Ville Vähänissi
@aalto.fi
Department of Electronics and Nanoengineering (Electron Physics Group) / School of Electrical Engineering
Aalto University
Scopus Publications
Scopus Publications
Pekka Laukkanen, Marko Punkkinen, Mikhail Kuzmin, Kalevi Kokko, Xiaolong Liu, Behrad Radfar, Ville Vähänissi, Hele Savin, Antti Tukiainen, Teemu Hakkarainen,et al.
IOP Publishing
Abstract Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.
Sören Schäfer, Xiaolong Liu, Patrick Mc Kearney, Simon Paulus, Behrad Radfar, Ville Vähänissi, Hele Savin, and Stefan Kontermann
Wiley
Charge carrier lifetime is a crucial material parameter in optoelectronic devices and knowing the dominant recombination channels points the way for improvements. The effective carrier lifetime τeff of surface‐passivated hyperdoped (hSi) and nonhyperdoped “black” (bSi) silicon by quasi‐steady‐state photoconductance decay (QSSPC) measurements and its evolution upon controlled wet‐chemical etching are studied. Sample preparation involves the irradiation of Si by numerous ultrashort laser pulses either in SF6 for hSi or ambient atmosphere for bSi. Findings suggest that the hSi is composed of a double layer: 1) an amorphous resolidified top layer with about 92% of the total incorporated sulfur that accounts for the sub‐bandgap absorptance and 2) a crystalline layer underneath in which sulfur concentration tails off toward the Si substrate. The effective lifetime is deconstructed by a 1D simulation to quantify the impact of the local lifetime of the defect‐rich top layer, τtop. It is found that by the QSSPC method, a maximum τtop for 1) can be estimated. For 2), τtop between 2 and 8 ns is estimated. The bSi sample shows a faster lifetime recovery upon etching which suggests that in hSi samples purely laser‐induced defects are not limiting the carrier lifetime compared to sulfur‐related defects.
Olli E. Setälä, Martin J. Prest, Konstantin D. Stefanov, Douglas Jordan, Matthew R. Soman, Ville Vähänissi, and Hele Savin
Wiley
Even though the recent progress made in complementary metal-oxide-semiconductor (CMOS) image sensors (CIS) has enabled numerous applications affecting our daily lives, the technology still relies on conventional methods such as antireflective coatings and ion-implanted back-surface field to reduce optical and electrical losses resulting in limited device performance. In this work, these methods are replaced with nanostructured surfaces and atomic layer deposited surface passivation. The results show that such surface nanoengineering applied to a commercial backside illuminated CIS significantly extends its spectral range and enhances its photosensitivity as demonstrated by >90% quantum efficiency in the 300-700 nm wavelength range. The surface nanoengineering also reduces the dark current by a factor of three. While the photoresponse uniformity of the sensor is seen to be slightly better, possible scattering from the nanostructures can lead to increased optical crosstalk between the pixels. The results demonstrate the vast potential of surface nanoengineering in improving the performance of CIS for a wide range of applications.
Annick Anctil, Meghan N. Beattie, Christopher Case, Aditya Chaudhary, Benjamin D. Chrysler, Michael G. Debije, Stephanie Essig, David K. Ferry, Vivian E. Ferry, Marina Freitag,et al.
SPIE-Intl Soc Optical Eng
Abstract. This report provides a snapshot of emerging photovoltaic (PV) technologies. It consists of concise contributions from experts in a wide range of fields including silicon, thin film, III-V, perovskite, organic, and dye-sensitized PVs. Strategies for exceeding the detailed balance limit and for light managing are presented, followed by a section detailing key applications and commercialization pathways. A section on sustainability then discusses the need for minimization of the environmental footprint in PV manufacturing and recycling. The report concludes with a perspective based on broad survey questions presented to the contributing authors regarding the needs and future evolution of PV.
Iris Mack, Kawa Rosta, Ulviyya Quliyeva, Jennifer Ott, Toni P. Pasanen, Ville Vähänissi, Zahra Sadat Jahanshah Rad, Juha-Pekka Lehtiö, Pekka Laukkanen, Caterina Soldano,et al.
