【作者】 王晓晖；

【导师】 常本康；

【作者基本信息】 南京理工大学 ， 光学工程， 2013， 博士

【摘要】 本文围绕纤锌矿结构GaN光电阴极的(0001)面光电发射模型、量子效率理论、材料结构设计与生长、制备工艺和阴极性能评估等方面开展了研究。针对现有的光电阴极表面发射模型的局限性,为了更好的解释NEAGaN光电阴极(0001)面光电发射机理,建立了基于双偶极子模型的[GaN(Mg)-Cs]:[0-Cs]光电发射模型,讨论了激活过程中Cs、O在GaN(0001)表面吸附的过程,认为第一个偶极层GaN(Mg)-Cs具有统一的有利于光电子逸出的指向性,所以进Cs后光电流上升幅度很大,第二个偶极层O-Cs没有统一的指向性,只是表面的缺陷使得一部分O-Cs偶极子具有有利于光电子逸出的方向,所以进O后光电流有增长但幅度不大。讨论了NEAGaN光电阴极(0001)面的光电发射过程,并通过求解载流子扩散方程的方法获得了NEAGaN光电阴极量子效率公式。根据公式,分析了GaN发射层吸收系数αhv、电子表面逸出几率P、电子扩散长度LD、GaN发射层厚度Te以及后界面复合速率Sv对量子效率的影响。分析了不同生长方法、不同衬底和不同缓冲层材料的优劣。设计了不同p型掺杂浓度的反射式GaN光电阴极、梯度掺杂的反射式GaN光电阴极、发射层厚度150nm的透射式GaN光电阴极、以及组分渐变Ga1-xAlxN作为缓冲层的透射式GaN光电阴极。对现有的NEA光电阴极制备与评估系统进行了升级改造,增加了紫外光源和透射式的测试光路,使其可以很好的完成紫外光电阴极制备及评估的工作。利用XPS研究了三种不同的化学清洗方法对GaN(0001)表面的净化效果,认为2：2：1的H2SO4(98%)：H202(30%)：去离子水混合溶液是一种有效的方法。采用同样的710℃进行了两次加热并且进行了激活实验,发现第一次加热中真空度的变化曲线成“W”型,而第二次加热过程中成“V”型,并且质谱仪记录的残气成分分压的变化情况也与之相符合。进行了不同光照下激活的实验,分别在全光谱的氘灯、70μW的300nm单色光和35μW的300nm单色光光照下对GaN光电阴极进行了激活实验。对不同结构的NEAGaN光电阴极性能进行了评估,发现GaN光电阴极最佳p型掺杂浓度在1017cm-3数量级,梯度掺杂的GaN光电阴极最大量子效率达到了56%,明显优于均匀掺杂,采用Ga1-xAlxN缓冲层可以使CaN光电阴极获得更为理想的透射式的量子效率,发现透射式GaN光电阴极最佳的发射层厚度应该在90nm左右。对比了不同制备工艺获得的NEAGaN光电阴极性能,验证了化学清洗方法的优劣,发现二次加热后GaN光电阴极的性能与第一次没有明显的变化,300nm单色光光照下激活的GaN光电阴极性能要优于氘灯的。最后对比了GaN与GaAs光电阴极的性能,发现NEAGaN光电阴极在稳定性上要好于GaAs。本文研究工作围绕NEAGaN光电阴极的相关理论和技术,在紫外光电阴极及紫外探测技术等方面具有参考和促进意义。更多还原

【Abstract】 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.更多还原