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相比于传统半导体材料，二维层状材料(如过渡金属硫族化合物)因其本征的面外方向量子限域效应，具有较大的激子结合能、较强的光与物质相互作用及层数相关的能带结构[1]。基于其独特的光学及光电子学特性，二维层状材料有望应用于高性能的光电探测器件[2-4]。
近年来，大量基于二维材料(如石墨烯[5]、MoS2[6]、WS2[7]、WSe2[8]、GaS[9]等)及其异质结(如石墨烯/MoS2[10]、MoS2/WSe2[11]、ReS2/Te[12]、石墨烯/MoS2/石墨烯[13]等)的高性能光电探测器被报道出来，其工作原理主要包括光电导效应[7]、光诱导栅控效应[6, 14]、光伏效应[15]、光-热电效应[16]等。其中，基于光诱导栅控效应的二维半导体平面型光电器件因其具有与传统晶体管相近的器件结构及工艺兼容性而备受关注。这些器件往往具有较高的响应度($R = {{{I_{ph}}} \mathord{\left/ {\vphantom {{{I_{ph}}} {{p_{ph}}}}} \right. } {{p_{ph}}}}$，其中${I_{ph}}$为光电流，$ {p_{ph}} $为辐照光功率)，但因受到高暗电流的限制而导致比探测率低($D = {{R\sqrt A } \mathord{\left/ {\vphantom {{R\sqrt A } {\sqrt {2 q{I_{dark}}} }}} \right. } {\sqrt {2 q{I_{dark}}} }}$，其中$A$为器件的有效面积，$q$为单位电荷，${I_{dark}}$为暗电流)。
为了进一步追求光电探测性能的提升，2017年，成均馆大学Young Hee Lee课题组构建了MoS2/h-BN/石墨烯垂直异质结构，通过调节载流子隧穿实现具有超高探测率的光电探测器，响应度达到180 A·W−1，比探测率达到2.6×1013 Jones[17]。2021年，哈尔滨工业大学胡平安教授课题组利用石墨烯/h-BN/SnS2垂直异质结亦实现了较高的光电探测性能[18]。此后，北京理工大学刘瑞斌教授课题组构建了WSe2/石墨烯/h-BN/MoS2范德华垂直异质结构，并在零偏压状态下得到了3.6 A·W−1的光响应度和1.2×105的开关比[19]。在范德华半导体异质结中(如上述MoS2/石墨烯或SnS2/石墨烯)插入h-BN绝缘层构建隧穿型光电探测器，可以有效抑制暗电流，实现高性能的光电探测。
在前述研究的基础上，文中进一步构建石墨烯/h-BN/MoS2/石墨烯垂直异质结构，提出并验证一种基于光生载流子FN隧穿的范德华垂直异质结光电探测器，以实现暗电流的抑制和响应速度的提升。
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石墨烯/MoS2/h-BN/石墨烯范德华垂直异质结器件结构如图1(a)所示，其中MoS2作为光吸收层，石墨烯(Gra)作为MoS2层的隧穿电极，在石墨烯与MoS2间插入的h-BN为其提供了一个有效的、厚度可调的隧穿势垒。器件测试时底部石墨烯(BGra)接地，顶部与MoS2直接相连的石墨烯(TGra)接电源的输出端，主要测量器件从TGra到BGra的隧穿电流。
图 1 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件结构与表征。(a) 器件结构示意图；(b) 器件光学图片及原子力显微镜表征；(c1), (c2) 结区MoS2、Gra拉曼光谱图及h-BN拉曼光谱图
Figure 1. Structure and characterization of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device; (a) Schematic of the device structure; (b) Corresponding optical microscopy image and AFM characterization of the real fabricated heterostructure device; (c1), (c2) Raman spectra of MoS2, graphene and h-BN
实际器件结构如图1(b)中光学图片所示，其中石墨烯、MoS2及h-BN皆采用机械剥离单晶方法获取。文中以14 nm厚度的h-BN垂直异质结为代表性器件展开研究，h-BN厚度利用原子力显微镜表征确定。图1(c)分别为石墨烯、MoS2及h-BN的拉曼光谱表征，从图中可以看出，MoS2的特征峰${\rm{E}}^1_ {\rm{2g}} $和A1g分别位于383 cm−1和408 cm−1处，石墨烯的G峰和2D峰分别位于1582 cm−1和2713 cm−1处，h-BN的E2g峰位于1364 cm−1处，与文献报道情况相符[17]。
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为了研究器件的光-电隧穿输运特性，分别测试了器件在暗场条件及光照条件下的输出曲线，如图2(a)、(b)所示。考虑到MoS2的光学带隙小于2 eV，在此选用实验室较为常用的波长532 nm (能量2.33 eV) 激光作为辐照光源，激光强度为1.48 μW。在I-V曲线中，不管是施加正向偏压还是负向偏压，都能明显观测到两个特征段，这与理论设计中石墨烯/MoS2/h-BN/石墨烯范德华垂直异质结在低偏压下的直接隧穿以及高偏压下的FN隧穿输运机理相符。
