Dr.-Ing. Wolfgang Mertin

Electrical Engineering and Information Technology
University of Duisburg-Essen

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  • Direct growth of graphene on GaN via plasma-enhanced chemical vapor deposition under N2 atmosphere
    Mischke, J. and Pennings, J. and Weisenseel, E. and Kerger, P. and Rohwerder, M. and Mertin, W. and Bacher, G.
    2D Materials 7 (2020)
    One of the bottlenecks in the implementation of graphene as a transparent electrode in modern opto-electronic devices is the need for complicated and damaging transfer processes of high-quality graphene sheets onto the desired target substrates. Here, we study the direct, plasma-enhanced chemical vapor deposition (PECVD) growth of graphene on GaN-based light-emitting diodes (LEDs). By replacing the commonly used hydrogen (H2) process gas with nitrogen (N2), we were able to suppress GaN surface decomposition while simultaneously enabling graphene deposition at <800 °C in a single-step growth process. Optimizing the methane (CH4) flow and varying the growth time between 0.5 h and 8 h, the electro-optical properties of the graphene layers could be tuned to sheet resistances as low as ∼1 kΩ/D with a maximum transparency loss of ∼12%. The resulting high-quality graphene electrodes show an enhanced current spreading effect and an increase of the emission area by a factor of ∼8 in operating LEDs. © 2020 The Author(s).
    view abstract10.1088/2053-1583/ab8969
  • Direct growth of graphene on Ge(100) and Ge(110) via thermal and plasma enhanced CVD
    Bekdüz, B. and Kaya, U. and Langer, M. and Mertin, W. and Bacher, G.
    Scientific Reports 10 (2020)
    The integration of graphene into CMOS compatible Ge technology is in particular attractive for optoelectronic devices in the infrared spectral range. Since graphene transfer from metal substrates has detrimental effects on the electrical properties of the graphene film and moreover, leads to severe contamination issues, direct growth of graphene on Ge is highly desirable. In this work, we present recipes for a direct growth of graphene on Ge via thermal chemical vapor deposition (TCVD) and plasma-enhanced chemical vapor deposition (PECVD). We demonstrate that the growth temperature can be reduced by about 200 °C in PECVD with respect to TCVD, where usually growth occurs close to the melting point of Ge. For both, TCVD and PECVD, hexagonal and elongated morphology is observed on Ge(100) and Ge(110), respectively, indicating the dominant role of substrate orientation on the shape of graphene grains. Interestingly, Raman data indicate a compressive strain of ca. − 0.4% of the graphene film fabricated by TCVD, whereas a tensile strain of up to + 1.2% is determined for graphene synthesized via PECVD, regardless the substrate orientation. Supported by Kelvin probe force measurements, we suggest a mechanism that is responsible for graphene formation on Ge and the resulting strain in TCVD and PECVD. © 2020, The Author(s).
    view abstract10.1038/s41598-020-69846-7
  • Graphene growth through a recrystallization process in plasma enhanced chemical vapor deposition
    Bekdüz, B. and Beckmann, Y. and Mischke, J. and Twellmann, J. and Mertin, W. and Bacher, G.
    Nanotechnology 29 (2018)
    Thermal chemical vapor deposition (TCVD) is the current method of choice to fabricate high quality, large area graphene films on catalytic copper substrates. In order to obtain sufficiently high growth rates at reduced growth temperatures an efficient dissociation of the precursor molecules already in the gas phase is required. We used plasma enhanced chemical vapor deposition (PECVD) to fabricate high quality graphene films at various temperatures. The efficient, plasma-induced dissociation of the precursor molecules results in an activation energy of 2.2 eV for the growth rate in PECVD, which is reduced by almost a factor of 2 compared to TCVD growth in the same reactor. By varying the growth time, we demonstrate that crystalline graphene grains surrounded by amorphous carbon formed during the early stage of growth merge into an almost defect-free graphene film with growth time via a recrystallization process. Almost defect-free graphene is prepared with negligible (I D/I G < 0.1) contributions of the D peak in Raman spectroscopy and with a sheet resistance down to 470 Ω/sq. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aadd74
