Size dependence of photoluminescence quantum efficiency of Si nanocrystals

Miura, Satoru; Nakamura, Toshihiro; Fujii, Minoru; Inui, M.; Hayashi, Shinji

doi:10.1103/physrevb.73.245333

Cited by 73 publications

(98 citation statements)

References 25 publications

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“…We do that by using Eq. (1) and the radiative recombination rates as reported by Miura et al 7 As already mentioned, the results for samples A, B, and C, plotted in Fig. 4, clearly demonstrate that the heat treatment significantly increases the average IQE.…”

Section: Further Analysis and Discussionsupporting

confidence: 67%

“…Such a possibility has been suggested in the past 7,22,23 and concerns exciton diffusion between NCs by means of energy transfer resulting from dipole-dipole interaction, 24 the F€ orster resonant energy transfer (FRET). The nonradiative recombination is accomplished by energy transfer from an emitting NC to a dNC, which then effectively terminates further diffusion.…”

Section: -4mentioning

confidence: 99%

“…Past investigations established that the radiative rate of Si NCs is predominantly determined by the level of quantum confinement, i.e., the size of a NC, with only minor changes due to particulars of the preparation procedure. 7 Consequently, the IQE of Si NCs is emission wavelength dependent. Equation (1) implies that in order to determine the IQE for a specific emission wavelength (i.e., a particular NC size), it is necessary to uncouple the measured effective recombination rate into the radiative and nonradiative recombination components.…”

Section: Preliminary Considerationsmentioning

confidence: 99%

“…Equation (1) implies that in order to determine the IQE for a specific emission wavelength (i.e., a particular NC size), it is necessary to uncouple the measured effective recombination rate into the radiative and nonradiative recombination components. Miura et al 7 determined radiative rates for Si NCs of different sizes by measuring their emission in vicinity of an Au layer, which modifies the radiative rates, leaving the nonradiative recombination unaffected. Since the materials used in the present study have been prepared in the same way, we will assume that the radiative rates determined in Ref.…”

Section: Preliminary Considerationsmentioning

confidence: 99%

“…Which EQE values eventually should be reached depend on the quality of the solar cell; however, in order to realize the net increase in the number of emitted photons by the conversion layer, the EQE of emission (measured at low excitation energies, i.e., not involving the afore-mentioned space-separated quantum cutting process) should be well in excess of 50%, herewith yielding an EQE of at least 115% for the high energy end of the solar spectrum. 6 Previous research showed that while individual Si NCs can reach near 100% efficiencies, 7 the ensembles do not. The two major and mutually connected reasons for this discrepancy are the so-called "blinking" and formation of "dark" Si NCs (dNCs).…”

Section: Introductionmentioning

confidence: 99%

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Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

Limpens

Gregorkiewicz

2013

Journal of Applied Physics

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The percentage of dark silicon nanocrystals, i.e., the nanocrystals that are not able to radiatively recombine after absorption of a photon, is investigated by combining measurements of external and internal quantum efficiencies. The study is conducted on samples prepared by co-sputtering and subsequent heat treatments. We show that the external quantum efficiency is mainly limited by the presence of dark nanocrystals, which induce losses after direct excitation and also, as we propose, by indirect excitation enabled by energy migration. The percentage of dark nanocrystals can be decreased by high quality surface passivation as a result of low-temperature annealing in ambients of O 2 and H 2 . By using a non-passivated sample as a reference, the relation between the size of a nanocrystal and its probability of being dark is studied. Larger nanocrystals are demonstrated to function more likely as dark centers. The study shows that high external quantum efficiencies of Si nanocrystal ensembles can be realized for small, well passivated Si nanocrystals under suppression of excitation diffusion. V C 2013 AIP Publishing LLC. [http://dx

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Section: Further Analysis and Discussionsupporting

confidence: 67%

Section: -4mentioning

confidence: 99%

Section: Preliminary Considerationsmentioning

confidence: 99%

Section: Preliminary Considerationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

Limpens

Gregorkiewicz

2013

Journal of Applied Physics

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Optical Properties of Intrinsic and Shallow Impurity‐Doped Silicon Nanocrystals

