In equilibrium reaction processes, the precise control of composition ratios in multi-component thin films is problematic due to the changes in the state of precursor materials, which are caused by differences in metal ion stabilities. We have identified a strategy to resolve this problem by first developing a theory and then constructing a fine-channel type mist chemical vapor deposition (mist-CVD) system with two solution chambers and a mist-mixing chamber. Zn 1-x Mg x O thin films were fabricated using the conventional and the developed mist-CVD systems to verify the theory. The properties of the Zn 1-x Mg x O thin films produced using the two systems were significantly different. The Mg composition ratio in the film was clearly influenced by the Mg ion stability and all the measurement results support the theory.
The growth mechanism of ZnO films fabricated via mist CVD was proposed by analyzing the growth rates, crystallinities, surface morphologies, and chemical states of the films that were grown when the precursor material supply amounts were fixed and the H2O concentrations were changed. At appropriate conditions, e.g. [H2O]/[Zn] ≈ 60–70 and [Zn] = 20 mM, high-quality ZnO thin films with a high growth rate, (001) dominant orientation, smooth surface, and low oxygen-related defects were obtained. These conditions provided a sufficient diffusion length for the precursor materials on the surface and an appropriate collision probability for the precursor and oxygen sources.
A MoS2 layered thin film was grown by atmospheric-pressure solution-based mist CVD on a 30 mm2 large-area substrate at 400 °C, which is a significantly lower growth temperature than that of conventional CVD. The film formation was reliably confirmed by Raman spectroscopy, grazing incidence X-ray diffraction (GIXD), and transmission electron microscope (TEM). Possibly, film formation energy was decreased by dissolving (NH3)6Mo7O24 ∙ 4H2O in methanol owing to formation of an intermediate product, whose decomposition temperature was lower than 760 °C — the decomposition temperature observed in thermogravimetric-differential thermal analysis (TG-DTA). Optical constants were derived by spectroscopic ellipsometry measurements, whose results indicate the reasonably good quality of MoS2 and the suitability of mist CVD in response to the large-area low-temperature growth request from industry.
Ammonium dichromate and chromium chloride were examined via thermogravimetric and differential thermal analyses and used to grow α-Cr2O3 via mist chemical vapor deposition. Only the former resulted in noticeable film formation. Using ammonium dichromate, α-Cr2O3 single crystals were grown in a wide growth-temperature range of 300–440 °C. The dependences of the lattice constants a and c on the growth temperature and precursor concentration were explained by combination of (i) lattice reconstruction and (ii) unit-cell volume variation. A maximum growth rate of 10 nm/min; a low (0006) rocking-curve full width at half maximum of 150 arcsec; and low a and c lattice mismatches (with α-Ga2O3) of 0.7 and 1.4%, respectively, were achieved.
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