Dr.-Ing. Claudia Bock

Electronic Materials and Nanoelectronics
Ruhr-Universität Bochum

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  • A study on the influence of ligand variation on formamidinate complexes of yttrium: New precursors for atomic layer deposition of yttrium oxide
    Beer, S.M.J. and Boysen, N. and Muriqi, A. and Zanders, D. and Berning, T. and Rogalla, D. and Bock, C. and Nolan, M. and Devi, A.
    Dalton Transactions 50 (2021)
    The synthesis and characterization of a series of closely related Y(iii) compounds comprising the formamidinate ligands (RNCHNR) (R = alkyl) is reported, with the scope of using them as prospective precursors for atomic layer deposition (ALD) of yttrium oxide (Y2O3) thin films. The influence of the side chain variation on the thermal properties of the resulting complexes is studied and benchmarked by thermal analysis and vapor pressure measurements. Density functional theory (DFT) studies give theoretical insights into the reactivity of the compounds towards water, which was targeted as a co-reactant for the deposition of Y2O3via thermal ALD in the next step. Among the four complexes analyzed, tris(N,N′-di-tert-butyl-formamidinato)yttrium(iii) [Y(tBu2-famd)3] 1 was found to possess enhanced thermal stability and was selected for Y2O3 ALD process development. A broad ALD window ranging from 200 °C to 325 °C was obtained, yielding films of high compositional quality. Furthermore, with a film density of (4.95 ± 0.05) g cm-1 close to the bulk value, polycrystalline fcc Y2O3 layers with a smooth topography resulted in promising dielectric properties when implemented in metal insulator semiconductor (MIS) capacitor structures. © 2021 The Royal Society of Chemistry.
    view abstract10.1039/d1dt01634b
  • Atomic layer deposition of dielectric Y2O3thin films from a homoleptic yttrium formamidinate precursor and water
    Boysen, N. and Zanders, D. and Berning, T. and Beer, S.M.J. and Rogalla, D. and Bock, C. and Devi, A.
    RSC Advances 11 (2021)
    We report the application of tris(N,N′-diisopropyl-formamidinato)yttrium(iii) [Y(DPfAMD)3] as a promising precursor in a water-assisted thermal atomic layer deposition (ALD) process for the fabrication of high quality Y2O3 thin films in a wide temperature range of 150 °C to 325 °C. This precursor exhibits distinct advantages such as improved chemical and thermal stability over the existing Y2O3 ALD precursors including the homoleptic and closely related yttrium tris-amidinate [Y(DPAMD)3] and tris-guanidinate [Y(DPDMG)3], leading to excellent thin film characteristics. Smooth, homogeneous, and polycrystalline (fcc) Y2O3 thin films were deposited at 300 °C with a growth rate of 1.36 Å per cycle. At this temperature, contamination levels of C and N were under the detectable limits of nuclear reaction analysis (NRA), while X-ray photoelectron spectroscopy (XPS) measurements confirmed the high purity and stoichiometry of the thin films. From the electrical characterization of metal-insulator-semiconductor (MIS) devices, a permittivity of 13.9 at 1 MHz could be obtained, while the electric breakdown field is in the range of 4.2 and 6.1 MV cm-1. Furthermore, an interface trap density of 1.25 × 1011 cm-2 and low leakage current density around 10-7 A cm-2 at 2 MV cm-1 are determined, which satisfies the requirements of gate oxides for complementary metal-oxide-semiconductor (CMOS) based applications. © 2021 The Royal Society of Chemistry.
    view abstract10.1039/d0ra09876k
  • Cobalt Metal ALD: Understanding the Mechanism and Role of Zinc Alkyl Precursors as Reductants for Low-Resistivity Co Thin Films
    Zanders, D. and Liu, J. and Obenlüneschloß, J. and Bock, C. and Rogalla, D. and Mai, L. and Nolan, M. and Barry, S.T. and Devi, A.
