A “visible-blind” solution-processed UV photodetector is realized on the basis of colloidal ZnO nanoparticles. The devices exhibit low dark currents with a resistance >1 TΩ and high UV photocurrent efficiencies with a responsivity of 61 A/W at an average intensity of 1.06 mW/cm2 illumination at 370 nm. The characteristic times for the rise and fall of the photocurrent are <0.1 s and about 1 s, respectively. The photocurrent of the device is associated with a light-induced desorption of oxygen from the nanoparticle surfaces, thus removing electron traps and increasing the free carrier density which in turn reduces the Schottky barrier between contacts and ZnO nanoparticles for electron injection. The devices are promising for use in large-area UV photodetector applications.

Solution-processed ultraviolet photodetectors based on colloidal ZnO nanoparticles.

During the last years, ZnO thin films have been studied extensively due to their potential applications in e.g. piezoelectric and optoelectronic devices or photovoltaic cells. Ordered c-axis orientation of ZnO crystallites is desirable for applications where crystallographic anisotropy is a prerequisite such as for short-wavelength semiconductor diode lasers (SDLs), and piezoelectric surface acoustic wave or acousto-optic devices. Many works were dedicated to c-axis oriented ZnO thin films elaboration and the study of their properties, including physical and chemical methods. For instance, sol–gel processes are particularly well adapted to produce ZnO films in a simple, low-cost and highly controlled way. This review summarizes the main chemical routes used in the sol–gel synthesis of undoped ZnO thin films and highlights the chemical and physical parameters influencing their structural properties. In this process, the ZnO films synthesis includes three principal steps: (i) solution preparation, (ii) coating and (iii) heat treatment. For the first step, the particle formation is discussed including nucleation and growth, particle size, morphology and colloids stability. These three steps involve several parameters such as: (i) nature and concentration of precursor, solvent and additive, and solution aging time, for the chemical system, (ii) coating method, thickness and substrate for the coating step, and (iii) pre-and post-heat treatment for the last step. The influence of these steps and synthesis parameters on ZnO thin films orientation is discussed.

Sol–gel-deposited ZnO thin films: A review

A solution-processed ultraviolet photodetector with a nanocomposite active layer composed of ZnO nanoparticles blended with semiconducting polymers can significantly outperform inorganic photodetectors

/pdf/a-nanocomposite-ultraviolet-photodetector-based-on-5f1mlg6qi5.pdf

A nanocomposite ultraviolet photodetector based on interfacial trap-controlled charge injection.

Ultraviolet (UV) photodetection has drawn a great deal of attention in recent years due to a wide range of civil and military applications. Because of its wide band gap, low cost, strong radiation hardness and high chemical stability, ZnO are regarded as one of the most promising candidates for UV photodetectors. Additionally, doping in ZnO with Mg elements can adjust the bandgap largely and make it feasible to prepare UV photodetectors with different cut-off wavelengths. ZnO-based photoconductors, Schottky photodiodes, metal–semiconductor–metal photodiodes and p–n junction photodetectors have been developed. In this work, it mainly focuses on the ZnO and ZnMgO films photodetectors. We analyze the performance of ZnO-based photodetectors, discussing recent achievements, and comparing the characteristics of the various photodetector structures developed to date.

https://www.mdpi.com/1424-8220/10/9/8604/pdf

ZnO-Based Ultraviolet Photodetectors

The effect of substrate-induced strain in polycrystalline ZnO thin films on different substrate, e.g., GaN epilayer, sapphire (0001), quartz glass, Si(111)∕SiO2, and glass deposited by sol-gel process, has been investigated by x-ray diffraction, scanning electron microscope, electrical resistivity, and photoluminescence measurements. A strong dependence of orientation, crystallite size, and electrical resistivity upon the substrate-induced strain along the c axis has been found. The results of structural and morphological studies indicate that relatively larger tensile strain exists in ZnO deposited on sapphire and glass, while a smaller compressive strain appears in film deposited on GaN and the strain is relaxed in larger crystallite size. The electrical resistivity of the films increases exponentially with increasing strain. The excitonic peak positions are found to shift slightly towards lower energy side with increasing strain. The analysis shows that GaN being a closely lattice-matched substrate produces ZnO films of better crystallinity with a lower resistivity.

S.K. Sen

Papers

Photoconductive UV detectors on sol–gel-synthesized ZnO films