  • Influence of atmospheric species on the electrical properties of functionalized graphene sheets
    Bekdüz, B. and Kampermann, L. and Mertin, W. and Punckt, C. and Aksay, I.A. and Bacher, G.
    RSC Advances 8 (2018)
    We report on the time-dependent influence of atmospheric species on the electrical properties of functionalized graphene sheets (FGSs). When exposed to laboratory air, FGSs exhibit a significant, irreversible decrease in electrical conductance with time, strongly depending on the oxygen content of the FGSs. To separate the roles of charge carrier density and mobility in this aging process, we performed electron transport measurements using a back-gate field-effect transistor architecture. Investigating the position of the Dirac point under different atmospheres, we found that adsorbed atmospheric species result in pronounced p-doping, which-on a short time scale-can be reversed under nitrogen atmosphere. However, on a time scale of several days, the resistance increases irreversibly, while the Dirac point voltage remains constant. From these experiments, we conclude that the aging of FGSs is related to the chemisorption of atmospheric species leading to enhanced carrier scattering due to an increasing amount of sp 3 - regions and thus to a reduced charge carrier mobility. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ra08227h
  • Low Resistive Edge Contacts to CVD-Grown Graphene Using a CMOS Compatible Metal
    Shaygan, M. and Otto, M. and Sagade, A.A. and Chavarin, C.A. and Bacher, G. and Mertin, W. and Neumaier, D.
    Annalen der Physik (2017)
    view abstract10.1002/andp.201600410
  • Relation between growth rate and structure of graphene grown in a 4″ showerhead chemical vapor deposition reactor
    Bekdüz, B. and Beckmann, Y. and Meier, J. and Rest, J. and Mertin, W. and Bacher, G.
    Nanotechnology 28 (2017)
    The chemical vapor deposition (CVD) growth of graphene on copper is controlled by a complex interplay of substrate preparation, substrate temperature, pressure and flow of reactive gases. A large variety of recipes have been suggested in literature, often quite specific to the reactor, which is being used. Here, we report on a relation between growth rate and quality of graphene grown in a scalable 4″ CVD reactor. The growth rate is varied by substrate pre-treatment, chamber pressure, and methane to hydrogen (CH4:H2) ratio, respectively. We found that at lower growth rates graphene grains become hexagonal rather than randomly shaped, which leads to a reduced defect density and a sheet resistance down to 268 Ω/sq. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/aa68a8
  • On the origin of contact resistances in graphene devices fabricated by optical lithography
    Chavarin, C.A. and Sagade, A.A. and Neumaier, D. and Bacher, G. and Mertin, W.
    Applied Physics A: Materials Science and Processing 122 (2016)
    The contact resistance is a key bottleneck limiting the performance of graphene-based electronic and optoelectronic devices. Using a combined approach of atomic force microscopy patterning, Kelvin probe force microscopy and micro-Raman mapping, we study the influence of optical lithography resists on the contact resistance in graphene devices. We find that devices fabricated by optical lithography show a significantly larger contact resistance compared to devices produced by electron beam lithography using polymethylmethacrylate as resist. This difference is attributed to a 3–4-nm-thick residual layer remaining in between the contact metal and the graphene after optical lithography. © 2016, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00339-015-9582-5
  • Implementation of graphene multilayer electrodes in quantum dot light-emitting devices
    Wolff, S. and Jansen, D. and Terlinden, H. and Kelestemur, Y. and Mertin, W. and Demir, H.V. and Bacher, G. and Nannen, E.
    Applied Physics A: Materials Science and Processing 120 (2015)
    Graphene is a highly attractive candidate for implementation as electrodes in next-generation large-area optoelectronic devices thanks to its high electrical conductivity and high optical transparency. In this study, we show all-solution-processed quantum dot-based light-emitting devices (QD-LEDs) using graphene mono- and multilayers as transparent electrodes. Here, the effect of the number of graphene layers (up to three) on the QD-LEDs performance was studied. While the implementation of a second graphene layer was found to reduce the turn-on voltage from 2.6 to 1.8 V, a third graphene layer was observed to increase the turn-on voltage again, which is attributed to an increased roughness of the graphene layer stack. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00339-015-9304-z