Wiley
Oxide–semiconductor interface quality has often a direct impact on the electrical properties of devices and on their performance. Traditionally, the properties are characterized through metal–oxide–semiconductor (MOS) structures by depositing a metal layer and measuring the capacitance–voltage (C–V) characteristics. However, metal deposition process itself may have an impact on the oxide and the oxide–semiconductor interface. The impact of magnetron sputtering, e‐beam evaporation, and thermal evaporation on an interface is studied, where atomic layer deposited (ALD) is used, by MOS C–V and corona oxide characterization of semiconductors (COCOS) measurements. The latter allows characterization of the interface also in its original state before metallization. The results show that sputtering induces significant damage at the underlying interface as the measured interface defect density increases from to cm−2 eV. Interestingly, sputtering also generates a high density of positive charges at the interface as the charge changes from to cm. Thermal evaporation is found to be a softer method, with modest impact on and . Finally, Alnealing heals the damage but has also a significant impact on the charge of the film recovering the characteristic negative charge of (∼−4 × 1012 cm).
M. Garín, T. P. Pasanen, G. López, V. Vähänissi, K. Chen, I. Martín, and H. Savin
Wiley
Cutting costs by progressively decreasing substrate thickness is a common theme in the crystalline silicon photovoltaic 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), that is, ideal light trapping, is reached with wafer thicknesses as low as 40, 20, and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results are 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 cm- 2 , 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.
Tsun Hang Fung, Joonas Isometsä, Juha-Pekka Lehtiö, Toni P Pasanen, Hanchen Liu, Oskari Leiviskä, Pekka Laukkanen, Hele Savin, and Ville Vähänissi
IOP Publishing
Abstract Germanium (Ge) is a vital element for applications that operate in near-infrared wavelengths. Recent progress in developing nanostructured Ge surfaces has resulted in >99% absorption in a wide wavelength range (300–1700 nm), promising unprecedented performance for optoelectronic devices. However, excellent optics alone is not enough for most of the devices (e.g. PIN photodiodes and solar cells) but efficient surface passivation is also essential. In this work, we tackle this challenge by applying extensive surface and interface characterization including transmission electron microscopy and x-ray photoelectron spectroscopy, which reveals the limiting factors for surface recombination velocity (SRV) of the nanostructures. With the help of the obtained results, we develop a surface passivation scheme consisting of atomic-layer-deposited aluminum oxide and sequential chemical treatment. We achieve SRV as low as 30 cm s−1 combined with ∼1% reflectance all the way from ultraviolet to NIR. Finally, we discuss the impact of the achieved results on the performance of Ge-based optoelectronic applications, such as photodetectors and thermophotovoltaic cells.
Vladyslav Matkivskyi, Oskari Leiviskä, Sigurd Wenner, Hanchen Liu, Ville Vähänissi, Hele Savin, Marisa Di Sabatino, and Gabriella Tranell
MDPI AG
Two widely used atomic layer deposition precursors, Tetrakis (dimethylamido) titanium (TDMA-Ti) and titanium tetrachloride (TiCl4), were investigated for use in the deposition of TiOx-based thin films as a passivating contact material for solar cells. This study revealed that both precursors are suited to similar deposition temperatures (150 °C). Post-deposition annealing plays a major role in optimising the titanium oxide (TiOx) film passivation properties, improving minority carrier lifetime (τeff) by more than 200 µs. Aluminium oxide deposited together with titanium oxide (AlOy/TiOx) reduced the sheet resistance by 40% compared with pure TiOx. It was also revealed that the passivation quality of the (AlOy/TiOx) stack depends on the precursor and ratio of AlOy to TiOx deposition cycles.
Olli E. Setälä, Kexun Chen, Toni P. Pasanen, Xiaolong Liu, Behrad Radfar, Ville Vähänissi, and Hele Savin
American Chemical Society (ACS)
Detection 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.
