Publications

Karsai, Ferenc; Engel, Manuel; Kresse, Georg & FlageLarsen, Espen (2018). Electronphonon coupling in semiconductors within the GW approximation. New Journal of Physics.
ISSN 13672630.
20(12), s 1 13 . doi:
10.1088/13672630/aaf53f

Karsai, Ferenc; Humer, Moritz; FlageLarsen, Espen; Blaha, Peter & Kresse, Georg (2018). Effects of electronphonon coupling on absorption spectrum: K edge of hexagonal boron nitride. Physical review B (PRB).
ISSN 24699950.
98(23), s 2352051 23520510 . doi:
10.1103/PhysRevB.98.235205

Musland, Lars & FlageLarsen, Espen (2018). Thermoelectric effect in superlattices; applicability of coherent and incoherent transport models. Computational materials science.
ISSN 09270256.
153, s 88 96 . doi:
10.1016/j.commatsci.2018.05.044
Full text in Research Archive.

Musland, Lars & FlageLarsen, Espen (2017). Thermoelectric transport calculations using the Landauer approach, ballistic quantum transport simulations, and the Buttiker approximation. Computational materials science.
ISSN 09270256.
132, s 146 157 . doi:
10.1016/j.commatsci.2017.02.016

FlageLarsen, Espen (2015). Charge carrier passivating nitrogenphosphorus defects in crystalline silicon. Computational materials science.
ISSN 09270256.
98, s 220 225 . doi:
10.1016/j.commatsci.2014.11.018
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In this work, the geometric and electronic structure of the neutral and charged nitrogen–phosphorus defects were for the first time rigorously investigated by density functional theory. The ground state structures were located by screening all possible geometric configurations of the defects. It is shown that the modified selfinterstitial nitrogen–phosphorus defect passivate the free carrier states of isolated substitutional phosphorus, known to be an excellent dopant in crystalline silicon. Furthermore, the band gap is shown to be similar in magnitude to bulk silicon, but direct. However, this study indicate that the nitrogen–phosphorus defect is possibly less stable than the selfinterstitial nitrogen dimer at high nitrogen defect concentrations. Finally, the vibrational spectra were analyzed by means of linear response theory and phonon calculations. The resulting vibrational spectra yield a peak split of the modified selfinterstitial nitrogen mode of 4 THz compared to the isolated selfinterstitial nitrogen defect.
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Løvvik, Ole Martin; FlageLarsen, Espen & Skomedal, Gunstein (2018). Screening silicide thermoelectric materials using ab initio transport calculations.

Musland, Lars; FlageLarsen, Espen; Prytz, Øystein & Bergli, Joakim (2018). Charge carrier transport in multilayered structures; thermoelectric applications.

Prytz, Øystein; FlageLarsen, Espen & Bilden, Sindre Rannem (2018). Simulation of momentum resolved Electron Energy Loss Spectroscopy in the low loss region using model band structures.
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One of the key limiting factors to progress within nano science is the ability to measure properties on the relevant length scale. The probe size provided by optical measurements is often larger than the individual nanoscale structures, and the resulting measurement is an average over some large volume, thus other methods must be applied. Electron Energy Loss Spectroscopy (EELS) in a Transmission Electron Microscope (TEM) provides a probe size suitable for measuring on nanoscopic structures, but the physics of the probe change when using electrons instead of photons. The fast electrons passing through the sample carry a significant momentum in addition to energy, and both can be transferred to an electron in the sample. The possible transfer of momentum in addition to energy increases the number of possible excitations immensely thus making the spectra of EELS more complex than its optical equivalent. The EELSspectra also provide useful information about properties earlier methods could not measure such as excitations resolved by momentum, and a straight measure of transitions across indirect band gaps. However, simulations are key in interpretation of EELS where transitions with momentum transfer contribute. Most simulation software for EELS focus on the optical limit and a production ready software for momentum resolved simulations is so far missing. In the present project a simulation software for EELS is developed for a momentum and energy resolved spectrum. Based on existing theory, a full framework for EELS simulation is developed in the dielectric formulation, strongly depending on the dielectric permittivity. The framework has been implemented with focus on a interactive visualization and interpretation of the result which should be easy to handle. Some limitations have been encountered when it comes to computational cost when mapping both momentum and energy. To limit the computational cost, the permittivity was heavily simplified by treating only its longitudinal component. When applying the software on parabolic bands it was found that the calculated joint density of states reproduced analytically derived results. In calculations of joint density of states of parabolic bands with indirect band gap it was found that the intensity onset had different shape when probing a range of momentum transfers opposed to single momentum transfers. When applied to electronic structure models from tight binding calculations, it was found that the longitudinal permittivity was not sufficient to describe the full response of the systems. The longitudinal permittivity is found insufficient in the presence of transverse electric fields and in nonisotropic systems, thus a correction to the permittivity has been presented, this has not been implemented. To conclude, the developed software indicates that momentum resolved calculations can provide useful information in its simplest manner, and be comparable to experiment with further development.

