List of publications

Published Papers
      1. Annett Thøgersen, Marit Stange, Ingvild J. T. Jensen, Arne Røyset, Alexander Ulyashin and Spyros Diplas.  Structure and optical properties of aSiAl and aSiAlHx magnetron sputtered thin films. APL Materials, 4, 036103 (2016)

        Abstract: Thin films of homogeneous mixture of amorphous silicon and aluminum were produced with magnetron sputtering using 2-phase Al–Si targets. The films exhibited variable compositions, with and without the presence of hydrogen, aSi1−xAlx and aSi1−xAlxHy. The structure and optical properties of the films were investigated using transmission electron microscopy, X-ray photoelectron spectroscopy, UV-VisNIR spectrometry, ellipsometry, and atomistic modeling. We studied the effect of alloying aSi with Al (within the range 0–25 at. %) on the optical band gap,refractive index, transmission, and absorption. Alloying aSi with Al resulted in a non-transparent film with a low band gap (1 eV. Variations of the Al and hydrogen content allowed for tuning of the optoelectronic properties. The films are stable up to a temperature of 300 °C. At this temperature, we observed Al induced crystallization of the amorphous silicon and the presence of large Al particles in a crystalline Si matrix.

      2. Mikael Syväjarvi, Quanbao Ma, Valdas Jokubavicius, Augustinas Galeckas,Jianwu Sun, Xinyu Liu, Mattias Jansson, Peter Wellmann, Margareta Linnarsson,Paal Runde, Bertil Andre Johansen, Annett Thøgersen, Spyros Diplas,Patricia Almeida Carvalho, Ole Martin Løvvik, Daniel Nilsen Wright, Alexander Yu Azarov, Bengt G. Svensson.  Cubic silicon carbide as a potential photovoltaic material. Solar Energy Materials & Solar Cell. DOI: doi:10.1016/j.solmat.2015.08.029 (2015)

        Abstract: In this work we present a significant advancement in cubic silicon carbide (3C-SiC) growth in terms of
        crystal quality and domain size, and indicate its potential use in photovoltaics. To date, the use of 3C-SiC for photovoltaics has not been considered due to the band gap of 2.3 eV being too large for conventional solar cells. Doping of 3C-SiC with boron introduces an energy level of 0.7 eV above the valence band. Such energy level may form an intermediate band (IB) in the band gap. This IB concept has been presented in the literature to act as an energy ladder that allows absorption of sub-bandgap photons to
        generate extra electron–hole pairs and increase the efficiency of a solar cell. The main challenge with this concept is to find a materials system that could realize such efficient photovoltaic behavior. The 3C-SiC bandgap and boron energy level fits nicely into the concept, but has not been explored for an IB behavior. For a long time crystalline 3C-SiC has been challenging to grow due to its metastable nature. The material mainly consists of a large number of small domains if the 3C polytype is maintained. In our work a crystal growth process was realized by a new approach that is a combination of initial nucleation and step-flow growth. In the process, the domains that form initially extend laterally to make larger 3C-SiC domains, thus leading to a pronounced improvement in crystalline quality of 3C-SiC. In order to explore the feasibility of IB in 3C-SiC using boron, we have explored two routes of introducing boron impurities; ion implantation on un-doped samples and epitaxial growth on un-doped samples using pre-doped source material. The results show that 3C-SiC doped with boron is an optically active material, and thus is interesting to be further studied for IB behavior. For the ion implanted samples the crystal quality was maintained even after high implantation doses and subsequent annealing. The same was true for the samples grown with pre-doped source material, even with a high concentration of boron impurities. We present optical emission and absorption properties of as-grown and boron implanted 3C-SiC. The low-temperature photoluminescence spectra indicate the formation of optically active deep boron centers, which may be utilized for achieving an IB behavior at sufficiently high dopant concentrations. We also discuss the potential of boron doped 3C-SiC base material in a broader range of applications, such as in photovoltaics, biomarkers and hydrogen generation by splitting water

      3. X. Song, K. Valset, J.S. Graff, A. Thøgersen, A.E. Gunnæs, S. Luxsacumar, O.M. Løvvik,G.J. Snyder, and T.G. Finstad. Nanostructuring of Undoped ZnSb by Cryo-Milling. Journal of Electronic Materials DOI: 10.1007/s11664-015-3708-6 (2015)

        Abstract: We report the preparation of nanosized ZnSb powder by cryo-milling. The effect of cryo-milling then hot-pressing of undoped ZnSb was investigated and compared with that of room temperature ball-milling and hot-pressing under different temperature conditions. ZnSb is a semiconductor with favorable thermoelectric properties when doped. We used undoped ZnSb to study the effect of nanostructuring on lattice thermal conductivity, and with little contribution at room temperature from electronic thermal conductivity. Grain growth was observed to occur during hot-pressing, as observed by transmission electron microscopy and x-ray diffraction. The thermal conductivity was lower for cryo-milled samples than for room-temperature ball-milled samples. The thermal conductivity also depended on hot-pressing conditions. The thermal conductivity could be varied by a factor of two by adjusting the process conditions and could be less than a third that of single-crystal ZnSb.

      4. Trygve Mongstad, Annett Thøgersen, Aryasomayajula Subrahmanyam, and Smagul Karazhanov. The electronic state of thin films of yttrium, yttrium hydrides and yttrium oxide. Solar Energy Materials & Solar Cells 128, 270 (2014)

        Abstract: Thin films of yttrium hydride have for almost 20 years been under investigation for optoelectronic and solar energy applications due to the hydrogen-induced switching in electronic state from the metallic elemental yttrium and yttrium dihydride to the transparent semiconductor material yttrium trihydride. In this study, we investigate the electronic structure of yttrium, yttrium hydride and yttrium oxide by using X-ray photoelectron spectroscopy and kelvinprobe measurements. The investigated samples have been prepared by reactive sputtering deposition. We show that the electronic workfunction of transparent yttrium hydride is of 4.76 eV and that the recently discovered photochromic reaction lowers the electronic workfunction of the transparent hydride by 0.2 eV.

