Spark-processing has been shown recently to generate a silicon-based substance which intensely photoluminesces in the UV/blue or green spectral range, depending on whether the preparation was performed in flowing or stagnant air, respectively. This study reports about significant differences for the radiative properties of spark-processed silicon after preparations in oxygen, nitrogen, and mixtures of both gases. Whereas there is essentially no photoluminescence (PL) after processing in pure nitrogen, an orange PL band at 1.89 eV occurs after preparation in pure oxygen. The orange PL degrades with time and when exposed to laser light. Furthermore, it switches immediately to a more intense blue emission centered at 2.61 eV when the sample is subjected to lower gas pressures. A similar 2.6 eV PL is observed after heat treatments up to 350°C. If, however, spark-processing of Si is carried out in nitrogen and oxygen is added, a green PL arises, centered at 2.35 eV. Moreover, for a volume ratio of about 1:1 N2/O2 a PL band at 3.22 eV emerges, superimposing the green emission. Both bands are about two orders of magnitude more intense than the orange PL, when prepared under otherwise identical processing conditions.

Si nanocrystals were deposited in a helium atmosphere by the gas-evaporation technique. Their average size is 3.5 nm, much smaller than those of the Si nanocrystals deposited in an argon atmosphere. The PL spectra of the as-deposited and the HF-treated Si nanocrystals were compared. A great increase in the PL intensity of the HF-treated Si nanocrystals is attributed to the hydrogen passivation of Si surface dangling bonds. A good correlation between the amount of Si-O bonds and the PL intensity suggests that the oxygen-passivation of dangling bonds is required for the red-band PL. The PL spectra of the HF-treated Si nanocrystals resemble those of porous Si and clearly indicate that the HF-treated Si nanocrystals well simulate the porous Si.

We have prepared silicon nanocolloids by trapping silicon nanocrystallites produced by a gas evaporation method into an organic liquid. Blue photoluminescence (PL) at a peak maximum around 480 nm from silicon nanocolloids has been observed at room temperature. The external quantum efficiency of blue PL is 0.16 %. A room temperature UV PL has been observed by dropping alkaline solids such as sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)2) into silicon nanocolloids. The mechanism of blue and UV PL from silicon nanocolloids is discussed.

Density of states changes in the valence and conduction band of silicon nanoclusters were monitored using soft x-ray emission and absorption spectroscopy as a function of cluster size. A progressive increase in the valence band edge toward lower energy is found for clusters with decreasing diameters. A similar but smaller shift is observed in the near-edge x-ray absorption data of the silicon nanoclusters.

We studied the optical and electronic properties of silicon nanocrystals derived from two distinct fabrication procedures. One technique uses a controlled chemical reaction. In the other case, silicon nanocrystals are produced by ultrasonic fracturing of porous silicon layers. We report on the photoluminescence, photoluminescence excitation, and absorption spectroscopy of various size distributions derived from these techniques. We compare the different optical properties of silicon nanocrystals made this way and contrast them with that observed in porous silicon. Our results emphasize the dominant role of surface states in these systems as manifested by the different surface passivation layers present in these different fabrication techniques. Experimental absorption measurements are compared to theoretical calculations with good agreement. Our results provide compelling evidence for quantum confinement in both types of Si nanocrystals. Our results also indicate that the blue emission from very small Si nanocrystals corresponds to the bandedge emission, while the red emission arises from traps.

Amorphous Si/SiO2 superlattices with periodicities from 2 to 5 nm have been grown on (100) Si wafers by several different techniques: molecular beam epitaxy, magnetron sputtering, and plasma enhanced chemical vapor deposition (PECVD). With the first two methods little or no hydrogen was incorporated during growth and visible photoluminescence (PL) was obtained at wavelengths from 520 to 800 nm. The shift in the PL peak position with Si layer thickness is consistent with quantum confined emission. Annealing the sputtered superlattices at temperatures up to 1100°C produced a very bright red PL that is similar in intensity to that found in porous Si. The PL was also considerably enhanced by deposition on aluminum-coated glass substrates. For large numbers of periods (e.g., 425) the PL was strongly modulated in intensity owing to optical interference within the superlattice. Similar quantum-confined PL was also observed in the PECVD grown superlattices, where the amorphous Si layers were heavily hydrogenated. The blue-red cathodoluminescence observed from sputtered superlattices is due primarily to defects in the SiO2 layers.

We discuss the mechanism of efficient photoluminescence (PL) from Si quantum dots and wells. Luminescence properties of SiO2-capped Si nanocrystals are different from those of H-terminated Si nanocrystals, but are very similar to those of Si quantum wells sandwiched by SiO2 layers. The size-dependence of PL properties and resonantly excited PL spectra of SiCb-capped Si dots and wells indicate that excitons are localized near the interface between the crystalline Si core and surface oxide layer, and the strong coupling of electronic and vibrational excitations causes the broad PL spectrum. The exciton localization plays an essential role in the efficient luminescence process in nanoscale Si/SiO2 systems.

The nc-Si/a-SiO2 composite thin films doped with tungsten show very fast and efficient photoluminescence (PL). In order to obtain insight into the PL mechanism we have performed a comparative study with other metals. The results lend support to the suggested mechanism which includes the photogeneration of charge carriers due to efficient absorption of the excitation UV light in the silicon nanocrystals followed by energy transfer to the Wn+ radiative center from which the light emission occurs.

