UV Photodoping and Remote Hydrogen Plasma Treatment of ZnO Nanocrystal Films

<b>Flexible and transparent electronics such as flexible displays and biosensors have drawn great interest in the past decade. Due to their high optical transparencies and good electrical performance, ZnO-based materials are widely used in semiconductor devices. Synthesis methods range from sol-gel processes, to hydrothermal synthesis and chemical vapor deposition. However, due to the temperature sensitivity of plastic substrates like polyethylene terephthalate (PET), low synthesis temperatures are required for flexible and transparent devices, which limits the choice of synthesis approaches and treatment techniques to enhance the electrical performance. </b><br/><b>Here, room temperature deposition of thin films comprised of about 10 nm ZnO nanocrystals (NCs) is achieved by non-thermal plasma synthesis using diethylzinc (DEZ) as precursor. Although the produced ZnO nanocrystals possess a large number of native defects like oxygen vacancies serving as donors, the electrical conductivity is inhibited by the abundance of surface groups, namely hydroxyl and carboxylate groups, which capture the free electrons. We demonstrate that by using UV (ultraviolet) exposure at room temperature, the electron-trapping surface hydroxyls are removed and electrical conductivity is greatly improved due to the increased carrier density. This photodoping effect is found to be independent of photon energy in the UVA range but gets weaker in the UVB range presumably because newly formed surface groups localize electrons. However, the UV exposure does not affect carboxylates. The role of carboxylates for the electronic properties is further investigated with the help of hydrogen remote plasma treatment. As the duration of H2 plasma treatment increases, more carbon-oxygen single bond groups are converted to carboxylates and the carrier density after UV photodoping is increased, which indicates that carboxylate groups act as electron donors; however, the possibility of induced hydrogen interstitials cannot be ruled out. We also find that the UV photodoping effect is quenched by forming new hydroxyl groups due to water absorption from air. To maintain the UV photodoping effect, atomic layer deposition (ALD) is applied to cover the ZnO NCs with Al2O3 coatings which insulates and protects NCs from the surrounding atmosphere.</b><br/><b>In summary, highly conductive ZnO NC thin films are achieved by non-thermal plasma synthesis, UV photodoping, H2 remote plasma treatments and atomic layer deposition (ALD) at low temperatures without intentional doping, enabling applications for flexible and transparent devices. This strategy may also be applicable to other metal oxides nanocrystal films.</b>