Volume 52 Issue 3
Mar.  2023
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Guo Sitong, Qiu Kaifang, Wang Wenyan, Li Guohui, Zhai Aiping, Pan Deng, Ji Ting, Cui Yanxia. Enhancing broadband response of hot-electron photodetectors by Au/TiO2 composite nanostructure[J]. Infrared and Laser Engineering, 2023, 52(3): 20220464. doi: 10.3788/IRLA20220464
Citation: Guo Sitong, Qiu Kaifang, Wang Wenyan, Li Guohui, Zhai Aiping, Pan Deng, Ji Ting, Cui Yanxia. Enhancing broadband response of hot-electron photodetectors by Au/TiO2 composite nanostructure[J]. Infrared and Laser Engineering, 2023, 52(3): 20220464. doi: 10.3788/IRLA20220464

Enhancing broadband response of hot-electron photodetectors by Au/TiO2 composite nanostructure

doi: 10.3788/IRLA20220464
Funds:  National Natural Science Foundation of China (U21A20496, 61922060, 61905173, 61775156, 61805172, 12104334, 62174117); the Natural Science Foundation of Shanxi Province (20210302123154, 20210302123169); Research Project Supported by Shanxi Scholarship Council of China (2021-033); Research Program Supported by Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2021SX-FR008); Introduction of Talents Special Project of Lvliang City (Rc2020206, Rc2020207)
  • Received Date: 2022-07-05
  • Rev Recd Date: 2022-11-28
  • Publish Date: 2023-03-25
  •   Objective   Hot electron photodetectors (HEPDs) with wide spectrum responses are promising in the fields of image sensors and optical communications, etc. Metallic micro/nano-structures can efficiently generate hot carriers by exciting surface plasmons. It is helpful to realize low-cost and wide-spectrum response photodetectors when the heterostructures form by combining metallic micro/nano-structures with wide bandgap semiconductors. This approach can also be applied to improve the performance of HEPDs made of other semiconductors. This work contributes to the development of advanced plasmonic devices.  Methods   During the fabrication, the wet-cleaned FTO glass substrates were first subjected to the surface plasma treatment for increasing the work function of FTO substrates. Then, TiO2 and Au films were prepared by radio frequency (RF) and direct current (DC) magnetron sputtering, respectively. In detail, a TiO2 layer with a thickness of 20 nm was deposited onto the FTO substrate, followed by the deposition of an ultrathin Au film with its thickness varying from 2 nm to 8 nm. Then, the as-prepared multiplayer samples were annealed in air at 500 °C. The annealing process could, simultaneously, transform the ultrathin Au film into a layer of Au NPs, and transform the amorphous TiO2 film into its polycrystalline anatase film structure with a rough profile. After that, another thin Au film was deposited onto the annealed samples by DC magnetron sputtering. Here, the thin Au film can act as the transparent electrode with its thickness fixed to 20 nm.  Results and Discussions   The proposed hybrid plasmonic nanostructure based HEPD has an architecture as shown in Figure 1. Here, the TiO2 layer formed a concave-convex nanostructure with a scale of about 100 nm after annealing process, the nanostructure constructed by the Au nanoparticle layer and the conformal Au film used as electrode is for exciting surface plasmons. With the assistance of the Au/TiO2 composite nanostructure, the device has a wide spectrum absorption in the range of 400 nm to 900 nm, and the average absorption efficiency is 33.84%. Therefore, the proposed device can detect the incident photons outside the intrinsic absorption band of TiO2. The responsivity and linear dynamic range of the device under the wavelength of 600 nm separately are 9.67 μA/W and 60 dB (Fig.2). Besides, the corresponding rise/fall response speed are 1.6 ms and 1.5 ms respectively. (Fig.3). The finite element method is also used for simulation calculation, and the electric field distribution diagrams verify that the rich surface plasmon resonances excited in the Au/TiO2 composite nanostructure, which is the reason for realizing the wide spectrum and high efficiency detection (Fig.5).  Conclusions   In summary, we demonstrated a TiO2-based HEPD by incorporating a hybrid plasmonic nanostructure made of Au NPs together with a conformal Au film. Different from other similar approaches that were designed for high-efficiency hydrogen generation in the photocatalysts, a hybrid plasma nanostructure was used in photodetectors for realizing wide spectrum response. With the structural diversity of the hybrid plasmonic nanostructure, different surface plasmon resonances were excited, so that the device can respond to incident photons in a broadband wavelength range, covering UV-visible-NIR. The method of constructing hybrid plasmonic nanostructures has a guidance in developing high-performance optoelectronic devices.
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Enhancing broadband response of hot-electron photodetectors by Au/TiO2 composite nanostructure

