Yajuan Xiong

Variation of spectral response for exponential-doped transmission-mode GaAs photocathodes in the preparation process

To confirm the actual effect of an exponential-doped structure on cathode performance, an exponential-doped structure was applied to the preparation of a transmission-mode GaAs photocathode, and spectral response curves after high-temperature activation, low-temperature activation, and the indium sealing process were separately measured by use of the on-line spectral response measurement system. The results show that, compared to the previously uniform-doped photocathode, the exponential-doped photocathode can obtain higher cathode performance and photoemission capability because of the built-in electric field. Nevertheless, cesium desorption and impurity of gas during the sealing process can cause the degeneration of spectral response in the entire response waveband, especially in the long-wavelength region, with the decrease in surface electron escape probability related to the adverse evolution of the surface potential barrier profile.

Influence of exponential doping structure on the performance of GaAs photocathodes

Obtaining higher quantum efficiency and more stable GaAs photocathodes has been an important developmental direction in the investigation of GaAs photocathodes. One significant approach to this problem is to improve the electron diffusion length. We put forward and investigate an exponential doping mode GaAs photocathode. It was proved by theoretical and experimental results that, because the exponential doping structure is in favor of forming a directional constant built-in electric field, the electron diffusion and drift length of the cathode material can accordingly be enhanced. The mathematical expression of the electron diffusion and drift length L(DE) of an exponential doping photocathode was deduced, and the relationship between the doping coefficient and the electron diffusion and drift length is made certain. This investigation contributes to the understanding of varied doping GaAs photocathodes and provides guidance to optimize the doping structure of GaAs photocathodes for higher quantum efficiency.

Annealing study of carrier concentration in gradient-doped GaAs/GaAlAs epilayers grown by molecular beam epitaxy

We measured the carrier concentration distribution of gradient-doped GaAs/GaAlAs epilayers grown by molecular beam epitaxy before and after annealing at 600 degrees C, using electrochemical capacitance voltage profiling, to investigate the internal variation of transmission-mode GaAs photocathodes arising from the annealing process. The results show that the carrier concentration increased after annealing. As a result, the total band-bending energy in the gradient-doped GaAs emission layer increased by 25.24% after annealing, which improves the photoexcited electron movement toward the surface. On the other hand, the annealing process resulted in a worse carrier concentration discrepancy between the GaAs and the GaAlAs, which causes a lower back interface potential barrier, decreasing the amount of high-energy photoelectrons.

Spectral response variation of exponential-doping transmission-mode GaAs photocathodes in the preparation process

The exponential-doping structure was applied to prepare the transmission-mode GaAs photocathode, and spectral response curves after high-temperature activation, low-temperature activation and the indium sealing process were respectively measured by use of the on-line spectral response measurement system, to research into the practical effect of the exponential-doping structure on cathode performance. The results show that a high photosensitivity ranging from 560 nm to 880 nm with an ascending trend can be obtained after the high-low temperature activation. In the region of longwave threshold, there is a distinct inflexion indicating a better photoemission capability than the former uniform-doping photocathodes. Besides, the spectral response curve in the whole response waveband, especially the long-wave region obviously decreases after indium seal. Compared with the fitted surface electron escape probability after Cs-O activation, it decreases after indium seal according to the quantum efficiency formula of exponential-doping transmission-mode GaAs photocathodes. Based on the double dipole model, the reasons for the variation of spectral response shape are explained on account of the relation between surface escape probability and the evolution of surface potential barrier profile.

Comparison of structure and performance between extended blue and standard transmission-mode GaAs photocathode modules

Extended blue and standard transmission-mode GaAs photocathode modules were prepared, respectively, by metal organic chemical vapor deposition. The experimental reflectivity, transmissivity, and spectral response curves were measured and compared separately. The integral sensitivities are 1980 μA/lm and 2022 μA/lm for both the modules. By use of the revised quantum yield formula, the experimental spectral response curves are fitted to obtain the structure parameters. The fitted results show that the Ga(1-x)Al(x)As window layer with varied aluminum components is beneficial to improve extended blue GaAs photocathode module. In addition, the layer-thickness and aluminum component in the window layer determine the extended blue performance, while the thickness of the GaAs active layer settles the long-waveband performance for the transmission-mode GaAs photocathode module.

