Relaxing the Conductivity/Transparency Trade-Off in MOCVD ZnO Thin Films by Hydrogen Plasma

Increasing the conductivity of polycrystalline zinc oxide films without impacting the transparency is a key aspect in the race to find affordable and high quality material as replacement of indium-containing oxides. Usually, ZnO film conductivity is provided by a high doping and electron concentration, detrimental to transparency, because of free carrier absorption. Here we show that hydrogen post-deposition plasma treatment applied to ZnO films prepared by metalorganic low-pressure chemical vapor deposition allows a relaxation of the constraints of the conductivity/transparency trade-off. Upon treatment, an increase in electron concentration and Hall mobility is observed. The mobility reaches high values of 58 and 46 cm2V−1s−1 for 2-μm- and 350-nm-thick films, respectively, without altering the visible range transparency. From a combination of opto-electronic measurements, hydrogen is found, in particular, to reduce electron trap density at grain boundaries. After treatment, the values for intragrain or optical mobility are found similar to Hall mobility, and therefore, electron conduction is found to be no longer limited by the phenomenon of grain boundary scattering. This allows to achieve mobilities close to 60 cm2V−1s−1, even in ultra-transparent films with carrier concentration as low as 10^19 cm−3.