Browsing by Author "Pasanen, Toni P."
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Item Al-neal Degrades Al2O3 Passivation of Silicon Surface(WILEY-V C H VERLAG GMBH, 2021-12) Setälä, Olli E.; Pasanen, Toni P.; Ott, Jennifer; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupAtomic layer deposited (ALD) aluminum oxide (Al2O3) has emerged as a useful material for silicon devices due to its capability for effective surface passivation and ability to generate p+ region underneath the oxide as active or passive component in semiconductor devices. However, it is uncertain how Al2O3 films tolerate the so-called Al-neal treatment that is a necessary process step in devices that also contain silicon dioxide (SiO2) passivation layers. Herein, it is reported that the Al-neal process is harmful for the passivation performance of Al2O3 causing over eightfold increase in surface recombination velocity (SRV) (from 0.9 to 7.3 cm s−1). Interestingly, it is also observed that the stage at which the so-called activation of Al2O3 passivation is performed impacts the final degradation strength. The best result is obtained when the activation step is done at the end of the process together with the Al-neal thermal treatment, which results in SRV of 1.7 cm s−1. The results correlate well with the measured interface defect density, indicating that the Al-neal affects defects at the Si/SiOx/Al2O3 interface. The root causes for the defect reactions are discussed and possible reasons for the observed phenomena are suggested.Item Black silicon significantly enhances phosphorus diffusion gettering(2018-01-31) Pasanen, Toni P.; Laine, Hannu; Vähänissi, Ville; Schön, Jonas; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Fraunhofer Institute for Solar Energy SystemsBlack silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1e13 cm−3 to less than 1e10 cm−3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.Item Compatibility of 3-D Printed Devices in Cleanroom Environments for Semiconductor Processing(2019-01-01) Pasanen, Toni P.; von Gastrow, Guillaume; Heikkinen, Ismo T. S.; Vähänissi, Ville; Savin, Hele; Pearce, Joshua; Department of Electronics and Nanoengineering; Hele Savin Group3-D printing has potential to revolutionize manufacturing of customized low-cost scientific equipment, and numerous self-designed applications have already been realized and demonstrated. However, the applicability of 3-D printed devices to cleanrooms used for semiconductor processing is not as straightforward, as the controlled environment sets strict requirements for the allowed materials and items. This work investigates the opportunity to utilize 3-D printing in cleanrooms by analyzing three potentially suitable polymers (polylactic acid (PLA), acrylonitrile butadiene styrene (ABS) and polypropylene (PP)) for two applications that do not require particular chemical compatibility: a custom single wafer storage box and a wafer positioner for a metrology system. The designed equipment supplements commercial selection by introducing support for samples with non-standard shape or size and simultaneously reduces the price of often extensively expensive cleanroom equipment. The results show that the single wafer boxes 3-D printed from PLA and ABS generate as little particles as a commercial equivalent, whereas slightly more particles are found from a wafer stored in the self-printed PP box. Nevertheless, the number of particles on all wafers is in the same order of magnitude, indicating that 3-D printed boxes are not significant particle sources. The 3-D wafer positioner seems to cause a negligible particle increase on the manipulated wafer, while abrasion of the mechanical parts generate larger numbers of particles that may disperse in the environment. Regular cleaning of those parts is thus recommended, and applicability in a cleanroom environment will depend on the cleanliness constraints. Elemental analysis reveals that 3-D printed objects contain no other harmful metal impurities than those originating from colorants. Thus, 3-D printing filaments with natural color should be preferred for purposes, where metal contamination could be an issue, including semiconductor processing. Finally, 3-D printing filaments considered in this study are shown to be resistant to isopropanol and deionized water, which is critical for efficient cleaning for use of 3-D printed objects in cleanrooms. The results demonstrate that simple 3-D printed objects, such as wafer boxes or tweezers, are not notable contamination sources, and hence, are equally suitable for use in cleanrooms as the commercial equivalents.Item Economic Advantages of Dry-Etched Black Silicon in Passivated Emitter Rear Cell (PERC) Photovoltaic Manufacturing(2018-09-05) Modanese, Chiara; Laine, Hannu; Pasanen, Toni P.; Savin, Hele; Pearce, Joshua; Department of Electronics and Nanoengineering; Hele Savin GroupIndustrial Czochralski silicon (Cz-Si) photovoltaic (PV) efficiencies have routinely reached >20% with the passivated emitter rear cell (PERC) design. Nanostructuring silicon (black-Si) by dry-etching decreases surface reflectance, allows diamond saw wafering, enhances metal gettering and may prevent power conversion efficiency degradation under light exposure. Black-Si allows a potential for >20% PERC cells using cheaper multicrystalline silicon (mc-Si) materials, although dry-etching is widely considered too expensive for industrial application. This study analyzes this economic potential by comparing