Synthesis
and Modeling of Uniform Complex Metal Oxides
by Close-Proximity Atmospheric Pressure Chemical Vapor Deposition
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Abstract
A close-proximity atmospheric pressure
chemical vapor deposition
(AP-CVD) reactor is developed for synthesizing high quality multicomponent
metal oxides for electronics. This combines the advantages of a mechanically
controllable substrate-manifold spacing and vertical gas flows. As
a result, our AP-CVD reactor can rapidly grow uniform crystalline
films on a variety of substrate types at low temperatures without
requiring plasma enhancements or low pressures. To demonstrate this,
we take the zinc magnesium oxide (Zn<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O) system as an example. By
introducing the precursor gases vertically and uniformly to the substrate
across the gas manifold, we show that films can be produced with only
3% variation in thickness over a 375 mm<sup>2</sup> deposition area.
These thicknesses are significantly more uniform than for films from
previous AP-CVD reactors. Our films are also compact, pinhole-free,
and have a thickness that is linearly controllable by the number of
oscillations of the substrate beneath the gas manifold. Using photoluminescence
and X-ray diffraction measurements, we show that for Mg contents below
46 at. %, single phase Zn<sub>1–<i>x</i></sub>Mg<sub><i>x</i></sub>O was produced. To further optimize the growth
conditions, we developed a model relating the composition of a ternary
oxide with the bubbling rates through the metal precursors. We fitted
this model to the X-ray photoelectron spectroscopy measured compositions
with an error of Δ<i>x</i> = 0.0005. This model showed
that the incorporation of Mg into ZnO can be maximized by using the
maximum bubbling rate through the Mg precursor for each bubbling rate
ratio. When applied to poly(3-hexylthiophene-2,5-diyl) hybrid solar
cells, our films yielded an open-circuit voltage increase of over
100% by controlling the Mg content. Such films were deposited in short
times (under 2 min over 4 cm<sup>2</sup>)