Characteristics of Al-doped ZnO Nanowire Arrays Ultraviolet Photodetectors

ZnO，an II-VI semiconductor，has a direct wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV. Also ZnO has many advantages，such as low cost，nontoxicity，rich raw materials，good chemical stability and excellent photoelectric properties. Up till now， ZnO has been widely investigated for various potential applications，such as solar cells，catalyses，piezoelectric devices，gas sensors，light-emitting diodes and Ultraviolet（UV）photodetectors. UV photodetectors have received great attention because they play key roles in many important fields，such as military， civilian， space communication， industrial automation， wastewater treatment and environmental monitoring. In particular，UV photodetectors based on some wide bandgap semiconductors，such as gallium nitride，titanium dioxide，diamond，silicon carbide，gallium oxide and ZnO，have attract increasing interest due to their excellent performances. Among these wide bandgap semiconductors，ZnO is considered as one of the most ideal candidates for UV photodetectors. In order to achieve high-performance ZnO UV photodetectors，various nanostructured ZnO，such as nanoparticles，nanorods，nanowires and nanoflowers， were used as photosensitive layers to fabricate UV photodetectors. Well-aligned ZnO nanowire arrays，a unique structure，have high specific surface area，good photoelectronic performance as well as a more direct and effective conduction path for electrons，high-performance UV photodetectors based on well-aligned ZnO nanowire arrays are easy to realize. Unfortunately，the performances of the reported well-aligned ZnO nanowire arrays UV photodetectors have still been poor so far. Normally，the performance of photoconductive well-aligned ZnO nanowire arrays UV photodetector mainly depends on electric property of ZnO nanowire arrays，the metallic elements doping，such as Ga，In，Mg and Al，is one of the most well-known means for improving electric property of ZnO nanowire arrays. Among these metallic elements，the radius of Al3+ ions is smaller than that of Zn2+ ions，Al are easily doped into ZnO matrix，which results in improving electric property of Al-doped ZnO nanowire arrays. Low-cost and high-performance of ultraviolet photodetectors with good reliability have widely applications in many important fields. In this paper，fluorine-doped tin oxide coated glass substrates were cut into 2 cm×2 cm in size and etched into interdigital patterns using the laser technology，the substrates were rinsed in detergent，deionized water，ethanol，isopropanol for 15 min via an ultrasonic cleaning bath，respectively，and air-dried at 50 ℃. ZnO thin films were first deposited on the clean interdigital patterned fluorine-doped tin oxide coated glass substrates by the spin coating technique and acted as seed layers，zinc nitrate hexahydrate and hexamethylenetetramine was used as precursors，aluminum nitrate nonahydrate acted as an Al dopant，deionized water was the solvent，several well-aligned Al-doped ZnO nanowire arrays were then grown on the ZnO seed layers by a facile hydrothermal method at low temperature，five Al doping concentrations were designed，the atomic ratios of Al to Zn were 0%（undoped ZnO nanowire arrays），0.5%，1%，2% and 4%，respectively. Firstly，the crystal structures of all the Al-doped ZnO nanowire arrays were measured using an X-Ray Diffractometer（XRD），the XRD results indicated that all the Al-doped ZnO nanowire arrays were hexagonal wurtzite structure of ZnO，no any other impurities and compounds were found. Then the morphology of all the Al-doped ZnO nanowire arrays were observed by a Scanning Electron Microscope（SEM），it was found that a large number of ZnO∶Al nanowires grew onto the surface of substrate along the approximate vertical direction to the surface. Energy dispersive spectroscopy results of the 1% Al-doped ZnO nanowire arrays demonstrated that Al ions were substitutionally incorporated into the ZnO lattice. As a result，well-aligned Al-doped ZnO nanowire arrays were successfully built. Furthermore，the resulting five Al-doped ZnO nanowire arrays were used as photosensitive layers， respectively， the corresponding UV photodetectors were fabricated， their performances were investigated in detail. I-V characteristics of all the Al-doped ZnO nanowire arrays UV photodetectors in the dark and under 365 nm light illumination revealed that the currents of all the devices under illumination were much larger than their dark currents，indicating that all the devices showed good responses to 365 nm light. Also，the main performance parameters of all the devices were obtained by calculation，their performance parameters were compared，it was noted that the 1% Al-doped ZnO nanowire arrays UV photodetector had the optimal performance among these devices. For 1% Al-doped ZnO nanowire arrays UV photodetector，at a wavelength of 365 nm，its responsivity，specific detectivity，sensitivity，external quantum efficiency，response time and decay time were 6 180 mA/W（5 V），1.51× 1012 Jones（5 V），83.2（0 V），6 090%（5 V），4.12 s and 14.45 s，respectively. These results indicated that high-performance Al-doped ZnO nanowire arrays UV photodetectors could be achieved when ZnO nanowire arrays were doped with a proper Al concentration. This work will be helpful for developing low-cost， high-performance and large-area UV photodetectors and other related devices based on Al-doped ZnO nanowire arrays. Also，it can provide some reference for investigating other related UV photodetectors. © 2024 Chinese Optical Society. All rights reserved.