F.R. Chen

Fabrication, characterization and studies of annealing effects on ferromagnetism in Zn1?xCoxO nanowires

Diluted magnetic semiconductor Zn1−xCoxO (x≤0.11) nanowires with average diameter of ~40xa0nm were prepared by thermal evaporation, followed by high-energy Co ion implantation. Bombardment by Co ions produced a good number of structural defects (stacking faults and orientational variations) in the nanowires. The as-implanted nanowires were paramagnetic. We performed two types of thermal annealing, one in 1xa0atm argon flow and the other in a high vacuum, at 600u2009°C, and studied the effects of annealing on the magnetic properties of these nanowires. Argon annealing removed structural defects in the nanowires and the nanowires then revealed ferromagnetic ordering. This result suggests that structure defects are harmful to the occurrence of ferromagnetism in the Co-implanted ZnO. The structure of the as-implanted and the annealed nanowires was inspected in detail by using scanning electron microscopy, energy dispersive x-ray spectroscopy, maps of electron energy loss spectra, x-ray diffraction, and high-resolution transmission electron microscopy. Taken together, these studies suggested that no second phase existed on the scale down to the spatial resolution of ~0.5xa0nm. Noticeably, the nanowires even displayed largely enhanced ferromagnetism after annealing in a high vacuum. A subsequent annealing in oxygen has also been performed on those vacuum-annealed nanowires to study the roles played by the O vacancies in determining the ferromagnetic properties of the nanowires. Our results indicate that both the improved structural quality and the increased number of O vacancies are key factors for the occurrence of ferromagnetic ordering in the Zn1−xCoxO nanowires.

Observations of Al segregation around dislocations in AlGaN

Transmission electron microscopy has been used to observe Al segregation around the threading dislocations in Al0.1Ga0.9N and Al0.3Ga0.7N grown by metalorganic chemical vapor deposition on 6H–SiC. Dislocation lines were found to have up to 70% more Al concentration than those regions free of dislocations in the matrix. The Al-depleted regions around the dislocations are shown to be within a few nanometers from the dislocation lines. The results also show that more Al segregate to edge dislocations than to screw ones.

Segregation of Bismuth to Triple Junctions in Copper

Bismuth segregation in copper has been studied using energy-dispersive X-ray spectrometry (EDX) in a JEOL 2010F transmission electron microscope. In addition to the expected solute enrichment at grain boundaries, we have observed extremely high concentrations of bismuth at certain triple junctions, with significantly greater enrichment factors than in the adjacent grain boundaries. It is shown here that the triple junction segregation is a function of the parameters of the grain boundaries at the triple line, and existence of this type of segregation implies that the affected triple junctions embody excess free energy. At least one of the observed triple junctions may not obey the usual X-product rule, as a result of deviations from the exact coincidence misorientations.

Room-temperature ferromagnetism in self-assembled (In, Mn)As quantum dots

Self-assembled In1−xMnxAs quantum dots (0.19⩽x⩽0.45) have been grown on GaAs (100) substrates by low-temperature molecular beam epitaxy. The microstructure analysis revealed that the uniformly distributed In1−xMnxAs dots have a zinc blende structure as x⩽0.38. Furthermore, all samples exhibit ferromagnetic state at 5K, and their Curie temperatures range from 260to340K varying with x. These (In, Mn)As quantum dots are promising for room-temperature spintronic devices.

Direct observations of heteroepitaxial diamond on a silicon(110) substrate by microwave plasma chemical vapor deposition

Heteroepitaxial diamond has been successfully deposited on Si (110) substrate by microwave plasma chemical vapor deposition method. The pretreatment consisted of carburization and bias-enhanced nucleation steps. Cross-sectional transmission electron microscopy reveals that diamond can be in the cube-on-cube epitaxial relationship with the Si substrate. Various orientation relationships between diamond and Si substrates have also been observed, depending on the location where the plasma applied. Near the center of the plasma, twins were rarely observed in cube-on-cube epitaxial regions. Away from the center of the plasma ball, {Sigma}3 twins are seen first, and then additional {Sigma}9 and {Sigma}27 twins occur near the edge of the plasma. In general, defect density in the epitaxial films is less than that observed in polycrystalline ones. No interlayer could be observed between diamond and silicon. In addition, 2H-type hexagonal diamond has also been found, and is in epitaxy with the Si substrate. {copyright} {ital 1996 Materials Research Society.}

