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评价光电探测器性能依赖多个重要的基本参数。因此,理解关键性能参数的准确含义非常重要。以下列举出文献中经常给出的一些性能参数,并给予适当的描述。
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光响应度(R)是最常用的表征光电探测器灵敏度的一个参数,定义为照射在光电探测器光敏面上单位功率引起的光电流,即:
$$ R = \frac{{{I_{{\text{ph}}}}}}{P} $$ (1) 式中:Iph为对应的光电流(光电流为光照下器件的电流减去暗态下的电流,即
$ {I_{{\text{ph}}}} = {I_{{\text{light}}}} - {I_{{\text{dark}}}} $ ,Ilight为光照下的电流,Idark为暗态下的电流);P为照射在光敏面上的光功率。如果信号用电压来表示,则称为电压响应度,可定义为一定波长光照下,光电探测器的输出平均光电流Iph,与平均入射光功率P之比,也可用电压来表示,即:$$ R = \frac{{{V_{\text{ph}}}}}{P} $$ (2) 光响应度表征了探测器入射光信号转换为电信号的能力,光响应度越大,说明单位光功率照射下可以获得更大的光电流/光电压。因此,一般而言光响应度越大越好。光响应度一般是入射光波长和功率的函数,即R = R(λ, P)。
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量子效率一般又分为内量子效率和外量子效率。内量子效率指材料中产生的光生电子-空穴对同吸收的光子数之比。一般而言,器件层面,更值得关注的是入射到材料上的总光子数引起的光生载流子数目,所以往往不计算内量子效率,而是给出外量子效率(η),即光生载流子量和入射的光子数之比。对于光电探测器,外量子效率可以写为:
$$ \eta = \frac{{{I_{{\text{ph}}}}/e}}{{P/hv}} $$ (3) 式中:Iph为光生电流;e为单个电子所带的电荷量;h为普朗克常量;v为入射光频率; P为入射光功率。量子效率是描述光电器件光电转换能力的参数,与材料的光吸收系数与载流子迁移率有着密不可分的联系。一般来说,对于结型器件,如肖特基光电二极管和pn结二极管,没有发生雪崩的情况下,增益小于1,外量子效率小于1;对于光电导器件,由于存在增益,外量子效率可大于1。由外量子效率的定义可知光响应度和外量子效率存在如下关系:
$$ \eta = \frac{{hv}}{e} \cdot R $$ (4) 由于R是波长和光功率的函数,η也同样是波长和光功率的函数。
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对于光电探测器而言,并非响应度越高,探测器探测微弱信号的能力越强。当器件对微弱信号的响应和噪声处于相同的水平时,信号就难以同噪声区分开。因此,噪声的大小限制了探测器所能探测的微弱信号的水平。为了表征噪声的大小,引入了噪声等效功率(NEP),其定义为当信号电流Iph同噪声电流in相等时对应的辐照在探测器上的光功率,即探测器的输出信号与噪声功率比值为1时对应的光功率。然而,在通常情况下,信噪比为1时,信号很难直接测量,因此转而在较高信号电平上通过线性外延得到NEP,如公式(5)所示:
$$ NEP = \frac{P}{{{I_{{\text{ph}}}}/{i_{\text{n}}}}} $$ (5) 式中:
$ {i_{\text{n}}} $ 为噪声电流。由响应度的定义,可以得到NEP同响应度R的关系:$$ NEP = i_{{\text{n}}}/R $$ (6) 显然,NEP正比于噪声电流,反比于响应度。
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为了更好地表征光电探测器的性能,将响应度和等效噪声功率结合,定义了一个新的灵敏度参数:探测度(D),其定义为噪声等效功率的倒数,即D = 1/NEP。显然,NEP越小,D越大,说明器件探测性能越好。因为NEP与探测器的光敏面积(A)和工作带宽相关(Δf ),因此D也与A和Δf相关。由于D的这个特性,不同探测器之间的优劣比较难以进行,所以需要把与探测器光敏面积和工作带宽相关的参数排除。为此,定义了比探测度D*(又称为归一化探测度):
$$ {D^*} = \frac{{{{(A\Delta f)}^{1/2}}}}{{NEP}} $$ (7) 比探测度D*的物理含义是当探测器具有单位面积的光敏面积,工作带宽为1 Hz时,单位入射光功率所能获得的信号噪声比,其单位为cm·Hz1/2/W,或写为Jones。比探测度D*避免了由于光敏面积、工作带宽带来的影响,在一定程度上可区分光电探测器的性能。
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响应时间分为上升时间和下降时间。上升时间指的是从给予光照探测器输出信号从0升高到稳定值的1−1/e倍,即稳定值的63%所用的时间。下降时间则是指去除光照后,探测器输出信号由稳定值下降为原来的1/e倍,即稳定值的37%所用时间。除此以外,另外一种常用的响应时间的定义为上升阶段从稳定值的10%变到90%所用时间为上升时间,下降阶段从稳定值的90%下降到10%所用时间为下降时间。
响应时间表征了器件对光照变化的响应速度,对于实际应用来说非常重要。在大多数场合都要求具有很快的响应速度。限制光子型器件的响应速度因素主要有:载流子迁移率、载流子寿命和电路的RC常数。对于光电导型器件,主要受到载流子寿命的限制,对于肖特基和pn结光电二极管,限制因素主要是载流子迁移率,只有在响应速度非常快的情况下电路的RC常数将成为约束响应时间的瓶颈。
