Study of Ultrasonically Sprayed ZnO Films: Thermal Annealing Effect

ZnO thin films were deposited on microscope glass substrates by ultrasonic spray pyrolysis technique. The effects of annealing under various temperatures on the optical and structural properties of ZnO thin films were analyzed. The as-deposited and annealed ZnO thin films were investigated by UV/VIS spectrophotometer and X-ray diffractometer. Some of the optical properties of the films such as transmittance, absorbance and band gap energy were investigated by UV/VIS spectrophotometer. The crystallinity levels of the films were investigated, the structural parameters such as diffraction angle, full-width at half maximum, lattice parameters, grain size and dislocation density were calculated and structural properties were analyzed. X-ray diffraction patterns indicated that the ZnO films had a polycrystalline hexagonal wurtzite structure

Electroluminescent Thin Film Phosphors

In the later part of the twentieth century, classic cathode ray tubes (CRTs) are replaced by flat panel displays (FPDs) with a thinner, smaller size and with a significant reduction in power consumption. Such displays are currently used in several applications including mobile phones, laptops, and televisions. Among other FPDs, electroluminescent displays perform better than its counterparts such as liquid crystal, field emission, and plasma displays. Electroluminescent (EL) display is a common term for devices such as light-emitting diodes (LEDs), powder phosphor devices, thin film, and thick dielectric electroluminescent devices. Thin film EL devices (TFEL) show advantages such as quick response, high resolution, wide viewing angles and wide operating temperatures and so on. TFEL displays are all solid state and based on electroluminescence in which luminescent material gives light under high electric field. Phosphors act as luminescent materials both in powder as well as in thin film forms in these devices. ZnS doped with Mn is one of the earliest and still in use electroluminescent phosphor. Monochrome display was developed using ZnS:Mn sandwiched between single or double multi-stack dielectric layers. An efficient yellow emission was generated when a high voltage was applied between front indium tin oxide (ITO) and rear aluminium (Al) electrodes. Multiple color displays were then achieved by combination of different rare earth-doped ZnS layers emitting different colors. Similar displays were developed more efficiently with other materials. Mostly oxides, sulphides, and fluorides are highly preferred host for such phosphors because of wider bandgap that allows the transmission of visible light. Oxides have been found to have more stability compared to sulphides which suffer degradation under high electron bombardment due to sulphur depletion. TFEL phosphors have been grown by physical vapour deposition techniques such as pulsed laser deposition, sputtering, and chemical reaction techniques such as spray pyrolysis, chemical vapour deposition, sol–gel process followed by spraying, dipping, or spinning. Over the years, extensive research has been carried out to develop multi-color TFEL devices using combination of red, green, blue emitting phosphors. Optimum growth techniques, dopant concentration, choice of rare earths, and hosts have been investigated and led to the production of efficient, high luminance display devices. This chapter introduces basics of phosphors, minimum requirements to be an efficient phosphor, different fabrication techniques, and a review of different color electroluminescent phosphors for display devices