Low-temperature synthesis of crystalline GeSn with high Sn concentration by electron excitation effect

The low-temperature synthesis of high-Sn-concentration GeSn is challenging in realizing flexible thin-film transistors and solar cells. Because of athermal processes, irradiation with energetic particles is anticipated to significantly reduce the processing temperature for device fabrication. Here, we demonstrated that polycrystalline Ge with ~30 at. % Sn can be realized at room temperature by the electron-beam-induced recrystallization of amorphous GeSn. We found that inelastic electronic stopping, the so-called electron excitation effect, plays an important role in the recrystallization of amorphous GeSn

Transmission electron microscopy study on FeSi₂ nanoparticles synthesized by electron-beam evaporation

We have synthesized epitaxially grown iron disilicide (FeSi₂) nanoparticles using an electron-beam evaporation technique and characterized them by transmission electron microscopy (TEM). An Fe film was deposited on a Si(100) substrate, followed by thermal annealing at 1073 K for 2 h. It was found that epitaxially grown nanoparticles with an average size of ∼ 10 nm were formed just beneath the Si surface, suggesting that the deposited Fe atoms diffuse into the substrate. Every single phase of nanoparticles was examined in detail by TEM observation, nanobeam electron diffraction, and energy-dispersive x-ray spectroscopy. Plan-view and cross-sectional TEM observations revealed that these nanoparticles consist of α-, α-, and γ-FeSi ₂. It was found that the morphology of nanoparticles is closely related to the phases. The α and β phases consist of angled hemisphere and asymmetric triangle-shaped nanoparticles, respectively, while the γ phase consists of hemispherical or columnar-shaped nanoparticles. These particle morphologies are discussed with respect to the lattice mismatches between the particles and the matrix. © 2006 American Institute of Physics.Jong Han Won, Kazuhisa Sato, Manabu Ishimaru, and Yoshihiko Hirotsu, "Transmission electron microscopy study on FeSi₂ nanoparticles synthesized by electron-beam evaporation", Journal of Applied Physics 100, 014307 (2006) https://doi.org/10.1063/1.2209751

Direct observations of crystallization processes of amorphous GeSn during thermal annealing: A temperature window for suppressing Sn segregation

The solubility limit of tin (Sn) in germanium (Ge) is very small, and, therefore, it is difficult to synthesize high Sn concentration GeSn crystals by conventional methods. An amorphous phase can contain elements beyond the solubility limit of the crystal state, and, therefore, recrystallization of the amorphous alloy is one of the possible ways to realize materials far from the equilibrium state. To suppress Sn precipitation during thermal annealing, knowledge of crystallization processes is required. In the present study, amorphous GeSn thin films with different Sn concentrations were prepared by sputtering, and their crystallization processes were examined by in situ transmission electron microscopy. It was found that the crystallization temperature decreases with increasing Sn concentration, and it became lower than the eutectic temperature when the Sn concentration exceeded ∼25 at. %. Radial distribution function analyses revealed that phase decomposition occurs in the amorphous state of the specimens which crystallize below the eutectic temperature, and Sn crystallites were simultaneously precipitated with crystallization. On the other hand, no remarkable phase decomposition was detected in amorphous GeSn with <25 at. % Sn. Sn precipitation occurred at a higher temperature than the crystallization in these specimens, and the difference between the crystallization and Sn precipitation temperatures became large with decreasing Sn concentration. Because of the existence of this temperature difference, a temperature window for suppressing Sn segregation existed. We demonstrated that large GeSn grains with high Sn concentration could be realized by annealing the specimens within the temperature window

Effects of surface step and substrate temperature on nanostructure of L1₀-FePt nanoparticles

An investigation on the variation of the particle size, particle density and its relation to the hard magnetic properties of FePt nanoparticles with respect to substrate surface morphologies and substrate temperatures was performed. At a substrate temperature below 573 K, densely dispersed FePt nanoparticles with a particle density of 1012 cm -2 were obtained. It was found that for obtaining well-oriented and well-isolated L10-FePt nanoparticles with large coercivity, substrate temperatures between 623 and 823 K were necessary.Kazuhisa Sato, Takenori Kajiwara, Masaru Fujiyoshi, Manabu Ishimaru, and Yoshihiko Hirotsu, "Effects of surface step and substrate temperature on nanostructure of L10–FePt nanoparticles", Journal of Applied Physics 93, 7414-7416 (2003) https://doi.org/10.1063/1.1541641

Deposition-Temperature Dependence of Vortex Pinning Property in YBa2Cu3O7+BaHfO3 Films

Improvement of critical current density (Jc) in magnetic fields is required in YBa2Cu3O7 films, and process parameters should be optimized for controlling pinning centers. In the present study, a deposition temperature was varied in pulsed laser deposition of YBa2Cu3O7+BaHfO3 films to control the nanorod structure, and its influence on Jc was analyzed. The YBa2Cu3O7+BaHfO3 film deposited at 850°C exhibited pinning force maximum (Fp,max) as high as 413 GN/m3 at 40 K, while the Fp,max for the deposition temperature of 850°C at 77 K was smaller than that in the YBa2Cu3O7+BaHfO3 film deposited at 900°C. A critical temperature decreased and matching field increased with decreasing the deposition temperature. Increase in deposition temperature is effective in improving the Fp,max in high temperatures, since the critical temperature and matching field dependences of Jc value dominate the Fp,max. On the other hand, low deposition temperature improves the Fp,max in low temperatures since the Fp shift in accordance with matching field is dominant to the Fp,max. Thus, the deposition temperature should be set in pulsed laser deposition of YBa2Cu3O7 films containing nanorods considering the Jc variation with critical temperature and matching field

