Annealing to achieve lower resistivity in Ga-doped ZnO epitaxial films grown from low-temperature aqueous solution

Various annealing conditions were examined for enhancing the electrical conductivity of gallium-doped ZnO (ZnO:Ga) epitaxial films grown from a low-temperature aqueous solution. The resistivity of the films decreased with increasing annealing temperature, accompanying increases in both the carrier concentration and mobility. However, the improvement obtained from 0.5 h of annealing was limited because the films spalled when the temperature exceeded ∼300 °C. The temperature at which spalling of the films occurred depended on the annealing atmosphere, and it was lower in 5%H2-Ar than in air or vacuum. Thermal desorption spectroscopy showed that the cause of spalling was expulsion of H2O vapor from the interior of the film, indicating that water molecules or hydroxides were incorporated into the crystal during film growth. Analysis of the variations in electrical properties with increasing annealing duration revealed that the electrical properties were quickly improved upon heating before the film spalled. Thus, by limiting the annealing duration to ≤20 s, annealing could be performed at a temperature as high as 580 °C without spalling of the film, which yielded the lowest resistivity of 4.7 × 10−4 Ω cm. The annealed film exhibited transmittance as high as >75% in the visible spectrum.