### Browsing by Author "Anttu, Nicklas"

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Item Absorption modeling with FMM, FEM and FDTD(2019-07-01) Anttu, Nicklas; Mantynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern Finland; Hinzer, Karin; Piprek, JoachimAbsorption modeling is at the core of the design process of nanostructured solar cells and photodetectors. We compare the performance of three of the most popular numerical modeling methods: the Fourier modal method (FMM), the finite element method (FEM) and the finite-difference time-domain (FDTD) method. We find that the numerically most efficient method depends on the geometry of the system, as well as on which physical quantities are needed for further analysis. From our study, we will highlight the optimum choice of method for various current nanostructures. With these guidelines, we enable design optimization that would otherwise be impossible with a suboptimal method choice.Item Absorption of light in a single vertical nanowire and a nanowire array(IOP PUBLISHING LTD, 2019-01-18) Anttu, Nicklas; Department of Electronics and Nanoengineering; Harri Lipsanen GroupBoth a single III-V semiconductor nanowire and an array of such nanowires have shown promise for solar cell applications. However, the correspondence between the optical properties of the single nanowire and the nanowire array has not been studied. Here, we perform electromagnetic modeling of InP nanowires to study this relationship. We find that a single nanowire can show at an absorption peak, a remarkably high absorption cross-section that is more than 50 times the geometrical cross-section. With optimization of the diameter of the single nanowire, the short-circuit current density is 30 times higher than in a bulk solar cell. With such a strong absorption, we predict an apparent efficiency >500% for the single nanowire solar cell. In contrast, we show that an efficient nanowire array solar cell cannot rely on strong absorption just through the absorption peak. Instead, the nanowires need to be packed rather closely to enhance the absorption of the full solar spectrum. At the optimum diameter for the nanowire array, neighboring nanowires compete strongly for absorption of incident photons at the absorption peak, which limits the absorption per nanowire by a factor of 18. As a result, the single InP nanowire is optimized at a diameter of 110 nm while the nanowires in the array are optimized at a considerably larger diameter of 180 nm. Importantly, we show analytically the coupling efficiency of incident light into the fundamental HE11 guided mode and consecutive absorption of the mode in the nanowires. With that analysis, we explain that a single nanowire shows two different absorption pathways-one through coupling into the guided mode and another by coupling into the nanowire through the sidewall. This analytical analysis also shows at which period the neighboring nanowires in an array start to compete for absorption of incident photons.Item Absorption of light in finite semiconductor nanowire arrays and the effect of missing nanowires(Multidisciplinary Digital Publishing Institute (MDPI), 2021-09) Anttu, Nicklas; Department of Electronics and NanoengineeringWhen modelling the absorption in semiconductor nanowire (NW) arrays for solar cell and photodetector applications, the array is typically assumed to be infinitely periodic such that a single unit cell suffices for the simulations. However, any actual array is of a finite extent and might also show varying types of localized defects such as missing or electrically non-contacted individual NWs. Here, we study InP NWs of 2000 nm in length and 180 nm in diameter, placed in a square array of 400 nm in period, giving a rather optimized absorption of sunlight. We show that the absorption in the center NW of a finite N x N array converges already at N = 5 close to the value found for the corresponding infinite array. Furthermore, we show that a missing NW causes an enhanced absorption in neighboring nanowires, which compensates for 77% of the absorption loss due to the missing NW. In other words, an electrically non-contacted NW, which absorbs light but cannot contribute to the external short-circuit current, is a four times worse defect than a missing NW.Item Applied electromagnetic optics simulations for nanophotonics(American Institute of Physics, 2021-04-07) Anttu, Nicklas; Mäntynen, Henrik; Sorokina, Anastasiia; Turunen, Jari; Sadi, Toufik; Lipsanen, Harri; Department of Electronics and Nanoengineering; Department of Neuroscience and Biomedical Engineering; Harri Lipsanen Group; University of Eastern FinlandNanophotonics—the science and