Prof. Dr.-Ing. Ulrich Kunze

Electronic Materials and Nanoelectronics
Ruhr-Universität Bochum

Author IDs

  • Ballistic rectification based on inhomogeneous magnetic stray fields
    Szelong, M. and Ludwig, Ar. and Wieck, A.D. and Kunze, U.
    Journal of Applied Physics 125 (2019)
    We present a ballistic rectification effect in an orthogonal four-terminal cross junction where the symmetry is broken by local magnetic fields. The input current is injected between opposing branches and the current-free branches serve as potential probes. The local magnetic field is induced by two permalloy (Py) stripes with a magnetic single-domain structure, where one end of each stripe is positioned close to the junction center. The Py stripes are oriented such that an external in-plane magnetic field can magnetize them into two different main configurations having either equally or oppositely magnetized ends. Equal magnetic ends are expected to result in a Hall-effect device, while for opposite magnetic ends, the stray field should deflect the electrons into the same output lead for both current polarities, leading to a rectifying behavior. Here, we present the proof of concept for stray-field controlled transfer characteristics. First, we show by magnetic force microscopy that both configurations are stable and the Py stripes exhibit a remanent magnetic single-domain structure. Second, we demonstrate the influence of the remanent magnetization on the low-temperature dc characteristics which are superimposed by a parasitic background. Third, we present the extracted Hall and the rectified voltage which are, respectively, linearly and parabolically dependent on the input current up to ±55 μA. © 2019 Author(s).
    view abstract10.1063/1.5085714
  • 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
  • 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
  • Identification and separation of rectifier mechanisms in Si/SiGe ballistic cross junctions
    Von Pock, J.F. and Salloch, D. and Wieser, U. and Hackbarth, T. and Kunze, U.
    Journal of Applied Physics 121 (2017)
    Depending on the detailed geometry, gate voltage, and circuitry, nanoscale Si/SiGe cross junctions at low temperatures exhibit full-wave rectification arising from different mechanisms like change in the number of current-carrying modes, stationary ballistic charging of a current-free voltage lead, and hot-electron thermopower. We study the rectifier structures on high-mobility Si/SiGe heterostructures consisting of a straight voltage stem and oblique current-injecting leads. Local gate electrodes are used to control the electron density in the voltage or current channel. Compared to three-terminal Y-branch junctions, the four-terminal cross junction eliminates the mode effect. A gradual increase of output voltage as gate-voltage is reduced until threshold voltage is identified as contribution of hot-electron thermopower. Heating the initially cold reservoir from a second orthogonal cross junction eliminates the electron temperature gradient and suppresses the thermopower. Even if the operation as six-terminal device re-induces a mode-controlled contribution, we demonstrate that it is negligible. As expected, the ballistic signal can be reliably separated from other mechanisms by measurements under positive gate voltage. The ballistic voltage can be described by a parabolic function of the injected current and is proportional to the cosine of the injection angle. © 2017 Author(s).
    view abstract10.1063/1.4973279
  • Input-Current Addition in Closely Positioned Dual-Stage Ballistic Rectifiers
    Von Pock, J. and Wieser, U. and Kunze, U.
    Physical Review Applied 7 (2017)
    Injection-type ballistic rectifiers are realized as asymmetric cross junctions. Their output-voltage (Vout) input-current (Iin) characteristics are known as parabola Vout=α2Iin2, where α2 is the curvature coefficient. Previous experiments on widely separated dual-stage ballistic rectifiers yielded the total output signal to be the sum of that of the single stages Vout=α12Iin12+α22Iin22. Starting from an ostensive model containing the ballistic length as critical size, we show that sufficiently closely positioned stages lead to input-current addition Vout=(α1Iin1+α2Iin2)2. We demonstrate two consequences of current addition. First, the excess voltage with respect to voltage addition Vexc=2α1α2Iin1Iin2 is proportional to the product Iin1Iin2. Second, sign-dependent current cancellation results in a shift of the Vout (Iin1) parabola vertex at fixed Iin2 or, at Iin1=-Iin2, to vanishing output voltage. The regime of intermediate-stage separation is discussed in terms of partial overlap of the injected charge clouds, leading to a smaller excess voltage. The results should provide both a deeper insight into the microscopic mechanism of ballistic rectification and novel device applications. © 2017 American Physical Society.
