Research On ZnO Vertical Nanorod Arrays Photodetector

One-dimensional(1D)nanostructures with well-defined composition have been widely utilized to enhance the photodetector performance and additionally to miniaturize the device components.Taking working mode into account,the photodetectors can be classified into two categories:photoconductors and photodiodes.As for the photoconductor,a photon with energy larger than the band-gap energy is absorbed to generate an electron-hole pair,thereby changing the electrical conductivity of the photoactive materials.Normally,the high-mobility materials are favorable for high-responsivity photoconductors,in which the photocarriers can recirculate many times through an external circuit.Contradictorily,this inevitably results in high dark current and low ON/OFF ratio due to the high carrier concentration and thus limits the device performance including signal-to-noise ratio,detectivity(D*)and so on.What is worse,the optical and electrical areas of the photoconductive device are the same,namely,the light absorption layer and device channel form an inseparable whole.Consequently,it is impossible to suppress the dark current while maintaining the high photocurrent by reducing the channel area or to enhance the photocurrent while maintaining the low dark current by increasing the area of light absorption layer.Herein,we design a ZnO vertical nanorod arrays based photoconductor with light absorption layer separated from the device channel.At the bottom,every nanorod contacts to each other and there exists gap between every two adjacent ZnO nanorods above the dense layer.As for this device,therefore,the photo-generated carriers along the axial ZnO nanorods drive to the external electrodes through nanorod-nanorod junctions at the bottom.The results show that the ON/OFF ratio increase with increasing the ratio between ZnO nanorod length and the thickness of dense layer and the ON/OFF ratio reaches to the maximum of 790 as the ZnO nanorod with length of1.9?m has the ratio L/T of 3.8,which is two orders of magnitude larger than the value for the ZnO horizontal nanowire based photoconductor.The as-produced ZnO vertical nanorod arrays based photoconductors possess a high responsivity of 1.3×10~5 A/W,high detectivity of 1.3×10~133 Jones and low detectable light intensity of 1?W/cm~2.More importantly,the developed approach combining top down and bottom up strategy allows the integration of ZnO vertical nanorod arrays based photodetector arrays as image sensors with 100%device yield and uniform device-to-device performance(photocurrent ranging from 2 to 12?A),which is of great importance for image sensors.