图 2 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件的光电输运特性。(a)施加负向偏压 和(b)正向偏压时，器件在暗场和532 nm光照下的电流-电压特性；(c) 施加负向偏压和(d) 正向偏压时，器件在暗场和532 nm光照下的FN特性曲线
Figure 2. Photocurrent characteristics of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device. The I-V characteristics of the device under dark and 532 nm laser illumination with applied (a) negative bias and (b) positive bias; FN tunneling fitting curves of the device under dark and 532 nm laser illumination with applied (c) negative bias and (d) positive bias
为了进一步证明石墨烯/MoS2/h-BN/石墨烯垂直异质结器件的输运机理，对器件的I-V曲线进行FN隧穿模型分析(图2(c)、(d))。在大偏压状态下的Fowler-Nordheim隧穿(FNT)满足以下模型：
$$ \begin{split} \\ {I_{{\text{FNT}}}} = \frac{{{A_{{\text{eff}}}}{q^3}m{V_{{\text{ds}}}}^2}}{{8\pi h{\varphi _{\text{B}}}{d^2}{m^*}}}\exp \left[ {\frac{{ - 8\pi \sqrt {2{m^*}} \varphi _{\text{B}}^{\tfrac{3}{2}}d}}{{3hq{V_{{\text{ds}}}}}}} \right] \end{split} $$ (1) 式中：Aeff、q、m、Vds、h、φB 、d、m*分别为有效接触面积、电子电荷、自由电子质量、施加偏压、普朗克常数、势垒高度、势垒宽度和有效电子质量。对公式(1)进行变化，得到：
$$ \ln \frac{{{I_{{\text{FNT}}}}}}{{{V_{{\text{ds}}}}^2}} = \ln \frac{{{A_{{\text{eff}}}}{q^3}m}}{{8\pi h{\varphi _{\text{B}}}{d^2}{m^*}}} - \frac{{8\pi \sqrt {2{m^*}} \varphi _{\text{B}}^{\tfrac{3}{2}}d}}{{3hq}}\frac{1}{{{V_{{\text{ds}}}}}} $$ (2) 根据公式(2)，将器件的输出特性以−1/Vds或1/Vds为横坐标、 ln(Ids/Vds2)为纵坐标重新绘制，得到图2(c)、(d)所示的结果。从图中可以明显观察到，随着$ \left|{V}_{ds}\right| $的增大，曲线的斜率从正变为负，并且在较大偏压时，ln(Ids/Vds2)与$\left|{V}_{{\rm{ds}}}^{-1}\right|$呈现较好的线性关系，符合FN隧穿模型规律，证明了器件的载流子输运机理是从小偏压时的直接隧穿主导转变为大偏压时的FN隧穿主导，其中转变电压记为VD-FN。从图2(c)、(d)中可以分别得到器件在施加负(正)向偏压时的转变电压VN-D-FN(VP-D-FN)，同时利用FN 隧穿曲线的拟合斜率可以得到势垒高度${\varphi _{\rm{B}}}$。考虑到与 h-BN 直接接触的石墨烯以及 MoS2 的能带结构更加靠近 h-BN 的价带，相比于电子隧穿，空穴隧穿的势垒高度更低，更容易发生FN隧穿。其中，当施加负向偏压时，主要考虑h-BN一侧石墨烯中空穴的直接隧穿或FN隧穿对电流的贡献，此时从FN曲线的拟合斜率可以得到石墨烯一侧空穴的隧穿势垒高度φh_Gra；而当施加正向偏压时，主要考虑MoS2中空穴的直接隧穿或FN隧穿对电流的贡献，此时从FN曲线的拟合斜率可以得到MoS2一侧空穴的隧穿势垒高度φh_MoS2。具体结果如表1所示。
表 1 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件在532 nm光照时的转变电压和势垒高度
Table 1. The transition bias and barrier height of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device under 532 nm laser illumination
VN-D-FN/V φh_Gra/eV VP-D-FN/V φh_MoS2/eV BNM-14 −8.4 2.4 eV 5.3 1.6 另一方面，在图2中，对比暗场I-V曲线及光照下的I-V曲线，可以观察到在施加正向偏压时(图2(b))存在明显的光电响应，光电流相比暗电流有明显提升；而在施加负向偏压时，光电响应不明显(图2(a))。表明光照对施加负向偏压时MoS2中空穴的直接隧穿或FN隧穿有显著影响，而对施加正向偏压时石墨烯中空穴的直接隧穿或FN隧穿影响不明显。
根据表1中的势垒绘制了器件的能带结构图(图3)，以下结合能带结构图对上述现象进行详细阐述。
如图3(a)所示，在不施加偏压时，h-BN为矩形势垒，并且整体能带结构更加偏向于h-BN的价带。
在暗场条件下，如图3(b)中左图所示，施加大负偏压时，BGra/h-BN处的势垒从梯形变成三角形，输出电流主要是由BGra中的空穴FN隧穿所贡献的；而在施加大正偏压时，如图3(c)中左图所示，MoS2中空穴FN隧穿通过MoS2/h-BN处的三角形势垒形成的隧穿电流占据主导地位。由于石墨烯中的空穴密度明显高于MoS2中的空穴密度，在暗状态下当施加大负偏压时器件暗电流较施加大正偏压时更大，这与图2(a)、(b) I-V曲线中的实验结果是相符的。
