-
完成器件制备后,首先利用光学显微镜(Optical microscopy,OM)和扫描电子显微镜(Scanning electron microscopy,SEM)表征器件结构。图3(a)~(c)、(d)~(f)分别为悬空的和塌陷的两个石墨烯/h-BN器件的显微镜照片。其中图3(a)和图3(d)分别为50倍放大倍数下两个器件的光学显微镜照片,其余图片为相对应的SEM照片。通过光学显微镜照片,可以判断器件的完整性以及转移后范德华异质结中石墨烯和电极接触的位置,如图3(a)和图3(d)所示,沟道部分异质结颜色可以观察到条纹状变化,这一现象是由于异质结与衬底高度不一致产生了光学薄膜干涉引起的。图3(b)、(e)为低放大倍数下器件整体的SEM照片,图(c)、(f)为高放大倍数下悬空部分细节的SEM照片。根据器件的SEM照片阴影变化,可以判断器件的悬空与塌陷状态,见图3(c),悬空的器件沟道部分异质结的阴影连续均匀。说明,薄层的h-BN在范德华力的作用下,紧密吸附石墨烯,有效地增强了悬空石墨烯的机械力学稳定性,避免了在制备、测试和表征过程中石墨烯由于外部应力(重力、静电力等)而导致的坍塌、断裂和卷曲等失效。此外,致密的h-BN晶体结构可以有效隔绝外部环境对石墨烯的大部分影响,避免污染和掺杂,保证了器件的高质量。
图 3 悬空石墨烯器件的显微镜表征。(a)完成制备流程后悬空的石墨烯器件光学显微镜照片;(b)~(c)悬空石墨烯器件对应不同放大倍数的扫描电子显微镜照片;(d)完成制备流程后坍塌的石墨烯器件光学显微镜照片;(e)~(f)坍塌石墨烯器件对应不同放大倍数的扫描电子显微镜照片
Figure 3. Microscopy characterizations of suspended graphene devices. (a) Optical microscopy image of graphene devices suspended after the completion of the fabrication process; (b)-(c) Corresponding scanning electron microscopy images of different magnifications; (d) Optical microscopy image of graphene devices collapsed after the completion of the fabrication process; (e)-(f) Corresponding scanning electron microscopy images of different magnifications
在室温条件下用真空探针台测试器件电学性能。在测试腔达到2.2×10−5 hPa真空度后,使用源表2636B在源漏极加偏压得到电流电压的输出曲线(I-Vb)以及电阻值,而后固定偏压改变栅压,分别得到0.1 V、0.5 V和1 V偏压下的场效应曲线(I-Vg)。完成退火前器件的电学性能测试之后,将器件置于高真空退火炉进行退火处理。以10 ℃/min的升温速度加热到400 ℃,保温3 h,之后再降温到室温,整个退火操作在4.5×10−4 hPa的真空度下进行,以防止高温下石墨烯被氧化。退火之后,对器件进行与退火前相同的测试表征,包括光学显微镜观察,探针台电学测试等。对比分析测试结果,计算载流子迁移率,确定器件退火前后的变化。载流子迁移率计算公式为:
$$ \begin{split} \\ \mu =\frac{{{\rm{d}}I}_{b}}{{{\rm{d}}V}_{g}}\cdot \frac{L}{W}\cdot \frac{1}{{C}_{g}\cdot {V}_{b}} \end{split}$$ (1) 式中:$ {I}_{b} $为电流;$ {V}_{g} $为栅极电压;L与W分别为悬空石墨烯的长度与宽度;Vb为偏置电压;Cg为介质电容。
-
退火前后,对同一器件进行了相同的电学测试。得到0 V栅压下的输出特征曲线和0.1 V偏压下的转移特征曲线,如图4所示,黑色为退火前数据,红色为退火后数据。图4(a)为悬空石墨烯焦耳热红外辐射器件的输出特性曲线,偏压变化范围为−0.1~0.1 V,退火前的电阻为4 249 Ω,退火之后电阻减少至770 Ω。图4(b)为器件在0.1 V偏压下的转移特性曲线,根据上述公式(1)计算得到,退火前后悬空石墨烯的迁移率分别为277 cm2/(V·s)和5040 cm2/(V·s),退火后石墨烯的电阻降低到退火前的六分之一,载流子迁移率提高到退火前的18倍。电学测试结果说明高真空热退火可以使石墨烯更好地接触金属电极,同时能有效去除悬空石墨烯器件中大部分的气泡、污染等缺陷,降低接触电阻的同时,提高了石墨烯本身的电导率。
图 4 悬空石墨烯器件退火前后的电学特性比较。(a)退火前后的I-Vb曲线对比;(b)退火前后的场效应I-Vg曲线对比(黑色为退火前,红色为退火后)
Figure 4. Comparison of electrical properties of suspended graphene devices before and after annealing. (a) Comparison of I-Vb curves before and after annealing; (b) Comparison of field effect I-Vg curves before and after annealing (black is before annealing, red is after annealing)
完成退火后的电学测试之后,研究了悬空石墨烯在偏置电压焦耳热作用下的温度特性和红外热辐射特性,其中石墨烯的温度通过拉曼光谱测试得到,而红外热辐射特性则通过发光光谱测试得到。石墨烯拉曼光谱主要的特征峰有G峰和2D峰。其中G峰位于1582 cm−1附近,与温度呈线性相关,当温度升高时,石墨烯G峰位置向低波数方向移动。其关系表达式为[1, 27]:
