The resolution model of graded doping and graded composition reflection-mode AlGaAs/GaAs photocathode is solved numerically from the two-dimensional continuity equations. According to the model, the theoretical modulation transfer functions (MTFs) of different structure reflection-mode photocathodes were calculated, and the effects of doping concentration, Al composition, AlGaAs and GaAs layer thickness on the resolution of cathodes were analyzed. The simulation results show that both graded composition and graded doping structures can increase the resolution of photocathode, and the effect of graded composition structure is more pronounced. The resolution improvement is attributed to the built-in electric field induced by a graded composition or doping structure. The simulation results also show that the MTFs of cathodes are affected by the AlGaAs and GaAs layer thickness.
The quantum efficiency equations of two kinds of reflcetion-mode GaAs photocathodes (GaAs-GaAs and
AlGaAs-GaAs) with back interface recombination velocity have been solved from the diffusion equations. According to
these quantum efficiency equations, the integral sensitivities as a function of active layer thickness, electron diffusion
length and back interface recombination velocity for both kinds of cathodes are simulated. Through the theoretical
simulation, we found the active layer thickness for AlGaAs-GaAs cathodes has an optimum value at which the cathodes
achieve the maximum sensitivity. Under most conditions, the theoretical integral sensitivities of AlGaAs-GaAs cathodes
are greater than that of GaAs-GaAs cathodes. This is attributed to that AlGaAs-GaAs interface barrier reflects most
photoelectrons back into the active layer. The theoretical spectral response of both kinds of cathodes is also simulated.
We found that the increase in integral sensitivity of AlGaAs-GaAs cathodes mainly reflects in the increase of spectral
response of long wavelength photons.
An exponential-doping GaAs photocathode was designed and activated, the achieved integral sensitivity for the
exponential-doping cathode is 1956μA/lm, which is much higher than that of gradient-doping cathode with identical
thickness of epitaxial layer. According to the quantum efficiency theory of exponential-doping cathode, we analyzed the
reason responsible for the increase in integral sensitivity of exponential-doping cathode, which are mainly attributed to
the invariable induced electric field, the photoelectrons driven by the field move towards the cathode surface by way of
diffusion and drift. Accordingly, increase the average distance that photoelectrons transport and reduce the influence of
the back-interface recombination velocity on photoemission.
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.
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.
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, on the base of simple introduction of inner structure of 320×240 pixels UFPA in electronics and
calorifics, the relationship of NETD (noise equivalent temperature difference) and bias voltage are researched and
presented through the formulas about noise and NETD. The relation between NETD and four kinds of temperatures is
presented. Moreover the two bias voltages are adjusted to observe the changing of NETD. Some experiments on power
consumption and image quality of thermal imaging system is done, the result data is given. On the basis of the theory and
experiments, how to enhance the NETD performance of UFPA (Focal Plane Array) at much lower or higher than room
temperature is researched by analyzing experiment data. At last, the conclusion is summarized: in order to get the best
image and the lest power consumption, we should adjust these parameters to find the optimized configuration at different
A variety of gradient-doping reflection-mode GaAs photocathode materials are designed and are prepared into negative electron affinity (NEA) GaAs photocathodes by (Cs,O) activation technique for the first time. These gradient-doping photocathodes are grown by molecular beam epitaxy (MBE), in which from GaAs bulk to surface doping concentration is distributed gradiently from high to low. The activation experimental results show that for gradient-doping GaAs photocathodes that are grown over p-type GaAs (100) wafer, where the epitaxial layer doping concentration range is 1019 to 1018 cm–3 and the epitaxial layer thickness is 1 µm, can achieve high integral sensitivity. The highest integral sensitivity 1798 µA/lm is achieved for gradient-doping GaAs photocathodes, which is much higher than that of a common uniform-doping GaAs photocathode under identical cleaning and activation condition. The inherent mechanism responsible for the fact that a gradient-doping GaAs photocathode can obtain higher quantum efficiency is also discussed.