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金属与硅形成肖特基结的过程是一个涉及电荷重新分布和电子能级调整的物理过程。如图1 (a)所示,对于N型硅,金属与硅接触后,N型硅由于费米能级一般高于金属的费米能级,导致N型硅的电子流向金属。在此过程中,半导体与金属接触的界面因电子转移而形成电场,电场的方向由半导体指向金属,又由于电场方向是电势降低的方向(也是电子的电势能升高的方向),因此N型硅的能带向上翘曲,形成如图1 (b)所示的能带结构。在理想情况下,金属与N型硅形成的肖特基结的势垒高度可由公式φ = Wmetal −χsi计算获得,其中φ为肖特基结势垒高度,Wmetal为金属的功函数,χsi为硅中电子的亲和势,代表硅导带底到真空的能量距离。如图1(c)所示,金属与P型硅形成肖特基结的过程与N型硅的情况类似,两者主要的不同点在于:N型硅肖特基结对应的热载流子一般为热电子,而P型硅肖特基结对应的热载流子通常是热空穴。
图 1 金属-硅肖特基结形成的原理示意图。 (a)金属和硅接触前的能带图; (b)金属和N型硅接触后的肖特基结能带图;(c)金属和P型硅接触后的肖特基结能带图;(d)肖特基结的典型电流-电压特性曲线
Figure 1. Schematic diagram of the formation principle of metal-silicon Schottky junction. (a) Energy band diagram before the contact between metal and silicon; (b) Energy band diagram of Schottky junction after the contact between metal and N-type silicon; (c) Energy band diagram of Schottky junction after the contact between metal and P-type silicon; (d) Typical current-voltage characteristic curve of Schottky junction
外加偏置电压的引入会打破肖特基结的热平衡状态,导致金属和硅之间的费米能级不再相等,从而改变了费米能级与肖特基结势垒的相对高度。具体而言,正向偏置时会使势垒相对高度降低,允许电子从金属流向硅中,形成正向电流;反向偏置则使势垒相对高度增加,阻止电子流动,电流几乎为零,展现了整流特性,如图1(d)所示。在实际情况中,肖特基势垒的高度由很多复杂因素决定,包括界面的性质、材料性质和制备条件等。在金属和硅之间形成肖特基势垒时,这些因素可以显著改变其高度。尤其是界面态可能会引发费米能级的钉扎效应,导致实际的肖特基势垒高度与理论预测值存在较大偏差。因此,在实际应用中,肖特基势垒的高度一般是通过实验测量来确定。
当红外光照射到金属-硅肖特基结探测器时,光子与金属内的电子发生相互作用,激发电子从低能量态跃迁至高能量态,形成热电子。这些热电子具备一定的初始动量分布,并在金属内部进行传输。其中,部分热电子会朝着肖特基结方向移动,并有一定几率越过肖特基势垒注入到硅材料中。成功注入硅中的热电子会继续向电极方向移动,最终形成可测量的电流,即光电流。如图2所示,光电流的形成过程涉及多个过程:首先是光子吸收导致热电子的产生,其次是热电子在金属内的传输,最后是热电子越过肖特基势垒注入硅材料。这些过程中均存在能量损耗,包括辐射损耗、热损耗、电子-电子、电子-声子相互作用引起的损耗等,导致大部分能量转化为热能而非电能。为了提高金属-硅肖特基结探测器的光电转换效率,须深入理解各阶段的能量损耗机制,并确立有效的策略来抑制这些损耗,增强光电流的产生。
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在金属中热电子的产生主要有两种方式[13−15]。第一种方式是入射光直接激发块体金属中的热电子。在这种情况下,光的吸收率对热电子的激发效率起着决定性作用,而块体金属会反射绝大部分的入射光,因而该种方式的热电子激发效率较低。第二种方式是光照射到金属纳米结构上激发表面等离激元共振[16−22]。这些等离激元在超快的时间尺度上(1~10 μs)发生弛豫。在这一过程中,等离激元可能通过多种方式衰减,包括通过朗道阻尼[23−24]退化成热电子-热空穴对,通过辐射衰减将能量以光子的形式发射,或者直接形成热损耗[25]。这些衰减机制的比例受到金属纳米结构的尺寸、材料种类以及热载流子的寿命等因素的影响[26−29]。因此,在热电子的产生过程中,光反射引起的损耗、等离激元的辐射损耗以及热损耗构成了主要的能量损失途径。为了提升热电子的产生效率,降低这些损耗机制至关重要。这需要对金属纳米结构[30−35]进行优化设计,以实现更高的光吸收率和更有效的等离激元转化过程。
激发的热电子的空间分布、初始能量和动量分布都对金属-硅肖特基探测器的光电转换效率具有重要的影响。首先,热电子在空间上的分布上并不均匀,这归因于金属不同区域光强度的差异,导致光强度较高的区域产生更多热电子。具体来说,热电子的空间分布可由下列公式给出:
$$ n\left(\stackrel{\rightharpoonup }{r},\omega \right)\propto \frac{1}{2\hslash }\mathrm{Im}\left(\varepsilon \right){\left|E\left(\stackrel{\rightharpoonup }{r},\omega \right)\right|}^{2} $$ 式中:n为热电子数目;E为入射光的电场;ω为入射光的频率;ε为金属的介电函数。由于入射光通常由多个频率组成,因此在计算热电子分布n时,需对频率进行积分。在探测器的设计中,优化热电子的空间分布,以确保其尽可能靠近肖特基结界面,可提升热电子注入硅中的概率。
其次,热电子的初始能量和动量分布也可能存在非均匀性[36−37]。在金属内部,原本位于费米能级(EF)以下的电子,在吸收入射光能量或由入射光激发的等离激元退化成热电子后,其能量将增加∆E,从而热电子的能量范围位于EF与EF+∆E之间。当电子的激发过程发生在同一能带内(即带内跃迁)时,受激发的电子态密度在EF以下通常呈现较为均匀的分布,使得激发后的热电子能量也在EF+∆E范围内均匀分布。然而,当电子发生带间跃迁时,情况则截然不同。此时,电子的态密度可能变得极为不均匀,导致热电子的能量分布也不均匀,如图3所示。
图 3 金属材料类型对热电子初始能量分布的影响
Figure 3. Effect of metal material type on initial energy distribution of hot electrons