$$ {\omega }_{G}\left(T\right)={\omega }_{0}+\chi T $$ (2) 式中:$ {\omega }_{G}\left(T\right) $为温度为T时G峰的峰位;$ {\omega }_{0} $为室温(300 K)下G峰的拉曼位移;$ \chi $为一阶温度系数。通过加热真空腔样品台改变悬空石墨烯的环境温度,将温度从室温升至800 ℃,每50 ℃在悬空石墨烯中心点测一次拉曼光谱,得到不同温度下石墨烯G峰峰位。如图5(a)所示,绘制了G峰峰位随外界温度变化的散点图,通过线性拟合得到斜率为−0.0185 cm−1/℃,即为温度系数$ \chi $。
图 5 悬空石墨烯器件在偏压下的拉曼光谱和红外辐射光谱。(a)不同环境温度下石墨烯拉曼G峰位置的变化,红色实线是线性拟合结果;(b)不同偏压下石墨烯拉曼光谱的变化,黑色虚线标注出G峰和2D峰位置的变化;(c)偏压为6 V时,不同聚焦位置的红外辐射光谱,分别为石墨烯(Gr)、六方氮化硼(h-BN)、金电极(Au)和二氧化硅(SiO2);(d)不同偏压下石墨烯的红外辐射光谱,Y轴为对数坐标,X轴为线性坐标
Figure 5. Raman spectra and infrared radiation spectra of suspended graphene devices under bias voltage. (a) Evolution of graphene Raman G peak position at different ambient temperatures, the red solid line is the linear fitting; (b) Evolution of graphene Raman spectrum under different bias voltages, the black dotted line marks the G peak and 2D peak positions; (c) Infrared radiation spectra of different focus positions for graphene (Gr), hexagonal boron nitride (h-BN), gold electrode (Au) and silicon dioxide (SiO2) at a bias voltage of 6 V; (d) Infrared radiation spectra of graphene under different bias voltages, with logarithmic coordinates on the Y axis and linear coordinates on the X axis
完成石墨烯拉曼G峰随环境温度变化的线性拟合后,进行石墨烯器件焦耳热的实验研究。首先石墨烯器件需置于真空腔中避免高温下的氧化失效,而后在器件源漏两端加上偏压,此时悬空石墨烯在偏置电流焦耳热效应的作用下被加热,温度升高,并以黑体辐射的形式向外发射光子。如图5(b)所示,随着偏压从0 V增加到8 V, G峰位置向低波数的方向移动,表明悬空石墨烯的温度升高。根据公式(2)可以推算出8 V偏置电压下的悬空石墨烯中心的温度为836 K。通过发光光谱的测试发现,在偏置电压为6 V时,悬空石墨烯器件辐射光谱出现明显的红外辐射波峰,如图5(c)所示。通过测量悬空器件不同位置(石墨烯中心、二氧化硅衬底、六方氮化硼、金电极)处的辐射光谱发现,Au、SiO2、h-BN的辐射光谱基本重合,而石墨烯的辐射光谱有明显不同的特征峰,从而排除了外界环境的干扰。说明在6 V的偏置电压下,石墨烯温度升高并形成明显的热辐射,通过拉曼光谱计算得到此时石墨烯的温度为645 K。随着偏压的增大,悬空石墨烯热辐射的强度逐渐增强,且向短波长方向移动,如图5(d)所示,在8 V偏压下悬空石墨烯温度升高到836 K,并在波长955 nm处表现出强烈的红外辐射信号。由于悬空石墨烯/氮化硼异质结与硅片衬底构成了一个光学Fabry–Pérot谐振腔(F-P腔),当石墨烯在焦耳热的作用下温度升高并辐射电磁波时,电磁波在腔内经过多次反射透射而相干叠加,导致悬空石墨烯的焦耳热辐射光谱与经典的黑体辐射光谱有所区别。这一现象为进一步通过设计光学微腔和光子晶体等周期性微纳结构实现对石墨烯辐射光谱的有效调控(如1550 nm光通信波段),并构建硅基集成石墨烯纳米光子器件提供了新的思路。
Controllable fabrication and characterization of suspended graphene/hexagonal boron nitride heterostrcuture Joule heating infrared radiation devices (invited)
-
摘要: 石墨烯具有优异的光、电、热以及力学性质,而悬空石墨烯避免了衬底带来的褶皱、载流子散射和掺杂等影响因素,可以充分展现石墨烯的本征物理特性,因此在高性能石墨烯微电子和光电子器件研究中具有重要意义。然而,目前悬空石墨烯器件还存在着制备方法复杂、成品率低、性能不稳定等挑战。文中提出了一种利用六方氮化硼吸附石墨烯,将其定点转移到金属电极,制备悬空石墨烯焦耳热红外辐射器件的新方法。六方氮化硼对悬空石墨烯具有良好的支撑悬挂作用,有效提高了悬空石墨烯的力学稳定性,避免了坍塌、断裂等失效情况。真空热退火处理后悬空石墨烯的电阻降低到退火处理前的约六分之一,载流子迁移率比退火前提高了约18倍。当偏置电压为8 V时,拉曼光谱测试发现石墨烯温度为836 K,器件在955 nm波长处表现出强烈的红外辐射信号。Abstract:
