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Quasi-two-dimensional van der Waals ferromagnetic semiconductor CrGeTe3 studied by THz spectroscopy

The quasi-two-dimensional van der Waals intrinsic ferromagnetic semiconductor CrGeTe3 processes both a narrow semiconductor band gap and ferromagnetic properties, which makes it have broad application prospects in the fields of spintronics and optoelectronics. In recent years, CrGeTe3 has been paid extensive attention from researchers at domestic and abroad. So far as we learn about, these studies have mainly focused on the optical response in near infrared and visible light range, but little was 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 in the THz frequency. Herewith, we have employed 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 following 780 nm optical excitation. The fundamental characterizations were carried out on a 33 μm thick CrGeTe3 wafer by Fourier infrared spectroscopy, X-ray diffraction and Raman scattering, we 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 refractive index and absorption coefficient of 3.2 and 380 cm-1, respectively, both of which shows almost negligible dispersion in the investigated THz frequency. Following 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. Pump fluence and delay time dependence of THz photoconductivity dispersion can be well fitted with Drude-Smith model, and the fitting results demonstrate that the plasma frequency increases with pump fluence at a fixed delay time, and decreases with delay time at a fixed pump fluence. The momentum scattering time shows decrease with pump fluence and increase with delay time. These pump fluence and delay time dependence of the fitting microscopic parameters show similar tendency as a conventional semiconductor. In a word, the experimental study here demonstrates that narrow band-gap CrGeTe3 wafer shows good transparent and disperionless in THz frequency. Following above bandgap photoexcitation, 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.