S. P. Withrow

Growth of Ge, Si, and SiGe nanocrystals in SiO2 matrices

Nanocrystals of group‐IV semiconductor materials (Si, Ge, and SiGe) have been fabricated in SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystals has been studied by transmission electron microscopy. Critical influences of the annealing temperatures and implantation doses on the nanocrystal size distributions are demonstrated with the Ge‐implanted systems. Significant roughening of the nanocrystals occurs when the annealing temperature is raised above the melting temperature of the implanted semiconductor material.

Controlling the size, structure and orientation of semiconductor nanocrystals using metastable phase recrystallization

Materials engineering at the nanometre scale should provide smaller technological devices than are currently available,. In particular, research on semiconductor nanostructures with size-dependent optical and electronic properties is motivated by potential applications which include quantum-dot lasers and high-speed nonlinear optical switches,. Here we describe an approach for controlling the size, orientation and lattice structure of semiconductor nanocrystals embedded in a transparent matrix. We form nanocrystalline precipitates by implanting ions of the semiconductor into a single-crystal alumina substrate and applying thermal annealing. Control over the microstructure of the nanocrystals is achieved using substrate amorphization and recrystallization. In essence, the substrate microstructure is manipulated using ion beams to induce changes in impurity solubility, crystal symmetry and cation bonding, which exert a profound influence on the microstructure of the embedded precipitates—a concept familiar in metallurgy. This approach can be extended to exercise control over virtually any type of precipitate (such as metals, insulators or magnetic clusters) as well as epitaxial thin films.

Direct measurements of the velocity and thickness of ‘‘explosively’’ propagating buried molten layers in amorphous silicon

Simultaneous infrared (1152 nm) and visible (633 nm) reflectivity measurements with nanosecond resolution were used to study the initial formation and subsequent motion of pulsed KrF laser‐induced ‘‘explosively’’ propagating buried molten layers in ion implantation‐amorphized silicon. The buried layer velocity decreases with depth below the surface, but increases with KrF laser energy density; a maximum velocity of about 14 m/s was observed, implying an undercooling‐velocity relationship of ∼14 K/(m/s). Z‐contrast scanning transmission electron microscopy was used to form a direct chemical image of implanted Cu ions transported by the buried layer and showed that the final buried layer thickness was <15 nm.

GaAs nanocrystals formed by sequential ion implantation

Sequential ion implantation of As and Ga into SiO2 and α‐Al2O3 followed by thermal annealing has been used to form zinc‐blende GaAs nanocrystals in these two matrices. In SiO2, the nanocrystals are nearly spherical and randomly oriented, with diameters less than 15 nm. In Al2O3, the nanocrystals are three dimensionally aligned with respect to the crystal lattice. Infrared reflectance measurements show evidence for surface phonon modes in the GaAs nanocrystals in these matrices.

Ion implantation in β-SiC: Effect of channeling direction and critical energy for amorphization

Damage in single-crystal ..beta..-SiC(100) as a result of ion bombardment has been studied using Rutherford backscattering/channeling and cross-section transmission electron microscopy. Samples were implanted with Al (130 keV) and Si (87 keV) with doses between 4 and 20 x 10/sup 14/ cm/sup -2/ at liquid nitrogen and room temperatures. Backscattering spectra for He/sup +/ channeling as a function of implantation dose were initially obtained in the (110) direction to determine damage accumulation. However, the backscattered yield along this direction was shown to be enhanced as a result of uniaxial implantation-induced strain along (100). Spectra obtained by channeling along this latter direction were used along with the computer program t-smcapsr-smcapsIm-smcaps to calculate the critical energy for amorphization. The results for amorphization of ..beta..-SiC at liquid nitrogen and room temperature are approx.14.5 eV/atom and approx.22.5 eV/atom, respectively.

Oriented ferromagnetic Fe-Pt alloy nanoparticles produced in Al2O3 by ion-beam synthesis

Oriented Fe1−xPtx nanoparticles have been formed in single-crystal Al2O3 host matrices by the sequential implantation of Fe and Pt ions followed by thermal annealing. For x in the range of ∼35–55 at. % Pt, these nanoparticles are in the chemically ordered tetragonal L10 structure of FePt and appear to be fully ordered. The nanoparticles are ferromagnetic, and the magnetic coercivity is a strong function of the alloy composition, reaching values in excess of 20 kOe for x∼45%. The crystallographic orientation and morphology of the nanoparticles are strongly dependent on the implantation conditions. Under certain implantation conditions, a buried amorphous layer can be formed in the Al2O3 matrix which crystallizes during annealing giving rise to the formation of an interconnected network of large FePt particles with a single orientation. Oriented nanoparticles of Fe3Pt and FePt3 were also synthesized. The Fe3Pt and FePt3 particles have the ordered, cubic L12 structure with an order parameter of 0.5–0.8; and ...

Fabrication of single-crystal diamond microcomponents

We have combined a technique for the lift‐off of thin diamond films from a bulk diamond with a technique for engraving diamond with a focused excimer laser to produce free‐standing single‐crystal diamond microstructures. One microcomponent that has been produced is a 12 tooth gear ∼400 μm in diameter and ∼13 μm thick. Other microstructures have also been demonstrated, showing the versatility of this method. This process should be applicable to producing diamond microcomponents down to spatial dimensions (width and thickness) of a few micrometers.

Ion‐beam synthesis and stability of GaAs nanocrystals in silicon

Sequential implantation of Ga and As into silicon followed by thermal annealing has been used to synthesize GaAs buried inside silicon. The GaAs exists in the form of nanocrystals which are three‐dimensionally oriented with respect to the silicon matrix. Thermodynamic criteria which are important in determining whether the desired compound will form are discussed.

Formation of CdS and CdSe nanocrystals by sequential implantation

Abstract Nanocrystals of CdS and CdSe have been formed in SiO2, Al2O3, and Si by sequential ion implantation and annealing. In SiO2, the nanocrystals have the hexagonal wurtzite structure and are randomly oriented. In Al2O3 and Si, nanocrystals are three dimensionally oriented. They have the cubic zincblende structure in Si and can be produced with either structure in Al2O3 by controlling the implantation conditions. In SiO2 and Al2O3, nanocrystals exhibit strong optical absorption and photoluminescence (PL). Evidence for quantum confinement is observed, and the PL results for CdSe are strongly temperature dependent.

Magnetic force microscopy of ferromagnetic nanoparticles formed in Al2O3 and SiO2 by ion implantation

Magnetic force microscopy (MFM) has been used to investigate the properties of ferromagnetic FePt nanoparticles produced by the implantation of Fe and Pt ions into single-crystal Al2O3 or fused SiO2 followed by thermal processing. The MFM results are compared to cross-section and plan view transmission electron microscopy images of the same samples. We demonstrate that MFM can detect magnetism in nanosized particles that are situated several hundred nm below the sample surface. MFM is shown to be a promising tool for studying the characteristics of magnetic nanoparticles produced by ion implantation.