Self-vacancies in Gallium Arsenide: an ab initio calculation

We report here a reexamination of the static properties of vacancies in GaAs by means of first-principles density-functional calculations using localized basis sets. Our calculated formation energies yields results that are in good agreement with recent experimental and {\it ab-initio} calculation and provide a complete description of the relaxation geometry and energetic for various charge state of vacancies from both sublattices. Gallium vacancies are stable in the 0, -, -2, -3 charge state, but V_Ga^-3 remains the dominant charge state for intrinsic and n-type GaAs, confirming results from positron annihilation. Interestingly, Arsenic vacancies show two successive negative-U transitions making only +1, -1 and -3 charge states stable, while the intermediate defects are metastable. The second transition (-/-3) brings a resonant bond relaxation for V_As^-3 similar to the one identified for silicon and GaAs divacancies.Comment: 14 page

Managing the supercell approximation for charged defects in semiconductors: finite size scaling, charge correction factors, the bandgap problem and the ab initio dielectric constant

The errors arising in ab initio density functional theory studies of semiconductor point defects using the supercell approximation are analyzed. It is demonstrated that a) the leading finite size errors are inverse linear and inverse cubic in the supercell size, and b) finite size scaling over a series of supercells gives reliable isolated charged defect formation energies to around +-0.05 eV. The scaled results are used to test three correction methods. The Makov-Payne method is insufficient, but combined with the scaling parameters yields an ab initio dielectric constant of 11.6+-4.1 for InP. Gamma point corrections for defect level dispersion are completely incorrect, even for shallow levels, but re-aligning the total potential in real-space between defect and bulk cells actually corrects the electrostatic defect-defect interaction errors as well. Isolated defect energies to +-0.1 eV are then obtained using a 64 atom supercell, though this does not improve for larger cells. Finally, finite size scaling of known dopant levels shows how to treat the band gap problem: in less than about 200 atom supercells with no corrections, continuing to consider levels into the theoretical conduction band (extended gap) comes closest to experiment. However, for larger cells or when supercell approximation errors are removed, a scissors scheme stretching the theoretical band gap onto the experimental one is in fact correct.Comment: 11 pages, 3 figures (6 figure files). Accepted for Phys Rev

Density functional theory calculations of defect energies using supercells

Reliable calculations of defect properties may be obtained with density functional theory using the supercell approximation. We systematically review the known sources of error and suggest how to perform calculations of defect properties in order to minimize errors. We argue that any analytical error-correction scheme relying on electrostatic considerations is not appropriate to derive reliable defect formation energies, especially not for relaxed geometries. Instead we propose finite size scaling of the calculated defect formation energies, and compare the use of this with both fully converged and "Gamma" (Γ) point only k-point integration. We give a recipe for practical DFT calculations which will help to obtain reliable defect formation energies and demonstrate it using examples from III-V semiconductors

Vacancies in SiGe: Jahn–Teller distortion and spin effects

The electronic structure of a vacancy in silicon-germanium is studied using ab initio total-energy minimization methods. The calculations are based on density-functional theory in the local-spin-density approximation. We report ionic relaxations, defect formation energies and ionization levels of Si and Gevacancies in a zinc blende model structure (SiGe). The Gevacancy in SiGe is characterized by symmetry-lowering Jahn–Teller (JT) distortions and a negative-effective-U effect, in those respects resembling the vacancy in elemental silicon. For Si vacancy, the exchange-coupling energy is found to overcome the JT energy, and symmetric high-spin ground states are predicted.Peer reviewe

OZO v.1.0 : software for solving a generalised omega equation and the Zwack-Okossi height tendency equation using WRF model output

A software package (OZO, Omega-Zwack-Okossi) was developed to diagnose the processes that affect vertical motions and geopotential height tendencies in weather systems simulated by the Weather Research and Forecasting (WRF) model. First, this software solves a generalised omega equation to calculate the vertical motions associated with different physical forcings: vorticity advection, thermal advection, friction, diabatic heating, and an imbalance term between vorticity and temperature tendencies. After this, the corresponding height tendencies are calculated with the Zwack-Okossi tendency equation. The resulting height tendency components thus contain both the direct effect from the forcing itself and the indirect effects (related to the vertical motion induced by the same forcing) of each physical mechanism. This approach has an advantage compared with previous studies with the Zwack-Okossi equation, in which vertical motions were used as an independent forcing but were typically found to compensate the effects of other forcings. The software is currently tailored to use input from WRF simulations with Cartesian geometry. As an illustration, results for an idealised 10-day baroclinic wave simulation are presented. An excellent agreement is found between OZO and the direct WRF output for both the vertical motion and the height tendency fields. The individual vertical motion and height tendency components demonstrate the importance of both adiabatic and diabatic processes for the simulated cyclone. OZO is an open-source tool for both research and education, and the distribution of the software will be supported by the authors.Peer reviewe

The impact of new housing construction on rental housing market in 2020s Helsinki