以金属金为例,其d带电子主要位于费米能级下约2.6 eV的位置。当入射光子的能量超过此值时,会触发大量的d带电子的带间跃迁。这些跃迁后的电子大多位于费米能级附近,无法形成有效的热电子以越过肖特基势垒,从而对光电转换效率产生不利影响。另一方面,热电子的初始动量分布同样受到多种因素的影响,包括金属的晶体相对于硅界面的取向、入射光的电场方向以及带间跃迁等[36]。对于光电转换而言,理想的情形是电子的初始能量都高于肖特基势垒(而不是在EF+∆E范围均匀分布,或在势垒高度以下),电子的初始动量(速度)方向能够朝向肖特基界面,这样可增加其到达肖特基结界面的几率。因此,为了提升探测器的光电转换效率,须充分考虑热电子的初始能量分布以及动量分布特点。
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热电子在产生后,必须传输至肖特基结的界面,才有可能成功注入硅中。在传输过程中,热电子会经历多种散射过程,主要包括电子-电子散射和电子-声子散射[38−43]。从动力学角度来看,热电子激发后在不同的时间尺度内展现出三种主要状态[36−44]。如图4 (a)所示,在小于10 μs的时间内,为金属吸收光后的热电子激发。而在大约10~100 μs的时间尺度内,热电子主要受到电子-电子散射过程的影响,导致其能量逐步降低,电子间的能量进行重新分配,形成一个准费米分布。当时间尺度超过100 μs时,电子-声子相互散射过程占据主导地位,使得电子与晶格之间发生能量交换,热电子逐渐冷却至平衡态,整个系统恢复到初始状态。因此,在热电子的传输过程中,电子-电子散射和电子-声子散射两个过程至关重要,它们直接决定了热电子是否能够成功到达金属-硅界面。这两种散射过程可以通过平均自由程来描述,其中电子-电子散射对应的平均自由程为lee,而电子-声子散射对应的平均自由程为lep。特别地,电子-电子散射的平均自由程lee并非一个常数,而是与热电子的能量高低密切相关。具体而言,能量较高的热电子更易于与其他电子发生碰撞,因此其自由程相对较短,而能量较低的热电子则拥有较长的自由程。另一方面,电子-声子散射的平均自由程lep主要受到声子数量的影响,与电子能量的关系并不显著[45−47]。由于声子的数量与温度有关,因此在特定温度下,可以认为lep是一个相对稳定的值。图4 (b)展示了金的平均自由程lee和lep,可以看出,在热电子能量小于2 eV的区间内,两种自由程均达到20 nm以上。
图 4 (a)热电子受到的散射过程及时间尺度; (b)热电子受电子和声子散射的平均自由程与热电子能量的关系
Figure 4. (a) The scattering process and time scale of hot electrons; (b) the relationship between the mean free path of hot electrons scattered by electrons and phonons and the energy of hot electron
受到电子-电子散射、电子-声子散射后,热电子从初始位置传输到肖特基界面的概率与激发点-界面的距离呈指数衰减关系,可由下面的公式给出[48]:
$$ P\left(\stackrel{\rightharpoonup }{r},E\right)=\frac{1}{2\pi }{\displaystyle \underset{\theta 1}{\overset{\theta 2}{\int }}\mathrm{exp}\left(-\frac{d\left(\stackrel{\rightharpoonup }{r}\right)}{l\left(E\right)\left|\mathrm{cos}\theta \right|}\right)}{\mathrm{d}}\theta $$ (1) 式中:$ P\left(\stackrel{\rightharpoonup }{r},E\right) $为r处激发的能量为E的热电子达到金-硅界面的概率;$ d\left(\stackrel{\rightharpoonup }{r}\right) $为r处热电子到金-硅界面的距离;$ \theta $为热电子的动量的方向;l(E)为热电子的自由程,由lee和lep根据马西森(Matthiessen)规则可以计算获得,即1/l(E)=1/lee + 1/lep。
因此,为提升热电子成功到达肖特基结界面的概率,可以从两个方面入手。首先是增加热电子的自由程。在金属材料内部缺陷较少的前提下,热电子的自由程主要由材料本身的类型所决定,因而可以采用具有较长自由程的材料。其次是缩短热电子初始位置与肖特基界面之间的距离。可通过调节入射光的电场分布使热电子的产生位置尽量靠近肖特基结界面,或者采用超薄金属材料将热电子限制在肖特基界面附近。
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热电子到达金属-硅肖特基界面时并不一定都能够注入到硅中,其能量还必须满足一定的条件。具体而言,根据福勒(Fowler)模型,热电子垂直于肖特基界面的动能分量要高于势垒高度才能注入到硅中[49]。可认为热电子高于费米能级EF的能量部分为动能,在这种情况下,设热电子高于费米能级的能量为Ed,则有[49]:
$$ {E_{\rm{d}}} = \frac{{{{\left( {\hbar {k_{\rm{d}}}} \right)}^2}}}{{2m}} $$ (2) 式中:$\hbar {k_{\rm{d}}}$为热电子的动量大小,${k}_{{\rm{d}}}=\stackrel{\rightharpoonup }{\left|{k}_{{\rm{d}}}\right|}$为动量对应的波矢的模。根据上式,垂直于金-硅界面的能量部分可以表示为[49]:
$$ {E_{{\rm{d}} \bot }} = \frac{{{{\left( {\hbar {k_{{\rm{d}} \bot }}} \right)}^2}}}{{2m}} $$ (3) 式中:${k_{{\rm{d}} \bot }}$表示垂直于金-硅界面的动量。因此,只有这部分能量大于肖特基势垒高度的热电子才有可能注入到硅中,即满足如下的条件[37]:
$$ \hbar {k_{{\rm{d}} \bot }} > \sqrt {2m\varphi } $$ (4) 上式表示的注入条件对应的图像如图5所示,即在动量空间中,只有动量分布在红色锥形区内的那些热电子才可能注入。这个锥形区对应的临界角度为Ω,满足关系${k_{\rm{d}}}\cos \varOmega = \sqrt {2 m\varphi /{\hbar ^2}}$。因此,电子注入的概率就是对这个阴影区的立体角进行积分,再除以总的立体角,即有[49]:
图 5 能够注入到硅中的热电子动量的锥形分布
Figure 5. The conical distribution of the momentum of hot electrons that can be injected into silicon
$$ P\left( {{E_{\rm{d}}}} \right) = \frac{{{\varOmega _{\rm{s}}}}}{{4\pi }} = \frac{1}{{4\pi }}\int\limits_0^{2\pi } {\int\limits_0^\varOmega {\sin \theta {\rm{d}}} } \theta {\rm{d}}\varphi = \frac{1}{2}\left( {1 - \cos \varOmega } \right) $$ (5) 又由于${k_{\rm{d}}}\cos \varOmega = \sqrt {2 m\varphi /{\hbar ^2}}$,可化简为$\cos \varOmega = \sqrt {\varphi /({\hbar ^2}{k_{\rm{d}}}/2 m)} = \sqrt {\varphi /{E_{\rm{d}}}}$。因此,上式可进一步简化为[49]:
$$ P\left( {{E_{\rm{d}}}} \right) = \frac{1}{2}\left( {1 - \sqrt {\frac{\varphi }{{{E_{\rm{d}}}}}} } \right) $$ (6) 从公式(6)可以看出,热电子的注入概率与其本身的能量Ed密切相关,Ed越大,注入效率越高。当Ed远远大于势垒高度φ时,注入概率趋近于0.5。这意味着高能量的热电子,只要其动量方向指向金属-硅界面,就有机会实现注入;但能量稍低的热电子要在动量方向在立体角Ω之内才能注入,如图5所示。未能成功注入的热电子会在界面处被反射回金属内部,其能量因各种散射过程而逐渐降低,这大大降低了它们再次注入到硅中的可能性。为了提高热电子的注入几率,可以采取多种策略。一种策略是采用双肖特基结、体肖特基结、粗糙表面等,这些结构通过增加金属与硅之间的接触面,扩大了可注入的角度范围,进而提高了注入效率。另一种策略是降低肖特基结势垒高度,但同时也会提高暗电流,进而影响探测器的信噪比。因此需综合考虑探测器的信噪比来决定是否采用该方法。
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综上所述,硅基热电子短波红外探测技术近年来取得了显著进展,表1详尽地总结了代表性硅基热电子探测器的性能。这些探测器响应度通常在mA/W的量级,暗电流一般在10−4~10−8 A/cm²的范围内,对应的比探测率在106~1010 Jones的区间。值得注意的是,部分采用创新结构设计的硅基热电子器件展现出了更为卓越的性能,例如Feng等人提出的一种基于超表面天线结构的器件[73],其比探测率达到了1011 Jones的量级,这说明硅基热电子探测器在性能提升方面仍然具有潜力。
表 1 近年来硅基热电子短波红外探测器汇总
Table 1. Summary of silicon-based hot electron shortwave infrared detectors in recent years
Device type/
Enhancement strategyResponsivity Dark current/
Dark current densityDetectivity/
JonesNEP/
W·Hz−1/2Year Au film on Si nanobowl[35]
Hot electron generation0.34 mA·W−1@1300 nm
0.29 mA·W−1@1500 nm2.5×10−7 A None 8.3×10−10 2021 Disordered Au/
Si nanoneedles[50]
Hot electron generation2.56 mA·W−1@1300 nm
0.33 mA·W−1@1500 nm4×10−10 A None 4.4×10−12
@1300 nm2023 Au antennas/Ti/Si[51]
Hot electron generation10 μA·W−1@1250 nm
3 μA·W−1@1550 nmNone None None 2011 Deep-trench/thin Au/
Si antenna[52]
Hot electron generation3 mA·W−1@1300 nm
1.25 mA·W−1@1400 nm
0.5 mA·W−1@1550 nm8.7×10−9 A·cm−2 5.68 ×1010
2.37 ×1010
9.47 ×109None 2014 Metamaterial Au/Si[53]
Hot electron generation3 mA·W−1@1300 nm None None None 2014 Two distributed Bragg reflectors (DBRs)
Al2O3/TiO2 Al2O3/TiO2/Au[54]
Hot electron generation27 mA·W−1@813 nm None None None 2021 Disordered Au Si NHs[55]
Hot electron generation1.5-13 mA·W−1
@1100-1500 nm1×10−6 A None 4.75×10−11-
3.77×10−102018 Au grating/Si[16]
Hot electron generation0.6 mA·W−1@1460 nm None None None 2013 Material-embedded
Trenchlike thin Au/Si[61]
Hot electron generation5854 nA·mW−1@1310 nm
693 nA·mW−1@1550 nmNone None None 2019 Au/Si pyramid [22]
Hot electron generation5.2 mA·W−1@1200 nm None None None 2020 Au NWs embedded in Si[66]
Hot electron injection0.065 mA·W−1@1500 nm 1×10−9 A 3.63×106 2.75×10−10 2013 TiN/thin Au stripe
embedded in Si[68]
