Filip Tuomisto

Filip Tuomisto
University of Helsinki | HY · Department of Physics

D.Sc. (Tech.)

About

263
Publications
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5,315
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Publications

Publications (263)
Article
Full-text available
The authors have applied positron annihilation spectroscopy to study the vacancy defects in undoped and Si-doped Ga2O3 thin films. The results show that Ga vacancies are formed efficiently during metal-organic vapor phase epitaxy growth of Ga2O3 thin films. Their concentrations are high enough to fully account for the electrical compensation of Si...
Article
Full-text available
Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the ma...
Article
Full-text available
We have developed a method called optical transient positron spectroscopy and apply it to study the optically induced carrier trapping and charge transfer processes in natural brown type IIa diamond. By measuring the positron lifetime with continuous and pulsed illumination, we present an estimate of the optical absorption cross section of the vaca...
Article
In-grown vacancy defects in bulk and quasi-bulk GaN crystals have been extensively studied with positron annihilation spectroscopy. High concentrations of Ga-vacancy-related defects are found irrespective of the growth method used in crystals with a high O contamination or intentional O doping, and they act as the dominant compensating native defec...
Article
Positron annihilation signals from VMCO-like samples grown by atomic layer deposition at different temperatures are utilized for the characterization of differences in open volume defects in TiN/TiO[Formula: see text]/a-Si heterostructures. Doppler and coincidence Doppler mode of positron annihilation spectroscopy combined with a monoenergetic posi...
Article
Understanding chemical disorder in many concentrated solid solution alloys (CSAs) at the levels of electrons and atoms has attracted increasing attention as a path forward to reveal and identify underlying mechanisms for extraordinary mechanical properties and improved radiation tolerance. Single-phase NiFeCoCr CSA is a common base for many high-en...
Presentation
Full-text available
The ENEN+ project proposed and demonstrated various cost-effective actions to attract, develop and retain new talents in nuclear professions. This is a contribution of the ENEN aisbl, supported by the European Commission, to the common strategic goal of all nuclear stakeholders: to preserve, maintain and further develop the valuable nuclear knowled...
Article
The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrel...
Article
Full-text available
We present evidence of homogenization of atomic diffusion properties caused by C and N interstitials in an equiatomic single-phase high entropy alloy (FeMnNiCoCr). This phenomenon is manifested by an unexpected interstitial-induced reduction and narrowing of the directly experimentally determined migration barrier distribution of mono-vacancy defec...
Article
Positron annihilation spectroscopy, Fourier transform-infrared absorption spectroscopy, and secondary ion mass spectrometry have been used to study the behavior of gallium vacancy-related defects and hydrogen in deuterium (D) implanted and subsequently annealed β-Ga2O3 single crystals. The data suggest the implantation generates a plethora of VGa-r...
Article
We report a positron annihilation study using state-of-the-art experimental and theoretical methods in n-type and semi-insulating β - Ga 2 O 3. We utilize the recently discovered unusually strong Doppler broadening signal anisotropy of β - Ga 2 O 3 in orientation-dependent Doppler broadening measurements, complemented by temperature-dependent posit...
Article
We report a systematic first-principles study on positron annihilation parameters in the β−Ga2O3 lattice and Ga monovacancy defects complemented with orientation-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the β−Ga2O3 lattice and the considered defects exhibit unusually strong anisotropy...
Article
We report positron annihilation results on vacancy defects in Si-doped Al0.90Ga0.10N alloys grown by metalorganic vapor phase epitaxy. By combining room temperature and temperature-dependent Doppler broadening measurements, we identify negatively charged in-grown cation vacancies in the concentration range from below 1 × 10 16 cm⁻³ to 2 × 10 18 cm⁻...
Article
We have applied positron annihilation spectroscopy to study the formation of Ga vacancy related defects in Mg and Mn doped bulk GaN crystals grown by the ammonothermal method. We show that Mn doping has little or no effect on the formation of Ga vacancies, while Mg doping strongly suppresses their formation, in spite of both dopants leading to high...
Article
We present a systematic study of the positron lifetime as a function of measurement temperature in strontium titanate (SrTiO3) single crystals grown in different conditions and by different synthesis methods. We combine our experimental results with state-of-the-art theoretical calculations of positron annihilation parameters. We find that the esse...