      5. Spyros Diplas, Andriy Romanyuk, Annett Thøgersen, and Alexander Ulyashin. An in situ XPS study of growth of ITO on amorphous hydrogenated Si: Initial stages of heterojunction formation upon processing of ITO/a-Si:H based solar cell structures. Phys. Status Solidi A, 1–4 (2014)

        Abstract: In this work we studied the interface growth upon deposition of indium-tin oxide (ITO) on amorphous hydrogenated Si (a-Si: H)/crystalline Si (c-Si) structures. The analysis methods used were X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) in combination with in situ film growth with magnetron sputtering. The analysis was complemented with transmission electron microscopy (TEM) of the deposited films. The sputtering equipment was attached to the XPS spectrometer and hence early stage film growth was observed without breaking the vacuum. It was shown that during early deposition stages ITO is reduced by a-Si:H. The reduction is accompanied with formation of metallic In and Sn at the interface. Formation of Sn is more enhanced on a-Si substrates whilst formation of In is more dominant on c-Si substrates. The reduction effect is less intense for amorphous hydrogenated Si as compared to crystalline Si and this is attributed to stronger presence of dangling bonds in the latter than the former.

      6. I.J.T. Jensen, A. Thøgersen, O.M. Løvvik, H. Schreuders, B. Damd, S. Diplas. X-ray photoelectron spectroscopy investigation of magnetron sputtered Mg-Ti-H thin films. International Journal of Hydrogen Energy 38, 10704 (2013)

        Abstract: Thin film samples of Mg80Ti20 (MgeTi) and Mg, both with and without H, were investigated in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were covered with a thin protective layer of Pd, which was removed by Arþ sputtering prior to data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct, previously unknown peak was revealed in the Mg KLL spectrum of the Mg-Ti-H samples, located between the metallic and the MgO component. This peak was attributed to trapping of H in very stable interstitial sites at the interface between Ti nano-clusters and the Mg matrix, based on earlier density functional theory calculations and supported by so-called Bader analysis. The latter was performed in order to study the theoretical charge distribution between Mg, Ti and H, establishing a link between the position of the previously unknown peak and the effect of H on the valence state of Mg. The composition of the samples was studied both by energy dispersive spectroscopy using transmission electron microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were obtained for metallic Mg, MgO and MgH2, as well as Mg affected by trapped H. No difference between the AP values from the metallic components was found between the Mg and the Mg-Ti samples. The AP values for MgO and MgH2 were consistent with previous reports in literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen, on the other hand, showed a more metallic character, with its corresponding AP value less than 1 eV below the AP for pure Mg.

      7. Annett Thøgersen and Georg Muntingh. Solar induced growth of silver nanocrystals. Journal of Applied Physics 113, 144301 (2013)(Arxiv page).

        Abstract: The effect of solar irradiation on plasmonic silver nanocrystals has been investigated using Transmission Electron Microscopy and size distribution analysis, in the context of solar cell applications for light harvesting. Starting from an initial collection of spherical nanocrystals on a carbon film whose sizes are log-normally distributed, solar irradiation causes the nanocrystals to grow, with one particle reaching a diameter of 638 nm after four hours of irradiation. In addition some of the larger particles lose their spherical shape. The average nanocrystal diameter was found to grow as predicted by the Ostwald ripening model, taking into account the range of area fractions of the samples. The size distribution stays approximately log-normal and does not reach one of the steady- state size distributions predicted by the Ostwald ripening model. This might be explained by the system being in a transient state.

      8. Annett Thøgersen, Marie Syre, Birger Retterstol Olaisen, and Spyros Diplas Studies of the oxidation states of phosphorus gettered Si substrates using XPS and TEM. Journal of Applied Physics, 113, 044307 (2013).

        Abstract: Phosphorus diffusion in p-type silicon wafers with Fe or Cr impurities has been investigated using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Silicon wafers doped with phosphorous are heavily used in semiconductor devices. It is therefore of crucial importance to determine their compositions profile. The XPS P2p spectra revealed presence of elemental phosphorus (P0), donor ion (P+) and interstitial or substitutional diffused P− from phosphorus oxides P2O5 and P4O10 as residues of the diffusion process. The surface of the Si wafers was oxidized during the deposition of P2O5 and SiO2 (with a subsequent heating). This resulted in the formation of a 1.3-1.4 nm thick SiOx layer. Pile-up of elemental P was found near the surface of the wafer. This pile-up was larger for Fe contaminated samples compared to Cr contaminated ones. The pile-up may have been caused by a decrease in the diffusion length of P+ donor ions, which could only be found in the first few nm near the surface of the silicon wafer. The observed diffusion length was DL(P+) < DL(P0) < DL(P−).

      9. Guobin Jia, Bjorn Eisenhawer, Jan Dellith, Fritz Falk, Annett Thøgersen, Alexander Ulyashin. Multiple Core-Shell Silicon Nanowire Based Heterojunction Solar Cells. Journal of Physical Chemistry C, 2013, 117 (2), pp 1091–1096 (2013). 