    Chemistry of Materials (2021)
    In this work, we report a new and promising approach toward the atomic layer deposition (ALD) of metallic Co thin films. Utilizing the simple and known CoCl2(TMEDA) (TMEDA = N,N,N′,N′-tetramethylethylenediamine) precursor in combination with the intramolecularly stabilized Zn aminoalkyl compound Zn(DMP)2 (DMP = dimethylaminopropyl) as an auxiliary reducing agent, a thermal ALD process is developed that enables the deposition of Zn-free Co thin films. ALD studies demonstrate the saturation behavior of both precursors and linearity depending on the applied number of cycles as well as temperature dependency of film growth in a regime of 140-215 °C. While the process optimization is carried out on Si with native oxide, additional growth studies are conducted on Au and Pt substrates. This study is complemented by initial reactivity and suitability tests of several potential Zn alkyl-reducing agents. For the CoCl2(TMEDA)-Zn(DMP)2 combination, these findings allow us to propose a series of elemental reaction steps hypothetically leading to pure Co film formation in the ALD process whose feasibility is probed by a set of density functional theory (DFT) calculations. The DFT results show that for reactions of the precursors in the gas phase and on Co(111) substrate surfaces, a pathway involving C-C coupling and diamine formation through reductive elimination of an intermediate Co(II) alkyl species is preferred. Co thin films with an average thickness of 10-25 nm obtained from the process are subjected to thorough analysis comprising atomic force microscopy, scanning electron microscopy, and Rutherford backscattering spectrometry/nuclear reaction analysis as well as depth profiling X-ray photoemission spectroscopy (XPS). From XPS analysis, it was found that graphitic and carbidic carbon coexist in the Co metal film bulk. Despite carbon concentrations of ∼20 at. % in the Co thin film bulk, resistivity measurements for ∼22 nm thick films grown on a defined SiO2 insulator layer yield highly promising values in a range of 15-20 μω cm without any postgrowth treatment. © 2021 American Chemical Society.
    view abstract10.1021/acs.chemmater.1c00877
  • From Precursor Chemistry to Gas Sensors: Plasma-Enhanced Atomic Layer Deposition Process Engineering for Zinc Oxide Layers from a Nonpyrophoric Zinc Precursor for Gas Barrier and Sensor Applications
    Mai, L. and Mitschker, F. and Bock, C. and Niesen, A. and Ciftyurek, E. and Rogalla, D. and Mickler, J. and Erig, M. and Li, Z. and Awakowicz, P. and Schierbaum, K. and Devi, A.
    Small 16 (2020)
    The identification of bis-3-(N,N-dimethylamino)propyl zinc ([Zn(DMP)2], BDMPZ) as a safe and potential alternative to the highly pyrophoric diethyl zinc (DEZ) as atomic layer deposition (ALD) precursor for ZnO thin films is reported. Owing to the intramolecular stabilization, BDMPZ is a thermally stable, volatile, nonpyrophoric solid compound, however, it possesses a high reactivity due to the presence of Zn-C and Zn-N bonds in this complex. Employing this precursor, a new oxygen plasma enhanced (PE)ALD process in the deposition temperature range of 60 and 160 °C is developed. The resulting ZnO thin films are uniform, smooth, stoichiometric, and highly transparent. The deposition on polyethylene terephthalate (PET) at 60 °C results in dense and compact ZnO layers for a thickness as low as 7.5 nm with encouraging oxygen transmission rates (OTR) compared to the bare PET substrates. As a representative application of the ZnO layers, the gas sensing properties are investigated. A high response toward NO2 is observed without cross-sensitivities against NH3 and CO. Thus, the new PEALD process employing BDMPZ has the potential to be a safe substitute to the commonly used DEZ processes. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smll.201907506
  • Femtosecond laser patterning of graphene electrodes for thin-film transistors
    Kasischke, M. and Subaşı, E. and Bock, C. and Pham, D.-V. and Gurevich, E.L. and Kunze, U. and Ostendorf, A.