doi: 10.3788/IRLA20220464
  • 1. College of Physics, Taiyuan University of Technology, Taiyuan 030024, China
  • 2. College of Optoelectronics, Taiyuan University of Technology, Taiyuan 030024, China
  • 3. Aluminum-Magnesium based new material R&D Co. Ltd. -Subsidiary of Xing Xian County Economic and Technological Development Zone, Lvliang 035300, China
Fund Project:  National Natural Science Foundation of China (U21A20496, 61922060, 61905173, 61775156, 61805172, 12104334, 62174117); the Natural Science Foundation of Shanxi Province (20210302123154, 20210302123169); Research Project Supported by Shanxi Scholarship Council of China (2021-033); Research Program Supported by Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (2021SX-FR008); Introduction of Talents Special Project of Lvliang City (Rc2020206, Rc2020207)

Abstract:   Objective   Hot electron photodetectors (HEPDs) with wide spectrum responses are promising in the fields of image sensors and optical communications, etc. Metallic micro/nano-structures can efficiently generate hot carriers by exciting surface plasmons. It is helpful to realize low-cost and wide-spectrum response photodetectors when the heterostructures form by combining metallic micro/nano-structures with wide bandgap semiconductors. This approach can also be applied to improve the performance of HEPDs made of other semiconductors. This work contributes to the development of advanced plasmonic devices.  Methods   During the fabrication, the wet-cleaned FTO glass substrates were first subjected to the surface plasma treatment for increasing the work function of FTO substrates. Then, TiO2 and Au films were prepared by radio frequency (RF) and direct current (DC) magnetron sputtering, respectively. In detail, a TiO2 layer with a thickness of 20 nm was deposited onto the FTO substrate, followed by the deposition of an ultrathin Au film with its thickness varying from 2 nm to 8 nm. Then, the as-prepared multiplayer samples were annealed in air at 500 °C. The annealing process could, simultaneously, transform the ultrathin Au film into a layer of Au NPs, and transform the amorphous TiO2 film into its polycrystalline anatase film structure with a rough profile. After that, another thin Au film was deposited onto the annealed samples by DC magnetron sputtering. Here, the thin Au film can act as the transparent electrode with its thickness fixed to 20 nm.  Results and Discussions   The proposed hybrid plasmonic nanostructure based HEPD has an architecture as shown in Figure 1. Here, the TiO2 layer formed a concave-convex nanostructure with a scale of about 100 nm after annealing process, the nanostructure constructed by the Au nanoparticle layer and the conformal Au film used as electrode is for exciting surface plasmons. With the assistance of the Au/TiO2 composite nanostructure, the device has a wide spectrum absorption in the range of 400 nm to 900 nm, and the average absorption efficiency is 33.84%. Therefore, the proposed device can detect the incident photons outside the intrinsic absorption band of TiO2. The responsivity and linear dynamic range of the device under the wavelength of 600 nm separately are 9.67 μA/W and 60 dB (Fig.2). Besides, the corresponding rise/fall response speed are 1.6 ms and 1.5 ms respectively. (Fig.3). The finite element method is also used for simulation calculation, and the electric field distribution diagrams verify that the rich surface plasmon resonances excited in the Au/TiO2 composite nanostructure, which is the reason for realizing the wide spectrum and high efficiency detection (Fig.5).  Conclusions   In summary, we demonstrated a TiO2-based HEPD by incorporating a hybrid plasmonic nanostructure made of Au NPs together with a conformal Au film. Different from other similar approaches that were designed for high-efficiency hydrogen generation in the photocatalysts, a hybrid plasma nanostructure was used in photodetectors for realizing wide spectrum response. With the structural diversity of the hybrid plasmonic nanostructure, different surface plasmon resonances were excited, so that the device can respond to incident photons in a broadband wavelength range, covering UV-visible-NIR. The method of constructing hybrid plasmonic nanostructures has a guidance in developing high-performance optoelectronic devices.

