A high performance ultraviolet photocathode was needed urgently with the evolution of UV detector and lithographic manufacturing, gallium nitride(GaN) is a semiconductor of wide band gap and has many excellent characters including resistance to high temperature, corrosion and radiation, the negative electron affinity (NEA) based-GaN photocathodes has high quantum efficiency, low dark emission and concentrated emission electron energy, because of these advantages NEA GaN photocathodes is a good candidate for ultraviolet detection and electron emission source [1,2]. Transmission-mode photocathodes are more convenient and easily than reflection-mode in applied practice, so the transmission-mode GaN photocathodes are used more and widely in the practical application and to be worthy researching. In this paper transmission-mode GaN photocathodes was activated by Cs/O, its quantum efficiency curve and the factors which influenced the quantum efficiency mostly were studied.
Transmission-mode GaN photocathodes was grown by Metal-Organic Chemical VaporDeposition (MOCVD), Ah03 was used as the substrate and a 20nm thickness AIN buffer layer was grown on Ah03 firstly, GaN was extended on AIN to reduce the crystal lattice mismatch, the thickness of GaN active-layer was 150nm and was p-type doped by element Mg, the doping concentration is L6x 10 17 cm-3 . The GaN photocathodes were activated in the ultrahigh vacuum (URV ) system by Cs/O, the photocurrent curve was obtained during the experiments and shown in Fig. I.After the activation spectral responses of the NEA GaN photocathodes were tested and quantum yield was calculated, the quantum yield curve was drawn according to the data from calculation as shown in Fig.2. The quantum yield curves verified the transmission-mode GaN photocathodes have flat and high response between 250 and 365nm, there was the highest quantum yield of 13% appeared around 280nm, and the quantum yield was decreased acutely around 365nm because this wavelength is the threshold of GaN, it reflects the character of "solar blind" exactly. Unlike reflection-mode the quantum yield of transmission-mode GaN photocathodes decreased at short wavelength less than 250nm, because in the transmission-mode light reach substrate layer firstly then go through the AIN buffer layer and reach the GaN active-layer finally, because AIN has wide band gap of 6.2eV, most of the short wavelength light was absorbed in this layer, there is little short wavelength light can reach GaN to produce photoelectron, and resulted in the quantum yield of transmission-mode GaN photocathodes decreased at short wavelength.The quantum yield formula of transmission-mode GaN photocathodes have been solved from the diffusion equations. The quantum yield formula shows that the factors influenced the transmission-mode quantum yield mostly are the electron escape probability P, electron diffusion length LD, back-interface recombination velocity Sy and the active-layer thickness Te.The quantum yield change higher with the increase of P and LD and mi...
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