    Applied Surface Science 478 (2019)
    The aim of this study is to assess femtosecond laser patterning of graphene in air and in vacuum for the application as source and drain electrodes in thin-film transistors (TFTs). The analysis of the laser-patterned graphene with scanning electron microscopy, atomic force microscopy and Raman spectroscopy showed that processing in vacuum leads to less debris formation and thus re-deposited carbonaceous material on the sample compared to laser processing in air. It was found that the debris reduction due to patterning in vacuum improves the TFT characteristics significantly. Hysteresis disappears, the mobility is enhanced by an order of magnitude and the subthreshold swing is reduced from S sub = 2.5 V/dec to S sub = 1.5 V/dec. © 2019 Elsevier B.V.
    view abstract10.1016/j.apsusc.2019.01.198
  • Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Tin(IV) Oxide from a Functionalized Alkyl Precursor: Fabrication and Evaluation of SnO2-Based Thin-Film Transistor Devices
    Mai, L. and Zanders, D. and Subaşl, E. and Ciftyurek, E. and Hoppe, C. and Rogalla, D. and Gilbert, W. and Arcos, T.D.L. and Schierbaum, K. and Grundmeier, G. and Bock, C. and Devi, A.
    ACS Applied Materials and Interfaces (2019)
    A bottom-up process from precursor development for tin to plasma-enhanced atomic layer deposition (PEALD) for tin(IV) oxide and its successful implementation in a working thin-film transistor device is reported. PEALD of tin(IV) oxide thin films at low temperatures down to 60 °C employing tetrakis-(dimethylamino)propyl tin(IV) [Sn(DMP)4] and oxygen plasma is demonstrated. The liquid precursor has been synthesized and thoroughly characterized with thermogravimetric analyses, revealing sufficient volatility and long-term thermal stability. [Sn(DMP)4] demonstrates typical saturation behavior and constant growth rates of 0.27 or 0.42 Å cycle-1 at 150 and 60 °C, respectively, in PEALD experiments. Within the ALD regime, the films are smooth, uniform, and of high purity. On the basis of these promising features, the PEALD process was optimized wherein a 6 nm thick tin oxide channel material layer deposited at 60 °C was applied in bottom-contact bottom-gate thin-film transistors, showing a remarkable on/off ratio of 107 and field-effect mobility of μFE ≈ 12 cm2 V-1 s-1 for the as-deposited thin films deposited at such low temperatures. © 2019 American Chemical Society.
    view abstract10.1021/acsami.8b16443
  • PEALD of HfO2 Thin Films: Precursor Tuning and a New Near-Ambient-Pressure XPS Approach to in Situ Examination of Thin-Film Surfaces Exposed to Reactive Gases
    Zanders, D. and Ciftyurek, E. and Subaşl, E. and Huster, N. and Bock, C. and Kostka, A. and Rogalla, D. and Schierbaum, K. and Devi, A.
    ACS Applied Materials and Interfaces 11 (2019)
    A bottom-up approach starting with the development of new Hf precursors for plasma-enhanced atomic layer deposition (PEALD) processes for HfO2 followed by in situ thin-film surface characterization of HfO2 upon exposure to reactive gases via near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) is reported. The stability of thin films under simulated operational conditions is assessed, and the successful implementation of HfO2 dielectric layers in metal-insulator-semiconductor (MIS) capacitors is demonstrated. Among the series of newly synthesized mono-guanidinato-tris-dialkyl-amido class of Hf precursors, one of them, namely, [Hf{2-(iPrN)2CNEtMe}(NEtMe)3], was representatively utilized with oxygen plasma, resulting in a highly promising low-temperature PEALD process at 60 °C. The new precursors were synthesized in the multigram scale and thoroughly characterized by thermogravimetric analyses, revealing high and tunable volatility reflected by appreciable vapor pressures and accompanied by thermal stability. Typical ALD growth characteristics in terms of linearity, saturation, and a broad ALD window with constant growth of 1.06 Å cycle-1 in the temperature range of 60-240 °C render this process very promising for fabricating high-purity smooth HfO2 layers. For the first time, NAP-XPS surface studies on selected HfO2 layers are reported upon exposure to reactive H2, O2, and H2O atmospheres at temperatures of up to 500 °C revealing remarkable stability against degradation. This can be attributed to the absence of surface defects and vacancies. On the basis of these promising results, PEALD-grown HfO2 films were used as dielectric layers in the MIS capacitor device fabrication exhibiting leakage current densities less than 10-7 A cm-2 at 2 MV cm-1 and permittivities of up to 13.9 without postannealing. © 2019 American Chemical Society.
    view abstract10.1021/acsami.9b07090
  • Role of gallium and yttrium dopants on the stability and performance of solution processed indium oxide thin-film transistors
    Jaehnike, F. and Pham, D.V. and Bock, C. and Kunze, U.
