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在按照需求设计具有特定功能的复合介质时,常使用等效介质理论来快速计算复合介质的等效电磁参数[16],这种方式大大提高了整个设计流程的效率。等效介质理论的思想是,当电介质中的掺杂物大小远小于入射波的波长,该复合材料就可以从宏观的角度分析其电磁特性,此时可以用准静态来近似[17]。
当电介质中掺杂物的含量较低,微粒间的相互作用和更高的多级作用可以忽略,此时通常采用Maxwell-Garnett理论(下面简称M-G)来计算复合介质的等效电磁参数[18]。另外,参考文献[19]实验证明了M-G模型在填充因子低于5%情况下的可靠性。如果设掺杂物为半径为$R$的球形,介电常数为${\varepsilon _m}$,根据Clausius-Mossotti公式可知,在由微粒和介质构成的二元体系下,微粒的电极化度$\alpha $与材料宏观介电常数${\varepsilon _e}$之间的关系为[20]:
$$ \frac{{{\varepsilon _e} - {\varepsilon _h}}}{{{\varepsilon _e} + 2{\varepsilon _h}}} = \frac{{n\alpha }}{{3{\varepsilon _0}}} $$ (1) 式中:$n$表示单位体积内的粒子数目;${\varepsilon _h}$表示电介质的介电常数。其中电极化度为:
$$ \alpha = 4\pi {\varepsilon _0}{R^3}\frac{{{\varepsilon _m} - {\varepsilon _h}}}{{{\varepsilon _m} + 2{\varepsilon _h}}} $$ (2) 将公式(2)代入公式(1),得:
$$ \frac{{{\varepsilon _e} - {\varepsilon _h}}}{{{\varepsilon _e} + 2{\varepsilon _h}}} = p\frac{{{\varepsilon _m} - {\varepsilon _h}}}{{{\varepsilon _m} + 2{\varepsilon _h}}} $$ (3) 这就是M-G等效介质公式,其中,$p$为掺杂物所占的体积分数。
根据银和金的折射率实验数据,可以拟合出它们的折射率曲线,如图1所示(图中点表示实验数据,线表示拟合结果)。然后,利用关系$\varepsilon {\text{ = }}{n^2}$,可以得到两种金属的介电常数[21]。接下来将纳米银粒子的体积分数(以1%为例)、介电常数以及硅的介电常数分别代入公式(3),可以得到Si掺杂Ag复合介质在可见光和近红外波段的等效介电常数。最后,将结果转换为复折射率,并与介质硅的复折射率进行对比,如图2所示。从图中可以看出,两者的主要区别为760 nm附近的波段范围,说明掺杂物对于复合介质电磁性能的影响主要发生在这个波段。掺杂对于复折射率实部的影响是,会在750 nm附近出现值剧烈的折射率变化,产生一个极大和一个极小值。掺杂对于复折射率虚部的影响是,会在760 nm附近产生一个峰值。出现该情况的原因如下,将公式(3)转变为:
$$ {\varepsilon _e} = {\varepsilon _h}\left[ {1 - \frac{p}{{{{{\varepsilon _h}} \mathord{\left/ {\vphantom {{{\varepsilon _h}} {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}}} \right. } {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}} - \frac{1}{3}\left( {1 - p} \right)}}} \right] $$ (4) 图 1 金属的复折射率与拟合曲线。(a) 银的折射率;(b) 金的折射率
Figure 1. Complex refractive index and fitting curve of metals. (a) Refractive index of silver; (b) Refractive index of gold
当$ {{{\varepsilon _h}} \mathord{\left/ {\vphantom {{{\varepsilon _h}} {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}}} \right. } {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}} $的实部取(0,1)之间时,分母可能会消失,而此时$ {{{\varepsilon _h}} \mathord{\left/ {\vphantom {{{\varepsilon _h}} {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}}} \right. } {\left( {{\varepsilon _h} - {\varepsilon _m}} \right)}} $的虚部保持有限。这种情况称为几何共振,由复合材料的微观几何和组分的介电系数决定[17]。
Silicon based near-infrared absorption enhancement structure with gradient doping of nano metal particles
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摘要: 硅基光电子与CMOS工艺兼容,借助成熟的微电子加工工艺平台可以实现大规模批量生产,具有低成本、高集成度、高可靠性的优势。其中,硅基半导体探测器是目前应用最为广泛的可见光波段探测设备,将其工作频段拓展到近红外波段具有重要意义。由于硅的禁带宽度,硅基材料在近红外波段电磁波吸收存在明显限制,硅基探测器在近红外波段的应用受到挑战。根据纳米金属粒子发生局域表面等离子共振时产生的近场增强效应,提出了一种纳米金属粒子梯度掺杂的硅基结构。通过应用等效介质理论,模拟了复合硅基结构在可见光与近红外波段的吸收特性。结果表明:该结构在近红外波段具有电磁波吸收提升效果,并且当选择纳米金粒子梯度递增掺杂时,可以在610~1450 nm波段提升吸收性能,最高提升可达到10.7 dB。所提出的结构可以有效增强硅基材料在近红外波段的吸收效率,研究结果为硅基半导体探测器在近红外波段的应用提供了重要参考。Abstract:
