Transmission-mode Photocathode Research Articles

UV–Vis–NIR broadband response of GaAs-based photocathode with multilayer graded-band cascade structure

To improve the lattice matching, quantum efficiency and response range of the transmission-mode GaAs-based photocathode , a novel photocathode with multilayer graded-band cascade emission layer is proposed. The one-dimensional steady-state continuity equations and finite-difference time-domain methods are utilized to obtain the quantum efficiency and optical absorption characteristics of the proposed photocathode, respectively. The results show that, the built-in electric fields generated by the varying-composition and varying-doping structure can assist the photoelectrons generated in the sublayer itself and the front sublayers to transport towards the emitting surface and escape to the vacuum. Compared with the conventional AlGaAs/InGaAs structure, the proposed structure possesses better lattice matching and enhanced quantum efficiency in the UV and NIR wavelength range. A three-fold increase of quantum efficiency can be realized at 400 nm and the theoretical quantum efficiency at 1064 nm can reach 2%. Besides, the integral absorptivity is increased by 6.8% in the visible wavelength range, and 12.9% in the NIR wavelength range. It is found that the increase of the thickness of AlGaAs sublayer and GaAs sublayer mainly reduces the quantum efficiency in the UV–Vis wavelength range. When the InGaAs sublayer is thin enough, the increase of thickness would enhance the quantum efficiency in the NIR wavelength range and deteriorate that in the UV–Vis wavelength range. This work can contribute to the performance improvement of UV–Vis–NIR broadband GaAs-based photocathode. • A transmission-mode photocathode with multilayer graded-band cascade structure is proposed. • The quantum efficiency model of the proposed multilayer structure is deduced. • The quantum efficiency and optical absorption characteristics are investigated. • The lattice matching, quantum efficiency and response range are improved.

Quantum efficiency of transmission-mode graded bandgap AlxGa1-xAs/GaAs photocathode

Graded bandgap AlxGa1-xAs/GaAs photocathode with enhanced quantum efficiency is analyzed in this study. We present the relevant quantum efficiency equations by solving onedimensional continuity equations for transmission-mode graded bandgap AlxGa1-xAs/GaAs and standard AlGaAs/GaAs photocathodes. The results show that the built-in electrical field from bandgap gradation efficiently collects photogenerated electrons in the buffer layer such that quantum efficiencies in the short regions are improved in transmission-mode AlxGa1-xAs/GaAs photocathode. The results also show that a thinner buffer layer improves the short-wavelength response of transmission-mode photocathode. Increasing the active layer thickness improves long-wavelength responses but reduces short-wavelength responses. The method presented in this work may provide better estimate of performance and guide the optimum design of graded bandgap AlxGa1-xAs/GaAs photocathode.

High quantum efficiency transmission-mode GaAlAs photocathode with a nanoscale surface structure

To obtain a high quantum efficiency transmission-mode GaAlAs photocathode, a photocathode with a graded Al composition structure is designed, in which the Al composition of the emission layer is decreased gradually from bulk to surface, and the outermost layer is GaAs. Based on the graded Al composition structure, GaAlAs photocathodes with different thicknesses of GaAs layers are prepared. The experimental results show that the quantum efficiency of the GaAlAs photocathode in blue-green light significantly increased when the thickness of the outermost GaAs achieves a dozen nanometers. The peak quantum efficiency of a GaAlAs photocathode with a nanoscale GaAs layer could achieve 31%, which appears at 620 nm. The nanoscale surface structure has a quantum confinement effect, which is the main factor in the increased quantum efficiency of the GaAlAs photocathode.

Quantum efficiency of heterostructured AlN/AlxGa1−xN photocathodes with graded bandgap emission layer

A heterostructured AlN/AlxGa1−xN photocathode consisting of a sapphire substrate layer, a AlN buffer layer, and a graded bandgap emission layer is proposed to improve the quantum efficiency of AlGaN photocathode. The theoretical models for transmission-mode and reflection-mode photocathodes were deduced based on one-dimensional continuity equations to analyze the characteristics of the devices. Results show that the multilevel built-in electric field induced by the bandgap gradation in the emission layer can improve the quantum efficiency because of the longer electron diffusion length and the reduction of the back-interface recombination losses. The effect of thicknesses of photocathodes and Al composition in the sublayers on efficiency is discussed. This work would provide theoretical guidance for better performance of AlGaN photocathodes.

