Benkang Chang

Evaluation system of negative electron affinity photocathode

This article first describes the background of the research and manufacture of evaluation system of Negative Electron Affinity photocathode. This article designs a set of super high vacuum system for activating NEA photocathode on the base of activation theory, the process of design and debugging is given. The system is composed of three parts: super high vacuum system for GaAs material activation, multi-meter testing system, surface analysis system. The system is used for on-line evaluation of activating of NEA photocathode. The technical parameters and structure of the evaluation system of NEA photocathode are given in the paper. The system is finished and experiments are made. At last the picture of the system is given.

Measurement of spectral response of photocathodes and its application

Spectral response is an important parameter of photocathodes. By analyzing measured spectral response curves, much information about the photocathodes can be obtained which is useful to investigation of photocathodes. The principle measuring the spectral response of photocathodes is expounded in this paper. The on-line measurement system was developed, which can measure the spectral response of optoelectronic devices within range of 400nm~1800nm. It can also measure the reflectance of monochromatic light, the monochromatic photocurrent, and integral sensitivity of photocathodes. The measurement system was used to on-line measure spectral response of multi-alkali photocathodes(Na2KSb:Cs) when they are being prepared. Combining measurement of reflectance of monochromatic light, by which the thickness of photocathodes can be timely obtained, the optimum thickness of photocathode is looked for. The measurement system also used in the investigation of GaAs:Cs-O NEA photocathodes. Surface escape probability, electron diffusion length and back-interface recombination velocity are the factors that influence the quantum yield of NEA photocathode. It is difficult to directly measure these parameters. But they can be obtained by simulation of measured spectral response. The reflective GaAs samples were activated and evaluated. The results were given and analyzed.

Analysis of online measured spectral responses of NEA photocathodes

In this paper, on the base of explaining the importance of spectral response, the way to derive characteristic parameters of NEA photocathode from its spectral response is introduced. Then, an automatic spectral response recording system is described. The system is used to take spectral response curves during the activation procedure. For the first time, on-line measured spectral response curves of GaAs:Cs-O NEA photocathodes are presented. These curves indicate the typical variation in spectral response caused by the deposition of cesium and oxygen. By the way of comparing and analyzing these curves, many important parameters of photocathodes are obtained, and the reasons that cause the bad behavior of NEA photocathodes are discussed.

The design of CY-1R night vision helmet

In this paper, the research intention and design principle of CY-1R night vision helmet are explained which fills the gaps in active-passive combined night vision field in our country. The structure, composition, mechanism and overall performance of the goggle are analyzed. It is a new type device consisting of laser illuminator system, special optical system and high performance low-level-light intensifier. Based on these characteristics, the sensitivity of the system is high and the image observed is very clear. Taking advantage of it, we can complete the military operation under any atrocious weather conditions.

A study on the activation technique of GaAs:Cs-O NEA photocathodes

In this paper, a large activation and analysis system is introduced , which consists of automatic spectral response recording equipment, x-ray photoelectron spectrometer and ultrahigh vacuum chamber. Then, activation and on-line spectral response measurement are described. For the first time, on-line measured spectral response curves of GaAs:Cs-O NEA photocathodes are presented. These curves indicate the typical variation in spectral response caused by the deposition of cesium and oxygen. By the way of comparing and analyzing those curves, some valuable data obtained which are helpful in investigating the mechanism of the activation.

Experiment and analysis of (Cs,O) activation for NEA photocathode preparation

On-line spectral response measurement technology and angular-dependent X-ray photoelectron spectroscopy (XPS) technology are firstly used to research the (Cs,O) activation of Gallium arsenide (GaAs) negative electron affinity (NEA) photocathode. Dynamic spectral response can be obtained from on-line spectral response measurement system and by use of the computer program for quantitative angular-dependent XPS, atom concentration and layer thickness of activation layer can be calculated. The photocathode properties for evaluating (Cs,O) activation, such as the surface electron escape probability, diffusion length and so on are also calculated by simulation method. From our experimental results, optimum cesium deposition quantity is obtained when photocurrent arrives at peak value and then decreases to 10% of the maximum, and optimum oxygen deposition quantity is obtained when photocurrent arrives at peak value and then decreases to 90% of the maximum. And the photoemission peaks at 0.71 monolayer of Cs coverage. In our experimental system the optimum thickness of activation layer is 0.82 nm and interfacial oxidation layer is 0.2 nm for successfully activated GaAs NEA photocathode. Mechanism of interfacial barrier formation and effect of oxygen on the surface of activation layer are explained.