technology of confining, guiding, and making photons interact with matter at the nanoscale—is an active research field. By varying the geometry and constituent materials, nanostructures allow precise control of the scattering of incident light and tailoring of emitted light. In this Tutorial, we outline the use of the Maxwell equations to model the optical response of nanostructures. This electromagnetic optics approach uses the refractive indices of the constituent materials and the geometry of the nanostructures as input. For most nanostructure geometries, analytical solutions to the Maxwell equations are not available. Therefore, we discuss varying computational methods for solving the equations numerically. These methods allow us to simulate the optical response of nanostructures, as needed for design optimization and analysis of characterization results.Item Comparison of absorption simulation in semiconductor nanowire and nanocone arrays with the Fourier modal method, the finite element method, and the finite-difference time-domain method(IOP Publishing, 2020) Anttu, Nicklas; Mäntynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern Finland; Department of Electronics and NanoengineeringFor the design of nanostructured semiconductor solar cells and photodetectors, optics modelling can be a useful tool that reduces the need of time-consuming and costly prototyping. We compare the performance of three of the most popular numerical simulation methods for nanostructure arrays: the Fourier modal method (FMM), the finite element method (FEM) and the finite-difference time-domain (FDTD) method. The difference between the methods in computational time can be three orders of magnitude or more for a given system. The preferential method depends on the geometry of the nanostructures, the accuracy needed from the simulations, whether we are interested in the total, volume-integrated absorption or spatially resolved absorption, and whether we are interested in broadband or narrowband response. Based on our benchmarking results, we provide guidance on how to choose the method.Item Comparison of FMM, FEM and FDTD for absorption modeling of nanostructured solar cells and photodetectors(2019-01-01) Anttu, Nicklas; Mantynen, Henrik; Sadi, Toufik; Matikainen, Antti; Turunen, Jari; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; University of Eastern FinlandWe compare FMM, FEM and FDTD for absorption modeling. We discuss optimum choice of modeling method for varying nanostructures, enabling solar cell and photodetector design optimization that would be impossible with a suboptimal method choice.Item Electro-Optical Coupling in Double Diode Structures(American Physical Society, 2023-06) Anttu, Nicklas; Dagytė, Vilgailė; Behaghel, Benoît; Radevici, Ivan; Sadi, Toufik; Kivisaari, Pyry; Oksanen, Jani; Åbo Akademi University; Department of Neuroscience and Biomedical EngineeringAlternative types of artificial cooling techniques are of large interest for multiple applications. Here, we develop a framework for studying the role of electro-optical coupling in the analysis of solid-state refrigerators based on electroluminescent cooling (ELC) by combining device measurements with optical simulations. The studied device consists of a light-emitting diode (LED) epitaxially connected to a photodetector (PD) in a double-diode structure (DDS). Previous results of the DDS have indicated that the LED side already operates at conditions corresponding to ELC, but Ohmic losses and imperfect photodetection of the LED light in the PD have prevented observing the effect directly. Here, to break down the detection losses of the DDS, we report on the electro-optical response of the LED and the PD in detail, as well as the role of the spectral coupling from the LED to the PD. We present a detailed framework for combining measurements and simulations of the DDS to gain quantitative insight of the electro-optical response of the LED and PD, as well as the coupling between them, including the analysis of effects that are not directly accessible by standard measurements. The developed approach allows identifying the different photodetection loss mechanisms from the current-voltage and electroluminescence measurements and thereby gives guidance for designs toward a direct demonstration of ELC at practically relevant cooling powers. Somewhat surprisingly, the results show that an imperfect spectral absorption efficiency of the PD, in addition to its below unity quantum efficiency, are together required to explain the previously observed low photodetection efficiency of the DDS even for several microns thick PD structures. In comparison, the LED top mirror