Hot electron injectionExceed 1.0 A·W−1@1550 nm None None None 2016 Thin film TiN/p-Si[69]
Hot electron injection1 mA·W−1@1250 nm 3×10−10 A 6.12×108 9.8×10−12 2019 Waveguide-based Al/Si[70]
Hot electron injection12.5 mA·W−1
@1550 nm (0.1 V)3×10−8 A
(0.1 V)None 7.84×10−12 2012 Metasurface Au/Si[73]
Hot electron transfer
and injection94.5 mA·W−1
@1150 nm (1.5 V)1.45×10−7
A·cm−24.38 ×1011 None 2019 Partially metalizing the
pyramid Al/SiO2/Si[34]
Dark current suppressionNone Reduce by
2 timesNone None 2021 Graphene/Al2O3/Ge[74]
Dark current suppression1.2 A·W−1@1550 nm (2 V) 1×10−6 A None None 2021 Interface engineering
assisted graphene/Si[75]
Dark current suppressionNone@890 nm 7.2×10−10 A 9.3×1012 1.8×10−12 2022 ITO/ Thin Ag/n-Si[76]
Dark current suppression0.05 A·W−1@1550 nm (2 V) 2.4×10−6
A/cm2 (−1 V)None None 2018 Thin ITO/Au/
Au Nanoparticle/n-Si[43]
Dark current suppression2.82 mA·W−1@1310 nm
(−1 V)4.4×10−5
A/cm2 (−1 V)None None 2022 Graphene with
polyethyleneimine/p-Si[77]
Dark current suppression0.3 A·W−1@850 nm 2.4×10−10 A 5.9×1010 None 2021 NanoalloysAu40Ag60/Si[78]
Hot electron transfer7.3 mA·W−1@1310 nm
1.9 mA·W−1@1550 nmNone None None 2024 Au/crystallized Ge/Si[79]
Hot electron injection0.71 A·W−1@1310 nm
0.64 A·W−1@1550 nm (1 V)None None None 2022 TalrTe4/Si[80]
Hot electron transfer14 mA·W−1@1310 nm
1.32 mA·W−1@1550 nmNone None None 2022 Mo2C/MoGeSiN4/Si[81]
Hot electron transfer176 mA·W−1@1550 nm None None None 2022 尽管硅基热电子探测器在兼容CMOS制造工艺和成本效益方面具有显著优势,但与铟镓砷、锗、量子点等传统短波红外探测器相比,商用铟镓砷光电探测器具备高响应度,通常达到A/cm2的量级,同时暗电流保持在10−9 A/W的级别,其比探测率普遍在1012~1014 Jones范围内。同样,锗和量子点短波红外探测器在响应度、暗电流和比探测率等方面均表现出与铟镓砷探测器相近的性能。
展望未来,硅基热电子短波红外探测技术的发展仍具有广阔的前景。通过不断优化器件结构、提高材料性能以及探索新的机制,有望进一步提升硅基热电子探测器的性能,缩小与传统短波红外探测器之间的性能差距。具体而言,可从以下几个方面进一步开展研究:
1)在新材料研究方面,可开发性能更为卓越的热电子材料。重点分析材料的热电子自由程和电子态密度两个核心属性,它们对硅基热电子探测器的光电转换效率有着显著的影响。一方面,平均自由程越长,热电子越容易穿越材料内部并成功注入到肖特基界面,从而提升光电转换效率。另一方面,材料的电子态密度亦至关重要,它决定了热电子的初始能量分布。理想情况下,期望材料在费米能级附近表现出很高的电子态密度,这样电子在受到光子激发时,更可能形成高能量的电子,进而进一步提升光电转换效率。为了获得上述材料性质,可考虑探索金属合金材料[78]、导电氧化物、金属硅化物、金属氮化物和新型二维材料等,这些材料可能提供优化热电子发射性能的新途径[79−80],推动硅基热电子短波红外探测器性能的进一步提升。
2)在新结构研究方面,需解决两个重要的问题。首先是如何通过创新的微纳结构设计,实现宽带宽红外光的高效吸收。尽管当前在提升探测器吸收率方面的工作已经较为丰富,且吸收率也能达到较高水平,但如何在保证宽带宽的同时实现高吸收率,仍然是一个具有挑战性的任务。其次,需深入探索肖特基界面结构与其性质之间的关系。肖特基界面的特性不仅直接关系到热电子的注入效率,还会对探测器的暗电流产生显著影响。因此,如何精心设计界面结构,例如通过引入二维材料、调控界面表面态和粗糙度等手段,提升热电子注入效率,同时保持或降低暗电流水平,也是值得深入研究的课题。
3)在新机制研究方面,可探索更高效的光电转换原理,以突破当前硅基热电子探测器的性能瓶颈。目前,多数报道的硅基热电子探测器遵循着热电子产生、传输和注入的经典理论框架。然而,在这一框架下的每一个环节,均存在多个热电子能量的损耗路径,这难以避免地导致了光电转换效率的低下。因此,需寻求改变这种经典光电转换机制。例如,近期有报道指出,热电子在受到光激发后能够直接从金属跃迁到硅材料中,这种直接且高效的过程有望显著提升光电转换效率[82−83]。此外,还可以借鉴近年来在半金属二维材料中发现的热电子的光热电效应等新型机制[84−91]。这些探索可能会从根本上突破现有原理的限制,为未来的红外探测技术发展开辟新的路径。
总之,硅基热电子探测技术能够将硅的响应波段拓展至短波红外波段,同时与硅基半导体工艺兼容,具有低成本和高均一性等优势,有望在军事、安防、环境监测等多个领域发挥重要的作用。文中从肖特基热电子探测技术的光电转换机制出发,对热电子的产生、传输及注入三个核心环节进行系统回顾。并针对上述三个环节所存在的能量损耗过程分析了相应的应对措施或优化策略。此外,也探讨了暗电流的抑制方法,为硅基热电子短波红外探测器的进一步优化与应用提供参考。