    view abstract10.1103/PhysRevApplied.7.044023
  • Positive centre voltage in T-branch junctions on n-type GaAs/AlGaAs based on hydrodynamics
    Szelong, M. and Ludwig, Ar. and Wieck, A. and Kunze, U.
    Semiconductor Science and Technology 32 (2017)
    Nanoscale three-terminal T-branch junctions operated in pushpull fashion (VL=-VR = V0) commonly exhibit a nonzero voltage VC at the centre electrode. In principle, their sign corresponds to the conduction type of the semiconductor material. For example is VC < 0 for n-type conduction, independent of the origin of the effect which could be ballistic or diffusive mode control, hot-electron thermopower or, in Y-shaped junctions, ballistic charging. We report on orthogonal four-terminal junctions on high-mobility n-type GaAs/AlGaAs, with currentcarrying branches of varying length at constant widths of 410 nm and 320 nm, respectively. When operated at low temperatures as three-terminal devices, we show that under sufficiently large gate voltage the result is VC > 0. This is particularly pronounced at short branches where the mode effect is weak. Up to a current ILR ∼ 25 mA we observe VC ϵ ILR independent of the branch length, where VC ILR ∼ 70 W. Around V0 = 0 the centre voltage exhibits a parabolic behaviour, VC V0 , = k 2 where the curvature k is independent of electron density in the range n ≥ 4.4 × 1011 cm-2. As temperature rises k monotonically decreases, staying positive up to 77 K. The observation of the Gurzhi effect as a signature of the hydrodynamic transport regime suggests an explanation of the mechanism in terms of the electronic analogue of the Venturi effect. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6641/aa7d44
  • Quantization and anomalous structures in the conductance of Si/SiGe quantum point contacts
    Von Pock, J.F. and Salloch, D. and Qiao, G. and Wieser, U. and Hackbarth, T. and Kunze, U.
    Journal of Applied Physics 119 (2016)
    Quantum point contacts (QPCs) are fabricated on modulation-doped Si/SiGe heterostructures and ballistic transport is studied at low temperatures. We observe quantized conductance with subband separations up to 4 meV and anomalies in the first conductance plateau at 4e2/h. At a temperature of T = 22 mK in the linear transport regime, a weak anomalous kink structure arises close to 0.5(4e2/h), which develops into a distinct plateau-like structure as temperature is raised up to T = 4 K. Under magnetic field parallel to the wire up to B = 14 T, the anomaly evolves into the Zeeman spin-split level at 0.5(4e2/h), resembling the "0.7 anomaly" in GaAs/AlGaAs QPCs. Additionally, a zero-bias anomaly (ZBA) is observed in nonlinear transport spectroscopy. At T = 22 mK, a parallel magnetic field splits the ZBA peak up into two peaks. At B = 0, elevated temperatures lead to similar splitting, which differs from the behavior of ZBAs in GaAs/AlGaAs QPCs. Under finite dc bias, the differential resistance exhibits additional plateaus approximately at 0.8(4e2/h) and 0.2(4e2/h) known as "0.85 anomaly" and "0.25 anomaly" in GaAs/AlGaAs QPCs. Unlike the first regular plateau at 4e2/h, the 0.2(4e2/h) plateau is insensitive to dc bias voltage up to at least VDS = 80 mV, in-plane magnetic fields up to B = 15 T, and to elevated temperatures up to T = 25 K. We interpret this effect as due to pinching off one of the reservoirs close to the QPC. We do not see any indication of lifting of the valley degeneracy in our samples. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4945116
  • 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
  • Visibility of two-dimensional layered materials on various substrates
    Müller, M.R. and Gumprich, A. and Ecik, E. and Kallis, K.T. and Winkler, F. and Kardynal, B. and Petrov, I. and Kunze, U. and Knoch, J.