Objective Graphene exhibits superior optical, electrical, thermal, and mechanical properties, while the suspended structure avoids external factors such as wrinkles, carrier scattering and doping caused by rough substrates, and can maximize the intrinsic physical properties of graphene, which is of great significance in the research of high-performance graphene microelectronics and optoelectronic devices. However, the current research on suspended graphene devices is yet limited by the complicated fabrication methods, low yield, and unstable electrical and thermal properties of devices. Methods In order to improve the yield rate of suspended graphene nano devices and the comprehensive performance of the device, this paper develops a method by using two-dimensional material hexagonal boron nitride (h-BN) to pick up graphene, then transfers graphene directly to the surface of pre-fabricated metal electrodes, and finally prepares suspended graphene Joule heating infrared radiation devices (Fig.1). In order to further reduce the defects and improve the device quality, a high-vacuum thermal annealing treatment was performed on the suspended graphene device. Based on the high-quality suspended graphene device after annealing, we used Raman spectroscopy and luminescence spectroscopy to study the temperature characteristics and thermal radiation spectral characteristics of the device under the Joule heating effect caused by bias voltage. Result and discussion The experimental results show that the h-BN covers the upper surface of the graphene and plays a critical role in supporting and suspending the graphene, which effectively improves the stability of the suspended graphene and avoids device failures such as collapse and fracture. After the thermal annealing at 400 ℃/3 h in high vacuum of 4.5×10−4 hPa, the resistance of suspended graphene decreased to one-sixth of that before annealing, and the carrier mobility increased eighteen times compared with that before annealing (Fig.4). When the bias voltage is 8 V, the temperature of suspended graphene measured by Raman spectroscopy is 836 K, and it shows a strong infrared radiation signal at 955 nm wavelength (Fig.5). Conclusions This paper presents a controllable fabrication method of high-quality suspended graphene Joule heating radiation devices, and investigates the electrical, temperature, and thermal radiation characteristics of suspended graphene devices. The h-BN in the device structure demonstrates a good support and adhesion effect for suspended graphene, which greatly improves the device performance. The impurities attached to the surface of graphene can be effectively removed through high vacuum thermal annealing, which greatly improves the electrical performance of suspended graphene devices. It was observed that the temperature of graphene increased with the increase of bias voltage, showing a blue shift in the Raman spectrum and strong thermal radiation emission. The research results of this paper provide an important reference for deepening the understanding of the intrinsic physical properties of suspended graphene and developing optoelectronic applications based on suspended graphene devices. -
