Atomic Layer Deposition of TiO_2 ultrathin films on 3D substrates for energy applications

摘要

In the present global environmental context, it becomes more and more critical to find efficient solutions to lower our energy consumption on one hand, and to produce energy from clean renewable sources on the other hand. Consequently, research efforts on materials for energy applications are intensifying. The present work aims at developing optoelectrical components usable for both energy saving (light emitting diodes) and renewable energy production (solar cells) by fabricating p-n heterojunctions based on a single semiconductor, titanium dioxide. TiO_2 is indeed a very promising candidate: it is chemically and physically stable under irradiation, transparent to visible and near-infrared light (E_g = 3 - 3.5 eV), presents photocatalytic activity, is non-toxic and low cost, which permits to envisage its large scale use. In the present paper, the proposed architecture for both solar cells and LEDs is original as well as common for both applications: a three-dimensional architecture based on an anodic alumina nanoporous membrane which serves as nanomask for TiO_2 growth in order to enlarge the effective surface of the components. TiO_2 is synthesized by Atomic Layer Deposition (ALD), a technique particularly well adapted to the deposition of ultrathin films (from one monolayer to few tens of nanometers) on 3D porous substrates patterned with high aspect ratio nanopores. In this work, the capacity of synthesizing 3D nanostructures is demonstrated. TiO_2 ultrathin films (10 to 100 nm) were grown by ALD on flat, micropatterned, microporous and nanoporous anodic alumina membranes (AAM) substrates. The films were highly conformal, as confirmed by SEM and TEM imaging. Both EDS and XPS analyses validated the dioxide film stoichiometry.