Browsing by Author "Chen, Kexun"
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- Adsorption Induced Bipolar Excitation at Semiconductor Surface
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2024-07) Li, Shengyang; Chen, Kexun; Alizadeh, Mahdi; Vähänissi, Ville; Savin, Hele; Oksanen, JaniAdsorption on metal surfaces has been shown to lead to chemisorption induced electronic excitation but direct experimental evidence of chemisorption induced excitation on semiconductor surfaces is still missing. Here, we design and use high-quality Silicon (Si) p-n diodes to in-situ probe the charge transfer process taking place during vapor and liquid phase chemisorption of iodine molecules on H-Si (100) surface. We find that the diodes can generate electricity and feed an external circuit during the chemisorption process, demonstrating chemisorption-induced electron and hole generation and the means to electrically monitor the process. A bipolar semiconductor electrochemical model where a single semiconductor surface hosts the spontaneous oxidation (electron injection) and reduction (hole injection) reactions simultaneously on its conduction and valence bands without any electrolytes is proposed to explain the observations. Our work provides new insight on the energy relaxation processes of chemisorption and also calls for further studies on the observed chemovoltaic effect. Overall, the results indicate a possibility for developing electrolyte-free vapor phase chemical energy-to-electricity converters and detectors. - Black ultra-thin crystalline silicon wafers reach the 4n2 absorption limit – application to IBC solar cells
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-09-27) Garin, Moises; Pasanen, Toni; López, G.; Vähänissi, Ville; Chen, Kexun; Martín, I.; Savin, HeleCutting costs by progressively decreasing substrate thickness has been a common theme in the crystalline silicon PV industry for the last decades, since drastically thinner wafers would significantly reduce the substrate-related costs. In addition to the technological challenges concerning wafering and handling of razor-thin flexible wafers, a major bottleneck is to maintain high absorption in those thin wafers. For the latter, advanced light-trapping techniques become of paramount importance. Here we demonstrate that by applying state-of-the-art black-Si nanotexture produced by DRIE on thin uncommitted wafers, the maximum theoretical absorption (Yablonovitch’s 4n2 absorption limit), i.e. ideal light trapping, is reached with wafer thicknesses as low as 40 µm, 20 µm and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results were implemented into an actual thin interdigitated back contacted solar cell. The proof-of-concept cell, encapsulated in glass, achieved a 16.4% efficiency with an JSC = 35 mA/cm², representing a 43% improvement in output power with respect to the reference polished cell. These results demonstrate the vast potential of black silicon nanotexture in future extremely-thin silicon photovoltaics. - Boron-implanted black silicon photodiode with close-to-ideal responsivity from 200 to 1000 nm
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-06-21) Setälä, Olli; Chen, Kexun; Pasanen, Toni; Liu, Xiaolong; Radfar, Behrad; Vähänissi, Ville; Savin, HeleDetection of UV light has traditionally been a major challenge for Si photodiodes due to reflectance losses and junction recombination. Here we overcome these problems by combining a nanostructured surface with an optimized implanted junction and compare the obtained performance to state-of-the-art commercial counterparts. We achieve a significant improvement in responsivity, reaching near ideal values at wavelengths all the way from 200 to 1000 nm. Dark current, detectivity, and rise time are in turn shown to be on a similar level. The presented detector design allows a highly sensitive operation over a wide wavelength range without making major compromises regarding the simplicity of the fabrication or other figures of merit relevant to photodiodes. - Decreasing Interface Defect Densities via Silicon Oxide Passivation at Temperatures Below 450 C
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-10-14) Rad, Zahra Jahanshah; Lehtiö, Juha-Pekka; Mack, Iris; Rosta, Kawa; Chen, Kexun; Vähänissi, Ville; Punkkinen, Marko P.J.; Punkkinen, Risto; Hedman, Hannu-Pekka; Pavlov, Andrei; Kuzmin, Mikhail V.; Savin, Hele; Laukkanen, Pekka J.; Kokko, KaleviLow-temperature (LT) passivation methods (<450oC) for decreasing defect densities in the material combination of silica (SiOx) and silicon (Si) are relevant to develop a diverse technology (e.g. electronics, photonics, medicine), where defects of SiOx/Si cause losses and malfunctions. Many device structures contain the SiOx/Si interface(s), of which defect densities cannot be decreased by the traditional, beneficial high temperature treatment (> 700oC). Therefore, the LT passivation of SiOx/Si has been, since long, a research topic to improve applications performance. Here, we demonstrate that a LT (<450oC) ultrahigh-vacuum (UHV) treatment is a potential method that can be combined with current state-of-the-art