图 1 悬空石墨烯/六方氮化硼异质结器件制备流程示意图。(a)清洗硅片;(b)旋涂AZ光刻胶;(c)光刻图形化AZ光刻胶;(d)蒸镀Cr/Au金属电极;(e)剥离电极;(f)将石墨烯/六方氮化硼异质结直接转移到电极上制备悬空结构器件
Figure 1. Schematic diagram of the fabrication process of suspended graphene/hexagonal boron nitride heterojunction devices. (a) Cleaning the silicon wafer; (b) Spin-coating AZ photoresist; (c) Photolithographic patterning of AZ photoresist; (d) Evaporation of Cr/Au metal electrodes; (e) Lift-off electrodes; (f) Graphene/hexagonal boron nitride heterojunctions are directly transferred to electrodes to prepare suspended structure devices
2 机械剥离单层石墨烯和多层六方氮化硼的表征。(a)单层石墨烯的光学显微镜照片,红色十字标注了拉曼光谱的聚焦位置;(b)单层石墨烯的拉曼光谱,其中G峰强度明显低于2D峰强度,证明为单层石墨烯;(c)多层六方氮化硼的光学显微镜照片,白色实线标注了原子力显微镜的测量位置;(d)多层六方氮化硼的厚度,约为36 nm
2. Characterization of mechanically exfoliated monolayer graphene and multilayer hexagonal boron nitride. (a) Optical microscope image of monolayer graphene, the red cross marks the focus position of Raman spectroscopy; (b) Raman spectrum of monolayer graphene, in which the G peak intensity is significantly lower than the 2D peak intensity, proving it is a monolayer graphene; (c) Optical microscope image of a multilayer hexagonal boron nitride, the white solid line marks the measurement position of atomic force microscope; (d) The thickness of multilayer hexagonal boron nitride is about 36 nm
图 3 悬空石墨烯器件的显微镜表征。(a)完成制备流程后悬空的石墨烯器件光学显微镜照片;(b)~(c)悬空石墨烯器件对应不同放大倍数的扫描电子显微镜照片;(d)完成制备流程后坍塌的石墨烯器件光学显微镜照片;(e)~(f)坍塌石墨烯器件对应不同放大倍数的扫描电子显微镜照片
Figure 3. Microscopy characterizations of suspended graphene devices. (a) Optical microscopy image of graphene devices suspended after the completion of the fabrication process; (b)-(c) Corresponding scanning electron microscopy images of different magnifications; (d) Optical microscopy image of graphene devices collapsed after the completion of the fabrication process; (e)-(f) Corresponding scanning electron microscopy images of different magnifications
图 4 悬空石墨烯器件退火前后的电学特性比较。(a)退火前后的I-Vb曲线对比;(b)退火前后的场效应I-Vg曲线对比(黑色为退火前,红色为退火后)
Figure 4. Comparison of electrical properties of suspended graphene devices before and after annealing. (a) Comparison of I-Vb curves before and after annealing; (b) Comparison of field effect I-Vg curves before and after annealing (black is before annealing, red is after annealing)
图 5 悬空石墨烯器件在偏压下的拉曼光谱和红外辐射光谱。(a)不同环境温度下石墨烯拉曼G峰位置的变化,红色实线是线性拟合结果;(b)不同偏压下石墨烯拉曼光谱的变化,黑色虚线标注出G峰和2D峰位置的变化;(c)偏压为6 V时,不同聚焦位置的红外辐射光谱,分别为石墨烯(Gr)、六方氮化硼(h-BN)、金电极(Au)和二氧化硅(SiO2);(d)不同偏压下石墨烯的红外辐射光谱,Y轴为对数坐标,X轴为线性坐标