    Journal of Materials Chemistry C 7 (2019)
    We study the effect of gallium and yttrium doping on both the electrical performance and the stability of indium based metal-oxide thin-film transistors (MOTFTs) at varied concentrations. As the Ga (Y) content in the In1.0GaxOy (In1.0YxOy) channel material was increased to x = 0.1 the mobility of the MOTFTs degrades by a factor of 4. Thereby the temperature stress stability is clearly enhanced by increasing doping concentration: the onset voltage shift is reduced by a factor of 3 for both In1.0Ga0.1Oy and In1.0Y0.1Oy films compared to that in indium-oxide TFTs. Also the stability during negative bias stress (NBS) is improved since the strong oxygen binders Ga and Y prevent the desorption of oxygen at the surface. In contrast, the onset voltage shift during positive bias stress (PBS) of doped metal oxide TFTs is higher ΔVon = 12 V for InGaO (100:10) TFTs and ΔVon = 15 V for InYO ((100:10) TFTs) compared to that of intrinsic indium oxide TFTs (ΔVon = 6 V), which could be attributed to the generation of flat trap states at the dielectric/semiconductor interface. Doping with Ga and Y significantly enhances the temperature and NBS stability of TFTs and simultaneously degrades the performance. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8tc06270f
  • Direct Growth of MoS2 and WS2 Layers by Metal Organic Chemical Vapor Deposition
    Cwik, S. and Mitoraj, D. and Mendoza Reyes, O. and Rogalla, D. and Peeters, D. and Kim, J. and Schütz, H.M. and Bock, C. and Beranek, R. and Devi, A.
    Advanced Materials Interfaces 5 (2018)
    For the growth of 2D transition metal dichalcogenides, such as molybdenum (MoS2) and tungsten disulfides (WS2), metalorganic chemical vapor deposition (MOCVD) routes are favorable due to their superior scalability, the possibility to tune the processing temperatures by a proper choice of reactants thus avoiding the need for a postdeposition treatment. Herein, the first example of a promising MOCVD route for the direct fabrication of MoS2 and WS2 layers under moderate process conditions is reported. This straightforward route is successfully realized by the combination of metalorganic precursors of Mo or W bearing the amidinato ligand with just elemental sulfur. The formation of stoichiometric hexagonal 2H-MoS2 and 2H-WS2 is demonstrated which is confirmed by Raman, X-ray diffraction, and X-ray photoelectron spectroscopy studies. The deposited layers are evaluated for their electrocatalytic activity in hydrogen evolution reaction as a proof of principle for application in water splitting devices. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/admi.201800140
  • Water assisted atomic layer deposition of yttrium oxide using tris(N,N0-diisopropyl-2-dimethylamido-guanidinato) yttrium(III): Process development, film characterization and functional properties†
    Mai, L. and Boysen, N. and Subaşı, E. and De Los Arcos, T. and Rogalla, D. and Grundmeier, G. and Bock, C. and Lu, H.-L. and Devi, A.
    RSC Advances 8 (2018)
    We report a new atomic layer deposition (ALD) process for yttrium oxide (Y2O3) thin films using tris(N,N0-diisopropyl-2-dimethylamido-guanidinato) yttrium(III) [Y(DPDMG)3] which possesses an optimal reactivity towards water that enabled the growth of high quality thin films. Saturative behavior of the precursor and a constant growth rate of 1.1 Å per cycle confirm the characteristic self-limiting ALD growth in a temperature range from 175 C to 250 C. The polycrystalline films in the cubic phase are uniform and smooth with a root mean squared (RMS) roughness of 0.55 nm, while the O/Y ratio of 2.0 reveal oxygen rich layers with low carbon contaminations of around 2 at%. Optical properties determined via UV/Vis measurements revealed the direct optical band gap of 5.56 eV. The valuable intrinsic properties such as a high dielectric constant make Y2O3 a promising candidate in microelectronic applications. Thus the electrical characteristics of the ALD grown layers embedded in a metal insulator semiconductor (MIS) capacitor structure were determined which resulted in a dielectric permittivity of 11, low leakage current density (z107 A cm2 at 2 MV cm1) and high electrical breakdown fields (4.0–7.5 MV cm1). These promising results demonstrate the potential of the new and simple Y2O3 ALD process for gate oxide applications. © The Royal Society of Chemistry 2018.