processes in a scalable way, to decrease the defect densities at the SiOx/Si interfaces. The studied LT-UHV approach includes a combination of wet chemistry followed by UHV-based heating and pre-oxidation of silicon surfaces. The controlled oxidation during the LT-UHV treatment is found to provide an until now not reported crystalline Si oxide phase. This crystalline SiOx phase can explain the observed decrease in the defect density by halve. Furthermore, the LT-UHV treatment can be applied in a complementary, post-treatment way to ready components to decrease electrical losses. The LT-UHV treatment has been found to decrease the detector leakage current by factor of two. - Effect of MACE Parameters on Electrical and Optical Properties of ALD Passivated Black Silicon
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-07-01) Chen, Kexun; Pasanen, Toni; Vähänissi, Ville; Savin, HeleMetal-assisted chemical etching (MACE) enables efficient texturing of diamond-wire sawn multicrystalline silicon (mc-Si) wafers. However, the excellent optics are often sacrificed by polishing the surface to achieve better surface passivation with chemical-vapor-deposited (CVD) silicon nitride (SiNx). In this work, we show that a polishing step is not required when CVD SiNx is replaced with atomic-layer-deposited (ALD) aluminum oxide (Al2O3). Indeed, while polishing increases reflectance, it has in general only very modest effect on surface recombination velocity of ALD-passivated b-Si. Furthermore, since ALD Al2O3 is compatible with various surface morphologies due to its excellent conformality, the MACE parameters can be more freely adjusted. First, the concentration of silver nitrate (AgNO3) in AgNO3/H2O solution that is used to deposit Ag nanoparticles is shown to affect the final b-Si morphology. Instead of needle-shaped b-Si produced by 5 mmol/L AgNO3 concentration, two orders of magnitude lower AgNO3 concentration produces porous structures, which are more challenging to passivate. Additionally, we demonstrate that a separate Ag nanoparticle removal step in nitric acid (HNO3) is not a prerequisite for high carrier lifetime. Instead, Ag nanoparticles present during polishing in a HF/HNO3/H2O solution affect the final b-Si morphology by accelerating the etching of Si. The results demonstrate that no trade-offs are necessary between optical and electrical properties of MACE b-Si when using ALD. - Effects of post oxidation of SiO2/Si interfaces in ultrahigh vacuum below 450 °C
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-08) Rad, Zahra Jahanshah; Lehtiö, Juha Pekka; Chen, Kexun; Mack, Iris; Vähänissi, Ville; Miettinen, Mikko; Punkkinen, Marko; Punkkinen, Risto; Suomalainen, Petri; Hedman, Hannu Pekka; Kuzmin, Mikhail; Kozlova, Jekaterina; Rähn, Mihkel; tart, university; Savin, Hele; Laukkanen, Pekka; Kokko, KaleviGrowing SiO2 layer by wet-chemical oxidation of Si surfaces before growth of another insulating film(s) is a used method to passivate Si interfaces in applications (e.g., solar cell, photodiode) at low temperatures (LT) below 450 oC. We report on potential of LT ultrahigh-vacuum (UHV) treatments combined with the wet-chemical oxidation, by investigating effects of LT-UHV oxidation after the wet-chemical growth of SiO2 and before growing Al2O3 film on top of SiO2/Si. This method modifies the SiO2/Si and is found to (i) decrease defect- level density, (ii) increase negative fixed charge density, and (iii) increase carrier lifetime for Al2O3/SiO2/p-Si, as compared to state-of-the-art SiO2/p-Si reference interfaces without LT-UHV. X-ray photoelectron spectroscopy shows that the LT-UHV treatment decreases amount of Si+3 oxidized atoms in chemically grown SiO2 and also amount of carbon contamination. In order to pave the way for further tests of LT-UHV in silicon technology, we present a design of simple UHV instrument. The above-described benefits are reproduced for 4-inch silicon wafers by means of the instrument, which is further utilized to make LT-UHV treatments for complementary SiO2/Si interfaces of the native oxide at silicon diode sidewalls to decrease the reverse bias leakage current of the diodes. - Efficient photon capture on germanium surfaces using industrially feasible nanostructure formation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-01-15) Chen, Kexun; Isometsä, Joonas; Pasanen, Toni P.; Vähänissi, Ville; Savin, HeleNanostructured 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. - Electron Injection in Metal Assisted Chemical Etching as a Fundamental Mechanism for Electroless Electricity Generation
