The liquid compound pentamethylcyclopentadienyl gallium (GaCp*) exhibits several notable characteristics, including a relatively high vapor pressure and thermal stability up to 250 °C. Using it as a precursor, Ga2O3 thin films can be deposited by atomic layer deposition (ALD), with the combination of oxidants of H2O followed by O2 plasma (WpO), where H2O is used to desorb the ligand, and O2 plasma then oxidizes the surface Ga species. A self-limiting surface reaction during the ABC-type ALD process applying GaCp*, H2O, and O2 plasma in order was observed with no delay in nucleation for pulse times for GaCp*, H2O, and O2 plasma of 0.1, 3.0, and 50 s, respectively, at a growth temperature of 200 °C. The growth rate was 0.06 nm/cycle. ALD-Ga2O3 deposition was possible even if only O2 plasma was used as an oxidant, though it takes a very long time for the oxidation by O2 plasma to reach saturation. On the other hand, when no O2 plasma was used, no film could be deposited using H2O and O2 in any combination. Ga2O3 films deposited by ALD using the WpO oxidant exhibited a stoichiometric O/Ga ratio of 1.5, with negligible residual carbon and an amorphous structure. These results show that GaCp* is a promising candidate precursor for forming high quality Ga2O3 films.
Liquid ethylcyclopentadienyl indium (InEtCp) was synthesized, and this compound exhibited superior characteristics, including a relatively high vapor pressure and thermal stability up to 250 °C. In2O3 thin films were subsequently deposited by atomic layer deposition (ALD) using the InEtCp as a precursor together with combinations of oxidants: H2O followed by O2 plasma (WpO), H2O followed by O2 (WO), O2 plasma alone (pO), and O2 plasma followed by H2O (pOW). The growth rates for In2O3 thin films using the pO and pOW processes were much smaller than those using the WO and WpO processes. A self-limiting surface reaction during the WpO process was observed with no delay in nucleation when the pulse times for InEtCp, H2O, and O2 plasma were 0.1, 2.0, and 14 s, respectively, at a growth temperature of 200 °C. For the WO and WpO processes, the In2O3 films are considered to be formed by quite different mechanisms, due to oxidation or lack thereof in the presence or absence of the -O-In-OH* intermediate product. This is due to the difference in the oxidation strength of O2 gas and O2 plasma during the oxidation step in the ALD process. As a result, In2O3 thin films deposited by WpO were obtained with a stoichiometric O/In ratio of 1.5 and a negligibly low residual carbon concentration of around TOF-SIMS detection limit, below 1 ppm. Therefore, InEtCp is one of the promising candidate precursors to form a high quality In2O3 film.
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