    view abstract10.1039/c7ra13417g
  • Double-pulse injection seeding of a terahertz quantum cascade laser
    Markmann, S. and Nong, H. and Pal, S. and Hekmat, N. and Scholz, S. and Kukharchyk, N. and Ludwig, Ar. and Dhillon, S. and Tignon, J. and Marcadet, X. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    Double-pulse injection seeding is used to modify the spectral emission of a terahertz quantum cascade laser (THz QCL). Two broad-band THz pulses delayed in time imprint a modulation on the single THz pulse spectrum. The resulting seed enables modification of the QCL emission spectrum, even though, the spectral bandwidth of each THz pulse is much broader than the QCL gain bandwidth. For a proper time delay between the pulses, the seeded THz QCL emission can even be switched from a multimode to a single mode regime. © 2015 IEEE.
    view abstract10.1109/IRMMW-THz.2015.7327478
  • High-Quality Solution-Processed Silicon Oxide Gate Dielectric Applied on Indium Oxide Based Thin-Film Transistors
    Jaehnike, F. and Pham, D.V. and Anselmann, R. and Bock, C. and Kunze, U.
    ACS Applied Materials and Interfaces 7 (2015)
    A silicon oxide gate dielectric was synthesized by a facile sol-gel reaction and applied to solution-processed indium oxide based thin-film transistors (TFTs). The SiO<inf>x</inf> sol-gel was spin-coated on highly doped silicon substrates and converted to a dense dielectric film with a smooth surface at a maximum processing temperature of T = 350 °C. The synthesis was systematically improved, so that the solution-processed silicon oxide finally achieved comparable break downfield strength (7 MV/cm) and leakage current densities (<10 nA/cm2 at 1 MV/cm) to thermally grown silicon dioxide (SiO<inf>2</inf>). The good quality of the dielectric layer was successfully proven in bottom-gate, bottom-contact metal oxide TFTs and compared to reference TFTs with thermally grown SiO<inf>2</inf>. Both transistor types have field-effect mobility values as high as 28 cm2/(Vs) with an on/off current ratio of 108, subthreshold swings of 0.30 and 0.37 V/dec, respectively, and a threshold voltage close to zero. The good device performance could be attributed to the smooth dielectric/semiconductor interface and low interface trap density. Thus, the sol-gel-derived SiO<inf>2</inf> is a promising candidate for a high-quality dielectric layer on many substrates and high-performance large-area applications. (Graph Presented). © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b03105
  • Spectral modification of the laser emission of a terahertz quantum cascade laser induced by broad-band double pulse injection seeding
    Markmann, S. and Nong, H. and Pal, S. and Hekmat, N. and Scholz, S. and Kukharchyk, N. and Ludwig, Ar. and Dhillon, S. and Tignon, J. and Marcadet, X. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    Applied Physics Letters 107 (2015)
    We demonstrate by injection seeding that the spectral emission of a terahertz (THz) quantum cascade laser (QCL) can be modified with broad-band THz pulses whose bandwidths are greater than the QCL bandwidth. Two broad-band THz pulses delayed in time imprint a modulation on the single THz pulse spectrum. The resulting spectrum is used to injection seed the THz QCL. By varying the time delay between the THz pulses, the amplitude distribution of the QCL longitudinal modes is modified. By applying this approach, the QCL emission is reversibly switched from multi-mode to single mode emission. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4930993
  • Strong coupling of intersubband resonance in a single triangular well to a THz metamaterial
    Pal, S. and Nongt, H. and Markmann, S. and Kukharchyk, N. and Valentin, S.R. and Scholz, S. and Ludwig, Ar. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    IRMMW-THz 2015 - 40th International Conference on Infrared, Millimeter, and Terahertz Waves (2015)
    We investigate the strong light-matter interactions of intersubband resonances (ISRs) in a triangular quantum well to a THz metamaterial. The large tuning possibility of ISRs with a high quality epitaxial gate enables the device to be electrically driven in-and-out of the coupling regime. © 2015 IEEE.