在光照条件下，石墨烯中产生热载流子，MoS2中产生光生载流子和热载流子。如图3(b)中右图所示，在大负偏压状态下，BGra中会产生一定量的热载流子，但由于石墨烯中本征载流子密度很高，热载流子并不能引起载流子浓度的明显提升，因而光照条件下隧穿电流变化不大。此外，具有较高能量的热载流子也能参与热载流子发射过程，但较宽的势垒能够有效抑制热载流子发射，使得热载流子发射对器件总电流的贡献较小，也不会引起光电流的明显变化。而在大正偏压下，如图3(c)中右图所示，光照条件下光生载流子使得MoS2中的载流子浓度显著提升，提高了隧穿通过势垒的载流子数目，总体光电流以空穴的FN隧穿电流为主，得到显著提高。
图 3 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件能带示意图。(a) 不施加偏压时器件能带结构图；(b) 偏压小于负向转变偏压时，器件在暗场和532 nm光照时的能带结构图；(c) 偏压大于正向转变偏压时，器件在暗场和532 nm光照时的能带结构图
Figure 3. Energy band diagrams of the graphene/MoS2/h-BN/graphene heterostructure. (a) Energy band diagram of the device with zero bias. (b) Energy band diagrams of the device under dark and 532 nm laser illumination at Vds < VN-D-FN; (c) Energy band diagrams of the device under dark and 532 nm laser illumination at Vds > VP-D-FN
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为了进一步研究器件的光电探测性能，在不同功率的532 nm波长激光照射下，测量了正向偏压下器件的输出特性。如图4(a)所示，随着光功率密度的增加，电流值也随之提高。从图4(b)中的FN特性曲线可以看出，直接隧穿转变成FN隧穿的拐点也随着入射光功率的增大而提前。对不同光功率下的光生电流进行拟合，可见器件的光电响应在该功率范围内呈现一定的线性关系(图4(c))。如图4(d)所示，器件的响应度随着光功率的增加而减小，在4.5 mW/cm2功率密度的光照射下，器件的响应度达到~140 mA/W；器件的光开关比随着光功率的增加而增大，在41.4 mW/cm2时达到~2.3。
图 4 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件光电探测特性。(a) 器件在暗场和不同强度532 nm激光光照下I-V特性曲线；(b) 器件在暗场和不同强度光照下的FN特性曲线；(c) Vds = 6.75 V时，光生电流随激光功率的变化关系曲线；(d) 响应度和光开关比随激光功率的变化关系曲线
Figure 4. Optoelectronic properties of the graphene/MoS2/h-BN/graphene vertical heterostructure device. (a) The I-V characteristics of the device under dark and 532 nm laser illumination with different power densities; (b) ln(Ids/V2)–1/Vds curves of the device under dark and 532 nm laser illumination with different power densities; (c) Power-dependent photocurrent of the device at Vds = 6.75 V; (d) Responsivity and Ilight/Idark ratio along with illumination intensities
为了测试石墨烯/h-BN/MoS2/石墨烯异质结光电探测器的响应速度，对器件进行了开-关循环测试，如图5(a)所示。器件工作在正偏压范围内，保持以MoS2中的空穴作为隧穿过程的主要载流子。上升时间(下降时间)通过测量器件在激光开启(关闭)后从最大光生电流的10%上升到90%的时间(从90%下降到10%)获得。结果表明，石墨烯/h-BN/MoS2/石墨烯异质结器件的上升时间和下降时间均约为0.3 s，与文献报道的MoS2/h-BN/石墨烯异质结0.23 s （0.25 s）的上升（下降）时间相近[17]。作为对比，单独对器件中石墨烯(TGra)/MoS2异质结部分也进行了开-关循环测试。如图5(b)所示，单独石墨烯/MoS2异质结器件性能的循环稳定性较差，输出电流整体呈现上升的趋势，利用$y = {y_0} + A{{\rm{e}}^{{{ - t} \mathord{\left/ {\vphantom {{ - t} \tau }} \right. } \tau }}}$公式进行拟合，得到上升时间和下降时间约为20 s[20]。可见基于光生载流子FN隧穿效应的石墨烯/h-BN/MoS2/石墨烯垂直异质结光电探测器的响应速度相比于传统石墨烯/MoS2异质结器件有近两个数量级的提升。
