CC BY
15THz-I-16
Terahertz spectroscopy of nanowires
V.N. Trukhin1, I.A. Mustafin1, A.D. Bouravleuv 2, G.E. Cirlin2, H. Lipsanen3
1- Ioffe Institute, St.Petersburg, Russia 2- St.Petersburg Academic University, St.Petersburg, Russia
3- Department of Electronics and Nanoengineering, Aalto University, FIN-02150, Espoo, Finland
valera.truchin@mail.ioffe.ru
Despite some successes in the creation, synthesis, and application of semiconductoi nanostructures such as nanowires (NW), the industry is still being in its infancy. Understanding the fundamental properties of such structures is the crucial key factor for the implementation of these systems in the field of nano-photonics and nano-optoelectronics, requiring further research.
Our international team is dedicated to the synthesis and investigation of the properties of semiconductor nanowires. Over the last years, our technology team has been able to achieve remarkable success in the development of these structures - we have succeeded in synthesizing ordered arrays oi nanowires on various substrates and various A3B5 compounds, which geometrical and physical parameters are highly repeatable and stable. In this report, the main results of the investigation of these structures using time-resolved THz spectroscopy methods will be presented.
The synthesis of semiconductor NWs was carried out both by molecular-beam epitaxy and metal-organic vapour-phase epitaxy techniques. The results of studies of THz generation in GaAs-based NWs with n- and p-alloying have shown, that the mechanism of THz generation in n-type conductivity nanocrystals is due to the drift (movement of carriers in the contact electric field at the upper face of the NW and in the electric field of the n-n+ transition between the NW and the substrate) and diffusion (diffusion of carriers due to non-uniform excitation of photocarriers along the nanocrystal) currents oi photoexcited charge carriers having the same direction. THz generation in p-type conductive nanocrystals is associated with drift and diffusion currents of photoexcited charge carriers moving in the opposite directions. The contribution to THz radiation generation from the drift current is stronger than the diffusion current for GaAs nanowires with catalyzed Au droplets. It has been demonstrated thai THz generation in NWs is much more efficient (several-fold increase) than from the surface of a bulk semiconductor, even without taking into account the nanocrystal filling factor of the substrate surface.
For the first time it was experimentally demonstrated that the efficiency of THz generation in semiconductor NWs has a linear dependence on the NW filling factor when the NW period is larger than the wavelength of the excitation light, and does not grow to infinity when the maximum packing of nanocrystals is reached, but reaches a maximum value for the NW period of the order of the excitation light wavelength. The experimental results indicate that the near-field interaction of excited nanocrystals, when the NW array is essentially a metamaterial, leads to the attenuation of the electromagnetic field in the nanocrystal. In addition, it was shown experimentally that the efficiency of THz generation depends not only on the NW filling factor, but for a certain diameter of the nanocrystal at a given frequency of excitation light, it increases significantly. The study of the effect of polarization and angle of incidence of excitation light on the process of THz generation in periodic arrays of GaAs NWs, confirmed the essential role of resonant absorption of excitation light. The dependence of the light absorption cross section on the nanowire diameter, angle of incidence, polarization, and frequency of the excitation light obtained as a result of the simulation based on the Lorentz-Mi theory is in agreement with the experimental results The results of studies of terahertz excitation spectra in samples with different fill factor and nanocrystal diameters further confirmed the effect of resonant excitation of the leaky modes (so-called Mie modes) in a semiconductor nanocrystal on the THz generation process. Efficient THz generation in a GaAs-based NW periodic array was demonstrated experimentally when compared with THz generation in a p-InAs semiconductor.