    Journal of Applied Physics 118 (2015)
    For the investigation of 2D layered materials such as graphene, transition-metal dichalcogenides, boron nitride, and their heterostructures, dedicated substrates are required to enable unambiguous identification through optical microscopy. A systematic study is conducted, focusing on various 2D layered materials and substrates. The simulated colors are displayed and compared with microscopy images. Additionally, the issue of defining an appropriate index for measuring the degree of visibility is discussed. For a wide range of substrate stacks, layer thicknesses for optimum visibility are given along with the resulting sRGB colors. Further simulations of customized stacks can be conducted using our simulation tool, which is available for download and contains a database featuring a wide range of materials. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4930574
  • Electron waveguide interferometers for spin-dependent transport experiments
    Chiatti, O. and Buchholz, S. and Kunze, U. and Reuter, D. and Wieck, A. and Fischer, S.
    Physica Status Solidi (B) Basic Research 251 (2014)
    Semiconductor nanostructures are of interest in the field of spintronics, because they allow us to employ special features of low-dimensional transport for spin polarization, manipulation, and detection. Proposals for spintronic devices are based on coherent transport and interference effects in extended electron waveguide (EWG) structures. We present the state-of-the-art of low-dimensional EWGs and Aharonov-Bohm (AB) interferometers, fabricated in GaAs/AlGaAs heterostructures. Low-temperature measurements in etched quantum point contacts (QPCs) show large subband spacing and, in the presence of a source-drain DC bias, the differential conductance is resonantly enhanced due to a many-body modification of the barrier potential because of spin fluctuations. Multiterminal asymmetric quantum wire rings (QRs) have been fabricated and AB conductance oscillations are used to detect electrostatically induced continuous phase shifts and π-phase jumps, due to resonances and reflections at cross-junctions. The effects of measurement configuration and non-equilibrium in QR structures are also studied. By embedding a QPC into the leads of a QR, coherent mode-filtered injection of electrons into a few-mode AB interferometer is demonstrated. Single-and few-mode spin transport in EWG interferometers is discussed and the opportunities offered by embedding quantum dots for studying interaction and spin effects are presented. The results encourage the investigation of spin-dependent quantum transport in extended EWG interferometers. © 2014 The Authors. Phys. Status Solidi B is published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssb.201350229
  • 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
  • Spin-resolved conductance quantization in InAs
    Lehmann, H. and Benter, T. and Von Ahnen, I. and Jacob, J. and Matsuyama, T. and Merkt, U. and Kunze, U. and Wieck, A.D. and Reuter, D. and Heyn, C. and Hansen, W.
    Semiconductor Science and Technology 29 (2014)
    We report on the quantized conductance through side- and top-gated InAs quantum point contacts and discuss its dependence on the temperature and on a magnetic field applied perpendicular to the sample plane. Even in the absence of a magnetic field we observe besides the integer steps in units of 2e 2/h spin-resolved steps in units of e2/h up to the highest occupied mode. A conductance anomaly at 0.7 × 2e2/h is found as well. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0268-1242/29/7/075010
  • 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
  • Nano photoelectron ioniser chip using LaB 6 for ambient pressure trace gas detection
    Zimmer, C.M. and Kieschnick, M. and Kallis, K.T. and Schubert, J. and Kunze, U. and Doll, T.