    view abstract10.1109/IRMMW-THz.2015.7327816
  • Ultrawide electrical tuning of light matter interaction in a high electron mobility transistor structure
    Pal, S. and Nong, H. and Markmann, S. and Kukharchyk, N. and Valentin, S.R. and Scholz, S. and Ludwig, Ar. and Bock, C. and Kunze, U. and Wieck, A.D. and Jukam, N.
    Scientific Reports 5 (2015)
    The interaction between intersubband resonances (ISRs) and metamaterial microcavities constitutes a strongly coupled system where new resonances form that depend on the coupling strength. Here we present experimental evidence of strong coupling between the cavity resonance of a terahertz metamaterial and the ISR in a high electron mobility transistor (HEMT) structure. The device is electrically switched from an uncoupled to a strongly coupled regime by tuning the ISR with epitaxially grown transparent gate. The asymmetric potential in the HEMT structure enables ultrawide electrical tuning of ISR, which is an order of magnitude higher as compared to an equivalent square well. For a single heterojunction with a triangular confinement, we achieve an avoided splitting of 0.52THz, which is a significant fraction of the bare intersubband resonance at 2THz. © 2015, Nature Publishing Group. All rights reserved.
    view abstract10.1038/srep16812
  • Improved morphology and performance of solution-processed metal-oxide thin-film transistors due to a polymer based interface modifier
    Weber, C. and Oberberg, M. and Weber, D. and Bock, C. and Pham, D.V. and Kunze, U.
    Advanced Materials Interfaces 1 (2014)
    The influence of a polymer interface modifier on the performance of solution-processed indium-based metal-oxide (MO) thin-film transistors (TFTs) is investigated. We use the polymer ethoxylated polyethylenimine (PEIE). Compared to a reference sample this modification enhances the mobility by a factor of four, clearly reduces the contact and the sheet resistance, and decreases the charge carrier activation energy by about 20%. The improved electrical performance originates from both a reduced contact and a reduced sheet resistance of the TFTs. The molecular dipole of PEIE reduces the work function of the electrodes. Adversely the dipole enhances the off current and the trap density at the semiconductor/dielectric interface for bottom-contact transistors with small channel length. The substrate becomes highly polar with a PEIE-treatment. Accordingly, topographical studies of bottom-contact TFTs show a very similar MO film morphology on the electrodes and in the channel for modified TFTs, whereas in the untreated samples the film has a higher roughness on the electrodes than in the channel. TFTs in top-contact configuration with the polymer interface layer at the dielectric/semiconductor interface also show higher mobility compared to the reference MOTFTs which displays that the improved performance is due to the improved morphology of the MO film. The role of the polymer-based interface modifier ethoxylated polyethylenimine (PEIE) in metal-oxide thin-film transistors is studied in detail via a combination of morphology studies, work function determination, and electrical measurements. The enhanced performance of PEIE-treated TFTs results directly from both a reduced contact resistance and from an improved morphology leading to a reduced sheet resistance. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/admi.201400137
  • Influence of grain orientation on the local deformation mode induced by cavitation erosion in a CuSnNi alloy
    Stella, J. and Pohl, M. and Bock, C. and Kunze, U.