    • 光电探测器在图像传感、光通信、安全监测等领域应用广泛。传统光电探测器主要基于半导体感光原理,这类器件只能吸收能量高于半导体带隙的光子,导致其响应波段受到了限制[1]。为了拓宽其工作带宽,可利用金属作为感光材料,通过引入微纳结构激发表面等离激元共振效应,能够增强光吸收。当金属微纳结构吸收光子后,会产生大量的高能载流子,即热载流子,包含热电子与热空穴两类。这些热电子或热空穴越过金属-半导体界面,进入半导体中后被传输到对电极,便形成了光电流[2-6]。其中,由金属与n型半导体构成的热载流子型光电探测器中收集的是热电子,可称之为热电子光电探测器,较之由金属与p型半导体构成的热空穴型光电探测器更为常见。该策略在光催化等光电子器件中也得到了广泛关注[3,7-9]。需要指出的是,基于热载流子的光电探测器通常具有比较低的外量子效率,只有当器件的暗电流足够低时,才能表现出相对可观的亮暗电流比[10-16]。对于多数金属-半导体-金属结构,只有形成肖特基接触才能有效地抑制暗电流,达到热载流子信号不被背景噪声淹没的目的[17-21]

      已经报道的有关热电子光电探测器的工作中,常见的是采用周期性金属微纳结构实现宽谱光电响应。例如,Wu等人设计了一种基于一维周期性Ag纳米光栅结构的TiO2热电子光电探测器,通过相位匹配优化了其在1550 nm波长处的表面等离激元共振强度,实现了0.1 mA/W的响应率[22]。Knight等人提出了一种基于二维周期性Au纳米矩形阵列结构的Si热电子光电探测器,通过激发局域表面等离激元共振模式有效拓宽了器件的响应光谱,使其在1200~1600 nm波段内均具有光电响应,并在1250 nm波长下实现了10 µA/W的峰值响应率[2]。Fang等人在两片单层石墨烯之间插入了七聚体Au纳米圆盘二维阵列结构,在可见光及近红外波段内实现了20%的内量子效率,其中器件在785 nm 波长下响应率达到了13 mA/W[23]。但这类金属纳米结构尺寸单一,激发的表面等离激元共振波段相对较窄,并且这些规则金属纳米结构的制作工艺较为复杂,成本较高。

      相比于周期性金属纳米结构[8,24-25],通过热退火工艺制得的随机分布金属纳米颗粒(NPs)的尺寸多样,能够在宽谱范围内激发表面等离激元共振模式,基于此可有效拓宽热载流子型光电探测器的响应带宽。例如,2014年,Nazirzadeh等人在平直Si衬底上经退火制备了一层Au NPs,在Au NPs上继续沉积一层掺铝的氧化锌 (AZO) 透明阳极后制得光电探测器,该器件在Si本征吸收波段以外具有显著的光电响应,其响应率在1300 nm和1550 nm波长下分别为2 mA/W和600 μA/W[26]。2017年,Qi等人在Si金字塔纳米结构上经退火制备了随机分布Au NPs,并在其上方继续制备氧化铟锡 (ITO) 透明阳极后制得光电探测器,该器件在1000~1600 nm波段内的平均光吸收效率接近70%,且在1300 nm波长下器件响应率达到5 mA/W[27]。在这些研究中,随机分布的金属NPs起到了宽谱感光的作用,使得热载流子型光电探测器的响应带宽得到了有效拓展。

      文中设计了一种由Au纳米颗粒 (Au NPs) 与顶层保形Au膜 (Au-film) 共同组成的表面等离激元纳米结构,该结构作为感光元件,与具有凹凸纳米结构的TiO2薄膜和FTO(Fluorine-doped Tin Oxide)衬底共同构建了结构为FTO/TiO2/Au NPs/Au-film的热电子光电探测器。利用Au/TiO2复合纳米结构充分激发的表面等离激元共振效应使器件在400~900 nm入射波长范围内具有宽谱光电响应性能,且器件平均光吸收效率为33.84%,在600 nm波长处的峰值外,量子效率和响应率分别为0.002%、9.67 μA/W,线性动态范围为 60 dB,器件在其响应波段范围内表现出较快的光电响应速度。将设计的金属/介质复合纳米结构引入热电子光电探测器中,为制备工艺简单的宽光谱热电子光电探测技术提供了一种新思路。

