Study On The Surface Activation Mechanism And Evaluation Of Near-infrared InGaAs Photocathod

In Ga As photocathode materials can be widely used in near-infrared photoelectric devices due to the advantages of controllable response cutoff in the near-infrared range,the low dark current and concentrated electron emission energy.At present,because In P material is not resistant to high temperature and is not conducive to high temperature purification,it is necessary to study the In Ga As photocathode material with low In concentration epitaxially grown on Ga As substrate.Based on this background,this paper focuses on the mechanism and evaluation of surface activation for In Ga As photocathode,including surface activation adsorption mechanism,residual gas adsorption mechanism,activation effect evaluation and attenuation performance characterization,specifically as follows:Firstly,the Ga-rich In_0.19Ga_0.81As（100）β₂（4×2）clean surface and Zn doping model at different locations are constructed based on first-principles calculation.In the model with the lowest work function,Cs atoms with different coverage,O atoms and NF₃molecules levels were adsorbed,and it was found that with the increase of Cs coverage,whether single Cs,Cs/O or Cs/NF₃,the surface work function showed a trend of first decreasing and then increasing.When adsorbing 6Cs,all of them reach the minimum value.The work function of Cs/O models is slightly larger than that of Cs/NF₃models.At the same time,it can be found that with the increase of the number of Cs atoms,single Cs atom becomes more and more unstable.Cs/NF₃models is easier to adsorb on the surface than Cs/O models.The single Cs,Cs/O or Cs/NF₃adsorption is conducive to the near-infrared absorption of the surface,but leads to the decrease of the ultraviolet-visible absorption coefficient.Secondly,the 6Cs/1O adsorption model and 6Cs/1NF₃adsorption model of In_0.19Ga_0.81As（100）β₂（4×2）surface activation layer model was selected,and the five common residual gas structures including CO₂,CO,CH₄,H₂O and H₂were introduced into the surface activated layer model.By comparing the photoelectric properties of the residual gas activated layer model,it was found that CO₂and CH₄residual gases had a strong destructive effect on the cathode surface,H₂have little effect on cathode surface destruction,while the influence of H₂O and CO is different for the two models.The influence of Cs/NF₃stratification model on the surface of In_0.19Ga_0.81As（100）β₂（4×2）material is much greater than that on Cs/O stratification model.And it is found that various residual gas molecules on the surface of the layered adsorption model will be far away from the surface and the dimers will change.Finally,by using the ultra-high vacuum interconnection system and X-ray photoelectron spectroscopy（XPS）and ultraviolet photoelectron spectroscopy（UPS）,different Cs/O activation experiments and Cs/NF₃activation experiments are designed.The time of the peak value of photocurrent,the time when photocurrent drops to 80%of its peak value and the time when photocurrent drops to the valley value are selected respectively.The second activation source like O or NF₃is opened at these occasions.Then the sample were attenuated for 24hours after activation.Evaluating the activation results,it can be found that the Cs/NF₃activation experiment can obtain a photocathode with better near-infrared photoemission performance than the Cs/O activation experiment,which is consistent with the slightly smaller work function of Cs/NF₃model.Compared with the performance attenuation results,it is found that the Cs/O activation experiment after 24 hours of attenuation is more stable than Cs/NF₃activation experiment,which is consistent with that the residual gas has a worse impact on the Cs/NF₃activation surface than Cs/O.Using surface analysis technology,it can be found that for the surface with the Ga-rich surface and the higher Cs content,the work function of the cathode surface after attenuation is lower.