Publications Search Results

Now showing 1 - 10 of 13
  • Publication
    Raman Spectroscopy as a Key Method to Distinguish the Ferroelectric Orthorhombic Phase in Thin ZrO2-Based Films
    ( 2022)
    Materano, M.
    ;
    Reinig, P.
    ;
    Kersch, A.
    ;
    Popov, M.
    ;
    Deluca, M.
    ;
    Mikolajick, T.
    ;
    Boettger, U.
    ;
    Schroeder, U.
    Inducing and detecting the polar orthorhombic phase are crucial for the establishment of ferroelectricity in HfO2- and ZrO2-based thin films. Unfortunately, commonly used structural characterization techniques such as grazing incidence angle X-ray diffraction (GIXRD) only partially allow an accurate detection of this crystalline phase, whose characteristic pattern almost coincides with the one of the tetragonal phase. As a consequence, phase determination is commonly based on peak deconvolution tracing the position of the main peak at 2TH values of around 30°, which can be assigned both to the t(101) and the o(111) plane directions and additionally be influenced by mechanical stress in the layers. Alternatively, epitaxial layers are required to differentiate the phase. Herein, using an integrated experimental-computational approach, it is shown how Raman spectroscopy can distinguish between the monoclinic, the tetragonal, and the orthorhombic phase of ZrO2. The Raman spectra calculated from first principles match the experimentally measured data and thus enable an unambiguous phase assignment. Therefore, Raman spectroscopy proves to be a powerful technique for discerning the three main crystalline phases in these materials. As demonstrated by the good agreement between structural and electrical data, it can therefore be used to predict ferroelectricity in the addressed layers.
  • Publication
    Symmetry and structure of carbon-nitrogen complexes in gallium arsenide from infrared spectroscopy and first-principles calculations
    ( 2018)
    Künneth, C.
    ;
    Kölbl, S.
    ;
    Wagner, H.E.
    ;
    Häublein, V.
    ;
    Kersch, A.
    ;
    Alt, H.C.
    Molecular-like carbon-nitrogen complexes in GaAs are investigated both experimentally and theoretically. Two characteristic high-frequency stretching modes at 1973 and 2060 cm−1, detected by Fourier transform infrared absorption (FTIR) spectroscopy, appear in carbon- and nitrogen-implanted and annealed layers. From isotopic substitution, it is deduced that the chemical composition of the underlying complexes is CN2 and C2N, respectively. Piezospectroscopic FTIR measurements reveal that both centers have tetragonal symmetry. For density functional theory (DFT) calculations, linear entities are substituted for the As anion, with the axis oriented along the ⟨1  0  0⟩ direction, in accordance with the experimentally ascertained symmetry. The DFT calculations support the stability of linear N-C-N and C-C-N complexes in the GaAs host crystal in the charge states ranging from + 3 to -3. The valence bonds of the complexes are analyzed using molecular-like orbitals from DFT. It turns out that internal bonds and bonds to the lattice are essentially independent of the charge state. The calculated vibrational mode frequencies are close to the experimental values and reproduce precisely the isotopic mass splitting from FTIR experiments. Finally, the formation energies show that under thermodynamic equilibrium CN2 is more stable than C2N.
  • Publication
    Impact of field cycling on HfO2 based non-volatile memory devices
    ( 2016)
    Schroeder, U.
    ;
    Pesic, M.
    ;
    Schenk, T.
    ;
    Mulaosmanovic, H.
    ;
    Slesazeck, S.
    ;
    Ocker, J.
    ;
    Richter, C.
    ;
    Yurchuk, E.
    ;
    Khullar, K.
    ;
    Müller, J.
    ;
    Polakowski, P.
    ;
    Grimley, E.D.
    ;
    LeBeau, J.M.
    ;
    Flachowsky, S.
    ;
    Jansen, S.
    ;
    Kolodinski, S.
    ;
    Bentum, R. van
    ;
    Kersch, A.
    ;
    Künneth, C.
    ;
    Mikolajick, T.
