Growth of Bi₂O₃ Films by Thermal- and Plasma-Enhanced Atomic Layer Deposition Monitored with Real-Time Spectroscopic Ellipsometry for Photocatalytic Water Splitting

Ultrathin Bi2O3 films were grown by thermal- and plasma-enhanced ALD by using [Bi(tmhd)3] as bismuth precursor. The use of an O2 plasma instead of H2O as oxidant increases the growth per cycle (GPC) from 0.24 to 0.35 Å/cycle and shifts the temperature window from 210–270 to 270–330 °C. The deposited films were grown conformally and are oxygen rich, dense, and amorphous with bandgaps in the range 2.7–2.9 eV. Real-time spectroscopic ellipsometry reveals a superior reactivity of the plasma toward Bi(tmhd)x surface groups, explaining the increased GPC. Furthermore, the bulky precursor allows individual thickness determination of a Bi2O3 bulk and a Bi(tmhd)x surface layer during plasma-enhanced deposition using an optical double-layer model. The thickness of the Bi(tmhd)x surface layer is about 2.8 Å. With this double-layer model we precisely follow the cyclic ALD and, for the first time, report the refractive index of an adsorbed precursor monolayer during ALD. These results will help the development of for example Bi-containing photoelectrodes for solar energy conversion.