Uniform-doping Photocathode Research Articles

Quantum Yield of Reflection Mode Varied Doping GaN Photocathode

Using the NEA photocathode activation and evaluation experiment system, the varied doping GaN photocathode has been activated and evaluated. According to the diffusion and orientation drifting equation, the quantum yield formula of reflection mode varied doping NEA GaN photocathode was gotten. The factors affecting the quantum efficiency of varied doping GaN photocathode were studied. For the varied doping GaN photocathode, the quantum efficiency is mainly decided by the escape probability of electron P, he absorption coefficient α, the electron diffuse length L D , the reflectance of cathode materials for incident light R, emission layer thickness Te and the inside electric field E . The experiment and analysis results show: With the directional inside electric field in the bulk, the varied doping NEA GaN photocathode has better photoemission performance than uniform doping photocathode.

Surface activation behavior of negative-electron-affinity exponential-doping GaAs photocathodes

In view of the effect of doping structure on the surface activation behavior, the Cs–O activation experiments under the same preparation condition were performed on the exponential-doping and uniform-doping GaAs photocathodes. The activation results show that, compared with the uniform-doping photocathode, the exponential-doping photocathode presents a lower photocurrent peak after the initial Cs activation and a higher photocurrent peak after the Cs–O alternate activation. Based on the double dipole model, the parameters of surface potential barrier are obtained from the calculation of longitudinal energy distribution of emitted electrons according to the ratio of photocurrent peak after Cs–O activation to that after Cs activation. The discrepancy in the photocurrent evolution during the entire activation process is related to the different variations in surface potential barrier profile.

Influence of the antireflection, window, and active layers on optical properties of exponential-doping transmission-mode GaAs photocathode modules

In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si 3N 4/Ga 1 − x Al x As/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si 3N 4 antireflection layer of 0.1 μm, the Ga 1 − x Al x As window layer of more than 0.4 μm, and the GaAs active layer of 1.5 μm–2 μm and low average doping concentration.

Influence of varied doping structure on photoemissive property of photocathode

The built-in electric fields within a varied doping GaAs photocathode may promote the transport of electrons from the bulk to the surface, thus the quantum efficiency of the cathode can be enhanced remarkably. But this enhancement, which might be due to the increase in either the number or the energy of electrons reaching the surface, is not clear at present. In this paper, the energy distributions of electrons in a varied doping photocathode and uniform doping photocathode before and after escaping from the cathode surface are analysed, and the number of electrons escaping from the surface in different cases is calculated for the two kinds of photocathodes. The results indicate that the varied doping structure can not only increase the number of electrons reaching the surface but also cause an offset of the electron energy distribution to high energy. That is the root reason for the enhancement of the quantum efficiency of a varied doping GaAs photocathode.

Effect of exponential-doping structure on quantum yield of transmission-mode GaAs photocathodes

The quantum yield formulas for exponential-doping and uniform-doping transmission-mode GaAs photocathodes are modified by adding a shortwave constraint factor to the photoelectron generation function in a one-dimensional continuity equation. Based on the modified transmission-mode quantum yield formulas, the experimental exponential-doping and uniform-doping transmission-mode quantum yield curves are respectively fitted, and the fitted curves are consistent well with the experimental curves. In addition, the fitted cathode performance parameters indicate that as compared with the uniform-doping photocathode, the exponential-doping photocathode can obtain a higher cathode performance because of the built-in electric field. The exponential-doping structure can effectively increase the quantum yield of transmission-mode photocathode.

Influence of exponential-doping structure on photoemission capability of transmission-mode GaAs photocathodes

In order to verify the actual effect of an exponential-doping structure on cathode performance, an exponential-doping structure has been applied to the preparation of the transmission-mode GaAs photocathode via molecular beam epitaxy technique. Compared with the uniform-doping photocathode, the activation and spectral response results show that the exponential-doping photocathode can achieve a higher photoemission capability. In addition, based on the revised uniform-doping and exponential-doping transmission-mode quantum yield equations, the cathode performance parameters such as electron average transport length and electron escape probability of the exponential-doping photocathode are obtained, which are greater than those of the uniform-doping one. The improvement in the cathode performance is attributed to the built-in electric field arising from this special doping structure, which effectively increases the electron transport efficiency and escape probability.

Study on the model of quantum efficiency of reflective varied doping GaAs photocathode

The theoretical model of quantum efficiency of the reflective varied doping GaAs photocathode is proposed, which adopts the weighted mean method based on the quantum efficiency formula of exponential doping and uniform doping photocathode. By this model the quantum efficiency curves of a reflective GaAs photocathode under varied doping structure have been fit and we obtained the precise best fit in different incident photon wavebands. The phenomena that the contributions of doping structures to the cathode quantum efficiency are different was discovered. This phenomena has been analysed and discussed detailedly in the paper.

Resolution characteristic of exponential-doping GaAs photocathodes

The modulation transfer function （MTF） of transmission-mode exponential-doping photocathodes has been solved from the 2-dimensional continuity equations. According to the MTF equation，we calculated and analyzed the theoretical resolution characteristic of the exponential-doping photocathodes. The calculated results show that，compared with the uniform-doping photocathodes，the exponential-doping structure can increase the resolution of photocathodes evidently. For the spatial frequency franges between 100 and 400 lp/mm，the increase of resolution is more pronounced. Let f=200 lp/mm，the resolution of exponential-doping photocathodes generally increases by 20%—50%. The resolution improvement of exponential-doping photocathodes，as well as the quantum efficiency improvement，is also attributed to the built-in electric field.