    The discovery of ferroelectricity in HfO2 and ZrO2 based dielectrics enabled the introduction of these materials in highly scalable non-volatile memory devices. Typical memory cells are using a capacitor or a transistor as the storage device. These scaled devices are sensitive to the local structure of the storage material, here the granularity of the dielectric doped HfO2 layer, varying the local ferroelectric properties. Detailed studies are conducted to correlate these structural properties to the electrical performance to further optimize the devices for future applications.
  • Publication
    Stabilizing the ferroelectric phase in doped hafnium oxide
    ( 2015)
    Hoffmann, M.
    ;
    Schroeder, U.
    ;
    Schenk, T.
    ;
    Shimizu, T.
    ;
    Funakubo, H.
    ;
    Sakata, O.
    ;
    Pohl, D.
    ;
    Drescher, M.
    ;
    Adelmann, C.
    ;
    Materlik, R.
    ;
    Kersch, A.
    ;
    Mikolajick, T.
    The ferroelectric properties and crystal structure of doped HfO2 thin films were investigated for different thicknesses, electrode materials, and annealing conditions. Metal-ferroelectric-metal capacitors containing Gd:HfO2 showed no reduction of the polarization within the studied thickness range, in contrast to hafnia films with other dopants. A qualitative model describing the influence of basic process parameters on the crystal structure of HfO2 was proposed. The influence of different structural parameters on the field cycling behavior was examined. This revealed the wake-up effect in doped HfO2 to be dominated by interface induced effects, rather than a field induced phase transition. TaN electrodes were shown to considerably enhance the stabilization of the ferroelectric phase in HfO2 compared to TiN electrodes, yielding a P-r of up to 35 mu C/cm(2). This effect was attributed to the interface oxidation of the electrodes during annealing, resulting in a different density of oxygen vacancies in the Gd:HfO2 films. Ab initio simulations confirmed the influence of oxygen vacancies on the phase stability of ferroelectric HfO2.
  • Publication
    Ferroelectricity and antiferroelectricity of doped thin HfO2-based films
    ( 2015)
    Park, M.H.
    ;
    Lee, Y.H.
    ;
    Kim, H.J.
    ;
    Kim, Y.J.
    ;
    Moon, T.
    ;
    Do Kim, K.
    ;
    Müller, J.
    ;
    Kersch, A.
    ;
    Schroeder, U.
    ;
    Mikolajick, T.
    ;
    Hwang, C.S.
    The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O-3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferroelectricity or antiferroelectricity in thin HfO2 fi lms. They have large remanent polarization of up to 45 mu C cm(-2), and their coercive field (approximate to 1-2 MV c(-1)) is larger than conventional ferroelectric fi lms by approximately one order of magnitude. Furthermore, they can be extremely thin (<10 nm) and have a large bandgap (>5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin fi lms is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.
  • Publication
    Evidence of single domain switching in hafnium oxide based FeFETs: Enabler for multi-level FeFET memory cells
    ( 2015)
    Mulaosmanovic, H.
    ;
    Slesazeck, S.
    ;
    Ocker, J.
    ;
    Pesic, M.
    ;
    Müller, S.
    ;
    Flachowsky, S.
    ;
    Müller, J.
    ;
    Polakowski, J.
    ;
    Paul, J.
    ;
    Jansen, S.
    ;
    Kolodinski, S.
    ;
    Richter, C.
    ;
    Piontek, S.
    ;
    Schenk, T.
    ;
    Kersch, A.
    ;
    Künneth, C.
    ;
    Bentum, R. van
    ;
    Schröder, U.
    ;
    Mikolajick, T.
    Recent discovery of ferroelectricity in HfO2 thin films paved the way for demonstration of ultra-scaled 28 nm Ferroelectric FETs (FeFET) as non-volatile memory (NVM) cells [1]. However, such small devices are inevitably sensible to the granularity of the polycrystalline gate oxide film. Here we report for the first time the evidence of single ferroelectric (FE) domain switching in such scaled devices. These properties are sensed in terms of abrupt threshold voltage (VT) shifts leading to stable intermediate VT levels. We emphasize that this feature enables multi-level cell (MLC) FeFETs and gives a new perspective on steep subthreshold devices based on ferroelectric HfO2.
  • Publication
    Ferroelectric hafnium oxide: A CMOS-compatible and highly scalable approach to future ferroelectric memories
    ( 2013)
    Müller, J.