        Abstract: Silicon nanowire based solar cells received increasing attention due to their enhanced light harvesting properties. However, the very large surface deteriorates the performance of nanowire solar cells due to surface recombination. A multiple core-shell silicon nanowire based heterojunction solar cell has been fabricated in which this problem is strongly reduced. To this end an ultrathin passivating Al2O3 tunnel layer was deposited on the highly doped p-type a-Si:H emitter prior to a transparent conducting oxide by atomic layer deposition (ALD). Both open circuit voltage and current density increase significantly due to the insertion of the ultrathin Al2O3 layer. An efficiency of 10.0 % has been reached by using this multiple core-shell structure.

      10. M. H. Rein, M. V. Hohmann, A. Thogersen, J. Mayandi, A. O. Holt, A. Klein, and E. V. Monakhov An in situ XPS study of the initial stages of rf magnetron sputter deposition of indium tin oxide on p-type Si substrate. Appl. Phys. Lett. 102, 021606 (2013).

        Abstract: The interface between indium tin oxide and p-type silicon is studied by in situ X-ray Photoelectron Spectroscopy. This is done by performing XPS without breaking vacuum after deposition of ultrathin layers in sequences. Elemental tin and indium are shown to be present at the interface, both after 2 and 10 s of deposition. In addition, the silicon oxide layer at the interface is shown to be composed of mainly silicon suboxides rather than silicon dioxide.

      11. T. Mongstad, C. C. You, A. Thøgersen, J. P. Maehlen, Ch. Platzer-Björkman, B. C. Hauback, S. Zh. Karazhanov. 
        MgyNi1-y (Hx) thin films deposited by magnetron co-sputtering
        .  Journal of Alloys and Compounds, Volume 527, 25 June 2012, pp. 76 – 83. (
        Arxiv page, journal page)

        Abstract: In this work we have synthesised thin
        MgyNi1-y (Hx) metal and metal hydride
        films with y between 0 and 1. The films are deposited by magnetron
        co-sputtering of metallic targets of Mg and Ni. Metallic
        MgyNi1-y  films were deposited with pure Ar plasma while
        MgyNi1-yHx  hydride films were
        deposited reactively with 30% H2 in the Ar plasma. The depositions were
        done with a fixed substrate carrier, producing films with a spatial
        gradient in the Mg and Ni composition. This combinatorial method gives
        an insight into the phase diagram of MgyNi1-y and MgyNi1-yHx,
        and allows us to investigate the structural, optical and electrical
        properties of the resulting alloys. Reactive deposition results in
        direct synthesis of metal hydride films, with high purity in the case of
        Mg2NiH4. We have observed limited oxidation under
        ambient conditions and several months of exposure. The films desorb
        hydrogen at 240 ºC (Mg2NiH4) and 380 ºC (MgH2). The current
        work has been carried out to investigate the non-reactive and reactive
        co-sputtering process and the properties of the resulting films. MgyNi1-y and MgyNi1-yHx films can be applied for optical control in smart windows, optical sensors and
        Mg2NiH4  as a semiconducting material for photovoltaic solar cells.

      12. Annett Thøgersen, Josefine H. Selj, and Erik S. Marstein. Oxidation effects on graded porous silicon anti-reflection coatings. J. Electrochem. Soc., Volume 159, Issue 5, pp. D276 – D281 (2012). (Arxiv page, journal page)

        AbstractEfficient anti-reflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband anti-reflection properties can be fabricated. In this work, ageing of PS has been studied using Spectroscopic Ellipsometry, Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS elements such as pure Si (SiO), Si2(Si+), SiO (Si2+), Si2O3 (Si3+), and SiO2 (Si4+) are present. In addition both hydrogen and carbon is introduced to the PS in the form of Si3SiH and CO. The oxide grows almost linearly with time when exposed to oxygen, from a thickness of 0 – 3.1 nm for the surface PS. The oxidation is then correlated to the optical stability of multi-layered PS ARCs. It is found that even after extensive oxidation, the changes in the optical properties of the PS structures are small.

      13. Carl Huseby Fosli, Annett Thøgersen, Smagul Karazhanov and Erik Stensrud Marstein. Plasmonics for light trapping in thin silicon solar cells. Energy
        Procedia, European Materials Research Society Conference Symp. Advanced
        Inorganic Materials and Concepts for Photovoltaics, Volume 10, 2011, pp. 287 – 291. (journal page)

        Abstract: The
        cost of the wafer represents a significant share of the cost of a solar
        module. One way to reduce the cost of solar electricity is to reduce
        the wafer thickness. However, reduced wafer thicknesses lead to
        increases in the transmission through the cell. Therefore, light
        trapping structures must be incorporated in the solar cell. In recent
        years, metallic nanoparticles have been shown to enhance absorption in a
        solar cell through surface plasmon-related e.ects. Such particles have
        been used to increase the absorption of light in both thick and thin
        solar cells. In the present work, a method of producing silver
        nanoparticles has been investigated; evaporation of silver followed by
        annealing. Electron microscopy was used to determine how particle sizes
        and shapes depend on process conditions. Process parameteres like
        depositied silver thickness and time and temerature of the annealing
        step were investigated. The size of the silver particles can be
        controlled by varying the deposited silver thickness. The change in
        reflection when adding silver nanoparticles on the silicon wafers was
        measured. Reflection was found to be particle size dependent, exhibiting
        a red shift and an intensity increase of the reflection peak for larger
        particles.  