    • 图1 (a) 为所设计的基于Au/TiO2复合纳米结构热电子光电探测器的结构示意图,其结构式为FTO/TiO2/Au NPs/Au-film。其中, Au NPs/Au-film为产生和发射热电子的表面等离激元纳米结构,半导体TiO2用来接收和传输热电子。由于Au与TiO2两者的能级差为0.9 eV,从而形成了肖特基结,器件中各功能层的能级结构如图1 (b) 所示。器件的实验制备流程为:在清洁好的FTO玻璃基底上通过真空磁控溅射法依次沉积厚度分别为20 nm和4 nm的TiO2膜和Au膜,之后将制备好的FTO/TiO2/Au-film样品置于500 ℃的空气环境中退火处理3 h,退火后TiO2膜和Au膜将分别形成具有百纳米凹凸结构的TiO2膜层和Au纳米颗粒层(Au NPs,粒径约为15 nm),形成FTO/TiO2/Au NPs样品,最后在该样品上同样采用真空磁控溅射法沉积20 nm厚的Au膜(Au-film),至此器件FTO/TiO2/Au NPs/Au-film制备完成。随后,通过扫描电子显微镜 (SEM,Hitachi SU8010) 以及原子力显微镜 (AFM,Park NX10) 分别对实验制备器件的侧视和俯视形貌进行表征,结果如图1 (c) 和 (d) 所示。可以看出,TiO2膜层表面呈现出百纳米尺度的凹凸结构,如图中黄色箭头标记所示,并且器件顶部的Au NPs/Au-film复合金属纳米结构的表面具有明显的颗粒状凸起形貌,如图中白色箭头标记所示,(图1 (d)中的白色颗粒为Au NPs和Au-film共同构成的纳米结构,材料为Au),证明器件顶部的Au NPs/Au-film复合纳米结构具有明显的保形特征。

      Figure 1.  (a) Schematic diagram of the proposed HEPD based on Au/TiO2 composite nanostructure; (b) Structural diagram of the energy levels of the materials used in the proposed HEPD; (c), (d) Cross-sectional and top-view SEM images of the proposed device respectively

    • 图2 (a) 为具有不同TiO2膜层厚度(tTiO2)器件在暗态下的I-V曲线图,可以看出,当tTiO2较小时 (如15 nm),器件的暗电流随外加电压升高而显著增加,这是由于较薄的TiO2膜层经退火后变得不连续,导致部分Au纳米颗粒与FTO直接接触形成欧姆接触而引起。此时器件在非零偏压下TiO2层中产生的自由载流子数量低于外部电路注入的载流子,因此,为保证热载流子信号不被背景噪声所淹没,文中的研究器件均工作在0 V偏压下,且TiO2膜厚度设置为20 nm。使用紫外可见分光度计 (Agilent Cary5000) 对器件的光吸收谱进行表征,从图2 (b) 中可以看出,器件在400~900 nm波段内具有宽谱光吸收特性,平均光吸收效率为33.84%。器件通过半导体分析仪 (Agilent B1500A) 测量获得的响应率、外量子效率以及线性动态范围均呈现宽谱响应特征,如图2 (c)、(d) 所示。且在600 nm照射条件下,响应率、外量子效率达到峰值,分别为9.67 μA/W、0.002%。导致外量子效率较低的因素主要有以下几点:首先,热退火形成的 Au NPs 尺寸较大,不利于热载流子的激发[28];其次, Au-TiO2 界面处形成了肖特基势垒,在减小暗电流的同时也降低了热载流子的注入效率[29];最后,在外加偏置电压为 0 V的条件下,TiO2薄膜的导电率较低,进一步阻碍了热电子的传输[30]。此外,器件的线性动态范围为60 dB,如图2 (e) 所示。众所周知,TiO2是一种宽禁带半导体材料,其本征光吸收波段在400 nm以下[31],故该器件在400~900 nm波段内的光电响应性能来源于表面等离激元纳米结构Au NPs/Au-film激发的高能电子(即热电子),热电子在Au/TiO2肖特基结界面处的产生及传输过程如图2 (f) 所示。

      Figure 2.  (a) I-V curves of devices with different thicknesses of TiO2 film (tTiO2) in the dark state. The response characteristics of the device with tTiO2 = 20 nm under 0 V bias: (b) Absorption spectra; (c) External quantum efficiency (EQE); (d) Responsivity; (e) Linear dynamic range. (f) Schematic diagram of hot electron generation and transfer process over the Au/TiO2 Schottky barrier

      进一步,通过半导体分析仪 (Agilent B1500A) 测试了器件的响应速度。从图3 (a) 中可以看出,器件在850 nm光照条件下的上升与下降响应速度分别为1.6 ms和1.5 ms,且该器件在375~850 nm波段内的上升与下降响应速度平稳地分布在1.5~1.8 ms之间,如图3 (b) 所示。

      Figure 3.  The rise and fall response speeds of the device under 0 V bias: (a) Under the illumination wavelength of 850 nm; (b) Under the illumination wavelength range varying from 375 nm to 850 nm

