Robert M. Dickerson

Light amplification in semiconductor nanocrystals: Quantum rods versus quantum dots

We perform side-by-side comparison of optical gain properties of spherical and elongated nanocrystals (quantum dots and quantum rods, respectively). This comparison indicates that elongated nanoparticles provide several features beneficial for lasing applications, such as enhanced absorption cross sections (and hence reduced lasing threshold and improved photostability), increased optical gain lifetime, and extended optical gain spectral range through the use of transitions that involve both ground and excited electronic states.

Controlling heterojunction abruptness in VLS-grown semiconductor nanowires via in situ catalyst alloying.

For advanced device applications, increasing the compositional abruptness of axial heterostructured and modulation doped nanowires is critical for optimizing performance. For nanowires grown from metal catalysts, the transition region width is dictated by the solute solubility within the catalyst. For example, as a result of the relatively high solubility of Si and Ge in liquid Au for vapor-liquid-solid (VLS) grown nanowires, the transition region width between an axial Si-Ge heterojunction is typically on the order of the nanowire diameter. When the solute solubility in the catalyst is lowered, the heterojunction width can be made sharper. Here we show for the first time the systematic increase in interface sharpness between axial Ge-Si heterojunction nanowires grown by the VLS growth method using a Au-Ga alloy catalyst. Through in situ tailoring of the catalyst composition using trimethylgallium, the Ge-Si heterojunction width is systematically controlled by tuning the semiconductor solubility within a metal Au-Ga alloy catalyst. The present approach of alloying to control solute solubilities in the liquid catalyst may be extended to increasing the sharpness of axial dopant profiles, for example, in Si-Ge pn-heterojunction nanowires which is important for such applications as nanowire tunnel field effect transistors or in Si pn-junction nanowires.

Surface area of respirable beryllium metal, oxide, and copper alloy aerosols and implications for assessment of exposure risk of chronic beryllium disease

The continued occurrence of chronic beryllium disease (CBD) suggests the current occupational exposure limit of 2 microg beryllium per cubic meter of air does not adequately protect workers. This study examined the morphology and measured the particle surface area of aerodynamically size-separated powders and process-sampled particles of beryllium metal, beryllium oxide, and copper-beryllium alloy. The beryllium metal powder consisted of compact particles, whereas the beryllium oxide powder and particles were clusters of smaller primary particles. Specific surface area (SSA) results for all samples (N=30) varied by a factor of 37, from 0.56 +/- 0.07 m(2)/g (for the 0.4-0.7 microm size fraction of the process-sampled reduction furnace particles) to 20.8 +/- 0.4 m(2)/g (for the </=0.4 microm size fraction of the metal powder). Large relative differences in SSA were observed as a function of particle size for the powder of beryllium metal, from 4.0 +/- 0.01 m(2)/g (for the particle size fraction >6 microm) to 20.8 +/- 0.44 m(2)/g (for the particle size fraction </=0.4 microm). In contrast, little relative difference in SSA (<25%) was observed as a function of particle size for the beryllium oxide powder and particles collected from the screening operation. The SSA of beryllium metal powder decreases with increasing particle size, as expected for compact particles, and the SSA of the beryllium oxide powders and particles remains constant as a function of particle size, which might be expected for clustered particles. These associations illustrate how process-related factors can influence the morphology and SSA of beryllium materials. To avoid errors in predicting bioavailability of beryllium and the associated risks for CBD, the mechanisms of particle formation should be understood and the SSA of beryllium particles should be measured directly.

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

High-dose oxygen ion implantation into 6H-SiC

Microstructures of oxygen ion implanted SiC have been examined using transmission electron microscopy (TEM) and scanning transmission electron microscopy equipped with an energy-dispersive x-ray spectrometer. 6H-SiC (0001) substrates were implanted with 180 keV oxygen ions at 650 °C to fluences of 0.7×1018 and 1.4×1018/cm2. A continuous buried oxide layer was formed in both samples, while the surrounding 6H-SiC contained minimal damage. These results suggest that oxygen implantation into SiC is a useful technique to establish SiC-on-insulator structures. In bright-field TEM images, the amorphous layer possessed uniform contrast in the low-dose sample, while it consisted of three distinct layers in the high-dose sample: (1) a bubbled or mottled layer; (2) a dark contrast layer; and (3) a light contrast layer. Chemical measurements revealed that the bubbled and light contrast regions have low silicon and oxygen contents, while carbon enrichment was found in these layers.

Physicochemical characteristics of aerosol particles generated during the milling of beryllium silicate ores: implications for risk assessment.

