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  • 151.
    Yakimova, Rositsa
    et al.
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Yazdi, Gholam Reza
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Nguyen, Son Tien
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Ivanov, Ivan Gueorguiev
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Syväjärvi, Mikael
    Linköping University, The Institute of Technology. Linköping University, Department of Physics, Chemistry and Biology, Materials Science .
    Sun, S.
    Tompa, G.
    Kuznetsov, A.
    Svensson, B.
    Optical and Morphological Features of Bulk and Homoepitaxial ZnO2006In: Superlattices and Microstructures, ISSN 0749-6036, E-ISSN 1096-3677, Vol. 39, p. 247-256Article in journal (Refereed)
    Abstract [en]

    ZnO substrate crystals from two different sources, and epitaxial layers have been studied by SEM, AFM, photoluminescence (T=2-135K) and EPR. Although fabricated by the same growth principle, i.e. the hydrothermal technique, the substrates differ in terms of purity and structural quality. In the PL spectra of all samples the dominating emission originates from the donor bound exciton (BE) recombination positioned at about 3361 meV. The temperature dependence of the spectra confirms the assignment of the free exciton emission in the purest sample, the line at 3376 meV evolves into a broad peak at higher temperatures, probably including both A and B excitons. Another FE-related emission appears as a shoulder on the high-energy side of FEA,B above 40 K. It is expected and associated with the crystal-field split-off counterpart of the valence band. Free-exciton related emission in the less pure sample can only be seen if the temperature is above 45 K. At T=135K all bound excitons are quenched and the spectrum in both samples consists of the free exciton no-phonon lines and their replicas. However, the emission from the pure samples is several orders of magnitude stronger than that from the other sample, which indicates strong non-radiative quenching of the excitons in the latter sample. The EPR measurements reveal a possible scenario of impurity re-arrangement, e.g. annealing at 950 °C may dissociate existing complexes and release Fe as isolated ions. The AFM and SEM investigations of an epilayer grown by MOCVD on one of the studied substrates have indicated growth instabilities and structural irregularities, thus pointing to the need for substrate quality and epitaxial process optimization.

  • 152.
    Yang, Li Li
    et al.
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Zhao, Qingxiang
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Xing, G. Z.
    Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, NanyangTechnological University, Singapore.
    Wang, D. D.
    Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, NanyangTechnological University, Singapore.
    Wu, T.
    Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, NanyangTechnological University, Singapore.
    Willander, Magnus
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology. Linköping University, The Institute of Technology.
    Yang, J. H.
    Institute of Condensed State Physics, Jilin Normal University, Siping, 136000, People's Republic of China.
    Mg diffusion in Zn0.94Mg0.06O/ZnO heterostructures grown by MOCVDManuscript (preprint) (Other academic)
    Abstract [en]

    Zn0.94Mg0.06O/ZnO heterostructures were grown on 2 inch sapphire wafer by MOCVD equipment. Photoluminescence mapping demonstrated that Mg uniformly distributed on the entire wafer with average concentration of ~6%. The annealing effects on the Mg diffusion behaviors were investigated by secondary ion mass spectrometry (SIMS). All Mg SIMS depth profiles were fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn0.94Mg0.06O layer deposited at 700 oC was two order of magnitude lower than that of annealed samples, which indicated that the deposition temperature of 700 oC is much more beneficial to grow ZnMgO/ZnO heterostructures or quantum wells.

  • 153.
    Yang, Li-Li
    et al.
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Zhao, Qingxiang
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Willander, Magnus
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Yang, J H
    Jilin Normal University.
    Ivanov , Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Materials Science . Linköping University, The Institute of Technology.
    Annealing effects on optical properties of low temperature grown ZnO nanorod arrays2009In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 105, no 5, p. 053503-Article in journal (Refereed)
    Abstract [en]

    Vertically well-aligned ZnO nanorods on Si substrates were prepared by a two-step chemical bath deposition method. The structure and optical properties of the grown ZnO nanorods were investigated by Raman and photoluminescence spectroscopy. The results showed that after an annealing treatment at around 500 degrees C in air atmosphere, the crystal structure and optical properties became much better due to the decrease in surface defects. The resonant Raman measurements excited by 351.1 nm not only revealed that the surface defects play a significant role in the as-grown sample, which was supported by low temperature time-resolved photoluminescence measurements, but also suggested that the strong intensity increase in some Raman scatterings was due to both outgoing resonant Raman scattering effect and deep level defect scattering contribution for ZnO nanorods annealed from 500 to 700 degrees C.

