Transmission-mode GaAs Photocathode Research Articles

Improvement and structure optimization of transmission-mode GaAs photocathode performance

In order to improve the performance of transmitted GaAs photoelectric cathode, the quantum efficiency curve of Chinese transmitted GaAs photoelectric cathode is compared with that of the product of American ITT company, showing that the integration sensitivity of Chinese transmitted photoelectric cathode is 2130 μA/lm, and the American ITT company’s reaches 2330 μA/lm. Through the matrix method to solve the three membranes, the theoretical reflectivity is obtained. Based on the uniform doping transmission GaAs photocathode quantum efficiency formula, by replacing the fixed value <i>R</i> with variable value <inline-formula><tex-math id="M1">\begin{document}$ {R}_{{\mathrm{t}}{\mathrm{h}}{\mathrm{e}}} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231542_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20231542_M1.png"/></alternatives></inline-formula>, adding the short wave constraint factor, and modifying the quantum efficiency formula, a modified uniform doping transmission GaAs photocathode quantum efficiency formula is obtained. Using the revised quantum efficiency, optical performance and integral sensitivity theory model, through fitting the quantum efficiency curve of American ITT company product, introducing the ITT cathode component performance parameters, comparing the performance parameters of Chinese product, the results show that the Chinese photocathode in the window layer, the thickness of the emission layer, electron diffusion length and rear interface composite rate has a certain gap with ITT’s. In order to shorten the gap between the two and optimize the cathode structure parameters, the transmission GaAs photocathode optical structure software is designed to further analyze the influence of the electron diffusion length and the emission layer thickness on the quantum efficiency of the photocathode. The results show that with an electron diffusion length of 7 μm and emission layer thickness of 1.5 μm, the transmitted GaAs photocathode sensitivity can be more than 2800 μA/lm. However, the large electron diffusion length has high requirements for cathode materials and preparation level. The reasons responsible for the performance gap between Chinese product and other country’s are that in China the growth process of cathode materials is not jet matureand the cathode preparation equipment is out of date . In this paper, we study the relationship between GaAs photocathode optical performance and photoemission performance, and further optimize the structural design of cathode components, which has certain guiding significance for improving the cathode quantum efficiency and the level of image intensifier.

Modelling the Light Scattering Behavior of Transmission-Mode GaAs Photocathode Under the AlGaAs Window Layer Rough Surface

To improve the understanding of the optical image resolution of transmission-mode GaAs photocathode, by establishing the surface scattering analysis model of AlGaAs window layer, we have researched the effect of the surface height distribution variance σ on scattering surface transfer function (STF) and point spread function (PSF) of transmission-mode GaAs photocathode. The simulation results show that the decrease of surface height distribution variance σ is beneficial to improve the optical image resolution. This improvement is mainly attributed to the decrease in input light energy attenuation and spatial diffusion caused by smaller surface height distribution variance σ, and thus the signal to noise ratio of scattering degraded optical image is increased.

Effect of Incident Light Scattering at AlGaAs Window Layer Interface on Resolution of Transmission-mode GaAs Photocathode

Effect of Incident Light Scattering at AlGaAs Window Layer Interface on Resolution of Transmission-mode GaAs Photocathode

Study on the time-resolved characteristics of the transmission-mode GaAs photocathode

Study on the time-resolved characteristics of the transmission-mode GaAs photocathode

In situ characterization of thermal cleaned surface for preparing superior transmission-mode GaAs photocathodes.

Considering that it is impractical to utilize in situ surface diagnostic means to determine the surface cleanliness of transmission-mode GaAs photocathodes in the vacuum device manufacturing process, the thermal desorption technique with the aid of the quadrupole mass spectrometer during the thermal cleaning process is employed to in situ characterize the thermal cleaned surface. The desorption behaviors for various impurity gases during the thermal cleaning process are analyzed. The experimental results show that the amount of desorbed impurity gases varies due to the different heat treatment temperatures. Through the verification of Cs/O activation and quantum efficiency measurement, it is found that the desorption behaviors of the specific impurity gases including AsH3 and As2 are crucial to surface cleanliness of transmission-mode GaAs photocathodes, which relate to the final photoemission capability. This simple and reliable criterion provides an effective way to guide the thermal cleaning process of transmission-mode GaAs photocathodes, and the desorption behaviors assist in in situ evaluation of surface cleanliness.