costs of standard texturized Cz-Si and black mc-Si PERC cells. Manufacturing sequences are divided into steps, and costs per unit power are individually calculated for all different steps. Baseline costs for each step are calculated and a sensitivity analysis run for a theoretical 1 GW/year manufacturing plant, combining data from literature and industry. The results show an increase in the overall cell processing costs between 15.8% and 25.1% due to the combination of black-Si etching and passivation by double-side atomic layer deposition. Despite this increase, the cost per unit power of the overall PERC cell drops by 10.8%. This is a significant cost saving and thus energy policies are reviewed to overcome challenges to accelerating deployment of black mc-Si PERC across the PV industry.Item Effect of anode sheet resistance on rise time of black silicon induced junction photodiodes(SPIE - The International Society for Optical Engineering, 2022-03-04) Heinonen, Juha; Haarahiltunen, Antti; Vähänissi, Ville; Pasanen, Toni P.; Savin, Hele; Juntunen, Mikko A.; Department of Electronics and Nanoengineering; Hele Savin GroupBlack silicon induced junction photodiodes have been shown to have nearly ideal responsivity across a wide range of wavelengths. Another important characteristic of a high-quality photodiode is rise time which can be used to approximate bandwidth of the photodiode. We show experimentally that the rise time of black silicon photodiodes is shorter than in planar photodiodes when alumina layer with similar charge is used to make an induced junction in both. Additionally, we show that the rise time can be rather well approximated using an analytical equation, which combines Elmore delay from equivalent circuit with standard RC-delay arising from series and load resistances.Item Effective passivation of p+ and n+ emitters using SiO2/Al2O3/SiNx stacks: Surface passivation mechanisms and application to industrial p-PERT bifacial Si solar cells(2018-11-01) Huang, Haibing; Modanese, Chiara; Sun, Shenghua; von Gastrow, Guillaume; Wang, Jianbo; Pasanen, Toni P.; Li, Shuo; Wang, Lichun; Bao, Yameng; Zhu, Zhen; Sneck, Sami; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; China Sunergy; Beneq OyIn this paper, we present an effective emitter passivation scheme using SiO2/Al2O3/SiNx stacks. Our study shows that SiO2/Al2O3/SiNx stacks can well passivate both p+ and n+ emitters due to an excellent chemical passivation combined with a weak field-effect passivation. Good quality boron and phosphorus emitters were achieved over a broad emitter-doping range, as demonstrated by post-fired emitter saturation current of 20 and 30 fA cm−2, respectively. Based on the results obtained with SiO2/Al2O3/SiNx emitter passivation, we present an industrial roadmap for a p-PERT bifacial cell structure. Using this roadmap, we demonstrate industrial p-PERT bifacial cells with front side efficiency of 20.5%, rear side efficiency of 19.8% (bifaciality factor BF = 0.98) for rear textured cells and 17.5% (BF = 0.85) for rear planar cells. In particular, the cells with bifacial SiO2/Al2O3/SiNx passivation on both p+ and n+ emitters also demonstrate promising performance and a simplified cell process. The results show that SiO2/Al2O3/SiNx emitter passivation scheme is a promising candidate for photovoltaic industry.Item Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation(IOP Publishing Ltd., 2021-01-15) Chen, Kexun; Isometsä, Joonas; Pasanen, Toni P.; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupNanostructured surfaces are known to provide excellent optical properties for various photonics devices. Fabrication of such nanoscale structures to germanium (Ge) surfaces by metal assisted chemical etching (MACE) is, however, challenging as Ge surface is highly reactive resulting often in micron-level rather than nanoscale structures. Here we show that by properly controlling the process, it is possible to confine the chemical reaction only to the vicinity of the metal nanoparticles and obtain nanostructures also in Ge. Furthermore, it is shown that controlling the density of the nanoparticles, concentration of oxidizing and dissolving agents as well as the etching time plays a crucial role in successful nanostructure formation. We also discuss the impact of high mobility of charge carriers on the chemical reactions taking place on Ge surfaces. As a result we propose a simple one-step MACE process that results in nanoscale structures with less than 10% surface reflectance in the wavelength region between 400 and 1600 nm. The method consumes only a small amount of Ge and is thus industrially viable and also applicable to thin Ge layers.Item Impact of doping and silicon substrate resistivity on the blistering of atomic-layer-deposited aluminium oxide(Elsevier Science B.V., 2020-08-30) Ott, Jennifer; Pasanen, Toni P.; Gädda, Akiko; Garín, Moises; Rosta, Kawa; Vähänissi, Ville; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupAluminium oxide (Al2O3) thin films grown at low temperatures using atomic layer deposition (ALD) are known to often suffer from local delamination sites, referred to as “blisters”, after post-deposition annealing during device processing. In this work, we report our observation that doping of the silicon substrate has an effect on blister formation. The introduction of a highly doped layer by diffusion or implantation is found to significantly reduce blistering, compared to the non-doped regions in the immediate