introduces only a minuscule drop in photodetection efficiency. Put in plain numbers, our analysis reveals that in the current DDS designs, there is headroom by 14% in the spectral matching between the LED and the PD, 5% in the charge collection efficiency of the PD, and 4% in the efficiency at which photons emitted from the LED reach the PD.Item Emission enhancement, light extraction and carrier dynamics in InGaAs/GaAs nanowire arrays(IOP Publishing Ltd., 2018) Kivisaari, Pyry; Chen, Yang; Anttu, Nicklas; Department of Neuroscience and Biomedical Engineering; Lund University; Department of Electronics and NanoengineeringNanowires (NWs) have the potential for a wide range of new optoelectronic applications. For example, light-emitting diodes that span over the whole visible spectrum are currently being developed from NWs to overcome the well known green gap problem. However, due to their small size, NW devices exhibit special properties that complicate their analysis, characterization, and further development. In this paper, we develop a full optoelectronic simulation tool for NW array light emitters accounting for carrier transport and wave-optical emission enhancement (EE), and we use the model to simulate InGaAs/GaAs NW array light emitters with different geometries and temperatures. Our results show that NW arrays emit light preferentially to certain angles depending on the NW diameter and temperature, encouraging temperature- and angle-resolved measurements of NW array light emission. On the other hand, based on our results both the EE and light extraction efficiency can easily change by at least a factor of two between room temperature and 77 K, complicating the characterization of NW light emitters if conventional methods are used. Finally, simulations accounting for surface recombination emphasize its major effect on the device performance. For example, a surface recombination velocity of 104 cm s−1 reported earlier for bare InGaAs surfaces results in internal quantum efficiencies less than 30% for small-diameter NWs even at the temperature of 30 K. This highlights that core–shell structures or high-quality passivation techniques are eventually needed to achieve efficient NW-based light emitters.Item Geometry Tailoring of Emission from Semiconductor Nanowires and Nanocones(MDPI AG, 2020-06-01) Anttu, Nicklas; Mäntynen, Henrik; Sorokina, Anastasiia; Kivisaari, Pyry; Sadi, Toufik; Lipsanen, Harri; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; Department of Electronics and NanoengineeringSemiconductor nanowires are of interest as light emitters in applications such as light-emitting diodes and single-photon sources. Due to the three-dimensional geometry in combination with a size comparable to the wavelength of the emitted light, nanowires have shown strong scattering effects for the emitted light. Here, we demonstrate with electromagnetic modeling that the emission properties of nanowires/nanocones show a complicated dependence on the geometry of the nanowire/nanocone, the shape and position of the emitter region, and the polarization of the emitter. We show that with proper design, the extraction efficiency can close in on 80% for as-grown single nanowires/nanocones. Importantly, we demonstrate how the internal quantum efficiency of the emitter plays a large role in the design process. A considerably different geometry design approach should be undertaken at low and high internal quantum efficiency. Due to the complicated design optimization, we strongly recommend the use of electromagneticmodeling of the emission to give guidance for suitable designs before starting the fabrication and processing of nanowire/nanocone-based light emitters.Item Management of light and scattering in InP NWs by dielectric polymer shell(IOP Publishing Ltd., 2020-09-18) Haggren, Tuomas; Anttu, Nicklas; Mäntynen, Henrik; Tossi, Camilla; Kim, Maria; Khayrudinov, Vladislav; Lipsanen, Harri; Department of Electronics and Nanoengineering; Zhipei Sun Group; Harri Lipsanen Group; Department of Neuroscience and Biomedical EngineeringUnderstanding and management of light is of great importance for nanoscale devices. This report demonstrates enhanced absorption, photoluminescence and scattering in InP nanowires when coated with dielectric polymer shell. The shells increase absorption and emission by a factor of ~2 and photoluminescence by a factor of ~4. A thorough optical characterization is provided, including reflectance, transmission, luminescence and scattering to incident and transmitted directions. From this characterization, we derive the distribution of absorbed light within the structure (InP nanowires, Au seed particles and