Review—Growth of Al-, Ga-, and In-Doped ZnO Nanostructures via a Low-Temperature Process and Their Application to Field Emission Devices and Ultraviolet Photosensors

Abstract

In recent decades, ZnO-based nanostructures have attracted considerable attention because of their various possible morphologies, large surface-to-volume ratios, non-toxicity, easy synthesis, low cost, and excellent physical properties for fabricating high-performance electronic, magnetic, and optoelectronic devices. This review article focuses on recent advances in Al-, Ga-, and In-doped ZnO nanostructures, including a brief overview of the hydrothermal method and aqueous solution methods. Among these strategies, ZnO nanostructures synthesized via the hydrothermal method exhibits distinct advantages, such as low growth temperature, low cost, and large resultant surface area, as well as applicability in fabricating field emission (FE) devices and photosensors. FE devices have gained widespread attention for their applications in flat-panel displays and other electronic devices, such as microwave amplifiers, X-ray tubes, cathode-ray tube monitors, and electron microscopes. FE devices are influenced by interrelated factors, including emitter geometry, crystal structure, conductivity, work function, spatial distribution of emitting centers, and nanostructure density of nanorods. An applied electric field bends the surface barrier to form a triangular barrier, and the excited electrons tunnel easily to generate the emission current. A large number of excited electrons are stored at the tips of nanowires, thereby causing subsequent point discharge that reduces the electric field. ZnO-based ultraviolet (UV) photosensors are frequently used in environmental monitoring, securing space-to-space communication, flame sensing, and chemical biological analysis. ZnO UV photosensors are used to manufacture various structures, such as p–n heterojunction photodiodes, metal–semiconductor–metal photosensors, and Schottky photodiodes. The resistance of pure n-type ZnO nanostructure is high. ZnO nanostructures are commonly doped with other elements. To improve the electrical properties of ZnO nanostructures, donor dopants for ZnO, such as group III elements (Al, Ga, and In), can be used. The optical and electrical properties of fabricated ZnO optoelectronic devices can also be improved by doping with group III elements. Furthermore, as a material with a direct bandgap (3.37 eV), ZnO can easily absorb wavelengths in the UV range. Thus, illumination under UV can enhance the FE capacity of ZnO nanowires. Methods that enhance the performance of field emitters and photosensors are introduced in this review paper. We hope that this review paper can provide a useful reference to those who are interested in ZnO-based field emission devices and photosensors.