Temperature dependent photoluminescence (PL) spectroscopy along with structural investigations of luminescent porous Si enable us to experimentally distinguish between the relative contributions of band-to-band and oxide interface mediated electronic transitions responsible for light emission from these nanostructures. Porous Si samples formed using high current densities (J ≥ 80 mA/cm2) have large porosities (P ≥ 85%) and consequently smaller (∼1-6 nm) average crystallite sizes. The PL spectra of these high porosity samples are characterized by multiple peaks. Two dominant peaks—one in the blue regime and one in the yellow/orange regime, along with a very low intensity red/NIR peak, are observed for these samples. The high energy peak position is nearly independent of temperature, whereas the yellow/orange peak red-shifts with increasing temperature. Both the peaks blue shift with ageing and with increasing porosity. The intensity of the blue peak increases whereas the yellow/orange peak decreases with increasing temperature, while the intensity and peak position of the very low intensity red/NIR peak appears to be unaffected by temperature, porosity, and ageing. The low porosity samples (P ≤ 60%) on the other hand exhibit a single PL peak whose intensity decreases and exhibits a very small red spectral shift with increase in temperature. From the variation of intensity and PL peak positions, it is established that both quantum confinement of excitons and oxide related interfacial defect states play dominant role in light emission from porous Si and it is possible to qualitatively distinguish and assign their individual contributions.
2668 2916 ** These authors contributed equally to this work.Random arrays of oxide-passivated silicon nanorods have been obtained by natural oxidation of electrochemically etched porous silicon in air. The charge transport through these nanorods exhibits intriguing characteristics. The I-V characteristics are non-linear, asymmetric, hysteretic, and exhibit resistive switching. Three different charge transport mechanisms dominate in three different ranges of bias and temperature. At high bias, the Fowler-Nordheim tunneling through the oxide barrier is the dominant conduction mechanism. The Pool-Frenkel emission takes over at moderate bias, while at low bias, trap controlled space-charge-limited conduction is the governing mode of charge transport. The bias voltage for cross-over from one transport mechanism to another is sensitively dependent on temperaturethe increase of temperature lowers the cross-over voltage. The observed phenomena can be explained in the framework of lateral transport through a disordered assembly of interconnected semiconducting nanorods. Such multiple transport mechanisms along with tunable cross-over from one mechanism to another simply by changing the bias or temperature, render these nanostructures amenable for a variety of applications. Additionally, the observed resistive switching makes them extremely promising candidates for low power-consuming resistive random access memory devices.
Unipolar resistive switching (URS) is observed in isolated Si-SiO core-shell nanostructures. I-V characteristics recorded by a conductive atomic force microscope tip show SET and RESET processes with self compliance behavior. Hopping of carriers through defect states in the high resistance state (HRS) and space charge limited conduction in the low resistance state (LRS) are found to be the dominant carrier transport mechanisms in Si-SiO core-shell nanostructures. URS between LRS and HRS may be attributed to the transition between hydrogen bridge (Si-H-Si) and hydrogen doublet (Si-HH-Si) defects. During RESET process, charge carriers tunnel through the nanostructure giving rise to oscillatory conduction.
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