Deposition Kinetics in the Magnetron Sputter Deposition of Aluminum Doped Zinc Oxide Thin Films

We study the growth mechanism of aluminum-doped zinc oxide films deposited using magnetron sputtering by examining the effects of precursor gas phase scattering and substrate temperature on the film properties. The best quality films, deposited at a substrate temperature of 240° C and an argon pressure substrate distance product of 80 mTorr-cm, have a resistivity of 5.4 x 10^-4 Ohm-cm and an average transparency in the visible-near IR spectrum (400-1100 nm) of 90%. In general the properties of films deposited on unheated substrates were much more sensitive to working gas pressure than films deposited on heated substrates. The unheated substrate films exhibit a pronounced maximum in carrier concentration and mobility at a pressure-target distance product of 50-80 mTorr-cm, but surprisingly x-ray diffraction and optical transmission results imply that this maximum corresponds to a minimum in film structural quality. This result is consistent with modulation of Zn concentration in the films and electron transport limited by grain boundary scattering. We propose that the strong dependence of film properties with working gas pressure on unheated substrates is due to changes in the properties of the precursor flux due to the differential scattering with the working gas. The unheated films retain a strong memory of the properties of the incident flux, whereas the films deposited at high temperatures are less sensitive to the incident flux due to temperature activated surface reactions. We will present preliminary gas transport and film growth simulation results to support this proposal.