Ultrahigh Vacuum Transmission Electron Research Articles

Characterization Investigation of β-FeSi2 Semiconductor by In Situ Ultrahigh-Vacuum Transmission Electron Microscopy

In an integrated ultrahigh-vacuum system, a thin Fe layer was deposited on a Si (111) 7×7 surface at room temperature by ultrahigh-vacuum molecular beam epitaxy (UHV-MBE) and annealed at 600°C. The characterization of the as-deposited and annealed specimen for microstructure and electronic structure was investigated using an analytical ultrahigh-vacuum transmission electron microscope (UHV-TEM) and an electron energy-loss spectroscope (EELS). After deposition of a 5 ML Fe layer, an epitaxial Fe layer formed on the Si surface, which suggests that the Si substrate may induce the arrangement of Fe atoms in the thin Fe layer. When annealed at 600°C, epitaxial β-FeSi2 islands with moiré fringes formed on the Si substrate. The Fe 3d occupancy change for as-deposited Fe and β-FeSi2 was calculated by the empirical method after background subtraction by the power law and deconvolution by the Fourier ratio. The calculated results show that there is a 0.95±0.06 electron/atom decrease for Fe 3d occupancy from as-deposited Fe to β-FeSi2. Fe white lines intensity ratios for the two states were calculated from the second derivative spectra, which are 2.89 and 2.51 for as-deposited Fe and β-FeSi2, respectively. The results for the intensity ratio show that the higher intensity ratio is related to stronger magnetism.

Initial oxidation kinetics of copper (1 1 0) film investigated by in situ UHV-TEM

Previous studies of the initial stage of oxidation on clean single crystal of Cu(1 0 0) have been extended to the case of the Cu(1 1 0) surface. The dynamic observation of the nucleation and growth of Cu oxide by means of in situ ultra high vacuum transmission electron microscopy (UHV-TEM) shows a highly enhanced oxidation rate on Cu(1 1 0) surface as compared to Cu(1 0 0). The kinetic data on the nucleation and growth of the three-dimensional oxide islands agree well with our heteroepitaxial model of surface diffusion of oxygen.

Temperature effect on the Cu 2O oxide morphology created by oxidation of Cu(0 0 1) as investigated by in situ UHV TEM

The temperature effect on the Cu 2O oxide morphology was investigated by oxidizing Cu(1 0 0) thin films at the temperature ranging from 150 to 1000 °C and constant oxygen partial pressure of 5×10 −4 Torr. The evolution of the oxide island size and shape was followed inside an in situ ultrahigh vacuum transmission electron microscope (UHV TEM). Of particular interest, we find that the oxide morphology can be triangular, hut, rod or pyramid shaped depending only on the oxidation temperature.

Self-assembly of metal-oxide nanostructures: oxidation of Cu films by In situ UHV-TEM

The effect of temperature on the epitaxial Cu2O oxide morphology has been investigated by oxidizing Cu(100) thin films at temperatures ranging from 150° to 1000°C. The evolution of the island size and shape was visualized within an in situ ultra high vacuum transmission electron microscope. Dramatically different morphologies of oxide nanostructures can be achieved by an appropriate choice of the oxidation temperature. During oxidation at low temperatures (T < 400°C), the oxide islands adopt triangular shape, but at temperatures higher than 400°C, the islands have a more symmetrical geometry. Of particular interest, quasi-one-dimensional Cu2O structures with aspect ratios as large as 40:1 were formed at the oxidation temperature of 600°C. The in situ observation data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interfacial energies and the elastic stress relaxation in the three dimensional islands.