Inhalation of beryllium dusts generated during milling of ores and cutting of beryl-containing gemstones is associated with development of beryllium sensitization and low prevalence of chronic beryllium disease (CBD). Inhalation of beryllium aerosols generated during primary beryllium production and machining of the metal, alloys, and ceramics are associated with sensitization and high rates of CBD, despite similar airborne beryllium mass concentrations among these industries. Understanding the physicochemical properties of exposure aerosols may help to understand the differential immunopathologic mechanisms of sensitization and CBD and lead to more biologically relevant exposure standards. Properties of aerosols generated during the industrial milling of bertrandite and beryl ores were evaluated. Airborne beryllium mass concentrations among work areas ranged from 0.001 μg/m3 (beryl ore grinding) to 2.1 μg/m3 (beryl ore crushing). Respirable mass fractions of airborne beryllium-containing particles were < 20% in low-energy input operation areas (ore crushing, hydroxide product drumming) and > 80% in high-energy input areas (beryl melting, beryl grinding). Particle specific surface area decreased with processing from feedstock ores to drumming final product beryllium hydroxide. Among work areas, beryllium was identified in three crystalline forms: beryl, poorly crystalline beryllium oxide, and beryllium hydroxide. In comparison to aerosols generated by high-CBD risk primary production processes, aerosol particles encountered during milling had similar mass concentrations, generally lower number concentrations and surface area, and contained no identifiable highly crystalline beryllium oxide. One possible explanation for the apparent low prevalence of CBD among workers exposed to beryllium mineral dusts may be that characteristics of the exposure material do not contribute to the development of lung burdens sufficient for progression from sensitization to CBD. In comparison to high-CBD risk exposures where the chemical nature of aerosol particles may confer higher bioavailability, respirable ore dusts likely confer considerably less. While finished product beryllium hydroxide particles may confer bioavailability similar to that of high-CBD risk aerosols, physical exposure factors (i.e., large particle sizes) may limit development of alveolar lung burdens.

Axial bandgap engineering in germanium-silicon heterostructured nanowires

Large composition changes along the nanowire axial direction provide an additional degree of freedom for tailoring charge transport in semiconductor devices. We utilize 100% axial composition modulated germanium to silicon semiconductor nanowires to demonstrate bandgap-engineered Schottky barrier heterostructured field-effect transistors that outperform their homogenous counterparts. The built-in electric field in the channel provided by the compositional change and asymmetric Schottky barrier heights enables high carrier injection in one transport direction but not the other, resulting in high on-currents of 50 μA/μm, 107 Ion/Ioff ratios, and no ambipolarity in transfer characteristics.

Effects of Xe ion irradiation and subsequent annealing on the structural properties of magnesium-aluminate spinel

Abstract Single crystals of magnesium-aluminate spinel MgAl 2 O 4 were irradiated with 340 keV Xe ++ ions at −173°C (∼100 K). A fluence of 1×10 20 Xe/m 2 created an amorphous layer at the surface of the samples. The samples were annealed for 1 h at different temperatures ranging from 130°C to 880°C. Recrystallization took place in the temperature interval between 610°C and 855°C. Transmission electron microscopy (TEM) images show two distinct layers near the surface: (1) a polycrystalline layer with columnar grain structure; and (2) a buried damaged layer epitaxial with the substrate. After annealing at 1100°C for 52 days, the profile of implanted Xe ions did not change, which means that Xe ions are not mobile in the spinel structure up to 1100°C. The thickness of the buried damaged layer decreased significantly in the 1100°C annealed sample comparing to the sample annealed for 1 h at 855°C.

Scanning Transmission Electron Microscopy‐Energy Dispersive X‐Ray/Electron Energy Loss Spectroscopy Studies on SiC‐on‐Insulator Structures

Elemental distributions and chemical bonding states of oxygen-ion-implanted SiC have been examined using scanning transmission electron microscopy (STEM) equipped with an energy dispersive X-ray spectrometer (EDX) and an electron energy loss spectrometer (EELS). 6H-SiC single crystals with [0001] orientation were implanted with 180 keV oxygen ions at 650 C to fluences of 0.7 x 10{sup 18} and 1.4 x 10{sup 18} cm{sup 2}. STEM-EDX/EELS measurements show that the low-dose sample possesses a buried amorphous SiC{sub x}O{sub y} layer, and oxygen concentration peaks around the center of the buried amorphous layer. On the other hand, a well-defined SiO{sub 2} layer including self-bonded carbon atoms is formed in the high-dose sample, and this amorphous region has a layered structure due to compositional variations of silicon, carbon, and oxygen. A slight chemical disordering induced by implantation is also confirmed to exist in topmost SiC layer.

Zero- to one-dimensional transition and Auger recombination in semiconductor quantum rods

Studies of Auger recombination in size/shape-controlled CdSe nanoparticles reveal that elongated nanocrystals provide almost independent control of the emission wavelength (diameter) and the Auger recombination rate (length). The results suggest that shape control may be the key to developing practical lasing applications.