Exponential-doping Photocathode Research Articles

Thermally enhanced external photoelectric emission theoretical study for transmissive exponentially doped GaAs photocathode

Thermally enhanced photoelectric emission (TEPE) is a new proposed mechanism to improve solar cell efficiency at low temperatures (< 400K). In this paper, the mechanism of TEPE phenomenon is discussed based on the transmission-mode GaAs photocathode with exponential-doping structure. Parameters varying with temperature are quantitatively studied in the photoelectric emission process, such as diffusion length, absorption coefficient, escape probability and the drift length. Combining the variation of these parameters by temperature with the quantum efficiency of the transmissive-mode exponential-doping photocathode, the final TEPE quantum efficiency formula is derived, which proves and explains that the temperature has an auxiliary effect on the improvement of the photoemission quantum efficiency in low-temperature environment.

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.

Effect of exponential doping structure on diffusion length of reflection-mode GaN NEA photocathodes

In order to improve the performance of the photocathode, a kind of photocathode with exponential doping structure is designed and grown; meanwhile two other kinds of photocathodes with uniform doping structure and gradient doping structure are also designed. Through comparing their quantum efficiencies, the exponential doping photocathode has the best quantum efficiency. The inherent mechanism leading to the increase of the diffusion length is analyzed in detail. The diffusion length formula of the photo excited electron in the exponential doping structure is deduced, and the diffusion length in the activation layer is calculated. The result has significance to further study the GaN photocathode.

The High Quantum Efficiency of Exponential-Doping AlGaAs/GaAs Photocathodes Grown by Metalorganic Chemical Vapor Deposition

An exponential-doping structure is successfully applied to the preparation of AlGaAs/GaAs photocathodes through the metalorganic chemical vapor deposition (MOCVD) technique. The experimental results show that the quantum efficiency in the entire waveband region for the exponential-doping photocathodes grown by MOCVD is remarkably enhanced as compared to those grown by molecular beam epitaxy. As a result of the improved built-in electric fields and cathode performance parameters, the photoemission characteristics for the MOCVD-grown transmission-mode and reflection-mode AlGaAs/GaAs photocathodes are different over the wavelength region of interest.

Comparison between exponential-doping reflection-mode GaAlAs and GaAs photocathodes

A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.

Negative Electron Affinity AlGaAs/GaAs Photocathodes with Exponential-Doping Structure

Obtaining photocathodes with high quantum yield has been the focus during the process of photocathode development. With the limitation of basic industrial manufacturing level, the further performance improvement of the negative electron affinity photocathode is subject to the quality of grown material itself. For this reason, according to the band engineering science, we have proposed an exponential-doping structure applied to the active-layer of reflection-mode and transmission-mode AlGaAs/GaAs photocathodes via molecular beam epitaxy technique, to increase the photocathode emission efficiency. A series of theoretical and experimental researches including structure design, material growth, surface cleanness, Cs-O activation and performance evaluation have been carried out to confirm the actual effect of exponential-doping photocathodes. As a result of the built-in electric field, the cathode performance was enhanced for exponential-doping AlGaAs/GaAs photocathodes.

Influence of built-in electric filed on the energy and emergence angle spreads of transmission-mode GaAs photocathode

In order to research the transport characteristic of photoelectrons in different-structure transmission-mode GaAs photocathodes, the energy and emergence angle spreads of photoelectrons reaching the band-bending region are calculated and the photoemission properties are analyzed. Based on the established atomic configuration models and ionized impurity scattering formulas of the uniform-doping and exponential-doping photocathodes, the trajectories of photoelectrons in different GaAs photocathodes have been calculated. The results show that, the emergence angle spread of the exponential-doping photocathode is more centralized than that of the uniform-doping one. The influence of the built-in electric field on the photoemission is obvious in the short-wave region. The built-in electric field not only increases the quantum efficient, but also improves the resolution of photocathode. This research can be propitious to investigate the photoemission mechanism, and to analyze the effect of the excited photoelectrons on the image intensifier performance.

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.

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.

Theoretical calculation of quantum yield for exponential-doping GaAs photocathodes

Exponential-doping photocathodes, in which from the GaAs bulk to the surface the doping concentration is distributed exponentially from high to low, can form a stable built-in electric field in the active layer, and the electric field facilitates the excited photoelectron emission. The quantum yield formulas of both reflection-mode and transmission-mode of exponential-doped photocathodes have been solved from the 1-diemnsion continuity equations, in which the build-in electric field is considered. According to these formulas, we calculate the theoretical quantum yield of the exponential-doped photocathodes. The calculated results show that the exponential-doping structure can increase the quantum yield of photocathodes significantly. To compare with the uniformly-doped photocathodes, the integral sensitivity of the reflection-mode exponential-doped photocathodes increases by nearly 20%, and for transmission-mode photocathodes the increase is more than 30%. The performance improvements of exponential-doping photocathodes are mainly attributed to the built-in electric field, the photoelectrons driven by the field move towards the cathode surface by way of diffusion and drift, accordingly, decrease the influence of the back-interface recombination velocity on photoemission and increase the equivalent electron diffusion length of cathodes.