Browsing by Author "Martikainen, Jani-Petri"

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  • Annealing Process Optimization for the Surface Passivation of Germanium Substrates
    (2024-12-13) Gosalvez, Sofia
     Perustieteiden korkeakoulu | Bachelor's thesis
    Presently, germanium is being studied as an alternative for silicon in near-infrared sensor applications. The main focus of this study is to improve the annealing process of n-type planar germanium wafers by uncovering empirically which annealing parameters give optimal surface passivation results. The parameters to vary include annealing temperature, annealing time as well as ambient gas. Surface recombination decreases the electrical quality of germanium significantly, and it can be reduced by passivating the surface with a dielectric layer. As a con- sequence, this study focuses on the surface passivation of germanium using two different passivation thin films. In this study, planar germanium wafers were passivated with either aluminum oxide (Al2O3) or a silicon dioxide and aluminum oxide stack (SiO2/Al2O3). In practice, the role of surface passivation is to maximize the minority carrier lifetime in the substrate by maximizing the thin film charge density and minimizing the interface defect density. Ultimately, the goal is to suppress surface recombination so that the photo-generated carriers are not lost to defects near the surface, allowing these carriers to be utilized in various applications, such as generating photocurrent, which can be detected as a signal by a photodetector. Surface passivation was characterized by performing measurements of both photo-conductance decay (PCD), as well as, corona oxide characterization of semiconductors (COCOS). This gave access to the effective minority carrier lifetime (τeff), the total charge (Qtot) in the dielectric layer, and the interface trap density (Dit) of each sample. After processing the data from these measurements, the best surface passivation results for the Al2O3-passivated samples were obtained by annealing the samples at either 425 °C for 10 min or 400 ° for 30 min, achieving surface recombination velocities (SRV) of about 13-14 cm/s. In addition, the use of forming gas was seen to further improve surface passivation. In the case of the SiO2/Al2O3 stack, only the effect of annealing temperature was studied and the optimal temperature was found to be 400 °C (30 min, N2 (g)) with a SRV of around 7cm/s.
  • Automated lock-in amplifier measurements using a matrix switch
    (2021-09-30) Räty, Topi
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Black holes and wormholes
    (2020-12-09) Ni, Anqi
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Computed Correction Coefficients for True Coincidence Summing
    (2019-12-16) Kantola, Konsta
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Conceptualizing the thermal management system of a high-performance battery pack for electric aviation
    (2021-11-24) Saikkonen, Veikko
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Condensation of surface lattice resonance excitations
    (2015-03-31) Julku, Aleksi
     Perustieteiden korkeakoulu | Master's thesis
  • Contact angle goniometry: From wetting theory to instrument automation
    (2024-02-29) Saine, Reetta
     Perustieteiden korkeakoulu | Bachelor's thesis
    Wetting is a multiscale phenomenon describing the behavior of a liquid in contact with a solid surface, which has significance in numerous biological systems and technological applications. Traditionally, surface wettability is characterized by a contact angle, a quantity arising from the interfacial interactions of a liquid, a gas and a surface. The predominant method used to measure contact angles is contact angle goniometry, where an optical subsystem captures the profile of a droplet. Despite its apparent simplicity, the method is highly susceptible to errors and has inherent limitations, which proposes a need for more sophisticated experimental setups than those commercially available. Incorporating automation and developing control software become crucial in instrumentation development, allowing instrument capabilities to be pushed beyond the current state-of-the-art. Increased precision and reporoducibility of the measurement techniques advances wetting characterization, improving our understanding of surface properties and aiding the design of functional surfaces. The aim of this thesis is to develop operational control software for a custom-made contact angle goniometer. Contact angles are discussed from a theoretical perspective as a background for the technique, and an overview of the experimental methodology is given. The custom-made setup and the implemented control software are introduced. To demonstrate the operability of the instrument, three surfaces with distinct wetting properties are measured. The results are validated against those measured with a commercial instrument and the performance of the software is evaluated. The control software developed in this thesis allows intentional interaction between the user and the measurement and meets the responsibilities for conducting reproducible and reliable contact angle measurements. As a final result, the experimental setup is operational and the conducted measurements are comparable to those performed with a commercial instrument. In addition to being more convenient to operate, it demonstrates more favorable features than a commercial one. Overall, the setup appears promising and holds potential for advanced wetting measurements.
  • Creep compression of wood
    (2019-12-05) Saariniemi, Eero
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Determination of continuous axial power distribution from discrete Aeroball Measurement System data
    (2022-05-18) Cederlöf, Nina
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Determining the bathymetric and topographic performance of a novel multiwavelength LiDAR scanner
    (2023-11-24) Kousa, Joonas
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Development of CorticokinematicCoherence Function between MEGSignals and a Kinematic Signal withPython
    (2021-04-16) Tuomaala, Sara
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Dispersion relation and density of states for surface lattice resonance excitations
    (2016-04-05) Moilanen, Antti
     Perustieteiden korkeakoulu | Master's thesis
    Surface lattice resonances (SLRs) are electromagnetic excitations in a periodic array of metallic nanoparticles. They originate from radiative coupling between localized surface plasmon resonances and diffractive orders of an incident optical field. SLRs form an interesting platform for observing macroscopic quantum-coherent phenomena. Providing strong spatial confinement of electric field in the lattice, the SLR excitations can interact with fluorescent molecules residing on top of the array. In this thesis, we study dispersion relation (DR) and density of states (DOS) for SLR excitations theoretically. For this we use the methods of Green’s function and discrete dipole approximation. We present two approaches for obtaining the DR: one based on mode analysis and the other based on transmission spectrum. We also show two ways to calculate the DOS: one based on DR and the other based on Green’s function. In addition, we investigate the effect of DR and DOS on condensation of photons in a dynamic system. We observe an agreement between the solved transmission spectra and experimental ones, and successfully calculate the relative DOS from dispersion curves fitted to the transmission spectra. We also find that the mode analysis for obtaining the DR and the Green’s function method for calculating the DOS are not able to capture the physics of band-gap formation. The form of DR and DOS are found to have an important role in achieving condensation.