Salim Asbah, Richard; FlageLarsen, Espen & Prytz, Øystein (2018). Quadratic integration over the three dimensional Brillouin zone.

Zhan, Wei; Prytz, Øystein; Kuznetsov, Andrej & FlageLarsen, Espen (2018). Band gap mapping of alloyed ZnO using probecorrected and monochromated STEMEELS. Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo.. 15017710.
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The band gap of semiconducting ZnO can be readily tuned through alloying it with other relevant oxides, such as CdO, consequently extending the performance of the corresponding materials and devices. In this context, one of the challenges is to establish the methodology for twodimensional band gap measurements on the nanometer scale. Here, monochromated electron energy loss spectroscopy (EELS) in combination with probecorrected scanning transmission electron microscopy (STEM) can be applied, potentially with much greater success compared to traditional techniques with low spatial resolution. However, up to now, the EELS based band gap mapping technique has not seen widespread use, primarily due to its experimental and data processing complexities. In this work, utilizing stateoftheart probecorrected and monochromated STEMEELS platform without particular instrumental design, we developed and applied methods for acquiring large band gap maps with high spatial resolution. A newlydeveloped efficient computing method was employed to extract band gap maps from the EELS data after proper background subtraction. All these advances are highlighted by the band gap mapping of Zn1xCdxO/ZnO hetero structure with a spatial resolution well below 10 nm and a high spectral precision. Nevertheless, band gap measurement by EELS are also restricted in spatial resolution, which is fundamentally determined by the delocalization length (L50) of the inelastic scattering process. The origin of this delocalization is the long range electrostatic interactions between the atomic electrons of the sample and the incident highenergy electrons. The EELS plasmon energy map has obviously higher spatial resolution than the band gap map, and its experiment as well as data extraction is also much easier to perform. In order to push the spatial resolving power in EELS band gap analysis further, the relationship between the band gaps and plasmon energies in Zn1xCdxO was investigated based on the fact that both depend strongly on the unit cell parameter. A robust quantitative correlation was established, providing a simple and straightforward way to calculate the band gap variations just from the easily measured plasmon energy, with improved spatial resolving ability as compared with the conventional EELS approach. In order to further verify the success of the probecorrected and monochromated STEMEELS technique, it was put into application to a new system, namely separate ZnCr2O4 nanoinclusions embedded in ZnO matrix. Band gap mapping of ZnCr2O4 nanoparticles in ZnO matrix and their interface was successfully achieved, confirming the validity of this STEMEELS approach. In addition, probecorrected STEM enables subångström imaging, from which the realistic structure can be revealed. We employed atomicresolution images together with geometric phase analysis (GPA) to analyze the structure and strain at ZnCr2O4/ZnO interfaces, which is of critical importance for thin film growth and may affect band gap.
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Published Aug. 17, 2018 11:07 AM
 Last modified Oct. 22, 2019 9:51 AM