Silicon based hot electron short wave infrared detection technology (cover paper·invited)
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摘要: 短波红外具有穿透烟雾的特性,可在低光照环境或恶劣天气条件下成像,在军事、安防、环境监测等多个领域展现出重要应用价值。硅基热电子短波红外探测技术因具备与CMOS半导体工艺兼容、响应波段灵活可调等独特优势,已成为当前研究的热点。文中系统地回顾了该领域的国内外研究进展,剖析了与光电转换效率密切相关的热电子产生、传输与注入等物理过程中的能量损耗机制。在此基础上,总结了针对性的改进策略,包括通过光学吸收增强和热损耗抑制来增加热电子的产生效率;通过精确调控热电子的初始位置、能量、动量分布及自由程来优化其传输过程;以及利用肖特基结和界面调控等技术来提高热电子的注入概率。此外,文中还讨论了暗电流的抑制方法,以期提升探测器的整体性能。最后,展望了硅基热电子红外光电探测器未来的发展方向。Abstract:
Significance Short wave infrared detectors, as a very important type of detector, play a crucial role in sensing and obtaining target image information. Their notable features include the ability to penetrate smoke, high spatial recognition, all-weather working ability, and applicability in harsh weather conditions, making it widely applicable in multiple fields of national major needs and national economic development. In the military field, shortwave infrared detectors, with their unique night vision and covert reconnaissance functions, have become a key tool for enhancing combat capabilities at night and in adverse weather conditions. In the field of security monitoring, it provides strong technical support for video monitoring under low or no light conditions, significantly enhancing security capabilities. In terms of environmental monitoring, these detectors provide valuable data support for environmental protection and climate research by accurately measuring specific components in the atmosphere. In addition, in the medical field, the application of shortwave infrared detectors in disease diagnosis has opened up new paths for medical technology innovation. Therefore, in-depth research on shortwave infrared detectors has important practical significance. Progress This article systematically reviews the photoelectric conversion mechanism of Schottky photodetector, and summarizes and analyzes recent research results at home and abroad around the basic physical processes of hot electrons. This article first introduces the formation and basic characteristics of metal silicon Schottky junctions, and explores the three core processes of hot electron generation, transmission, and injection. Next, in terms of the generation of hot electrons, a review is conducted on the relevant work of researchers to improve the efficiency of hot electron generation through methods such as light absorption enhancement and thermal loss suppression. In terms of the transfer of hot electrons, the current proposed methods to control the initial position, initial energy and momentum, and mean-free path of hot electrons have been summarized to improve the transfer efficiency of hot electrons. In the injection method of hot electrons, strategies to improve injection efficiency such as multiple Schottky junctions and interface engineering were introduced. In addition, considering the crucial impact of dark current on