At present, two kinds of activation techniques for preparing GaAs NEA photocathode are available. In this paper,
according to two kinds of photocurrent curve arising in the activation, the characteristic and mechanism of the two kinds
of craft were summed up and compared with each other, and the further theoretical investigation on the mechanism of
activation was carried out based on the recent research of NEA surface model for GaAs photocathode. It is proposed as a
process principle that during (Cs, O) alternation phase of the activation process of GaAs photocathode, Cs should always
be in excessive state. Besides, it is also indicated that whether Cs is excessive during (Cs, O) alternation phase and the
Cs/O ratio may affect directly the final property of photocathode. Finally, a method to modify the craft parameters to
guard against the deviation from the principle is presented. The presented study is very necessary and significative for
optimizing the activation techniques so as to enhance the performances of GaAs NEA photocathodes.
The exponential-doping structure was applied to prepare the transmission-mode GaAs photocathode, and spectral
response curves after high-temperature activation, low-temperature activation and the indium sealing process were
respectively measured by use of the on-line spectral response measurement system, to research into the practical effect of
the exponential-doping structure on cathode performance. The results show that a high photosensitivity ranging from 560
nm to 880 nm with an ascending trend can be obtained after the high-low temperature activation. In the region of longwave
threshold, there is a distinct inflexion indicating a better photoemission capability than the former uniform-doping
photocathodes. Besides, the spectral response curve in the whole response waveband, especially the long-wave region
obviously decreases after indium seal. Compared with the fitted surface electron escape probability after Cs-O activation,
it decreases after indium seal according to the quantum efficiency formula of exponential-doping transmission-mode
GaAs photocathodes. Based on the double dipole model, the reasons for the variation of spectral response shape are
explained on account of the relation between surface escape probability and the evolution of surface potential barrier
profile.
Taking GaAs and GaN as representation, negative electron affinity (NEA) photocathode has many virtues,
such as high quantum efficiency, low dark current, concentrated electrons energy distribution and angle distribution,
adjustive long-wave threshold, great potential to extend the long-wave spectral response waveband. Therefore it plays
more and more important effect in high performance image intensifiers and polarized electron sources. GaN NEA
photocathode and GaAs NEA photocathode are very similar because they all belong to III-V compound. But, GaN
photocathode and GaAs photocathode have many difference in such aspects as preparation process, activation manners,
stability and application field etc. In this paper, using the multi-information measurement and evaluation system of
photocathode, the preparation processes of native reflection-mode GaN photocathode and GaAs photocathode are
studied. The different activation manners of GaN photocathode and GaAs photocathode are compared and analyzed. The
spectral response and stability of the two kind of photocathode are compared also. The experiments indicate: the
atomically clean degree of NEA photocathode surface and the structure of activation layer are the main factors that
influence photocathode sensitivity and stability after activation. GaN photocathode and GaAs photocathode have good
NEA property and large quantum yield. Compare with GaAs photocathode, GaN photocathode has high stability, and the
decay of the quantum yield is comparatively slow.
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.
Two gradient-doping GaAs photocathodes were designed and activated, the achieved highest integral sensitivity for the
gradient-doping cathode is 2178μA/lm, which is much higher than that of uniform-doping cathode. The increase in the
integral sensitivity is attributed to the electric field induced in the active layer of gradient-doping cathode. We analyze
the transported mechanism of gradient-doping cathodes and solve the quantum efficiency equations of exponential-doping
cathode, which is a special gradient-doping cathode with a constant induced electric field, from the one-dimensional
continuity equations. According to these equations, we calculate the theoretical quantum yield of the
exponential-doping cathodes, and compare the performance of exponential-doping cathodes with that of uniform-doping
cathodes. The theoretical results show that the exponential-doping structure can increase the quantum yield of
photocathodes evidently, for the transmission-mode cathodes the increase is even more pronounced.