  • 154.
    Yang, L.L.
    et al.
    Jilin Normal University.
    Zhao, Qingxiang
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Xing, G.Z.
    Nanyang Technology University.
    Wang, D.D.
    Nanyang Technology University.
    Wu, T.
    Nanyang Technology University.
    Willander, Magnus
    Linköping University, Department of Science and Technology. Linköping University, The Institute of Technology.
    Gueorguiev Ivanov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Yang, J.H.
    Jilin Normal University.
    A SIMS study on Mg diffusion in Zn(0.94)Mg(0.06)O/ZnO heterostructures grown by metal organic chemical vapor deposition2011In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 257, no 20, p. 8629-8633Article in journal (Refereed)
    Abstract [en]

    Zn(0.94)Mg(0.06)O/ZnO heterostructures have been grown on 2 in. sapphire wafer using metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) mapping demonstrates that Mg distribution on the entire wafer is very uniform (standard deviation of Mg concentration/mean Mg concentration = 1.38%) with average concentration of similar to 6%. The effect of annealing on the Mg diffusion in Zn(0.94)Mg(0.06)O/ZnO heterostructures has been investigated in detail by using secondary ion mass spectrometry (SIMS). All the Mg SIMS depth profiles have been fitted by three Gaussian distribution functions. The Mg diffusion coefficient in the as-grown Zn(0.94)Mg(0.06)O layer deposited at 700 degrees C is two orders of magnitude lower than that of annealed samples, which clearly indicates that the deposition temperature of 700 degrees C is much more beneficial to grow ZnMgO/ZnO heterostructures and quantum wells.

  • 155.
    Yazdanfar, Milan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Gueorguiev Ivanov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Kordina, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Reduction of structural defects in thick 4H-SiC epitaxial layers grown on 4° off-axis substrates2013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 22Article in journal (Refereed)
    Abstract [en]

    By carefully controlling the surface chemistry of the chemical vapor deposition process for silicon carbide (SiC), 100 μm thick epitaxial layers with excellent morphology were grown on 4° off-axis SiC substrates at growth rates exceeding 100 μm/h. In order to reduce the formation of step bunching and structural defects, mainly triangular defects, the effect of varying parameters such as growth temperature, C/Si ratio, Cl/Si ratio, Si/H2 ratio, and in situ pre-growth surface etching time are studied. It was found that an in-situ pre growth etch at growth temperature and pressure using 0.6% HCl in hydrogen for 12 min reduced the structural defects by etching preferentially on surface damages of the substrate surface. By then applying a slightly lower growth temperature of 1575 °C, a C/Si ratio of 0.8, and a Cl/Si ratio of 5, 100 μm thick, step-bunch free epitaxial layer with a minimum triangular defect density and excellent morphology could be grown, thus enabling SiC power device structures to be grown on 4° off axis SiC substrates.

  • 156.
    Yazdanfar, Milan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kalered, Emil
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Danielsson, Örjan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kordina, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Nilsson, Daniel
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Ojamäe, Lars
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Brominated chemistry for chemical vapor deposition of electronic grade SiC2015In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 3, p. 793-801Article in journal (Refereed)
    Abstract [en]

    Chlorinated chemical vapor deposition (CVD) chemistry for growth of homoepitaxial layers of silicon carbide (SiC) has paved the way for very thick epitaxial layers in short deposition time as well as novel crystal growth processes for SiC. Here, we explore the possibility to also use a brominated chemistry for SiC CVD by using HBr as additive to the standard SiC CVD precursors. We find that brominated chemistry leads to the same high material quality and control of material properties during deposition as chlorinated chemistry and that the growth rate is on average 10 % higher for a brominated chemistry compared to chlorinated chemistry. Brominated and chlorinated SiC CVD also show very similar gas phase chemistries in thermochemical modelling. This study thus argues that brominated chemistry is a strong alternative for SiC CVD since the deposition rate can be increased with preserved material quality. The thermochemical modelling also suggest that the currently used chemical mechanism for halogenated SiC CVD might need to be revised.

