Compositional stability of FePt nanoparticles on SiO2/Si during annealing

The loss of Fe due to oxidation or diffusion into the substrate can prevent the successful preparation of well-ordered, stoichiometric, FePt nanoparticles. In this work we report the composition changes during annealing observed for small ( \u3c 10 nm) FePt nanoparticles on thermally grown SiO2 layers on Si wafer substrates. Additionally, we describe the use of a controlled reducing gas mixture, Ar+H-2+H2O, to reduce the loss of Fe

Diffusion profiles of high dosage Cr and V ions implanted into silicon

The depth profiles of high dosage Cr-52(+) and V-51(+) ions implanted in (100) crystalline silicon after thermal anneal at temperatures between 300 degreesC and 1000 degreesC are studied by secondary ion mass spectrometry and cross-sectional transmission electron microscopy. At dosages of 1x10(15) ions/cm(2) and above, the surface layer of silicon substrate is amorphorized. During the subsequent thermal annealing, the depth profiles of the implanted ions are strongly coupled with the solid phase epitaxial growth of amorphous silicon. Silicide precipitate formation is important to understand the differences between Cr and V diffusion. After anneal of the 1x10(15) ions/cm(2) implanted samples at 900 degreesC and 1000 degreesC, most of the Cr has left the silicon, but only 10% of the V has escaped. The 1x10(14) ions/cm(2) Cr-implanted sample shows Cr ions exist only near the surface after 1000 degreesC anneal. The V-implanted sample, on the other hand, only shows a narrowing of the V profile after 1000 degreesC anneal

Antiphase ordering and surface phases in lithium aluminate

Antiphase domains are seen in single crystal gamma lithium aluminate (gamma-LiAlO(2)) with 16.7 nm periodicity in the \u3c 110 \u3e direction. Alternate domains have a 1/2 [001] shift. Beta phase lithium aluminate (beta-LiAlO(2)) is seen to form on the surface of the as-received wafers with an epitaxial strain limited relationship with the bulk gamma phase. The orthorhombic beta phase aligns with the a and b axes (0.528 and 0.630 nm) matching with the tetragonal gamma phase\u27s a and c axes (0.5168 and 0.6268 nm). The gamma and beta phases are seen to have different etch rates. The beta phase converts back to the gamma phase above 450 degrees C

Defects in m-face GaN films grown by halide vapor phase epitaxy on LiAlO2

Free-standing wafers (50 mm diameter) of GaN were grown by halide vapor phase epitaxy on lattice-matched gamma-LiAlO2. We report a transmission electron microscopy study of defects and defect densities in these wafers. The growth direction is [10 (1) over bar0]. Stacking faults in the basal plane are seen when viewing the specimen in the [1 (2) over bar 10] direction with an average spacing of less than 100 nm. Convergent beam electron diffraction measurements show no switch in the polarity and thus the faults are proposed to be ABABACAC changes in the stacking. Threading dislocations are found to have a correlated arrangement with a density of 3x10(8) cm(-2) when viewing the [1 (2) over bar 10] direction and widely varying (depending upon location) when viewing in the [0001] direction. These dislocations act as seeds for postgrowth surface features that directly exhibit the correlated nature of these threading dislocations

Pressure-induced structural transformations in Si:V and Si:V, Mn

Semiconductors doped with magnetically active atoms are expected to find application in spintronics. Si samples implanted with Mn⁺ (Si:Mn) or with V⁺ (Si:V) can order magnetically after processing at high temperature (HT) and also under enhanced hydrostatic pressure (HP). This work presents new results on structure-related properties of single crystalline Si implanted at 200 keV with V⁺ as well as that co-implanted additionally with Mn⁺ ions (Si:V, Mn), with dosages DV⁺ ≤ 5·10¹⁵ cm⁻² and DMn⁺ = 1·10¹⁵ cm⁻². The samples were processed for 1–5 h at HT ≤ 1270 K under HP ≤ 1.1 GPa. Secondary Ion Mass Spectrometry, Transmission Electron Microscopy, X-ray and related methods were applied for sample characterization. The HT- (HP) treatment affects, among others, solid phase epitaxial re-growth (SPER) of amorphous silicon created at implantation and distribution of implanted species

Disordering of small metal particles in a scanning transmission electron microscope

Small metal particles in the range of a few nanometers in diameter are seen to progressively disorder when the 100 keV electron beam of a Scanning Transmission Electron Microscope (STEM) is held stationary on the particle. The diffraction pattern of the individual particle is seen to progress from an initial array of indexable diffraction spots to a mixture of diffraction spots and amorphous-like rings and finally to rings with no persistent diffraction spots. Only particles below a critical size are seen to fully disorder. We have observed this disordering in Platinum, Palladium, Rhodium, and Iridium and have developed a model for the disordering process. In this model, electrons scattering from surface atoms transfer enough energy to break the surface atoms from their binding site. A competing process of disordered atoms rebinding to crystalline sites is also included. Because small particles have large fractions of their atoms on the surface, the beam driven disorder, under certain conditions, is able to propagate into the core of the particle. For Platinum, surface disordering requires energy transfers from the electrons to the Platinum atoms of0.54 eV.U of I Onlydissertation/thesi

Sol-Gel Synthesis And Phase Evolution Behavior Of Sterically Stabilized Nanocrystalline Zirconia

Nanocrystalline as well as submicron sized, non-agglomerated, spherical ZrO2 particles have been successfully synthesized using the sol-gel technique utilizing hydroxypropyl cellulose (HPC) as a polymeric steric stabilizer. The effect of various parameters such as the ratio of molar concentration of water and alkoxide (R), the molar concentration [HPC] and the molecular weight (MWHPC) of HPC polymer as well as the calcination temperature on ZrO2 nanocrystallites size and their phase evolution behavior is systematically studied. The phase evolution behavior of nanocry stalline ZrO2 is explained and correlated with the adsorption behavior of HPC polymer on ZrO2 nanoparticles surface, which is observed to be a function of R, [HPC], MWHPC and the calcination temperature. Optimum synthesis parameters for obtaining 100% tetragonal phase in nanocry stalline ZrO2 are identified for the present sol-gel method of synthesizing nanoparticles

Atomic-Scale Imaging Of Dopant Atom Distributions Within Silicon Δ-Doped Layers

We report measurements of the distribution of Sb atoms in δ-doped Si, over a wide 2-D concentration range. Both annular dark-field imaging and electron energy loss spectroscopy proved sufficiently sensitive to locate Sb atoms at the atomic scale. Improvements in both detector sensitivities and specimen preparation were necessary to achieve these results, which offer a surprising explanation for the dramatic difference in electrical activity between 2-D and 3-D dopant distributions at the same effective volume concentrations. The prospects for the general identification of individual dopant atoms will be discussed. © 2001 Materials Research Society