    Wear 316 (2014)
    The local deformation induced by vibratory cavitation erosion in a CuSnNi alloy was studied employing electron back-scattering diffraction and three-dimensional profiling. The study focused on evaluating initial plastic deformation corresponding to a group of 57 grains in order to correlate crystal orientation and local deformation morphologies. While detectable slip markings developed in grains oriented close to the 〈111〉 and 〈101〉 crystallographic directions during the incubation phase of the cavitation test, grains oriented close to 〈001〉 exhibited no visible slip markings and extensive formation of craters and hills. Furthermore, an analysis based on the average Schmid factors calculated for each grain satisfactorily reflects the transition for the mentioned deformation modes. The quantitative evaluation of the Schmid factor for all slip systems suggests a novel interpretation of the degradation phenomena observed in face-centered cubic materials exposed to a cavitating liquid. © 2014 Elsevier B.V.
    view abstract10.1016/j.wear.2014.04.010
  • Atomic layer deposition of Gd 2O 3 and Dy 2O 3: A study of the ALD characteristics and structural and electrical properties
    Xu, K. and Ranjith, R. and Laha, A. and Parala, H. and Milanov, A.P. and Fischer, R.A. and Bugiel, E. and Feydt, J. and Irsen, S. and Toader, T. and Bock, C. and Rogalla, D. and Osten, H.-J. and Kunze, U. and Devi, A.
    Chemistry of Materials 24 (2012)
    Gd 2O 3 and Dy 2O 3 thin films were grown by atomic layer deposition (ALD) on Si(100) substrates using the homoleptic rare earth guanidinate based precursors, namely, tris(N,N′- diisopropyl-2-dimethylamido-guanidinato)gadolinium(III) [Gd(DPDMG) 3] (1) and tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)dysprosium(III) [Dy(DPDMG) 3] (2), respectively. Both complexes are volatile and exhibit high reactivity and good thermal stability, which are ideal characteristics of a good ALD precursor. Thin Gd 2O 3 and Dy 2O 3 layers were grown by ALD, where the precursors were used in combination with water as a reactant at reduced pressure at the substrate temperature ranging from 150 °C to 350 °C. A constant growth per cycle (GPC) of 1.1 Å was obtained at deposition temperatures between 175 and 275 °C for Gd 2O 3, and in the case of Dy 2O 3, a GPC of 1.0 Å was obtained at 200-275 °C. The self-limiting ALD growth characteristics and the saturation behavior of the precursors were confirmed at substrate temperatures of 225 and 250 °C within the ALD window for both Gd 2O 3 and Dy 2O 3. Thin films were structurally characterized by grazing incidence X-ray diffraction (GI-XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) analyses for crystallinity and morphology. The chemical composition of the layer was examined by Rutherford backscattering (RBS) analysis and Auger electron spectroscopy (AES) depth profile measurements. The electrical properties of the ALD grown layers were analyzed by capacitance-voltage (C-V) and current-voltage (I-V) measurements. Upon subjection to a forming gas treatment, the ALD grown layers show promising dielectric behavior, with no hysteresis and reduced interface trap densities, thus revealing the potential of these layers as high-k oxide for application in complementary metal oxide semiconductor based devices. © 2012 American Chemical Society.
    view abstract10.1021/cm2020862
  • Atomic layer deposition of HfO 2 thin films employing a heteroleptic hafnium precursor
    Xu, K. and Milanov, A.P. and Parala, H. and Wenger, C. and Baristiran-Kaynak, C. and Lakribssi, K. and Toader, T. and Bock, C. and Rogalla, D. and Becker, H.-W. and Kunze, U. and Devi, A.