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对于红外光电探测器,要求其截止波长在红外光范围之内。对于光子型红外探测器,探测器的截止波长一般由材料的能带宽度(Eg)决定,工作波长满足下式:
$$ \lambda < {\lambda _{\text{c}}} = \frac{{1.24}}{{{E_{\text{g}}}}} $$ (8) 式中:波长λ的单位为μm;能带宽度Eg的单位为eV。对于近红外探测器,要求截止波长λc > 1.1 μm (对应带隙Eg < 1.13 eV);短波红外探测器,要求λc > 2.5 μm (对应带隙Eg < 0.5 eV);中波红外探测器,要求λc > 5 μm(对应带隙Eg < 0.25 eV);长波红外探测器,要求λc > 14 μm (对应带隙Eg < 0.09 eV)。由此可见,随着截止波长的增加,需要半导体的带隙更窄。除了利用带间跃迁产生光生载流子的方法,也可以利用缺陷能级到带边的跃迁产生光生载流子,这样就可以在带隙较大的材料中实现对红外光的吸收。
Recent advances in two-dimensional materials in infrared photodetectors (invited)
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摘要: 二维材料,具有原子厚度,以其独特的物理、化学性质,吸引了广大研究人员的关注,成为了众多研究领域(如物理、材料、电子、光电子和化学等)的明星材料。因二维材料具有较高的载流子迁移率、强的光-物质相互作用、电/光学性质各向异性等,使其在光电子领域具有光明的应用前景。其中,窄带隙二维材料,如黑磷、黑磷砷等,在红外光电探测器中表现出巨大的应用潜力,成为了红外探测领域的新宠。文中将对二维材料在红外探测器中的应用,特别是光子型光电探测器的最新进展进行介绍。首先对二维材料的背景进行介绍;然后介绍表征光电探测器的关键参数;接着介绍二维材料在红外探测器中的最新进展,分别展示了单二维材料红外探测器、异质结红外探测器和光波导红外探测器方面的进展;最后对二维材料在红外探测器中的应用进行展望。Abstract: Two-dimensional (2D) materials, which have a thickness on the atomic scale, have attracted wide attention due to their unique physical and chemical properties. Because of their high carrier mobility, strong light-matter interaction, and anisotropic electronic/optical properties, etc., 2D materials show promising applications in optoelectronics. Among the 2D materials, narrow band gap semiconductors, such as black phosphorus, black arsenic phosphorus, etc., have shown huge potential in infrared photodetectors and have become star materials in infrared photodetectors. In this review, recent advances in 2D materials in infrared photodetectors are introduced, with an emphasis on photodetectors depending on the inner photoelectronic effect. First, the background of 2D materials is introduced. Then, the key parameters for infrared photodetectors, such as the responsivity, quantum efficiency, specific detectivity, and response speed, are listed. This is followed by the presentation of the recent advances of 2D materials in infrared photodetectors, which is divided into three parts: single component 2D material photodetectors, heterostructure infrared photodetectors, and waveguide photodetectors. Finally, a summary and outlook are provided for a guideline. We hope the present review will show the huge potential of 2D materials in infrared photodetectors and attract more exciting work on infrared photodetectors based on 2D materials in the future.
