Photoemission Performance Of Wurtzite Structure GaN (0001) Surface

The (0001) plane photoemission model, quantum efficiency theory, material structural designing and growth, preparation technique evaluation, and performance evaluation of the wurtzite structure GaN photocathode were researched in this thesis.Because of the limitations of the existing photocathode surface emission model, in order to explain (0001) surface photoemission mechanism of the NEA GaN photocathode better,[GaN (Mg)-Cs]:[O-Cs] photoemission model based on dual-dipole model is established. Cs, O adsorption process on GaN (0001) surface during the activation is discussed, and the GaN (Mg)-Cs dipole layer is found having a uniform direction which is conducive for photoelectron escaping, so photocurrent has a big rise when Cs is introduced. There is not a uniform direction for the O-Cs dipole layer, but because of the surface defects, part of the O-Cs dipoles have the direction which is conducive for photoelectron escaping, so the photocurrent has a modest growth after introducing O.Photoemission process on (0001) surface of NEA GaN photocathode is discussed, and NEA GaN photocathode quantum efficiency formula is obtained by solving the carrier diffusion equation. According to the formula, influences of GaN emission layer absorption coefficient μhv, electron surface escape probability P, electron diffusion length LD, GaN emission layer thickness Te, and the recombination rate Sv of the back interface on quantum efficiency are analyzed. The pros and cons of different growth methods, different substrates, and different buffer layer materials are discussed. The different p-type doping concentration and gradient doping are designed for the reflection mode GaN photocathode.150nm thick GaN emission layer and a composition graded Ga1-xAlxN buffer layer are designed for the transmission mode GaN photocathode.The existing NEA photocathode preparation and evaluation system is upgraded, a UV light source and transmission testing optical path are added, so the new system can play the preparation and evaluation work well for the ultraviolet cathode. Three different chemical cleaning methods are studied by XPS, and the2:2:1H2SO4(98%):H2O2(30%):deionized water solution is found to be an effective method to clean the GaN (0001) surface. GaN samples are annealed by the same temprature of710℃for two times and activated, respectively. The vacuum level curve has a shape "W" during the first annealing, while the second has a "V" shape, which is consistent with the partial pressure of the residual gas recored by a quadrupole mass spectrometer. Activation experiments under different light are carried out, a deuterium lamp with full spectrum,70μW of300nm monochromatic light, and35μW of300nm monochromatic light are used.The performances NEA GaN photocathode of different structure are evaluated. The best p-type doping concentration of GaN photocathode is found to be at the magnitude of1017cm-3. The maximum quantum efficiency of gradient-doped GaN photocathode reaches56%, which is better than the uniform doping significantly. GaN photocathode with Ga1-xAlxN buffer layer can obtain more ideal transmission mode quantum efficiency, and the optimal thickness of the emission layer for the transmission mode GaN photocathode should be about90nm. The performances of NEA GaN photocathode obtained by different preparation methods are compared, which verifies the merits of chemical cleaning methods. The performance of GaN photocathode after the secondary annealing has no obvious change with the first, and GaN photocathode activated under the300nm monochromatic light has higher quatunm efficiency than that under the deuterium lamp. Finally, the performance of GaN and GaAs photocathode are compared, and the stability of NEA GaN photocathode is better than GaAs.Theory and technology of NEA GaN photocathode are studied in this thesis, which has reference and positive significance for the ultraviolet photocathode and UV detection technology.