the substrate). Additionally, reflectance, transmission and emission are shown to become increasingly diffuse with the dielectric shells. The results are thought to provide better understanding in light-matter interaction in nanostructures, as well as to provide valuable tools for light and scattering management in nanoscale optoelectronics.Item Measurement of Nanowire Optical Modes Using Cross-Polarization Microscopy(2017-12-19) Kakko, Joona-Pekko; Matikainen, Antti; Anttu, Nicklas; Kujala, Sami; Mäntynen, Henrik; Khayrudinov, Vladislav; Autere, Anton; Sun, Zhipei; Lipsanen, Harri; Department of Electronics and NanoengineeringA method to detect optical modes from vertical InGaAs nanowires (NWs) using cross-polarization microscopy is presented. Light scattered from the optical modes in the NWs is detected by filtering out the polarized direct reflection with a crossed polarizer. A spectral peak and a valley were seen to red-shift with increasing NW diameter in the measured spectra. The peak was assigned to scattering from the TE01 optical mode and the valley was an indication of the HE11 mode, based on finite-element and scattering matrix method simulations. The cross-polarization method can be used to experimentally determine the spectral positions of the TE01 and HE11 optical modes. The modes are significantly more visible in comparison to conventional reflectance measurements. The method can be beneficial in the characterization of NW solar cells, light-emitting diodes and lasers where precise mode control is required.Item Modal analysis of resonant and non-resonant optical response in semiconductor nanowire arrays(2019-01-11) Dagyte, Vilgaile; Anttu, Nicklas; Department of Electronics and Nanoengineering; Harri Lipsanen GroupNanowire array solar cells have reached efficiencies where it becomes feasible to talk about creating tandem solar cells in order to achieve even higher efficiencies. An example of such a tandem solar cell could be a nanowire array embedded in a membrane and integrated on top of a Si bottom cell. Such a system, however, requires understanding and control of its interaction with light, especially to make sure that the low energy photons are transmitted to the bottom cell. The dependence of the optical response of a nanowire array on the nanowire length, diameter, array pitch, materials surrounding the nanowires, and absorption coefficient of the nanowire material is very strong and possibly resonant, indicating the complexity of the optical response. In this work, we use an eigenmode-based analysis to reveal underlying physics that gives rise to observed resonant and non-resonant behavior. First, we show that an effective refractive index can be defined at long wavelengths, where only a single mode propagates. Second, we analyze the origin of the resonant reflection when the next optical mode becomes propagating and can be 'trapped' in the array and interact with the fundamental mode. Additionally, we define two simple boundaries for the wavelength range of the resonant response: the resonances can only occur if there is more than 1 propagating mode in the array, and they disappear if the 1st diffracted order is propagating in the top or bottom material. Such resonance effects could be detrimental for tandem solar cells. We thus provide recommendations for tuning the geometry of the array and the nanowire materials in order to push the resonant regime to the absorbing regime of the nanowire, where absorption in the nanowires dampens the resonances. Finally, this work demonstrates the strength of an eigenmode-based analysis of the optical response of periodic nanostructures in terms of simplifying the analysis of a complex system.Item Nanowire oligomer waveguide modes towards reduced lasing threshold(MDPI AG, 2020-12-01) Mäntynen, Henrik; Anttu, Nicklas; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen GroupSemiconductor nanowires offer a promising route of realizing nanolasers for the next generation of chip-scale optoelectronics and photonics applications. Established fabrication methods can produce vertical semiconductor nanowires which can themselves act both as a gain medium and as a Fabry–Pérot cavity for feedback. The lasing threshold in such nanowire lasers is affected by the modal confinement factor and end facet reflectivities, of which the substrate end reflectivity tends to be limited due to small refractive index contrast between the nanowire and substrate. These modal properties, however, also depend strongly on the modal field profiles. In this work, we use numerical simulations to investigate waveguide modes in vertical nanowire oligomers (that is, arrangements of few vertical