Comparative study on the influence of Al component at GaAlAs layer for GaAs/AlGaAs photocathode**Project supported by the National Natural Science Foundation of China (Nos. 91433108, 61301023).

We designed two transmission-mode GaAs/AlGaAs photocathodes with different AlxGa1−x As layers, one has an AlxGa1−xAs layer with the Al component ranging from 0.9 to 0, and the other has a fixed Al component 0.7. Using the first-principle method, we calculated the electronic structure and absorption spectrum of AlxGa1−xAs at x = 0, 0.25, 0.5, 0.75 and 1, calculation results suggest that with the increase of the Al component, the band gap of AlxGa1−xAs increases. Then we activated the two samples, and obtained the spectral response curves and quantum efficiency curves; it is found that sample 1 has a better shortwave response and higher quantum efficiency at short wavelengths. Combined with the band structure diagram of the transmission-mode GaAs/AlGaAs photocathode and the fitted performance parameters, we analyze the phenomenon. It is found that the transmission-mode GaAs/AlGaAs photocathode with variable Al component and various doping structure can form a two-stage built-in electric field, which improves the probability of shortwave response photoelectrons escaping to the vacuum. In conclusion, such a structure reduces the influence of back-interface recombination, improves the shortwave response of the transmission-mode photocathode.

Intrinsic emittance reduction in transmission mode photocathodes

High quantum efficiency (QE) and low emittance electron beams provided by multi-alkali photocathodes make them of great interest for next generation high brightness photoinjectors. Spicer's three-step model well describes the photoemission process; however, some photocathode characteristics such as their thickness have not yet been completely exploited to further improve the brightness of the generated electron beams. In this work, we report on the emittance and QE of a multi-alkali photocathode grown onto a glass substrate operated in transmission and reflection modes at different photon energies. We observed a 20% reduction in the intrinsic emittance from the reflection to the transmission mode operation. This observation can be explained by inelastic electron-phonon scattering during electrons' transit towards the cathode surface. Due to this effect, we predict that thicker photocathode layers will further reduce the intrinsic emittance of electron beams generated by photocathodes operated in transmission mode.

Structural Design and Experiment of Narrow-Band Response GaAlAs Photocathodes**Supported by the National Natural Science Foundation of China under Grant Nos 61171042 and 61301023, and the Introducing Talent Scientific Initial Foundation of Nanjing Institute of Technology of China under Grant Nos YKJ201320, YKJ201322, and YKJ201323.

To obtain the peak response at 532 nm, narrow-band response GaAlAs photocathodes with two GaAlAs active layers of different aluminum compositions are designed in consideration of the maximum absorptivity and quantum efficiency. The transmission-mode and the corresponding reflective-mode photocathodes are grown by metalorganic chemical vapor deposition. The results indicate that the peak response and the cut-off wavelength occur at 532 nm for the two kinds of photocathodes respectively. The response of the reflection-mode photocathode is an order of magnitude higher than that of the transmission-mode photocathode, whereas the better growth quality and the thicker second GaAlAs active layer can improve the transmission-mode response.

Comparative study of Al(x)Ga(1-x)As/GaAs photocathodes with different aluminum concentrations by surface photovoltage spectroscopy.

The influence of aluminum concentration in an Al(x)Ga(1-x)As window layer on the performance of Al(x)Ga(1-x)As/GaAs photocathodes was investigated. Three types of transmission-mode photocathode materials with different aluminum concentrations were designed for the comparative research. The surface photovoltage technique was applied to prepare samples. After the Cs-O activation process, spectral response curves of Al(x)Ga(1-x)As/GaAs photocathodes were obtained. Comparative studies show that a higher aluminum composition in the window layer is beneficial to improve the response of Al(x)Ga(1-x)As/GaAs photocathodes in the shortwave region. The surface photovoltage calculation formula of photocathode materials was put forward and used to obtain key performance parameters of Al(x)Ga(1-x)As/GaAs photocathodes by fitting calculations. Through calculations, the Al(x)Ga(1-x)As/GaAs interface recombination velocity, the minority carrier diffusion length of the window layer, and the emission layer were deduced, and there is a positive correlation between the aluminum composition in the window layer and the Al(x)Ga(1-x)As/GaAs interface recombination velocity, which is negative with the performance of photocathodes.

