Quantum confinement and recombination dynamics in silicon nanocrystals embedded in Si/SiO2 superlattices

Abstract

In this study the structural and optical properties of nanocrystalline Si/SiO2 superlattices have been investigated and discussed. Ordered planar arrays of silicon nanocrystals (Si-nc) have been formed by thermal annealing of ten period amorphous Si/SiO2 superlattices prepared by plasma enhanced chemical vapor deposition. Thermal processing of the superlattices results in well separated (by about 5 nm of SiO2) nanocrystalline Si layers, when the annealing temperature does not exceed 1200 °C. The photoluminescence (PL) properties of these layers have been studied in details. The PL peaks wavelength has been found to depend on the laser pump power; this intriguing dependence, consisting in a marked blueshift for increasing power, has been explained in terms of the longer lifetime characterizing larger Si-nc. It is also observed that these decay lifetimes exhibit a single exponential behavior over more than two orders of magnitude, in clear contrast with the typical, nonsingle exponential trends observed for Si-nc uniformly dispersed inside an insulating matrix. We attributed this peculiar behavior to the lack of interaction among nanocrystals, due to their large reciprocal distance. In agreement with the carrier quantum confinement theory, we have found that the wavelength of the PL peak can be properly tuned by changing the annealing temperature and/or the thickness of the Si layers of the superlattices, and, in turn, the Si-nc mean size. Moreover, the observed lifetimes remain very long (about 0.3 ms) even at room temperature, revealing the absence of relevant nonradiative decay processes in these samples. Furthermore, we have used the experimental PL intensities and decay times to evaluate the radiative rate as a function of the temperature; the obtained data are in good agreement with a model proposed by Calcott in the case of porous silicon. All of these data are presented, discussed, and explained within a consistent picture.

Photoluminescence of undoped and erbium-doped SiO/SiO/sub 2/ multilayers

“…In correlation with the structural study, this luminescence can be attributed to the crystallized silicon nanoclusters which have appeared in the active layer. The PL energy (equal to 1.57 eV) correlated to the Si-nc mean size (3.2 nm) is in good agreement with previous works [9,10]. Moreover to demonstrate that the PL originates from the Sinc, the size effect on the PL energy was studied.…”

Silicon-rich SiO2/SiO2 multilayers: A promising material for the third generation of solar cell

“…Among all the fabrication techniques reported in the literature for providing photoluminescent Sincl, the most original ways to control the Si grain size are: ͑i͒ the laser pyrolisis of silane leading to the creation of free Si nanocrystals 18 or ͑ii͒ the deposition of Si/ SiO 2 multilayers ͑MLs͒ in which the Si sublayer thickness should not exceed the critical value predicted by the QC model. [19][20][21][22] In addition to the control of the Si-ncl size, the ML approach offers the possibility to control the SiO 2 sublayer thickness, which is a key point for allowing the carrier transport through the multilayered structure.…”

Photoelectron and Photosensitization Properties of Silicon Nanocrystal Ensembles

“…Thus, the exciton lifetime depends on the transfer rate; as a result, the kinetics of PL intensity drop "stretches". This assumption is supported by the study of the PL time characteristics of nc-Si ensembles separated by sufficiently thick (about 5 nm) spatial SiO 2 barriers (Vinciguerra, et al, 2000). In this case, nc-Si crystals are supposed to be well isolated; therefore, no energy exchange occurs between them and the lifetime is constant for nc-Si of a given size.…”

“…In this case, nc-Si crystals are supposed to be well isolated; therefore, no energy exchange occurs between them and the lifetime is constant for nc-Si of a given size. Indeed, the PL kinetics detected in such systems turns out to be monoexponential with a characteristic time of 0.2-0.8 ms (Vinciguerra, et al, 2000). However, recent theoretical investigation (Chen, 2003) predicts a stretched exponent for PL kinetics even for individual (isolated) nanocrystals in the case where the rate of trapping of photoexcited charge carriers is higher than the rate of their recombination.…”