The production and properties of TCO coatings prepared by pulsed magnetron sputtering from powder targets
Mith their unique transparent and conductive properties, TCO (transparent conductive oxide) coatings are becoming increasingly studied These commercially important coatings have a promising future due to their various applications as components in optoelectronic devices, photovoltaic solar cells, flat panel displays, electroluminescent devices, etc. Their high transmittance and low resistivity are generally achieved through the use of specific dopant materials, whilst adjustments are made to the deposition processes to improve the structure of the coatings. TCO coatings are commonly deposited by the magnetron sputtering process. Sputtering normally takes place from a solid plate, known as the target, of the material to be deposited. Clearly, each solid target can only be of a single composition. Thus, to change the compositions ofthe coatings, the whole target has to be replaced Furthermore, alloy, or doped targets can be very expensive and the choice ofavailable compositions is likely to be limited. In this project, instead of using solid targets, metal or ceramic powder blends were used as the targets. The powder blends were spread across the surface of a magnetron and lightly tamped down to produce a smooth surface. The benefits of this approach are that any material that is available in powderform can be considered as a target material and alloy or multi-component compositions can be readily blended The basic aim of this project, therefore, was to produce novel TCO coatings by magnetron sputteringfrom powder targets. The coatings were deposited in a specially designed rig with a number of important features, including a pulsed DC power supply and a closed magnetic field. The project concentrated on the production of commercially useful zinc oxide-based TCO coatings. Coatings were produced with different dopant materials and concentrations, and their optical and electrical properties were measured. After the coatings were annealed at 500 *Cfor 2 hours in vacuo, aluminum and gallium doped zinc oxide coatings showed their low resistivity, which were no larger than 5.19xI0-3S2cm, and the lowest resistivity was obtained from 3at% A 1-doped ZnO coating; 1.95 xI 00cm. The average transmittance in the visible range of the ZnO coatings was 90%. From this, optimal compositions were identified For comparison purposes, coatings were also produced of the TCO material most commonly used at present; namely ITO (Indium tin oxide). The results showed that ITO coatings generally had lower resistivity and visible transmittance, (4-6xlO-492cm and 80-8216), than doped ZnO coatings. Also, the electrical and optical properties of ITO coatings were very sensitive to the content of oxygen in the deposition atmosphere. Finally, theflexibility offered by this approach was exploited through the use of multi-component target compositions to produce TCO coatings with tailored optical and electrical properties.