Hongchang Cheng

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.

Photoemission characteristics of (Cs, O) activation exponential-doping Ga0.37Al0.63As photocathodes

Based on the studies of the GaAs photocathode, the surface model of the Ga0.37Al0.63As photocathode is investigated and the energy distributions of electrons reaching the surface charge region, reaching the surface and emitting into vacuum are calculated. The (Cs, O) adsorption and photoemission characteristics of the Ga0.37Al0.63As photocathode are studied according to the experiments. We use the quantum efficiency formula to fit the experimental curve, and obtain the performance parameters of the photocathode and the surface barrier parameters. The results show that the surface barrier of the Ga0.37Al0.63As photocathode is similar to that of the GaAs photocathode. The prepared reflection-mode Ga0.37Al0.63As photocathode responds to the blue-green light, while the transmission-mode Ga0.37Al0.63As photocathode is sensitive to the 532 nm light.

Effect of Cs adsorption on the photoemission performance of GaAlAs photocathode

The effect of Cs adsorption on the photoemission performance of a reflection-mode GaAlAs photocathode in an ultrahigh vacuum chamber has been investigated. The experiments for Cs/O activation, multiple recaesiation, and degradation are performed on a GaAlAs photocathode. Meanwhile, the Cs/O activated and recaesiated photocurrent curves, degraded photocurrent curves, and spectral response curves are measured and analyzed. Besides, the performance parameters of the photocathodes are obtained by using the formula to fit with the experimental quantum efficiency curves. The results show that the Cs atoms not only make the atomically clean surface form the negative electron affinity, but also make the degraded photocathode recover to a good level. The quantum efficiency and the lifetime of GaAlAs photocathode become lower with increasing the recaesiation times.

Photoemission performance of thin graded structure AlGaN photocathode.

In order to research a high-efficiency AlGaN photocathode, the AlGaN photocathodes with varied Al composition (0.68 and 0.4) and uniform Al composition (0.24) were grown. The photocathodes were activated by Cs adsorption and received their spectral response via multi-information system. Results show that the absorption rate of the AlGaN photocathode with varied Al composition is half of the AlGaN photocathode with uniform Al composition, but the quantum efficiency of the photocathode with varied Al composition is approximately 29% higher than that of the photocathode with uniform Al composition. The built-in field within the emission layer of the AlGaN photocathode with varied Al composition is much higher than that of the photocathode with uniform Al composition, which is the main factor that promotes the photoelectron movement toward the photocathode surface and improves photoemission performance of the AlGaN photocathode.

Comparison of resolution characteristics between exponential-doping and uniform-doping GaN photocathodes

The studies of quantum efficiency, electronic energy distribution and stability are highly concerned in the application of Negative electron affinity (NEA) gallium nitride (GaN) photocathodes while the resolution of photocathodes are concerned rarely. The resolutions of some image intensifiers are smaller than computational value partly because of ignoring the resolution of photocathodes. To a certain extent, the resolutions of image intensifiers are influenced by photocathodes. Electronic transverse diffusion is the main cause of decreasing the resolution of photocathodes whereas the exponential-doping structure can reduce its influence. In this paper, the resolution characteristics of photocathodes have been studied by using the modulation transfer function (MTF) method. The MTF expressions of transmission-mode exponential-doping photocathodes have been obtained by solving the two-dimensional continuity equations. According to the MTF expressions, the resolution characteristics between exponential-doping and uniform-doping GaN photocathodes are calculated theoretically and analyzed comparatively. At the same time, the relationships between resolution and thickness of the emission layer Te, electron diffusion length LD are researched in detail. The calculated results show that, compared with the uniform-doping photocathode, the exponential-doping structure can increase the resolution of photocathode evidently. The resolution of exponential-doping GaN photocathode is improved distinctly when the spatial frequency varies from 400 to 800 lp/mm. The MTF characteristics approach gradually when f increases or decreases. Let f =600 lp/mm, the resolution increases by 20%-48% approximately. The constant built-in electric field for exponential-doping GaN photocathode can increase the resolution of photocathode. The improvement of resolution is different from decreasing Te, LD or increasing the recombination velocity of back-interface which are at the cost of reducing the quantum efficiency of photocathode. Therefore, the MTF expressions of transmission-mode exponential-doping photocathode play a positive role in improving the resolution of ultraviolet detector and optimizing the structural design of GaN photocathode.