detector performance, this article also explores current methods for suppressing dark current. Finally, this article provides an outlook on the future development direction of this field. Conclusions and Prospects Silicon-based hot electron detection technology holds the potential to broaden the response band of silicon to include the short-wave infrared band, while maintaining compatibility with silicon-based semiconductor processes. Its advantages, including low cost and high uniformity, bode well for its significant role in diverse fields such as military applications, security, and environmental monitoring. Looking ahead, it is imperative to delve deeper into the research of novel materials, structures, and mechanisms to further enhance the detector's performance. By focusing on developing new materials that can enhance the mean-free path of electrons and optimize the density of states, the transport efficiency of hot electrons can be boosted. Concurrently, the pursuit of innovative structures that efficiently absorb wide-spectrum infrared light, coupled with the optimization of the Schottky interface to increase hot electron injection efficiency and minimize dark current, is paramount. Moreover, exploring novel photoelectric conversion mechanisms that transcend the constraints of classical frameworks offers a promising avenue for pioneering advancements in infrared detection technology. -
Key words:
- short wave infrared /
- hot electrons /
- detector /
- Schottky junction
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图 1 金属-硅肖特基结形成的原理示意图。 (a)金属和硅接触前的能带图; (b)金属和N型硅接触后的肖特基结能带图;(c)金属和P型硅接触后的肖特基结能带图;(d)肖特基结的典型电流-电压特性曲线
Figure 1. Schematic diagram of the formation principle of metal-silicon Schottky junction. (a) Energy band diagram before the contact between metal and silicon; (b) Energy band diagram of Schottky junction after the contact between metal and N-type silicon; (c) Energy band diagram of Schottky junction after the contact between metal and P-type silicon; (d) Typical current-voltage characteristic curve of Schottky junction
图 6 (a)、(b)基于一维和二维超材料完美吸收器的光电探测器示意图;(b)、(c)和(e)、(f)分别为(a)和(b)图探测器对应的吸收和响应曲线
Figure 6. (a), (b) Schematic diagram of a photodetector based on a one - and two-dimensional metamaterial perfect absorber; (b), (c) and (e), (f) are the absorption and response curves corresponding to the detectors in figure (a) and (b), respectively
图 8 (a)随机结构增强热电子探测器示意图;(b)不同退火温度下制备的Au/SiNH结构的吸收光谱;(c)不同Au涂层厚度的Au/SiNH器件的响应度;(d)不同照明模式下的响应度;(e)不同温度退火制备的Au/SiNH的扫描电镜(SEM)图像
Figure 8. (a) Schematic diagram of a random structure enhanced thermionic detector; (b) Absorption spectra of Au/SiNH structures prepared at different annealing temperatures; (c) The responsiveness of Au/SiNH devices with different Au coating thicknesses; (d) Responsiveness under different lighting modes; (e) Scanning electron microscopy (SEM) images of Au/SiNH prepared by annealing at different temperatures
