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.
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 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.
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.
We provide the expression of the probability density function (pdf) of polarization mode dispersion (PMD) in the optical fiber system concatenated with different pdf distribution sections. In recursive form for the first time, this makes the calculation of the characteristic pdf much easier. This calculation method is confirmed by the experiment.
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