From ZnO Colloids to Nanocrystalline Highly Conductive Films

Base‐catalyzed hydrolysis and condensation of zinc acetate dihydrate in alcoholic environments yields highly concentrated ZnO colloids with molarities near 3 M (50–70 wt % solid content) and particle sizes between 3 and 6 nm. These solutions were optimized to be stable against coagulation for several months and to yield optically transparent 0.8–2 μm thick films of variable electronic conductivity in a single coating step. 2 atom % or , if present in these nano‐porous air‐sintered layers, produce a high free‐carrier concentration of about without noticeable changes in sheet resistance R sh > 10 6 Ω □ −1 , whereas sintering under vacuum or (90/10)‐atmosphere results in R sh values around 150 Ω □ −1 Furthermore, infiltration of small ZnO clusters into the porous films increases the refractive index from 1.8 to 2.2 and substantially lowers R sh to values of about 20 Ω −1 . From Hall measurements, the charge‐carrier concentration has been determined to be and the corresponding electron mobility approaches values of about 9 cm 2 /V s. Electron microscopic investigations revealed that the ZnO/Al crystallites have a typical size of and are preferentially oriented with their (100) planes parallel to the substrate. Taking into account a preferential (002) orientation in physically deposited highly conductive ZnO/Al films, it appears that the chemical control of nanocrystallite boundaries is more decisive than their orientation within the films.