Se Ahn Song

Ge nitride formation in N-doped amorphous Ge2Sb2Te5

The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3Nat.% doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.

Interface structure and non-stoichiometry in HfO2 dielectrics

High-resolution electron microscopy, electron energy-loss spectroscopy, and first-principles theory are used to investigate the composition and electronic structure of HfO2 dielectric layers deposited directly onto Si. A thin, nonstoichiometric, but Hf-free SiO2 layer forms between the HfO2 dielectric and the substrate, consistent with one-dimensional spinodal decomposition. Rapid thermal annealing crystallizes the HfO2, and the resulting grain boundaries within the HfO2 are found to be O-depleted, with localized states within the bandgap. These localized states are thought to act as significant leakage pathways, and may be responsible for Fermi-level pinning at the dielectric/contact interface.

Observation of molecular nitrogen in N-doped Ge2Sb2Te5

Ge2Sb2Te5 (GST) film in the crystalline state was nitrogen doped using the reactive sputtering method in order to increase sheet resistance. High-resolution x-ray absorption spectroscopy revealed that molecular nitrogen (N2) existed in the N-doped GST film. This finding implies that both molecular nitrogen and atomic-state nitrogen should be taken into account in understanding the structures of N-doped GST film. The molecular nitrogen is believed to exist at interstitial and vacancy sites, and more likely at grain boundaries.

Magnetoresistance and interlayer diffusion in PtMn spin valves upon postdeposition annealing

We report annealing time effects on the microstructural evolution and resultant magneto-transport property changes in Ta/NiFe/CoFe/Cu/CoFe/PtMn/Ta spin valves comprising PtMn layer thicknesses ranging from 10 to 30 nm. Postdeposition annealing was performed at 270 °C up to 35 h. The blocking temperatures of samples with 20 nm PtMn and 30 nm PtMn layers were found to be 350 °C and 400 °C, respectively. The magnetoresistance and interlayer coupling field changes became large as annealing time increased, in particular, for samples with relatively thicker PtMn layers. The main cause of microstructural changes and property degradation was due to interlayer diffusion of atomic constituents such as Mn, most likely through grain boundaries. Light B doping (1 at. %) in both free and pinned CoFe layers was proven effective in terms of blocking diffusion processes.

Enhanced room temperature ferromagnetism in Co- and Mn-ion-implanted silicon

The authors report on ferromagnetism at room temperature in cluster-free, cobalt- and manganese-ion-implanted crystalline silicon. Through magnetic and structural analysis it is shown that the ion-implanted Si consists of two layers of Co- and Mn-containing silicon: (1) an amorphous Si layer on the surface and (2) single crystalline Si beneath. The amorphous layer shows very little magnetism by itself but seems to be responsible for partially canceling out or masking the ferromagnetism in the crystalline Si. The authors also observe that etching of the amorphous Si layer dramatically enhances the measured magnetism by as much as 400%.

Application of high-resolution electron microscopy for visualization and quantitative analysis of strain fields in heterostructures

The general principles, possibilities, and limitations of the classical geometric phase method have been considered. A new generalizing approach, implying analysis of all available spatial frequencies of a high-resolution electron microscopy image, is developed to expand the possibilities of the geometric phase method. Test models of strained Si-GeSi-Si heterostructures are developed and the effect of scattering by phonons and the surface amorphous layer on visualization of lattice distortions is studied within these models. A simple method is proposed for measuring elastic strains in multicomponent heterostructures with pseudomorphic layers.