    Chemical Vapor Deposition 18 (2012)
    The application of a heteroleptic hafnium amide-guanidinate precursor for the deposition of HfO 2 thin films via a water-assisted atomic layer deposition (ALD) process is demonstrated for the first time. HfO 2 films are grown in the temperature range 100-300 °C using the compound [Hf(NMe 2) 2(NMe 2-Guan) 2] (1). This compound shows self-limiting ALD-type growth characteristics with growth rates of the order of 1.0-1.2 Å per cycle in the temperature range 100-225 °C. The saturation behavior and a linear dependence on film thickness as a function of number of cycles are verified at various temperatures within the ALD window. The as-deposited HfO 2 films are characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS), X-ray photoelectron spectroscopy (XPS), and electrical measurements. For a direct comparison of the precursor performance with that of the parent alkyl amide [Hf(NMe 2) 4] (2), ALD experiments are also performed employing compound 2 under similar process conditions, and in this case no typical ALD characteristics are observed. The application of a heteroleptic hafnium amide-guanidinate precursor [Hf(NMe 2) 2(NMe 2-Guan) 2] for the deposition of HfO 2 thin films via a water assisted ALD process has been demonstrated for the first time. This compound showed self-limiting ALD type growth characteristics with the growth rates as high as 1.0-1.2 Å per cycle in the temperature range 100-225 °C. Typical ALD characteristics such as saturation behavior and linear dependence on the film thickness as a function of number of cycles were verified at different temperatures within the ALD window. The as-deposited HfO 2 films were characterized by AFM, SEM, RBS, XPS and electrical measurements. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201106934
  • Influence of structural properties on ballistic transport in nanoscale epitaxial graphene cross junctions
    Bock, C. and Weingart, S. and Karaissaridis, E. and Kunze, U. and Speck, F. and Seyller, T.
    Nanotechnology 23 (2012)
    In this paper we investigate the influence of material and device properties on the ballistic transport in epitaxial monolayer graphene and epitaxial quasi-free-standing monolayer graphene. Our studies comprise (a)magneto-transport in two-dimensional (2D) Hall bars, (b)temperature- and magnetic-field-dependent bend resistance of unaligned and step-edge-aligned orthogonal cross junctions, and (c)the influence of the lead width of the cross junctions on ballistic transport. We found that ballistic transport is highly sensitive to scattering at the step edges of the silicon carbide substrate. A suppression of the ballistic transport is observed if the lead width of the cross junction is reduced from 50nm to 30nm. In a 50nm wide device prepared on quasi-free-standing graphene we observe a gradual transition from the ballistic into the diffusive transport regime if the temperature is increased from 4.2 to about 50K, although 2D Hall bars show a temperature-independent mobility. Thus, in 1D devices additional temperature-dependent scattering mechanisms play a pivotal role. © 2012 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/23/39/395203
  • MOCVD of ZnO films from bis(ketoiminato)Zn(II) precursors: Structure, morphology and optical properties
    Bekermann, D. and Ludwig, Ar. and Toader, T. and MacCato, C. and Barreca, D. and Gasparotto, A. and Bock, C. and Wieck, A.D. and Kunze, U. and Tondello, E. and Fischer, R.A. and Devi, A.
    Chemical Vapor Deposition 17 (2011)
    Two closely related bis(ketoiminato) zinc precursors, which are air stable and possess favorable properties for metal-organic (MO)CVD, are successfully employed for the growth of ZnO films on silicon and borosilicate glass substrates at temperatures between 400 and 700 °C. The as-deposited films are investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), nuclear reaction analysis (NRA), as well as by UV-vis absorption spectroscopy and photoluminescence (PL) measurements. The structure, morphology, and composition of the as-grown films show a strong dependence on the substrate temperature. The formation of pure and (001)-oriented wurtzite-type stoichiometric ZnO is observed. PL measurements are performed both at room temperature and 77 K, revealing a defect-free emission of ZnO films. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cvde.201006898
  • Influence of the growth conditions of epitaxial graphene on the film topography and the electron transport properties
    Weingart, S. and Bock, C. and Kunze, U. and Emtsev, K.V. and Seyller, Th. and Ley, L.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    In this work we report on temperature-dependent magnetotransport measurements on epitaxial graphene grown on SiC(0 0 0 1) under different preparation conditions. We demonstrate that the temperature dependence of the charge carrier density and mobility is correlated to the annealing conditions during the graphitization process. As recently shown, SiC substrates annealed in an Ar atmosphere near atmospheric pressure exhibit continuous monolayer graphene films over 2 - 3 μ m wide and more than 50 μ m long terraces. For these films we determine a constant charge carrier density in the range from 1.4 K up to room temperature and an electron mobility exceeding 3000 cm2 (V s)- 1 at low temperatures. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2009.11.006
  • atomic force microscopy

  • atomic layer deposition

  • laser

  • terahertz waves

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