Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO2 layers followed by thermal annealing post-treatment. Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on 28Si. Indeed, for high incident energy, nuclear levels of 32S and 31P can be reached, and cross-sections for (α,α) and (α,p0) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO2, even for low fluences. In this work, (α,α) and (α,p0) reactions are applied to study depth distributions of 70 keV 28Si+ ions implanted in 200 nm SiO2 layers with fluences of 1 × 1017 and 2 × 1017 cm-2. Analysis is performed above ER = 3864 keV to take advantage of resonances in both (α,α) and (α,p0) cross-sections. We show how (α,p0) reactions can complement results provided by resonant backscattering measurements in this complex case. © 2010 Elsevier B.V. All rights reserved

Depth-profiling of implanted 28Si by (α,α) and (α,p0) reactions

Silicon nanocrystals enclosed in thin films (Si quantum dots or Si QDs) are regarded to be the cornerstone of future developments in new memory, photovoltaic and optoelectronic products. One way to synthesize these Si QDs is ion implantation in SiO2 layers followed by thermal annealing post-treatment. Depth-profiling of these implanted Si ions can be performed by reactions induced by α-particles on 28Si. Indeed, for high incident energy, nuclear levels of 32S and 31P can be reached, and cross-sections for (α,α) and (α,p0) reactions are more intense. This can help to increase the signal for surface silicon, and therefore make distinguishing more easy between implanted Si and Si coming from the SiO2, even for low fluences. In this work, (α,α) and (α,p0) reactions are applied to study depth distributions of 70 keV 28Si+ ions implanted in 200 nm SiO2 layers with fluences of 1 × 1017 and 2 × 1017 cm-2. Analysis is performed above ER = 3864 keV to take advantage of resonances in both (α,α) and (α,p0) cross-sections. We show how (α,p0) reactions can complement results provided by resonant backscattering measurements in this complex case. © 2010 Elsevier B.V. All rights reserved

Method for fabricating third generation photovoltaic cells based on Si quantum dots using ion implantation into SiO2

In this paper, we report on the synthesis of silicon quantum dots for photovoltaic applications by means of ion implantation followed by annealing. Nucleation was achieved by implanting Si ions into SiO2 thin films, previously thermally grown on a Si(100) substrate, and annealing to 1100 °C. Passivation was used for photoluminescence (PL) measurements. The thickness of the oxide layer, the stoichiometry of the implanted layer, and the depth profiles of the implanted ions were determined for all samples by both Rutherford backscattering spectroscopy (RBS) and ellipsometry techniques. Characterization by transmission electron microscopy (TEM) indicates that the diameter of the silicon quantum dots (Si-QDs) varies from 2 to 4 nm, which is less than the Bohr radius of bulk crystalline Si(∼5 nm). Optical and electrical properties have been investigated by PL and I-V measurements. When passivated silicon nanocrystals (Si-nc) embedded into SiO2 are excited using a 450 nm diode laser, they exhibit a strong PL emission in the range of 650-1000 nm. Based on these investigations, p-type Si-QDs/n-type c-Si junctions were fabricated and electrically characterized in the dark as well as under an AM1.5G terrestrial solar spectrum for nonimplanted, as-implanted, and implanted-annealed samples for different implantation fluences. The electrical curves of the structures under illumination demonstrate the photovoltaic behavior of the Si-QDs. Despite the weak light conversion of these devices, these results remain very promising and offer potentially unprecedented, vast improvements to third generation solar cells. © 2011 American Institute of Physics

Silicon nanocrystal synthesis by implantation of natural Si isotopes

Implantations of pure 28Si+, 29Si+, and 30Si+ into SiO2 can provide significant insight into the formation of silicon nanocrystals (Si-nc) and their light emission properties. Si-nc produced with different fractions of the heavier Si isotopes have been characterized by Raman and photoluminescence spectroscopy. Weak Stokes shifts of the Si-nc phonon peaks indicate that both the implanted Si and the native Si from the SiO2 substrate contribute to Si-nc nucleation. The Raman measurements also indicate that the Si isotopic composition of the Si-nc is similar to the Si isotopic fraction of the implanted SiO2. The Si-nc photoluminescence (PL) spectra are shifted towards the blue with increasing Si isotope mass, an indication that the increase of the Si-nc effective mass enhances the excitonic bandgap. Measurements from samples implanted with heavy isotopes at high Si excess concentrations indicate that the Si-nc isotope fraction evolves with annealing time such that the heaviest Si isotope are more concentrated in the vicinity of the Si-nc / SiO2 interface, which can modify the energy states involved in the radiative transitions associated with Si-nc. © 2009 Elsevier B.V. All rights reserved

Silicon nanocrystal synthesis by implantation of natural Si isotopes

Implantations of pure 28Si+, 29Si+, and 30Si+ into SiO2 can provide significant insight into the formation of silicon nanocrystals (Si-nc) and their light emission properties. Si-nc produced with different fractions of the heavier Si isotopes have been characterized by Raman and photoluminescence spectroscopy. Weak Stokes shifts of the Si-nc phonon peaks indicate that both the implanted Si and the native Si from the SiO2 substrate contribute to Si-nc nucleation. The Raman measurements also indicate that the Si isotopic composition of the Si-nc is similar to the Si isotopic fraction of the implanted SiO2. The Si-nc photoluminescence (PL) spectra are shifted towards the blue with increasing Si isotope mass, an indication that the increase of the Si-nc effective mass enhances the excitonic bandgap. Measurements from samples implanted with heavy isotopes at high Si excess concentrations indicate that the Si-nc isotope fraction evolves with annealing time such that the heaviest Si isotope are more concentrated in the vicinity of the Si-nc / SiO2 interface, which can modify the energy states involved in the radiative transitions associated with Si-nc. © 2009 Elsevier B.V. All rights reserved