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-06-23) Li, Shengyang; Chen, Kexun; Vähänissi, Ville; Radevici, Ivan; Savin, Hele; Oksanen, JaniMetal-assisted chemical etching (MACE) is a widely applied process for fabricating Si nanostructures. As an electroless process, it does not require a counter electrode, and it is usually considered that only holes in the Si valence band contribute to the process. In this work, a charge carrier collecting p-n junction structure coated with silver nanoparticles is used to demonstrate that also electrons in the conduction band play a fundamental role in MACE, and enable an electroless chemical energy conversion process that was not previously reported. The studied structures generate electricity at a power density of 0.43 mW/cm2 during MACE. This necessitates reformulating the microscopic electrochemical description of the Si-metal-oxidant nanosystems to separately account for electron and hole injections into the conduction and valence band of Si. Our work provides new insight into the fundamentals of MACE and demonstrates a radically new route to chemical energy conversion by solar cell-inspired devices. - Excellent Responsivity and Low Dark Current Obtained with Metal-Assisted Chemical Etched Si Photodiode
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-04-01) Chen, Kexun; Setälä, Olli; Liu, Xiaolong; Radfar, Behrad; Pasanen, Toni; Serue, Michael; Heinonen, Juha; Savin, Hele; Vähänissi, VilleMetal-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. - Harnessing Carrier Multiplication in Silicon Solar Cells Using UV Photons
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2021-12-15) Chen, Kexun; Setala, Olli E.; Radfar, Behrad; Kroth, Udo; Vahanissi, Ville; Savin, HeleSilicon solar cells are known to suffer from poor emitter performance that is seen as reduced external quantum efficiency at wavelengths below 500 nm. This is due to common tradeoff between electrical and optical performance. Here we demonstrate that no such tradeoff is needed when optimized boron implantation parameters are combined with non-reflective nanostructures and atomic layer deposited Al2O3 surface passivation. As a result, in our solar cells the external quantum efficiency actually increases with decreasing wavelength and reaches even above 100% at short wavelengths. This result indicates that carrier multiplication caused by absorption of high energy photons could be utilized for energy production in solar cells. - MACE nano-texture process applicable for both single- and multi-crystalline diamond-wire sawn Si solar cells
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2019-03) Chen, Kexun; Zha, Jiawei; Hu, Fenqin; Ye, Xiaoya; Zou, Shuai; Vähänissi, Ville; Pearce, Joshua; Savin, Hele; Su, XiaodongThe 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. - Millisecond-Level Minority Carrier Lifetime in Femtosecond Laser-Textured Black Silicon
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-08-15) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Pälikkö, Elmeri; Pasanen, Toni; Vähänissi, Ville; Savin, HeleFemtosecond laser-textured black silicon (fs-bSi) is known to suffer from heavy minority carrier recombination resulted from laser irradiation. In this paper, we demonstrate that the thermal annealing step, generally used to recover the crystal damage, could improve the minority carrier lifetime of the fs-bSi wafers only from 8 μs to 12 μs, even when using as high temperature as 800 °C. However, with an optimized wet chemical etching process, we obtain a high minority carrier lifetime of 2 ms without sacrificing the optical properties of the samples, i.e., the absorptance remains above 90% in the studied wavelength range (250–1100 nm). Increasing the etching time further leads to a total recovery of the lifetime up to 10.5 ms, which proves that the damage originating from the fs-laser texturing extends only to the near-surface layer (a few μm) of silicon. - Nanostructured germanium with >99% absorption at 300-1600 nm wavelengths
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2020-06-01) Pasanen, Toni; Isometsä, Joonas; Garin, Moises; Chen, Kexun; Vähänissi, Ville; Savin, HeleNear-infrared (NIR) sensors find numerous applications within various industry fields, including optical communications and medical diagnostics. However, the state-of-the-art NIR sensors made of germanium (Ge) suffer from rather poor response, largely due to high reflection from the illuminated device surface. This work demonstrates a method to increase the sensitivity of Ge sensors by implementing nanostructures to the wafer surfaces. The absorbance of nanostructured Ge wafers is measured to be >99% in the whole UV–vis–NIR spectrum up to 1600 nm wavelength, which is a significant improvement to bare Ge wafers that reach absorption of only 63% in maximum. The process is shown to be capable of producing uniform nanostructures covering full 100 mm diameter substrates as well as wafers with etch mask openings of different sizes and shapes, which demonstrates its applicability to complementary metal oxide semiconductor (CMOS) sensor manufacturing. The results imply that nanostructured Ge has potential to revolutionize the sensitivity of Ge-based sensors. - Optoelectronic properties of black silicon fabricated by femtosecond laser in ambient air: exploring a large parameter space