      14. J. Selj, E. S. Marstein, A. Thøgersen, and S. E. Foss. Porous Silicon Multilayer antireflaction coatings for Solar Cells; process considerations. Physica status solidi (C) – Current Topics in Solid State Physics, Volume 8, no. 6, pp. 18601864, 2010. (journal page)

        Abstract: Efficient
        AntiReflection Coatings (ARC) improve the light collection and thereby
        increase the current output of solar cells. For solar cells, broadband
        ARCs are desirable for efficient application over the entire solar
        spectrum. As previously demonstrated, such broadband ARCs can be made by
        electrochemical etching of Porous Silicon (PS) with graded refractive
        index. However, for efficient production there are a number of
        processing considerations which needs to be addressed. In this work the
        effects of electrolyte aging and sample inhomogeneities are investigated
        and quantified, using spectroscopic ellipsometry. A range of PS
        properties are extracted from the ellipsometric data by use of a graded
        Bruggeman Effective Medium model. Significant changes in porosity and
        thickness are detected and attributed to electrolyte aging. A positive
        correlation between thickness and porosity indicates that
        inhomogeneities across the sample are due to local variations in current
        density or HF concentration. Multilayered PS samples show smaller
        horizontal variations in porosity and thickness than single PS layers in
        our setup.

      15. A. Holt, A. Thøgersen, C. Rohr, J. I. Bye, Ø. Nordseth, S. A. Jensen, L. Norheim and Ø. Nielsen. Etch rates in alkaline solutions of mono-crystalline silicon wafers produced by diamond wire sawing. Published in the 25th European Photovoltaic Solar Energy Conference proceedings,  session 2CV .1.54, pp. 16171620, 2010. (journal page)

        Abstract: The
        objective of this work has been to identify the root cause of the
        reduced etch rate of fixed abrasive sawing (FAS) cut wafers during
        damage etching. The etch rate in alkaline solutions was measured as a
        function of time, temperature and different pre-cleaning processes, both
        for FAS and standard slurry wafers. The results showed a maximum etch
        rate for KOH concentrations around 20-30 wt%. The etch rate of FAS
        wafers was lower compared to slurry wafers during the initial 5-10
        minutes of etching, dependent upon KOH concentration and temperature. In
        order to characterize the wafer surface, scanning electron microscopy
        (SEM), transmission electron microscopy (TEM), atomic force microscopy
        (AFM), as well as reflectance measurements were used. By comparing the
        cross sections of the surface structure of as cut slurry and as cut FAS
        wafers, both an amorphous silicon layer and a defect layer were
        observed, but is concluded to not limiting initial higher etch rate.
        Furthermore during the initial stage of damage etching inverted square
        pillars were always formed. The walls of the pillars were constructed by
        the fast etching {110} planes. The depth of the pillars is found to be
        shallower for FAS cut wafers due to initially lower surface roughness.
        Therefore, the surface area exposed for the damage etch solution is
        larger for the slurry cut wafers, which explains the larger etching rate
        for the slurry cut wafers during the initial stage of the etching.

      16. A. Holt, A. Thøgersen, C. Rohr, J.I. Bye, G. Helgesen, Ø. Nordseth, S. A. Jensen, L. Norheim and Ø. Nielsen. Surface
        structure of mono-crystalline silicon wafers produced by diamond wire
        sawing and by standard slurry sawing before and after etching in
        alkaline solutions
        . Published in the 35th IEEE Photovoltaic Specialists Conference Proceedings, Honolulu (2010). (journal page)

        Abstract: Fixed
        abrasive sawing (FAS) using diamond coated steel wires is an
        interesting alternative for commercial production of silicon wafers, as
        it has potential for increasing productivity and reducing consumables
        costs. The objective of this study has been to understand the
        differences in surface structure of Cz mono-crystalline silicon wafers
        produced by diamond wire sawing and by standard slurry sawing, both
        before and after alkaline etching. Both as-cut wafers and wafers etched
        in 47% KOH at 75 °C for different etching times have been studied.
        Transmission electron microscope (TEM) investigations of the as-cut
        slurry wafers show an un-even surface and up to 4 μm deep micro cracks
        compared to the FAS wafers. Atomic force microscope (AFM) and scanning
        electron microscope (SEM) analyses of the ascut FAS wafers show a smooth
        wave-like pattern on the surface with a period of about 15 μm and
        amplitude of about 1 μm. During the initial part of the etching, square
        inverted pillars were formed for both types of wafers. The inverted
        pillars were initially deeper for the slurry cut wafers. The size in
        lateral direction of the inverted pillars increases with etching time.

      17. A. Thøgersen, J. Bonsak, C. Huseby Fosli, G. Muntingh. Size distributions of chemically synthesized Ag nanocrystals. Journal of Applied Physics, Volume 110, Issue 4. (Arxiv pagepost-print PDFjournal page)

        Abstract: Silver nanocrystals made by a chemical reduction of silver salts (AgNO3) by sodium borohydride (NaBH4) were studied using Transmission Electron Microscopy (TEM) and light scattering simulations. For various AgNO3/NaBH4 molar
        ratios, the size distributions of the nanocrystals were found to be
        approximately log-normal. In addition, a linear relation was found
        between the mean nanocrystal size and the molar ratio. In order to
        relate the size distribution of Ag nanocrystals of the various molar
        ratios to the scattering properties of Ag nanocrystals in solar cell
        devices, light scattering simulations of Ag nanocrystals in Si, SiO2, SiN, and Al2O3 matrices
        were carried out using Mie Plot. These light scattering spectra for the
        individual nanocrystal sizes were combined into light scattering
        spectra for the fitted size distributions. The evolution of these
        scattering spectra with respect to an increasing mean nanocrystal size
        was then studied. From these findings, it is possible to find the molar
        ratio for which the corresponding nanocrystal size distribution has
        maximum scattering at a particular wavelength in the desired matrix.

