Browsing by Department "Hele Savin Group"
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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 AlOx surface passivation of black silicon by spatial ALD: Stability under light soaking and damp heat exposure(AVS Science and Technology Society, 2020-03-01) Heikkinen, Ismo T. S.; Koutsourakis, George; Virtanen, Sauli; Yli-Koski, Marko; Wood, Sebastian; Vähänissi, Ville; Salmi, Emma; Castro, Fernando A.; Savin, Hele; Hele Savin Group; National Physical Laboratory (NPL); Beneq Oy; Department of Electronics and NanoengineeringScientific breakthroughs in silicon surface passivation have enabled commercial high-efficiency photovoltaic devices making use of the black silicon nanostructure. In this study, the authors report on factors that influence the passivation stability of black silicon realized with industrially viable spatial atomic layer deposited (SALD) aluminum oxide (AlOx) under damp heat exposure and light soaking. Damp heat exposure conditions are 85 °C and 85% relative humidity, and light soaking is performed with 0.6 sun illumination at 75 °C. It is demonstrated that reasonably thick (20 nm) passivation films are required for both black and planar surfaces in order to provide stable surface passivation over a period of 1000 h under both testing conditions. Both surface textures degrade at similar rates with 5 and 2 nm thick films. The degradation mechanism under damp heat exposure is found to be different from that in light soaking. During damp heat exposure, the fixed charge density of AlOx is reduced, which decreases the amount of field-effect passivation. Degradation under light soaking, on the other hand, is likely to be related to interface defects between silicon and the passivating film. Finally, a thin chemically grown SiOx layer at the interface between the AlOx film and the silicon surface is shown to significantly increase the passivation stability under both light soaking and damp heat exposure. The results of this study provide valuable insights into surface passivation degradation mechanisms on nanostructured silicon surfaces and pave the way for the industrial production of highly stable black silicon devices.Item Black Silicon(2020-04-24) Pasanen, Toni; Hele Savin Group; Department of Electronics and NanoengineeringItem Cast Monocrystalline Silicon: New Alternative for Micro- and Nano-electromechanical Systems(Institute of Electrical and Electronics Engineers, 2019-08) Liu, Zhengjun; Vähänissi, Ville; Chekurov, Nikolai; Tittonen, Ilkka; Savin, Hele; Hele Savin Group; Oxford Instruments Technologies Oy; Department of Electronics and NanoengineeringCasted silicon wafers dominate the current photovoltaic (PV) market due to much lower fabrication costs as compared to well-known Czochralski (Cz) –growth. Traditionally casted silicon ingots have been multicrystalline, but recent developments in casting technology have enabled also the growth of single crystalline (sc) silicon ingots. While the resulting sc-Si ingot quality is naturally high enough for PV, it is not sufficient for the integrated circuit (IC) industry, mainly due to the increased amount of intrinsic point defects and dislocations in comparison to Cz-Si. However, many applications that do not have such stringent requirements for substrates, such as micro- and nano-electromechanical systems (MEMS, NEMS), could potentially find this material beneficial. Indeed, here we take the first step in studying the applicability of cast mono-Si for such applications. More specifically, we focus on advanced focused ion beam lithography combined with deep reactive ion etching for NEMS and wet etching for MEMS. Our results show that the quality of cast monoSi is high enough for successful patterning in both micro- and nanoscale. Sub-micron resolution is achieved and the Ga+ doses required for successful patterning are comparable to conventional Cz-Si. The preliminary results presented here thus show great promise for cast mono-Si as a low-cost alternative for micro- and nanoelectromechanical systems.Item Characterization of Heavily Irradiated Dielectrics for Pixel Sensors Coupling Insulator Applications(FRONTIERS MEDIA SA, 2022-01-19) Bharthuar, S.; Golovleva, M.; Bezak, M.; Brücken, E.; Gädda, A.; Härkönen, J.; Karadzhinova-Ferrer, A.; Kramarenko, N.; Kirschenmann, S.; Koponen, P.; Luukka, P.; Mizohata, K.; Ott, J.; Tuominen, E.; Helsinki Institute of Physics; University of Helsinki; Hele Savin Group; Department of Electronics and NanoengineeringAn increase in the radiation levels during the high-luminosity operation of the