Ramsha Khan, Hannu P. Pasanen, Harri Ali-Löytty, Hussein M. Ayedh, Jesse Saari, Ville Vähänissi, Mika Valden, Hele Savin, and Nikolai V. Tkachenko
Elsevier BV
Hanchen Liu, Toni P. Pasanen, Oskari Leiviskä, Joonas Isometsä, Tsun Hang Fung, Marko Yli-Koski, Mikko Miettinen, Pekka Laukkanen, Ville Vähänissi, and Hele Savin
AIP Publishing
The excellent field-effect passivation provided by aluminum oxide (Al2O3) on germanium surfaces relies on the high negative fixed charge present in the film. However, in many applications, a neutral or a positive charge would be preferred. Here, we investigate the surface passivation performance and the charge polarity of plasma-enhanced atomic layer deposited (PEALD) silicon oxide (SiO2) on Ge. The results show that even a 3 nm thick PEALD SiO2 provides a positive charge density (Qtot, ∼2.6 × 1011 cm−2) and a relatively good surface passivation (maximum surface recombination velocity SRVmax ∼16 cm/s). When the SiO2 thin film is capped with an ALD Al2O3 layer, the surface passivation improves further and a low midgap interface defect density (Dit) of ∼1 × 1011 eV−1 cm−2 is achieved. By varying the SiO2 thickness under the Al2O3 capping, it is possible to control the Qtot from virtually neutral (∼2.8 × 1010 cm−2) to moderately positive (∼8.5 × 1011 cm−2) values. Consequently, an excellent SRVmax as low as 1.3 cm/s is obtained using optimized SiO2/Al2O3 layer thicknesses. Finally, the origin of the positive charge as well as the interface defects related to PEALD SiO2 are discussed.
Joonas Isometsä, Zahra Jahanshah Rad, Tsun H. Fung, Hanchen Liu, Juha-Pekka Lehtiö, Toni P. Pasanen, Oskari Leiviskä, Mikko Miettinen, Pekka Laukkanen, Kalevi Kokko,et al.
MDPI AG
Germanium is an excellent material candidate for various applications, such as field effect transistors and radiation detectors/multijunction solar cells, due to its high carrier mobilities and narrow bandgap, respectively. However, the efficient passivation of germanium surfaces remains challenging. Recently, the most promising results have been achieved with atomic-layer-deposited (ALD) Al2O3, but the obtainable surface recombination velocity (SRV) has been very sensitive to the surface state prior to deposition. Based on X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), we show here that the poor SRV obtained with the combination of HF and DIW surface cleaning and ALD Al2O3 results from a Ge suboxide interlayer (GeOx, x < 2) with compromised quality. Nevertheless, our results also demonstrate that both the composition and crystallinity of this oxide layer can be improved with a combination of low-temperature heating and a 300-Langmuir controlled oxidation in an ultrahigh vacuum (LT-UHV treatment). This results in a reduction in the interface defect density (Dit), allowing us to reach SRV values as low as 10 cm/s. Being compatible with most device processes due to the low thermal budget, the LT-UHV treatment could be easily integrated into many future devices and applications.
Kexun Chen, Olli E. Setala, Xiaolong Liu, Behrad Radfar, Toni P. Pasanen, Michael D. Serue, Juha Heinonen, Hele Savin, and Ville Vahanissi
Institute of Electrical and Electronics Engineers (IEEE)
Shengyang Li, Hussein M. Ayedh, Marko Yli-Koski, Ville Vähänissi, Hele Savin, and Jani Oksanen
American Chemical Society (ACS)
Behrad Radfar, Kexun Chen, Olli E. Setälä, Ville Vähänissi, Hele Savin, and Xiaolong Liu
Optica Publishing Group
We 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.
Anastasia Matuhina, G. Krishnamurthy Grandhi, Fang Pan, Maning Liu, Harri Ali-Löytty, Hussein M. Ayedh, Antti Tukiainen, Jan-Henrik Smått, Ville Vähänissi, Hele Savin,et al.
American Chemical Society (ACS)
Zahra Jahanshah Rad, Mikko Miettinen, Marko Punkkinen, Pekka Laukkanen, Kalevi Kokko, Ville Vähänissi, and Hele Savin
Trans Tech Publications, Ltd.
