S. M. Prokes

Light emission in thermally oxidized porous silicon: Evidence for oxide‐related luminescence

The luminescence behavior of thermally oxidized porous silicon has been examined at various temperatures and times. No blue shifting of the photoluminescence has been noted with extended oxidation time (3–120 min), in a range where a 30% oxide thickness increase has been reported. This result does not easily fit the quantum confinement model, since the luminescence does not appear to depend on particle sizes. An oxide related luminescence, which is broad, in the red, and stable at high temperatures will be discussed as a possible source of this light emission.

Luminescence cycling and defect density measurements in porous silicon: Evidence for hydride based model

Changes in dangling bond densities in porous silicon were measured and results indicate a relatively low dangling bond density (roughly 3×1016 bonds/cm3) in as‐prepared samples, which increases by a factor of 6–7 upon quenching of the photoluminescence (PL). The electron spin resonance (ESR) data suggest the presence of significant disorder in the as‐prepared 1 Ω cm porous silicon samples, which may correlate with an enhanced PL intensity. The results of heat cycling and HF dipping experiments suggest that a continuous decrease in particle size does not result in a continuous PL blue shift, as would be expected in the quantum confinement model. These results will be discussed in terms of a hydride/polysilane luminescence mechanism.

Oxygen defect center red room temperature photoluminescence from freshly etched and oxidized porous silicon

Electron spin resonance and photoluminescence experiments have been performed on freshly etched and oxidized porous silicon. Results indicate the presence of oxygen‐related centers (nonbridging oxygen‐hole center clusters), which consist of similar core structures in as‐made and oxidized porous silicon (PSi) samples. A direct correlation exists between the presence of these centers and a red photoluminescence observed in both freshly anodized and oxidized PSi, suggesting that this emission process is the result of optical transitions in the oxygen‐hole centers.

Study of the luminescence mechanism in porous silicon structures

Measurements of n‐ and p‐type porous silicon indicate no direct correlation between particle size and photoluminescence (PL) energy. Controlled continuous removal of silicon does not result in a continuous PL blueshift, which would be expected in the quantum confinement model. Also, high temperature (1200 °C) anneals of porous silicon lead to a material consisting of 100–200 nm silicon spheres, with very low dangling bond densities, similar to crystalline silicon. This material does not exhibit noticeable PL in the visible range but when dipped in hydrofluoric acid (HF) for 1 s, strong visible PL appears with no structural changes noted. Polysilane/hydride complexes appear with the HF treatment, leading to the conclusion that the visible PL may be the result of a surface phenomenon related to the polysilane/hydride complexes, and not to a bulk Si quantum confinement effect. Anneals of porous silicon to 690 °C also show a significant redshifting of the PL, exhibiting identical behavior to measurements of s...

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.

Design and performance of a simple, room-temperature Ga2O3 nanowire gas sensor

Ga2O3 nanowire gas sensors were fabricated using the vapor-liquid-solid method of nanowire growth over platinum interdigitated electrodes. While cheaply and easily fabricated, the sensors are capable of detecting various analytes at room temperature. As analyte is adsorbed onto the nanowire surfaces, a change in the device capacitance is measured. Fast recovery of the sensing devices, without the use of an external heat source, allows these devices to operate at low power. Capacitance is seen to increase following a Freundlich isotherm in response to increasing concentrations of analyte vapors.

The effect of size and size distribution on the oxidation kinetics and plasmonics of nanoscale Ag particles

We employed a simple and effective electroless (EL) plating approach to produce silver nanoparticles (NPs) on bare silicon, on dielectric ZnO nanowires (NWs) and on Si NWs, respectively. The surface stability of the homogeneous Ag NPs formed on the ZnO NW surfaces was investigated by surface enhanced Raman spectroscopy (SERS), which show that the attachment of thiol to the Ag surface can slow down the oxidation process, and the SERS signal remains strong for more than ten days. To further examine the Ag NP oxidation process in air, the oxygen content in the silicon nanowire core/Ag sheath composites was monitored by the energy dispersive x-ray (EDX) method. The amount of oxygen in the system increases with time, indicating the silver NPs were continuously oxidized, and it is not clear if saturation is reached in this time period. To investigate the influence of the Ag NPs size distribution on the oxidation process, the oxygen amount in the NPs formed by EL deposition and e-beam (EB) evaporation on a bare silicon surface was compared. Results indicate a faster oxidation process in the EL formed Ag NPs than those produced by EB evaporation. We attribute this observation to the small diameter of the EL produced silver particles, which results in a higher surface energy.

Interdiffusion measurements in asymmetrically strained SiGe/Si superlattices

Interdiffusion measurements are reported for Si0.65Ge0.35/Si asymmetrically strained superlattices grown by molecular beam epitaxy at 530 °C. The temperature‐dependent interdiffusion coefficient obtained from x‐ray diffraction can be described by D=675 exp(−4.4 eV/kT)cm2/s in the temperature range 700–880 °C. Initially, an enhanced diffusion was observed, especially near the superlattice surface. This is attributed to the presence of nonequilibrium defects. Bulk interdiffusion measurements were made only after isoconfigurational conditions were attained. The diffusion analysis first formulated by J. W. Cahn [Acta Metall. 9, 795 (1961)] is applied here, and the relative importance of both gradient energy effects and coherency strain effects will be discussed.

Stress-driven formation of Si nanowires

We present an alternate mechanism for the growth of Si nanowires directly from a silicon substrate, without the use of a metal catalyst, silicon vapor or chemical vapor deposition (CVD) gasses. Since the silicon wires grow directly from the silicon substrate, they do not need to be manipulated or aligned for subsequent applications. Wires in the 20–50 nm diameter range with lengths over 80μm can be easily grown by this technique. The critical parameters in the growth of these nanowires are the surface treatment and the carrier gas used. A model is proposed involving stress-driven wire growth, which is enhanced by surface Si atom diffusion due to the presence of hydrogen gas.

Growth of epitaxial InAs nanowires in a simple closed system

The epitaxial growth of InAs nanowires on an InAs(111) substrate in a sealed quartz tube is described. The method is quite simple and fast, and uses only a bare InAs substrate and a gold colloid coated InAs(111) substrate. High quality InAs nanowires can be produced by this technique, with the nanowire diameter controllable by the variation of growth temperature. The composition of the seed particle at the tip of the nanowire indicates that the nanowires grew via the vapor-liquid-solid growth mechanism but with Au–In as the liquid alloy.