    Microelectronic Engineering 98 (2012)
    A detector including a nanoscaled ioniser chip that surmounts the limitation of conventional photo ionisation detectors is presented. Here, ionisable gaseous substances can be detected by photoelectrons accelerated to the ionisation potential of the incoming gaseous molecules. Thin lanthanum hexaboride (LaB 6) films deposited by pulsed laser technique (PLD) serve as the air stable photocathode material representing the basis of the ioniser chip of the detector. Besides the analysis of the emission behaviour of the photocathode in vacuum and at atmospheric pressure, the detection of different volatile alcohols using the detector with a low-energy UV LED instead of a PID (VUV photon source) was also performed successfully. © 2012 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2012.07.094
  • Influence of channel width on the performance of an injection-type ballistic rectifier: Carrier injection versus hot-electron thermopower
    Salloch, D. and Wieser, U. and Kunze, U. and Hackbarth, T.
    Microelectronic Engineering 88 (2011)
    Injection-type ballistic rectification is achieved in an asymmetric Si/SiGe cross junction made from narrow channels, where the injectors are inclined with respect to the straight voltage stem. In this geometry an inertial-ballistic signal establishes due to the momentum direction of the injected electrons towards the lower part of the central voltage stem. Additionally, a diffusion hot-electron thermopower signal is superimposed which arises from an electron temperature gradient over a gate-confined region of the stem channel. We investigate the influence of the stem width on the rectifier performance. The inertial-ballistic signal disappears at a stem width larger than the 2D electron mean free path in contrast to the diffusion thermopower, which is independent of the stem width. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.mee.2011.02.079
  • Many-body enhanced nonlinear conductance resonance in quantum channels
    Han, J.E. and Fischer, S.F. and Buchholz, S.S. and Kunze, U. and Reuter, D. and Wieck, A.D. and Bird, J.P.
    Physical Review B - Condensed Matter and Materials Physics 84 (2011)
    We measure a strong enhancement of the nonlinear differential conductance (g=dI/dV), the amplitude of which exceeds the universal quantum conductance (2e2/h), under finite bias voltage in quantum point contacts (QPCs). By developing a spin-based model in the low-electron-density limit, we demonstrate that this resonance is an intrinsic nonequilibrium phenomenon that arises from many-body induced modifications to the QPC potential. A comparison with the linear conductance (G=I/V) shows that this phenomenon is driven by many-body dynamics within a single one-dimensional sub-band. © 2011 American Physical Society.
    view abstract10.1103/PhysRevB.84.193302
  • 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
  • Mode-filtered electron injection into a waveguide interferometer
    Buchholz, S.S. and Kunze, U. and Reuter, D. and Wieck, A.D. and Fischer, S.F.
    Applied Physics Letters 98 (2011)
    Injection of mode-filtered electrons into a phase-sensitive four-terminal waveguide Aharonov-Bohm (AB) ring is studied. An individually tuneable quantum point contact (QPC) in a waveguide lead of the GaAs/AlGaAs-ring allows to selectively couple to one-dimensional modes in the ring. Thus, we demonstrate single-mode transport in a multimode waveguide structure. Coherent mode-filtering by the lowest QPC subband is verified by nonlocal bend resistance and phase-sensitive AB interference measurements. © 2011 American Institute of Physics.
    view abstract10.1063/1.3563714
  • Nanoscale photoelectron ionisation detector based on lanthanum hexaboride
    Zimmer, C.M. and Schubert, J. and Hamann, S. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 (2011)
    A nanoscale ioniser is presented exceeding the limitation of conventional photoionisation detectors. It employs accelerated photoelectrons that allow obtaining molecule specificity by the tuning of ionisation energies. The material lanthanum hexaboride (LaB 6) is used as air stable photo cathode. Thin films of that material deposited by pulsed laser deposition (PLD) show quantum efficiency (QE) in the range of 10 -5 which is comparable to laser photo stimulation results. A careful treatment of the material yields reasonable low work functions even after surface reoxidation which opens up the possibility of using ultraviolet light emitting diodes (UV LEDs) in replacement of discharge lamps. Schematic diagram of a photoelectron ionisation detector (PeID) operating by an electron emitter based on the photoelectric effect of lanthanum hexaboride. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201000966
  • Separation of the inertial-ballistic signal from hot-electron thermopower in an injection-type ballistic rectifier
    Salloch, D. and Wieser, U. and Kunze, U. and Hackbarth, T.