Objective Silicon based optoelectronics are compatible with CMOS technology, and with the help of mature microelectronic processing platforms, large-scale mass production can be achieved. It has the advantages of low cost, high integration, and high reliability. Among them, the application of silicon based semiconductor detectors in the visible light band has become more mature. However, the commonly used semiconductor materials for near-infrared band detectors have drawbacks such as difficulties in compatibility with existing CMOS processes and high prices. Therefore, expanding the operating frequency range of silicon based semiconductor detectors to the near-infrared band is of great significance. Due to the bandgap width of silicon, there are significant limitations in the absorption of electromagnetic waves by silicon based materials in the near-infrared band, posing serious challenges for the application of silicon based detectors in the near-infrared band. Methods In order to break through the bandgap width limitation of silicon and improve the absorption performance of silicon materials in the near-infrared band, a silicon based structure based on gradient doping of nanometallic particles was proposed, based on the near-field enhancement effect generated by local surface plasmon resonance of nanometallic particles. The slow change in doping concentration can effectively solve the severe change in reflectivity caused by refractive index mutation. By applying the Maxwell Garnett equivalent medium theory, the absorption characteristics of composite silicon based structures in the visible and near-infrared bands were simulated, and the effects of two doping concentration changes and two doping metals on the absorption enhancement effect of silicon based materials were compared. Results and Discussions The results indicate that the structure has a significant improvement in electromagnetic wave absorption in the near-infrared band. When the doped metal is silver, both decreasing and increasing doping can bring absorption improvement in the 640-1080 nm wavelength range. However, increasing doping can avoid the drastic change in reflectivity caused by refractive index mutations, and its effect is significantly better than decreasing doping(Fig.6). When comparing the effects of different metals, the absorption enhancement band brought by the doping of gold nanoparticles is wider than that of silver nanoparticles. So when choosing gradient increasing doping of gold nanoparticles, the effect is optimal, and the absorption performance can be improved in the 610-1450 nm wavelength range, with a maximum improvement of 10.7 dB(Fig.7). Conclusions A silicon based structure that can break through the bandgap width limitation of silicon was proposed, near-infrared absorption enhancement was achieved, and the absorption enhancement effect under different conditions was simulated and analyzed. The proposed structure can effectively enhance the absorption efficiency of silicon based materials in the near-infrared band, which helps to improve the performance of silicon based devices. And by comparing different doping methods and metal selection, it is concluded that gradient increasing doping of gold nanoparticles is the optimal choice. The research results of this article provide important references for the application of silicon based semiconductor detectors in the near-infrared band. -
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