表2总结了不同异质结构光电探测器性能，将上述石墨烯/h-BN/MoS2/石墨烯垂直异质结光电探测器以及作为对比例的石墨烯/MoS2异质结与文献中已报道的类似器件结构进行对比，可见基于光生载流子FN隧穿效应的石墨烯/h-BN/MoS2/石墨烯垂直异质结光电探测器对暗电流的抑制以及响应速度的提升都有显著的改善。需要指出的是，由于机械堆叠的范德华异质结在加工过程中各界面间常常存在有机残留（如图1中AFM图所示），同时范德华材料本身也可能存在一定的缺陷，这两类杂质或缺陷相关的陷阱态引起的存储效应会在很大程度上影响器件的性能，这也是该研究中器件的光电响应速度相比于成熟的商用器件仍旧较大的原因。然而，相比于文献中已经报道的以存储效应为主导的光电探测器件，如石墨烯/岛状MoS2异质结器件（响应时间~1.5 s）[22]，该研究中基于光生载流子FN隧穿效应的石墨烯/h-BN/MoS2/石墨烯垂直异质结光电探测器的响应速度有近5倍的提升。今后器件性能的进一步提升将着手于范德华界面的优化以及减少范德华材料中的缺陷。
图 5 石墨烯/MoS2/h-BN/石墨烯垂直异质结与石墨烯/MoS2异质结光电响应速度及稳定性的对比。(a) 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件的开-关循环测试，右图为11~16 s间的局部放大图；(b) 石墨烯/MoS2异质结器件的开-关循环测试，右图为60~100 s间的局部放大图
Figure 5. Comparison of the response time and cyclic stability between graphene/MoS2/h-BN/graphene vertical heterostructure and graphene/MoS2 heterostructure devices. (a) The dynamic temporal photoresponse of the graphene/MoS2/h-BN/graphene heterostructure device with the enlarged current profile between 11 s and 16 s; (b) The dynamic temporal photoresponse of the graphene/MoS2 heterostructure device with the enlarged current profile between 60 s and 100 s
表 2 不同异质结构光电探测器性能对比
Table 2. Comparison between the performances of typical vertical heterostructure photodetector devices
Device structure Dark current/A Response time/s Laser wavelength/nm Reference Gra/h-BN/MoS2/Gra ~10−12 ~0.3 532 This work Gra/MoS2 ~10−6 ~20 532 This work Gra/h-BN/MoS2 ~10−14 ~0.25 405 [17] Gra/h-BN/SnS2 ~10−12 ~2 300 [18] WSe2/Gra/h-BN/MoS2 ~10−14 ~0.4 532 [19] Gra/MoS2 ~10−5 1.2 632.8 [21] Gra/MoS2 island ~10−5 1.5 632.8 [22] Gra/MoS2/Gra ~10−4 0.59 405-2 000 [23] -
制备了石墨烯/h-BN/MoS2/石墨烯垂直异质结，研究了器件的光电响应特性，并分析了器件在不同状态、不同偏压情况下的工作机理。得益于h-BN的存在，器件暗电流得到了很好地抑制。通过合理选择工作偏压，可以使器件保持在以MoS2中空穴的FN隧穿为主导的工作状态。在此状态下，器件具有皮安量级的暗电流，响应时间约为0.3 s。实验结果验证了利用范德华垂直异质结中光生载流子FN隧穿效应实现高性能光电探测器件的可行性。
Photodetection properties of van der Waals vertical heterostructures based on photogenerated carrier-dominated FN tunneling
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摘要: 过渡金属硫族化合物及其范德华异质结在光电探测方面具有重要的应用前景。近年来，基于光电导效应、光诱导栅控效应、光伏效应、光-热电效应等机理的器件被提出并广泛研究。其中，基于光诱导栅控效应的过渡金属硫族化合物平面型光电器件因其与晶体管相近的器件结构、工艺兼容性以及较高的光电探测响应率而备受关注，然而往往存在响应速度慢、不施加栅压时暗电流大等缺点，制约了器件性能的进一步提升。因此，针对过渡金属硫族化合物光诱导栅控型光电器件，如何提高其响应速度、降低暗电流成为亟需解决的重要问题。该研究通过实验构建石墨烯/MoS2/h-BN/石墨烯垂直异质结构，在传统石墨烯/MoS2异质结中插入宽禁带h-BN势垒层以抑制器件暗电流，同时利用光照条件下光生载流子的FN隧穿效应提升器件的光电响应速度。该研究成功实现了皮安量级的暗电流以及相对较快的光电探测响应速度（响应时间约为0.3 s），相比于传统石墨烯/MoS2异质结器件（响应时间约为20 s）有近两个数量级的提升，同时验证了基于FN隧穿效应的范德华垂直异质结构对于增强光电探测性能的积极作用。Abstract:
