Terahertz photoconductivity and photocarrier dynamics in graphene–mesoporous silicon nanocomposites

Abstract

We investigate charge transport and photocarrier dynamics in graphene--mesoporous silicon nanocomposites using optical-pump terahertz-probe measurements. The nanocomposite material consists of a free-standing mesoporous silicon membrane whose specific surface is coated with a few-layer graphene shell. Temporal decays of the differential transmission measurements are reproduced using a biexponential function with an initial decay time of 5 ps and a longer decay time of about 25 ps. These decay times are significantly reduced compared to the values of ${\ensuremath{\tau}}_{1}\ensuremath{\sim}74$ ps and ${\ensuremath{\tau}}_{2}\ensuremath{\sim}730$ ps obtained for the uncoated mesoporous silicon membrane and this 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 time-resolved differential transmission curves, a capture/recombination model is proposed to describe the photocarrier dynamics in these nanocomposite materials. 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 taking into account diffusive-restoring currents. The $c$ parameter of this model, which describes the degree of carrier localization, is about $\ensuremath{-}0.73$ for the uncoated porous Si membrane and is approaching $\ensuremath{-}1$ for graphene--mesoporous Si nanocomposites formed at temperatures above $800{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. 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.