Atomic layer deposition towards novel device applications

Giovanni Marin
Abstract & Cover

Atomic layer deposition (ALD) is a gas-phase thin film deposition technique that has gained increasing popularity in the last 20 years because of its unique properties. It is based on self-limiting chemical reactions that ensure the layer-by-layer growth of the film. This unique growth mode is fundamental to the fine control of both film thickness and structure. The film grows conformally on the substrate, following the morphology of the surface. ALD can grow films at low temperature, making possible the use of temperature-sensitive substrates. A slightly modified technique called molecular layer deposition (MLD) utilises organic precur-sors to deposit fully organic films. Hybrid inorganic-organic materials can be deposited with a combination of ALD and MLD. The aim of this research was to utilise the unique characteris-tics of ALD/MLD in two different applications, thermoelectrics and barrier coatings.

Thermoelectric devices were fabricated on flexible plastic, glass, and textile. Testing of the barrier properties of ALD-grown films were carried out on 3D printed plastic substrates. The conformality of the deposition is fundamental in both applications. The films needed to coat the single fibres within the textile substrate as well as the porous surface of the 3D printed plastic. The low deposition temperature made it possible to use cotton as well as various plas-tics as substrates. The fine control over the film thickness and structure, enabled the deposi-tion of inorganic-organic superlattice hybrid materials. Zinc oxide (ZnO) and hydroquinone (HQ) were chosen for the fabrication of the thermoelectric devices while aluminium oxide (AlOx) was the chosen barrier material. Hydroquinone was utilised as monomolecular layers within the ZnO matrix to lower thermal conductivity and enhance the thermoelectric perfor-mance.

The ALD-deposited AlOx coating was shown to successfully lower the vacuum degassing of the 3D printed plastics compared to commercial sealants. These superior performances open the way to inexpensive and personalised, 3D printed, laboratory tools coated with ALD which pro-vide degassing protection to the vacuum environment.

Thermoelectric devices were fabricated on several substrates (silicon, flexible plastic, flexible glass, and textile) using the n-type ZnO as thermoelectric. On textile, the device was made with both n-type (ZnO or ZnO-HQ) and p-type (poly(3,4-ethylenedioxythiophene) - PEDOT) components to improve performance. The ZnO-HQ superlattice outperformed the bare ZnO films, proving that the hybrid approach is worth pursuing to reduce thermal conductivity. The best device fabricated on textile, produced an open-circuit voltage around 150 mV at a ΔT of 20 °C with a power output in the order of pW. These numbers, although low, are paving the way for future application of the ALD/MLD in the fabrication of thermoelectric devices inte-grated into smart clothing.

Source of Information
Aalto University
(Espoo, Finland)
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