O. J. Glembocki

Enhanced plasmon coupling in crossed dielectric/metal nanowire composite geometries and applications to surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) was performed on Ga2O3∕Ag and ZnO∕Ag nanowires, which were arranged in either a crossover or noncrossing geometry. Results indicate a high SERS sensitivity (near 0.2pg) for nanowires arranged in a crossing geometry. It is suggested that this is due to the dielectric core/metal shell structure, as well as to the nanowire crossings, which are regions of very high electric fields. Finite element simulations of the electric field near two crossed wires confirm an enhanced plasmon resonance in the vicinity of the crossing, which extends spatially in the crossings and around the nanowires.

Optical characterization of the electrical properties of processed GaAs

Abstract Optical spectroscopies such as photoreflectance offer unique and non-invasive tools that can be used to probe the bulk and surface electrical properties of processed semiconductors. Because of their contactless nature, they can also serve as in-situ, as well as ex-situ diagnostics of processing effects during various stages of optoelectronic device formation. In this paper, we will focus on the applications of photoreflectance to study the effects of various processing procedures such as dry etching, surface passivation and metallization on the electrical properties of GaAs. We will show how certain information, such as barrier heights and densities of surface states can be obtained in a contactless fashion.

An Alternate Mechanism for Porous Si Photoluminescence: Recombination in SiH x Complexes

Porous silicon layers have been formed which exhibit photoluminescence (PL) peaks that do not blueshift with increasing porosity. Hydrogen desorption experiments have been performed in vacuum under anneals between 230°C–390°C. Simultaneous, in-situ PL measurements show that the PL intensity decreases and disappears with decreasing hydrogen content. Infrared spectroscopy (IR) measurements also show loss of hydride species in the same temperature range. These results indicate that hydrogen is important in the PL process. We suggest that silicon hydrides are the source of PL in porous SI.

Far-infrared spectroscopic, magnetotransport, and x-ray study of athermal annealing in neutron-transmutation-doped silicon

We present evidence that the energy introduced by a short laser pulse focused to high intensity on a small spot on the surface of neutron-transmutation-doped silicon electrically activates impurities far away from the focal spot. The activation of the impurities is measured by far-infrared spectroscopy of shallow donor levels and by magnetotransport characterization. Electrical activity is comparable to that obtained with conventional thermal annealing. X-ray rocking curve measurements show strain in the area of the focal spot, but none at large distances from the focal spot.

Effect of crossing geometry on the plasmonic behavior of dielectric core/metal sheath nanowires

We have shown that dielectric/metal composite nanowires exhibit very strong surface enhanced Raman spectroscopy (SERS) signals when arranged in a random three-dimensional geometry. Since the intersections of the nanowires are critical in generating the high electric fields necessary for this enhancement, we are investigating this effect under more controlled conditions. We examined nanowire arrays formed by e-beam lithography and we have examined the plasmonic effects, both longitudinal and transverse, due to changes in crossing geometry by specific placements of dielectric/metal nanowires on these arrays. Results indicate significant angular effects on the SERS enhancement supported by electric field calculations.

Growth and characterization of single crystal Ga2O3 nanowires and nano-ribbons for sensing applications

The growth of monoclinic Ga2O3 nanowires, nano-ribbons and nano-sheets has been investigated. Results indicate that high quality single crystal nanowires can be grown at 900°C using an Au catalyst, while single crystal nano-ribbons and nano-sheets require no metal catalyst for growth. Since bulk Ga2O3 is a promising material for high temperature sensing, Ga2O3 nanowires and nano-ribbons may prove to enhance the sensing capability due to the high surface area. We have investigated the nature of defects in this material using Electron Spin Resonance, in as grown material, as well as under annealing in a reducing gas (H2) at various high temperatures. Results indicate the presence of an oxygen deficiency in the material, resulting in a conduction electron signal that becomes enhanced with annealing. An alternate use of these nanowires for sensing applications will also be presented, involving Surface Enhanced Raman Spectroscopy.

Electronic properties of GaAs surfaces etched in an electron cyclotron resonance source and chemically passivated using P2S5

Photoreflectance has been used to study the electronic properties of (100) GaAs surfaces exposed to a Cl2/Ar plasma generated by an electron cyclotron resonance source and subsequently passivated by P2S5. The plasma etch shifts the Fermi level of p-GaAs from near the valence band to midgap, but has no effect on n-GaAs. For ion energies below 250 eV, post-etch P2S5 chemical passivation removes the surface etch damage and restores the electronic properties to pre-etch conditions. Above 250 eV, the etch produces subsurface defects which cannot be chemically passivated. Auger electron spectroscopy shows that etching increases As at the GaAs/oxide interface, while passivation reduces it.

Nanoscale photovoltaic imaging using the scanning tunneling microscope

Abstract Optical interactions in scanning tunneling microscopy (STM) have been considered. It is shown that when STM is used as a proximal probe of photovoltages it is possible to obtain local information about the physical properties of the subsurface semiconducting material. We find that photovoltaic STM measurements allow us to detect relative changes in bulk properties, such as band gap and doping density, on a nanometer scale. These results show that the combination of STM and surface photovoltage are capable of providing not only valuable information about the surface of a material, but also about its bulk properties.

GaN light emitting diode as a photoreflectance pump source

A GaN light emitting diode is used as a photoreflectance pump source to acquire optical impedance spectroscopy data. Such a pump source has the advantage of having a controllable pump wave form (intensity, modulation depth, and shape) over a broad frequency range. Given the ready availability of light emitting diodes, many different wavelengths are potentially available.

Improved GaN materials and devices through confined epitaxy

An approach to reduce vertical threading dislocations in the active regions of III-nitride devices is described. The approach involves confined homo- or heteroepitaxy of GaN materials using sputtered oxide masks to delineate growth regions and conventional metal-organic chemical vapor deposition. The resulting confined epitaxial material is terminated with equilibrium crystal facets, which form hexagonal mesas, and contains a reduced dislocation density and reduced strain compared to the underlying template layer for homoepitaxial growth. Characterization of pn junction diodes grown with this approach reveals significantly reduced leakage currents in as-grown, unpassivated structures (as low as 1×10−7Acm−2).