    AIP Conference Proceedings 1399 (2011)
    In a ballistic rectifier consisting of an asymmetric cross junction the injection of electrons induce a nonlocal voltage in the current-free voltage stem, whose polarity is determined by the geometry and not by the direction of the input current. This inertial-ballistic signal is imposed by a hot-electron thermopower arising from a gradient in the electron temperature over a gate-confined region of the stem channel. We succeeded to separate both signal components by adding an orthogonal cross junction to the stem at the other side of the confined region. Current-induced heating of the electrons in the orthogonal leads purely causes hot-electron thermopower of opposite polarity. Appropriately designed geometry yields a difference signal which exclusively represents the inertial-ballistic component. © 2011 American Institute of Physics.
    view abstract10.1063/1.3666383
  • Ultrathin metal oxidation for vacuum monitoring device applications
    Mäder, S. and Haas, T. and Kunze, U. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 208 (2011)
    The oxide growth on thin metal films at room temperature has been investigated in terms of resistance change during oxidation. These data have been interpreted using the extended Cabrera-Mott theory of oxidation by Boggio. The resulting oxide thickness as well as the oxidation kinetics was found to depend on pressure. According to this dependence, oxidation of ultrathin metal films can be applied for monitoring the vacuum quality inside an evacuated environment. The performance of aluminum and copper sensing layers are compared with respect to sensor lifetime and response. Furthermore, the theoretically evaluated and resistively measured oxide thicknesses are verified by TEM studies. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201000921
  • Aharonov-Bohm oscillation phase shift in a multi-terminal asymmetric quantum ring
    Buchholz, S.S. and Fischer, S.F. and Kunze, U. and Reuter, D. and Wieck, A.D.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    We investigate coherent electron transport in a four-terminal asymmetric waveguide quantum ring with the focus on a controllable electrostatic Aharonov-Bohm (AB) oscillation phase shift. In the AlGaAs/GaAs device, electron wave interference has been detected in all possible four-terminal measurement configuration at a temperature as high as T = 1.5 K. We present a series of AB measurements in small magnetic fields for successive global gate voltages, which strongly suggest an electrostatic AB phase shift. Such a phase shift can be explained by the asymmetry of the ring and indicates that the design and positioning of voltage and current probes to the ring have a significant influence. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2009.11.123
  • Co-adsorption processes, kinetics and quantum mechanical modelling of nanofilm semiconductor gas sensors
    Velasco-Vélez, J.-J. and Kunze, U. and Haas, T. and Doll, T.
    Physica Status Solidi (A) Applications and Materials Science 207 (2010)
    A quantum mechanical model of co-adsorption on semiconductor surfaces is developed and successfully adopted towards exposure to several gases. It is related to nanofilms and thus allows the application of electric fields altering the electronic surface properties of adsorption centres (electroadsorptive effect, EAE). The model is matched against experimental data with O 2, NO 2 and CO measurements under the hypothesis of no direct interaction among the species. However the sequence of adsorption plays an important role where the adsorption of one gas species is opening up other sites that are filled by another sort of impinging molecules. Quantum mechanical modelling of co-adsorption: (a) NO 2 and CO present at the SnO 2 surface. (b) Simplified model. (c) Probability of adsorption. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.200983322
  • Control of the transmission phase in an asymmetric four-terminal Aharonov-Bohm interferometer
    Buchholz, S.S. and Fischer, S.F. and Kunze, U. and Bell, M. and Reuter, D. and Wieck, A.D.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Phase sensitivity and thermal dephasing in coherent electron transport in quasi-one-dimensional (1D) waveguide rings of an asymmetric four-terminal geometry are studied by magnetotransport measurements. We demonstrate the electrostatic control of the phase in Aharonov-Bohm resistance oscillations and investigate the impact of the measurement circuitry on decoherence. Phase rigidity is broken due to the ring geometry: orthogonal waveguide cross junctions and 1D leads minimize reflections and resonances between leads allowing for a continuous electron transmission phase shift. The measurement circuitry influences dephasing: thermal averaging dominates in the nonlocal measurement configuration while additional influence of potential fluctuations becomes relevant in the local configuration. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.045432
  • Efficient injection-type ballistic rectification in Si/SiGe cross junctions
    Salloch, D. and Wieser, U. and Kunze, U. and Hackbarth, T.
