C. Y. Liao

Mechanism of stress relaxation in Ge nanocrystals embedded in SiO2

Ion-beam-synthesized {sup 74}Ge nanocrystals embedded in an amorphous silica matrix exhibit large compressive stresses in the as-grown state. The compressive stress is determined quantitatively by evaluating the Raman line shift referenced to the line position of free-standing nanocrystals. Post-growth thermal treatments lead to stress reduction. The stress relief process is shown to be governed by the diffusive flux of matrix atoms away from the local nanocrystal growth region. A theoretical model that quantitatively describes this process is presented.

Stable, freestanding Ge nanocrystals

Freestanding Ge nanocrystals that are stable under ambient conditions have been synthesized in a two-step process. First, nanocrystals with a mean diameter of 5nm are grown in amorphous SiO2 by ion implantation followed by thermal annealing. The oxide matrix is then removed by selective etching in diluted HF to obtain freestanding nanocrystals on a Si wafer. After etching, nanocrystals are retained on the surface and the size distribution is not significantly altered. Freestanding nanocrystals are stable under ambient atmospheric conditions, suggesting formation of a self-limiting native oxide layer. For freestanding as opposed to embedded Ge nanocrystals, an additional amorphouslike contribution to the Raman spectrum is observed and is assigned to surface reconstruction-induced disordering of near-surface atoms.

High-purity, isotopically enriched bulk silicon

The synthesis and characterization of dislocation-free, undoped, single crystals of Si enriched in all 3 stable isotopes is reported: {sup 28}Si (99.92%), {sup 29}Si (91.37%), and {sup 30}Si (89.8%). A silane-based process compatible with the relatively small amounts of isotopically enriched precursors that are practically available was used. The silane is decomposed to silicon on a graphite starter rod heated to 700-750 C in a recirculating flow reactor. A typical run produces 35 gm of polycrystalline Si at a growth rates of 5 {micro}m/min and conversion efficiency >95%. Single crystals are grown by the floating zone method and characterized by electrical and optical measurements. Concentrations of shallow dopants (P and B) are as low as mid-10{sup 13} cm{sup -3}. Concentrations of C and O lie below 10{sup 16} and 10{sup 15} cm{sup -3}, respectively.

Reversible phase changes in Ge–Au nanoparticles

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80u2009°C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.

Processing route for size distribution narrowing of ion beam synthesized nanoclusters

Ion beam synthesis of nanocrystals is explored using a recently developed kinetic Monte Carlo model for the process. The model suggests that temperature can be used to engineer nanocrystal size distributions. Specifically, by initiating implants at low temperature and then ramping the temperature upward, one can both tune the average size of the nanocrystals and restrict size distribution widths to less than 20% of the average size.

Photoluminescence enhancement of Er-doped silica containing Ge nanoclusters

The photoluminescence (PL) of Er-doped silica films containing Ge nanoclusters synthesized by ion implantation was investigated. The area of the 1540 nm Er3+ PL peak was enhanced by up to a factor of 200 by the addition of Ge nanoclusters. The PL enhancement was found to be proportional to the concentration of Ge atoms. Control experiments with argon ion implantation were used to show that the enhancement is due to the presence of Ge and not radiation damage. Furthermore, the Er3+ PL was found to be strongly influenced by the postgrowth annealing and the crystallinity of the Ge nanoclusters.

Germanium Nanocrystals Embedded in Sapphire

{sup 74}Ge nanocrystals are formed in a sapphire matrix by ion implantation followed by damage. Embedded nanocrystals experience large compressive stress relative to bulk, as embedded in sapphire melt very close to the bulk melting point (Tm = 936 C) whereas experience considerably lower stresses. Also, in situ TEM reveals that nanocrystals ion-beam-synthesized nanocrystals embedded in silica are observed to be spherical and measured by Raman spectroscopy of the zone center optical phonon. In contrast, reveals that the nanocrystals are faceted and have a bi-modal size distribution. Notably, the matrix remains crystalline despite the large implantation dose and corresponding thermal annealing. Transmission electron microscopy (TEM) of as-grown samples those embedded in silica exhibit a significant melting point hysteresis around T{sub m}.

Melting Kinetics of Confined Systems at the Nanoscale: Superheating and Supercooling

In situ electron diffraction measurements of silica‐embedded Ge nanocrystals reveal a melting/solidification hysteresis of 470 K which is approximately symmetric about the bulk melting point. This surprising behavior, which is thought to be impossible in bulk systems, is well described by a simple, classical thermodynamic model. Surface pre‐melting, which occurs for materials with free surfaces, is suppressed by the presence of the host matrix, thereby allowing both kinetic supercooling and kinetic superheating of the embedded nanocrystals.

In Situ Characterization of Ge Nanocrystals Near the Growth Temperature

We present in situ electron diffraction data indicating that Ge nanocrystals embedded in a silica matrix can be solid at temperatures exceeding the bulk Ge melting point. Supercooling is observed when returning from temperatures above the melting point of the Ge nanocrystals. Since melting point hysteresis is observed, it is not clear if nanoclusters are solid or liquid during the initial growth process. Raman spectra of as‐grown nanocrystals give a measure of compressive stress and in‐situ Raman spectroscopy further confirms the presence of crystalline Ge above 800 °C.