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.

Research on quantum efficiency for reflection-mode InGaAs photocathodes with thin emission layer.

In order to understand the photoemission mechanism of the reflection-mode InGaAs photocathode with a thin emission layer, the formula describing reflection-mode quantum efficiency is revised by solving the one-dimensional continuity equation, in which the electrons generated in the GaAs buffer layer are considered. Compared with the conventional formula, the revised formula is proved to be more suitable for the reflection-mode InGaAs photocathode with a thin emission layer. In experiment, the InGaAs sample goes through two-step surface preparation including a wet chemical cleaning process and a heat treatment process. Then the sample is activated by Cs/O and the experimental quantum efficiency curves are measured simultaneously every other hour. The measured results show that the shapes of the quantum efficiency curves degrade with time because of the contamination of residual gases in the vacuum system. All the quantum efficiency curves are well fitted by the revised formula.

Improved activation technique for preparing high-efficiency GaAs photocathodes

In order to replace an uncontrollable gaseous oxygen source and obtain the required computer-controlled evaporation flow, two types of solid oxygen dispensers composed of silver oxide powder and barium peroxide powder are employed to prepare GaAs photocathodes. The experimental results show that the barium peroxide-based dispenser can release oxygen more effectively and deliver better photoemission performance than the silver oxide-based one. For the silver oxide-based dispenser with a long first warm-up time, an improved activation technique is proposed to avoid the Cs over-saturation and achieve the desired symmetry of photocurrent curve shape. This effective activation technique based on current-driven cesium and oxygen sources can facilitate the realization of automatic activation technology.

Improved performance of GaAs photocathodes using effective activation technique

To achieve negative-electron-affinity state, the atomically clean surface of GaAs-based photocathode is usually activated by cesium and oxygen in the ultrahigh vacuum environment. In view of the required computer-control of evaporation flow rates, the solid oxygen dispenser instead of gaseous oxygen is urgently needed just as the regular cesium dispenser. Accordingly, the solid cesium and oxygen dispensers were applied to activate epitaxial GaAs cathode samples. Two types of solid oxygen dispensers composed of barium peroxide powder and silver oxide powder respectively are employed to improve cathode photoemission performance. The experimental results show that the barium peroxidebased oxygen dispenser can release more oxygen and bring in higher activation photocurrent and spectral response than the silver oxide-based one. The unsatisfactory feature is that the silver oxide-based oxygen dispenser released effectual oxygen gas more slowly than the barium peroxide-based oxygen dispenser. Therefore, an effective activation technique was proposed to ameliorate this unfavorable phenomenon for the silver oxide-based dispenser, which can bring out the desired symmetry of photocurrent curve shape during the Cs/O alternate activation process. The improved activation technique would provide guidance for the optimization of activation craft.

A theoretical and experimental evaluation of III–nitride solar-blind UV photocathode

We have developed a superior solar-blind ultraviolet (UV) photocathode with an AlGaN photocathode () in semi-transparent mode, and assessed spectra radiant sensitivity related to practical use. Before being grown over a basal plane sapphire substrate by low-pressure metal organic chemical vapor deposition (MOCVD), a reasonable design was made to the photocathode epitaxy structure, focusing on the AlGaN: Mg active layer, then followed by a comprehensive analysis of the structural and optical characterization. The spectra radiant sensitivity is peaked of 41.395 mA/W at wavelength 257 nm and then decreases by about 3 to 4 decades at 400 nm demonstrating the ability of this photocathode for solar-blind application prospects.