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C. O. Kim,T. U. Nahm,J. P. Hong 한국진공학회(ASCT) 1999 Journal of Korean Vacuum Science & Technology Vol.3 No.2
Possible role of hydrogen atoms on the formation of microcrystalline silicon films was schematically investigated using a plasma enhanced chemical vapor deposition system. A layer-by-layer technique that can alternate deposition of α-Si thin film and then exposure of H₂ plasma was used for this end. The experimental process was extensively carried out under different hydrogen plasma times (t₂) at a fixed number of 20 cycles in the deposition. Structural properties, such as crystalline volume fractions and grain shapes were analyzed by using a Raman spectroscopy and a scanning electron microscopy. Electrical transports were characterized by the temperature dependence of the dark conductivity that gives rise to the calculation of activation energy (E_a). Optical absorption was measured using an ultra violet spectrophotometer, resulting in the optical energy gap (E_(opt)). Our experimental results indicate that both of the hydrogen etching and the structural relaxation effects on the film surface seem to be responsible for the growth mechanism of the crystallites in the μc-Si thin films.
T-U. Nahm 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.10
High-temperature oxidation of iron thin films deposited on Pt(100) surfaces was studied by using X-ray photoelectron spectroscopy (XPS). Upon an oxygen exposure of 300 Langmuir onto a 7.5- monolayer (ML) Fe film at 830 K, about 2 monolayers of the Fe film were oxidized as Fe3O4 while the remaining Fe atoms diffused into the substrate. For 1.25-, 2.5-, and 3.75-monolayer Fe films, only about a monolayer of the Fe film was oxidized as FeO, regardless of the number of Fe atoms. The oxide layers on the 7.5-monolayer Fe film were observed to be stable upon post-annealing at 1030 K.
Magnetic and structural properties of ultrathin magnetic films : Ni / Pt(111)
T.-U. Nahm,S.-J. Oh 한국진공학회(ASCT) 2003 Applied Science and Convergence Technology Vol.12 No.S1
We have studied magnetic and structural properties of ultrathin Ni films grown on Pt(111) surface using in situ surface magneto-optic Kerr effect and X-ray photoelectron spectroscopy. Perpendicular magnetic anisotropy was absent, and longitudinal Kerr signal was only detectable for Ni films thicker than 6 monolayers. Enhancement in longitudinal Kerr signal by 30% was achieved by post-annealing at temperatures below 800K, but upon annealing at 820K, surface alloy was formed. By using core-level binding-energy shifts, the composition was determined to be Ni 70 at.%.
Electronic Structure of Fe-Pd Alloys Studied by Using Photoemission Spectroscopy
T-U. Nahm 한국물리학회 2017 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.71 No.2
We investigated the electronic structure of FexPd1−x (x = 0.25, 0.5, and 0.75) alloys by measuring valence-band and core-level photoelectron spectra. The Fe 3d and Pd 4d partial spectral weights were determined by using the Cooper minimum phenomenon of the Pd 4d photoionization cross section. We found that the experimentally determined Fe partial spectral weight of Fe50Pd50 alloy differ much from the band calculation results, and we could not observe a spectral structure due to the Pd 4d states mixed with the Fe 3d majority states at the binding energy of 0.9 eV. We suggest that a plausible explanation for these discrepancies should be the spin-dependent lifetime of the Fe 3d states.
Magnetic and structural properties of ultrathin magnetic films: Ni/Pt(111)
Nahm, T.U.,Oh, S.J. The Korean Vacuum Society 2003 Applied Science and Convergence Technology Vol.12 No.1
We have studied magnetic and structural properties of ultrathin Ni films grown on PI(lII) surface using in situ surface magneto-optic Kerr effect and X-ray photoelectron spectroscopy. Perpendicular magnetic anisotropy was absent, and longitudinal Kerr signal was only detectable for Ni films thicker than 6 monolayers. Enhancement in longitudinal Kerr signal by 30% was achieved by post-annealing at temperatures below 800K, but upon annealing at 820K, surface alloy was formed. By using core-level binding-energy shifts, the composition was determined to be Ni 70 at. %.
Photoemission Spectroscopic Study on the Electronic Structure of Fe-Pt Alloys
S.-H. Song,T.-U. Nahm,B.-H. Choi,H.-J. Noh,Joo-Sang Park,S.-J. Oh 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.45 No.1
We investigated the electronic structure of FexPt1x (x = 0.25, 0.5, 0.75)alloys by photoemission spectroscopy by using synchrotron radiation. The Fe 3d and Pt 5d partial spectral weights are determined by taking advantage of the Cooper minimum phenomenon of Pt 5d photoionization cross section and taking the matrix element eect into consideration. Comparison with the previous band calculations is discussed.
Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes
Cho, S.G.,Nahm, T.U.,Kim, E.K. Elsevier 2014 Current Applied Physics Vol.14 No.3
Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at E<SUB>c</SUB>-0.51 eV and E<SUB>c</SUB>-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.