Defect complexes in silicon : electronic structures and positron annihilation

In silicon processing technology one of the most important current objectives is to achieve a controlled impurity doping in the crystal. Point defects and defect complexes present in the crystal influence in an important way the electrical activity and the diffusion properties of the dopants. In this thesis, defect complexes in silicon are studied by using quantum-mechanical electronic-structure calculations and by modeling positron annihilation experiments. The electronic-structure calculations are based on the density-functional theory and its state-of-the-art implementations, such as a plane-wave pseudopotential computer code. For the calculation of the momentum density of annihilating electron-positron pairs a new method is presented and tested. It is based on a two-particle description of the correlated pair so that the contact density depends explicitly on the whole spatial distribution of the electron state in question. The new method is found to be superior to the state-independent methods for the momentum density and provides a basis for identifying defect complexes with different chemical surroundings from their momentum distribution fingerprint. In this work, the computational methods are used to study the positron annihilation characteristics at small vacancy clusters in silicon and the properties of typical dopant atoms, which include arsenic and boron. In highly arsenic-doped silicon an electrically inactive defect complex consisting of a vacancy decorated by three arsenic atoms is identified. In boron-doped silicon the defect structures containing one boron atom are analyzed and an estimate is given for the activation energy of boron diffusion.reviewe

Computationally efficient implementation of hybrid functionals in SIESTA

In this work we have implemented hybrid functionals into the SIESTA code, with the main goal to implement a fast general solver within the SIESTA framework that performs efficiently and scales linearly with increasing system size. We describe the implementation of the solver and apply it to study the properties of five insulating materials; NaCl, CaF2, CeO2, TiO2 and HfO2. We show that a systematic improvement in the basic description of the properties of these materials over standard Density Functional approaches can be obtained at a reasonable additional computational cost

