Kwang-Seok Kim

Samsung Advanced Institute of Technology, Usan-ri, Gyeonggi Province, South Korea

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Publications (12)22.64 Total impact

  • Journal- Korean Physical Society 07/2009; 55(1). · 0.51 Impact Factor
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    ABSTRACT: We investigated the top and bottom interfaces of a Co Fe B / Mg O / Co Fe B tunnel junction using transmission electron microscope (TEM) and x-ray photoemission spectroscopy (XPS) in order to understand the origin of the asymmetry of dI/dV in terms of bias polarity. It was found, from a TEM image, that there is no clear cut at the top interface, while the bottom interface has relatively clean boundary. Furthermore, XPS data show that more hydroxides were formed at the top interface than at the bottom interface. These indicate that the hydroxides would hinder the epitaxial crystallinity at the interface in Co Fe B / Mg O / Co Fe B tunnel junctions. Therefore, it is most likely that the asymmetry of dI/dV is caused by the disappearance of minority Bloch state, which is closely correlated with the existence of hydroxides at the top interface of a Co Fe B / Mg O / Co Fe B tunnel junction.
    Journal of Applied Physics 05/2009; · 2.21 Impact Factor
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    ABSTRACT: We investigated the out-of-plane anisotropy of Co<sub>50</sub>Fe<sub>50</sub>/Tb multilayer film with changing the thickness of Tb layer and applying an inplane filed of 200 Oe during film deposition. A strong out-of-plane anisotropy was found only for the multilayer with small thickness of Tb layer (t<sub>Tb</sub> = 1 nm) and was suppressed significantly by the application of an inplane magnetic field of 200 Oe during deposition, resulting in an inplane anisotropy. It was also found that the application of the magnetic field caused the enhanced intermixing between CoFe and Tb layers to form Co-Tb and Fe-Tb alloys at the interface of the layers. While the origin of the out-of-plane anisotropy and the change of the out-of-plane to inplane anisotropy with the application of inplane field of 200 Oe are not understood completely, it seems that there is a strong correlation between the out-of-plane anisotropy and the formation of the alloys and the stress at the interface.
    IEEE Transactions on Magnetics 12/2008; · 1.42 Impact Factor
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    ABSTRACT: Variation in the quality of the interface in a CoFeB/MgO/CoFeB tunnel junction during thermal annealing was investigated using x-ray photoemission spectroscopy. The formation of B oxide and the reduction of Fe oxide at the bottom interface after thermal annealing near Ta = 300 °C were found to enhance the tunneling magnetoresistance ratio significantly. At the same time, an asymmetry of the conductance (dV/dI) in the bias polarity and a local minimum of conductance in a positive bias state were measured which were attributed to the presence of a minority state at the bottom interface. The authors believe that the existence of the Bloch state was also responsible for the failure of the application of the Brinkman-Dynes-Rowell or Simmons models to the CoFeB/MgO/CoFeB junction.
    Applied Physics Letters 09/2007; 91(10):102104-102104-3. · 3.52 Impact Factor
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    ABSTRACT: Thermally activated magnetization reversals of submicron sized magnetic tunnel junctions with NiFe single and NiFe / Ru / NiFe synthetic antiferromagnet (SAF) free layers are investigated by varying the pulse durations and current amplitudes in switching pulse measurements. The measured data show good agreement with the switching probability predicted by the Arrhenius-Neel theory and switching behaviors with a single activation energy barrier. Estimated activation barriers for magnetization reversals are higher for magnetic tunnel junctions (MTJs) with SAF free layers than MTJs with single free layers. It is believed that the high activation barrier is achieved by the larger magnetic volume of SAF layers.
    Journal of Applied Physics 05/2006; · 2.21 Impact Factor
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    ABSTRACT: Time-dependent dielectric breakdown (TDDB) measurements under constant voltage stress with positive and negative bias polarities are carried out for magnetic tunnel junctions (MTJs) with different oxidation status (under-, optimal, and overoxidation). We found that there is significant polarity dependence in the TDDB and speculated that the polarity dependence is due to both intrinsic and extrinsic origins. Optimally oxidized MTJs with positive bias on the top electrode show shorter times to breakdown (tBD’s) and lower barrier height than with negative bias, indicating that asymmetric band structure, in part, causes the polarity dependence. On the other hand, under- and overoxidized MTJs show much shorter tBD’s than optimally oxidized one and show a higher 1/f noise power density for positive bias than for negative bias, indicating that the polarity dependence is also, in part, due to the interface states, which acts like precursors for the dielectric breakdown.
    Journal of Applied Physics 04/2006; 99(8):08K705-08K705-3. · 2.21 Impact Factor
  • Kwang-Seok Kim, Ki-Su Lee, C.H. Nam, B.K. Cho
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    ABSTRACT: Magnetic tunnel junctions (MTJs) were fabricated using nitrogen-mixed oxygen plasma (O<sub>2</sub>:N<sub>2</sub>=10:1). From the measurements of time-dependent dielectric breakdown (TDDB) under a constant voltage stress, the reliability of MTJs with an oxy-nitride barrier was investigated and compared with that of MTJs fabricated using pure oxygen plasma. The reliability of MTJs with an oxy-nitride barrier is much improved with the incorporation of nitrogen to oxidation process. In addition, the reliability of the oxy-nitride barrier is gradually enhanced with increasing oxy-nitridation time even after the time exceeds the optimal value. It is believed that the enhancement is due to the bonding of nitrogen to electron traps both in the oxide barriers and at the bottom interface. The characteristics of the bonding were examined by XPS measurements, which reveal a nitrogen 1s peak in Al-N bond. The lifetime of the two barriers was also estimated for comparison.