Nonlocal self-organization of long stacking faults from highly strained nanocomposite film of complex oxide

Elastic strain and defects are important key words for controlling structure and properties in films. While epitaxial strain and misfit dislocations have been discussed in conventional films, the evolution of strain and defect can be significantly varied by nanocomposite strain and complicated defects in oxides. In the present study, long stacking faults with a spacing of 5–30 nm and a length of >500 nm were self-organized by ex situ annealing highly strained nanocomposite films of YBa2Cu3O7–δ (YBCO) + BaMO3 (M = Hf, Sn). It is surprising that the nonlocal nature of stacking faults, namely, the structural correlation over >500 nm, was observed in spite of the local configuration of the nanocomposite interface. This kind of structural variation was not observed in the pure YBCO film without nanorods, even when the same annealing was performed. A strain energy analysis showed that the stacking fault formation led to the strain energy minimum by reducing the nanocomposite strain. The layered structure of YBCO stacking faults and the large nanocomposite strain realized the present nonlocal self-organization, which is not observed in the conventional systems with epitaxial strain and misfit dislocations

Influence of matching field on critical current density and irreversibility temperature in YBa2Cu3O7 films with BaMO3(M=Zr, Sn, Hf) nanorods

The influence of the matching field (BΦ) on critical current density (Jc) and irreversibility temperature (Tirr) in YBa2 Cu3O7 films containing BaMO3 (M=Zr, Sn, Hf) nanorods was investigated. It was revealed that the irreversibility temperature normalized by the critical temperature (Tirr/Tc) was influenced by BΦ, for B>BΦ, but Tirr/Tc did not depend on which BaMO3 material was used for B<BΦ, i.e., there was no dependence on nanorod density, diameter, interface sharpness, or Tc in the case of ideal nanorods. However, Jc/Jc(0 T) was found to decrease with increasing BΦ at low magnetic field strengths and to improve at high magnetic field strengths. In addition to Jc being dependent on BΦ, the Tc term in Tirr and Jc(0 T) were also found to have an effect on Jc

Strong atomic ordering in Gd-doped GaN

Gd-doped GaN (Ga 1-xGd xN) thin films were grown on a GaN(001) template by radio frequency plasma-assisted molecular beam epitaxy and characterized by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM). Three samples with a different Gd composition were prepared in this study: x = 0.02, 0.05, and 0.08. XRD and TEM results revealed that the low Gd concentration GaN possesses the wurtzite structure. On the other hand, it was found that an ordered phase with a quadruple-periodicity along the [001] direction in the wurtzite structure is formed throughout the film with x = 0.08. We proposed the atomistic model for the superlattice structure observed here. © 2012 American Institute of Physics.Manabu Ishimaru, Kotaro Higashi, Shigehiko Hasegawa, Hajime Asahi, Kazuhisa Sato, and Toyohiko J. Konno, "Strong atomic ordering in Gd-doped GaN", Appl. Phys. Lett. 101, 101912 (2012) https://doi.org/10.1063/1.4751245

Liquid-mediated crystallization of amorphous GeSn under electron beam irradiation

Crystallization processes of amorphous germanium–tin (GeSn) under low-energy electron-beam irradiation were examined using transmission electron microscopy (TEM). Freestanding amorphous GeSn thin films were irradiated with a 100 keV electron beam at room temperature. The amorphous GeSn was athermally crystallized by electron-beam irradiation, when the electron flux exceeded the critical value. Heterogeneous structures consisting of nano- and micro-crystallites were formed after crystallization of amorphous GeSn with ∼24 at. % Sn in the as-sputtered amorphous state. In situ TEM observations of structural changes under electron-beam irradiation revealed that random nucleation and growth of nanocrystallites occur at the early stage of crystallization, followed by rapid formation of micro-grains surrounding the nanocrystals. It has been suggested that the growth of micro-grains progresses via supercooled liquid Sn at the amorphous/crystalline interface. The resultant GeSn grains with a size of a few micrometers contained ∼15 at. % Sn, much larger than the solubility limit of Sn in Ge (∼1 at. % Sn)

Direct imaging of atomic clusters in an amorphous matrix: A Co-C granular thin film

The atomic structure of extremely small cobalt (Co) nanoparticles embedded in an amorphous carbon (C) matrix has been studied by spherical aberration (C s) corrected high-resolution transmission electron microscopy and focal-series restoration. The Co nanoparticles, 1-3 nm in diameter, are crystalline with the face centered cubic structure, while the radial distribution function analysis revealed the existence of a Co-C bond. The reconstructed phase images of the exit-wave function clearly show the projected potential distribution within the Co nanoparticles. The C s-correction has hence a benefit to visualize embedded crystalline clusters unambiguously, which are responsible for the magnetotransport properties of the Co-C films. © 2012 American Institute of Physics.Kazuhisa Sato, Masaki Mizuguchi, Ruihe Tang, Jung-Goo Kang, Manabu Ishimaru, Koki Takanashi, and Toyohiko J. Konno, "Direct imaging of atomic clusters in an amorphous matrix: A Co-C granular thin film", Appl. Phys. Lett. 101, 191902 (2012) https://doi.org/10.1063/1.4765362