Figure 5. Raman spectra and infrared radiation spectra of suspended graphene devices under bias voltage. (a) Evolution of graphene Raman G peak position at different ambient temperatures, the red solid line is the linear fitting; (b) Evolution of graphene Raman spectrum under different bias voltages, the black dotted line marks the G peak and 2D peak positions; (c) Infrared radiation spectra of different focus positions for graphene (Gr), hexagonal boron nitride (h-BN), gold electrode (Au) and silicon dioxide (SiO2) at a bias voltage of 6 V; (d) Infrared radiation spectra of graphene under different bias voltages, with logarithmic coordinates on the Y axis and linear coordinates on the X axis
-
[1] Calizo I, Balandin A A, Bao W, et al. Temperature dependence of the Raman spectra of graphene and graphene multilayers [J]. Nano Letters, 2007, 7(9): 2645-2649. doi: 10.1021/nl071033g [2] Berciaud Stéphane, Han Melinda Y, Mak Kin Fai, et al. Electron and optical phonon temperatures in electrically biased graphene [J]. Physical Review Letters, 2010, 104(22): 227401. doi: 10.1103/PhysRevLett.104.227401 [3] Kim Young Duck, Gao Yuanda, Shiue Ren-Jye, et al. Ultrafast graphene light emitters [J]. Nano Letters, 2018, 18(2): 934-940. doi: 10.1021/acs.nanolett.7b04324 [4] Yang Qi, Shen Jun, Wei Xingzhan, et al. Recent progress on the mechanism and device structure of graphene-based infrared detectors [J]. Infrared and Laser Engineering, 2020, 49(1): 0103003. (in Chinese) [5] Liu Zhi, Chen Jimin, Li Dongfang, et al. Laser-induced transformation of carbon nanotubes into graphene nanoribbons and their conductive properties [J]. Infrared and Laser Engineering, 2020, 49(9): 20200298. (in Chinese) [6] Geim A K, Novoselov K S. The rise of graphene [J]. Nature Materials, 2007, 6(3): 183-191. [7] Bae Myungho, Ong Zhunyong, Estrada David, et al. Imaging, simulation, and electrostatic control of power dissipation in graphene devices [J]. Nano Letters, 2010, 10(12): 4787-4793. [8] Freitag Marcus, Steiner Mathias, Martin Yves, et al. Energy dissipation in graphene field-effect transistors [J]. Nano Letters, 2009, 9(5): 1883-1888. [9] Freitag Marcus, Chiu Hsin-Ying, Steiner Mathias, et al. Thermal infrared emission from biased graphene [J]. Nature Nanotechnology, 2010, 5(7): 497-501. [10] Mahlmeister N H, Luxmoore I J, Poole T, et al. Thermal emission from large area chemical vapor deposited graphene devices [J]. Applied Physics Letters, 2013, 103(13): 131901-131906. [11] Kim Young Duck, Kim Hakseong, Cho Yujin, et al. Bright visible light emission from graphene [J]. Nature Nanotechnology, 2015, 10(8): 676-681. [12] Tchon K, Go Ral I. Graphene hot-electron light bulb: Incandescence from hBN-encapsulated graphene in air [J]. 