  • 157.
    Yazdanfar, Milan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Sukkaew, Pitsiri
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Danielsson, Örjan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kordina, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    On the use of methane as a carbon precursor in Chemical Vapor Deposition of silicon carbide2014In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 390, p. 24-29Article in journal (Refereed)
    Abstract [en]

    It is generally considered that methane is not a suitable carbon precursor for growth of silicon carbide (SiC) epitaxial layers by Chemical Vapor Deposition (CVD) since its use renders epitaxial layers with very high surface roughness. In this work we demonstrate that in fact SiC epitaxial layers with high-quality morphology can be grown using methane. It is shown that a key factor in obtaining high-quality material is tuning the C/Si ratio of the process gas mixture to a region where the growth is limited neither by carbon nor by silicon supplies. From the growth characteristics presented here, we argue that the reactivity of methane with the SiC surface is much higher than generally assumed in SiC CVD modeling today.

  • 158.
    Yazdanfar, Milan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Stenberg, Pontus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Don Booker, Ian
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Gueorguiev Ivanov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Kordina, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Process stability and morphology optimization of very thick 4H-SiC epitaxial layers grown by chloride-based CVD2013In: Journal of Crystal Growth, ISSN 0022-0248, E-ISSN 1873-5002, Vol. 380, p. 55-60Article in journal (Refereed)
    Abstract [en]

    The development of a chemical vapor deposition (CVD) process for very thick silicon carbide (SiC) epitaxial layers suitable for high power devices is demonstrated by epitaxial growth of 200 nm thick, low doped 4H-SiC layers with excellent morphology at growth rates exceeding 100 nm/h. The process development was done in a hot wall CVD reactor without rotation using both SiCl4 and SiH4+HCl precursor approaches to chloride based growth chemistry. A C/Si ratio andlt;1 and an optimized in-situ etch are shown to be the key parameters to achieve 200 nm thick, low doped epitaxial layers with excellent morphology.

  • 159.
    Yazdanfar, Milan
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Stenberg, Pontus
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Don Booker, Ian
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Gueorguiev Ivanov, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Pedersen, Henrik
    Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, The Institute of Technology.
    Kordina, Olle
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Janzén, Erik
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, The Institute of Technology.
    Morphology optimization of very thick 4H-SiC epitaxial layers2013In: SILICON CARBIDE AND RELATED MATERIALS 2012, Trans Tech Publications , 2013, Vol. 740-742, p. 251-254Conference paper (Refereed)
    Abstract [en]

    Epitaxial growth of about 200 gm thick, low doped 4H-SiC layers grown on n-type 8 degrees off-axis Si-face substrates at growth rates around 100 mu m/h has been done in order to realize thick epitaxial layers with excellent morphology suitable for high power devices. The study was done in a hot wall chemical vapor deposition reactor without rotation. The growth of such thick layers required favorable pre-growth conditions and in-situ etch. The growth of 190 gm thick, low doped epitaxial layers with excellent morphology was possible when the C/Si ratio was below 0.9. A low C/Si ratio and a favorable in-situ etch are shown to be the key parameters to achieve 190 gm thick epitaxial layers with excellent morphology.

  • 160.
    Yazdi, Gholamreza
    et al.
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Akhtar, Fatima
    IHP, Germany.
    Ivanov, Ivan Gueorguiev
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Schmidt, Susann
    Linköping University, Department of Physics, Chemistry and Biology, Thin Film Physics. Linköping University, Faculty of Science & Engineering.
    Shtepliuk, Ivan
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Zakharov, Alexei
    Lund Univ, Sweden.
    Iakimov, Tihomir
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Yakimova, Rositsa
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    Effect of epitaxial graphene morphology on adsorption of ambient species2019In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 486, p. 239-248Article in journal (Refereed)
    Abstract [en]

    This work illustrates the impact of atmospheric gases on the surface of epitaxial graphene. The different rate of adsorption on different parts of graphene samples provides a concrete evidence that the surface morphology of graphene plays a significant role in this process. The uneven adsorption occurs only on the surface of the monolayer graphene and not on bilayer graphene. The second monolayer is distinguished and verified by the phase contrast mode of atomic force microscopy and the low energy electron microscopy, respectively. Raman spectroscopy is used to study the strain on the surface of graphene; results indicate that monolayer and bilayer graphene exhibit different types of strain. The bilayer is under more compressive strain in comparison with monolayer graphene that hinders the process of adsorption. However, the wrinkles and edges of steps of the bilayer are under tensile strain, hence, facilitate adsorption. Samples were subjected to X-ray photoelectron spectroscopy which confirms that the adsorbates on the epitaxial graphene are carbon clusters with nitrogen and oxygen contamination. For reversing the adsorption process the samples are annealed and a method for the removal of these adsorbates is proposed.

1234 151 - 160 of 160
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