Ville Vähänissi

Scopus Publications

Nanostructuring SiC by sequential plasma oxidation and reactive ion etching
Joonas Isometsä, Auguste Bieleviciute, Xiaolong Liu, Ville Vähänissi, Hele Savin
Applied Surface Science, 2026
Challenges in Simultaneous Microstructuring and Hyperdoping of Germanium with Ultrafast Laser
Dmytro Gnatyuk, Sara Hamed, Masoud Ebrahimzadeh, Hanchen Liu, Ville Vähänissi, et al.
Physica Status Solidi A Applications and Materials Science, 2026
Germanium's compatibility with Complementary Metal‐Oxide‐Semiconductor (CMOS) and strong near‐infrared response make it an attractive platform for infrared photonics, but its intrinsic material properties hinder straightforward extension of absorption beyond the band edge. In this perspective, we synthesize recent and new experiments and analyses on femtosecond‐laser approaches that attempt to combine surface microstructuring and hyperdoping of Ge in a single step. We argue that, unlike silicon, Ge's high optical absorption at visible/green wavelengths, shallow energy deposition, lower melting point, and reduced thermal conductivity favor intense localized heating, evaporation, and redeposition—conditions that both produce high baseline sub‐bandgap absorption from damage and prevent effective incorporation of thin‐film dopant precursors. In a case example, Ti shows only trace incorporation from qualitative measurements. We discuss why laser‐induced structural disorder, rather than stable deep dopant incorporation, dominates the optical response, and we outline practical pathways forward: exploring longer wavelengths or gas‐phase chemistries, applying separate in situ heating, or decoupling texturing from heavy doping.
Carrier mobility in crystalline germanium at high injection: experimental characterization of carrier-carrier scattering
Moisés Garín, Mansur Gamel, Marko Yli-Koski, Ville Vähänissi, Gerard Rivera, et al.
Solar Energy Materials and Solar Cells, 2026
Bipolar and unipolar charge transfer during the formation of silicon/iodine solution interfaces
Shengyang Li, Mahdi Alizadeh, Ville Vähänissi, Hele Savin, Guangwei She, et al.
Surfaces and Interfaces, 2026
Transforming Schottky to Ohmic Contacts via Ultrahigh-Vacuum Engineered Interfacial Alloying
Masoud Ebrahimzadeh, Perttu Piispanen, Sari Granroth, Mikko Miettinen, Ilari Angervo, et al.
ACS Applied Materials and Interfaces, 2025
Impact of Er-Based Atomic Layer Deposited Upconverting Oxides on Si Solar Cell Surface Passivation
Behrad Radfar, Amr Ghazy, Joona Pekkanen, Xiaolong Liu, Ville Vähänissi, et al.
Physica Status Solidi A Applications and Materials Science, 2025
Crystalline silicon solar cells face fundamental limitations in infrared (IR) absorption leading to significant solar energy losses. Using upconverting (UC) rare‐earth oxide layers is a potential solution to convert sub‐bandgap photons into higher‐energy photons that Si can absorb. However, integrating UC layers into solar cells can potentially affect surface passivation properties, which may result in recombination of photogenerated charge carriers at Si/UC interface. This study investigates the impact of atomic‐layer‐deposited mixed rare‐earth oxide (Y,Er,Ho)2O3 and Er2O3 UC layers on the passivation quality of Si interface, deposited directly on Si substrate or on the conventional passivation layers, i.e., SiO2 and Al2O3. Findings reveal that direct deposition provides very weak passivation (τeff ≈ 4.1 μs) which does not improve with post‐deposition annealing. Similarly, when deposited on top of SiO2 with optically relevant thickness, the resulting passivation is poor. However, UC layer on top of Al2O3 does not compromise the passivation performance, resulting in low surface recombination velocity of 1.5 cm s−1 for Er2O3 and 1.4 cm s−1 for (Y,Er,Ho)2O3. Consequently, the results indicate that it should be viable to integrate UC layer like Er2O3 into a state‐of‐the‐art solar cell to improve sub‐bandgap absorption without compromising the passivation properties.
Near-infrared germanium PIN-photodiodes with >1A/W responsivity
Hanchen Liu, Toni P. Pasanen, Tsun Hang Fung, Joonas Isometsä, Antti Haarahiltunen, et al.
Light Science and Applications, 2025
Even though efficient near-infrared (NIR) detection is critical for numerous applications, state-of-the-art NIR detectors either suffer from limited capability of detecting incoming photons, i.e., have poor spectral responsivity, or are made of expensive group III-V non-CMOS compatible materials. Here we present a nanoengineered PIN-photodiode made of CMOS-compatible germanium (Ge) that achieves a verified external quantum efficiency (EQE) above 90% over a wide wavelength range (1.2–1.6 µm) at zero bias voltage at room temperature. For instance, at 1.55 µm, this corresponds to a responsivity of 1.15 A/W. In addition to the excellent spectral responsivity at NIR, the performance at visible and ultraviolet wavelengths remains high (EQE exceeds even 100% below 300 nm) resulting in an exceptionally wide spectral response range. The high performance is achieved by minimizing optical losses using surface nanostructures and electrical losses using both conformal atomic-layer-deposited aluminum oxide surface passivation and dielectric induced electric field -based carrier collection instead of conventional pn-junction. The dark current density of 76 µA/cm2 measured at a reverse bias of 5 V is lower than previously reported for Ge photodiodes. The presented results should have an immediate impact on the design and manufacturing of Ge photodiodes and NIR detection in general.
