Deposition of aluminum-doped zinc oxide thin films for optical applications using rf and dc magnetron sputter deposition

Abstract

Aluminum-doped zinc oxide films were deposited by dc and rf magnetron sputtering from ZnO(98%)Al2O3(2%) target at room temperature on silicon and glass substrates under a variety of process conditions with the goal of attaining the highest transmittance and lowest resistivity for photovoltaic applications. The magnetron power and pressure were varied. For many dielectric deposition systems, added oxygen is necessary to achieve the appropriate stoichiometry. The effect of oxygen on film properties was then studied by varying the oxygen partial pressure from 1.5×10−5 to 4.0×10−5 T at a constant Ar pressure, with the result that any added oxygen was deleterious. Films deposited under power, pressure, and low-oxygen conditions were then characterized for electrical and optical properties. Following this, the dc and rf sputtered films were annealed at up to 400 °C seconds using rapid thermal annealing (RTA), and the influence of annealing on resistivity, transmittance, band gap, as well as grain growth and stress was studied. The effect of RTA was immediate and quite significant on dc films while the effect on rf films was not as profound. As-deposited rf films had a higher average transmittance (87%) and lower resistivity (5.5×10−4 Ω cm) compared to as-deposited dc films (84.2% and 8.9×10−4 Ω cm). On the other hand, after RTA at 400 °C for 60 s, dc films exhibited better average transmittance (92.3%) and resistivity (2.9×10−4 Ω cm) than rf films (90.7% and 4.0×10−4 Ω cm). The band gap of dc films increased from 3.55 to 3.80 eV while that of rf films increased from 3.76 to 3.85 eV. Finally, dc and rf films were textured in 0.1% HCl and compared to U-type Asahi glass for resistivity and transmittance.