nanowires close to each other) and their modal properties compared to single nanowire monomers. We solve for the oligomer waveguide eigenmodes which are understood as arising from interaction of monomer modes and further compute the reflectivity of these modes at the end facets of the nanowires. We consider either the nanowires or an additional coating layer as the gain medium. We show that both types of oligomers can exhibit modes with modal properties leading to reduced lasing threshold and also give directions for further research on the topic.Item Optical far-field extinction of a single GaAs nanowire towards in situ size control of aerotaxy nanowire growth(IOP PUBLISHING LTD, 2020-01-09) Chen, Yang; Anttu, Nicklas; Sivakumar, Sudhakar; Gompou, Eleni; Magnusson, Martin; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Lund UniversityA substrate-free approach of semiconductor nanowire growth has been achieved by the aerotaxy technique previously. In this work, we propose an in situ method to monitor the size of nanowires through non-destructive optical-extinction measurements. Our work aims to build a theoretical look-up database of extinction spectra for a single nanowire of varying dimensions. We describe the origin of possible peaks in the spectra, for example due to nanowire-length dependent Fabry–Perot resonances and nanowire-diameter dependent TM and TE mode resonances. Furthermore, we show that the Au catalyst on top of the nanowire can be ignored in the simulations when the volume of the nanowire is an order of magnitude larger than that of the Au catalyst and the diameter is small compared to the incident wavelength. For the calculation of the extinction spectra, we use the finite element method, the discrete dipole approximation and the Mie theory. To compare with experimental measurements of randomly oriented nanowires, we perform an averaging over nanowire orientation for the modeled results. However, in the experiments, nanowires are accumulating on the quartz window of the measurement setup, which leads to increasing uncertainty in the comparison with the experimental extinction spectra. This uncertainty can be eliminated by considering both a sparse and a dense collection of nanowires on the quartz window in the optical simulations. Finally, we create a database of extinction spectra for a GaAs nanowire of varying diameters and lengths. This database can be used to estimate the diameter and the length of the nanowires by comparing the position of a peak and the peak-to-shoulder difference in the extinction spectrum. Possible tapering of nanowires can be monitored through the appearance of an additional peak at a wavelength of 700–800 nm.Item Physics and design for 20% and 25% efficiency nanowire array solar cells(2019-02-15) Anttu, Nicklas; Department of Electronics and Nanoengineering; Harri Lipsanen GroupBottom-up fabricated single-junction III–V nanowire array solar cells have shown efficiency up to 15.3%, which is approximately half of the conventional Shockley–Queisser detailed balance efficiency limit of 33.6%. Here, based on numerical and analytical opto-electronics modeling and analysis, we give guidelines for (i) geometry that gives strong absorption as well as (ii) the design of efficient p–n junction and electrical contacts in the nanowires to reach 20% and 25% efficiency. We exemplify the impact of eight different optical and electrical loss mechanisms in a 15% and a 25% design. We also provide an analytical equation for estimating the efficiency drop due to resistive losses in the top contact layer for varying cell size.Item Single-photon sources with quantum dots in III-V nanowires(WALTER DE GRUYTER GMBH, 2019-05) Mäntynen, Henrik; Anttu, Nicklas; Sun, Zhipei; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun GroupSingle-photon sources are one of the key components in quantum photonics applications. These sources ideally emit a single photon at a time, are highly efficient, and could be integrated in photonic circuits for complex quantum system designs. Various platforms to realize such sources have been actively studied, among which semiconductor quantum dots have been found to be particularly attractive. Furthermore, quantum dots embedded in bottom-up-grown III-V compound semiconductor nanowires have been found to exhibit relatively high performance as well as beneficial flexibility in fabrication and integration. Here, we review fabrication and performance of these nanowire-based quantum sources and compare them to quantum dots in top-down-fabricated designs. The state of the art in single-photon sources with quantum dots in nanowires is discussed. We also present current