In situ dynamic HR-TEM and EELS study on phase transitions of Ge2Sb2Te5 chalcogenides

The phase transition phenomena of Ge2Sb2Te5 chalcogenides were investigated by in situ dynamic high-resolution transmission electron microscopy (HR-TEM) and electron energy loss spectroscopy (EELS). A 300kV field emission TEM and a 1250kV high voltage TEM were employed for the in situ heating experiments from 20 to 500 degrees C for undoped and 3wt% nitrogen-doped Ge2Sb2Te5 thin films deposited by DC sputtering. Crystallization of amorphous Ge2Sb2Te5 to its cubic structure phase started at 130 degrees C and then rapid crystal growth developed from cubic to hexagonal phase in the range of 130-350 degrees C; finally, the hexagonal crystals started to melt at 500 degrees C. For nitrogen-doped Ge2Sb2Te5, its crystallization from amorphous film occurred at higher temperature of ca. 200 degrees C, and the cubic and hexagonal phases were usually formed simultaneously without significant growth of crystals at further heating to 400 degrees C. EELS measurements showed that the electronic structures of Ge, Sb and Te stayed almost the same regardless of the amorphous, FCC and hexagonal phases. The nitrogen doped in Ge2Sb2Te5 was confirmed to exist as a nitride. Also, the doped nitrogen distributed homogeneously in both amorphous and crystalline phases. Localization of doped nitrogen was not found in the grain boundary of crystallized phases. The dynamic process of phase transition was enhanced by high-energy electron irradiation. Peeling of atomic layers in nitrogen-doped Ge2Sb2Te5 film was detected during heating assisted with electron beam irradiation.

Chemical state and atomic structure of Ge2Sb2Te5 system for nonvolatile phase-change random access memory

We present chemical state information on contamination-free Ge2Sb2Te5 thin film using high-resolution x-ray photoelectron spectroscopy (HRXPS) and the corresponding theoretical understanding of the chemical states, on both amorphous and metastable phases, illuminating the phase-change mechanism of the system. HRXPS data revealed that the Sb 4d shallow core level was split into two components having different binding energies and that the spin-orbit splitting feature of the Ge 3d level was enhanced as the system became metastable. Negligible change was observed in the Te 4d shallow core level, and in contrary to the previous report’s prediction less change in valance band spectra was observed. The results imply that Sb movement is also involved in the phase-change mechanism and that acquisition of shallow core-level spectra can be a useful measure for understanding phase-change mechanism. Hydrogenated SbTe6 octahedral-like cluster model was introduced to schematically interpret the generation of the two co...

Interface and microstructure evolutions in synthetic ferrimagnet-based spin valves upon exposure to postdeposition annealing

Synthetic ferrimagnet-based spin-valve (SV) films comprising Ta 5/NiFe 2/IrMn 9/CoFe (P1) 1.5/Ru 0.7/CoFe (P2) 3/Cu 2.8/CoFe 1.6/NiFe 3.2/Ta 5 (in nanometers) were examined with an emphasis given on understanding the interface evolution of IrMn/CoFe (P1) upon postdeposition annealing. A 300 °C, 5 h annealing resulted in 12% and 42% decreases in effective pinning field and 29% and 53% decreases in MR ratio for the bottom (B) and top (T) SV, respectively. Because both structures exhibit different thermal degradation behaviors, we were curious to uncover the underlying causes by employing various analytical methods. The column sizes of the B–SV and T–SV were about 30 and 15 nm, respectively. No major difference in the (111) crystal texture was observed. We found that the main cause was due to the interlayer diffusion of atomic constituents such as Mn, most likely through grain boundaries and IrMn/CoFe interface, where the degree of the diffusion was considerably higher for the T–SV. Due to the surface free e...

Observation of [110] surface band within {101} a-domain of heteroepitaxial PbTiO3 thin film fabricated by hydrothermal epitaxy

Heteroepitaxial PbTiO3 film on an Nb-doped (001) cubic SrTiO3 substrate was fabricated by hydrothermal epitaxy at 200°C. Piezoresponse force microscopy and x-ray Θ rocking curves confirmed that the film showed a c∕a∕c∕a multi-domain structure even though it did not undergo a cubic paraelectric (PE) to tetragonal ferroelectric (FE) phase transition. After heat treatment of this film at 600°C, we observed the [110] surface band within the a-domain, which was formed through the PE to FE phase transition. We also found that a [110] surface band existed along the (11¯1) plane within the a-domain. We predicted that the [110] surface band would be monoclinic phase due to the interaction of two different variants of a-domains in the presence of a c-domain in the heteroepitaxial PbTiO3 film.