vicinity. Similar behavior is observed for both phosphorus and boron doping. Further investigation of this phenomenon using substrates with different resistivities reveals that even when introduced already during silicon crystal growth, doping affects the blistering of aluminium oxide films. Changes in several properties of silicon affected by doping, most importantly surface terminating groups, native oxide growth, and passivation of defects with hydrogen, are discussed as potential reasons behind the observed effect on blistering.Item Impact of Standard Cleaning on Electrical and Optical Properties of Phosphorus-Doped Black Silicon(2018-03-19) Pasanen, Toni P.; Laine, Hannu; Vähänissi, Ville; Salo, Kristian; Husein, Sebastian; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin Group; Arizona State UniversityBlack silicon (b-Si) has been estimated to considerably grow its market share as a front texture of high-efficiency silicon solar cells. In addition to excellent optical properties, high-efficiency cell process requires extreme cleanliness of the bulk material, and thus cleaning of b-Si surfaces is often a critical process step. While standard clean (SC) 1 solution efficiently removes possible contamination from wafer surfaces, we show here that it may cause challenges in b-Si solar cells. First, the silicon etch rate in SC1 solution is shown to depend on the phosphorous concentration and as high rate as ∼1.4 nm/min is observed on planar emitter surfaces. When extending the study to b-Si, which has much larger surface area in contact with the cleaning solution, even higher volumetric Si consumption occurs. This is observed in significant changes in emitter doping profiles, for instance, a 10 and 30-min cleaning increases the sheet resistance from 47 to 57 Ω/□ and 127 Ω/□, respectively. Furthermore, the SC1 solution alters substantially the nanostructure morphology, which impacts the optics by nearly doubling and more than tripling the surface reflectance after a 30 and 60-min immersion, respectively. Thus, uncontrolled cleaning times may impair both the electrical and optical properties of b-Si solar cells.Item Low-T anneal as cure for LeTID in Mc-Si PERC cells(AMERICAN INSTITUTE OF PHYSICS, 2019-08-27) Yli-Koski, Marko; Pasanen, Toni P.; Heikkinen, Ismo; Serué, Michael; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupLight and elevated temperature induced degradation (LeTID) is known to be affected by the last dark anneal that the silicon wafers or cells experience prior to illumination. Here we study how low-temperature dark anneal performed on fully processed multicrystalline silicon (mc-Si) passivated emitter and rear solar cells (PERC) influences LeTID characteristics, both the intensity of the degradation and the degradation kinetics. Our results show that a relatively long anneal at 300 °C provides an efficient means to minimize LeTI D while too short dark anneal at the same temperature seems to have a negative impact on the subsequent degradation under light soaking. Finally, we compare the experimental results with the model originally developed for metal precipitation and discuss the possibility of metals being involved in LeTID mechanism.Item (poster) Low-T anneal as cure for LeTID in Mc-Si PERC cells(2019) Yli-Koski, Marko; Pasanen, Toni P.; Heikkinen, Ismo; Serué, Michael; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupItem Spatial uniformity of black silicon induced junction photodiode responsivity(SPIE, 2023-03-14) Heinonen, Juha; Haarahiltunen, Antti; Vähänissi, Ville; Pasanen, Toni P.; Savin, Hele; Toivanen, Juha; Juntunen, Mikko A.; Department of Electronics and Nanoengineering; Jiang, Shibin; Digonnet, Michel J.; Hele Savin GroupBlack silicon induced junction photodiodes have nearly ideal responsivity across a wide range of wavelengths between 175-1100 nm, with external quantum efficiency over 99 % at visible wavelengths, when a single spot is measured using light beam between 1 to 2mm in diameter. The spatial uniformity of responsivity is also an important characteristic of a high-quality photodiode, when considering its usage as a reference in photometry. We study here the spatial uniformity of responsivity of large area (8mmx8mm) black silicon photodiodes at 405 nm wavelength. Our results show that the spatial non-uniformity is less than 0.5 % over 90 % of the surface area, and thus the photodiodes meet the thigh criteria typically set for reference standards and are hence suitable for such application.Item Temperature dependency of responsivity and dark current of nearly ideal black silicon photodiodes(SPIE - The International Society for Optical Engineering, 2021-03-05) Heinonen, Juha; Haarahiltunen, Antti; Serue, Michael; Kriukova, Daria; Vähänissi, Ville; Pasanen, Toni P.; Savin, Hele; Juntunen, Mikko A.; Department of Electronics and Nanoengineering; Hele Savin Group; ElFys Inc.A high-quality photodiode has high signal-to-noise ratio (SNR), which is ultimately defined by the responsivity and dark current of the photodiode. Black silicon induced junction photodiodes have been shown to have nearly ideal responsivity across a wide range of wavelengths between 175-1100 nm at room temperature (RT). Here we present their spectral responsivity stability and dark current at different temperatures. Both quantities show temperature dependencies similar to conventional pn-junction photodiodes, proving that black silicon photodiodes maintain their improved SNR also at temperatures other than RT.
