High Rate Deposition of High Quality ZnO:Al by Filtered Cathodic Arc Page: 3 of 7
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EXPERIMENT
For a statistical study of DC cathodic arc growth of AZO, a large number of samples
were grown and characterized. Samples were deposited at room temperature, 200 C, and 425 C
both with and without the assistance of a 75 W oxygen plasma from a constricted hollow cathode
plasma source (7). For each growth condition, three samples were prepared (giving 18 total) in
order to observe the reproducibility of the films and to provide an accurate estimate of the overall
uncertainty of the growths. Samples were grown in a random order to minimize the effect of any
systematic uncertainties.
All samples were prepared in an oxygen ambient of around 5 mTorr with the oxygen
partial pressure set before growth using a differentially pumped residual gas analyzer. Base
pressure in the chamber was typically around 1-2x105 Torr once the substrates were at growth
temperature. Arc current was set at 70 A. The cathode was a low cost Zn:Al (<4% Al) alloy
which gives Al concentrations in the films of (1.0 + 0.3) at.% as measured by Rutherford
Backscattering (to be reported elsewhere). Once ignited, the arc consumed enough oxygen to
drop the pressure to 2.5-3.5 mTorr. An open particle filter in a 900 bend was used to filter the arc
plasma, which travelled a distance of approximately 30 cm from cathode to substrate.
Borosilicate microscope slides were used as substrates and were gently cleaned using
commercial glass cleaner (Liquinox) and thoroughly rinsed before loading into the chamber.
Prior to deposition, the substrates were exposed to a 75 W oxygen plasma for two minutes to
burn off any hydrocarbons on the glass surface. This procedure was chosen since it does not
produce waste solvents and could be easily integrated into an inline coating system. Films grown
on these substrates are very adherent and do not typically delaminate from the surface, even up
to several microns thick.
Structural, electrical, and optical characterization was performed on all the samples after
growth. Thickness was measured with a Dektak 150 profilometer. Simultaneous transmission
and reflection was measured at normal incidence over the visible range using an Ocean Optics
CCD system. Sheet resistance was measured by a 4-point probe, and Hall measurements at room
temperature were carried out in the Van Der Paw geometry in an Ecopia HMS-5000. Surface
morphology was studied with a Veeco MultiMode AFM in tapping mode and the structure was
investigated with a Bruker x-ray diffractometer equipped with an area detector.
DISCUSSION
Pulsed cathodic arc has already proven to grow high quality AZO (3) but the growth rate
is too slow to be industrially viable for large area window coatings. Running the arc with a
continuous direct current (DC) substantially increases the growth rate. Film thickness for 2
minute growth is shown in Figure 1. However, this is known to affect the reproducibility of the
growth rate since the cathode spot position fluctuates (8), altering the coupling of the plasma
stream into the always-necessary magnetic plasma filter.
In this work, no clear correlation was observed between deposition rate and substrate
temperature, oxygen plasma power, or any other growth parameter for that matter. A close look
does show some evidence that the oxygen plasma increases the thickness, but with only 3 data
points it is hard to remove outliers. Growth rate was between 100-400 nm/min, and also did not
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Mendelsberg, Rueben J.; Lim, S.H.N.; Milliron, D.J. & Anders, Andre. High Rate Deposition of High Quality ZnO:Al by Filtered Cathodic Arc, article, November 18, 2010; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc841827/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.