Synthesis of ALD Tungsten Trioxide Thin Films from W(CO)6 and H2O Precursors

Tungsten trioxide (WO₃) thin films are extensively investigated for their remarkable semiconducting properties in electrochromics, microelectrics, selective catalysts, solar cells, gas-sensors and environmental engineering. In addition, WO₃ has been particularly developed for electrochromic applications, such as smart windows for green buildings and airplanes, rear mirrors in automobiles, displays, and electro-devices.WO₃ thin films have been fabricated  by various methods, including physical vapor depostion (PVD), pulsed vapor deposition (PLD), RF magnetron sputtering deposition, and electrodeposition [1, 2].

Atomic layer deposition (ALD) is a novel  process  to synthesize   variaous  thin films, where the sequential alternating vapor pulses of chemical precursors are introduced into the  ALD reaction chamber, one at a time. Each precursor pulse was separated by inert gas, such as Ar or N₂. The excess precursors and reaction by-products were evacuated by vacuum pump from the ALD reaction chamber. The precursor is fully reacted with only one monoatomic layer on the reacting surface through a surface saturating self-terminationg process. Therefore, the ALD technique produces extremely uniform and conformal films in  atomic level thickness. In general ALD has been shown to be able to generate reproducible and well-defined thin films of binary oxides, nitrides, and sulfides.  In recent years  ALD has been used to deposit pure elemental metals, new types of ternary and quarternary materials.

Presently only a few processes have been reported on the synthesis of WO₃ films by atomic layer deposition (ALD) so far. ALD WO₃ thin films have been deposited using WF6 and H₂O as precursors. WO₃ films have also been deposited by ALD using  tungsten hexacarbonyl (W(CO)₆) and ozone as reactant precursors for tungsten and oxygen.

In this work, we report on the synthesis of WO₃ thin films on Si and quartz substrates by thermal ALD using a cross-flow reactor, with W(CO)₆ and DI water as the chemical ALD precursors for tungsten and oxygen, respectively. This was followed by 600 ^oC furnace annealing in regular  atmosphere. Characterization of WO₃ thin films has been performed using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), UV-VIS-NIR spectrometer, Raman spectroscopy (RS).

 Figure 1 shows the XRD pattern of the ALD WO₃ samples obtained after furnace annealing at 600 ^oC   under atmospheric conditions. All the diffraction peaks can be indexed into the monoclinic structure of the WO3 phase [space group: P21/n (14)], which is consistent with PDF 83-0950.

References:

 J.  Malm, T. Sajavaara, and M. Karppinen, Chem. Vap. Deposition 18, 245 (2012).

1. P. Tägtström, P. Mårtensson, U. Jansson, and J.-O. Carlsson, J. Electrochem. Soc. 146, 3139 (1999).