Xiaoqian Fu

Higher quantum efficiency by optimizing GaN photocathode structure

Higher quantum efficiency by optimizing GaN photocathode structure is reported. AlN was used to substitute for GaN as buffer layer, which could act as a potential barrier due to its higher energy gap and reflect electrons back toward the surface direction. Graded-doping structure was used to replace uniform-doping structure, which introduced three built-in electrical fields for the different doping concentrations.

Attenuation performance of reflection-mode AlGaN photocathode under different preparation methods

To research the attenuation performance of the AlGaN photocathode, three samples with the same structures grown by metalorganic chemical vapor deposition were activated with three different activation methods, which are called Cs-only, Cs-O, and Cs-O-Cs activation, respectively. The spectral responses and attenuated photocurrents of the three AlGaN photocathodes were measured. The results show that the Cs-O activated AlGaN photocathode exhibits the lowest attenuation speed in the first few hours, and the attenuation speed of the Cs-only activated one is fastest. After attenuating for 90 min, the attenuation photocurrent curve of the Cs-O-Cs activated sample is coincident with that of the Cs-O activated one. The main factor affecting the photocurrent attenuation is related to Cs atoms desorbed from the photocathode surface.

The possibly important role played by Ga2O3 during the activation of GaN photocathode

Three different chemical solutions are used to remove the possible contamination on GaN surface, while Ga2O3 is still found at the surface. After thermal annealing at 710 °C in the ultrahigh vacuum (UHV) chamber and activated with Cs/O, all the GaN samples are successfully activated to the effective negative electron affinity (NEA) photocathodes. Among all samples, the GaN sample with the highest content of Ga2O3 after chemical cleaning obtains the highest quantum efficiency. By analyzing the property of Ga2O3, the surface processing results, and electron affinity variations during Cs and Cs/O2 deposition on GaN of other groups, it is suggested that before the adsorption of Cs, Ga2O3 is not completely removed from GaN surface in our samples, which will combine with Cs and lead to a large decrease in electron affinity. Furthermore, the effective NEA is formed for GaN photocathode, along with the surface downward band bending. Based on this assumption, a new dipole model Ga2O3-Cs is suggested, and the exper...

Resolution properties of transmission-mode exponential-doping Ga 0.37 Al 0.63 As photocathodes

Using the modulation transfer function obtained by establishing and solving the two-dimensional continuity equation, we have calculated and comparatively analyzed the resolution characteristics of transmission-mode exponential-doping and uniform-doping Ga0.37Al0.63As photocathodes. The calculations show that compared with a uniform-doping Ga0.37Al0.63As photocathode, the exponential-doping structure can significantly improve not only the resolution, but also the quantum efficiency of the photocathode. This improvement is different from the approach for high resolution by reducing the emission layer thickness Te and electron diffusion length LD, or by increasing the recombination velocity of the back-interface, SV, which results in low quantum efficiency. Furthermore, the improvement in resolution and quantum efficiency for the transmission-mode exponential-doping Ga0.37Al0.63As photocathodes is the result of the effect of the built-in electric field on electron transport and lateral diffusion.

Photoemission stability of negative electron affinity GaN photocathode

The stability for reflection-mode GaN photocathode has been investigated by monitoring the photocurrent and the spectral response at room temperature. We watch that the photocurrent of the cathode decays with time in the vacuum system, and compare the spectral response curves after activation and after degradation. The photocurrent decay mechanism for reflection-mode NEA GaN photocathode was studied by the surface model ［GaN (Mg) :Cs］:O-Cs. The reduction of the effective dipole quantity, which is caused by harmful gases, is the key factor of the photocurrent reduction.

Comparative Study of uniform-doping and gradient-doping NEA GaN photocathodes

High temperature annealing and Cs, O activation is the formations of NEA GaN photocathode of external incentives, GaN material performance of the cathode of the internal factors are fundamental. In this paper, aiming at the difference of the uniform-doping and gradient-doping NEA GaN photocathode in structure, combined with the cathode active changes of the optical current and activated after the success of the spectral characteristics of the uniform-doping and gradient-doping performance of NEA photocathode similarities and differences. Experiments show that: compared to the uniform-doping cathode, graded-doping cathode activated with radiation in the current slow growth rate, activation time is relatively long, successful activation of higher quantum efficiency. Using field-assisted photocathode emission model can explain the differences existing between the two, built-in electric field gradient doping structure to increase the presence of electronic drift to the cathode surface movement; the electron reaches the cathode surface to improve the escape probability of electronic.

Higher quantum efficiency GaAs photocathode material with exponential-doping structure

To improve the performance of GaAs NEA photocathodes, an exponential-doping structure GaAs material has been put forward, in which from the GaAs bulk-to-surface doping concentration is distributed exponentially from high to low. We apply this exponential-doping GaAs structure to the transmission-mode GaAs photocathodes. This sample was grown on the high quality p-type Be-doped GaAs (100) substrate by MBE. We have calculated the band-bending energy in exponential-doping GaAs emission-layer, and the total band-bending energy is 59 meV which helps to improve the photoexcited electrons movement towards surface for the thin epilayer. The integrated sensitivity of the exponential-doping GaAs photocathode samples reaches 1547uA/lm.

Exponential-doping III–V photocathodes with negative electron affinity

With the limitation of epitaxial growth technique, the approximately exponential-doping structure is actually a special kind of gradient-doped structure, in which the doping concentration in the active-layer of the III-V photocathode varies exponentially from bulk to surface. This doping structure can result in a linearly downward band structure which helps form a constant built-in electric fields to facilitate photoexcited electron movement towards surface, and the band structure of the exponential-doping photocathode is shown in this paper. By using the MBE technique, the reflection-mode (RJVI) and transmission-mode (TM) exponential-doping GaAs photocathodes composed of many sublayers were grown.

Research of NEA GaN photocathode performance parameters on the effect of quantum efficiency

Research of negative electron affinity GaN ultraviolet photocathode performance parameters on the effect of quantum efficiency is reported. Electronic surface escaping probability is one of the important parameters in comprehensive measure the level of the preparation of GaN optoelectronic cathode.

Exponential-doping GaAs NEA photocathode grown by MBE

To improve the performance of GaAs NEA photocathodes, an exponential-doping structure GaAs material has been put forward, in which from the GaAs bulk-to-surface doping concentration is distributed gradiently from high to low. We apply this exponential-doping GaAs structure to the transmission-mode GaAs photocathodes. This sample was grown on the high quality p-type GaAs (100) substrate by MBE with p-type Be doping. We have calculated the band-bending energy in exponential-doping GaAs emission-layer, and the total band-bending energy is 59 meV which helps improve the photoexcited electrons movement towards surface for the thin epilayer. The integrated sensitivity of the two exponential-doping GaAs photocathode samples with different thickness reaches 1228uA/lm and 1547uA/lm respectively.