  • Durability of metal-supported solid oxide electrolyzer cells
    (2023-12-06) Muikku, Heidi-Maria
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Effect of agarose gel elasticity on the rooting of plant seedlings
    (2024-10-27) Saartoala, Seela
     Perustieteiden korkeakoulu | Bachelor's thesis
    Mechanical root-soil interaction, influenced by the properties of both the soil and the root system, is one of the deciding factors on the survival and success of plants. To anchor the growing plant to the ground and access water and nutrient resources, the roots are required to be able to penetrate into the soil. This root penetration is limited by the mechanical resistance of the soil. In this thesis, agarose gel, a standard and well-studied research culture medium, was used as a soil model, and the root penetration ability of young Arabidopsis thaliana seedlings was studied on different gel stiffnesses. A micropipette force sensor (MFS) indentation method was implemented to measure the Young's moduli of gels with varying concentrations of agarose and establish the relationship between agarose concentration and gel elasticity. Root response to different culture medium stiffnesses was studied by observing the ability of the emerging primary root to penetrate the agarose gel surface on different concentrations. The results for the gel elastic moduli followed a power law relation with respect to the agarose concentration and placed agreeably in the spectrum of literature values, affirming the suitability of the MFS indentation method. No consistent effect of storage time on Young's modulus was observed, suggesting that the gels could maintain a fairly constant elasticity over a time period of several weeks. A steep, steplike decrease of Arabidopsis primary root penetration ability with increasing agarose concentration was witnessed, indicating an upper threshold value of medium elasticity for root penetration. Future validation of the MFS indentation method should include the examination of the effect of micropipette spring constant and capillary wall thickness on measured elasticity values as well as the consideration of agarose gel viscoelastic properties. A direct experimental comparison between root penetration ability and gel Young's modulus is needed to find the critical gel stiffness and quantify the limit of root penetration more precisely.
  • Effect of single substitutes on plasmonic nanocatalysts
    (2022-11-30) Sorvisto, Daniel
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Encapsulation materials of crystalline silicon solar cells
    (2021-05-17) Markkanen, Magnus
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Experimental scheme and characterization for an iSWAP gate using tunable coupling
    (2021-01-11) Mylläri, Tuomas
     Perustieteiden korkeakoulu | Bachelor's thesis
  • Experimental study of a high permeability magnetic shield in a dilution refrigerator measurement system
    (2024-01-18) Vesala, Kasper
     Perustieteiden korkeakoulu | Bachelor's thesis
    Dilution refrigeration measurement (DR) systems are the current standard tool for performing measurements below the liquefying curve of 4He. For various measurements at ultra low temperatures, external magnetic fields are a distraction preferably to be minimized. A technically convenient method for magnetic shielding across wide temperature ranges are high permeability alloys. With the feature of high permeability, they can be used to direct magnetic fields away from the experimental region, a behaviour that can be mostly treated with classical electromagnetic theory. In this thesis, we study a cylindrical magnetic shield made of A4K nickel alloy in a standard Bluefors dilution refrigerator measurement system from the point of view of user applications requiring magnetic shielding. Thus, the objectives are to 1) estimate the shielding effectiveness against geomagnetic fields and ambient AC-fields at various temperatures 2) evaluate the overall usability of the sample in a DR-system. As computational complexity of real electromagnetic problems is considerable, we try to understand the situation mostly with simple, idealized models. For the experiments, a manual, mobile sensor mount is devised into the cryostat and equipped with a cryogenic fluxgate-magnetometer, which allows measurements of magnetic field inside the shielded space. Measurements are made under the geomagnetic field and artificial magnetic fields with 0 Hz and 50 Hz frequencies applied onto the shield. We demonstrate that the magnetometer can be operated in a dilution refrigerator measurement system with a spatial probe. Shielding properties of the sample seem to be better than what would be expected on a theoretical basis, which could prompt a more detailed study. We observe permanent magnetization of the shield and discuss demagnetization procedures, which should be accounted for in shielding applications. Data resulting from the AC-experiments show deficiencies in the models discussed. Significant temperature dependency of the shielding is observed. On the basis of the learnings from this study, more research iterations should be done in order to explain deviations observed and to minimize both environment- and instrument-based error.
  • Fabrication and characterization of asymmetric superconducting tunnel junctions
    (2023-08-04) Geisor, Joel
     Perustieteiden korkeakoulu | Bachelor's thesis
  • A feasibility study of one-layer coil designs for multi-coil transcranial magnetic stimulation
    (2023-10-18) Kellomäki, Julius
     Perustieteiden korkeakoulu | Bachelor's thesis