Measurement of phosphorus segregation in silicon at the atomic-scale using STM

In order to fabricate precise atomic-scale devices in silicon using a combination of scanning tunnelling microscopy (STM) and molecular beam epitaxy it is necessary to minimize the segregation/diffusion of dopant atoms during silicon encapsulation. We characterize the surface segregation/diffusion of phosphorus atoms from a $\delta$-doped layer in silicon after encapsulation at 250$^{\circ}$C and room temperature using secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES), and STM. We show that the surface phosphorus density can be reduced to a few percent of the initial $\delta$-doped density if the phosphorus atoms are encapsulated with 5 or 10 monolayers of epitaxial silicon at room temperature. We highlight the limitations of SIMS and AES to determine phosphorus segregation at the atomic-scale and the advantage of using STM directly

Ion-assisted deposition of silicon films

Die vorliegende Arbeit untersucht die Wachstumsvorgänge sowie die strukturellen und elektrischen Eigenschaften von Si-Epitaxieschichten aus der ionenassistierten Deposition (IAD). Bei der IAD werden Si-Atome durch einen Elektronenstrahlverdampfer bereitgestellt und in der Gasphase durch Elektronenemission aus einem Glühdraht teilweise ionisiert; der Ionisationsgrad beträgt ca. 1 %. Eine angelegte Spannung beschleunigt diese Si+ Ionen zum Substrat hin. Die Ko-Evaporation von Bor bzw. Phosphor ermöglicht die in-situ Dotierung der Epitaxieschichten zur Herstellung von pn-Übergängen. Die epitaktische Abscheidung von Si mittels IAD ist auf beliebigen Substratorientierungen möglich. Die Defektdichte und die Minoritätsträgerdiffusionslänge hängen aber stark von der Substratorientierung und der Beschleunigungsspannung ab. Dieses Ergebnis ist auf Unterschiede in der Oberflächenrekonstruktion und in den Aktivierungsenergien für atomare Diffusionsprozesse zurückzuführen. Bei der Betrachtung der Wachstumsmechanismen bei der IAD müssen zwei Temperaturbereiche unterschieden werden: Im Temperaturbereich < 400 °C unterstützen interstitielle Atome das epitaktische Wachstum, bei höheren Temperaturen dominiert die direkte Erhöhung der Adatommobilität durch Ionenbeschuß der Wachstumsoberfläche. Die optimale Ionenenergie liegt im Bereich 8 ... 20 eV für (100)-orientierte Epitaxieschichten. Diese Arbeit vertieft wesentlich das Verständnis der Wachstumsvorgänge bei der ionenassistierten Deposition von Si-Epitaxieschichten bei Depositionstemperaturen unterhalb von 650 °C und bietet erstmals eine grundlegende Evaluierung des Potentials von Si-Niedertemperaturepitaxieschichten. Eine umfassende Untersuchung struktureller und elektrischer Eigenschaften der Epitaxieschichten hat zur Herstellung von Schichten mit sehr guten Majoritäts- und Minoritätsträgereigenschaften bei einer Rekord-Depositionsrate von 0,8 µm/min geführt.This work investigates growth mechanisms as well as structural and electrical properties of epitaxial Si layers deposited by ion-assisted deposition (IAD). Ion-assisted deposition uses Si atoms supplied by an electron beam evaporator and ionized in the gas phase by electron bombardment from a hot tungsten wire; the fraction of ionized atoms is on the order of 1 %. A voltage applied between ionization stage and substrate accelerates these Si+ ions towards the substrate. Co-evaporation of boron and phosphorous enables in-situ doping of epitaxial layers for fabrication of pn-junctions. Silicon deposited by IAD grows epitaxially on various substrate orientations. However, defect density and minority carrier diffusion length strongly depend on substrate orientation and acceleration voltage. This result is attributed to differences in surface reconstruction and activation energies for atomic diffusion processes. Investigating the growth mechanisms in IAD, two regimes have to be distinguished: In the temperature range below 400 °C interstitial atoms support epitaxial growth. At deposition temperatures above 400 °C, direct enhancement of adatom mobility by ion bombardment dominates. The optimal ion energy is in the range of 8 ... 20 eV for (100)-oriented epitaxial layers. This work significantly enhances the understanding of growth processes operative during ion-assisted deposition of epitaxial Si layers at deposition temperatures below 650 °C and for the first time offers a basic evaluation of the potential of epitaxial Si layers deposited at low temperatures. A comprehensive investigation of structural and electrical properties of epitaxial layers resulted in the deposition of layers with very good majority and minority carrier properties at a record deposition rate of 0.8 µm/min for low temperature Si epitaxial growth