      18. J. H. Selj, A. Thøgersen, S. E. Foss, E. S. Marstein. Thin porous silicon films displaying a near-surface dip in porosity.
        The Electrochemical Society Transaction. 218th ECS Meeting, Volume 33,
        Issue 16, pp. 181
        189, October 10October 15, 2010, Las Vegas, NV. (journal page)

        Abstract: The
        simplicity of processing porous silicon (PS) layers is in stark
        contrast to the complexity of the fundamental questions regarding its
        formation mechanisms. Although a vast amount of literature is published
        on the subject, a number of issues are still in question, such as the
        exact dissolution chemistries of silicon and the origin of pore
        initiation.

      19. J. Selj, A. Thøgersen, S. E. Foss, and E. S. Marstein. Ellipsometric Study of the Influence of Chemical Etching on Thin Porous Silicon structures.
        Thin Solid Films, 519, Issue 9, 28 February 2011, pp. 2998
        3001,
        5th International Conference on Spectroscopic Ellipsometry (ICSE-V). (journal page

        Abstract: The
        effect of chemical etching on Porous Silicon (PS) samples is studied
        and quantified by using variable angle spectroscopic ellipsometry
        (VASE). The main aim of this work is to assess the impact of such
        etching on the physical properties of electrochemically etched, thin PS
        antireflection coatings (ARC) for solar cell applications. In this
        study, detailed models of PS layers etched at constant current densities
        are created using a graded uniaxial Bruggeman Effective Medium
        Approximation (BEMA). Changes in porosity, thickness, and optical
        anisotropy of the PS samples due to chemical etching are determined as a
        function of etching time after PS formation. Three series of PS films,
        etched at three different current densities, are investigated. It is
        shown that significant changes in physical properties occur for chemical
        etching times longer than ~ 60 s. The anodic etching process for
        fabricating PS ARC structures can be performed in less than 10 s.
        Therefore, chemical etching does not lead to significant deviations from
        the intended PS structure and is not seen as a hindrance to accurate
        control of processes for fabricating thin PS ARCs.

      20. Annett Thøgersen, Margrethe Rein, Jeyanthinath Mayandi, Spyros Diplas, and Edouard Monakhov. Elemental distribution and oxygen deficiency in magnetron sputtered ITO films. Journal of Applied Physics 109, 113532 (2011). (Arxiv page, journal page)

        Abstract: The
        atomic structure and composition of noninterfacial ITO and ITO-Si
        interfaces were studied with transmission electron microscopy and X-ray
        photoelectron spectroscopy (XPS). The films were deposited by dc
        magnetron sputtering on monocrystalline p-type (100) Si wafers. Both as
        deposited and heat treated films consisted of crystalline ITO. The
        ITO/Si interface showed a more complicated composition. A thin layer of
        SiOx was found at the ITO/Si interface together with In and
        Sn nanoclusters, as well as highly oxygen deficient regions, as observed
        by XPS. High energy electron exposure of this area crystallized the In
        nanoclusters and at the same time increased the SiOx interface layer thickness.

      21. Annett
        Thøgersen, Jeyanthinath Mayandi, Lasse Vines, Martin F. Sunding, Arne
        Olsen, Spyros Diplas, Masanori Mitome and Yoshio Bando. Composition and structure of Pd nanoclusters. Journal of Applied Physics 109, 084329 (2011). (Arxiv page, journal page

        Abstract: The nucleation, distribution, composition, and structure of Pd nanocrystals in SiO2
        multilayers containing Ge, Si, and Pd are studied using high resolution
        transmission electron microscopy (HRTEM) and X-ray photoelectron
        spectroscopy (XPS), before and after heat treatment. The Pd nanocrystals
        in the as deposited sample (sample ASD) seem to be capped by a layer of
        PdOx. A 1–2 eV shift in binding energy was found for the Pd-3d XPS
        peak, due to initial state Pd to O charge transfer in this layer. The
        heat treatment results in a decomposition of PdO and Pd into pure Pd
        nanocrystals and SiOx.

      22. Jack Bonsak, Jeyanthinath Mayandi, Annett Thøgersen, Erik Stensrud Marstein and M. Umadevi. Chemical synthesis of silver nanoparticles for solar cell applications. Phys. Status Solidi C 8, No. 3, 924927 (2011). (journal page)

        Abstract: A
        novel approach to fabricate silver nanoparticles for light trapping
        applications in silicon based solar cells has been demonstrated. Silver
        nanoparticles of various sizes have been synthesized by chemical
        reduction reactions. The particle size in the colloidal solutions was
        varied by using two different reducing compounds in addition to altering
        the compositions of the mixtures. In the present work, deposition of
        the silver nanoparticles onto a conventional bulk silicon solar cell
        shows an increase in the quantum efficiency at longer wavelengths,
        indicating the utilization of incident radiation that is normally lost
        in poor absorbing silicon.

      23. Annett Thøgersen, Jack Bonsak, Jeyanthinath Mayandi, Erik S. Marstein and M. Umadevi. Characterization of Ag nanocrystals for use in solar cell applications. MRS Proceedings, 2009, 1211 : 1211-R11-37. (journal page)

        Abstract: Ag
        nanocrystals made by chemical synthesis have been used in solar cell
        applications as a part of light trapping. The shape, crystal structure,
        defects and composition of these nanocrystals have been studied in
        detail. Samples with different ratios of silver solution (AgNO3) and
        reductant (NaBH4) were made, and a difference in nanocrystal size was
        observed. HRTEM and diffraction patterns showed that the samples
        contained mostly Ag nanocrystals, and some of them contained Ag2O
        nanocrystals as well. Some nanocrystals contained large defects, mostly
        twinning, which induced facets on the nanocrystal surface.