Large Hadron Collider calls for the development of silicon-based pixel detectors that are used for particle tracking and vertex reconstruction. Unlike the conventionally used conductively coupled (DC-coupled) detectors that are prone to an increment in leakage currents due to radiation, capacitively coupled (AC-coupled) detectors are anticipated to be in operation in future collider experiments suitable for tracking purposes. The implementation of AC-coupling to micro-scale pixel sensor areas enables one to provide an enhanced isolation of radiation-induced leakage currents. The motivation of this study is the development of new generation capacitively coupled (AC-coupled) pixel sensors with coupling insulators having good dielectric strength and radiation hardness simultaneously. The AC-coupling insulator thin films were aluminum oxide (Al2O3) and hafnium oxide (HfO2) grown by the atomic layer deposition (ALD) method. A comparison study was performed based on the dielectric material used in MOS, MOSFET, and AC-coupled pixel prototypes processed on high resistivity p-type Magnetic Czochralski silicon (MCz-Si) substrates. Post-irradiation studies with 10 MeV protons up to a fluence of 1015 protons/cm2 suggest HfO2 to be a better candidate as it provides higher sensitivity with negative charge accumulation on irradiation. Furthermore, even though the nature of the dielectric does not affect the electric field within the AC-coupled pixel sensor, samples with HfO2 are comparatively less susceptible to undergo an early breakdown due to irradiation. Edge-transient current technique (e-TCT) measurements show a prominent double-junction effect as expected in heavily irradiated p-type detectors, in accordance with the simulation studies.Item Characterization of magnetic Czochralski silicon devices with aluminium oxide field insulator: Effect of oxygen precursor on electrical properties and radiation hardness(IOP Publishing Ltd., 2021-05) Ott, J.; Bharthuar, S.; Gädda, A.; Arsenovich, T.; Bezak, M.; Brücken, E.; Golovleva, M.; Härkönen, J.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Kirschenmann, S.; Litichevskyi, V.; Luukka, P.; Martikainen, L.; Naaranoja, T.; Hele Savin Group; University of Helsinki; Advacam Oy; Ruder Boskovic Institute; Department of Electronics and NanoengineeringAluminium oxide (Al2O3) has been proposed as an alternative to thermal silicon dioxide (SiO2) as field insulator and surface passivation for silicon detectors, where it could substitute p-stop/p-spray insulation implants between pixels due to its negative oxide charge, and enable capacitive coupling of segments by means of its higher dielectric constant. Al2O3 is commonly grown by atomic layer deposition (ALD), which allows the deposition of thin layers with excellent precision. In this work, we report the electrical characterization of single pad detectors (diodes) and MOS capacitors fabricated on magnetic Czochralski silicon substrates and using Al2O3 as field insulator. Devices are studied by capacitance-voltage, current-voltage, and transient current technique measurements. We evaluate the influence of the oxygen precursors in the ALD process, as well as the effect of gamma irradiation, on the properties of these devices. We observe that leakage currents in diodes before the onset of breakdown are low forall studied ALD processes. Charge collection as measured by transient current technique (TCT) is also independent of the choice of oxygen precursor. The Al2O3 films deposited with O3 possess a higher negative oxide charge than films deposited by H2O, However, in diodes a higher oxide charge is linked to earlier breakdown, as has been predicted by simulation studies. A combination of H2and O3 precursors results in a good compromise between the beneficial properties provided by the respective individual precursors.Item Dissociation and Formation Kinetics of Iron-Boron Pairs in Silicon after Phosphorus Implantation Gettering(WILEY-V C H VERLAG GMBH, 2019-09) Khelifati, Nabil; Laine, Hannu S.; Vähänissi, Ville; Savin, Hele; Bouamama, Fatima Zohra; Bouhafs, Djoudi; Research Center in Semiconductor Technology for the Energetic (CRTSE); Department of Electronics and Nanoengineering; Hele Savin Group; Ferhat Abbas Sétif University 1This paper reports the results of a systematic study on the kinetics of dissociation and formation of iron-boron (FeB) pairs in boron-doped Czochralski silicon after phosphorus implantation gettering of iron at different temperatures. The aim of this study is threefold: (i) investigation of the dissociation kinetics of the FeB pairs by a standardized illumination as a function of the iron concentration after gettering process (ii) study of the kinetics of their association, and (iii) extraction of the characteristic parameters of these two phenomena for gettered samples, in particular the effective time constants of dissociation and association as well as the constant of material, which describes the dissociation rate well in the absence of other recombination channels. This article is protected by copyright. All rights reserved.