ASP Online Seminars: Terahertz dielectric properties, ultrafast photocarrier capture/recombination and transport dynamics in graphene-mesoporous silicon nanocomposites

by Dr Défi Jubgang (Université de Sherbrooke, Canada)

Tuesday 14 Sept 2021, 13:30 → 15:00 GMT

Abstract:

Terahertz (0.1-10 x 1012 Hz) spectroscopy techniques are powerful experimental methods used to determine the dielectric properties and ultrafast dynamics of photo-excited charges (photocarriers)  in a large variety of semiconductors. In this talk, we will present the dielectric properties, charge transport, and photocarrier dynamics in graphene–mesoporous silicon nanocomposites investigated using terahertz time-domain spectroscopy (THz-TDS) and optical-pump terahertz-probe (OPTP) measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell.  The silicon phase of the nanocomposite is formed by joined silicon nanocrystallites of 6 nm average size while the pores average size was estimated to 17 nm. THz-TDS results show an increase in the refractive index and absorption coefficient with the graphene deposition temperature. This increase was attributed to the increase in pores filling by carbon atoms as the temperature is increased [1]. The temporal decays of the photocarrier dynamics obtained from OPTP measurements are reproduced using a biexponential function with an initial decay time of τ1 ~5 ps and a longer decay time τ2 ~25 ps. These decay times are associated with the photocarrier lifetime in the conduction band of the nanocomposites and are significantly reduced compared to the values of τ1 ∼74 ps and τ2 ∼ 730 ps found for the uncoated mesoporous silicon membrane. This decrease is attributed to the introduction of additional surface defects formed during the graphene deposition process. Based on the influence of the laser fluence on the OPTP curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials [2]. Frequency-dependent complex photoconductivity data curves are extracted from the terahertz waveforms taken at different optical-pump THz-probe delays. These data curves are well reproduced using a modified Drude-Smith model that considers diffusive-restoring currents [3]. The c parameter of this model, which describes the degree of carrier localization is about −0.73 for the uncoated porous Si membrane and is approaching −1 for graphene–mesoporous Si nanocomposites formed at temperatures above 800◦C. For all the nanocomposites, the characteristics of the photoconductive material, in terms of photocarrier capture/recombination time and effective mobility, are of interest for the fabrication of pulsed terahertz devices.

[1] D. J. Jubgang Fandio, S. Sauze, A. Bouchérif, R. Arès and D. Morris, Nanoscale Adv., 2020, 340-346

[2] D. J. Jubgang Fandio, S. Sauze, A. Bouchérif, R. Arès, B. Ilahi  and D. Morris, PRB 102, 115407 (2020)

[3] T. L. Cocker, D. Baillie, M. Buruma, L. V. Titova, R. D. Sydora, F. Marsigio, and F. A. Hegmann, PRB 96, 205439 (2017). 
 

Defi-September 14 2021.pdf

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