CC BY
4THz-I-6
NONLINEAR AND TERAHERTS PHOTONICS
Photogalvanic currents in a -Sn/Ge QW
V.N. Trukhin1, I. А. Mustafin1, F. V. Kusmartsev2'3, A. Kusmartseva2, Y. Liu3, B. Zhang3' Y. Luo3
1-Ioffe Institute, 19021, St Petersburg, Russia 2- Department of Physics, Loughborough University, UK 3- Nano-fabrication laboratory, MTRC, China Email: valembr@mail.ru
In this report we present the results of an experimental study of photocurrents in nanostructures based on a-Sn/Ge quantum wells when they are excited by femtosecond optical pulses by the contact method and by recording the terahertz radiation induced by these photocurrents.
The studied samples were nanometer layers of Ge and a-Sn synthesized by electron-beam deposition on the surface of a silicon substrate with a top layer of SiO2. The number of a-Sn/Ge quantum wells in the investigated nanostructures was 10, the width of the quantum wells were 4 nm, 6 nm, 8.5 nm, and 10 nm, respectively. The separation between the quantum wells was 10 nm.
Raman spectroscopy studies were carried out at room temperature in the "backscattering" geometry on a Horiba Jobin-Yvon T64000 spectrometer equipped with a confocal optical microscope. When measuring the laser spot of continuous radiation from a YAG: Nd laser (^ = 532 nm), it was focused in an area with a diameter of ~ 1 ^m using a 100 x objective (NA = 0.9). The measurements were carried out using an optical pumping with a power in a range 0.04 - 1 mW. The key observation in the obtained Raman spectra is that, at the lowest pump power, an additional line in the 210 cm-1 region is observed, which is due to a-Sn mode. The mode disappears with increasing pump power. The observed disappearance of the line can be explained by the phase transition of a-Sn to P-Sn as the sample is heated with an increase in pump power.
The waveform of the THz pulse generated when the sample was excited by ultrashort laser pulses of femtosecond duration was recorded by terahertz time-resolved spectroscopy. During the study of samples containing a-Sn/Ge quantum wells with a width of 10 nm and 8.5 nm, it was found that when the polarization of the excitation light changes from TM to TE, the electric field of the THz radiation changes its sign and the dependence of the electric field of the THz radiation on the polarization vector rotation angle has a sinusoidal character. The sign of the electric field of the THz radiation excited by light with TM polarization corresponds to the movement of photoelectrons in the direction of light propagation. The amplitude of THz pulse is proportional to the intensity of excitation light. It was shown earlier that the observed dependences of the photocurrent on the polarization of light, the direction of the wave vector, and the intensity of the excited light are inherent in the drag current [1]. In samples with quantum well widths of 4 nm and 6 nm, the sign of the electric field of the THz radiation does not change when the polarization changes from TM to TE. At excitation by light with TE polarization the amplitude of THz pulse is less than at excitation of nanostructure by light with TM polarization. The amplitude of THz pulse is also proportional to the intensity of excitation light. Experiments on the study of photocurrents by the contact method showed a similar behavior of the corresponding dependences. The observed changes in the polarization dependences of the electric field of the THz radiation generated in a-Sn/Ge nanostructures when the width of quantum wells changes from 6 nm to 8. 5 nm appear to be related to the difference in electronic systems. Namely, nanostructures with a-Sn/Ge, 4 nm, and 6 nm quantum hole widths have the normal band structure of narrow-gap semiconductors, while nanostructures with a-Sn/Ge, 8.5 nm, and 10 nm quantum hole widths are two-dimensional topological insulators. Such a phase transition is predicted in the a-Sn/CdTe quantum well with the width of about 8 nm [2].
This work supported by the Russian Foundation for Basic Research and the Royal Society of London No. 21-5210015.
[1] Trukhin VN., Mustafin I.A., Gavrilova P.G., Kusmartsev F.V., Kusmartseva A., Liu Y., Zhang B., Luo Y., Generation of terahertz radiation in nanometer films:Ge/alpha-Sn, Proceedings of 45TH Int. Conf. Infrared, Millim., Terahertz Waves, IRMMW-THz, v.2020, pp. 915-915, 2020.
[2] S. Kufne, "Electronic and topological properties of low-dimensional a-Sn and HgTe structures", Dissertation (Ph.D.), 2015.
*
ALT'22