    ;
    Böscke, T.S.
    ;
    Müller, S.
    ;
    Yurchuk, E.
    ;
    Polakowski, P.
    ;
    Paul, J.
    ;
    Martin, D.
    ;
    Schenk, T.
    ;
    Khullar, K.
    ;
    Kersch, A.
    ;
    Weinreich, W.
    ;
    Riedel, S.
    ;
    Seidel, K.
    ;
    Kumar, A.
    ;
    Arruda, T.M.
    ;
    Kalinin, S.V.
    ;
    Schlösser, T.
    ;
    Boschke, R.
    ;
    Bentum, R. van
    ;
    Schröder, U.
    ;
    Mikolajick, T.
    With the ability to engineer ferroelectricity in HfO2 thin films, manufacturable and highly scaled MFM capacitors and MFIS-FETs can be implemented into a CMOS-environment. NVM properties of the resulting devices are discussed and contrasted to existing perovskite based FRAM.
  • Publication
    Role of defect relaxation for trap-assisted tunneling in high-K thin films
    ( 2012)
    Jegert, G.
    ;
    Popescu, D.
    ;
    Lugli, P.
    ;
    Häufel, M.J.
    ;
    Weinreich, W.
    ;
    Kersch, A.
    We assess the impact of structural relaxation of defects upon charging on trap-assisted tunneling in high- dielectric materials. ZrO 2/Al 2O 3/ZrO2 thin films are taken as an exemplary system. In our completely different approach, a first-principles defect model is derived from Hedins GW approximation calculations, which is then coupled to kinetic Monte Carlo charge transport simulations. Comparison between simulation and experiment demonstrates that it is often imperative to take structural relaxation processes into account when modeling nanoscale transport across defect states.
  • Publication
    Ultimate scaling of TiN/ZrO2/TiN capacitors: Leakage currents and limitations due to electrode roughness
    ( 2011)
    Jegert, G.
    ;
    Kersch, A.
    ;
    Weinreich, W.
    ;
    Lugli, P.
    In this paper, we investigate the influence of electrode roughness on the leakage current in TiN/high-K ZrO2/TiN (TZT) thin-film capacitors which are used in dynamic random access memory cells. Based on a microscopic transport model, which is expanded to incorporate electrode roughness, we assess the ultimate scaling potential of TZT capacitors in terms of equivalent oxide thickness, film smoothness, thickness fluctuations, defect density and distribution, and conduction band offset (CBO). The model is based on three-dimensional, fully self-consistent, kinetic Monte Carlo transport simulations. Tunneling transport in the bandgap of the dielectric is treated, which includes defect-assisted transport mechanisms. Electrode roughness is described in the framework of fractal geometry. While the short-range roughness of the electrodes is found not to influence significantly the leakage current, thickness fluctuations of the dielectric have a major impact. For thinner dielectric films they cause a transformation of the dominant transport mechanism from Poole-Frenkel conduction to trap-assisted tunneling. Consequently, the sensitivity of the leakage current on electrode roughness drastically increases on downscaling. Based on the simulations, optimization of the CBO is suggested as the most viable strategy to extend the scalability of TZT capacitors over the next chip generations.
  • Publication
    Monte Carlo simulation of leakage currents in TiN/ZrO2/TiN capacitors
    ( 2011)
    Jegert, G.
    ;
    Kersch, A.
    ;
    Weinreich, W.
    ;
    Lugli, P.
    Leakage currents in TiN/high-K-ZrO2/TiN capacitors were simulated by using a novel kinetic Monte Carlo algorithm specially designed to describe tunneling transport of charge carriers in high-K dielectrics, including defect-assisted transport mechanisms. Comparing simulation results with experimental data, a model for electronic transport was established and validated. Transport was found to be dominated by Poole-Frenkel emission from positively charged bulk trap states at medium voltages and trap-assisted tunneling at high voltages. Information on the conduction band offset at the TiN/ZrO2 interface as well as on the trap depth was extracted. The model accurately describes the scaling of the leakage current with temperature and with thickness of the dielectric film, and it provides insight into the mutual interdependence of the competing transport mechanisms.