Objective Compared with traditional 3D bulk semiconductors, 2D layered semiconductors (e.g. transition metal dichalcogenides) have the features of large exciton binding energy, strong light-matter interaction and layer-dependent band structure, due to the intrinsic quantum confinement effect in the out-of-plane direction. Owing to such special photonic and photo-electronic properties, transition metal dichalcogenides and their van der Waals heterostructures have great potential for high-performance photodetector applications. In recent years, photodetector devices based on mechanisms such as photogating effect, photoconductive effect, photovoltaic effect, and photothermoelectric effect have been proposed and widely studied. Transition metal dichalcogenides planar optoelectronic devices based on photogating effect have similar device structures with transistors and compatible fabrication, together with high responsivity, but suffer from slow response speed and large dark current without applying gate bias, which limits the improvement of the device performance. Therefore, improving the response speed and reducing the dark current of transition metal dichalcogenides optoelectronic devices becomes an urgent issue. Methods With mechanical exfoliation and dry transfer methods, van der Waals photodetectors with a graphene/MoS2/h-BN/graphene vertical heterostructure are constructed (Fig.1). In the devices, MoS2 performs as the photoabsorber with graphene as both top and bottom electrodes. The h-BN insulating layer is inserted between MoS2 photoabsorber and the bottom graphene electrode as an effective and tunable barrier. Both AFM and Raman characterizations are taken to confirm the thickness of the materials and the device structures. The tunneling current from the top graphene electrode to the bottom graphene electrode through MoS2 and h-BN under dark and laser illumination is measured with the home-built photocurrent measurement system. Results and Discussions Based on the I-V characteristics of the vertical heterostructure device under both dark and laser illumination, together with the Fowler-Nordheim (FN) tunneling fitting of the I-V curves, the transport mechanism of FN tunneling is verified in the graphene/MoS2/h-BN/graphene vertical heterostructure device (Fig.2). With the inserted wide bandgap h-BN insulating layer between the graphene electrode and MoS2 photoabsorber, dark current was highly suppressed, while photogenerated carriers (holes in MoS2) contributed effectively to the photocurrent through FN tunneling (Fig.3), which matches well with the observation of clear photocurrent when applying