Quantum efficiency study of the sensitive to blue–green light transmission-mode GaAlAs photocathode

The quantum efficiency of the blue–green transmission-mode GaAlAs photocathode has been studied. Two transmission-mode GaAlAs photocathodes with different structures are prepared, and the quantum efficiency curves are measured. We use the quantum efficiency formula to fit the experimental curves, and obtain the performance parameters of photocathodes such as the electron diffusion length, the back interface recombination velocity, and the surface electron escape probability. The effects of the Al compositions, the thickness of emission layer, and the electron diffusion length on quantum efficiency are investigated. The results show that both of transmission-mode GaAlAs photocathodes are sensitive to the blue–green light. The peak quantum efficiency of one photocathode appears at about 565nm, while that of another photocathode appears at about 535nm. The Al composition of emission layer plays a major role on the peak position of quantum efficiency of transmission-mode GaAlAs photocathode. Besides, the thickness of emission layer and the Al composition of window layer also have a large influence on the quantum efficiency.

Research on quantum efficiency of transmission-mode InGaAs photocathode

To investigate the quantum efficiency of high performance transmission-mode InGaAs photocathode, the quantum efficiency curve is fitted by using the multilayer optical thin film theory and the first principle calculation. A series of the performance parameters are obtained with the relative error less than 5%. It indicates that the thickness of the emission layer is 0.7–0.9μm, the thickness of the window layer is 0.1–0.3μm. Meanwhile the results from the first principle calculation are proved to be reliable as well. In addition, the long-wave response will increase and the highest integral sensitivity will be obtained when the thickness of the emission layer is the optimum value. For the InGaAs photocathode, the back interface recombination velocity mainly leads to the low integral sensitivity, which is caused by the material lattice mismatch.

Resolution characteristic of graded band-gap AlGaAs/GaAs transmission-mode photocathodes

The modulation transfer function (MTF) of graded band-gap AlGaAs/GaAs transmission-mode photocathodes is numerically solved from the two-dimensional continuity equations. According to the MTF model, we calculate the theoretical MTF of graded band-gap and uniform band-gap transmission-mode photocathodes, and analyze the effects of Al composition, wavelength of incident photon, and thickness values of AlGaAs and GaAs layer on the resolution. The calculated results show that compared with the uniform band-gap photocathode, the graded band-gap structure can increase the resolution of photocathode evidently. If the spatial frequency f ranges from 100 to 500 lp·mm-1, the increase of resolution is more pronounced. Let f=200 lp·mm-1, the resolution of graded band-gap photocathode generally increases 150%-260%. The resolution improvement of graded band-gap photocathode is attributed to the built-in electric field. While too high built-in electric field will influence the spectral response of long-wavelength photons due to higher Al composition in the AlGaAs/GaAs photocathodes.

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.

The optimal thickness of a transmission-mode GaN photocathode

A 150-nm-thick GaN photocathode with a Mg doping concentration of 1.6 × 1017 cm−3 is activated by Cs/O in an ultrahigh vacuum chamber, and a quantum efficiency (QE) curve of the negative electron affinity transmission-mode (t-mode) of the GaN photocathode is obtained. The maximum QE reaches 13.0% at 290 nm. According to the t-mode QE equation solved from the diffusion equation, the QE curve is fitted. From the fitting results, the electron escape probability is 0.32, the back-interface recombination velocity is 5 × 104 cm·s−1, and the electron diffusion length is 116 nm. Based on these parameters, the influence of GaN thickness on t-mode QE is simulated. The simulation shows that the optimal thickness of GaN is 90 nm, which is better than the 150-nm GaN.