    IEEE Transactions on Magnetics 02/2006; · 1.42 Impact Factor
  • Kwang-Seok Kim, B. K. Cho, T. W. Kim
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    ABSTRACT: Magnetoresistive random access memory (MRAM) is one of the promising universal memories, which combines the high speed of static RAM, high-density of dynamic RAM, and non-volatility of FLASH memory. However, MRAM faces several challenges prior to the appearance at the commercial market on a large scale. The most important challenge among them will be an issue, related to the magnetic switching. The conventional MRAM structures and writing methods cause several issues to be overcome for high packing density without cross-talk. Furthermore, small ferromagnetic elements will require high magnetic field, which will increase the power consumption of the devices. This article will review new writing schemes, such as thermal-assisted switching and spin-transfer switching methods. In addition, reliability characteristics and thermal stability of magnetic tunnel junctions will be presented, because ensuring the reliability of magnetic tunnel junctions emerges as an another challenging problem for the successful application of the new writing schemes to the high-density MRAM devices in the next generation.
    Current Applied Physics - CURR APPL PHYS. 01/2006; 6.
  • Kwang-Seok Kim, B. K. Cho
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    ABSTRACT: Time dependent dielectric breakdown (TDDB) measurements were carried out for magnetic tunneling junctions (MTJs) with different electron trap densities at the interface between a bottom electrode and an insulating barrier. The TDDB shows a strong bias-polarity dependence, which becomes bigger with increasing trap density. In addition, the current creep before total dielectric breakdown consistently shows bias-polarity dependence. The polarity dependence of the breakdown in MTJs with an ultrathin tunneling barrier (15–20 Å) is believed to be caused by precursor effect of the traps at the bottom interface, which enhances the trap generation rate in a tunneling barrier, resulting in acceleration of the breakdown process.
    Applied Physics Letters 03/2005; 86(14):142106-142106-3. · 3.52 Impact Factor
  • Kwang-Seok Kim, Heejea Shim, B. K. Cho
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    ABSTRACT: Time-dependent dielectric breakdown (TDDB) of magnetic tunnel junctions (MTJs) under a constant voltage stress, tunneling magnetoresistance (TMR), and barrier properties, e.g. effective barrier height and thickness, were investigated as a function of thermal annealing temperatures. A Weibull failure distribution function was plotted in terms of time to breakdown (TBD) of MTJs. The TBD when 63 fraction of as-fabricated MTJs cumulatively failed increased significantly after thermal annealing at 210 °C while the TMR also increased from 8.85% to 14.22% before and after thermal annealing at 210 °C, respectively. We believe that the enhanced reliability of the MTJs is due to healing effect of bulk defects in the barrier during the annealing process, likely leading to the reduction of defect trap density. The reduction of bulk defects in the barrier was also confirmed by the lowered /f noise power spectral density from voltage fluctuation measurements.
    physica status solidi (a) 06/2004; 201(8):1712-1715. · 1.21 Impact Factor
  • Kwang-Seok Kim, Y. M. Jang, J. M. Park, B. K. Cho
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    ABSTRACT: Time-dependent dielectric breakdown measurements under constant voltage stress were carried out for magnetic tunnel junctions (MTJs), prepared by different oxidation techniques. Insulating barriers in MTJs were fabricated by oxidation of a predeposited Al layer with different oxidation techniques, such as conventional O2 plasma, off-axis O2 plasma, Ar-mixed O2 plasma, and off-axis Ar-mixed O2 plasma oxidations. The time to breakdown (TBD) of the MTJs was estimated from Weibull failure distribution plot. The estimated TBD when 63% of the junctions formed by conventional O2 plasma oxidation failed was estimated to be about 55 s. The TBD increased up to about 250, 750, and 2,500 s for the junctions of off-axis, Ar-mixed, and off-axis Ar-mixed O2 plasma oxidation, respectively. We believe that the enhanced reliability of MTJs is due to the reduction of plasma damage during oxidation process, resulting in low electron trap density in the Al2O3 tunneling barrier. Low-frequency 1/f noise power density was measured to estimate the relative electron trap density as a function of the oxidation method, which supports our conjecture about the enhanced reliability of MTJs. © 2004 American Institute of Physics.
    Journal of Applied Physics 05/2004; 95(11):6786-6788. · 2.21 Impact Factor
  • Kwang-Seok Kim, B. K. Cho, T. W. Kim, W. J. Park
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    ABSTRACT: Breakdown characteristics of the magnetic tunnel junctions (MTJ) with different junction areas of S=200 μm2 and S=0.5 μm2 are investigated under constant voltage stress. The breakdown process is found to be quite different for the two junction areas. For the large junctions with S=200 μm2, magnetoresistance (MR) ratio decreases gradually with increasing time of constant voltage stress and lasts for 10–30 min. The low frequency 1/f noise power also increases with increasing stress time due to the Johnson and shot noises, caused by current flowing through ohmic shorts or pinholes across tunnel barrier. Meanwhile, the junctions with S=0.5 μm2 show abrupt junction breakdown after stress time of 1–7 h. There is no significant change in both MR ratio and its bias dependence during the voltage stress. In particular, soft-breakdown events are observed before total breakdown occurs. The junction breakdown of small junction area is of intrinsic nature of the junction while the one of large junction area is due to extrinsic factors. © 2003 American Institute of Physics.
    Journal of Applied Physics 05/2003; 93(10):8364-8366. · 2.21 Impact Factor

Publication Stats

32 Citations
22.64 Total Impact Points

Institutions

  • 2007–2009
    • Samsung Advanced Institute of Technology
      Usan-ri, Gyeonggi Province, South Korea
  • 2003–2008
    • Gwangju Institute of Science and Technology
      • School of Materials Science and Engineering
      Kwangju, Gwangju, South Korea