    Physica E: Low-Dimensional Systems and Nanostructures 42 (2010)
    Tunable inertial-ballistic rectification is studied in a nanoscale injection-type Si/SiGe rectifier in the hot-electron regime. The rectifier consists of a cascade of two nanoscale cross junctions in series. Two pairs of opposing current injectors merge under 30° into a straight central voltage stem. The electron densities in the injectors and the stem can be adjusted separately by two local top-gates. The measurements reveal a substantial efficiency increase for a nearly depleted stem. The efficiency of ballistic rectifiers can be expressed by the transfer resistance R T (output voltage divided by input current), the best value we achieve is 800Ω. © 2009 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.physe.2010.03.021
  • Full-wave rectification based upon hot-electron thermopower
    Wiemann, M. and Wieser, U. and Kunze, U. and Reuter, D. and Wieck, A.D.
    Applied Physics Letters 97 (2010)
    The hot-electron thermopower of a quantum point contact (QPC) is exploited for full-wave rectification at low temperatures. In a nanoscale AlGaAs/GaAs cross junction with orthogonal current and voltage leads the QPC is embedded into one voltage lead. The transfer resistance RT, given by the output voltage divided by input current, exhibits a distinct maximum at finite current and at gate voltages close to the QPC pinch-off voltage. Values in excess of RT =7k and output voltages up to 60% of the input voltages indicate an efficient ballistic rectification process. © 2010 American Institute of Physics.
    view abstract10.1063/1.3475922
  • 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
  • Nanofilm metal layers as vacuum quality sensors
    Mader, S. and Haas, T. and Kunze, U. and Doll, T.
    Procedia Engineering 5 (2010)
    A monitoring device for vacuum quality is realized by lowest cost single use oxygen sensors for vacuum insulation panels. They use the pressure dependence of oxide layer growth thickness on electrically measured metal nanofUms. These films were manufactured by e-beam evaporation , characterized in terms of resistance change with subsequent modeling of underlying mechanisms.
    view abstract10.1016/j.proeng.2010.09.313
  • Phase shifts and phase π jumps in four-terminal waveguide aharonov-bohm interferometers
    Kreisbeck, C. and Kramer, T. and Buchholz, S.S. and Fischer, S.F. and Kunze, U. and Reuter, D. and Wieck, A.D.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    Quantum coherent properties of electrons can be studied in Aharonov-Bohm (AB) interferometers. We investigate both experimentally and theoretically the transmission phase evolution in a four-terminal quasi-one-dimensional AlGaAs/GaAs-based waveguide AB ring. As main control parameter besides the magnetic field, we tune the Fermi wave number along the pathways using a top-gate. Our experimental results and theoretical calculations demonstrate the strong influence of the measurement configuration upon the AB-resistance- oscillation phase in a four-terminal device. While the nonlocal setup displays continuous phase shifts of the AB oscillations, the phase remains rigid in the local voltage-probe setup. Abrupt phase jumps are found in all measurement configurations. We analyze the phase shifts as functions of the magnetic field and the Fermi energy and provide a detailed theoretical model of the device. Scattering and reflections in the arms of the ring are the source of abrupt phase jumps by π. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.165329
  • magnetoelectronics

  • nanoelectronics

  • nanostructures

  • quantum dots

  • semiconductors

  • terahertz waves

  • thin films

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