Fujii

2010

Silicon Nanocrystals

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The electronic band structure of silicon (Si) crystals is significantly modified when the size is reduced to below the exciton Bohr radius ($4.9 nm) of bulk Si crystals due to the quantum size effect. The quantum size effect manifests itself as a high-energy shift of the luminescence band and an enhancement of the spontaneous emission rate. Furthermore, enlargement of the singlet-triplet splitting energy of exciton states with decreasing size has been demonstrated [1-4]. The modified band structure opens up new application of Si nanocrystals in the fields where bulk Si crystal has not been involved. Besides well-known high-efficiency visible photoluminescence (PL), one of the most exotic new feature of Si nanocrystals is their ability to generate a kind of active oxygen species called singlet oxygen by energy transfer from excitons to oxygen molecules (O 2 ) adsorbed onto the surface of Si nanocrystals [5]. This feature is due to the molecule-like energy structure of Si nanocrystals and is a direct consequence of the quantum size effects. Similarly, efficient photosensitization of rare earth ions by Si nanocrystals has been reported and this phenomenon is expected to lead to the development of an efficient planar waveguide-type optical amplifier operating at the optical telecommunication wavelength. Since almost all new features of Si nanocrystals arise from the modified electronic band structure, its deep understanding is indispensable to explore new nano-Si-based research fields and devices. Fortunately, the energy structure of Si nanocrystals is almost clarified, at least qualitatively, by detailed optical spectroscopy. In Section 3.2, we briefly summarize fundamental optical properties of intrinsic Si nanocrystals [1][2][3][4][5][6][7][8].The properties of Si nanocrystals can be controlled by the size, shape, surface termination, and so on. An additional freedom of material design can be introduced by impurity doping. The introduction of shallow impurities in a semiconductor significantly modifies its optical and electrical transport properties. Actually, a precise control of an impurity profile is key to achieve desirable functions in almost all kinds Silicon Nanocrystals: Fundamentals, Synthesis and Applications. Edited by Lorenzo Pavesi and Rasit Turan

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Photoluminescence Quantum Yield in Ensembles of Si Nanocrystals

Chung

Limpens

Gregorkiewicz

2017

Advanced Optical Materials

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energy exchange with defects. [2,12] The strength of the cooperative processes and their effect on the optical properties of an NC ensemble depend on the characteristics of the individual NCs themselves as well as on the ensemble properties, such as NC density and proximity, [13] confining potential of the embedding matrix [14] and its quality, etc. [6] The cooperative processes typically involve an energy barrier for their activation, and therefore will change with the excitation energy. On the other hand, it is well known that the Kasha-Vavilov rule, [15] which states that the PL QY is independent of the excitation energy, is frequently violated for colloidal semiconductor NCs, [16] since carriers that are photogenerated higher in the conduction/ valence bands experience an increased probability of capturing at defect states and/or escape to the outside of an NC, leading to its ionization [10] and a temporal loss of optical activity. In result, the PL QY of the NCs decreases typically at short pump wavelengths. In this study, we investigate the excitation energy dependence of the PL QY for Si NC layers and find that it varies strongly in different samples. We investigate and discuss possible physical mechanisms influencing the PL QY at different excitation energies and conclude on an important role of impact excitation and parasitic absorption. Sample PreparationThe samples used in this study were produced by a co-sputtering method in the form of multilayer (ML) structures, featuring multiple stacks (up to 100) of 3.5 nm thin active layers of Si NCs separated by SiO 2 barriers. In this process, two sputtering guns, with Si and SiO 2 targets, are used. By operating both guns simultaneously, an Si-rich substoichiometric SiO x "active" layer can be grown, while solely the gun with silicon dioxide is applied to develop a barrier layer of pure SiO 2 . The atomic composition of the active layer can be tuned by adjusting the power of the guns, and the layer thickness is controlled by the exposure time. For the samples investigated in this study, the silicon excess of 15%, 25%, and 30% was used in the active layer. The barrier thickness between two adjacent active layers was set at 5 nm to avoid exciton diffusion between these layers. [17] Extensive past research [18,19] has shown that, in contrast to thick homogeneous layers of Si NCs in SiO 2 , ML structures allow for a better size control and therefore yield ensembles with a more narrow size distribution.This study investigates the photoluminescence quantum yield for cosputtered solid-state dispersions of Si nanocrystals in SiO 2 with different size and density, and concludes that the absolute value of the photoluminescence quantum yield shows a varied dependence on the excitation energy. Physical mechanisms influencing the photoluminescence quantum yield at different excitation energy ranges are considered. Based on the experimental evidence, this study proposes a generalized description of the excitation dependence of photoluminescence quantum yield of Si nanocrys...

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Size dependence of photoluminescence quantum efficiency of Si nanocrystals

Cited by 73 publications

References 25 publications

Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

Optical Properties of Intrinsic and Shallow Impurity‐Doped Silicon Nanocrystals

Photoluminescence Quantum Yield in Ensembles of Si Nanocrystals

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