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2023-03-01) Radfar, Behrad; Chen, Kexun; Setälä, Olli; Vähänissi, Ville; Savin, Hele; Liu, XiaolongWe study the surface morphology, optical absorption (400–1100 nm), and carrier lifetime of black silicon fabricated by femtosecond (fs) laser in air. We explore a large laser parameter space, for which we adopt a single parameter ξ to describe the cumulative fluence delivered to the sample. We also study the laser-oxidized surface layer by measuring its photoluminescence spectra and comparing its effect on the aforementioned properties. Our study in a broad range of ξ is instructive in choosing laser parameters when targeting different applications. - (oral talk) Nanostructured germanium with >99 % absorption for near-infrared sensor applications
Abstract(2020) Pasanen, Toni; Isometsä, Joonas; Garin, Moises; Chen, Kexun; Vähänissi, Ville; Savin, Hele - Perspectives on Black Silicon in Semiconductor Manufacturing: Experimental Comparison of Plasma Etching, MACE and Fs-Laser Etching
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-08) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Setälä, Olli; Pasanen, Toni; Yli-Koski, Marko; Savin, Hele; Vähänissi, VilleIn semiconductor manufacturing, black silicon (bSi) has traditionally been considered as a sign of unsuccessful etching. However, after more careful consideration, many of its properties have turned out to be so superior that its integration into devices has become increasingly attractive. In devices where bSi covers the whole wafer surface, such as solar cells, the integration is already rather mature and different bSi fabrication technologies have been studied extensively. Regarding the integration into devices where bSi should cover only small selected areas, existing research focuses on device properties with one specific bSi fabrication method. Here, we fabricate bSi patterns with varying dimensions ranging from millimeters to micrometers using three common bSi fabrication techniques, i.e., plasma etching, metal-assisted chemical etching (MACE) and femtosecond-laser etching, and study the corresponding fabrication characteristics and resulting material properties. Our results show that plasma etching is the most suitable method in the case of µm-scale devices, while MACE reached surprisingly almost the same performance. Femtosecond-laser has potential due to its maskless nature and capability for hyperdoping, however, in this study its moderate accuracy, large silicon consumption and spreading of the etching damage outside the bSi region left room for improvement. - Robustness of electrical quality of ion implanted black silicon emitters: Comparison between different ion implantation service providers
A4 Artikkeli konferenssijulkaisussa(2024) Morozova, Olga; Chen, Kexun; Radfar, Behrad; Kentsch, Ulrich; Antwis, Luke; Savin, Hele; Vähänissi, VilleIon implantation provides precise control over the resulting dopant atom density, enabling high-quality optical and electrical performance of nanostructured (black silicon, b-Si) emitters. In this work, we study how sensitive the performance of Al2O3-passivated b-Si emitters is to small variations in the implantation conditions and the equipment used to perform it. We carried out boron emitter implantations for identical nanostructured and planar wafers at four different implantation service providers and with both beam line and parallel beam tool configurations. We then benchmarked the results against the earlier optimised b-Si emitter process. The results show that there are some differences in obtained sheet resistance and emitter saturation current depending on the service provider and the used tool configuration. Finally, there are also some deviations in terms of possible bulk contamination among the different service providers. - Superior gamma-detection and IR imaging via ALD-passivated germanium nanostructures
A4 Artikkeli konferenssijulkaisussa(2020) Savin, Hele; Pasanen, Toni; Isometsä, Joonas; Chen, Kexun; Vähänissi, Ville; Nurgalejevs, R; Savina, O; Gostilo, V.; Aurola, A - Tailoring Femtosecond-Laser Processed Black Silicon for Reduced Carrier Recombination Combined with >95% Above-Bandgap Absorption
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2022-01-17) Liu, Xiaolong; Radfar, Behrad; Chen, Kexun; Pasanen, Toni; Vähänissi, Ville; Savin, HeleThe femtosecond-pulsed laser processed black silicon (fs-bSi) features high absorptance in a wide spectral range but suffers from high amount of laser induced damage as compared to bSi fabricated by other methods. Here, we aim to minimize the charge carrier recombination in the fs-bSi caused by laser damage as indicated by the sub-bandgap absorption and as quantified by the carrier lifetime, while maintaining high absorption in the above-bandgap. The effect of the laser parameters, including the focal position, the average power, and the scan speed are systematically studied by characterizing the surface morphology, the absorptance spectra, and the minority-carrier recombination lifetime. For the surface passivation of fs-bSi we use the well-established atomic layer deposited (ALD) Al2O3. The results show that with the tailored laser parameters, high average absorptance of about 96% in the visible range and minority carrier lifetime of 54 μs at the injection level of Δn = 1 ∙ 1015 cm−3 can be obtained simultaneously. This work paves the way towards high-performance broadband optoelectronic devices based on surface passivated fs-bSi.