Muneyuki Naito

Local structure analysis of Ge-Sb-Te phase change materials using high-resolution electron microscopy and nanobeam diffraction

As-sputtered and melt-quenched amorphous structures together with the laser-induced crystallized structure of Ge-Sb-Te thin films were investigated using high-resolution electron microscopy (HREM) and nanobeam electron diffraction (NBED). Each of the Ge-Sb-Te thin films was embedded in a four-layered stack, which is the same as the layered structure of phase-change optical disks. Cross-sectional HREM revealed crystalline atomic clusters in the melt-quenched amorphous layer at a greater frequency than in the as-sputtered amorphous layer. Autocorrelation function analysis of the HREM images revealed similarity between the structures of atomic ordered regions in the amorphous phase and that of crystalline Sb. Atomic pair-distribution functions derived from halo NBED intensity analysis indicated that the atomic neighbor correlations developed more in the melt-quenched amorphous phase than in the as-sputtered phase. The development of locally ordered regions is considered to be closely related to the differenc...

Direct observations of thermally induced structural changes in amorphous silicon carbide

Thermally induced structural relaxation in amorphous silicon carbide (SiC) has been examined by means of in situ transmission electron microscopy (TEM). The amorphous SiC was prepared by high-energy ion beam irradiation into a single crystalline 4H-SiC substrate. Cross-sectional TEM observations and electron energy-loss spectroscopy measurements revealed that thermal annealing induces a remarkable volume reduction, so-called densification, of amorphous SiC. From radial distribution function analyses using electron diffraction, notable changes associated with structural relaxation were observed in chemical short-range order. It was confirmed that the structural changes observed by the in situ TEM study agree qualitatively with those of the bulk material. On the basis of the alteration of chemical short-range order, we discuss the origin of thermally induced densification in amorphous SiC.

Transmission electron microscopy study on ion-beam-synthesized amorphous Fe-Si thin layers

Ion-beam-synthesized amorphous Fe–Si thin layers have been characterized using transmission electron microscopy (TEM) in combination with imaging plate techniques. Si single crystals with a (111) orientation were irradiated with 120keV Fe+ ions to a fluence of 4.0×1017cm−2 at cryogenic temperature (120K). Cross-sectional TEM observations indicated the formation of an amorphous bilayer on the topmost layer of the Si substrate. It was found that the upper layer is an amorphous Fe–Si with the composition, in terms of atomic ratio, of Fe∕Si ∼1∕2, while the lower one is an amorphous Si. Atomic pair-distribution functions extracted from microbeam electron diffraction patterns revealed that the nature of short-range order in amorphous Fe–Si thin layer can be well described by the atomic arrangements of crystalline iron silicides.

Direct observations of Ge2Sb2Te5 recording marks in the phase-change disk

Atomistic structures of the Ge2Sb2Te5 thin film in the real phase-change disk have been investigated using transmission electron microscopy (TEM). As-deposited amorphous Ge2Sb2Te5 films were laser-irradiated for initialization (crystallization) and recording. Cross-sectional TEM observations revealed that the recording mark was fully amorphized by laser irradiation. A slight difference between the as-deposited and the laser irradiation-induced amorphous Ge2Sb2Te5 was observed in the intensity profile of nanobeam electron diffraction patterns and atomic pair distribution functions. This difference was attributed to structural relaxation of amorphous Ge2Sb2Te5, which gives rise to the alteration of chemical order.

Formation process of β-FeSi2∕Si heterostructure in high-dose Fe ion implanted Si

We have performed high-dose iron (Fe) ion implantation into a single crystalline silicon (Si) substrate in order to synthesize a β-FeSi2∕Si heterostructure. Si(001) substrates were implanted with 120keV Fe ions at 623K to a fluence of 4×1017∕cm2, followed by thermal annealing at temperatures ranging from 773to1073K. Implantation-induced microstructures as well as annealing-induced ones were examined by means of x-ray diffraction (XRD) and transmission electron microscopy (TEM) in combination with energy-dispersive x-ray spectroscopy. Grazing-incidence XRD and cross-sectional TEM observations indicated that a continuous iron silicide layer consisting of e-FeSi and β-FeSi2 is formed in as-implanted samples. With increasing annealing temperature the e-FeSi phase transformed into the β-FeSi2 one, and finally a continuous β-FeSi2 polycrystalline layer was formed on the topmost layer of the Si substrate. Pole figure XRD measurements revealed that the β-FeSi2 crystallites are not randomly oriented but possess a ...

