Yunsheng Qian

On-line measurement system of GaAs photocathodes and its applications

The preparation process of GaAs photocathodes is very complicated, in order to prepare the high performance cathodes, it is crucial to obtain information enough to evaluate the preparation process in real time. Based on a particular transfer light setup and a flexible communication network, we develop an on-line measurement system for GaAs cathode preparation, which is used to measure the pressure of activation chamber, sample temperature, photocurrent, spectral response curves, and currents heating Cs and oxygen dispensers during the heat-cleaning or activation processes of cathodes. According to these signals, we present some simple and real-time evaluation techniques for cathode preparation. Several peaks of pressure are observed in the pressure variations measured during heat cleaning. These peaks corresponding to the desorption of AsO, As2O3, Ga2O and Ga2O3 from the sample surface at different temperatures, respectively, are used to evaluate the effect of heat cleaning very well, while the signals measured during activation can be used to analyze and optimize the activation technique. Based on a revised quantum efficiency equation, many performance parameters of cathodes are obtained from the fitting of spectral response curves. According to these parameters, the performance of cathode material and the effect of activation can be evaluated.

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.

Performance analysis of low-cost uncooled microbolometer infrared detectors

Uncooled microbolometer infrared detectors are being developed for a wide range of thermal imaging applications. To design and manufacture high-performance microbolometer infrared detectors, numerical calculation and simulation is necessary. In this work, finite element methods are performed to simulate the transient temperature field of thermistor films of microbolometer infrared detectors. The varisized supporting legs impacts on the performance of detectors are discussed. At the same time, variation of the bias voltage and the substrate temperatures impacts on total noise, noise equivalent to temperature difference (NETD) and detectivity (D*) are also discussed in details. These performance analyses are helpful for optimum design of microbolometer infrared detectors structure and rational choice of working temperature of infrared focal plane arrays.

A study of the NEA photocathode activation technique on a [GaAs(Zn):Cs]: O-Cs model

In this paper we review simply the surface models. These models have several technical problems not solved appropriately except for having deficiency themselves. So we present a new negative electron affinity (NEA) photocathode photoelectric emission model: [GaAs (Zn): Cs]: O - Cs. After discussing photocathodes activation technique on the model, we design a activation technique, which increases the Cs current to decrease the first peak in appropriate degree after using smaller Cs current to achieve the first peak of photoemission (GaAs (Zn)-Cs dipole layer), then set out Cs-O alternation and do not end the technique until gaining maximal photoemission (Cs-O-Cs dipole layer), in the photocathodes with GaAs (Zn) (100)2×4 reconstruction surface. In the present material configuration and level of technique, it is difficult that the integral sensitivity of cathode excess 3500 μA/lm. However, it is likely to excess 4000 μA/lm by varied doping As-rich GaAs (Zn) (100)2×4 reconstruction surface.

Measurement method of the modulation transfer function of focal plane arrays

Focal plane arrays (FPA) of HgCdTe detectors with responses in the 3-5micrometers or 8-14micrometers bands have been successfully incorporated into infrared imagers. Using a variety of digital signal processing techniques, good imagery has been obtained but the image quality is limited by sampling effects at the focal plane that cause non-isoplanantism and aliasing. The verification of the operation of these detectors is an important step in the design and development process. Modulation transfer function (MTF) is a measurement of how well the system will faithfully reproduce the scene. As the MTF decreases, scene detail associated with those specific spatial frequencies will be reproduced with lower contrast. The development of focal plane arrays with considerable on-chip processing, notably time-delay integration, imposes new and severe requirements on the instrumentation to establish the MTF of the integrated focal plane arrays.1 This paper details a method for measuring the MTF for such arrays. Several methods to derive the MTF from the experimental data were investigated; the preferred approach is presented.

Intelligent measuring equipment for the width of fibers

In this paper, we describe high accurate non-contact measuring equipment together with its principle, hardware structure and idea for software design. Its principle is using optical modulation to modulate and amplify the figure of fibers width, then convert the optical signa into electrical signal. The electrical signal is processed by an automatic processor which comprises MCS-51 single-chip microprocessor and other interface circuits. Finally the processed data is displayed or printed. This equipment can exclude the system error with calibration. Multivariate linear regression algorithm is used in calibration software design. It can discriminate the overlapping and across of fibers and eliminate the system errors. All proceeding is automatically completed without the interference of people. It excludes the personal influence and ensures the reliability of measurement.

Comparison of photoemission behavior between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources [1,2], the difference in the photoemission behavior, namely the activation process and quantum yield decay, between the two typical types of III-V compound photocathodes has been investigated using the multi-information measurement system. The reflection-mode GaAs and GaN photocathode samples both were grown by metal organic chemical vapor deposition, wherein the former is a substrate/AlGaAs/GaAs structure and the latter is a sapphire/AlN/GaN structure. The active layer in both structures was uniformly doped with p-type zinc and magnesium respectively.

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.

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.