Browsing by Author "Kalliokoski, M."

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Characterization of magnetic Czochralski silicon devices with aluminium oxide field insulator: Effect of oxygen precursor on electrical properties and radiation hardness
(2021-05) Ott, J.; Bharthuar, S.; Gädda, A.; Arsenovich, T.; Bezak, M.; Brücken, E.; Golovleva, M.; Härkönen, J.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Kirschenmann, S.; Litichevskyi, V.; Luukka, P.; Martikainen, L.; Naaranoja, T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Aluminium oxide (Al2O3) has been proposed as an alternative to thermal silicon dioxide (SiO2) as field insulator and surface passivation for silicon detectors, where it could substitute p-stop/p-spray insulation implants between pixels due to its negative oxide charge, and enable capacitive coupling of segments by means of its higher dielectric constant. Al2O3 is commonly grown by atomic layer deposition (ALD), which allows the deposition of thin layers with excellent precision. In this work, we report the electrical characterization of single pad detectors (diodes) and MOS capacitors fabricated on magnetic Czochralski silicon substrates and using Al2O3 as field insulator. Devices are studied by capacitance-voltage, current-voltage, and transient current technique measurements. We evaluate the influence of the oxygen precursors in the ALD process, as well as the effect of gamma irradiation, on the properties of these devices. We observe that leakage currents in diodes before the onset of breakdown are low forall studied ALD processes. Charge collection as measured by transient current technique (TCT) is also independent of the choice of oxygen precursor. The Al2O3 films deposited with O3 possess a higher negative oxide charge than films deposited by H2O, However, in diodes a higher oxide charge is linked to earlier breakdown, as has been predicted by simulation studies. A combination of H2and O3 precursors results in a good compromise between the beneficial properties provided by the respective individual precursors.
FORESAIL-1 CubeSat Mission to Measure Radiation Belt Losses and Demonstrate Deorbiting
(2019-07) Palmroth, M.; Praks, J.; Vainio, R.; Janhunen, P.; Kilpua, E. K.J.; Afanasiev, A.; Ala-Lahti, M.; Alho, A.; Asikainen, T.; Asvestari, E.; Battarbee, M.; Binios, A.; Bosser, A.; Brito, T.; Dubart, M.; Envall, J.; Ganse, U.; Ganushkina, N. Yu; George, H.; Gieseler, J.; Good, S.; Grandin, M.; Haslam, S.; Hedman, H. P.; Hietala, H.; Jovanovic, N.; Kakakhel, S.; Kalliokoski, M.; Kettunen, V. V.; Koskela, T.; Lumme, E.; Meskanen, M.; Morosan, D.; Mughal, M. Rizwan; Niemelä, P.; Nyman, S.; Oleynik, P.; Osmane, A.; Palmerio, E.; Peltonen, J.; Pfau-Kempf, Y.; Plosila, J.; Polkko, J.; Poluianov, S.; Pomoell, J.; Price, D.; Punkkinen, A.; Punkkinen, R.; Riwanto, B.; Salomaa, L.; Slavinskis, A.; Säntti, T.; Tammi, J.; Tenhunen, H.; Toivanen, Petri; Tuominen, J.; Turc, L.; Valtonen, E.; Virtanen, Pasi; Westerlund, T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Today, the near-Earth space is facing a paradigm change as the number of new spacecraft is literally skyrocketing. Increasing numbers of small satellites threaten the sustainable use of space, as without removal, space debris will eventually make certain critical orbits unusable. A central factor affecting small spacecraft health and leading to debris is the radiation environment, which is unpredictable due to an incomplete understanding of the near-Earth radiation environment itself and its variability driven by the solar wind and outer magnetosphere. This paper presents the FORESAIL-1 nanosatellite mission, having two scientific and one technological objectives. The first scientific objective is to measure the energy and flux of energetic particle loss to the atmosphere with a representative energy and pitch angle resolution over a wide range of magnetic local times. To pave the way to novel model-in situ data comparisons, we also show preliminary results on precipitating electron fluxes obtained with the new global hybrid-Vlasov simulation Vlasiator. The second scientific objective of the FORESAIL-1 mission is to measure energetic neutral atoms of solar origin. The solar energetic neutral atom flux has the potential to contribute importantly to the knowledge of solar eruption energy budget estimations. The technological objective is to demonstrate a satellite deorbiting technology, and for the first time, make an orbit maneuver with a propellantless nanosatellite. FORESAIL-1 will demonstrate the potential for nanosatellites to make important scientific contributions as well as promote the sustainable utilization of space by using a cost-efficient deorbiting technology.
