1.Engineering Research Center of Nuclear Technology Application, Ministry of Education, East China Institute of Technology, Nanchang 330013, China 2.Engineering Research Center of New Energy Technology and Equipment of Jiangxi Province, East China Institute of Technology, Nanchang 330013, China 3.School of Electronic and Optical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
Abstract:Metallic photocathodes have drawn attention due to their outstanding performances of ultrafast photoelectric response and long operational lifetime. However, due to their high work function and the large number of scattering events, metallic photocathodes typically are driven by ultraviolet laser pulses and characterized by low intrinsic quantum efficiency (QE). In this work, a new type of Mie-type silver (Ag) nano-sphere resonant structure fabricated on an Ag/ITO composite substrate is used to enhance the photocathode QE, where Mie scattering resonance is used to enhance the local density of optical state and then to improve the light absorption and electron transporting efficiency in Ag nano-spheres. The cesium (Cs) activation layer is also used to lower the electron work function and then to excite photoemission in the visible waveband for Ag photocathode. The optical characteristics of Ag nano-sphere arrays are analyzed by using finite difference time domain method. For the investigated Ag nano-sphere array, theoretical results show that Mie-type electric dipole resonance modes can be obtained over the 400–600 nm waveband by adjusting the sphere diameter, and the large resonance-enhanced absorption can be achieved in nanospheres at the resonance wavelength. The Ag nano-spheres are fabricated on the Ag/ITO substrate by magnetron sputtering and annealing process, then the Cs activation layer is deposited on surface, and finally QE is measured in an ultra-high vacuum test apparatus. Experimental results show that over 0.35% of QE is obtained for Ag nano-sphere particle (with a diameter of 150 nm) at a wavelength of 425 nm, and the wavelength positions of QE maxima are in agreement with Mie resonance for corresponding geometry predicted from the computational model. Given these unique optoelectronic properties, Ag nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today. Keywords:optical-resonance/ Ag-nanospheres/ photocathode
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2.1.器件结构与仿真方法
图1(a)给出了金属光阴极的Spicer三步光电发射物理过程[10,27], 其中EFM, EVAC, W分别为金属的费米能级、真空能级和金属的功函数; $ E^{\prime}_{\rm VAC} $和W' 分别为综合考虑镜像电荷和激活层作用后金属表面的有效真空能级和有效功函数值. 结合图1(a), Spicer三步光电发射物理过程包括电子吸收能量大于W′的光子激发至高能态(过程A), 然后扩散输运至表面并维持在高能态(过程B), 最后跃过表面势垒实现逃逸(过程C), 因此光电发射的QE 由光吸收率、光电子输运效率和表面逃逸概率综合决定. 图 1 光学共振增强Ag纳米结构光阴极 (a) Spicer三步光电发射物理过程; (b) Ag纳米球结构光阴极; (c) Ag薄膜光阴极; (d) ITO衬底上Ag纳米球的FDTD光学仿真设置 Figure1. Optical resonance enhanced Ag nano-structured photocathode: (a) Spicer’s three-step model of photoemission; (b) illustration of the Ag nano-structured photocathode; (c) illustration of the Ag film photocathode; (d) cross-section of the FDTD setup used for simulating the optical properties of the Ag nanoparticles on ITO substrate.
图2(a)为Ag纳米球光阴极的制备工艺流程. 首先采用磁控溅射方法在ITO表面制备Ag薄膜, 溅射条件为: 背底真空度优于10–6 Torr (1 Torr≈133.322 Pa), 溅射工作气体为5 × 10–4 Torr的Ar气, 溅射离子能量为600 eV, 电流为0.16 A, 溅射时间在2—12 s之间变化以调节纳米球的尺寸. 将所溅射的Ag薄膜在Ar气氛围下退火约20 min, 退火温度300 °C, 使得ITO表面的Ag薄膜自组装为Ag纳米球颗粒. 图2(b)和图2(c)分别为退火前的Ag薄膜和退火后的Ag纳米球的扫描电子显微镜(scanning electron microscopy, SEM)照片. 将所制备的Ag纳米球光阴极样品安装在背底真空度优于10–11 Torr的腔体中加热至400 °C进行表面热清洗, 以去除表面氧化层. 然后将温度降低至室温, 采用Cs源进行激活. 激活工艺过程为: 将放置于电阻丝上的高纯Cs源(购于SAES公司)连接至电流源正负极并安装在真空激活腔体中, 电阻丝中有电流流过时可以加热Cs源从而释放Cs原子并沉积在Ag纳米球表面进行激活, Cs原子的释放速率可通过电流大小进行控制, 在整个激活过程中将铯原子的分压维持在(2—3) × 10–14 Torr范围内, 并采用波长λ = 532 nm的激光持续照射光阴极表面以激发可供监测的光电流. 图2(c)为实际Ag纳米球表面Cs激活过程中光电流随时间的变化, 可以看到光电流随时间持续上升, 在约 1 h时光电流达到最大, 说明已获得最佳激活效果, 随后光电流开始下降. 图 2 Ag纳米球光阴极的制备及Cs激活工艺 (a) Ag纳米球光阴极的制备工艺流程; (b)退火前Ag薄膜的SEM照片; (c)退火后Ag纳米球的SEM照片; (d) Ag纳米球表面Cs激活过程中光电流的演化过程 Figure2. Fabrication and activation process of the Ag nanosphere photocathode: (a) Schematics of the fabrication process for Ag nanosphere photocathode; (b) SEM image of the Ag film; (c) SEM image of the Ag nanosphere; (d) surface Cs activation process of the Ag nanosphere.