positive bias (Fig.2(b)). Detailed measurement of the photocurrent under laser illumination with various powers reveals the responsivity of the device of ~140 mA/W at laser power of 4.5 mW/cm2 and Ilight/Idark ratio of ~2.3 at laser power of 41.4 mW/cm2 (Fig.4) are achieved. A low dark current in the order of picoamperes and relatively high photodetection response speed with the response time of ~0.3 s are achieved, which is nearly two orders of magnitude higher than that of traditional graphene/MoS2 heterostructure with the response time of ~20 s (Fig.5). The achieved low dark current and high response speed confirm the principle design of van der Waals vertical heterostructures based on FN tunneling effect in promoting the photodetection performance of the devices. Conclusions A novel van der Waals vertical heterostructure with graphene/MoS2/h-BN/graphene is developed to achieve high-performance photodetector properties with a low dark current and relatively high photodetection response speed, which verifies the significance of FN tunneling of photogenerated carriers for the development of van der Waals heterostructure photodetectors based on 2D materials. -
Key words:
- photodetector /
- van der Waals vertical heterostructure /
- FN tunneling /
- MoS2 /
- h-BN /
- graphene
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图 1 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件结构与表征。(a) 器件结构示意图；(b) 器件光学图片及原子力显微镜表征；(c1), (c2) 结区MoS2、Gra拉曼光谱图及h-BN拉曼光谱图
Figure 1. Structure and characterization of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device; (a) Schematic of the device structure; (b) Corresponding optical microscopy image and AFM characterization of the real fabricated heterostructure device; (c1), (c2) Raman spectra of MoS2, graphene and h-BN
图 2 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件的光电输运特性。(a)施加负向偏压 和(b)正向偏压时，器件在暗场和532 nm光照下的电流-电压特性；(c) 施加负向偏压和(d) 正向偏压时，器件在暗场和532 nm光照下的FN特性曲线
Figure 2. Photocurrent characteristics of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device. The I-V characteristics of the device under dark and 532 nm laser illumination with applied (a) negative bias and (b) positive bias; FN tunneling fitting curves of the device under dark and 532 nm laser illumination with applied (c) negative bias and (d) positive bias
图 3 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件能带示意图。(a) 不施加偏压时器件能带结构图；(b) 偏压小于负向转变偏压时，器件在暗场和532 nm光照时的能带结构图；(c) 偏压大于正向转变偏压时，器件在暗场和532 nm光照时的能带结构图