Hakkuukonetiedon hyödyntäminen kuviorajauksessa ja ajouratunnusten laskennassa

Metsävaratiedon ajantasaisuus palvelee koko metsäalaa metsänomistajasta raakapuun ostajaan. Valtakunnan metsien inventoinnin sekä Suomen metsäkeskuksen kaukokartoitusaineiston ja maastokoealojen avulla kerätty metsävaratieto Suomen metsävaroista edesauttaa metsänhoidon laajamittaista strategista suunnittelua ja luo pohjan tilakohtaisten metsäsuunnitelmien laadinnalle ja metsävaratietojen ajantasaistukselle. Metsävaratiedon ylläpito ja kuviotason operatiivinen suunnittelu vaativat kuitenkin ajantasaista tietoa myös metsässä tehdyistä toimenpiteistä. Tähän Metsäkeskuksella on ollut tarve kehittää uusia työkaluja. Tavoitteena on, että vuosittain tehdyt metsänhoitotoimenpiteet ja hakkuut siirtyisivät mahdollisimman nopeasti ajantasaisena metsävaratietona eri toimijoiden käyttöön. Tämän työn päätavoitteena oli validoida Metsäteho Oy:n kehittämää menetelmää, jossa metsäkuviolle muodostetaan automaattinen kuviorajaus hakkuukonetiedon pohjalta (Melkas ym. 2020). Runkokohtaiseen hakkuukoneen sijaintitietoon perustuva kuviorajaus mahdollistaisi jatkuvan metsävaratiedon päivittämisen hakkuiden yhteydessä. Kuviorajaus on tärkeä muuttuja, koska kuvion pinta-alaa käytetään suoraan kertoimena arvioitaessa kuviokohtaisia puustotunnuksia ja hakkuukertymiä (Belbo & Talbot 2020). Tutkimuksen hakkuukonetieto oli kerätty 12/2017–06/2018 ja kattoi noin 3 000 korjuulohkoa ja 5 316 214 sijaintipistettä Manner-Suomessa. Hakkuukonetiedosta saatavaa runkokohtaista sijaintitietoa voidaan käyttää automaattisessa kuviorajauksessa. Tutkimuksessa korjuulohkojen sijaintitiedot luokiteltiin kuvioihin ja siirtymäuriin. Yksittäisen kuvion sijaintitiedot yhdistettiin kolmioimalla verkostoksi, minkä jälkeen muodostuneelle polygonille asetettiin puskurivyöhyke tasaamaan GNSS-paikannuksessa tapahtuvaa virhettä ja kuvastamaan samalla hakkuukoneen puomin ulottuvuutta. Hakkuukoneen sijaintitietojen avulla kuviolle voitiin muodostaa automaattisesti myös ajouraverkosto, josta pystyttiin laskemaan kuviokohtaisia ajouratunnuksia. Automaattinen kuviointi tuotti ilmakuvareferensseihin verrattuna luotettavat kuviorajaukset kolmella yleisimmällä hakkuutavalla, kun kuvion pinta-ala oli vähintään 0,75 ha. Hakkuukonekuvio oli keskimäärin kolme prosenttia suurempi kuin ilmakuvalta käsin digitoitu verrokkina toiminut referenssikuvio. Hakkuukonekuvioiden pinta-alojen suhdeluvut referenssikuvioihin nähden olivat ensiharvennuksilla 1,044, harvennuksilla 1,020, avohakkuilla 1,034 ja kaikki hakkuutavat huomioiden 1,031. Eri hakkuutapojen välillä vaihtelu oli pientä, ja hakkuukonekuvion pinta-alavastaavuus parani isompia kuvioita kohti. Yli 1,5 hehtaarin kuvioilla pinta-alojen suhteellinen ero oli keskimäärin enää yhden prosentin luokkaa. Tutkimuksen toinen tavoite oli automaattisen ajouralaskennan validointi. Sijaintipisteiden pohjalta kuvioille luotiin ajouraverkosto, jossa hakkuukoneen edestakaiset liikkeet jätettiin huomiotta. Automaattisesta ajouraverkostosta laskettiin keskimääräiset ajouravälit (m) ja uratiheys (m/ha) kuviokohtaisesti sekä kattavasti koko kuviolta että jäljittelemällä Suomen metsäkeskuksen maastokoealamittausta korjuujäljen laadun arvioinnissa. Laskenta tuotti ajouratunnukset, jotka olivat realistisia verrattaessa hyvän metsänhoidon suosituksiin. Ajouravälit olivat harvennuksilla keskimäärin 20,7 metriä ja avohakkuilla 17,1 metriä. Koko työn yhteenvetona voidaan todeta, että automaattinen kuviorajaus vastasi referenssikuvioita niin pinta-alaltaan kuin sijainniltaan varsin luotettavasti ja on tuotavissa osaksi luotettavaa ajantasaista metsävaratietoa. Ajouratunnusten laskenta antoi tuloksia, joiden avulla voidaan todentaa ajouraverkostolle asetettujen suositusten toteutumista.Up-to-date forest inventory benefits the entire forest industry, all the way from forest owners to buyers of raw wood. The forest inventory gathered through remote sensing data and field sample plots by both National Forest Inventory (NFI) and Finnish Forest Centre supports large-scale strategic planning of forestry management and creates a foundation for forest planning as well as up-to-date forest inventory. Operative planning and up-to-date forest inventory also require information about recent cuttings. Finnish Forest Centre has deemed it necessary to develop tools to monitor the realized cuttings on an annual basis. The aim is that data from annual forest operations and cuttings could be transferred into updated forest inventory as soon as possible. The main focus of this thesis was on a method developed by Metsäteho Oy, whereby a stand delineation is automatically created for each forest stand based on harvester data (Melkas ym. 2020). Stand delineation carried out on the basis of stem-specific harvester location data would enable to constantly update the forest inventory in conjunction with logging operations. Stand delineation is an important information because stand area is routinely used as a coefficient in the estimation of stand-specific logging accumulation (Belbo & Talbot 2020). The harvester data was gathered between December 2017 and June 2018 and comprised approximately 3,000 harvested objects and 5,316,214 locations all over continental Finland. The stem-spesific location data recorded by harvester is used in automated stand delineation. Using triangulation, the location data of the stands was combined into a network, and a buffer zone was created for the resulting polygon to reduce the contribution of errors in GNSS navigation while also reflecting the reach of the harvester boom. The use of harvester location data also made it possible to automatically create a strip road network, which in turn allowed to calculate stand-specific strip road variables. Compared to aerial photography references, automated delineation yielded reliable stand delineations when carried out with three most common logging methods and when the stand area was at least .75 hectares. The automated stand was on average three per cent larger than the reference stand manually created from digital aerial photographs. Compared to reference stands, the relative areas of the automated stands were as follows: 1.044 for the first thinning; 1.020 for later thinnings; 1.034 for clear cutting; and 1.031 for all of these harvesting methods combined. There was little variation between the various harvesting methods, and the correlation between automated stand areas and references increased with the size of the stand. For stands with an area more than 1.5 hectares the relative difference in areas was, on average, only around one per cent. Another aim was the validation of automated strip road calculation. On the basis of harvester locations, a strip road network was created, where the to-and-fro movement of the harvester was ignored. Next, the automatically created strip road network was used to calculate the average spacings between strip roads (in metres) and strip road density (in metres/hectare). This was done comprehensively for each stand. In addition, the strip road variables were calculated by emulating sample plot measurements carried out by the Finnish Forest Centre in the evaluation of the quality of harvesting sites objects. Both results were realistic when compared to best practices in forest management. On average, the spacing between strip roads in thinning areas was 20.7 metres and 17.1 metres in clear cuttings. To sum up, there was a reliable correlation between automated stand delineation and reference stands both in terms of area and location; thus, it would be viable to integrate the automatically delineated stands as part of reliable and up-to-date forest inventory. The results of strip road calculation are applicable to validate the implementation of the recommendations set for strip road networks