Modeling the impact of defects on the charge collection efficiency of a Cadmium Telluride detector
(2021-08) Golovleva, M.; Bezak, M.; Bharthuar, S.; Brücken, E.; Gadda, A.; Harkönen, J.; Kalliokoski, M.; Kirschenmann, S.; Luukka, P.; Ott, J.; Tuuva, T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Cadmium telluride is a favorable material for X-ray detection as it has an outstanding characteristic for room temperature operation. It is a high-Z material with excellent photon radiation absorption properties. However, CdTe single crystals may include a large number of extended crystallographic defects, such as grain boundaries, twins, and tellurium (Te) inclusions, which can have an impact on detector performance. A Technology Computer Aided Design (TCAD) local defect model has been developed to investigate the effects of local defects on charge collection efficiency (CCE). We studied a 1 mm thick Schottky-type CdTe radiation detector with transient current technique by using a red laser at room temperature. By raster scanning the detector surface we were able to study signal shaping within the bulk, and to locate surface defects by observing their impact on the CCE. In this paper we present our TCAD model with localized defect, and compare the simulation results to TCT measurements. In the model an inclusion with a diameter of 10 mu m was assumed. The center of the defect was positioned at 6 mu m distance from the surface. We show that the defect has a notable effect on current transients, which in turn affect the CCE of the CdTe detector. The simulated charge collection at the position of the defect decreases by 80 % in comparison to the defect-free case. The simulations show that the defects give a characteristic shape to TCT signal. This can further be used to detect defects in CdTe detectors and to estimate the overall defect density in the material.
Multispectral photon-counting for medical imaging and beam characterization — A project review
(2022-09-11) Kirschenmann, S.; Bezak, M.; Bharthuar, S.; Brücken, E.; Emzir, M.; Golovleva, M.; Gädda, A.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Karjalainen, A.; Koponen, P.; Kramarenko, N.; Luukka, P.; Ott, J.; Petrow, H.; Siiskonen, T.; Särkkä, S.; Tikkanen, J.; Turpeinen, R.; Winkler, A.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Central focus of the MPMIB project – funded via the Academy of Finland's RADDESS 2018–2021 programme – has been research towards a next-generation radiation detection system operating in a photon-counting (PC) multispectral mode: The extraction of energy spectrum per detector pixel data will lead to better efficacy in medical imaging with ionizing radiation. Therefore, it can be an important asset for diagnostic imaging and radiotherapy, enabling better diagnostic outcome with lower radiation dose as well as more versatile characterization of the radiation beam, leading for example to more accurate patient dosimetry. We present our approach of fabricating direct-conversion detectors based on cadmium telluride (CdTe) semiconductor material hybridized with PC mode capable application-specific integrated circuits (ASICs), and will give a review on our achievements, challenges and lessons learned. The CdTe crystals were processed at Micronova, Finland's national research infrastructure for micro- and nanotechnology, employing techniques such as surface passivation via atomic layer deposition, and flip chip bonding of processed sensors to ASIC. Although CdTe has excellent photon radiation absorption properties, it is a brittle material that can include large concentrations of defects. We will therefore also emphasize our quality assessment of CdTe crystals and processed detectors, and present experimental data obtained with prototype detectors in X-ray and Co-60 beams at a standards laboratory.
Processing of AC-coupled n-in-p pixel detectors on MCz silicon using atomic layer deposited aluminium oxide
(2020-04-01) Ott, Jennifer; Gädda, A.; Bharthuar, S.; Brücken, E.; Golovleva, M.; Härkönen, J.; Kalliokoski, M.; Karadzhinova-Ferrer, A.; Kirschenmann, S.; Litichevskyi, V.; Luukka, P.; Martikainen, L.; Naaranoja, T.
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We report on the fabrication of capacitively (AC) coupled n+-in-p pixel detectors on magnetic Czochralski silicon substrates. In our devices, we employ a layer of aluminium oxide (Al2O3) grown by atomic layer deposition (ALD) as dielectric and field insulator, instead of the commonly used silicon dioxide (SiO2). As shown in earlier research, Al2O3 thin films exhibit high negative oxide charge, and can thus serve as a substitute for p-stop/p-spray insulation implants between pixels. In addition, they provide far higher capacitance densities than SiO2 due to their high dielectric constant, permitting more efficient capacitive coupling of pixels. Furthermore, metallic titanium nitride (TiN) bias resistors are presented as an alternative to punch-through or poly-Si resistors. Devices obtained by the above mentioned process are characterized by capacitance–voltage and current–voltage measurements, and by 2 MeV proton microprobe. Results show the expected high negative charge of the Al2O3 dielectric, uniform chargecollection efficiency over large areas of pixels, and acceptable leakage current densities.