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.

Iterative correction of the thickness for transmission-mode GaAs photocathode optical performance

Transmission-mode GaAs photocathode sample was grown by molecular beam epitaxy with the structure of Glass/Si3N4/GaAlAs/GaAs. The experimental reflectivity and transmittance curves were measured by the spectrophotometer. In order to fitting the actual thicknesses, the correction method is proposed which is multiple iterations of the thicknesses for reducing the composition error. The results show that the composition error of reflectivity and transmittance curves decreased from 15.2% to 4.9% in the range of 600–1100 nm by the correction of the multiple iterations. In addition, the thickness structure approximating to the actual value can be deduced by fitting the optical performance.

Comparison of module structure of wideband response GaAs photocathode grown by MBE and MOCVD

In order to compare the structures of GaAs photocathodes grown by molecular beam epitaxy (MBE) and metal–organic chemical vapour deposition (MOCVD), four wideband response exponential doping photocathodes were prepared. Their reflectivity and transmittivity were measured by the spectrophotometer, and three thin layer thicknesses were fitted based on the matrix theory in Thin-film Optics. The comparison of the results indicated that for the GaAs photocathode grown by MBE, only one amendatory layer with the low Al component should be added between the Ga1−xAlxAs window layer and the GaAs active layer in order to the higher fitting accuracy. Opposite occurs for the MOCVD samples. In the case of accurately controlling the layered doping concentration, the material grown by MBE is the optimal, while that grown by MOCVD is suit for the exponential doping situation. These results are available for the material growth and the module preparation of the varied doping transmission-mode GaAs photocathodes.

Electron energy and angle distribution of GaAs photocathodes

A precise Monte Carlo model is developed to investigate the electron energy and angle distribution of the transmission-mode GaAs (100) photocathode at room temperature. Both distributions are important for high-quality electron sources. The results show that the energy loss (0.1309 eV) and the angle-dependent energy distribution curves fit well with experimental data. It is found that 65.24% of the emission electrons come from Γ valley, 33.62% from L valley, and 1.15% from X valley. The peak of the energy distribution curve is contributed by both Γ and L-valley electrons, while the high-energy part is contributed by Γ-valley electrons rather than L electrons, which is different from previous inference and can be attributed to the narrow energy range of L-valley electrons. However, L-valley electrons have a larger angular spread than Γ-valley electrons and lead to the spread of the emission cone. The further simulation indicates that increasing the hole concentration or the thickness of the first activation layer can improve the angle distribution, but the energy distribution will turn to be slightly more dispersive. Temperature effect on the two distributions is also analyzed. The energy distribution curve moves towards the higher energy while the angle distribution curve moves towards the lower value when the temperature declines.

Quantum efficiency conversion from the reflection-mode GaAs photocathode to the transmission-mode one

In the article, the transmission-mode negative electron affinity GaAs photocathode is compared with the reflection-mode one in structure, working principle and the quantum efficiency equation. The key point to establish relation between the two modes is that the active-layer is in full accord and the biggest difference is that the incident direction of photons is inverse. Based on the relation and difference above, we obtain the electron escape probability P and the spectrum absorption coefficient α of the reflection-mode GaAs photocathode by multilayer film matrix theory. The quantum efficiency of the transmission-mode GaAs photocathode can be estimated through the coefficients P and α of the reflection-mode one. Two groups of samples are analyzed, the one group contains uniform doping reflection-mode and transmission-mode GaAs photocathodes and the other contains exponential doping ones. Through the analysis and curve fitting, we find that the conversion difference of the uniform-doping is about 11.22% while that of the exponential-doping is about 5.46%. Another four groups of samples with exponential-doping active layer are experimentalized and the differences of them are all less than 6%. The results prove the suggested method is feasible and effective and it is more suitable for the conversion of GaAs photocathode with exponential-doping active layer.

