Fast growth of high mobility ZnO:Al by cathodic arc deposition (Vol. 42, No. 5)

image Transmittance of a 505 nm thick AZO film on 1 mm thick float glass showing a sheet resistance of 7Ω with a carrier mobility of 55 cm2/Vs, a very high value for doped ZnO on glass. The film is transparent not only in the visible but for most of the solar spectrum, which is also shown.

Transparent conducting oxides (TCOs) are increasingly important materials as the demand for high efficiency solar cells, displays, and smart windows increases. Currently, indium tin oxide (ITO) is the preferred material for its properties of low resistance and high visible light transmittance. Excessive indium demand justifies the search for abundant and low cost alternative materials to satisfy the growing need of coating (>108 m2/year). The best ZnO films doped with Ga or Al typically deposited at low rate by magnetron sputtering, are attractive but their performance is usually inferior to ITO.

Using a lesser known technique, called dc filtered cathodic arc deposition, we have shown that very high quality Al-doped ZnO (AZO) can be grown at rates 10 times higher than the rates for sputtering. Filtered cathodic arc produces a flux of fully ionized material, in stark contrast to sputtering which occurs at much lower power and predominantly produces a flux of neutral atoms. The arc-produced ions bring significant potential and kinetic energies to the surface, which leads to heating of the growing film just where the growth occurs while the substrate as a whole can remain at a much lower temperature. If the ion flux is high, the surface heat accumulates and anneals the film as it grows. The result is high quality AZO with electron mobility approaching the theoretical limit for polycrystalline AZO films. High electron mobility is what allows TCOs to transmit light throughout the solar spectrum while maintaining high conductivity since the electron concentration does not have to be very high.

Achieving high mobility ZnO:Al at very high growth rates by dc filtered cathodic arc deposition
R.J. Mendelsberg, S.H.N. Lim, Y.K. Zhu, J. Wallig, D.J. Milliron and A. Anders, J. Phys. D: Appl. Phys. 44, 232003 (2011)