Advances in the area of flexible optoelectronics demand an in-depth understanding of deposition as well as growth processes, which lead to high quality thin films of transparent conductive oxides on flexible substrates. This is fundamentally important in tuning structure and critical semiconductor attributes including carrier concentration and work function. Therefore, high quality indium-free zinc oxide, AZO (Zinc oxide-aluminum) and other zinc oxide related oxide films have been deposited successfully on polymer substrates for selected applications.
Moreover, significant research work has been proposed for the development of deposition procedures of zinc oxide and related materials on flexible plastics and the analysis of corresponding thin film materials. They include RT plasma sputtering, sol gel spin coating, atmospheric plasma deposition, direct writing, and RF magnetron sputtering. The attributes of the flexible substrate are as important as those of the active oxide layers for the optimization of selected applications.
A team of researchers at Dublin City University in Ireland, Saikumar Inguva, Enda McGlynn, and Jean-Paul Mosnier investigated the properties of zinc oxide and AZO films deposited by pulsed-laser at room temperature on Zeonor that is a form of cyclo olefin polymer flexible plastic. Zeonor possesses unique and excellent material attributes as opposed to other plastics, which include low water absorption, large hydrophobicity, and high optical transmission. This study exhibits the notable effects of the oxygen deposition pressure on the electrical and structural attributes of the oxide layer. Their research work is published in Thin Solid Films.
The research team grew the films on a PLD apparatus. They then used zinc oxide and AZO targets. Sheets of about 1.2mm thick Zeonor in 1x2cm rectangles were used as substrates and loaded onto the PLD apparatus. The experiment was done at a base pressure of about 3.9×10-6 for all the growths and at ambient oxygen growth pressure. The authors then measured, water contact angles film thicknesses and surface morphologies, structural attributes, optical transmittance, and electrical properties.
As the oxygen pressure decreased from 10Pa, the researchers observed that the deposit surface morphology changed from crystalline to a continuous film-like form while the surface roughness increased from 5nm to 65nm reference to the deep valley-shaped cracks. The degree of hydrophobicity was also observed to decrease with a transformation to hydrophilic behavior at 0.13Pa. The orientation of the crystalline fraction of the deposit shifted from c-plane to m-plane for both AZO and zinc oxide materials.
The visible optical transmittance dropped from about 95% to 65% and the bandedge onset and multiple-beam fringe patterns became diffuse owing to a rise in scattering as well as film non-uniformity. The pronounced orange-red emission in zinc oxide films was quenched. Electrical attributes were considerably improved. The resistivity of the zinc oxide films dropped from 105 Ω cm to 10-3 Ω cm and carrier concentration improved from about 109-1015 cm -3 to 1021 cm-3.
The results of their study demonstrates that the effect of oxygen pressure on the attributes and structure of AZO and ZnO films is an important aspect that enables researchers to tune and tailor the attributes of AZO and zinc oxide films on polymer substrates, for instance, Zeonor.
Saikumar Inguva, Enda McGlynn, and Jean-Paul Mosnier. Pronounced effects of oxygen growth pressure on structure and properties of ZnO and AZO films laser deposited on Zeonor polymer. Thin Solid Films, volume 621 (2017), pages 171–177.Go To Thin Solid Films