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To confirm the actual effect of an exponential-doped structure on cathode performance, an exponential-doped structure was applied to the preparation of a transmission-mode GaAs photocathode, and spectral response curves after high-temperature activation, low-temperature activation, and the indium sealing process were separately measured by use of the on-line spectral response measurement system. The results show that, compared to the previously uniform-doped photocathode, the exponential-doped photocathode can obtain higher cathode performance and photoemission capability because of the built-in electric field. Nevertheless, cesium desorption and impurity of gas during the sealing process can cause the degeneration of spectral response in the entire response waveband, especially in the long-wavelength region, with the decrease in surface electron escape probability related to the adverse evolution of the surface potential barrier profile. OCIS codes: 160.2100, 250.0250.

The spectral response and band features of cesium-covered GaAs have been investigated by using a spectral-response measuring instrument at room temperature. We find that the shape of the spectral-response curves is almost identical if the vacuum level is greater than the energy level of the conduction band minimum; otherwise, the shape changes with time. The preservation or change in the shape is attributed to the evolution of a surface barrier. By calculation of the derivatives of the spectral response, the band features of GaAs can be determined. We find six peaks in the spectral-response derivatives. These peaks are in excellent agreement with the photon energy positions determined by the transitions from different valence band peaks to different conduction band valleys. OCIS codes: 160.6000, 230.0250.

We measured the carrier concentration distribution of gradient-doped GaAs/GaAlAs epilayers grown by molecular beam epitaxy before and after annealing at 600[degrees]C, using electrochemical capacitance voltage profiling, to investigate the internal variation of transmission-mode GaAs photocathodes arising from the annealing process. The results show that the carrier concentration increased after annealing. As a result, the total band-bending energy in the gradient-doped GaAs emission layer increased by 25.24% after annealing, which improves the photoexcited electron movement toward the surface. On the other hand, the annealing process resulted in a worse carrier concentration discrepancy between the GaAs and the GaAlAs, which causes a lower back interface potential barrier, decreasing the amount of high-energy photoelectrons. OCIS codes: 160.2100, 250.0250.

We compared two reflection-mode negative electron affinity (NEA) GaAs photocathode samples that are grown by molecular beam epitaxy with p-type beryllium doping. One sample is uniform doping, and another is gradient doping. Experimental curves of spectral response sensitivity and quantum efficiency are obtained. The thicknesses of the two cathodes are both 2.6 [micro]m. The integrated sensitivity of the uniform doping one is 1966 [micro]A/lm, and that of the gradient-doping one is 2421 [micro]A/lm. The escape probability and diffusion length are fitted from the spectral response curves. For the uniform-doping sample, the escape probability is 0.45 and the diffusion length is 5 [micro]m. For the gradient-doping sample, the escape probability is 0.55 and the diffusion length is 5.5 [micro]m. OCIS codes: 0160.2100, 250.0250.

In order to research the spectral response variation of a negative electron affinity (NEA) photocathode in the preparation process, we have done two experiments on a transmission-type GaAs photocathode. First, an automatic spectral response recording system is described, which is used to take spectral response curves during the activation procedure of the photocathode. By this system, the spectral response curves of a GaAs:Cs-O photocathode measured in situ are presented. Then, after the cathode is sealed with a microchannel plate and a fluorescence screen into the image tube, we measure the spectral response of the tube by another measurement instrument. By way of comparing and analyzing these curves, we can find the typical variation in spectral-responses. The reasons for the variation are discussed. Based on these curves, spectral matching factors of a GaAs cathode for green vegetation and rough concrete are calculated. The visual ranges of night-vision goggles under specific circumstances are estimated. The results show that the spectral response of the NEA photocathode degraded in the sealing process, especially at long wavelengths. The variation has also influenced the whole performance of the intensifier tube. OCIS codes: 120.0120, 160.2100.

According to the response of photoelectric device to a light source, the formula of spectral matching factor of photo cathode-object combination is deduced. The spectral matching factors of Super [S.sub.25] and New [S.sub.25] photo cathodes for green vegetation and rough concrete are calculated and compared. For Super [S.sub.251], the spectral-matching factors for green vegetation and rough concrete are 0.707 and 0.743, respectively, under the full-moon radiation and are 0.504 and 0.577, respectively, under the clear-star radiation. For Super [S.sub.252], the full-moon factors are 0.561 (green vegetation) and 0.732 (rough concrete), and those for clear-star radiation are 0.338 (green vegetation) and 0.471 (rough concrete). For New $25, the full-moon factors are 0.741 (green vegetation) and 0.762 (rough concrete), and those for clear-star radiation are 0.603 (green vegetation) and 0.657 (rough concrete). An analysis of results shows that the spectral-matching factor has influence on the performance of a low-light level night vision system and is essential for evaluating the visual range of a night-vision system. OCIS codes: 120.0280, 300.6170, 120.5700, 040.3780, 040.5160.