图 9 (a)半无限表面、 (b) 40 nm、(c) 20 nm和(d) 10 nm直径球体中的热损耗、几何辅助、声子辅助和直接跃迁占吸收总能量的百分比随频率变化的关系
Figure 9. The frequency variation of the percentage of heat loss, geometrical assist, phonon assist and direct transition to the total absorbed energy in (a) semi-infinite surface, (b) 40 nm, (c) 20 nm and (d) 10 nm diameter spheres
图 12 (a) Au和Pt肖特基探测器在1510 nm波长下的三种主要热电子损耗机制占比;(b)定量比较了20 nm厚度的六种金属肖特基探测器的外量子效率;(c) Cu,Ni,Ag,Au,Pt 五种金属对应得吸收率,注入概率,平均自由程以及量子效率
Figure 12. (a) The proportion of three main thermoelectronic loss mechanisms of Au and Pt Schottky detectors at 1510 nm; (b) Quantitative comparison of the external quantum efficiency of six metal Schottky detectors with a thickness of 20 nm; (c) The five metals Cu, Ni, Ag, Au, Pt pair absorption rate, injection probability, mean free path and quantum efficiency
图 13 (a)从左到右依次是铝、银、铜和金的能带结构和热载流子能量分布与入射光子能量的关系(上图为热电子可能发生的跃迁在能带中的位置,下图为热载流子的能量分布); (b)从左到右依次是铝、银、铜和金的能量和动量方向分布(上图为热电子的能量和动量方向分布,下图为空穴能量和动量方向分布)
Figure 13. (a) From left to right, the band structure of aluminum, silver, copper and gold and the relationship between the hot carrier energy distribution and the incident photon energy are shown (The above figure shows the position of the possible transition of hot electrons in the energy band, and the following figure shows the energy distribution of hot carriers); (b) From left to right are the directional distributions of energy and momentum for aluminum, silver, copper and gold (The above figure shows the direction distribution of energy and momentum of hot electrons, and the following figure shows the direction distribution of energy and momentum of holes)
图 14 (a)铝、(b) 银、(c) 铜和 (d)金的能带结构和热载流子能量分布与入射光子能量的关系(其中,上图为热电子可能发生的跃迁在能带中的位置,下图为热载流子的能量分布)
Figure 14. The relationship between the energy distribution of (a) aluminum, (b) silver, (c) copper and (d) gold and the energy of incident photons(Among them, the above figure shows the position of the possible transition of hot electrons in the energy band, and the following figure shows the energy distribution of hot carriers)
图 15 (a)金属/TiO2−x/p-Si探测器结构及对光电响应的提升效果; (b)粗糙界面示意图;(c)在金属半导体界面的不同粗糙度下,热电子注入概率与电子能量的关系,Λ越小表示粗糙度越大;(d)金属-半导体结引入量子阱后的能带图。虚线为肖特基势垒,量子阱中的准离散能级用红色粗线表示
Figure 15. (a) Metal/TiO2−x/p-Si detector structure and its effect on photoelectric response; (b) Rough interface diagram; (c) The relationship between the probability of hot electron injection and electron energy under different roughness of metal semiconductor interface, the smaller Λ indicates the larger roughness. (d) Band diagram of metal-semiconductor junction after introduction of quantum well. The dashed line is the Schottky barrier, and the quasi-discrete energy levels in the quantum well are represented by the thick red line
表 1 近年来硅基热电子短波红外探测器汇总
Table 1. Summary of silicon-based hot electron shortwave infrared detectors in recent years