challenges and possible future research directions.Item Symmetry Reduction in FEM Optics Modeling of Single and Periodic Nanostructures(Multidisciplinary Digital Publishing Institute (MDPI), 2021-05) Mantynen, Henrik; Lipsanen, Harri; Anttu, Nicklas; Department of Electronics and Nanoengineering; Harri Lipsanen GroupNumerical optics modeling is an invaluable tool in the design of nanostructures for nanophotonics applications where diffraction effects often lead to complex dependency between the nanostructure geometry and its optical properties and response. In order to analyze, design, and optimize such nanostructures, computationally efficient numerical optics modeling methods are required. One way to improve the numerical performance is to exploit symmetries found in many optics problems. By identifying equivalencies and restrictions arising from symmetry, it can be possible to simplify the problem at hand, which is the essence of symmetry reduction. However, applying symmetry reduction in optics modeling problems is not trivial. To the best of our knowledge, symmetry reduction has so-far been applied in finite element method (FEM) optics models only in those specific cases where an incident plane wave shares symmetries with the nanostructure geometry. In this work, we show how to extend the symmetry reduction of FEM optics models to the case of nonsymmetric plane-wave incidence, demonstrate such reduction with numerical examples of incident plane wave absorption in a single nanowire and a periodic nanowire array, and discuss the achieved gains in computational efficiency.Item Tailored emission to boost open-circuit voltage in solar cells(Institute of Physics Publishing, 2019) Anttu, Nicklas; Kivisaari, Pyry; Chen, Yang; Harri Lipsanen Group; Department of Neuroscience and Biomedical Engineering; Lund University; Department of Electronics and NanoengineeringRecently, a lot of research focus has been on how to make solar cells more efficient. One direction is to enhance the open-circuit voltage V oc by optimizing the emission of photons in the cell, where emission is a necessary loss process due to the reciprocity between absorption and emission of light. Here, we performed a Shockley-Queisser detailed balance analysis to predict the benefit of managing emitted photons in a single-junction solar cell. First, at low internal luminescence efficiency η int, non-radiative recombination dominates, and management of emitted photons plays negligible role for V oc. Similarly, for an external luminescence efficiency η ext < 10%, externally emitted photons play negligible role, and V oc is set either by non-radiative recombination; or parasitic absorption of internally emitted photons. For higher η ext, the V oc can be boosted, maximally by 15%, by restricting the external emission to match the incidence cone of the AM1.5D sun light spectrum. Such emission restriction corresponds to lower escape probability of internally emitted photons, enhances photon recycling, drops η ext, and actually makes the solar cell into a worse LED. Finally, for partly diffuse incident light, by restricting the angular emission for photons in a 130 nm wavelength range around the bandgap, we predict a maximum 14% relative boost in solar cell efficiency. The results of this paper are intended to serve as a general guideline on how to utilize emission-tuning possibilities to develop highly efficient photovoltaic devices.Item Wafer-Scale Synthesis and Optical Characterization of InP Nanowire Arrays for Solar Cells(AMERICAN CHEMICAL SOCIETY, 2021-09-08) Hrachowina, Lukas; Anttu, Nicklas; Borgström, Magnus T.; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Lund UniversityNanowire solar cells have the potential to reach the same efficiencies as the world-record III-V solar cells while using a fraction of the material. For solar energy harvesting, large-area nanowire solar cells have to be processed. In this work, we demonstrate the synthesis of epitaxial InP nanowire arrays on a 2 inch wafer. We define five array areas with different nanowire diameters on the same wafer. We use a photoluminescence mapper to characterize the sample optically and compare it to a homogeneously exposed reference wafer. Both steady-state and time-resolved photoluminescence maps are used to study the material's quality. From a mapping of reflectance spectra, we simultaneously extract the diameter and length of the nanowires over the full wafer. The extracted knowledge of large-scale nanowire synthesis will be crucial for the upscaling of nanowire-based solar cells, and the demonstrated wafer-scale characterization methods will be central for quality control during manufacturing.