      24. J. H. Selj, A. Thøgersen, S. E. Foss, E. S. Marstein. Optimization of multilayer porous silicon antireflection coatings for silicon solar cells. Journal of Applied Physics 107, 074904 (2010). (journal page)

        Abstract: Efficient
        antireflection coatings (ARC) improve the light collection and thereby
        increase the current output of solar cells. In this work, multilayered
        refractive index stacks optimized for antireflection, in bare air and
        within modules, are modeled. The relation between porous silicon (PS)
        etching parameters and PS structure is carefully investigated using
        spectroscopic ellipsometry, gravimetry, X-ray photoelectron
        spectroscopy, and scanning electron microscopy. The close relation
        between porosity and refractive index, modeled using the Bruggeman
        effective medium approximation, allows PS multilayers to be tailored to
        fabricate the optimized ARCs. Limits imposed by efficient application in
        photovoltaics, such as thickness restrictions and the angular
        distribution of incident light, are examined and accounted for. Low
        reflectance multilayer ARCs are fabricated with integrated reflectances
        of ∼ 3% in air and 1.4% under glass in the wavelength range 400–1100 nm.

      25. Krister Mangersnes, Sean Erik Foss, and Annett Thøgersen. Damage free laser ablation of SiO2 for local contact opening on back-junction silicon solar cells using an a-Si:H buffer layer. Journal of Applied Physics 107, 043518 (2010). (journal page)

        Abstract: We
        have used a Q-switched Nd:YVO4, diode pumped 532 nm laser with
        nanosecond pulses, and a spot diameter of 40 μm to ablate a layer of
        plasma enhanced chemical vapor deposited (PECVD) SiO2 on n-type Cz
        silicon, with the aim of making local contact openings on back-junction
        silicon solar cells. Laser pulses within the ns range are usually
        believed to be incompatible with processing of high efficiency solar
        cells because such long pulses induce too much damage into the
        underlying silicon lattice. This is due to thermal dissipation. In this
        work, a PECVD layer of a-Si:H between the n-type silicon and the
        dielectric layer is shown to absorb much of the laser radiation and
        allows for ablation at laser fluences lower than the ablation threshold
        of crystalline silicon. In addition, the a-Si:H layer serves as an
        excellent surface passivation layer for the silicon substrate. We show
        that it is possible to ablate PECVD SiO2 in a damage free way with
        fluences five times lower than those needed to ablate crystalline Si.
        Our results are verified experimentally with high resolution
        transmission electron microscopy of the crystal structure in the laser
        irradiated areas, and quasi-steady-state photoconductance measurements
        of emitter saturation currents. In addition, we have simulated the
        energy transfer from a ns 532 nm Gaussian shaped laser beam to a SiO2
        covered Si lattice with and without the a-Si:H buffer layer. A model
        that coincides very well with the experiments is found.

      26. Annett Thøgersen. PhD Thesis: TEM and XPS studies of nanocrystals and clusters in nanostructured materials used for memory storage applications. (PDF, Duo Repository).

        Abstract:
        Nanoscaled electronic devices have attracted much attention due to
        their optical and electronic properties, especially related to MOS
        (Metal-Oxide-Semiconductor) devices used for memory storage
        applications. Improved electrical properties, longer retention, lower
        gate voltage and lower power consumption are assumed to be possible when
        replacing bulk floating gate in flash memory devices with nanocrystals.
        Multilayer samples with Si, Ge, Er-oxide, and Pd nanocrystals and
        clusters were studied in detail. The nucleation, distribution, defects,
        composition, and atomic and electronic structure are important factors
        to understand in order to improve performance of memory storage devices.
        These parameters were studied by high resolution transmission electron
        microscopy, energy filtered transmission electron microscopy, electron
        energy loss spectroscopy, X-ray photoelectron spectroscopy, energy
        dispersive spectroscopy, and secondary ion-mass spectrometry.

      27. Annett Thøgersen, Jeyanthinath Mayandi, Terje Finstad, Arne Olsen, Spyros Diplas, Masanori Mitome and Yoshio Bando, The formation of Er-oxide nanoclusters in SiO2 thin films with excess Si. Journal of Applied Physics 106, 014305 (2009). (Arxiv page, journal page)

        Abstract: The nucleation, distribution, and composition of erbium embedded in a SiO2–Si
        layer were studied with high resolution transmission electron
        microscopy (TEM), electron energy loss spectroscopy, energy filtered
        TEM, scanning transmission electron microscopy, and X-ray photoelectron
        spectroscopy. When SiO2 layer contains small amounts of Si
        and Er, nanoclusters of Er oxide are formed throughout the whole layer.
        The exposure of oxide to an electron beam with 1.56 × 106 electrons nm2
        s causes nanocluster growth. Initially this growth matches the Ostwald
        ripening model, but eventually it stagnates at a constant nanocluster
        radius of 2.39 nm.

      28. Grossner Ulrike, Avice Marc,
        Diplas Spyros, Thøgersen Annett, Christensen Jens S., Svensson Bengt G.,
        Nilsen Ola, Fjellvåg Helmer, Watts John. F. Influence of annealing on the Al2O3/4H-SiC interface.
        Materials Science Forum Volumes 600603. Silicon carbide and related materials 2007, PTS 1 and 2 (2009). (journal page)

        Abstract: Summarizing,
        after a post-deposition annealing at 1000 °C, correlation of XPS, SIMS
        and HRTEM data yields a scenario where the SiOx layer, occurring after ozone cleaning and Al2O3 deposition, breaks up and transforms into islands of SiO2,
        which is thermodynamically very stable, at the interface. As a result, a
        rather rough interface region evolves and excess of pure Si appears in
        the Al2O3 film. Moreover, a pronounced
        accumulation of H takes place in the rough interface region and this may
        at least partly be responsible for the low density of shallow electron
        states reported for annealed Al2O3/4H-SiC structures.