Device type/
Enhancement strategyResponsivity Dark current/
Dark current densityDetectivity/
JonesNEP/
W·Hz−1/2Year Au film on Si nanobowl[35]
Hot electron generation0.34 mA·W−1@1300 nm
0.29 mA·W−1@1500 nm2.5×10−7 A None 8.3×10−10 2021 Disordered Au/
Si nanoneedles[50]
Hot electron generation2.56 mA·W−1@1300 nm
0.33 mA·W−1@1500 nm4×10−10 A None 4.4×10−12
@1300 nm2023 Au antennas/Ti/Si[51]
Hot electron generation10 μA·W−1@1250 nm
3 μA·W−1@1550 nmNone None None 2011 Deep-trench/thin Au/
Si antenna[52]
Hot electron generation3 mA·W−1@1300 nm
1.25 mA·W−1@1400 nm
0.5 mA·W−1@1550 nm8.7×10−9 A·cm−2 5.68 ×1010
2.37 ×1010
9.47 ×109None 2014 Metamaterial Au/Si[53]
Hot electron generation3 mA·W−1@1300 nm None None None 2014 Two distributed Bragg reflectors (DBRs)
Al2O3/TiO2 Al2O3/TiO2/Au[54]
Hot electron generation27 mA·W−1@813 nm None None None 2021 Disordered Au Si NHs[55]
Hot electron generation1.5-13 mA·W−1
@1100-1500 nm1×10−6 A None 4.75×10−11-
3.77×10−102018 Au grating/Si[16]
Hot electron generation0.6 mA·W−1@1460 nm None None None 2013 Material-embedded
Trenchlike thin Au/Si[61]
Hot electron generation5854 nA·mW−1@1310 nm
693 nA·mW−1@1550 nmNone None None 2019 Au/Si pyramid [22]
Hot electron generation5.2 mA·W−1@1200 nm None None None 2020 Au NWs embedded in Si[66]
Hot electron injection0.065 mA·W−1@1500 nm 1×10−9 A 3.63×106 2.75×10−10 2013 TiN/thin Au stripe
embedded in Si[68]
Hot electron injectionExceed 1.0 A·W−1@1550 nm None None None 2016 Thin film TiN/p-Si[69]
Hot electron injection1 mA·W−1@1250 nm 3×10−10 A 6.12×108 9.8×10−12 2019 Waveguide-based Al/Si[70]
Hot electron injection12.5 mA·W−1
@1550 nm (0.1 V)3×10−8 A
(0.1 V)None 7.84×10−12 2012 Metasurface Au/Si[73]
Hot electron transfer
and injection94.5 mA·W−1
@1150 nm (1.5 V)1.45×10−7
A·cm−24.38 ×1011 None 2019 Partially metalizing the
pyramid Al/SiO2/Si[34]
Dark current suppressionNone Reduce by
2 timesNone None 2021 Graphene/Al2O3/Ge[74]
Dark current suppression1.2 A·W−1@1550 nm (2 V) 1×10−6 A None None 2021 Interface engineering
assisted graphene/Si[75]
Dark current suppressionNone@890 nm 7.2×10−10 A 9.3×1012 1.8×10−12 2022 ITO/ Thin Ag/n-Si[76]
Dark current suppression0.05 A·W−1@1550 nm (2 V) 2.4×10−6
A/cm2 (−1 V)None None 2018 Thin ITO/Au/
Au Nanoparticle/n-Si[43]
Dark current suppression2.82 mA·W−1@1310 nm
(−1 V)4.4×10−5
A/cm2 (−1 V)None None 2022 Graphene with
polyethyleneimine/p-Si[77]
Dark current suppression0.3 A·W−1@850 nm 2.4×10−10 A 5.9×1010 None 2021 NanoalloysAu40Ag60/Si[78]
Hot electron transfer7.3 mA·W−1@1310 nm
1.9 mA·W−1@1550 nmNone None None 2024 Au/crystallized Ge/Si[79]
Hot electron injection0.71 A·W−1@1310 nm
0.64 A·W−1@1550 nm (1 V)None None None 2022 TalrTe4/Si[80]
Hot electron transfer14 mA·W−1@1310 nm
1.32 mA·W−1@1550 nmNone None None 2022 Mo2C/MoGeSiN4/Si[81]
Hot electron transfer176 mA·W−1@1550 nm None None None 2022 -
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