Single crystalline ZnS nanotubes and their structural degradation under electron beam irradiation

ZnS nanotubes were synthesized using wet-chemistry method. These nanotubes appear to be extremely unstable under electron beam irradiation. Time dependent transmission electron diffraction patterns disclose the appearance of additional diffraction spots that belong to ZnO, with the prolonged e-beam irradiation duration. The experimental results suggest that displacement damage followed by oxidation is mainly responsible for the structural degradation of these ZnS nanotubes.

The effect of substrate position on the orientation and interfacial reaction of epitaxial diamond on silicon

Abstract Diamond was grown on silicon substrates by microwave plasma-enhanced chemical vapour deposition with pretreatment consisting of carburization and biasing. Cross-sectional transmission electron microscopy shows that epitaxial diamond can be directly grown on Si(110). The orientation of epitaxial diamond varies with the substrate position under the plasma ball. At the central part of the substrate, diamond is mainly in a cube-on-cube orientation relationship with silicon which is dia// Si and {111}dia//{111}Si. Away from the centre, five different orientation relationships are observed. The defect density of the diamond film is also dependent on the substrate position. For Si(100) substrates, the interfacial structure between diamond and Si also varies as a function of the substrate position. An SiC interlayer is formed after the bias and growth stages of deposition. The amorphous carbon, deposited in the carburization step, may react with Si to form SiC. Plasma inhomogeneity plays an important role in the variation of the diamond orientation. In addition, the composition and structure of the interlayer formed between diamond and silicon depend on the position under the plasma.

Heteroepitaxial diamond nucleation and growth on silicon by microwave plasma-enhanced chemical vapor deposition synthesis

Heteroepitaxial diamond films were successfully nucleated and deposited on 1-inch diameter Si(001) substrates by microwave plasma-enhanced chemical vapor deposition (MPECVD). The precursor gases for the synthesis were methane and hydrogen. Before the application of a negative d.c. bias to the substrate, an in-situ carburization pre-treatment on the silicon was found to be an indispensable step towards the heteroepitaxial diamond on the silicon. Morphologies of the films were characterized by scanning electron microscopy (SEM). Interface observations based on the cross-sectional HRTEM directly reveal the heteroepitaxial diamond nucleation phenomena in detail. No interlayers of silicon carbide and/or amorphous carbon phases were observed. Tilt and azimuthal misorientation angles between the heteroepitaxial diamond crystals and the substrate were determined by combining the Ewald sphere construction in the reciprocal lattice space and the selected area diffraction (SAD) patterns taken across the interface.

HRTEM and energy filtering TEM study of interfacial reaction between diamond film and silicon

Abstract Diamond was deposited on Si(100) substrates by microwave plasma assisted chemical vapor deposition in three steps: carburization, biasing and growth. High-resolution transmission electron microscopy (TEM) in cross-sectional view has been used to observe the evolution of microstructures around the interfacial region between diamond and Si in each processing step. The chemistry near the interface was characterized with elemental mapping using an energy-filtered TEM imaging technique. In the carburization stage, an amorphous SiC interlayer 1.5 nm thick was formed between an amorphous carbon layer and Si. In the following biasing stage, β-SiC can form in epitaxial orientation with Si. In the growth stage, diamond grains aligned in a strongly textured condition developed, while the initially grown diamond had polycrystalline characteristics. Processing conditions for epitaxial diamond deposition on Si substrates are briefly discussed.

Prospects for Bright Field and Dark Field Electron Tomography on a Discrete Grid

Extended abstract of a paper presented at the Pre-Meeting Congress: Materials Research in an Aberration-Free Environment, at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, July 31 and August 1, 2004.