Figure 3. Energy band diagrams of the graphene/MoS2/h-BN/graphene heterostructure. (a) Energy band diagram of the device with zero bias. (b) Energy band diagrams of the device under dark and 532 nm laser illumination at Vds < VN-D-FN; (c) Energy band diagrams of the device under dark and 532 nm laser illumination at Vds > VP-D-FN
图 4 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件光电探测特性。(a) 器件在暗场和不同强度532 nm激光光照下I-V特性曲线；(b) 器件在暗场和不同强度光照下的FN特性曲线；(c) Vds = 6.75 V时，光生电流随激光功率的变化关系曲线；(d) 响应度和光开关比随激光功率的变化关系曲线
Figure 4. Optoelectronic properties of the graphene/MoS2/h-BN/graphene vertical heterostructure device. (a) The I-V characteristics of the device under dark and 532 nm laser illumination with different power densities; (b) ln(Ids/V2)–1/Vds curves of the device under dark and 532 nm laser illumination with different power densities; (c) Power-dependent photocurrent of the device at Vds = 6.75 V; (d) Responsivity and Ilight/Idark ratio along with illumination intensities
图 5 石墨烯/MoS2/h-BN/石墨烯垂直异质结与石墨烯/MoS2异质结光电响应速度及稳定性的对比。(a) 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件的开-关循环测试，右图为11~16 s间的局部放大图；(b) 石墨烯/MoS2异质结器件的开-关循环测试，右图为60~100 s间的局部放大图
Figure 5. Comparison of the response time and cyclic stability between graphene/MoS2/h-BN/graphene vertical heterostructure and graphene/MoS2 heterostructure devices. (a) The dynamic temporal photoresponse of the graphene/MoS2/h-BN/graphene heterostructure device with the enlarged current profile between 11 s and 16 s; (b) The dynamic temporal photoresponse of the graphene/MoS2 heterostructure device with the enlarged current profile between 60 s and 100 s
表 1 石墨烯/MoS2/h-BN/石墨烯垂直异质结器件在532 nm光照时的转变电压和势垒高度
Table 1. The transition bias and barrier height of the graphene/MoS2/h-BN/graphene vertical heterostructure photodetector device under 532 nm laser illumination
VN-D-FN/V φh_Gra/eV VP-D-FN/V φh_MoS2/eV BNM-14 −8.4 2.4 eV 5.3 1.6 
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表 2 不同异质结构光电探测器性能对比
Table 2. Comparison between the performances of typical vertical heterostructure photodetector devices
Device structure Dark current/A Response time/s Laser wavelength/nm Reference Gra/h-BN/MoS2/Gra ~10−12 ~0.3 532 This work Gra/MoS2 ~10−6 ~20 532 This work Gra/h-BN/MoS2 ~10−14 ~0.25 405 [17] Gra/h-BN/SnS2 ~10−12 ~2 300 [18] WSe2/Gra/h-BN/MoS2 ~10−14 ~0.4 532 [19] Gra/MoS2 ~10−5 1.2 632.8 [21] Gra/MoS2 island ~10−5 1.5 632.8 [22] Gra/MoS2/Gra ~10−4 0.59 405-2 000 [23]
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