Electron irradiation-induced phase transformation in α-Fesi2

Structural changes of α-FeSi2 induced by electron beam irradiation have been investigated using transmission electron microscopy (TEM). Single crystals of Si(111) were implanted with 120 keV Fe ions at −150 °C to a fluence of 1.0×1017 /cm2, followed by thermally annealing at 350–550 °C. Cross-sectional and plan-view TEM observations revealed the formation of the metastable α-FeSi2 in the annealed samples. Under high-energy electron beam irradiation, the α-phase changed to a metastable crystalline phase whose structure is close to the CsCl structure. The phase transformation was caused mainly by displacement damage processes and suggests a low displacement energy for Fe atoms (<9 eV). To explain these observations, it was considered that vacancies in α-FeSi2 are responsible for the electron irradiation-induced phase transformation.

Electron microscopy study on amorphous Ge-Sb-Te thin film for phase change optical recording

Atomic structures of a sputtered amorphous Ge5Sb70Te25 thin film as a phase change optical recording material included in a rewritable optical disk have been studied by transmission electron microscopy (TEM) and nanobeam electron diffraction. Cross-sectional high-resolution TEM revealed that crystalline atomic cluster regions as small as 2 nm are formed in the as-sputtered amorphous Ge5Sb70Te25 thin film. Atomic pair-distribution functions derived from a halo diffraction intensity analysis suggested that the as-sputtered Ge5Sb70Te25 thin film including the crystalline clusters has an atomic configuration similar to that of the amorphous Sb structure.

Nano-Scale Tensile Testing and Sample Preparation Techniques for Silicon Nanowires

In this paper, we describe an experimental technique to achieve a highly reliable characterization of the mechanical properties of silicon (Si) nanowires (NWs). A reusable on-chip Si device consisting of comb-drive electrostatic actuator for generating tensile force and capacitive sensors for measuring tensile force and displacement was designed and developed for quasi-static tensile test of Si NWs. The combination of focused ion beam (FIB) fabrication, FIB-assisted chemical vapor deposition, and probe manipulation enabled us to directly fabricate the NWs on the device. This sampling technique led to high yielding percentage of nano-scale tensile testing. The NWs were made from 200-nm-thick Si membranes that were produced by using silicon-on-nothing membrane fabrication technique. Several Si NWs were annealed at 700 °C in ultrahigh vacuum (UHV) for 5 min in order to examine the influence of annealing on the mechanical characteristics. The mean Youngs modulus for nonannealed NWs was 129.1±10.1 GPa. After UHV annealing, the mean value was improved to be 168.1±1.3 GPa, comparable to the ideal value for Si(001)[110]. The annealing process gave rise to improving the Youngs modulus, whereas it degraded the strength. Transmission electron microscopy suggested that recrystallization and gallium nanoclusters formation by annealing would have changed the mechanical characteristics.

Electron irradiation-induced phase transformation in {alpha}-FeSi{sub 2}

Structural changes of α-FeSi2 induced by electron beam irradiation have been investigated using transmission electron microscopy (TEM). Single crystals of Si(111) were implanted with 120 keV Fe ions at −150 °C to a fluence of 1.0×1017 /cm2, followed by thermally annealing at 350–550 °C. Cross-sectional and plan-view TEM observations revealed the formation of the metastable α-FeSi2 in the annealed samples. Under high-energy electron beam irradiation, the α-phase changed to a metastable crystalline phase whose structure is close to the CsCl structure. The phase transformation was caused mainly by displacement damage processes and suggests a low displacement energy for Fe atoms (<9 eV). To explain these observations, it was considered that vacancies in α-FeSi2 are responsible for the electron irradiation-induced phase transformation.

Interfacial reaction of Si islands on SiO2 during high-temperature annealing

We examine the interfacial reaction between submicron Si islands and SiO2 during high-temperature annealing under ultrahigh vacuum, using atomic force microscopy and transmission electron microscopy. We show that Si island/SiO2 interfaces are much more reactive than interfaces of wetting thin Si films on SiO2. This indicates that important processes responsible for the reaction occur at the Si island edges. During the reaction removal of O atoms from the SiO2 side of the interface occurs, resulting in depression of the Si island/SiO2 interface. Our observations indicate that the interfacial reaction advances via O out-diffusion from SiO2 into the Si island, O lateral diffusion along the interface, SiO formation at the edge of the Si island, and SiO desorption from the surface.