| Photodetector is based on the photoelectric effect,the incident light signal is converted into electrical signal for output,to achieve the acquisition of light information.Based on the aim of improving the response properties of silicon photodetectors,fabrication,optical absorption and photoelectrical response properties of silicon nanowires areall studied.The results are all as follows.(1)Fabrication of nanowire silicon: Silicon nanowires were prepared with the length ranging from 2 ?m to 50 ?m by optimizing the process parameters such as silver plating time,etching time and reaction temperature in Metal-assisted chemical etching.It was found that the silver nanoparticles were transported horizontally during the etching process and then embedded in the surface of silicon nanowires forming non-equilibrium doping layer.(2)Optical absorption properties: The silicon nanowire showed excellently optical absorption properties in the wavelength of 400 nm-2500 nm.On the one hand,the columnar structure trapping light increased the optic absorptive path and played a key role of antireflection.On the other hand,the silver nanoparticles in the surface and bottom of the nanowires further improved the light trapping due to the absorption of the silver particles themselves.Also,surface defects formed during etching contributed to absorption.(3)Photoelectric response characteristics: The upper and lower contact electrodes were prepared by screen printing to obtain the prototype devices.These devices exhibited typical rectifying characteristics and good optoelectronic performance under wide spectral irradiation of254 nm-2500 nm.Under 980 nm laser irradiation with bias voltage of 5V,its switching ratio,response and corresponding gain factor were up to 131:1,28.23 A/W,and 35.72,respectively.And its time response located in the level of microsecond with rise and fall time of 25?s and 62?s,respectively.In addition,the device still had obvious response in the near infrared and short wave infrared,and its responsivity was 68.85 m A/W,714.29 ?A/W,77.14 ?A/W,50 ?A/W under 1200 nm,1310 nm,1650 nm,2200 nm light irradiation,respectively.(4)Photoelectric response mechanism:SEM and TEM tests showed that there were a large number of defects(dislocation and vacancy)on the surface of silicon nanowires,which captured most carriers(electrons)in n type semiconductors,resulting in the bending of the surface energy bands and the formation of built-in electric fields from inside(core)to outside(shell).Under the irradiation of incident light(wavelength less than 1100 nm),the valence band electrons were excited to the conduction band forming electron-hole pairs,which were separated by built-in electric field.With the irradiation wavelength more than 110 nm,intermediate energy levels formed by defects and silver doping mainly contributed to the photo current.(5)Effect of post-processing on photoelectric properties: There were two different effects of recovery of the surface defects and unbalanced doping of silver particles.The gradual decrease of the built-in field with the defect recovery led to reduction of the separation efficiency under reverse bias voltage,so the photoelectric response characteristic was gradually weakened.However,a increase of photocurrent under forward voltage with annealing were partly attributed to the gradual release of potoelectrons,captured in defects,formed by Ag particle under light,and partly to the non-equilibrium energy level formed after activation of doped Ag.A highest photoelectrical response could be obtained when a balance between these two effects were reached when the annealing temperature up to 500?.In addition,the current direction at zero bias voltage was reversed twice with annealing temperature.Ag non-equilibrium doping could introduce both donor and acceptor levels to form p and n semiconductors.Photocurrent direction indicated the reversal of the built-in field,which confirmed the transition of the nanowire silicon surface from N to P,and then to N type. |
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