In situ ultrahigh vacuum transmission electron microscopy studies of palladium silicide island formation on silicon (111) 7×7 surface

Pd was deposited onto Si (111) 7×7 surface at approximately 700 K inside an ultrahigh vacuum transmission electron microscope. Plan-viewed transmission electron microscopy (TEM) observation indicated that the islands have two kinds of shapes, round and rectangular (one-dimensional) ones. In a diffraction pattern for the rectangular islands, extra spots along the 〈110〉 direction of the Si surface, spacing of which is 1/8 times as long as that of Si (220) spots, were seen. A high resolution TEM image showed the corresponding superstructure in the rectangular islands. In situ observation of the growing process of the rectangular islands showed repeat of introduction and relief of strains during the growth, suggesting that such superstructure would be constructed by stacking compositionally different phases or introducing defects so that the periodically maximized strain is relieved.

Formation of quasi-one-dimensional Cu2O structures by in situ oxidation of Cu(100).

Epitaxial Cu2O islands on a Cu(100) surface formed through oxidizing Cu(100) films at 600 degrees C in an ultrahigh vacuum transmission electron microscope were observed to undergo a shape transition from initially square shaped islands to elongated islands at a critical size of approximately 110 nm. Our experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three-dimensional islands. We developed a kinetic model based on oxygen surface diffusion that fits well with the observed volume evolution of the Cu2O islands.

From nucleation to coalescence of Cu2O islands during in situ oxidation of Cu(001)

The nucleation, growth, and coalescence of Cu2O islands due to oxidation of Cu(001) films were visualized by in situ ultrahigh-vacuum transmission electron microscopy. We have previously demonstrated that the nucleation and initial growth of copper oxides is dominated by oxygen surface diffusion. These surface models have been extended to quantitatively represent the coalescence behavior of copper oxidation in the framework of the Johnson–Mehl–Avrami–Kolmogorov theory. An excellent agreement exists between the experimental data of nucleation to coalescence with the surface model. The implication could be an alternate paradigm for passivation and oxidation, since classic theories assume uniform film growth.

Analytical UHV transmission electron microscopy studies of electronic structure changes between as-deposited Mn and Mn silicide on Si( [formula omitted]) surface

Mn was deposited on Si(1 1 1) 7×7 surface at room temperature in an ultrahigh vacuum (UHV) chamber and then transferred to a UHV transmission electron microscope through a UHV transportation system, followed by investigation of changes in crystallographic structure and electronic structure of the deposited Mn layer before and after annealing at about 400 °C. Transmission electron microscopy observation showed that the as-deposited Mn layer had microcrystalline particles composed of pure Mn but the annealed one exhibited the round shaped MnSi islands with the size of several tens nanometers growing epitaxially on the Si(1 1 1) surface. In situ electron energy loss spectroscopy indicated the change of white lines Mn L 2,3 edges between the as-deposited Mn and MnSi. From intensity calculation of the white lines, it was estimated that 3d occupancies of Mn increased by 2.5±0.06 electron/atom for MnSi formation.

Surface Kinetics of Copper Oxidation Investigated by In Situ Ultra-high Vacuum Transmission Electron Microscopy.

We review our studies of the initial oxidation stages of Cu(001) thin films as investigated by in situ ultra-high vacuum transmission electron microscopy. We present our observations of surface reconstruction and the nucleation to coalescence of copper oxide during in situ oxidation in O2. We have proposed a semi-quantitative model, where oxygen surface diffusion is the dominant mechanism of the initial oxidation stages of Cu. We have also investigated the effect of water vapor on copper oxidation. We have observed that the presence of water vapor in the oxidizing atmosphere retards the rate of Cu oxidation and Cu2O is reduced when exposed directly to steam.

Development of Ultra High Vacuum Transmission Electron Microscope for In Situ Observation of Silicides And Island Formation on Silicon Surface at High Temperatures by Reflection And Transmission Electron Microscope

An ultra high vacuum transmission electron microscope (UHV-TEM) has been successfully developed for in situ observation of the islands and silicides formation on silicon surface at high temperatures by reflection electron microscope (REM) and TEM. The cleaning process of the contaminated Si(100) surface by heating to 1150°C was successfully observed by REM. The thinning process of the Si(100) specimen by heating to 900°C with the oxygen exposure is dynamically observed by TEM. The observation of the surface steps on Si(100) with TEM is also carried out with considerable success. The present results lead us to the confirmation that the developed UHV-TEM can be applied to the study of the islands and silicides formation by in situ REM and TEM observation.