Article
Full-text available
Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se2, one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se2 absorbers and subsequently exposing them to...
Article
Full-text available
Defect evolution under irradiation is investigated in a set of single-phase concentrated solid solution alloys (SP-CSAs) containing Ni with Co, Fe and/or Cr. We show that atomic segregation of Ni takes place already at very early stages of radiation damage in the 2-4 element SP-CSAs containing Fe or Cr, well below 1 dpa. We arrive at this conclusio...
Article
Full-text available
A unique experimental setup at the Accelerator Laboratory of the University of Helsinki enables in situ positron annihilation spectroscopy analysis on ion irradiated samples. In addition, the system enables temperature control (10 – 300 K) of the sample both during irradiation and during subsequent positron annihilation measurements. Using such a s...
Preprint
We report a systematic first principles study on positron annihilation parameters in the $\beta$-Ga$_2$O$_3$ lattice and Ga mono-vacancy defects complemented with direction-dependent experiments of the Doppler broadening of the positron-electron annihilation. We find that both the $\beta$-Ga$_2$O$_3$ lattice and the considered defects exhibit colos...
Chapter
Positron annihilation spectroscopy has been applied to study vacancy defects in Ga2O3. Both positron lifetime and Doppler broadening experiments have been performed, both in bulk single crystals and in thin films. The results show that Ga vacancy defects are efficiently formed in thin film growth and account for the electrical compensation in semi-...
Article
We show that N-polar GaN/(Al,Ga)N/GaN heterostructures exhibit significant N deficiency at the bottom (Al,Ga)N/GaN interface, and that these N vacancies are responsible for the trapping of holes observed in unoptimized N-polar GaN/(Al,Ga)N/GaN high electron mobility transistors. We arrive at this conclusion by performing positron annihilation exper...
Article
The thermal evolution of vacancies and vacancy clusters in tungsten (W) has been studied. W (100) single crystals were irradiated with 200 keV hydrogen (H) ions to a low damage level (5.8×10−3 dpa) at 290 K and then annealed at temperatures in the range of 500–1800 K. The resulting defects were characterized by positron annihilation lifetime spectr...
Article
Full-text available
Owing to their excellent radiation tolerance, some of the high-entropy alloys (HEAs) are considered as potential candidates for structural materials in extreme conditions. In order to shed light on the early-stage irradiation damage in HEAs, we performed positron annihilation spectroscopy on hydrogen implanted equiatomic FeMnNiCoCr and interstitial...
Article
In this work, pulsed photoionization as well as photoluminescence and positron annihilation spectroscopy were combined to detect different species of defects. The GaN crystals, grown by the ammono-thermal method, doped with Mn as well as Mg impurities and irradiated with different fluences of reactor neutrons, were examined to clarify the role of t...
Article
The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of n-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual...
Article
Reliable and accurate knowledge of the physical properties of elementary point defects is crucial for predictive modeling of the evolution of radiation damage in materials employed in harsh conditions. We have applied positron annihilation spectroscopy to directly detect mono-vacancy defects created in tungsten through particle irradiation at cryog...
Article
The influence of Fermi level position and annealing ambient on the zinc vacancy VZn generation and Al diffusion is studied in monocrystalline zinc oxide (ZnO). From secondary-ion mass spectrometry and positron annihilation spectroscopy results, a quadratic dependence between the concentrations of VZn and Al is established, demonstrating the Fermi l...
Article
Full-text available
Purpose The primary motivation of the ENEN + project is to substantially contribute to the revival of the interest of young generations in careers in the following nuclear disciplines: nuclear reactor engineering and safety; waste management and geological disposal; radiation protection and medical physics. Methods The main objectives are: attract...
Conference Paper
The European Nuclear Education Network (ENEN) was established in 2003 through an EU Fifth Framework Programme (FP) project, as a legal nonprofit-making body. Its main objective is the preservation and further development of expertise in the nuclear fields by higher education and training. This objective is realized through the cooperation between E...
Conference Paper
Full-text available
The European Nuclear Education Network (ENEN) was established in 2003 through an EU Fifth Framework Programme (FP) project, as a legal nonprofit-making body. Its main objective is the preservation and further development of expertise in the nuclear fields by higher education and training. This objective is realized through the cooperation between E...
