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准二维范德瓦耳斯本征铁磁半导体CrGeTe3兼具窄的半导体带隙和铁磁性质, 在自旋电子学和光电子学等领域具有广阔的应用前景, 近年来受到国内外研究人员的广泛关注. 本文利用傅里叶红外光谱得到CrGeTe3间接带隙的大小, 并采用超快太赫兹光谱(太赫兹时域光谱和光泵浦-太赫兹探测光谱)研究了准二维范德瓦耳斯本征铁磁半导体CrGeTe3的相关性质. 结果表明, 准二维CrGeTe3的间接带隙大小为0.38 eV; 在1 THz附近的折射率约为3.2, 吸收系数约为380 cm–1; 780 nm激光泵浦后的光载流子符合双指数弛豫过程, 存在快慢两个寿命, 由电子-空穴对的复合主导, 复光电导率的Drude-Smith模型拟合展示了微观系统的相关参量随时间的演化. 本文主要展示了CrGeTe3在太赫兹波段的光谱及其相关性质, 对光电子学等领域的研究具有借鉴意义.
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关键词:
- 范德瓦耳斯铁磁半导体 /
- 时间分辨光谱 /
- 太赫兹光谱
The quasi-two-dimensional van der Waals intrinsic ferromagnetic semiconductor CrGeTe3 possesses both a narrow semiconductor band gap and ferromagnetic properties, which makes it have a broad application prospect in the fields of spintronics and optoelectronics. In recent years, CrGeTe3 has received extensive attention from researchers. To the best of our knowledge, so far, these studies have mainly focused on the optical response in near infrared and visible light range, but little has been done in THz frequency range. Therefore, it is upmost importance to obtain the complex dielectric constant as well as the photocarrier dynamics of the CrGeTe3 at the THz frequency. Herewith, we use time-domain THz spectroscopy and time-resolved THz spectroscopy to investigate the fundamental properties of the CrGeTe3 crystal in the THz range, including refractive index and absorption coefficient in THz frequency, as well as the THz photocarrier dynamics under 780-nm optical excitation. The fundamental characterizations are carried out on a 33-μm-thick CrGeTe3 wafer by Fourier infrared spectroscopy, X-ray diffraction and Raman scattering. It is concluded that the CrGeTe3 wafer shows an indirect band gap of 0.38 eV and good crystalline quality. The THz time domain spectroscopy presents that the CrGeTe3 wafer has a refractive index and an absorption coefficient of 3.2 and 380 cm–1, respectively, both of which show almost negligible dispersion in the investigated THz frequency. Under the optical excitation of 780 nm, the subsequent photocarrier relaxation can be well reproduced by a double exponential function: the fast relaxation shows a lifetime of 1–2 ps, depending on pump fluence, which is contributed by electron-phonon coupling; the slow relaxation has a typical lifetime of 7–8 ps, which is due to phonon-assisted electron-phonon recombination. The Pump fluence and delay time dependence of THz photoconductivity dispersion can be well fitted with Drude-Smith model, and the fitted results demonstrate that the plasma frequency increases with pump fluence in a fixed delay time, and then decreases with delay time increasing at a fixed pump fluence. The momentum scattering time shows that it decreases with pump fluence increasing, and increases with delay time increasing. These pump fluence and delay time dependent fitting microscopic parameters show similar tendencies to those of a conventional semiconductor. In a word, the experimental study here demonstrates that the narrow band-gap CrGeTe3 wafer is well transparent and disperionless in a THz frequency range. From the above bandgap photoexcitation it follows that the wafer shows fast response and high modulation depth in THz radiation, providing a useful reference for the application of CrGeTe3 in optoelectronics and related fields.-
Keywords:
- van der Waals ferromagnetic semiconductor /
- time-resolved spectroscopy /
- terahertz spectroscopy
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图 1 时间分辨超快光泵浦-THz探测实验光路示意图
Fig. 1. Schematic diagram of the experimental setup for time-resolved ultrafast optical pump-terahertz probe spectroscopy.
图 2 (a) CGT原子结构的顶视图和侧视图; (b) 利用红外透射光谱计算得到的间接带隙; (c) 参考信号与透过样品后的THz时域信号; (d) 通过THz时域光谱得到的CGT晶体在THz波段的折射率和吸收系数
Fig. 2. (a) Top and side views of the atomic structure of CGT; (b) indirect band gap obtained from Fourier infrared spectroscopy; (c) the reference signal without placing sample and the THz-TDs signal through the sample; (d) the calculated refractive index and absorption coefficient of CGT crystal in the investigated THz frequency range.