Structural observation of Mn silicide islands on Si(1 1 1) 7×7 surface with UHV-TEM

Atomic structure and crystallography of Mn silicide islands formed on an Si(1 1 1) surface was investigated. Mn with 8–10 MLs was deposited on an Si(1 1 1) 7×7 surface at room temperature by means of an ultrahigh vacuum molecular beam epitaxy, and then transferred to an ultrahigh vacuum transmission electron microscope (UHV-TEM) without exposure to air. Annealing at 400°C caused the round shaped silicide islands composed of MnSi with a size of several tens of nanometers to grow epitaxially. From transmission electron microscopy analysis, it was found that the crystallography of the islands matches with that of the Si(1 1 1) substrate with the relation (1 1 1) Si∥(1 1 1) MnSi and [ 1 ̄ 0 1] Si ∥ [ 1 ̄ 2 1 ̄ ] MnSi , but their interface was incommensurate.

In situ transmission and reflection electron microscopy studies of palladium silicide islands grown on silicon (1 1 1) surface

Pd was deposited onto Si (1 1 1) 7×7 surface at about 700 K inside an ultrahigh vacuum transmission electron microscope. In reflection electron microscopy (REM) observation of Pd-deposition process, it was indicated that Pd silicide islands were formed by involving two kinds of growth behavior. It was found from plan view transmission electron microscopy (TEM) that the islands have two kinds of shape, one round and the other rectangular (one-dimensional). Comprehensive TEM/REM and diffraction analysis revealed that the two kinds of the Pd silicide islands with different shape and atomic structure grew independently.

In-situ observation of deposition process of Pd on clean Si surfaces by ultrahigh vacuum-transmission electron microscopy/scanning tunneling microscopy

Pd-deposited clean Si(111) and (110) substrates were examined with ultrahigh vacuum (UHV) transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) that are part of the newly developed UHV-TEM/STM integrated characterization system. Deposition at room temperature (RT) onto TEM (110) samples resulted in the formation of a Pd–Si mixed layer. The layer on the Si〈111〉 edge showed contrast typical for amorphous images while that on the high index edge showed a uniform Pd 2Si layer. Annealing of the Pd-deposited substrates at 673 K resulted in the formation of the Pd 2Si islands on a Pd 2Si layer. The formation of a 3×3 structure was observed on top of the Pd 2Si islands with STM. Pd deposition to TEM substrates at 673 K resulted in the formation of Pd 2Si islands. A superstructure with three times periodicity was found on the surface of the island. This structure is suggested to be Si-rich and has a sub-surface layer. After low coverage deposition at RT followed by annealing at 673 K, there was formation of peculiar surface structure on a Pd 2Si layer which was different from the 3×3 structure.

Surface Kinetics of Copper Oxidation Investigated by In Situ Ultra-high Vacuum Transmission Electron Microscopy

AbstractWe review our studies of the initial oxidation stages of Cu(001) thin films as investigated by in situ ultra-high vacuum transmission electron microscopy. We present our observations of surface reconstruction and the nucleation to coalescence of copper oxide during in situ oxidation in O2. We have proposed a semi-quantitative model, where oxygen surface diffusion is the dominant mechanism of the initial oxidation stages of Cu. We have also investigated the effect of water vapor on copper oxidation. We have observed that the presence of water vapor in the oxidizing atmosphere retards the rate of Cu oxidation and Cu2O is reduced when exposed directly to steam.