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.; Hele Savin Group; Department of Electronics and NanoengineeringBlack 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 Effect of Contamination Level and Gettering Parameters on Solar Cell Performance in Iron Contaminated Silicon(2011) Vähänissi, Ville; Haarahiltunen, Antti; Talvitie, Heli; Yli-Koski, Marko; Savin, Hele; Department of Electronics and Nanoengineering; Hele Savin GroupItem Effect of substrate pretreatments on the atomic layer deposited Al2O3 passivation quality(2015) Bao, Yameng; Li, Shuo; von Gastrow, Guillaume; Repo, Päivikki; Putkonen, Matti; Savin, Hele; Hele Savin Group; Department of Micro and NanosciencesItem Effect of Thermal Donors Induced in Bulk and Variation in P-stop Dose on the No-gain Region Width Measurements of LGADs(Physical Society of Japan, 2021-06-10) Bharthuar, Shudhashil; Ott, Jennifer; Brücken, Erik; Gädda, Akiko; Kirschenmann, Stefanie; Golovleva, Maria; Luukka, Panja; University of Helsinki; Hele Savin Group; Helsinki Institute of Physics; LUT University; Department of Electronics and NanoengineeringTo cope up with high pile-up rates during Phase-2 operation of the LHC, the CMS experiment will install a precision Minimum Ionising Particle Timing Detector (MTD), between the tracker and calorimeter, which will have a timing resolution of ∼30 ps. The endcap part of the MTD, called the Endcap Timing Layer (ETL), will be based on Low-Gain Avalanche Detector (LGAD) Technology. In this paper, the authors have performed a study on UFSD3.1 LGAD production from Fondazione Bruno Kessler (FBK). The objective of this production was to optimise the interplay between the p-stop dose and the inter-pad terminal strategy. Results show that the nominal no-gain region width value between pads does not affect the breakdown voltage of the sensors at low p-stop doses. A comparative study on the effect of thermal donors induced in the bulk of the sensor was performed by capacitance versus voltage measurements. Further, the property of the sensors were investigated using infra-red (IR) laser, by performing voltage scans and inter-pad profile measurements with gain variation, on changing temperature in Transient Current Technique (TCT). Results show no significant change in the charge collection efficiency and measured no-gain region width values of sensors irrespective of the nature of the bulk.Item Excellent Responsivity and Low Dark Current Obtained with Metal-Assisted Chemical Etched Si Photodiode(IEEE, 2023-04-01) Chen, Kexun; Setälä, Olli; Liu, Xiaolong; Radfar, Behrad; Pasanen, Toni; Serue, Michael; Heinonen, Juha; Savin, Hele; Vähänissi, Ville; Hele Savin Group; Department of Electronics and Nanoengineering; ElFys Inc.; Department of Electronics and NanoengineeringMetal-assisted chemical etched (MACE; also known as MacEtch or MCCE) nanostructures are utilized widely in the solar cell industry due to their excellent optical properties combined with a simple and cost-efficient fabrication process. The photodetection community, on the other hand, has not shown much interest toward MACE due to its drawbacks, including insufficient surface passivation, increased junction recombination, and possible metal contamination, which are especially detrimental to p-n photodiodes. Here, we aim to change this by demonstrating how to fabricate high-performance MACE p-n photodiodes with above 90% external quantum efficiency (EQE) without external bias voltage at 200-1000 nm and dark current less than 3 nA/cm2 at -5 V using industrially applicable methods. The key is to utilize an induced junction created by an atomic layer deposited (ALD) highly charged Al2O3 thin film that simultaneously provides efficient field-effect passivation and full conformality over the MACE nanostructures. Achieving close to ideal performance demonstrates the vast potential of MACE nanostructures in the fabrication of high-performance low-cost p-n photodiodes.Item High-sensitivity NIR photodiodes using black silicon(SPIE - The International Society for Optical Engineering, 