准二维范德瓦耳斯本征铁磁半导体CrGeTe_(3)的THz光谱

Quasi-two-dimensional van der Waals ferromagnetic semiconductor CrGeTe_(3) studied by THz spectroscopy

准二维范德瓦耳斯本征铁磁半导体CrGeTe_(3)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CrGeTe_(3)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CrGeTe_(3)的相关性质.结果表明,准二维CrGeTe_(3)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CrGeTe_(3)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.

The quasi-two-dimensional van der Waals intrinsic ferromagnetic semiconductor CrGeTe_(3) 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,CrGeTe_(3) 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 CrGeTe_(3) at the THz frequency.Herewith,we use time-domain THz spectroscopy and time-resolved THz spectroscopy to investigate the fundamental properties of the CrGeTe_(3) 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 CrGeTe_(3) wafer by Fourier infrared spectroscopy,X-ray diffraction and Raman scattering.It is concluded that the CrGeTe_(3) wafer shows an indirect band gap of 0.38 eV and good crystalline quality.The THz time domain spectroscopy presents that the CrGeTe_(3) 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 CrGeTe_(3) 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 CrGeTe_(3) in optoelectronics and related fields.