The Surface Kinetics of the Initial Oxidation Stages of Copper as Investigated by In Situ Ultra-High Vacuum Transmission Electron Microscopy

We have investigated the initial stages of Cu(001) oxidation using in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM) from 10 -5 torr O 2 to atmospheric pressures and temperature range from room temperature to 600°C. The advantages of in situ TEM experiments include visualization of oxidation processes in real time and information on buried interfaces. We have previously demonstrated that the initial stages of copper oxidation in dry oxygen is a surface-limited process. To investigate the effect of oxidizing environments, we have examined the oxidation of Cu(001) with water vapor and noted the surprising reduction of copper oxides in contrast to thermochemical data. We are also modelling the nudeation to coalescence of the oxide scale using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and noted a qualitative agreement.

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Burrowing of nanoparticles on clean metal substrates: Surface smoothing on a nanoscale

We have investigated soft landings of Co nanoparticles on clean Cu(001) surfaces. The nanoparticles, {approx}10 nm in size, were generated by dc magnetron sputtering in argon and transferred in the gas stream to an ultrahigh vacuum transmission electron microscope (TEM), where they were deposited on the thin-film substrate. The surface morphology created in this fashion exhibits a unique smoothing mechanism. The nanoparticles do not remain on the surface at temperatures as low as 600 K, but rather reorient and burrow into the substrate. By analyzing the TEM data in combination with molecular dynamics simulations, we were able to study this process in detail and to develop a model to quantify the temperature and cluster size dependence of burrowing.

Initial Oxidation Kinetics of Copper (110) Thin Films As Investigated By IN SITU UHV-TEM

In the study of metal oxidation, there is a wide gap between information provided by surface science methods and that provided by bulk oxidation studies. The former have mostly examined the adsorption of up to ∽1 monolayer (ML) of oxygen on the metal surface, where as both low and high temperature bulk oxidation studies have mainly focused on the growth of an oxide layer at the later stages of oxidation. Hence, we are visualizing the initial oxidation stages of a model metal system by in situ ultra-high vacuum (UHV) transmission electron microscopy (TEM), where the surfaces are atomically clean, in order to gain new understanding of these ambiguous stages of oxidation. We have previously studied the growth of Cu2O islands during initial oxidation of Cu(100) film. We are presently investigating the initial stages of Cu(110) oxidation, from 10−4 Torr O2 to atmospheric pressures and temperature range from room temperature to 700 °C.

UHV-TEM/REM Study of Palladium Silicide Islands Grown on Silicon (111) 7×7 Surface

The growth of self-organized nanoislands and nanowires on a substrate has been extensively studied with a view to fabricating the new functional materials and advanced electric devices. in the present work, Pd silicide islands and wires grown on Si (111) 7x77×7surface were observed in situ by ultrahigh vacuum transmission and reflection electron microscopy (UHV-TEM/REM). Pd was deposited on Si (111) 7×7 surface at about 700 K using an electron beam evaporator attached to the column of a UHV microscope. Two kinds of specimens were prepared: a &lt;111&gt; oriented rectangular specimen with a thin area, whose (111) top surface was observed by plan viewed TEM, and a &lt;110&gt; oriented bulk rectangular specimen, whose (111) side surface was observed by REMFigure la shows a REM image of Si (111) 7×7 surface. An incident electron beam is directed from the top to the bottom of the figure, which is foreshortened in the beam direction.

Ultrahigh-Vacuum Electron Microscopy for Gold Nanostructures

Ultrahigh-vacuum transmission electron microscopy (UHVTEM) has become increasingly popular for the direct observation of nanostructures having clean surfaces, since industrial requirements to make and research nano-scale materials have been rapidly growing for quantum or nanoscale electronic devices. Since we have first developed high resolution UHVTEM in 1986, the UHVTEMs have been evolved with steady advances such as UHV compatible goniometer, field emission gun or etc. Furthermore, the UHVTEM started to combine analytical capabilities such as energy dispersive X-ray spectrometer, in-column type energy filter and etc., and to combine STM (scanning tunneling microscope). The UHV technology is essential for the analysis, because the portion of contaminant in a nano-scale specimen increases as the size of the specimen goes down. This paper reports the results of gold nanostructures by recently the developed UHVTEM.Figure 1 shows recently developed UHVTEM with Schottky type field emission gun.