2020) Heinonen, Juha; Haarahiltunen, Antti; Serue, Michael; Vähänissi, Ville; Pasanen, Toni; Savin, Hele; Werner, Lutz; Juntunen, Mikko; ElFys Inc.; Department of Electronics and Nanoengineering; Hele Savin Group; Physikalisch-Technische BundesanstaltThere is an increasing demand for highly sensitive near infrared (NIR) detectors due to many rapidly growing application areas, such as LiDAR and optical communications. Despite the limited NIR absorption, silicon is a common substrate material in NIR detectors due to low cost and maturity of the technology. To boost the NIR performance of silicon devices, one option is texturizing the front and/or back surface to reduce reflectance and extend the optical path by scattering. Here we demonstrate silicon photodiodes with nanostructured front surface (i.e. black silicon fabricated with reactive ion etching (RIE) that exhibit significantly enhanced external quantum efficiency (EQE) at NIR wavelengths compared to typical state-of-the-art silicon photodiodes. The detectors exhibit over 90% EQE with wavelengths up to 1040 nm and above 60% at 1100 nm. Identical detectors with a planar surface are also investigated revealing that the enhancement from black silicon effectively corresponds to increasing the substrate thickness up to 43% at 1100 nm depending on the thickness of the active layer and back surface structure. This confirms that in addition to reduced reflectance, scattering of transmitted light induced by black silicon plays a key role in the EQE enhancement which benefits especially devices such as backside illuminated photodetectors where very thin substrates are required. We also demonstrate that the high EQE of the black silicon detectors is maintained at incidence angles up to 60 degrees allowing excellent performance in applications where the light is not always perpendicularItem 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; Hele Savin Group; Helsinki Institute of Physics; Department of Electronics and NanoengineeringAluminium 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 Increased surface recombination in crystalline silicon under light soaking due to Cu contamination(Elsevier Science B.V., 2021-10) Rauha, Ismo TS; Soeriyadi, Anastasia H; Kim, Moonyong; Yli-Koski, Marko; Wright, Brendan; Vähänissi, Ville; Hallam, Brett J; Savin, Hele; Department of Electronics and Nanoengineering; University of New South Wales; Hele Savin GroupLight-induced degradation (LID) can occur in crystalline silicon (Si) due to increased recombination in the bulk or at the surfaces. As an example, copper (Cu) is a contaminant that reportedly causes LID in the bulk of Si under illumination. In this article, we show that Cu contamination can also increase recombination at the surface under illumination using surface saturation current density (J 0) analysis. More specifically, in the presence of Cu we observed that J 0 increased from 14 fA/cm 2 to 330 fA/cm 2 in SiO 2 passivated Float Zone (FZ) Si, and from 11 fA/cm 2 to 200 fA/cm 2 in corresponding Czochralski (Cz) Si after illumination under an LED lamp (0.6 Suns, 80 °C). In reference samples without Cu contamination, the J 0 was unaffected. These results demonstrate that a significant increase in surface recombination is possible without the presence of hydrogen. Furthermore, hydrogen was not seen to affect the Cu-induced surface degradation as similar experiments made with hydrogenated silicon nitride (SiN x:H) did not show further increase in J 0. However, the timescale of the observed degradation was relatively fast (hours) indicating that Cu-induced surface degradation is a separate phenomenon from the earlier reported surface-related degradation.Item Industrial Applicability of Antireflection-Coating-Free Black Silicon on PERC Solar Cells and Modules(2018) Pasanen, Toni; Vähänissi, Ville; Wolny, Franziska; Oehlke, Alexander; Wagner, Matthias; Juntunen, Mikko A.; Heikkinen, Ismo T. S.; Salmi, Emma; Sneck, Sami; Vahlman, Henri; Tolvanen, Antti; Hyvärinen, Jaakko; Savin, Hele; Hele Savin Group; Aalto Nanofab; SolarWorld Industries GmbH; SolarWorld Innovations GmbH; Naps Solar Systems Inc.; Beneq Oy; Department of Electronics and Nanoengineering; Endeas OyBlack silicon (b-Si) is of particular interest within the photovoltaic community due to its ability to texture diamond wire-sawn multicrystalline silicon wafers. In this work, we apply a deep dry-etched black silicon nanotexture, which possesses less than 1 % solar-weighted reflectance with no antireflection coating (ARC), to industrial Passivated Emitter and Rear Cells (PERC). Additionally, the cells are processed further into small solar