Conference Paper
Full-text available
The ENEN+ project proposes cost-effective actions to attract, develop and retain new talents in nuclear professions. This is a contribution of the ENEN Association, supported by the European Commission, to the common strategic goal of all nuclear stakeholders: to preserve, maintain and further develop the valuable nuclear knowledge for todays and f...
Article
Various nominally undoped and Si-doped (InxGa1–x)2O3 thin films were grown by pulsed laser deposition in a continuous composition spread mode on c-plane α-sapphire and (100)-oriented MgO substrates. Positron annihilation spectroscopy in the Doppler broadening mode was used as the primary characterisation technique in order to investigate the effect...
Article
Single crystals of oxygen-free copper oriented to easy glide of dislocations were tensile tested in order to study the hydrogen effects on the strain localisation in the form of slip bands appearing on the polished specimen surface under tensile straining. It was found that hydrogen increases the plastic flow stress in Stage I of deformation. The d...
Article
Full-text available
We show that Be exhibits amphoteric behavior in GaN, involving switching between substitutional and interstitial positions in the lattice. This behavior is observed through the dominance of BeGa in the positron annihilation signals in Be-doped GaN, while the emergence of VGa at high temperatures is a consequence of the Be impurities being driven to...
Conference Paper
Full-text available
The ENEN+ project proposes cost-effective actions to attract, develop and retain new talents in nuclear professions. This is a contribution of the ENEN Association, supported by the European Commission, to the common strategic goal of all nuclear stakeholders: to preserve, maintain and further develop the valuable nuclear knowledge for todays and f...
Article
The process of oxidation in zirconium alloys is not straightforward. It includes cyclic periods, where the oxidation rate is decelerating as a function of increasing oxide thickness. Transition to the higher oxidation rate happens approximately after every 2 μm oxide growth and each cycle is associated with a layer of oxide with little or no cracki...
Article
Full-text available
Photoluminescence (PL) was used to estimate the concentration of point defects in GaN. The results are compared with data from positron annihilation spectroscopy (PAS), secondary ion mass spectrometry (SIMS), and deep level transient spectroscopy (DLTS). Defect-related PL intensity in undoped GaN grown by hydride vapor phase epitaxy increases linea...
Article
Full-text available
We study acceptor-type defects in grown by molecular beam epitaxy. The hole density of the layers, from capacitance-voltage measurements of Schottky diodes, is higher than that of the binary alloys and increases linearly up to 10¹⁹ with the Bi content. Positron annihilation spectroscopy and ab initio calculations show that both Ga vacancies and Ga...
Article
The charge compensation mechanisms in U1-xGdxO2 and Th1-xGdxO2-x/2 have been systematically studied using X-ray Absorption Spectroscopy (XAS) upon gradually increasing the Gd content. Gd doped nuclear fuels are widely used for optimizing the fresh core neutronics, yet when Gd³⁺ is substituted into U⁴⁺ or Th⁴⁺ lattice position in UO2 or ThO2, respec...
Article
Positron annihilation Doppler broadening spectroscopy characterizes lattice point defects and is sensitive to very small vacancy densities. High-purity germanium detectors are generally used for recording the Doppler broadening spectrum because they provide good energy resolution and stability. However, the energy resolution of a germanium detector...
Article
We demonstrate that the instability of the Sb vacancy in GaSb leads to a further increase in the acceptor-type defect concentration in proton irradiated undoped, p-type GaSb. Using positron annihilation spectroscopy in situ with 10 MeV proton irradiation at 35 K, we find that the irradiation produces both native vacancy defects in GaSb. However, th...
Article
The effect of radiation damage on the defect and alloy structure in InxGa1−xN thin films grown on Si substrates was studied using positron annihilation spectroscopy. Prior to the measurements, the samples were subjected to double He⁺ implantation at 40 and 100 keV. The results show the presence of cation vacancy-like defects in high concentrations...
Article
Native bulk gallium nitride (GaN) has emerged as an alternative for sapphire and silicon as a substrate material for III-N devices. While quasi-bulk GaN substrates are currently commercially available, single crystal GaN substrates are considered essential for future high performance light emitters and power devices. The ammonothermal method is cur...
Article
Full-text available
Positron annihilation spectroscopy and thermal desorption spectroscopy experiments were combined to ascertain the role of hydrogen on generation of vacancies and vacancy clusters in Ni alloys. The effects of grain size and deformation temperature are emphasized for pure Ni single crystals and polycrystalline Ni-201 alloy samples with two grain size...