    • 器件中各功能层对其光电响应性能至关重要,研究了TiO2膜厚度 (tTiO2)、顶层保形Au膜厚度 (tAu) 以及形成Au NPs的名义Au膜厚度 (tn-Au) 变化对器件瞬态光电流和光吸收性能的影响。瞬态光电流通过照射波长为850 nm的LED光源 (Thorlabs M850L4 LED) 测试获得,光功率密度为10.2 mW/cm2,器件有效面积为0.04 cm2。器件的光吸收谱同样通过紫外可见分光度计(Agilent Cary5000)获得。

      图4 (a) 所示,器件的瞬态光电流随TiO2膜厚度 (tTiO2) 从15 nm增加到90 nm时呈现出先增加后减弱的趋势。当tTiO2 = 20 nm时,瞬态光电流达到峰值15.8 nA。但随着tTiO2进一步增加 (30~90 nm) ,器件的瞬态光电流不断下降,对应器件的平均光吸收效率也呈不断减弱趋势,如图4 (b) 所示。主要原因有:一方面,较厚的TiO2膜表面在退火后更为平整均匀,凹凸结构特征减弱不利于在其上保形制备的Au纳米结构中表面等离激元的充分激发,导致光吸收性能减弱;另一方面,较厚的TiO2膜增加了热电子传输到FTO对电极的距离,不利于器件光电流的收集。这里,当tTiO2 = 15 nm 时,由于制备的TiO2膜太薄,在高温退火后会出现薄膜不连续的现象,导致Au纳米结构与FTO电极导通,形成了欧姆接触,从而无法获得相应器件的光吸收测试谱。

      Figure 4.  Comparison of response performance of devices with different tTiO2 and tAu respectively: (a), (c) Transient photocurrents; (b), (d) Photocurrents and the corresponding average light absorption efficiencies

      图4 (c)、(d) 中可以看出,随着顶层保形Au膜厚度 (tAu) 从10 nm逐渐增加到40 nm时,器件的瞬态光电流及平均光吸收效率均呈现先增加后减弱的趋势。当tAu = 20 nm时,瞬态光电流和平均光吸收效率均达到了峰值,分别为 26.1 nA和33.6%。当tAu较薄时,如tAu = 10 nm,尽管器件的平均光吸收效率为24.37%,但其瞬态光电流几乎为零 (0.0025 nA),这是由于较薄的tAu会导致其局部薄膜不连续,使得器件导电性能大幅降低而引起。而tAu较厚时,例如tAu = 40 nm,尽管器件的导电性能得到明显提升,但其瞬态光电流及平均光吸收效率相较于tAu = 20 nm时均下降明显。这是因为较厚的tAu使其保形特征减弱,以致表面等离激元共振增强器件光吸收效果减弱,从而热电子的产生率降低;同时,一定程度上增加了热电子到达肖特基结的传输距离,不利于热电子高效地注入到半导体TiO2中。

      接着,研究了形成Au NPs的名义Au膜厚度 (tn-Au) 对器件光电响应性能的影响,因为不同的tn-Au将直接影响该Au膜退火后形成的Au NPs尺寸及分布。如图5 (a) 所示,随着tn-Au从2 nm逐渐增加到15 nm,器件的瞬态光电流同样呈现出先增加后减弱的趋势,且均优于未掺入Au NPs的参比器件 (tn-Au = 0 nm,即器件FTO/TiO2/Au-film)。当tn-Au = 4 nm时,形成的Au NPs的粒径约为15 nm,对应器件瞬态光电流达到峰值21.6 nA,是参比器件的9.63倍。从图5 (b) 中可以看出,随着tn-Au值的变化,器件的平均光吸收效率的变化趋势类似于瞬态光电流曲线,当tn-Au = 4 nm时器件光吸收性能最优。太薄的tn-Au (如2 nm) 在退火操作后形成的Au NPs尺寸较小且排布均匀密集,不利于表面等离激元共振的激发。而当tn-Au大于4 nm时,形成的Au NPs尺寸较大且分布不均匀 (如tn-Au = 8 nm时,相应产生的Au NPs的粒径约为45 nm),不利于Au材料中产生的热电子有效传输至肖特基结区[32]

      Figure 5.  (a) Comparison of the transient photocurrents for the devices with different tn-Au; (b) Comparison diagram between the photocurrents and average absorption efficiencies of the devices with different tn-Au. The normalized electric field distributions (|E|2): (c) The optimal device with tn-Au = 4 nm and (d) the reference device