modules, which are characterized by their electrical and optical performance. The fragile nanostructures remain intact during industrial cell and module fabrication, maintaining the excellent optical properties until the final product. We show that b-Si modules with a typical cover glass retain their performance until incident angles larger than 60°, whereas the heavily increased reflectance of acidic-textured modules decreases their efficiency already after a 30° tilt. Furthermore, at an incidence angle of 70°, the efficiency of b-Si modules has reduced only 7 %, while that of the acidic-textured equivalents has decreased more than 25 %. Hence, the excellent optical properties of ARC-free b-Si are maintained also at module level. The results demonstrate that deep dry-etched b-Si nanostructures are fully applicable to current industrial PERC production facilities.Item Is Carrier Mobility a Limiting Factor for Charge Transfer in Tio2/Si Devices? A Study by Transient Reflectance Spectroscopy(Elsevier, 2023-06) Khan, Ramsha; Pasanen, Hannu; Ali-Löytty, Harri; Ayedh, Hussein; Saari, Jesse; Vähänissi, Ville; Valden, Mika; Savin, Hele; Tkachenko, Nikolai V.; Tampere University; Hele Savin Group; Department of Electronics and Nanoengineering; Department of Electronics and NanoengineeringTiO 2 coatings are often deposited over silicon-based devices for surface passivation and corrosion protection. However, the charge transfer (CT) across the TiO 2/Si interface is critical as it may instigate potential losses and recombination of charge carriers in optoelectronic devices. Therefore, to investigate the CT across the TiO 2/Si interface, transient reflectance (TR) spectroscopy was employed as a contact-free method to evaluate the impact of interfacial SiO x, heat-treatments, and other phenomena on the CT. Thin-film interference model was adapted to separate signals for Si and TiO 2 and to estimate the number of transferred carriers. Charge transfer velocity was found to be 5.2 × 10 4 cm s −1 for TiO 2 heat-treated at 300 °C, and even faster for amorphous TiO 2 if the interfacial SiO x layer was removed using HF before TiO 2 deposition. However, the interface is easily oversaturated because of slow carrier diffusion in TiO 2 away from the TiO 2/Si interface. This inhibits CT, which could become an issue for heavily concentrated solar devices. Also, increasing the heat-treatment temperature from 300 °C to 550 °C has only little impact on the CT time but leads to reduced carrier lifetime of ¡3 ns in TiO 2 due to back recombination via the interfacial SiO x, which is detrimental to TiO 2/Si device performance.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; Hele Savin Group; Department of Electronics and NanoengineeringLight 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 MACE nano-texture process applicable for both single- and multi-crystalline diamond-wire sawn Si solar cells(2019-03) Chen, Kexun; Zha, Jiawei; Hu, Fenqin; Ye, Xiaoya; Zou, Shuai; Vähänissi, Ville; Pearce, Joshua; Savin, Hele; Su, Xiaodong; Soochow University; Hele Savin Group; Department of Electronics and NanoengineeringThe photovoltaic (PV) industry requires efficient cutting of large single and multi-crystalline (sc- and mc-) silicon (Si) wafers. Historically multi-wire slurry sawing (MWSS) dominated, but the higher productivity of diamond-wire-sawing (DWS) holds promise for decreasing PV costs in the future. While surface texturing of DWS wafers is more complicated than of MWSS wafers, especially in mc-Si wafers, nanotexturing has been shown to overcome this challenge. While the benefit of nanotexturing is thus clearer in mc-Si, a universal nano-texture process that also works on sc-Si would simplify and reduce the investments costs of PV production-lines. In this paper, such a nano-texture process is developed using a metal-assisted chemical etch (MACE) technique. Step-by-step characterization of surface structure and reflectance of the MACE process is used after: 1) wafering, 2) standard acidic texturing etch, 3) silver nanoparticles deposition, and 4) MACE nanotexturing for both sc and mc-Si. The results show that the same MACE process works effectively for both sc-Si and mc-Si wafers. Finally, the nano-textured wafers are processed into PV cells in an industrial process line with conversion efficiencies of 19.4 % and 18.7%, for sc-Si and mc-Si solar cells, respectively.Item Magnetic polaron formation in graphene- based single- electron transistor(2014) Savin, Hele; Kuivalainen, Pekka; Novikov, S.; Lebedeva, Natalia; Hele Savin Group; Department of Micro and Nanosciences