Ultrahigh vacuum (UHV) environment has been widely used in surface science, but UHV technology has been often considered too complex and expensive methodology for large-scale industrial use. Because the preparation of atomically smooth and clean Si surfaces has become relevant to some industrial processes, we have re-addressed the question if UHV could be utilized in these surface tasks using industrially feasible parameters. In particular, we have studied how UHV treatments might be combined with the widely used semiconductor cleaning methodology of wet chemistry.
Juha Heinonen, Antti Haarahiltunen, Ville Vähänissi, Toni Pasanen, Hele Savin, Juha Toivanen, and Mikko Juntunen
SPIE
Black 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.
Hanchen Liu, Toni P. Pasanen, Tsun Hang Fung, Joonas Isometsä, Oskari Leiviskä, Ville Vähänissi, and Hele Savin
Wiley
Xiaolong Liu, Behrad Radfar, Kexun Chen, Elmeri Palikko, Toni P. Pasanen, Ville Vahanissi, and Hele Savin
Institute of Electrical and Electronics Engineers (IEEE)
Femtosecond laser-textured black silicon (fs-bSi) is known to suffer from heavy minority carrier recombination resulted from laser irradiation. In this letter, 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 <inline-formula> <tex-math notation="LaTeX">$8 \\mu \\text{s}$ </tex-math></inline-formula> to <inline-formula> <tex-math notation="LaTeX">$12 \\mu \\text{s}$ </tex-math></inline-formula>, 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 <inline-formula> <tex-math notation="LaTeX">$\\mu \\text{m}$ </tex-math></inline-formula>) of silicon.
Xiaolong Liu, Behrad Radfar, Kexun Chen, Olli E. Setala, Toni P. Pasanen, Marko Yli-Koski, Hele Savin, and Ville Vahanissi
Institute of Electrical and Electronics Engineers (IEEE)
In 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 $\\mu \\text{m}$ -scale devices, while MACE reaches 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 leave room for improvement.
Zahra Jahanshah Rad, Juha-Pekka Lehtiö, Kexun Chen, Iris Mack, Ville Vähänissi, Mikko Miettinen, Marko Punkkinen, Risto Punkkinen, Petri Suomalainen, Hannu-Pekka Hedman,et al.
Elsevier BV
Shengyang Li, Kexun Chen, Ville Vähänissi, Ivan Radevici, Hele Savin, and Jani Oksanen
American Chemical Society (ACS)
Metal-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.
Hussein M. Ayedh, Christopher W. Forbom, Juha Heinonen, Ismo T. S. Rauha, Marko Yli-Koski, Ville Vahanissi, and Hele Savin
Institute of Electrical and Electronics Engineers (IEEE)
Photoluminescence imaging (PLI) technique is conventionally used in silicon (Si) photovoltaics (PV) for device characterization and inline quality control, providing substantial assistance for a wafer-level process monitoring from as-cut wafers to fully fabricated devices. Surprisingly, employing this method has not spread outside PV, and thus, its potential remains largely unknown in other fields. In this case study, a fully processed Si photodetector wafer, consisting of photodiodes with various sizes, has been chosen as an example to explore the potential of PLI beyond PV. First, we show that the standard PLI measurement is able to provide a high-resolution full-wafer luminescence image of the complete devices only within a couple of seconds. The image reveals various types of inhomogeneities present in the devices, such as furnace contamination and other processing-induced defects. The measured data are then converted to an effective lifetime image followed by benchmarking with a conventionally measured recombination lifetime map obtained by microwave-detected photoconductance decay ( $\\mu $ -PCD), demonstrating further superiority of PLI in terms of the spatial resolution and the measurement time. Finally, correlation with diode leakage current and photoresponse measurements show that PLI is able to provide useful information on the final device performance without a need for traditional electrical contact measurements. While this study has focused on Si photodetectors, the results imply that PLI also has potential in other semiconductor devices for fast wafer-level process monitoring purposes as well as for a single device characterization either before or after wafer dicing.
Juha Heinonen, Antti Haarahiltunen, Ville Vähänissi, Toni P. Pasanen, Hele I. Savin, and Mikko A. Juntunen
SPIE
Black 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.