图 3 (a) 不同泵浦功率下的瞬态THz透过率((ΔT/T0)%); (b) 泵浦-探测零延迟时间泵浦功率依赖的调制深度, 实线是线性拟合的结果; (c) 快慢过程的振幅占比随泵浦功率的依赖关系; (d) 快慢寿命随泵浦功率的依赖关系
Fig. 3. (a) Transient dynamic evolution (ΔT/T0)% under different pump fluence; (b) pump power-dependent modulation depth at zero pump-probe time delay, the solid line is the result of a linear fit; (c) the fitting fast (A1) and slow (A2) amplitudes with respect to pump fluence; (d) the fitting fast (τ1) and slow (τ2) lifetimes with respect to pump fluence.
图 4 (a) 泵浦-探测延迟时间为2 ps、不同泵浦功率下光电导的色散曲线, 实线是Drude-Smith模型拟合的结果; (b) 75.3 μJ/cm2泵浦功率、不同泵浦-探测延迟时间下THz光电导色散曲线. 利用Drude-Smith模型拟合的在不同泵浦功率下随延迟时间演化的参数 (c) 等离子体频率ωp; (d) 背散射因子c; (e) 载流子动量散射时间τ
Fig. 4. (a) Real and imaginary parts of THz photoconductivity dispersion measured at delay time of 2 ps for different pump fluences, the solid lines are the fitting curves with of Drude-Smith model; (b) the real and imaginary parts of THz photoconductivity dispersion under pump fluence of 75.3 μJ/cm2 at various delay times. The fitting parameters obtained with Drude-Smith model with respect to delay time: (c) plasma frequency, ωp; (d) backscattering factor, c; (e) carrier momentum scattering time, τ.
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[1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V, Firsov A A 2004 Science 306 666
Google Scholar
[2] Tongay S, Schumann T, Miao X, Appleton B R, Hebard A F 2011 Carbon 49 2033
Google Scholar
[3] Tongay S, Schumann T, Hebard A 2009 Appl. Phys. Lett. 95 222103
Google Scholar
[4] Desai S B, Madhvapathy S R, Sachid A B, Llinas J P, Wang Q, Ahn G H, Pitner G, Kim M J, Bokor J, Hu C 2016 Science 354 99
Google Scholar
[5] Suo P, Yan S, Pu R, Zhang W, Li D, Chen J, Fu J, Lin X, Miao F, Liang S J 2022 Photonics Res. 10 653
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[6] Waters D, Nie Y, Lüpke F, Pan Y, Flsch S, Lin Y C, Jariwala B, Zhang K, Wang C, Lü H 2020 ACS Nano 14 7564
Google Scholar
[7] Tran K, Moody G, Wu F, Lu X, Choi J, Kim K, Rai A, Sanchez D A, Quan J, Singh A 2019 Nature 567 71
Google Scholar
[8] Cao Y, Fatemi V, Fang S, Watanabe K, Taniguchi T, Kaxiras E, Jarillo-Herrero P 2018 Nature 556 43
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[9] Ma Q S, Zhang W, Wang C, Pu R, Ju C W, Lin X, Zhang Z, Liu W, Li R 2021 J. Phys. Chem. C 125 9296
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[13] Deng Y, Yu Y, Song Y, Zhang J, Wang N Z, Sun Z, Yi Y, Wu Y Z, Wu S, Zhu J 2018 Nature 563 94
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[14] Liu B, Liu S, Yang L, Chen Z, Zhang E, Li Z, Wu J, Ruan X, Xiu F, Liu W 2020 Phys. Rev. Lett. 125 267205
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Google Scholar
[18] Tian Y, Gray M J, Ji H, Cava R J, Burch K S 2016 2 D Mater. 3 025035
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[19] Ji H, Stokes R A, Alegria L D, Blomberg E C, Tanatar M A, Reijnders A, Schoop L M, Tian L, Prozorov R, Burch K S 2013 J. Appl. Phys. 114 045302
Google Scholar
[20] Li Y F, Wang W, Guo W, Gu C Y, Sun H Y, He L, Zhou J, Gu Z B, Nie Y F, Pan X Q 2018 Phys. Rev. B 98 125127
Google Scholar
[21] Zhu F, Zhang L, Wang X, Dos Santos F J, Song J, Mueller T, Schmalzl K, Schmidt W F, Ivanov A, Park J T 2021 Sci. Adv. 7 eabi7532
Google Scholar
[22] 索鹏, 夏威, 张文杰, 朱晓青, 国家嘉, 傅吉波, 林贤, 郭艳峰, 马国宏 2020 69 207302
Google Scholar
Suo P, Xia W, Zhang W J, Zhu X Q, Guo J J, Fu J B, Lin X, Guo Y F, Ma G H 2020 Acta Phys. Sin. 69 207302
Google Scholar
[23] Tauc J, Grigorovici R, Vancu A 1966 Phys. Status Solidi B 15 627
Google Scholar
[24] Davis E, Mott N 1970 Philos. Mag. 22 0903
Google Scholar
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Google Scholar
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