  • The effect of degree of reduction on the electrical properties of functionalized graphene sheets
    Punckt, C. and Muckel, F. and Wolff, S. and Aksay, I.A. and Chavarin, C.A. and Bacher, G. and Mertin, W.
    Applied Physics Letters 102 (2013)
    We study the effect of carbon to oxygen ratio (C/O) on the electrical resistance of functionalized graphene sheets prepared by thermal exfoliation and reduction of graphite oxide at various temperatures. Using a 2-probe technique in conjunction with Kelvin probe force microscopy, we observe a transition from high-resistance (>400 kΩ/sq) nonlinear current/voltage characteristics at low C/O to low-resistance (<10 kΩ/sq) linear behavior at high C/O, indicating a transition from hopping to diffusive electron transport. Simultaneously, the metal-graphene contacts change from high-resistance Schottky-type behavior to nearly non-invasive metal-metal contact characteristics. © 2013 American Institute of Physics.
    view abstract10.1063/1.4775582
  • Material and doping contrast in III/V nanowires probed by Kelvin probe force microscopy
    Vinaji, S. and Bacher, G. and Mertin, W.
    NanoScience and Technology 79 (2012)
    We have studied the local surface potential and the voltage drop along individual VLS grown GaAs nanowires using Kelvin probe force microscopy. With the obtained information, we identify a core-shell structure inGaAs/GaP hetero structure nanowires, which we attribute to the difference in radial and vertical growth between the two semiconductor materials. In p-doped GaAs nanowires, qualitative and quantitative doping levels are estimated. Furthermore, we find a better incorporation of the zinc compared to the carbon to realize doping in partially p-doped GaAs nanowires by localizing the doping transitions and estimating the width of their depletion layers. Additionally, the p-n junction can be localized with a resolution better than 50nm and the bias dependence of the depletion layer width can be studied. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-29656-7_8
  • Local voltage drop in a single functionalized graphene sheet characterized by Kelvin probe force microscopy
    Yan, L. and Punckt, C. and Aksay, I.A. and Mertin, W. and Bacher, G.
    Nano Letters 11 (2011)
    We studied the local voltage drop in functionalized graphene sheets of subμm size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 × 10 5 S/m corresponding to a sheet resistance of 2.7 kΩ/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found to be < 6.3 × 10-7 Ωcm2. © 2011 American Chemical Society.
    view abstract10.1021/nl201070c
  • Potential distribution in functionalized graphene devices probed by Kelvin probe force microscopy
    Yan, L. and Punckt, C. and Aksay, I.A. and Mertin, W. and Bacher, G.
    AIP Conference Proceedings 1399 (2011)
    Kelvin probe force microscopy was used to study the impact of contacts and topography on the local potential distribution on contacted, individual functionalized graphene sheets (FGS) deposited on a SiO 2/Si substrate. Negligible contact resistance is found at the graphene/Ti interface and a graphene resistance of 2.3 kΩ is extracted for a single sheet with sub-μm size. Pronounced steps in the topography, which we attribute to a variation of the spacing between graphene and substrate, result in a significant change of the local resistivity. © 2011 American Institute of Physics.
    view abstract10.1063/1.3666628
  • Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes
    Lysov, A. and Vinaji, S. and Offer, M. and Gutsche, C. and Regolin, I. and Mertin, W. and Geller, M. and Prost, W. and Bacher, G. and Tegude, F.-J.
    Nano Research 4 (2011)
    The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm2, which is important for potential applications in concentrator solar cells. © 2011 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12274-011-0155-4
  • electrical nanoanalytics

  • graphene

  • Kelvin probe force microscopy

  • microelectronic materials and devices

  • nanocharacterization

  • optoelectronics

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