      针对上述瞬态光电流最优器件 (tn-Au = 4 nm) 以及参比器件 (tn-Au = 0 nm),进一步利用有限元法 (FEM) 对两者进行光学仿真计算,并对比研究了两者在850 nm波长下的电场分布。参比器件中顶层保形Au-film与TiO2膜凸起高度均为h (15 nm) ,晶格常数Λ= 200 nm。最优器件 (tn-Au = 4 nm) 在参比器件的基础上引入了简化的准周期Au NPs阵列结构,Au NPs的长短半径分别固定为a = 30 nm和b = 15 nm。仿真计算中,结构单元均沿x轴和y轴设周期性边界条件,顶部和底部为完美匹配层 (PML) ,入射光从顶层保形Au-film一侧照射,各功能层材料的折射率参考文献[33-34]。如图5 (c) 所示,参比器件的强电场分布集中在由TiO2膜凸起引起的保形顶层Au膜凹槽及拐角处。相比之下,图5 (d) 所示的最优器件的强电场更多地分布在由 Au NPs阵列排布引起的凹槽处,这归因于Au纳米结构激发的表面等离激元共振,特别在凹槽拐角处的强电场激发的热电子可更有效地进入TiO2膜层以提高光电流收集率。

      最后,研究了不同顶层保形金属薄膜(Au-film、Ag-film和Cu-film)对器件瞬态光电流和光吸收性能的影响。选取850 nm波长照射器件为例进行详细说明。从图6 (a) 中可以看出,Au-film器件的瞬态光电流最优,而Cu-film器件的瞬态光电流最低 (0.061 nA),且含有Au-film、Ag-film和Cu-film器件的平均光吸收效率也依次减弱,如图6 (b) 所示。其中,尽管Cu-film器件平均光吸收效率也可达到14.52%,但其瞬态光电流几乎为0,原因在于Cu的光吸收损耗较大且其极易被氧化生成CuO从而降低了器件的导电性所致。而Ag-film器件相比于Au-film器件的表面等离激元共振波长会蓝移,故Au-film器件在850 nm照射波长下表面等离激元共振效应对器件光吸收增强更为明显,相应的器件瞬态光电流也更强。可见,顶层金属薄膜的选择对器件光电流响应性能的影响极为重要。

      Figure 6.  Comparison of response performance of devices with top conformal metal films of Au-film, Ag-film and Cu-film: (a) Transient photocurrents; (b) Photocurrents and the corresponding average light absorption efficiencies

    • 综上所述,文中设计研究了一种含有Au/TiO2复合纳米结构的表面等离激元型热电子光电探测器,其结构为FTO/TiO2/Au NPs/Au-film。在Au/TiO2复合纳米结构的协同作用下,器件表现出了宽光谱 (400~900 nm) 光电响应性能,且器件在600 nm照射波长下的响应率、外量子效率均达到了峰值,分别为 9.67 μA/W、0.002%,器件的线性动态范围为60 dB。此外,当照射波长在375~850 nm范围内变化时,器件的上升与下降响应速度较为平稳地分布在1.5~1.8 ms之间。进一步就不同功能层厚度对器件瞬态光电流以及光吸收性能的影响进行了详细的分析与研究。优化后的Au NPs (其名义厚度为4 nm)/Au-film (20 nm) 纳米结构可高效产生热电子,并可调控热电子的分布以提升其进入TiO2膜层的注入效率;而20 nm厚的TiO2膜层不仅可以有效传输热电子到FTO对电极,且其表面的凹凸结构也可协助表面等离激元共振的激发,使器件最终实现宽光谱光电响应性能。此外,还对比研究了具有 Au-film、Ag-film和Cu-film三种顶层金属薄膜器件的光吸收效率及瞬态光电流,Au-film器件均表现最佳。所提出的基于金属/介质复合纳米结构的光电探测器对开发制备工艺简单的表面等离激元增强型热电子光电探测器件具有一定的指导意义。此外,可从以下几个方面改善器件外量子效率性能:首先,进一步优化金属纳米颗粒尺寸及分布,通过激发丰富的表面等离激元共振以提高热载流子产生率[35-36];其次,可通过界面修饰方法缩小界面处势垒的宽度,引入量子隧穿效应,提高界面处热电子的注入效率[2, 37-38];最后,在TiO2薄膜中掺杂金属离子,可减少电子-空穴对复合,进一步促进TiO2薄膜中热电子传输[15,39]

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