      29. Annett
        Thøgersen, Jeyanthinath Mayandi, Terje G. Finstad, Arne Olsen, Jens
        Sherman Christensen, Masanori Mitome and Yoshio Bando. Characterization of amorphous and crystalline silicon nanoclusters in ultra thin silica layers. Journal of Applied Physics 104, p. 094315 (2008). (Arxiv page,
        journal page)

        Abstract: The
        nucleation and structure of silicon nanocrystals formed by different
        preparation conditions and silicon concentrations (28 – 70 area %) have
        been studied using transmission electron microscopy (TEM), energy
        filtered TEM, and secondary ion mass spectroscopy. The nanocrystals were
        formed after heat treatment at high temperature of a sputtered 10 nm
        thick silicon rich oxide on 3 nm SiO2 layer made by rapid
        thermal oxidation (RTO) of silicon. Nanocrystals precipitate when the
        excess silicon concentration exceeds 50 area %. Below this percentage
        amorphous silicon nanoclusters were found. In situ heat treatment of the
        samples in the TEM showed that the crystallization requires a
        temperature above 800 °C. The nanocrystals precipitate in a 4 nm band, 5
        nm from the Si substrate, and 4 nm from the SiO2 sample
        surface. The silicon nucleates where the excess Si concentration is the
        highest. The top surface has less excess Si due to reaction with oxygen
        from the ambient during annealing. The SiO2-RTO layer is more Si rich due to Si diffusion from the SiO2–Si
        layer into RTO. Twinning and stacking faults were found in nanocrystals
        with 4 – 10 nm in diameter. These types of defects may have large
        effects on the usability of the material in electronic devices. Both
        single and double twin boundaries have been found in the nanocrystals by
        high resolution TEM. Image simulations were carried out in order to
        obtain more information about the defects and nanocrystals. The stacking
        faults are extrinsic and located in the twin boundaries.

      30. Spyros
        Diplas, Marc Avice, Annett Thøgersen, Jens S. Christensen, Ulrike
        Grossner, Bengt G. Svensson, Ola Nilsen, Helmer Fjellvåg, Steve Hinder,
        John F. Watts. Interfacial studies of Al2O3 deposited on 4H-SiC(0001). Surface and interface analysis 40 (3
        4), pp. 822825 (2008). (journal page)

        Abstract: Al2O3
        films deposited on 4H-SiC(0001) by atomic layer deposition (ALD) were
        characterized by XPS, and high-resolution transmission electron
        microscopy (HRTEM). The effect of medium and high temperature (873, 1273
        K) annealing on samples with oxide thicknesses of 5–8 and 100–120 nm
        was studied. XPS indicated the presence of a thin (∼1 nm) SiOx
        layer on the as-grown samples which increased to ∼3 nm after annealing
        above crystallization temperature (1273 K) in Ar atmosphere. Upon
        annealing the stoichiometry of the interfacial oxide approaches that of
        SiO2. HRTEM showed that the thickness of the interfacial
        oxide formed after annealing at 1273 K was not uniform. No significant
        increase in the thickness of the interfacial oxide, was observed after
        annealing at 873 K in a N2 (90%)/H2 (10%) atmosphere.

      31. Annett
        Thøgersen, Spyros Diplas, Jeyanthinath, Mayandi, Terje Finstad, Arne
        Olsen, John F. Watts, Masanori Mitome and Yoshio Bando. An experimental study of charge distribution in crystalline and amorphous Si nanoclusters in thin silica films. Journal of Applied Physics 103, p. 024308 (2008). (
        Arxiv page, journal page)

        Abstract: Crystalline
        and amorphous nanoparticles of silicon in thin silica layers were
        examined by transmission electron microscopy, electron energy loss
        spectroscopy, and X-ray photoelectron spectroscopy (XPS). We used XPS
        data in the form of the Auger parameter to separate initial and final
        state contributions to the Si2p energy shift. The electrostatic charging
        and electron screening issues as well as initial state effects were
        also addressed. We show that the chemical shift in the nanocrystals is
        determined by initial state rather than final state effects, and that
        the electron screening of silicon core holes in nanocrystals dispersed
        in SiO2 is inferior to that in pure bulk Si.

      32. M. Avice, S. Diplas, A. Thøgersen, J. S. Christensen, U. Grossner, B.G. Svensson, O. Nilsen, H. Fjellvåg and J. F. Watts. Rearrangement of the oxide-semiconductor interface in annealed Al2O3/4H-SiC structures. Applied Physics Letters 91, p. 052907 (2007). (journal page)

        Abstract: Al2O3
        films with different thicknesses have been deposited on n-type
        (nitrogen-doped) 4H-SiC(0001) epitaxial samples by atomic layer chemical
        vapor deposition at 300 °C and subsequently annealed in Ar atmosphere
        at temperatures up to 1000 °C. The Al2O3/4H-SiC
        structures were analyzed by X-ray photoelectron spectroscopy (XPS),
        secondary ion mass spectrometry (SIMS), and transmission electron
        microscopy (TEM). The XPS and SIMS results indicate that the average
        composition in the wider interface area does not significantly change
        due to the annealing. However, as revealed by the TEM investigations in
        combination with XPS, the as-grown samples exhibit a double interface
        created by an intermediate suboxide SiOx layer (x<2). After annealing, this intermediate suboxide layer breaks up and transforms into SiO2 islands, resulting in a rather rough interface region and a high concentration of pure Si in the Al2O3
        film. Furthermore, a pronounced accumulation of H is found in the rough
        interface region and this may play a key role for the low density of
        shallow electron states reported for annealed Al2O3/4H-SiC structures.