Modelling Recycling in Life Cycle Assessment of Perovskite/Silicon Tandem Modules

International audiencePerovskite on silicon tandem is a promising photovoltaic (PV) technology that can help achieve module power conversion efficiencies beyond 30%. To ensure its potential on the path towards sustainable development, an appropriate end-of-life (EoL) management is critical. The environmental consequences of such EoL management should be evaluated using widely accepted methods, if possible, objective and standardized. The aim of this research is to identify the most suitable methodological approaches to account for recycling when modelling the life cycle of PV modules. Life cycle assessment (LCA) was applied to determine the environmental impacts of a perovskite/silicon tandem system over its whole life cycle. A functional unit of 1 m2 of perovskite/silicon tandem modules was used. Six different approaches for EoL modelling were applied to analyze the effect on the LCA results. The results for the climate change category range from 41 to 58 kg CO2-eq/m2 and the Cumulative Energy Demand (CED) from 530 to 714 MJ/m2 . For the evaluated categories, the cut-off with economic allocation provides the highest results and the closed-loop allocation the lowest. Different EoL modelling approaches lead to non-negligible differences in the results. A clear justification of methodological choices related to EoL modelling is, therefore, essential to ensure the representativeness of the LCA results. With respect to the choice of EoL modelling for a prospective LCA of PV module including recycling, it is recommended: 1) choosing an intermediate solution for EoL modelling that provides incentives for both the use of recycled material as an input and the choice of recycling as the EoL preferred option, 2) using the cut-off method and the closed-loop allocation for the sensitivity analysis and to check the robustness of the results

Modelling recycling for the life cycle assessment of perovskite/silicon tandem modules

International audienceWith the massive growth of the global capacity of photovoltaics (PV) over the last decade, the PV waste is expected to increase dramatically in the near future. Having potential to reduce the use of raw materials and preserve natural resources, PV recycling is attracting more and more attention. This being said, the environmental impacts over the life cycle of PV technologies, including the end-of-life (EoL) stage, should be evaluated carefully. Life cycle assessment (LCA) is currently the most common methodology to assess the potential environmental impacts of a product throughout its entire life cycle. However, the modelling of recycling in LCA has always been a challenge and no consensus has yet been reached, since the treatment of recycling does not only involve an EoL management of waste, but also the production of recycled material. Perovskite on silicon tandem is a widely investigated emerging PV technology having the potential to overcome the power conversion efficiency (PCE) limit of the single-junction crystalline silicon technology. The EoL modelling seems more challenging in the case of emerging technologies for which the EoL is more uncertain than for established technologies. In this article, six common and important approaches of EoL modelling in LCA were applied to future perovskite/silicon tandem modules to analyze the effect of the different EoL modelling approaches on the LCA results. The aim was to identify the most suitable methodological approaches to account for recycling, when modelling the life cycle of PV modules. The environmental performance of perovskite/silicon tandem modules was assessed over their life cycle and expressed in terms of impacts per m2 of module. After testing the six EoL modelling approaches and comparing the LCA results, the EoL modelling choice was found to lead to non-negligible differences. For example, in terms of climate change, the impact of the tandem modules ranges from 45 to 59 kg CO2-eq/m2. Among the six EoL modelling options, the approaches of simple cut-off and cut-off with economic allocation are more oriented towards the promotion of high rates of recycled material integrated as an input to the assessed product among industrial actors, while the approach of closed-loop allocation provides incentives to maximize the ratio of recycling at the EoL, regardless the initial ratio of recycled content within the product. Some approaches such as the circular footprint formula (CFF) tend to provide both incentives to increase the content of recycled input material in the manufacturing of the product and the recycling ratio at the EoL of such product. After applying the different alternatives, a set of recommendations to select the relevant EoL modelling approaches are provided: 1) the CFF is recommended as a representative approach due to its wide applicability, tending to provide an intermediate result and reflecting the characteristics of materials; 2) sensitivity analysis should be applied to check the robustness of the results, 3) the cut-off approach and the closed-loop allocation should be used at least for the sensitivity analysis