Photoemission performance of gradient-doping transmission-mode GaAs photocathodes

With an attempt to improve the photoelectron emission efficiency, a gradient-doping structure proposed based on the Spicers three-step model has been applied to the preparation of the transmission-mode GaAs photocathode via molecular beam epitaxy technique. The Cs-O activation phenomenon suggests that the gradient-doping structure can bring a potential photoemission capability with the increase of activation time, and the spectral response curves show that the gradient-doping photocathode can obtain a higher response capability in the entire waveband region, especially in the regions of short-wavelength threshold and long-wavelength threshold. By fitting quantum yield curves, the obtained cathode performance parameters such as electron average diffusion length and electron escape probability of the gradient-doping photocathode are greater than those of the uniform-doping one. The electron average diffusion length of the gradient-doping photocathode achieves 3.2 μm. The improvement in cathode performance of the gradient-doping photocathode could be ascribed to the downward gradient band-bending structure.

Research on optical properties of transmission-mode GaAs photocathode module

In order to know performance of transmission-mode photocathode module completely, the GaAs photocathode with a structure of Glass/Si3N4/Ga1-xAlxAs/GaAs was prepared in the experiment and the reflectance and transmittance spectra were measured by the spectrophotometer. Meanwhile optical constants of the GaAs active layer and the Ga1-xAlxAs window layer in the photocathode are discussed by using piecewise polynomial fitting method. On this basis of analysis on the optical constants, the theoretical reflectance, transmittance and absorptivity of cathode module are calculated and revised with the aid of matrix formula in thin film optics. The thickness of each layer in the module is obtained by fitting the reflectance and transmittance curves simultaneously. The results indicates that the thicknesses of three thin films except Glass are respectively 110.14 μm, 1007.20 μm, 1480.81 μm with the relative curve error less than 5%, meanwhile the error of the module thickness in total is also controlled within 5%.

Exponential-doping III–V photocathodes with negative electron affinity

With the limitation of epitaxial growth technique, the approximately exponential-doping structure is actually a special kind of gradient-doped structure, in which the doping concentration in the active-layer of the III-V photocathode varies exponentially from bulk to surface. This doping structure can result in a linearly downward band structure which helps form a constant built-in electric fields to facilitate photoexcited electron movement towards surface, and the band structure of the exponential-doping photocathode is shown in this paper. By using the MBE technique, the reflection-mode (RJVI) and transmission-mode (TM) exponential-doping GaAs photocathodes composed of many sublayers were grown.

Simulation and spectral fitting of the transmittance for transmission-mode GaAs photocathode

Error modification is introduced to the theoretical calculation and the fitting thicknesses and deviations of the photocathode are obtained in the paper. Measured with the spectrophotometer typed UV-3600, the transmittance spectrum of the sample was achieved ranging from 400 nm to 1100 nm.

Contrast study on GaAs photocathode activation techniques

At present, two kinds of activation techniques for preparing GaAs NEA photocathode are available. In this paper, according to two kinds of photocurrent curve arising in the activation, the characteristic and mechanism of the two kinds of craft were summed up and compared with each other, and the further theoretical investigation on the mechanism of activation was carried out based on the recent research of NEA surface model for GaAs photocathode. It is proposed as a process principle that during (Cs, O) alternation phase of the activation process of GaAs photocathode, Cs should always be in excessive state. Besides, it is also indicated that whether Cs is excessive during (Cs, O) alternation phase and the Cs/O ratio may affect directly the final property of photocathode. Finally, a method to modify the craft parameters to guard against the deviation from the principle is presented. The presented study is very necessary and significative for optimizing the activation techniques so as to enhance the performances of GaAs NEA photocathodes.