      33. J. Mayandi, T.G. Finstad, S. Foss, A. Thøgersen, U. Serincan and R. Turan. Ion beam synthesized luminescent Si nanocrystals embedded in SiO2 films and the role of damage on nucleation during annealing.  Surface and Coatings Technology 201, pp. 84828485 (2007). (journal page)

        Abstract: Si
        nanocrystals in thermal oxide films (∼ 250 nm) were fabricated by 100
        keV Si ion implantation followed by high temperature annealing. Two
        different doses were compared after annealing at 1050 °C for 2 h. A
        sample implanted with a dose of 1 × 1017 cm−2 shows a broad photo luminescence peak centered around 880 nm after annealing. A dose of 5 × 1016 cm−2
        yields a considerable blue shift of about 100 nm relative to the higher
        dose as well as a reduction in intensity. Transmission electron
        microscopy study reveals a difference in the microstructure of the SiO2
        films. Nanocrystals are clearly identified in the middle of the film
        for the highest dose, but not for the lower dose. The difference is
        discussed in terms of concentration dependent nucleation rate and
        differences in defect concentration. It is argued that the latter effect
        has a strong effect on the depth distribution of nanocrystals.

      34. J. Mayandi, T.G. Finstad, A. Tøgersen, S. Foss, U. Serincan and R. Turan. Scanning probe measurements on luminescent Si nanoclusters in SiO2 films. Thin Solid Films 515, pp. 6375 – 6380 (2007). (journal page)

        Abstract: Embedded Si nanocrystals in SiO2
        have a large current interest due to the prospects for practical
        applications. For most of these it is essential to characterize and
        ultimately control the nanocrystal size, size distribution and spatial
        distribution. Here we present a study of Si nanocrystals and clusters in
        SiO2 studied by atomic force microscopy (AFM). Since it is
        an indirect method, it requires several other methods to establish a
        reliable description of the structure of the samples. We here compare
        the AFM results with photoluminescence (PL) and transmission electron
        microscopy (TEM). Si nanocrystals in thermal oxide films (∼ 250 nm) were
        fabricated by 100 keV Si ion implantation at a dose of 1 × 1017 cm−2
        followed by high temperature annealing. AFM micrographs were taken
        after different etching times of the oxide and compared to TEM
        measuerements of the nanocrystal size and distribution. The correlations
        observed strongly indicate AFM signatures connected to the
        nanocrystals. We have analyzed and modeled the etch sectioning
        technique. Comparisons with the experiments let us conclude that the
        sectioning technique has some memory effect, but yields a distribution
        of nanocrystals with depth. A dose of 5 × 1016 cm−2
        yields a PL blue shift of about 100 nm relative to the higher dose. No
        nanocrystals are observed with TEM in this latter case. However distinct
        signatures can be observed with AFM and is tentatively attributed to
        the presence of non-crystalline Si-rich nanoclusters.

      35. J. Mayandi, T. G. Finstad, C. L. Heng, Y. J. Li, A. Thøgersen, S. Foss and H. Klette. A comparison between 1.5 μm photoluminescence from Er-doped Si-rich SiO2 films and (Er,Ge) co-doped SiO2 films. ENS06 Paris, France, 1415 December 2006. (Arxiv page, journal page)

        Abstract: We
        have studied the 1.5 µm photoluminescence (PL) from Er ions after
        annealing two different sample sets in the temperature range 500 °C to
        1100 °C. The different sample sets were made by magnetron sputtering
        from composite targets of Si+SiO2+Er and Ge+SiO2+Er respectively for the
        different sample sets. The annealing induces Si – and Ge-nanoclusters
        respectively in the different film sets. The PL peak reaches its maximum
        intensity after annealing at 700 °C for samples with Ge nanoclusters
        and after annealing at 800 °C for samples with Si. No luminescence from
        nanoclusters was detected in neither sample sets. This is interpreted as
        an energy transfer from the nanocluster to Er atoms. Transmission
        electron microscopy shows that after annealing to the respective
        temperature yielding the maximum PL intensity both the Ge and Si
        clusters are non-crystalline. Here we mainly compare the spectral shape
        of Er luminescence emitted in these different nanostructured matrixes.
        The PL spectral shapes are clearly different and witness a different
        local environment for the Er ions.

      36. J. Mayandi, T. G. Finstad, S. Foss, A. Thøgersen, U. Serincan and R. Turan. Luminescence from silicon nanoparticles in SiO2: atomic force microscopy and transmission electron microscopy studies. Physica Scripta T126, pp. 7780 (2006). (journal page)

        Abstract: Si
        nanocrystals in thermal oxide films (~250 nm) were fabricated by 100
        keV Si ion implantation at various doses followed by high temperature
        annealing. After annealing a sample implanted with a dose of 1 × 1017 cm−2 at 1050 °C for 2 h, a broad photoluminescence peak centred around 880 nm was observed. A dose of 5 × 1016 cm−2
        yields a considerable blue shift of about 100 nm relative to the higher
        dose. Transmission electron microscopy and atomic force microscopy
        (AFM) are used to characterize the microstructures in the SiO2
        film. The limitations of these techniques for the study of the
        nanostructures are addressed in this paper and it is suggested that AFM
        combined with etching can yield a structural spectroscopy with very good
        sensitivity.

      37. A. Thøgersen. Master thesis: A TEM study of the structural changes in supermartensitic stainless steel during deformation and welding.
      38. A.
        Thøgersen, R. B. Johansen, P. T. Zagierski, A. E. Gunns og A. Olsen.
        Extended abstract. Structural changes in Supermartensittic, Stainless
        Steel under deformation and welding, Proceedings SCANDEM 2003.

Citations can be found on Google Scholar.