New Transparent Conducting Oxides Synthesized by a Novel ALD Process: Effect of Dopant Content and Growing Conditions
Authors: Gesesse Getaneh Diress , Debieu Olivier , Jolivet Aline , Frilay Cédric , Duprey Sylvain , Portier Xavier , Dufour Christian , Marie Philippe , Labbé Christophe , El roz Mohamad , Cardin Julien 
Published: February 2024

Titanium oxide (TiO2) is a wide bandgap (3.0-3.4 eV) material that presents a high transparency in the visible range, a high refractive index, and an insulator behavior. In order to vary its structural, optical, and electronic properties with the objective to synthesize transparent conductive oxide (TCO) films, we have investigated several doping strategies. Due to their extra electron and atomic radius close to that of Ti, respectively larger for Nb or smaller for V, these are among the interesting potential dopants of TiO2. In this objective to synthesize TiO2-based TCO films, we have performed atomic layer deposition (ALD) with different concentrations of Nb dopants and by varying the growth conditions.

Highly transparent and conducting Nb-doped TiO2 thin films were produced by a low-temperature (300 °C) new ALD process using titanium tetraisoproxide, niobium(V) ethoxide, and water as a reagent. The deposition was performed in a supercycle fashion, alternating Ti precursor and water cycles followed by Nb precursor and water cycles, which allowed controlling the dopant concentration. This study focused on non-annealed and annealed films under N2 and forming gas. The dopant concentration significantly affected the fabricated films' thickness, structure, morphology, composition, and optical and electrical properties. Indeed, the crystalline anatase phase was obtained before annealing for a low Nb content, while the films were found amorphous for a larger Nb content. The non-annealed film thickness and surface roughness decreased with increasing Nb doping, while the size of the anatase grains increased, and the films became amorphous. This progressive amorphization of the films with increasing Nb content was observed by several techniques (G-XRD, Raman, and FTIR). Surprisingly, films annealed at 600°C led to the formation of larger and more homogeneous grains than for non-annealed films. Moreover, the XRD, Raman, and FTIR results clearly indicated the substitution of Ti by Nb in the anatase structure. Furthermore, the XPS spectra showed that the amount of Ti and Nb in the Ti3+ and Nb4+ oxidation states increased with increasing the Nb content. These two latter points may explain the decrease in resistivity caused by the increase of density of shallow donor states in addition to the decrease of electrons scattering caused by the grain size increase. In addition, the film transmittance was increased up to 15% while the resistivity decreased to 10-3 Ω.cm leading to a TCO behavior. Using the figure of Merit (FoM) established over a wide spectral range from AM1.5g irradiance [*] giving the maximum photocurrent density of an ideal solar cell equipped with such TCO electrode, it showed an improvement by a factor of 8 by Nb doping, reaching a maximum of FoM for the film produced with a particular Nb doped rate of 0.33.

Other strategies of TCO improvement were investigated, such as growing V-doped, and Nb&V co-doped TiO2 films by innovative dual and triple precursors ALD processes, respectively. Once more, the dopant kind and amount strongly impacted the growth rate, structure, morphology, optical and electrical properties of those two new kinds of doped TiO2 materials. The full study of Nb-doped TiO2 films will be presented, while the comparative results obtained for V-doped and Nb/V-doped materials will be introduced.

The Nb-doped TiO2's TCO performances and future improvement based on V-doped, and Nb&V co-doped may pave the way for the use of those kinds of materials in solar radiation energy conversions as electrode or junction materials for Photovoltaic or photo(electro)catalytic systems.

[*] J.A. Mendez-Gamboa, R. Castro-Rodriguez, I. V. Perez-Quintana, R.A. Medina-Esquivel, A. Martel-Arbelo, A figure of merit to evaluate transparent conductor oxides for solar cells using photonic flux density, Thin Solid Films. 599 (2016) 14–18. https://doi.org/10.1016/j.tsf.2015. .12.038.

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