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Key words:
- two-dimensional materials /
- heterostructures /
- photodetectors /
- infrared
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图 1 BP和b-AsP光电探测器。(a) BP的晶体结构[36];(b) 电场对BP带隙的调控[38];(c) 电场对BP光响应的调控[37]。应变对BP能隙和光电探测的调控[39]:(d) 应变对光致发光的调控;(e) 在PETG柔性衬底上光电探测器示意图;(f) 光响应度同波长和应变的关系。b-AsP光导型探测器[42]:(g) 含P 17%的b-AsP吸收谱;(h) 光响应度和外量子效率同波长的关系
Figure 1. BP and b-AsP photodetectors. (a) Crystal structure of BP[36]; (b) Modulation of the bandgap of BP by electrical field[38]; (c) Modulation of the photoresponse of BP by electrical field[37]. Modulation of the bandgap and photoresponse of BP by strain[39]: (d) Modulation of photoluminescence by strain; (e) Schematic of the BP photodetector on PETG flexible substrate; (f) Photoresponsivity vs wavelength and strain. b-AsP photodetector[42]: (g) Absorbance spectrum of b-AsP with 17% P; (h) Photoresponsivity and EQE vs wavelength
图 2 过渡金属硫族化合物光电探测器。(a) MoTe2光导型探测器响应光谱[51];(b) Au空穴纳米棒增强MoTe2光导型探测器增强因子同波长的关系[52]。PtSe2光导型探测器[54]:(c) PtSe2晶体结构;(d) 单层PtSe2光导型探测器动态响应;(e) 双层PtSe2光导型探测器动态响应;(f) PtSe2带隙同缺陷浓度的关系
Figure 2. Photodetectors based on transition metal chalcogenides. (a) Photoresponse spectrum of MoTe2 photodetector[51]; (b) Enhancement factor vs wavelength of the Au hollow nanorod enhanced MoTe2 photodetector[52]. PtSe2 photodetector[54]: (c) Crystal structure of PtSe2; (d) Dynamic photoresponse of monolayer PtSe2 photodetector; (e) Dynamic photoresponse of bilayer PtSe2 photodetector; (f) Bandgap of PtSe2 vs defect concentration
图 3 Te纳米片光导型探测器。(a) Te的晶体结构[66]。基于水热法制备的Te纳米片光导型探测器[67]:(b) 器件结构和实物显微镜照片;(c) 响应度与波长和激光功率的关系;(d) 对2.3 μm光的各向异性响应。基于化学气相沉积法合成的Te纳米材料的光导型探测器[68]:(e) 纳米线和纳米片光电探测器的黑体辐射响应;(f) 纳米片光电探测器的各向异性光响应
Figure 3. Te nanosheet photodetector. (a) Crystal structure of Te[66]. Photodetector based on Te nanosheet from hydrothermal synthesis[67]: (b) Schematic of the photodetector; (c) Photoresponsivity vs wavelength and laser power; (d) Anisotropic photoresponse when irradiated by 2.3 μm light. Photodetectors based on Te nanostructures synthesized from chemical vapor deposition[68]: (e) Photoresponse of nanowire and nanosheet detector to black body irradiation; (f) Anisotropic photoresponse of nanosheet detector
图 4 Bi2O2Se探测器。(a) Bi2O2Se的晶体结构[69]。Bi2O2Se探测器性能[70]:(b) 光响应谱和光吸收谱;(c) 光响应度同偏压和入射光功率的关系;(d) 响应时间
Figure 4. Bi2O2Se photodetector. (a) Crystal structure of Bi2O2Se[69]. Performance of Bi2O2Se photodetector[70]: (b) Photoresponse spectrum and absorbance spectrum; (c) Photoresponsivity vs bias voltage and light power; (d) Response time
图 5 异质结光电探测器。InSe/BP异质结雪崩光电探测器[72]:(a) 器件结构示意图;(b) 弹道雪崩示意图;(c) 光响应特性。WS2/HfS2层间激子光电探测器[73]:(d) 器件结构示意图;(e) 光照下的电流-电压曲线;(f) 比探测度对比。BP/MoS2/石墨烯单极势垒光电探测器[28]:(g) 器件结构示意图;(h) 反向偏压下的能带结构示意图;(i) 比探测度随波长的变化
Figure 5. Heterostructure photodetectors. Avalanche photodetector based on InSe/BP heterostructure[72]: (a) Device structure; (b) Schematic of ballistic avalanche; (c) Photoresponse. Interlayer exciton photodetector based on WS2/HfS2 heterostructure[73]: (d) Device structure; (e) Current vs voltage under light irradiation; (f) Specific detectivity vs wavelength. Unipolar barrier photodetector based on BP/MoS2/graphene heterostructure[28]: (g) Device structure; (h) Band structure under reverse bias; (i) Specific detectivity vs wavelength
图 6 光波导探测器。 左右电极结构的MoTe2光波导探测器[74]:(a) 结构示意图;(b) 动态性能;(c) 鱼眼图(1 Gbit/s)。上下电极结构的MoTe2光波导探测器[75]:(d) 结构示意图;(e) 11 nm厚的MoTe2光波导探测器的动态性能。含应力的MoTe2光波导探测器[76]:(f) 器件结构示意图;(g) 器件实物图;(h) 器件中应变形成示意图
Figure 6. Waveguide photodetector. Photodetector based on MoTe2 with left and right electrodes configuration[74]: (a) Device structure; (b) Dynamic response; (c) Eye diagram (1 Gbit/s). Photodetector based on MoTe2 with top and down electrodes configuration[75]: (d) Device structure; (e) Dynamic response of the photodetector with 11 nm thick MoTe2. Photodetector based on strained MoTe2[76]: (f) Device structure; (g) Device picture; (h) Schematic of the formation of strain in the device
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