List of publications

Published Papers
    1. Quanbao Ma, Patricia Carvalho , Augustinas Galeckas , Alexander Azarov , Sigurd Hovden, Annett Thøgersen, Daniel N. Wright, Spyros Diplas, Ole M. Løvvik, Valdas Jokubavicius, Jianwu Sun, Mikael Syväjärvi, and Bengt G. Svensson. Characterization of B-implanted 3C-SiC for intermediate band solar cells. European Conference on Silicon Carbide & Related Materials (ECSCRM), Halkidiki, Greece, 2016, pp. 1-1. doi: 10.4028/www.scientific.net/MSF.897.299 (2016)

      Abstract: Sublimation-grown 3C-SiC crystals were implanted with B ions at elevated temperature (400 °C) using multiple energies (100 to 575 keV) with a total dose of 1.3×1017 atoms/cm2 in order to form intermediate band (IB) in 3C-SiC. The samples were then annealed at 1400 °C for 60 min. An anomalous area in the center was observed in the PL emission pattern. The SIMS analysis indicated that the B concentration was the same both within and outside the anomalous area. The buried boron box-like concentration profile can reach ∼3×1021 cm−3 in the plateau region. In the anomalous area a broad emission band (possible IB) emerges at around ∼1.7–1.8 eV, which may be associated with B-precipitates having a sufficiently high density.

    2. Kjeldstad, T., Thøgersen, A., Nilsen, O., Monakhov, E. and Galeckas, A. Controllable template approach for ZnO nanowire growth. Phys. Status Solidi A, 214: n/a, 1600480. doi:10.1002/pssa.201600480 (2017)

      Abstract: Anodic aluminum oxide (AAO) is an optically transparent material with uniform hexagonal pores, which has widely been used as a template for nanowire growth. This study examines the potential of utilizing commercially available AAO to fabricate sub-10 nm ZnO nanowires by atomic layer deposition. The pore diameter of the AAO framework is reduced by controlled deposition of math formula. Subsequently, ZnO is deposited and ZnO nanowires are obtained. The analysis of structural and optical properties shows that the nanowires are polycrystalline, and exhibit a pronounced near band edge emission. Photoluminescence (PL) measurements are performed to monitor the effect of additional coating with math formula and ZnO, on the optical stability of the AAO. The measurements indicate that the PL is highly influenced by the adsorption and desorption of species from the ambient at the surface. ALD deposition of a thin math formula layer reduces considerably the effect of ambient atmosphere. The layer of math formula facilitates PL characterization of very thin ZnO nanowires

    3. 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.

    4. 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

    5. 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.

    6. 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.

    7. 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.

    8. 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.

    9. 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.

    10. 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−).

    11. 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.

    12. 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.

    13. 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.

    14. 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.

    15. 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 effects. 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 parameters like deposited silver thickness and time and temperature 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.  

    16. 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.

    17. 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.

    18. 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.

    19. 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.

    20. 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. 181189, 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.

    21. 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.

    22. 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.

    23. 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 SiOmultilayers 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.

    24. 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.

    25. 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.

    26. 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.

    27. 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.

    28. 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.

    29. 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 nm2s causes nanocluster growth. Initially this growth matches the Ostwald ripening model, but eventually it stagnates at a constant nanocluster radius of 2.39 nm.

    30. 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.

    31. 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.

    32. 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) SiOlayer 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.

    33. 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.

    34. 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: Al2Ofilms 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 Al2Ofilm. 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.

    35. 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 SiOfilms. 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.

    36. 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 SiOhave 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.

    37. 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.

    38. 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 SiOfilm. 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.

    39. A. Thøgersen. Master thesis: A TEM study of the structural changes in supermartensitic stainless steel during deformation and welding.
    40. 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.

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