Ville Vähänissi

Scopus Publications

Near-infrared germanium PIN-photodiodes with >1A/W responsivity
Hanchen Liu, Toni P. Pasanen, Tsun Hang Fung, Joonas Isometsä, Antti Haarahiltunen, Steven Hesse, Lutz Werner, Ville Vähänissi, and Hele Savin
Springer Science and Business Media LLC
AbstractEven 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.

Femtosecond-laser-surface-nanostructured glass for building-integrated photovoltaics
Lingju Meng, Mohammad Awashra, Sara Hamed, Dmytro Gnatyuk, Ville Vähänissi, Ville Jokinen, Hele Savin, and Xiaolong Liu
Elsevier BV

Atomic-layer-deposited Cu<inf>x</inf>Cr<inf>y</inf>O<inf>z</inf> 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, Hele Savin, Maarit Karppinen, Daniel H. Macdonald, and Lachlan E. Black
Elsevier BV

Fs-laser significantly enhances both above- and below-bandgap absorption in germanium
Xiaolong Liu, Dmytro V. Gnatyuk, Julius Halmela, Ville Vähänissi, and Hele Savin
Optica Publishing Group
Fs-laser irradiation is a promising fabrication method for future broadband optoelectronic applications as it creates antireflective micro- and nanoscale structures on semiconductor surfaces and introduces below-bandgap absorption; however, its application has mainly been limited to silicon. This paper demonstrates that fs-laser technology enables high optical absorption both above and below the bandgap in germanium (Ge). With optimized laser parameters, we achieve a maximum above-bandgap absorptance of 95% and over 70% below-bandgap absorptance, due to the creation of surface microstructures and structural defects, respectively. Raman spectroscopy reveals that under intense laser irradiation, Ge may undergo a phase transition to structures with a narrower bandgap extending the absorption to the mid-infrared region. Furthermore, we develop a hyperdoping process using Ti coating pre-laser processing followed by rapid thermal annealing, which results in 90% above-bandgap absorption and a 12% relative increase in below-bandgap absorption along with a high degree of crystallinity. The increased below-bandgap absorption is attributed to Ti doping and is twice as high as reported earlier. Our findings should have significant implications for the future Ge-based infrared applications.

Passivation of Germanium Surfaces by HF:H<inf>2</inf>O<inf>2</inf> Aqueous Solution
Mariia Terletskaia, Joonas Isometsä, Mikko Miettinen, Pekka Laukkanen, Ville Vähänissi, and Hele Savin
Wiley
Promising intrinsic electronic properties, such as narrow bandgap and high charge carrier mobilities, make germanium (Ge) a good replacement for silicon in optoelectronic applications (e.g., photodetectors). However, successful fabrication of efficient Ge devices requires minimization of both reflectance and surface recombination losses. This work begins with an observation that metal‐assisted chemical etching (MACE) of Ge surfaces, used for optics improvement, reduces surface recombination without application of any intentional passivation. We proceed with investigation of the effect of MACE solution components and their mixtures on Ge surface passivation. The results demonstrate that HF:H2O2 aqueous solution leads to efficient and stable passivation. The film formed in this solution secures surface recombination velocity (Seff) of 14 cm s−1. Morphological and chemical characterization of the structure reveals porous germanium (PGe) layer with some GeOx included. Finally, we propose several hypotheses on a mechanism behind this passivation, among which are the presence of GeO2 at the film‐bulk Ge interface and appearance of a potential barrier due to the heterojunction formation. The presented Ge passivation with PGe layer provides a simple and cost‐efficient alternative to existing state‐of‐the‐art passivation schemes.

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, Ville Vähänissi, and Hele Savin
Elsevier BV