Charged thin film enables dopant free ohmic metal–semiconductor contact formation
Lassi Lahtiluoma, Olli E. Setälä, Ville Vähänissi, Hele Savin
Applied Surface Science, 2025
Effects of ultra-high vacuum treatments on n-type Si contact resistivity
Mikko Miettinen, Esa Vuorinen, Juha-Pekka Lehtiö, Zahra Jahanshah Rad, Risto Punkkinen, et al.
Applied Surface Science, 2025
Femtosecond-laser-surface-nanostructured glass for building-integrated photovoltaics
Lingju Meng, Mohammad Awashra, Sara Hamed, Dmytro Gnatyuk, Ville Vähänissi, et al.
Materials and Design, 2025
Passivation of Germanium Surfaces by HF:H2O2 Aqueous Solution
Mariia Terletskaia, Joonas Isometsä, Mikko Miettinen, Pekka Laukkanen, Ville Vähänissi, et al.
Physica Status Solidi Rapid Research Letters, 2025
Atomic-layer-deposited CuxCryOz thin films: Optoelectronic properties and potential application as hole-selective contacts for c-Si solar cells
Gabriel Bartholazzi, Topias Jussila, Jorit Obenlüneschloß, Ville Vähänissi, Anjana Devi, et al.
Applied Surface Science, 2025
Fs-laser significantly enhances both above- and below-bandgap absorption in germanium
Xiaolong Liu, Dmytro V. Gnatyuk, Julius Halmela, Ville Vähänissi, Hele Savin
Optical Materials Express, 2025
Elimination of dead layer in silicon particle detectors via induced electric field based charge collection
Olli E. Setälä, Toni P. Pasanen, Jennifer Ott, Igors Krainukovs, Juha Heinonen, et al.
Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment, 2025
Effective Carrier Lifetime in Ultrashort Pulse Laser Hyperdoped Silicon: Sulfur Concentration Dependence and Practical Limitations
Sören Schäfer, Xiaolong Liu, Patrick Mc Kearney, Simon Paulus, Behrad Radfar, et al.
Physica Status Solidi A Applications and Materials Science, 2024
Extended Infrared Absorption in Nanostructured Si Through Se Implantation and Flash Lamp Annealing
Behrad Radfar, Xiaolong Liu, Yonder Berencén, Mohd Saif Shaikh, Slawomir Prucnal, et al.
Physica Status Solidi A Applications and Materials Science, 2024
Status report on emerging photovoltaics
Annick Anctil, Meghan N. Beattie, Christopher Case, Aditya Chaudhary, Benjamin D. Chrysler, et al.
SPIE Selections 50 Years of Solar Energy Research, 2024
Impact of post-ion implantation annealing on Se-hyperdoped Ge
Xiaolong Liu, Patrick McKearney, Sören Schäfer, Behrad Radfar, Yonder Berencén, et al.
Applied Physics Letters, 2024
Adsorption induced bipolar excitation at semiconductor surface
Shengyang Li, Kexun Chen, Mahdi Alizadeh, Ville Vähänissi, Hele Savin, et al.
Surfaces and Interfaces, 2024
Contactless analysis of surface passivation and charge transfer at the TiO2-Si interface
Ramsha Khan, Xiaolong Liu, Ville Vähänissi, Harri Ali-Löytty, Hannu P. Pasanen, et al.
Physical Chemistry Chemical Physics, 2024
Bridging the gap between surface physics and photonics
Pekka Laukkanen, Marko Punkkinen, Mikhail Kuzmin, Kalevi Kokko, Xiaolong Liu, et al.
Reports on Progress in Physics, 2024
CMOS Image Sensor for Broad Spectral Range with >90% Quantum Efficiency
Olli E. Setälä, Martin J. Prest, Konstantin D. Stefanov, Douglas Jordan, Matthew R. Soman, et al.
Small, 2023
Quantifying the Impact of Al Deposition Method on Underlying Al2O3/Si Interface Quality
Iris Mack, Kawa Rosta, Ulviyya Quliyeva, Jennifer Ott, Toni P. Pasanen, et al.
Physica Status Solidi A Applications and Materials Science, 2023
Status report on emerging photovoltaics
Annick Anctil, Meghan N. Beattie, Christopher Case, Aditya Chaudhary, Benjamin D. Chrysler, et al.
Journal of Photonics for Energy, 2023
Black Ultra-Thin Crystalline Silicon Wafers Reach the 4n2 Absorption Limit–Application to IBC Solar Cells
M. Garín, T. P. Pasanen, G. López, V. Vähänissi, K. Chen, et al.
Small, 2023