| Fast response InP/InGaAs/InP infrared photocathode in addition to the generalⅢ-Ⅴcompound semiconductor photocathodes, has the advantages of electron energydistribution centralized, high quantμm efficiency, also has long threshold wavelengthand fast response. Regarding these characteristics, it is expect to be applied in infraredultrafast diagnostic devices in picosecond magnitude, i.e. streak camera, which hasmany application in photochemistry, photobiology, molecular dynamics, plasmaphysics, ultrafast spectroscopy etc.Firstly, the physical structure model of InP/InGaAs/InP infrared photocathodewas established in this paper. According to the electron transmission characteristicsfrom internal of the cathode to surface, influence of the structure parameters ofphotocathode on the band structure of the heterojunction was analyzed. Thephotocathode absorption layer doping concentraton should not be heavier than1016cm-3. The time response characterisitics of InP/In0.53Ga0.47As/InP heterojunctioninfrared photocathode was studyed. The results show that the response speed of thephotocathode are accelerated with increasing of the field-assisted bias voltage. WhileP-InGaAs photo-absorbing layer is thickneed, the response speed gets slow. Theresponse speed also gets slow when increasing the doping concentration of InGaAsphoto-absorbing layer. When increasing the thickness and doping concentration ofP-InP photoelectron-emitting layer, the respose time will be prolonged. Based on allthese conclusions, the structure parameters and doping concentration of each layerwere optimized. The optimization results show that when the thickness of thephoto-absorbing layer less than2μm and the photoelectron emitting layer about0.5μm respectively, and the doping concentration of photo-absorbing layer andphotoelectron emitting layer are about1.5×1015cm-3and1.0×1016cm-3respectively,the response time of photocathode can be reduced to less than100ps.According to photoelectron emission theory, the spectral responsecharacterisitics of InP/In0.53Ga0.47As/InP infrared photocathode was calculated, response range from0.95μm to1.7μm. The results show that when thephoto-absorbing layer thickness and doping concentration are0.2μm and1016cm-3respectively, cathode got the maximum internal quantum efficiency at λ=1.55μm andbetter spectral response in entire range. The cathode surface is equivalent to theone-dimensional potential barrier, The cathode surface is equivalent to aone-dimensional barrier, by solving the cathode surface electron escape probability,get external quantum efficiency of the cathode. The results showed that, the externalquantum efficiency of photocathode, which compared with zero bias voltage,increased about two orders of magnitude, the response time of photocathode can bereduced to40ps.At last, the structure of photocathode is further optimized, and the opticalproperties of the photocathode is analyzed. The effect of refractive index andthickness of AR coating on the optical properties of photocathode was calculated atλ=1.1μm and at λ=1.3μm, The results indicated that compared with no AR coating,absorption characteristics of cathode have a certain increase. Determine the range ofoptimum AR coating refractive index and thickness. Select theSi3N4antireflection(AR) coating, The absorption characteristics of photocathode wascalculated, Got the relationship between the optimum AR coating thickness andwavelength, Absorption of cathode relative increase about10%.According tothe optimization results, spectral response characteristics of cathode wascalculated, indicating that compared with the previous optimization, internal quantumefficiency of photocathode has greatly improved. |
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