Recently, the photoconducting response at different degrees of straining of ZnO–based nanostructured materials and devices has raised a lot of research interests. For a strained piezoelectric semiconductor, such as ZnO, an ionic displacement takes place inside ZnO inducing piezoelectric polarization that modulate the carrier concentration, transport behavior and barrier height at the interface. In this study, we report high performance ZnO-based photodetectors with enhanced photo-responsivity fabricated on flexible polyethylene terephthalate (PET) and polyimide (PI) substrates by sputtering at low temperatures. X-ray photoelectron spectroscopy results indicate that good stoichiometry composition transfer from the target to the films by using RF magnetron sputtering. The effect of growth temperature and sputtering power on the crystallinity, optical transparency and surface morphology of nanostructured ZnO thin films has been systematically investigated in this work. XRD results show that ZnO thin films grown on PI flexible substrates exhibit good (002) c-plane oriented crystallinity. Interdigital electrode (IDE) planar photodetector devices have been fabricated on PI substrates with Ag as the top electrodes. The average optical transparency is over 85% in the wavelength range of 400-800 nm for these ZnO photodetector devices on flexible PI substrates. The photocurrents of the ZnO flexible PDs on PI substrates realize an increase by three magnitudes from 10-9 under dark conditions to 10-6 A under a 35 mW laser excitation source.
Wide bandgap compound ZnS is one of the promising materials used in versatile applications including solar cells, UV photodetectors and luminescent devices. Compared to other II-VI sulfides, ZnS has several advantages of non-toxicity, wider solar spectrum transparency and earth-abundance. Recent progress on optimization of synthesis techniques of ZnS allows it to be effectively realized on transparent and flexible substrates. In this work, nanostructured ZnS films were deposited on bare quartz and ITO coated glass substrates using RF sputtering at various conditions. Crystal structural, optical and surface chemical properties of ZnS films have been systematically characterized. It is revealed that ZnS films exhibit cubic β-phase crystalline structures on both substrates. The optical transmittance of ZnS films is above 85% in the visible range. The substrate temperature plays an important role in crystalline quality of ZnS films. Sputtering power exhibits more effects on the thickness and optical properties of ZnS films. Optimization of deposition parameters has been realized by investigation of optical, microstructure and crystallinity of ZnS thin films in this study.
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