Abstract
Zinc oxide (ZnO) is a wide-band-gap semiconductor that is promising for use as a transparent conductive oxide film. To date, to improve their optoelectrical properties, pristine ZnO films have been doped with metals using various techniques. In this study, nanostructured Cu-ZnO thin films were synthesized using a modified two-step radio frequency magnetron sputtering technique with separate ZnO and metallic Cu targets. Controlling the timing of the Cu/ZnO co-sputtering and ZnO-only sputtering steps afforded a significant change in the resulting nanostructures, such as uniform Cu-ZnO and broccoli-structured Cu-ZnO thin films. Using various measurement techniques, the influence of Cu doping was analyzed in detail. Furthermore, a crystal growth model for the formation of the broccoli-like clusters was suggested. The Cu-ZnO thin films synthesized using this technique demonstrate a highly improved conductivity with some loss in optical transmittance.
Highlights
Zinc oxide (ZnO) is considered a promising material for optoelectronics and laser applications owing to its wide band gap (3.1–3.3 eV) and high exciton binding energy (60 meV) [1,2,3]
C2, while C10 had the lowest intensity. This implies that Cu-ZnO films with higher Cu contents have fewer oxygen vacancies, which might cause the lower compressive strain indicated by the W-H plots
scanning electron microscope (SEM) analysis showed that the pristine ZnO film had a homogeneous microstructure, whereas the highly Cu-doped
Summary
Zinc oxide (ZnO) is considered a promising material for optoelectronics and laser applications owing to its wide band gap (3.1–3.3 eV) and high exciton binding energy (60 meV) [1,2,3]. In radio frequency (RF)/direct current (DC) co-sputtering and/or sequential sputtering, the use of two separate sources has been proven to be effective [25,28] This approach has resulted in a decrease in crystal orientation [22], shrinkage of lattice constants [22,27], reduction in band gap [23,28], and broadening of Raman peaks [23,24]. In these previous reports, the diverse processing parameters were not controlled; only the sputtering ratio of the two sources has been considered using single-step sputtering. To explain the properties of the sputtered Cu-ZnO thin films and the unique growth mechanism, various optical, structural, and electrical observations were performed
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