High-performance reflection-mode GaAs photocathode (named cathode 1 for short) with the integral sensitivity of 2140μA/lm is prepared by adopting "high-low temperature" two-step activation and using heavily p-type Be-doped GaAs materials, which is grown by molecular beam epitaxy (MBE) technique. Moreover, spectral response characteristic and cathodes performance parameters of two cathodes are obtained by spectral response database we compiled, one is the reflection-mode photocathode (named cathode 2 for short) with the integral sensitivity of 1800μA/lm reported by G. H. Olsen in the 70s; the other is the transmission-mode photocathode (named cathode 3 for short) with the integral sensitivity 3070μA/lm reported by O. H. W. Siegmund in 2003. A transmission-mode cathode (named cathode 4 for short) is acquired by computer simulation on the basis of cathode 1, and its integral sensitivity is 1907μA/lm, then we compare the reflection-mode cathodes (cathode 1 and cathode 2) and the transmission-mode cathodes (cathode 3 and cathode 4), respectively, and analyze the cause for performance difference among these cathodes, the results show that the surface escape probability of cathode 1 reach to 0.62, which is lower slightly that of cathode 2, so preparation technique of cathode 1 has gotten higher the surface escape probability, but the electron diffusion length of cathode 1 and the back interface recombination velocity of cathode 4 is not better compared to cathode 2 or cathode 3. Which shows preparation technique of cathode 1 obtains better surface barrier, it need to be optimized all the same for achieving higher performance GaAs photocathodes.
The spectral response curves of reflection-mode GaAs (100) photocathodes are measured in activation chamber by multi-information measurement system at RT, and by applying quantum efficiency formula, the variation of spectral response curves have been studied. Reflection-mode GaAs photocathodes materials are grown over GaAs wafer (100) by MBE with p-type beryllium doping, doping concentration is 1×1019 cm-3 and the active layer thickness is 1.6μm. During the high-temperature activation process, the spectral response curves varied with activation time are measured. After the low-temperature activation, the photocathode is illuminated by a white light source, and the spectral response curves varied with illumination time are measured every other hour. Experimental results of both high-temperature and low-temperature activations show that the spectral response curve shape of photocathodes is a function of time. We use traditional quantum efficiency formulas of photocathodes, in which only the Γ photoemission is considered, to fit experimental spectral response curves, and find the theoretical curves are not in agreement with the experimental curves, the reason is other valley and hot-electron yields are necessary to be included in yields of reflection-mode photocathodes. Based on the two-minima diffusion model and the fit of escape probability, we modified the quantum efficiency formula of reflection-mode photocathodes, the modified formula can be used to explain the variation of yield curves of reflection-mode photocathodes very well.
The photocurrent curves and spectral response curves of GaAs photocathodes are measured by the multi-information
measurement system, and the photocurrent variation has been investigated as a function of Cs/O current ratios. The
identical Zn doped (1×1019cm-3) p-type GaAs (100) wafers, identical methods of chemical cleaning and heat cleaning of
wafers are used in the performed three experiments. From the experimental results, we find the envelopes of three
photocurrent curves approximately satisfy parabola after the exposure to oxygen, while the detailed variation process and
the ultimate photocurrent of them are different. The photocathode activated with the smallest Cs/O current ratio has the
least consumed time and the largest photocurrent during the first exposure to cesium, and the most alteration times. The
photocathode activated with the moderate ratio has the most rapid increase of photocurrent during the first exposure to
oxygen, and has the highest quantum efficiency and stability after activation. The photocathode activated with the largest
ratio has the fewest alteration times and the lowest quantum efficiency. These phenomena have a close relationship with
the coadsorption mechanism of cesium and oxygen on GaAs, and in which the oxygen plays an important role. Due to
the exposure to oxygen, the cesium atoms adsorbed on the surface becomes Cs+, their radius decrease to 1.67Å from
2.71Å, and form the dipoles with O-2, this is the main reason of above phenomena appeared.
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.
A multi-information measurement system is used to activate the GaAs photocathode. During the experiment, the curves that show the change of vacuum pressure and photocurrrent are recorded also. The cathode used in the experiment is heavily p-type GaAs (100). The doping concentration is 1×1019cm-3. The cathode is grown by molecular beam epitaxy (MBE) and the thickness is 1.6μm. GaAs cathode is degreased before being sent into ultra-high vacuum system to be heat cleaned. The activation technique is "high-low temperature" two-step activation. High temperature of heating is 600° and low temperature of heating is 410°. During the high temperature activation the integrated sensitivity is 1380μA/lm, the surface escape probability is 0.3 and the electron diffusion length is 3.1μm. During the low temperature activation the integrated sensitivity is 2140μA/lm, the surface escape probability is 0.6 and the electron diffusion length is 3.8μm.
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