Yujie Du

Quantum efficiency of GaN photocathode under different illumination

GaN samples are activated by Cs/O under illumination of deuterium lamp, 300 nm monochromatic light with power of 70 μW and 300 nm monochromatic light with power of 35 μW, respectively. Photocurrent is detected before activation under illumination of deuterium lamp. Quantum efficiency (QE) is tested after activation. The results indicate that GaN activated under 300 nm monochromatic light have higher QE than that under deuterium lamp, and no obvious difference is detected between different power 300 nm monochromatic light. The photocurrent before activation inhibits the adsorption of Cs on the GaN surface, which decrease the QE of GaN.

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.

Influence of vacancy defect on surface feature and adsorption of Cs on GaN(0001) surface.

The effects of Ga and N vacancy defect on the change in surface feature, work function, and characteristic of Cs adsorption on a (2 × 2) GaN(0001) surface have been investigated using density functional theory with a plane-wave ultrasoft pseudopotential method based on first-principles calculations. The covalent bonds gain strength for Ga vacancy defect, whereas they grow weak for N vacancy defect. The lower work function is achieved for Ga and N vacancy defect surfaces than intact surface. The most stable position of Cs adatom on Ga vacancy defect surface is at T1 site, whereas it is at BGa site on N vacancy defect surface. The E ads of Cs on GaN(0001) vacancy defect surface increases compared with that of intact surface; this illustrates that the adsorption of Cs on intact surface is more stable.

Influences of performance parameters of GaAs/AlGaAs materials on photoemission

Photocathode based on GaAs/AlGaAs heterojunction has been applied broadly in night vision imaging intensifiers for its broad spectral response wavelength, high quantum efficiency and low dark current. Especially in recent years the extension of response wavelength to near infrared makes GaAs/AlGaAs applied in laser imaging. To obtain negative electron affinity GaAs photocathode with high performance, reasonable selection of performance parameters of GaAs material is required. In this paper on basis of photoemission model, analysis of the influences of diffusion length of minor carrier, recombination velocity at the interface of GaAs/AlGaAs and thickness of active GaAs layer on spectral response of GaAs photocathode were detailed, and the optimizing principle and methods of these parameters were proposed. Our measurement results of compositions distributions of GaAs/AlGaAs heterojunction are also given to demonstrate influences of material performance parameters.

Comparison and research of spectral response characteristic of transmission-mode GaAs photocathode before and after indium seal

Based on the research of the standard second generation, the high capability third generation, the exceeding third generation and the fourth generation, the spectral response of the third generation LLL was carried out using a self-developing spectral response measurement instrument. Spectral response characteristics of third-generation LLL tube were obtained, and the material performance parameters of GaAs photocathode were calculated by curve simulation method. On-line measurement of GaAs photocathode after high-temperature activation and low-temperature activation was carried out, the results showed that spectral response in the whole response waveband decreased after indium seal, and long wave responsibility was most obviously influenced. Decrease was large, cut-off wavelength and peak value wavelength move towards short-wave, peak response value and integral sensitivity decreased, and the final spectral response curve became flat. By calculating photocathode parameters, it was found that indium seal, lead to the variations of surface activation layers of photocathode, and the long wave responded and sensitivity decreased accordingly. The influence factors on the surface activation layers during indium seal were also analyzed.

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.

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.

Study of structural and electronic properties of Cs adsorption on GaAs(100)−(2×4) surface

In this paper, the structural and electronic properties of Cs adsorption on GaAs surface were studied. The samples were prepared by MBE. The conduction bands, valence bands, band structure and density of states were calculated by first principles based on density functional theory. The effect of Zn doping on electronic structure were also analyzed.