The popularity of rental housing has grown steadily throughout the 2000s and today more than half of all households in Helsinki live in rented accommodation. From a landlord's point of view, determining the right rent level is of primary importance when renting an apartment. In the big picture, the key factor in determining the rent is the amount of demand in relation to supply. Many factors affecting supply and demand can be identified and taken into account. In the case of new construction, the difficulty in identifying the impact is that new construction not only increases the supply of housing in the area but may also increase the attractiveness of the area. The impact of new construction on rents for nearby housing has been studied in several US cities. The results of previous studies have varied from city to city, and different results have also been obtained in the same cities. Findings have ranged from a 7% increase in rents to almost a 7% decrease in rents for apartments within a few hundred meters of new construction. Since the impact of new construction is difficult to generalise due to the regional nature of the housing market, this study fills a gap in the research on the impact of new construction on rents of nearby dwellings in Helsinki. This study also adds to previous studies in this area by the fact that during the period of the study, significant macroeconomic events have taken place that have led to an oversupply of rental housing in the market. Five professionals working in the real estate sector are interviewed to formulate the hypotheses of the research problem and to gain an in-depth understanding of the topic. In the statistical analysis of the study, the hypotheses are tested using the quasi-experimental difference-in-differences method, which is well established in studies of the subject. The method uses a regression model to compare a dummy variable describing the interaction between the treatment and control groups before and after the completion of the new construction. The advantage of this approach is that it removes biases due to group differences and external factors. The dataset consists of new buildings constructed in Helsinki in 2022 and 2023, and rent observations within a radius of one kilometre around the new buildings from 2021 to 2024, with a sample size of 12 960. The results show that new construction increases rents for dwellings within 400 meters by 5,8% compared to dwellings within 400-1000 meters. The results are explained by the dominant demand effect. New housing attracts households, which in turn attracts new amenities and as a result the demand for the whole area increases. However, the analysis does not include discount campaigns, which were given more to dwellings close to new buildings. Therefore, rents in the area around new buildings have actually increased less because ten-ants have been attracted by one-off discounts instead of a permanent reduction in the monthly rent.Vuokra-asumisen suosio on kasvanut tasaisesti koko 2000-luvun ja nykyään jo yli puolet Helsingin asuntokunnista asuu vuokralla. Vuokranantajan näkökulmasta oikean vuokratason määrittäminen on ensisijaisen tärkeää asunnon vuokraamisessa. Isossa kuvassa vuokran määrityksessä keskeistä on kysynnän määrä suhteessa tarjontaan. Monet kysyntään ja tarjontaan vaikuttavat tekijät osataan tunnistaa ja ottaa huomioon. Uudisrakentamisen kohdalla vaikutusten tunnistamisesta vaikeaa tekee se, että uudisrakentaminen ei ainoastaan lisää alueen asuntojen tarjontaa, vaan se saattaa myös lisätä alueen vetovoimaa. Uudisrakentamisen vaikutuksia lähialueen asuntojen vuokriin on tutkittu useissa Yhdysvaltojen kaupungeissa. Aikaisempien tutkimusten tulokset ovat olleet vaihtelevia kaupungin mukaan ja erilaisia tuloksia on saatu myös samoista kaupungeista. Havainnot ovat vaihdelleet 7 % vuokria nostavasta vaikutuksesta lähes 7 % vuokria laskevaan vaikutukseen muutaman sadan metrin säteellä uudesta rakennuksesta sijaitsevissa asunnoissa. Koska uudisrakentamisen vaikutuksia on hankala yleistää asuntomarkkinoiden alueellisuuden takia, tämä tutkimus täyttää tutkimusaukon uudisrakentamisen vaikutuksista läheisten asuntojen vuokriin Helsingissä. Lisäpanoksen tämä tutkimus antaa aihepiirin aikaisempii tutkimuksiin myös sillä, että tutkimuksen ajanjaksolla on tapahtunut merkittäviä makrotalouden tapahtumia, jotka ovat johtaneet vuokra-asuntojen ylitarjontatilanteeseen markkinoilla. Viittä kiinteistöalalla työskentelevää ammattilaista haastatellaan tutkimusongelman hypoteesien muodostamista varten ja aiheen syvällisen ymmärtämisen saavuttamiseksi. Tutkimuksen tilastollisessa analyysissä hypoteesit testataan aiheen tutkimuksissa vakiintunutta kvasikokeellista difference-in-diffecenses -menetelmää käyttämällä. Menetelmän avulla regressiomallissa verrataan koe- ja vertailuryhmän vuorovaikutusta kuvaavaa dummy-muuttujaa ennen ja jälkeen uudisrakennuksen valmistumisen. Tämän lähestymistavan etuna on, että se poistaa ryhmien eroista ja ulkoisista tekijöistä johtuvat vääristymät. Aineisto koostuu Helsingissä vuosina 2022 ja 2023 rakennetuista uusista rakennuksista sekä vuokrahavainnoista kilometrin säteellä uudisrakennusten ympäriltä vuosilta 2021–2024, otoskoon ollessa 12 960. Tulokset osoittavat uudisrakennusten nostavan 400 metrin säteellä sijaitsevien asuntojen vuokria 5,8 % verrattuna 400–1000 metrin säteellä sijaitseviin asuntoihin. Tuloksia selittää hallitseva kysyntävaikutus eli, uudet asunnot houkuttelevat kotitalouksia, joka puolestaan houkuttelee uusia palveluita ja lopputuloksena koko alueen kysyntä paranee. Analyysi ei kuitenkaan ota huomioon alennuskampanjoita, joita oli annettu enemmän uudisrakennusten lähellä sijaitseville asunnoille. Näin ollen uudisrakennusten lähialueella vuokrat ovat nousseet todellisuudessa vähemmän koska, vuokralaisia on houkuteltu kertaluontoisilla alennuksilla kuukausivuokran pysyvän alentamisen sijaan