Abstract
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.
Highlights
Light emission form porous Si has been one of the most well researched areas since the discovery of visible photoluminescence (PL) in 1990.1 Despite two decades of extensive research on various aspects of this promising material, the mechanism of light emission has hitherto remained controversial.2–12. Though it is more-or-less accepted that both quantum confinement of excitons, as well as localized defect states at the surface/interface of nanocrystalline Si and the surrounding oxide layer have combined contributions towards visible PL from porous Si, it has remained difficult to isolate the relative contributions of interfacial defects and size
We show explicitly that primarily two competitive processes involving band-to-band and band-to-interface transitions are responsible for radiative emission from porous Si which account for the appearance of multiple peaks in high porosity porous Si, whereas low porosity samples are characterized by a single peak
Temperature dependent PL characteristics of porous Si nanostructures reveal that two competitive processes involving band-to-band and oxide related interface mediated transitions are responsible for radiative emission from porous Si
Summary
Light emission form porous Si has been one of the most well researched areas since the discovery of visible photoluminescence (PL) in 1990.1 Despite two decades of extensive research on various aspects of this promising material, the mechanism of light emission has hitherto remained controversial. Though it is more-or-less accepted that both quantum confinement of excitons, as well as localized defect states at the surface/interface of nanocrystalline Si (nc-Si) and the surrounding oxide layer have combined contributions towards visible PL from porous Si, it has remained difficult to isolate the relative contributions of interfacial defects and size.. Light emission form porous Si has been one of the most well researched areas since the discovery of visible photoluminescence (PL) in 1990.1 Despite two decades of extensive research on various aspects of this promising material, the mechanism of light emission has hitherto remained controversial.. Light emission form porous Si has been one of the most well researched areas since the discovery of visible photoluminescence (PL) in 1990.1 Despite two decades of extensive research on various aspects of this promising material, the mechanism of light emission has hitherto remained controversial.2–12 Though it is more-or-less accepted that both quantum confinement of excitons, as well as localized defect states at the surface/interface of nanocrystalline Si (nc-Si) and the surrounding oxide layer have combined contributions towards visible PL from porous Si, it has remained difficult to isolate the relative contributions of interfacial defects and size.. Low temperature PL spectra of these samples, the ones having high porosity allow us to clearly distinguish between the contributions of the two competing processes
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