Effect of surface potential barrier on electron escape probability of GaN photocathode

Aiming to inverstigate the effect of surface potential barrier on electron escape probability of negative electron affinity GaN photocathode, the electron escape probability is calculated by using the Boltzmann distribution and the method of transfer matrix based on Airy function. It is found that barrier I is a key influencing factor of electron escape probability, while barrier II has a limited influence. The activation photocurrent curve of the transmission-mode GaN photocathode is measured by using our built activation and evaluation experimental system of NEA GaN photocathode. The obvious increase of electron escape probability can be achieved mainly by activating Cs only. The increase of electron escape probability is not large in Cs/O activation process with only Cs activated sufficiently. The theoretical calculations are in good agreement with the photocurrent curves from experimental test. The reason is that the contribution of activating only Cs to the reducing of vacuum level for obtaining NEA state is much larger than that of activating Cs/O.

Research on surface photovoltage spectroscopy for GaAs photocathodes with AlxGa1?xAs buf fer layer

Surface photovoltage spectroscopy equations for cathode materials with an AlxGa1?xAs buffer layer are determined in order to effectively measure the body parameters for transmission-mode (t-mode) photocathode materials before Cs-O activation. Body parameters of cathode materials are well fitted through experiments and fitting calculations for the designed AlxGa1?xAs/GaAs structure material. This investigation examines photo-excited performance and measurements of body parameters for t-mode cathode materials of different doping structures. It also helps study various doping structures and optimize structure designs in the future.

High-efficiency graded band-gap AlxGa1−xAs/GaAs photocathodes grown by metalorganic chemical vapor deposition

To enhance the quantum efficiency over the wavelength region of interest, a special graded band-gap structure applied to the AlGaAs/GaAs photocathodes is developed, in which the compositional grade in the AlxGa1−xAs layer is associated with the doping grade in the GaAs layer. The experimental results show that this unique structure can be achieved by the metalorganic chemical vapor deposition technique. As a result of the built-in electric fields arising from the graded band-gap structure, the quantum efficiency in the short-wavelength and long-wavelength regions is significantly increased for the transmission-mode and reflection-mode photocathodes, respectively.

Theoretical Revision and Experimental Comparison of Quantum Yield for Transmission-Mode GaAlAs/GaAs Photocathodes

The quantum yield formula for uniform-doping GaAlAs/GaAs transmission-mode photocathodes is revised by taking into account the light absorption in the window layer. By using the revised quantum yield formula, the domestic and ITT's experimental quantum yield curves are fitted and the fitted curves match well with the experimental curves. In addition, the fit results show that the integral sensitivity and quantum yield of domestic image intensifier tube has achieved 2130μA/lm and 45%, nearly reaching ITT's third generation level in 2002, whereas the discrepancy in cathode performance is mainly embodied in the electron diffusion length and back interface recombination velocity.

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.

Study of spectral response for transmission-modeNEA GaN photocathodes

Transmission-mode GaN photocathodes with the emission layer thickness of 150 nm and the doping concentration of 1.6×1017 cm-3 were grown by metal-organic chemical vapor deposition (MOCVD) and were activated in ultra-high vacuum system. The result was tested by Multi-information test system. The shape of transmission-mode NEA GaN photocathode quantum yield curves looks like the Chinese charocter 门 for "door", the photocathode had flat and high response between 255 and 355 nm, the highest quantum yield of 13% appeared at 290 nm. When the wavelength was less than 255 nm the quantum yield was decreased because of the high absorption coefficient of AlN buffer layer at short wavelengths. The quantum yield was also decreased beyond 355 nm and fell to 3.5% at the threshold of 365 nm, the quantum yield at 385 nm was reduced to 0.1% and the cut-off character of long wave was well shown. The quantum yield formula of transmission-mode GaN photocathode has been solved from diffusion equations, and the main factors affecting the quantum yield mostly, including electron diffusion length, electron escape probability, active-layer thickness and the back-interface recombination velocity, were analysed and discussed. The future work is optimizing the structure of the photocathodes.

Transmission-mode GaN photocathode based on graded AlxGa1 xN buffer layer

Transmission-mode GaN photocathode based on graded AlxGa1 xN buffer layer