2D Materials, 2018, 5(1): 1910-1915. [13] Shiue Ren-Jye, Gao Yuanda, Tan Cheng, et al. Thermal radiation control from hot graphene electrons coupled to a photonic crystal nanocavity [J]. Nature Communications, 2019, 10 (1): 109. [14] Luo Fang, Fan Yansong, Peng Gang, et al. Graphene thermal emitter with enhanced joule heating and localized light emission in air [J]. ACS Photonics, 2019, 6(8): 2117-2125. [15] Brar Victor W, Sherrott Michellez, Jang Min Seok, et al. Electronic modulation of infrared radiation in graphene plasmonic resonators [J]. Nature Communications, 2015, 6 (1): 7032. [16] Meyer Jannik C, Geim A K, Katsnelson M I, et al. The structure of suspended graphene sheets [J]. Nature, 2007, 446: 60-63. [17] Fischbein Michael D, Drndic Marija. Electron beam nanosculpting of suspended graphene sheets [J]. Condensed Matter, 2008, 93(11): 113107. [18] Alyobi Mona, Barnett Chris, Cobley Richard. Effects of thermal annealing on the properties of mechanically exfoliated suspended and on-substrate few-layer graphene [J]. Crystals, 2017, 7(11): 349. doi: 10.3390/cryst7110349 [19] Li Qiang, Cheng Zengguang, Li Zhongjun. Fabrication of suspended graphene devices and their electronic properties [J]. Chinese Physics B, 2010, 19(9): 97307. [20] Watanabe Kenji, Taniguchi Takashi, Kanda Hisao. Direct-bandgap properties and evidence for ultraviolet lasing of hexagonal boron nitride single crystal [J]. Nature Materials, 2004, 3(6): 404-409. [21] Cassabois G, Valvin P, Gil B. Hexagonal boron nitride is an indirect bandgap semiconductor [J]. Nature Photonics, 2016, 10(4): 262-266. doi: 10.1038/nphoton.2015.277 [22] Wang L, Meric I, Huang P Y, et al. One-dimensional electrical contact to a two-dimensional material. [J]. Science, 2013, 342: 614-617. [23] Dean C R, Young A F, Meric I, et al. Boron nitride substrates for high-quality graphene electronics [J]. Nature Nanotechnology, 2010, 5(10): 722-726. [24] Gao Xin, Zheng Liming, Luo Fang, et al. Integrated wafer-scale ultra-flat graphene by gradient surface energy modulation [J]. Nature Communications, 2022, 13(1): 5410. [25] Fukamachi Satoru, Solís-fernández Pablo, Kawahara Kenji, et al. Large-area synthesis and transfer of multilayer hexagonal boron nitride for enhanced graphene device arrays [J]. Nature Electronics, 2023, 6(2): 126-136. [26] Li Xiaoli, Qiao Xiaofen, Han Wenpeng, et al. Layer number identification of intrinsic and defective multilayered graphenes up to 100 layers by the raman mode intensity from substrates [J]. Nanoscale, 2015, 7(17): 8135-8141. [27] Zhang T Y, Wang H W, Xia X X, et al. A monolithically sculpted van der waals nano-opto-electro-mechanical coupler [J]. Light Sci Appl, 2022, 11(1): 76-85.