Ultrafast photoconductivity of silicon nanostructures measured by time-resolved terahertz spectroscopy: carrier dynamics, excitons and charge localization

Abstract

Time-resolved terahertz spectroscopy (TRTS) has been applied to study the sub-picosecond to nanosecond carrier dynamics in silicon nanowires (Si NWs), silicon quantum dots (Si QDs) and silicon nanocrystals (Si NCs) surrounded by different organic and inorganic environments. In a TRTS experiment, the changes in the conductivity spectrum due to a photoexcitation event are measured over THz frequencies spanning 0.2 - 2 THz. The TRTS technique is thus employed as a non-contact electrical probe that can determine the nature of photoinduced carriers in a semiconductor (i.e. free-carriers, localized carriers or excitons) as well as the time scales the excited carriers exist in the material. We have developed a method to obtain the average photoconductivity response of an individual Si nanostructure from TRTS measurements on an ensemble of Si nanostructures dispersed in an organic solvent. These measurements indicate different relaxation pathways in Si NWs, Si QDs, and bulk c-Si. For the Si NWs we observe [greater than]1 ns carrier lifetimes that are dominated by surface recombination. Through a surface recombination model, we were able to determine surface recombination velocities ranging from ~1100-1700 cm s[superscript -1] which depend on the processing conditions of the Si NWs. The Si NCs have markedly different decay dynamics where we initially observe hot carriers that rapidly thermalize within 1 ps. The cooled carriers then form bound excitons within the Si QDs which are long lived. We also were able to extract the intrinsic mobility of the Si NWs, which agreed with extracted values from bulk c-Si. We observed size-dependent change in the hot carrier relaxation time in the Si QDs, where small Si QDs demonstrated slower decay times than large Si QDs. The size-dependent polarizabilities of excitons in Si QDs were obtained from time-domain THz spectroscopy measurements and followed an r[superscript 4] dependence. Ultrafast carrier dynamics measurements on hydrogenated nanocrystalline silicon (nc-Si:H) samples revealed a fast electron transfer process occurring for electrons initially excited in a-Si that then inject into Si NCs that are embedded in the a-Si matrix. The dynamics could be modeled with a trap-mediated electron transfer process where hot carriers in a-Si become trapped at defect states on the surface of the Si NCs and then can thermally emit into the Si NCs. We observed a dependence of this process on crystalline volume fraction, where the efficiency of the electron transfer process was greater for large volume fractions.