Relationship between X-Ray Relative Diffraction Intensity and Integral Sensitivity of GaAlAs/GaAs Photocathode

To establish a methode for predicting the integral sensitivity of transmission-mode GaAs photocathodes, the relationship between X-ray relative diffraction intensity and integral sensitivity of GaAlAs/GaAs photocathode material is researched. After thermocompression bonding Si3N4/GaAlAs/GaAs/GaAlAs/GaAs epitaxial material to glass window in the vacuum condition, and chemically etching the GaAlAs buffer-layer and GaAs substrate, the glass/Si3N4/GaAlAs/GaAs photocathode module is formed. The X-ray relative diffraction intensity of the photocathode module is tested and calculated respectively, then the photocathode surface was activated in the ultrahigh vacuum chamber using the Cs-O activation technique. Following that, the integral sensitivity of the transmission-mode GaAs photocathode is measured by the spectral response measurement instrument in situ. It is found that the GaAlAs/GaAs photocathode material and photocathode module have similar X-ray relative diffraction shapes. The higher the similar degree of X-ray relative diffraction shape is, the bigger the X-ray relative diffraction intensity of photocathode module is, which results in the better photoemission capability and higher photocathode integral sensitivity. This method can be used as an evaluation criterion for the quality of transmission-mode GaAs photocathode module material.

Study on X-Ray Integral Diffraction Intensity of GaAs Photocathode

In order to precisely predict the sensitivity of Ф18 mm transmission-mode GaAs photocathode, a concept of integral diffraction intensity is proposed based on X-ray diffraction principle after analyzing the predecessors′ limitations of testing the micro-area of such photocathode and GaAs photocathode of image intensifier tube is plane electron source in this paper. The integral diffraction intensity on the entire photocathode surface was obtained by multi-points detection in the effective area of the photocathode with integral method. The crystal quality of entire photocathode surface will be taken with the integral diffraction intensity. According to the principle, X-ray diffraction testing for 4 samples of GaAs photocathode modules was executed with high-resolution four-wafer X-ray Diffractometer whose test spot size is 4 mm×5 mm. The diffraction curves were obtained and the integral diffraction intensity was calculated. Subsequently the 4 photocathode modules was activation processed with Cs-O in ultra-high vacuum system simultaneity the photocurrent of photocathode modules was measured. Comparing the variation of diffraction curve with integral diffraction vs photocathode photocurrent curve, they show that the greater the integral diffraction intensity of is, the more photocurrent is in the photocathode module. The variation relation curve between X-ray integral diffraction intensity and photocurrent in the photocathode was fitted with least square method. The curve, which accords with logarithm curve and whose fitting degree is 0.878, was achieved. Since photocathode sensitivity is direct proportion to photocathode photocurrent. The above results prove that A Practicality Φ18mm photocathode of image intensifier tube is plane electron source, GaAs photocathode sensitivity and other photoelectric performance lies on entire photocathode surface crystal quality, the photocathode module integrality reflected by the integral diffraction intensity plays crucial role of GaAs photocathode sensitivity. So integral sensitivity of Ф18 mm transmission-mode GaAs photocathode can be precisely predicted with X-ray integral diffraction intensity, some feasible ideas for further research of GaAs photocathode was obtained in this paper.

Theoretical Modification on Optic Properties for Transmission-Mode GaAs Photocathode Module

To explore the optic properties of transmission-mode GaAs photocathode module, experimental and theoretical values of reflectance and transmittance of photocathode module has been compared. it showns that experemental curves cannot tally with theoretical curves completely. The variation range of initial values of thickness is firstly setted. Modifing transmittance formula by a fitting coefficient A, optical properties is fitted using the method of error control. R-T combined error reduced from 15.2% to 4.9% using R-T combined error control scheme. Optimal fitting values of thickness of photocathode module are obtained, which are d1= 110 nm, d2= 1019 nm, d3= 1491 nm. And the error of total thickness is 2.9%.