Uncertainty in multispectral lidar signals caused by incidence angle effects

Multispectral terrestrial laser scanning (TLS) is an emerging technology. Several manufacturers already offer commercial dual or three wavelength airborne laser scanners, while multispectral TLS is still carried out mainly with research instruments. Many of these research efforts have focused on the study of vegetation. The aim of this paper is to study the uncertainty of the measurement of spectral indices of vegetation with multispectral lidar. Using two spectral indices as examples, we find that the uncertainty is due to systematic errors caused by the wavelength dependency of laser incidence angle effects. This finding is empirical, and the error cannot be removed by modelling or instrument modification. The discovery and study of these effects has been enabled by hyperspectral and multispectral TLS, and it has become a subject of active research within the past few years. We summarize the most recent studies on multi-wavelength incidence angle effects and present new results on the effect of specular reflection from the leaf surface, and the surface structure, which have been suggested to play a key role. We also discuss the consequences to the measurement of spectral indices with multispectral TLS, and a possible correction scheme using a synthetic laser footprint.publishedVersionPeer reviewe

Excitons in van der Waals materials : From monolayer to bulk hexagonal boron nitride

We present a general picture of the exciton properties of layered materials in terms of the excitations of their single-layer building blocks. To this end, we derive a model excitonic Hamiltonian by drawing an analogy with molecular crystals, which are other prototypical van der Waals materials. We employ this simplified model to analyze in detail the excitation spectrum of hexagonal boron nitride (hBN) that we have obtained from the ab initio solution of the many-body Bethe-Salpeter equation as a function of momentum. In this way, we identify the character of the lowest-energy excitons in hBN, discuss the effects of the interlayer hopping and the electron-hole exchange interaction on the exciton dispersion, and illustrate the relation between exciton and plasmon excitations in layered materials.Peer reviewe

37 GHz observations of a large sample of BL Lacertae objects

We present 37 GHz data obtained at Metsahovi Radio Observatory in 2001 December - 2005 April for a large sample of BL Lacertae objects. We also report the mean variability indices and radio spectral indices in frequency intervals 5 - 37 GHz and 37 - 90 GHz. Approximately 34 % of the sample was detected at 37 GHz, 136 BL Lacertae objects in all. A large majority of the detected sources were low-energy BL Lacs (LBLs). The variability index values of the sample were diverse, the mean fractional variability of the sample being \Delta S_2 = 0.31. The spectral indices also varied widely, but the average radio spectrum of the sample sources is flat. Our observations show that many of the high-energy BL Lacs (HBL), which are usually considered radio-quiet, can at times be detected at 37 GHz.Comment: 12 pages, 5 figures + 5 tables. Published in Astronomical Journa

Variation in pronoun frequencies in early English letters: Gender-based or relationship-based?

Peer reviewe

The isotropic Compton profile difference across the phase transition of VO2_2

We studied the isotropic Compton profile of the prototypical oxide VO$_2$ across the temperature induced electronic and structural phase transition at T$_C$ $\approx$ 340 K. We show that the phase transition leaves an observable signal, which facilitates Compton scattering studies of electronic structure and phase transitions in complex solids in powder form. We compare the experimental results with density functional theory calculations and find agreement in the shape of the difference profile, although the amplitude of the observed features is overestimated. The origin of the disagreement is discussed and we argue that it mainly originates mostly correlation effects beyond our current calculations and possibly to some extent, from thermal motion