Spectral response and surface layer thickness of GaAs:Cs-O negative electron affinity photocathode

On-line spectral response curves of GaAs:Cs-O NEA photo- cathode of reflection model is first presented and the relation between spectral response curves and the thickness of Cs-O layer is discussed. When Cs and O is deposited on the surface of cleaning GaAs wafer, photo-cathodes spectral response rises sharply and long-wavelength threshold tends to a fixed value at the beginning of the activation. But, as Cs and O are deposited continually, spectral response rises slowly and the long-wavelength threshold tends to be shortened. When a fine thickness of Cs-O layer is reached, the optimum spectral response is obtained. As a quantity Cs-O is further increased, both the spectral response and the long-wavelength threshold decrease. The thickness of GaAs photo-cathode surface layer that consists of Cs-O layer and GaAs-O layer is researched by take-off angle XPS technology. Thickness of Cs-O layer is approximately equal to 0.7 nm, and the GaAs-O layer is approximately 0.2nm. Our experiments show when the thickness of Cs-O layer is approximately equal to 0.7nm, and the GaAs-O layer is approximately 0.2mm. Our experiments show when the thickness of Cs-O layer is 0.7nm or so and the GaAs-O layer tend to be disappeared, NEA photo-cathode with the optimum spectral response is achieved which can be used widely in low-light level imaging detectors.

Surface analysis for NEA GaAs photocathode

To study the effect surface defects imposed on NEA GaAs photocathode, a surface analysis instrument shown in Fig. 1 has been designed, which is composed of a microimager, a 2-D displacement platform, an auto-focus equipment, a computer.

Surface photovoltage spectroscopy characterization of varied-doping GaAs

Summary form only given. The quantum efficiency of negative electron affinity (NEA) GaAs photocathodes mainly depends on the performance of GaAs material. Usually, the uniform-doping structure are used in the photoemissive layer of cathodes. To increase quantum efficiency of cathodes, the varied-doping structure are used. The quantum efficiency can be increased by using the varied-doping structure in GaAs material. The electric field exists in the varied-doping GaAs material, that improves the movement of photoexcited electron toward the surface. To evaluate the performance of GaAs material before they are activated, the measurement system of surface photovoltage spectroscopy (SPS) of GaAs is developed. The system consists of the light source, optical chopper, monochromator, the quartz fiber team, photovoltage test fixture, Lock-In Amplifier, computer and software. The system can measure the SPS of the GaAs material with range of 500~1000nm. The SPS analyses offer the possibility of performing nondestructive evaluation for GaAs material. The diagram of measurement system is shown in Fig.1. The reflection mode uniform-doping and gradient-doping GaAs material are grown by molecular beam epitaxy(MBE) with p-type beryllium doping. The detailed material doping structure of grown samples is shown in Fig.2. Sample 1 is a uniform-doping structure. The top layer is the photoemissive layer, p-GaN. The thickness is about 2.6um. The dopant concentration is about 1.0x1019cm-3 . The second layer is the substrate, p-GaAs(100). Sample 2 is a gradient-doping structure. The epitaxial layers of samples 2 are divided into four sections. The total thickness is also 2.6μm. The dopant concentration of epitaxial layers varies from 1.0x1018 cm-3 to 1.0x1019 cm-3. SPS of the sample 1 and sample 2 are measured by developed measurement system. The measurement results are shown in Fig.3. The reflections of GaAs material under the different incident wavelength are measured by spectrophotometer. The electron diffusion length of GaAs material are obtained by equal light intensity method analysis of SPS[2,3,4]. Sample 1 and sample 2, respectively, the electron diffusion length is about 2.6 μm and 3.2 μm. The results show that the electron diffusion length of gradient-doping GaAs material is increased compared with uniform-doping GaAs material.

Decay characteristics and mechanism of GaN UV photocathode

Summary form only given. Recent progress in Gallium Nitride (GaN, AlGaN, InGaN) photocathodes show great promise for future ultraviolet (UV) detector applications. Stability has been one main performance parameters for photocathode. The photoemission decay characteristics of GaN photocathode after the end of activation in ultra-high vacuum system was investigated using photocurrent and spectral response on-line monitoring measurement. It was found that under low-level light irradiation, the photocurrent of GaN photocathode decays at a very slow rate during a long period, and then began to decay suddenly at a faster speed. The spectral response curve reflects the attenuation of long-wave response is greater than short-wave one. GaN photoemission performance can be restored by Cs replenishment, which indicates that Cs desorption is the main reason for GaN cathode degradation in vacuum system. The influences of Cs desorption on GaN cathode surface barrier and photoelectron surface escape probability were analyzed based on surface model of cesium-oxygen activated GaN cathode, and the reasons for observed GaN cathode decay behaviors were explained.