Extended Infrared Absorption in Nanostructured Si Through Se Implantation and Flash Lamp Annealing
Behrad Radfar, Xiaolong Liu, Yonder Berencén, Mohd Saif Shaikh, Slawomir Prucnal, Ulrich Kentsch, Ville Vähänissi, Shengqiang Zhou, and Hele Savin
Wiley
Nanostructured silicon can reduce reflectance loss in optoelectronic applications, but intrinsic silicon cannot absorb photons with energy below its 1.1 eV bandgap. However, incorporating a high concentration of dopants, i.e., hyperdoping, to nanostructured silicon is expected to bring broadband absorption ranging from UV to short‐wavelength IR (SWIR, <2500 nm). In this work, we prepare nanostructured silicon using cryogenic plasma etching, which is then hyperdoped with selenium (Se) through ion implantation. Besides sub‐bandgap absorption, ion implantation forms crystal damage, which can be recovered through flash lamp annealing. We study crystal damage and broadband (250–2500 nm) absorption from planar and nanostructured surfaces. We first show that nanostructures survive ion implantation hyperdoping and flash lamp annealing under optimized conditions. Secondly, we demonstrate that nanostructured silicon has a 15% higher sub‐bandgap absorption (1100–2500 nm) compared to its non‐hyperdoped nanostructure counterpart while maintaining 97% above‐bandgap absorption (250–1100 nm). Lastly, we simulate the sub‐bandgap absorption of hyperdoped Si nanostructures in a 2D model using the finite element method. Simulation results show that the sub‐bandgap absorption is mainly limited by the thickness of the hyperdoped layer rather than the height of nanostructures.

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, 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.

Status report on emerging photovoltaics
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.

Impact of post-ion implantation annealing on Se-hyperdoped Ge
Xiaolong Liu, Patrick McKearney, Sören Schäfer, Behrad Radfar, Yonder Berencén, Ulrich Kentsch, Ville Vähänissi, Shengqiang Zhou, Stefan Kontermann, and Hele Savin
AIP Publishing
Hyperdoped germanium (Ge) has demonstrated increased sub-bandgap absorption, offering potential applications in the short-wavelength-infrared spectrum (1.0–3.0 μm). This study employs ion implantation to introduce a high concentration of selenium (Se) into Ge and investigates the effects of post-implantation annealing techniques on the recovery of implantation damage and alterations in optical properties. We identify optimal conditions for two distinct annealing techniques: rapid thermal annealing (RTA) at a temperature of 650 °C and ultrafast laser heating (ULH) at a fluence of 6 mJ/cm2. The optimized ULH process outperforms the RTA method in preserving high doping profiles and achieving a fourfold increase in sub-bandgap absorption. However, RTA leads to regrowth of single crystalline Ge, while ULH most likely leads to polycrystalline Ge. The study offers valuable insights into the hyperdoping processes in Ge for the development of advanced optoelectronic devices.

Adsorption induced bipolar excitation at semiconductor surface
Shengyang Li, Kexun Chen, Mahdi Alizadeh, Ville Vähänissi, Hele Savin, and Jani Oksanen
Elsevier BV

Contactless analysis of surface passivation and charge transfer at the TiO<inf>2</inf>-Si interface
Ramsha Khan, Xiaolong Liu, Ville Vähänissi, Harri Ali-Löytty, Hannu P. Pasanen, Hele Savin, and Nikolai V. Tkachenko
Royal Society of Chemistry (RSC)
Investigating the effects of compositional and structural changes of interfacial SiOx and TiO2 films on the surface passivation and its correlation with the charge transfer (CT) across the TiO2–Si interface.

Bridging the gap between surface physics and photonics
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.

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, 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.

Quantifying the Impact of Al Deposition Method on Underlying Al<inf>2</inf>O<inf>3</inf>/Si Interface Quality
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).

Black Ultra-Thin Crystalline Silicon Wafers Reach the 4n<sup>2</sup> Absorption Limit–Application to IBC Solar Cells
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.

Efficient surface passivation of germanium nanostructures with 1% reflectance
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.

Atomic Layer Deposition of Titanium Oxide-Based Films for Semiconductor Applications—Effects of Precursor and Operating Conditions
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.

Boron-Implanted Black Silicon Photodiode with Close-to-Ideal Responsivity from 200 to 1000 nm
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.

Is carrier mobility a limiting factor for charge transfer in TiO<inf>2</inf>/Si devices? A study by transient reflectance spectroscopy
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

Plasma-enhanced atomic layer deposited SiO<inf>2</inf> enables positive thin film charge and surface recombination velocity of 1.3 cm/s on germanium
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.

Surface Passivation of Germanium with ALD Al<inf>2</inf>O<inf>3</inf>: Impact of Composition and Crystallinity of GeO<inf>x</inf> Interlayer
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.

Excellent Responsivity and Low Dark Current Obtained with Metal-Assisted Chemical Etched Si Photodiode
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)

Chemical Excitation of Silicon Photoconductors by Metal-Assisted Chemical Etching
Shengyang Li, Hussein M. Ayedh, Marko Yli-Koski, Ville Vähänissi, Hele Savin, and Jani Oksanen
American Chemical Society (ACS)