Article
Positron annihilation spectroscopy and secondary ion mass spectrometry have been applied to study the evolution of polishing-induced defects in hydrothermally grown ZnO samples depending on the annealing temperature. Annealing of the as-grown ZnO wafer at 1200–1500 °C is found to lead to Li accumulation in the sub-surface layer, and significant red...
Conference Paper
In this paper, the development of a new positron lifetime spectrometer dedicated to high radioactive materials such as nuclear power plant’s mixed oxide fuels and structural steels is discussed. In the case of nuclear fuels, the behaviour of nanoscale defects, which are mainly created by the high radioactivity and high irradiation flux of neutrons...
Article
Highly n-type Ge attained by shallow As implantation and excimer laser annealing was studied with positron annihilation spectroscopy and theoretical calculations. We conclude that a high concentration of vacancy–arsenic complexes was introduced by the doping method, while no sign of vacancies was seen in the un-implanted laser-annealed samples. The...
Article
Si nanocrystals embedded in a SiO 2 matrix were studied with positron annihilation and photoluminescence spectroscopies. Analysis of the S- and W-parameters for the sample annealed at 800 °C reveals a positron trap at the interface between the amorphous nanodots and the surrounding matrix. Another trap state is observed in the 1150 °C heat treated...
Article
We have determined positron annihilation characteristics (lifetime and Doppler broadening) in six basic vacancy-type defects of 6H-SiC and two nitrogen-vacancy complexes using ab initio calculations. The positron characteristics obtained allow us to point out which positron technique in the most adapted to identify a particular defect. They show th...
Article
Positron annihilation spectroscopy was performed to study defects in Ge doped with As, P and Sb. In each case, the samples had approximately the same dopant concentration ∼10(19) cm(-3). Results from the Doppler broadening and positron lifetime spectroscopies were compared to electronic structure calculations. The positron lifetime results show tha...
Article
Positron annihilation spectroscopy, when combined with supporting high-quality modeling of positron states and annihilation in matter, is a powerful tool for detailed defect identification of vacancy-type defects in semiconductors and oxides. Here we demonstrate that the Doppler broadening of the positron annihilation radiation is a very sensitive...
Article
Full-text available
Understanding the properties of the positronium atom in matter is of interest for the interpretation of positron annihilation experiments. This technique has a unique capability for the investigation of nanometer sized voids and pores in soft molecular materials (polymers, liquids or biostructures) and porous materials. However, detailed interpreta...
Article
In this work we present detailed studies of the influence of nitrogen and antimony on the optical quality of InNAs(Sb) alloys. We employed photoluminescence, photoreflectance and positron annihilation spectroscopy to reveal the role of antimony and nitrogen on the improvement/degradation of the optical qualities of InNAs(Sb) alloys. A series of 1 m...
Article
Full-text available
Positron annihilation Doppler broadening spectroscopy is one of the most popular positron annihilation vacancy characterization techniques in experimental materials research. The measurements are often carried out with a slow positron beam setup, which enables depth profiling of the samples. The key measurement devices of Doppler broadening spectro...
Article
Full-text available
Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (Cu3N) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind t...
Article
Point defects play an important role in present day semiconductor devices. Their ability to influence both the electrical and optical properties of a semiconductor together with the decreasing device size, makes the knowledge of their nature a crucial ingredient in the applicability of a specific semiconductor material. There are very few experimen...
Article
Full-text available
The large increase in the p-type conductivity observed when nitrogen is added to GaSb has been studied using positron annihilation spectroscopy and ab initio calculations. Doppler broadening measurements have been conducted on samples of GaN x Sb 1− x layers grown by molecular beam epitaxy, and the results have been compared with calculated firs...
Article
In this work, poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) was used as active layer in bilayer solar cell with C60 as electron acceptor. As cast devices already show reasonable power conversion efficiency (PCE) that increases to 4% upon annealing at 100 °C. Space charge limited measure...
Article
Full-text available
We present positron annihilation results on Sb-doped SnO2 and ZnO thin films. The vacancy types and the effect of vacancies on the electrical properties of these intrinsically n-type transparent semiconducting oxides are studied. We find that in both materials low and moderate Sb-doping leads to formation of vacancy clusters of variable sizes. Howe...