Comparative research on indium seal process for transmission-mode GaAs photocathodes

For night vision devices and other applications, the transmission-mode photocathodes must be sealed to tube by indium seal process (ISP) in practical application. But in early research, the integral sensitivity has large drop to about 30% amplitude after ISP process than after activation process. In order to well study the influence of ISP on surface barriers of activated GaAs photocathodes, we used the comparative research method by surface photovoltage spectroscopy (SPS) and spectral response current (SRC). Through fitting calculation, we can find that the impurity gas sources by micro channel plate and fluorescent screen have deep influence on the amplitude and width of surface barriers which lead to the large drop on SRC curves before and after ISP process. This on-line comparative research method can help to optimize ISP technique and vacuum degree optimization for transmission-mode GaAs photocathodes in the future.

Comparative research on the transmission-mode GaAs photocathodes of exponential-doping structures

Early research has shown that the varied doping structures of the active layer of GaAs photocathodes have been proven to have a higher quantum efficiency than uniform doping structures. On the basis of our early research on the surface photovoltage of GaAs photocathodes, and comparative research before and after activation of reflection-mode GaAs photocathodes, we further the comparative research on transmission-mode GaAs photocathodes. An exponential doping structure is the typical varied doping structure that can form a uniform electric field in the active layer. By solving the one-dimensional diffusion equation for no equilibrium minority carriers of transmission-mode GaAs photocathodes of the exponential doping structure, we can obtain the equations for the surface photovoltage (SPV) curve before activation and the spectral response curve (SRC) after activation. Through experiments and fitting calculations for the designed material, the body-material parameters can be well fitted by the SPV before activation, and proven by the fitting calculation for SRC after activation. Through the comparative research before and after activation, the average surface escape probability (SEP) can also be well fitted. This comparative research method can measure the body parameters and the value of SEP for the transmission-mode GaAs photocathode more exactly than the early method, which only measures the body parameters by SRC after activation. It can also help us to deeply study and exactly measure the parameters of the varied doping structures for transmission-mode GaAs photocathodes, and optimize the Cs-O activation technique in the future.

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.

Theoretical analysis of response characteristics for the large exponential-doping transmission-mode GaAs photocathodes

A new-type GaAs photocathode with ultrafast time response, that is, the large exponential-doping transmission-mode GaAs photocathode, is discussed in detail. The response characteristics, including quantum yield, time and spatial resolution, are numerically simulated. The analysis results show that the transit response time of the photo-excited electrons for the GaAs photocathode is extremely shortened, because the built-in electric field in GaAs layer formed by the large exponential-doping mode is benefitcial to the photoelectron transport process of GaAs photocathodes. The response time can reach about 10 ps when the thickness of GaAs dgorption layer is around, which shows that the novel NEA cathode has a better feature of temporal response than that of traditional GaAs photocathode. In addition, the quantum yield will reach ～10%-20% in the whole special response range, and the spatial resolution is improved obviously. The analysis results indicate that with high quantum efficiency guaranteed, the large exponential-doping NEA cathode overcomes the limitation of time response of traditional GaAs NEA cathode and improves the spatial resolution, which indicates that the new NEA cathode is expected to meet the demands of high-speed device and photoelectron device, and promote the further development and applications of NEA cathodes.

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.

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.

Improvement of photoemission performance of a gradient-doping transmission-mode GaAs photocathode

Two types of transmission-mode GaAs photocathodes grown by molecular beam epitaxy are compared in terms of activation process and spectral response, one has a gradient-doping structure and the other has a uniform-doping structure. The experimental results show that the gradient-doping photocathode can obtain a higher photoemission capability than the uniform-doping one. As a result of the downward graded band-bending structure, the cathode performance parameters, such as the electron average diffusion length and the surface electron escape probability obtained by fitting quantum yield curves, are greater for the gradient-doping photocathode. The electron diffusion length is within a range of from 2.0 to 5.4 μm for doping concentration varying from 1019 to 1018 cm−3 and the electron average diffusion length of the gradient-doping photocathode achieves 3.2 μm.