The kinetics of the decomposition of silicon suboxides (SiOx, x<2) to Sic + SiO2 was studied as a function of composition and post-deposition annealing. Amorphous hydrogenated SiOx films (0.8<x<1.4) were deposited by remote plasma enhanced chemical vapor deposition (RPECVD) and rapid thermal annealed (RTA) at temperatures of 500–1000°C. By monitoring the Si-O infra-red (IR) bond-stretch mode frequency, it was found that at temperatures below 850°C, or at a oxygen poor composition near SiO0.8, the decomposition reaction only proceeded to a metastable form of SiO1.6 + Si. Characterization by Raman and spectroscopie ellipsometry confirm similar trends. Cross sectional transmission electron microscopy (TEM) confirms that Si nanocrystals (Sine) are formed with anneals at 900°C (30 sec). As deposited suboxides show band edge photoluminescence at 1.6 eV which disappears upon annealing at 900°C, indicating a sharp suboxide free interface between Sinc and SiO2 matrix.

Electron energy loss spectroscopy (EELS) has been used to investigate the electronic states of isolated, nanocrystalline carbon particles. Small carbon nanocrys-tals were prepared via sputter deposition onto SiO2 substrates, followed by annealing to 700C. The structure and size distribution of the particles have been characterized by Raman scattering, Auger electron spectroscopy and electron microscopy. EELS observations indicate that a semimetal to semiconductor transition occurs for particles smaller than lnm. In addition, hydrogen adsorption is found to significantly affect the electronic states of these particles, indicating that both finite size and dangling bond effects modify the properties of small carbon nanocrystallites.

Small isolated nanocrystallites of carbon have been prepared on amorphous SiO2 and studied by interference enhanced Raman scattering (IERS) and scanning transmission electron microscopy (STEM). A new mode of dark field STEM, using a thin annular detector, has allowed imaging of 1–2 nm graphite-like particles using (002) diffraction. For such small particles, the Raman spectra provide the first evidence for changes in the phonon density of states of a nanocrystalline system.

Room temperature Raman scattering spectra are reported for the type II superconductors MxBaySi46 (M-Na, K) which were recently shown to exhibit Tc's ∼ 3.5 K. The spectra are compared to those of other Si46-clathrates which exhibit normal metallic behavior down to 2K. Thirteen of the twenty first-order Raman frequencies predicted by group theory have been detected, and the frequencies are found to be sensitive to the particular dopants. The Raman linewidths observed for the MxBaySi46 system are comparable to those observed for Na8Si46 and K7Si46. The data, taken collectively, suggest that the line broadening in the metallic Si-clathrates is due to important contributions from both the electron-phonon interaction as well as to a random filling of the Si cages.

The Vapor - Liquid - Solid (VLS ) technique allows the growth of high aspect ratio Si wires. The Si nanowires formed by this technique can be thinned down by oxidation. This approach allows the formation of very thin Si cores which may be used to research the properties of Si nanostructures. In this work the growth and oxidation of these wires is characterized.

In the growth a very thin layer of Au is deposited on a Si (111) surface, silane gas is introduced into the chamber as the Si source gas and the temperature is raised to 300 – 600°C. Initially a catalytically active Au surface phase leads to the growth of a defective epitaxial Si layer. As Au / Si molten alloy balls nucleate and grow in size to approach the threshold size for VLS wire growth, which is determined by the Gibbs - Thomson effect, the epitaxial layer growth rate decreases and a transition to Si nanowire growth occurs. The morphology and width of the wires is strongly dependent on the growth temperature and pressure. At low pressure and high temperature relatively thick well-formed wires grow straight up from the substrate surface along the [111] direction. As the temperature is decreased and the pressure is increased thinner wires (as thin as 10 nm ) grow which tend to exhibit growth defects. A light oxidation yields Si cores which are of the order of 5 nm in diameter.

The growth of Si dots on thermally-grown SiO2/c-Si from a thermal decomposition of pure silane has been systematically studied in the temperature range from 500 to 650°C. It has been suggested mat the Si dot height and dot diameter on as-grown SiO2 are rate-limited by the cohesive action of adsorbed precursors and the thermal decomposition of silane on Si nucleation sites, respectively. The nucleation site on as-grown SiO2 is likely to be generated by the thermal dissociation of surface Si-O bonds. It has been also found that in Si dot formation on OH-terminated SiO2 surface the nucleation density is dramatically enhanced and consequently the dot size and its distribution become small. This implies that surface OH bonds can provide the nucleation sites.

We report Raman spectroscopy measurements on Ge nanocrystals with average radii ranging from about 65 Å down to 10 Å (with a size dispersion lower than 20 %).

Ge has been deposited by UHV evaporation on an amorphous substrate, kept at such a temperature as to produce the Ge nanodroplets nucleation in the liquid phase.

A nanocrystalline size dependence of the Raman spectra has been observed and explained in the framework of a phonon confinement model. We have observed the softening of the TO Raman peak predicted by the theory when the dimensions of the particles are decreased. Moreover the observed inhomogeneous broadening of the Raman lines has been correlated with the size distribution of the particles in the samples.

Our results provide a characterization of Ge nanoparticles exhibiting a good crystalline nature, down to about 10 Å, and in conditions of substantial absence of perturbations of the environment.

The surface morphology of Ge grown on Si (001) and porous Si(001) by molecular beam epitaxy at 380 °C is examined using atomic force microscopy (AFM). For layer thicknesses of 30 nm, the surface shows islanding while still maintaining some of the underlying roughness of the surface of porous Si. For thicknesses in the 100 nm range, the surface roughness is not visible, but the islanding persists. Unlike the case of silicon where islands tend to merge and nearly disappear as the thickness of the deposited layer rises, we observe on the porous layer the persistence of the islands with no merging even for macroscopic thicknesses as large as 0.73 microns.

The Si/Si1-xGex quantum well system is attractive for high temperature thermoelectric applications and for demonstration of proof-of-principle for enhanced thermoelectric figure of merit Z, since the interfaces and carrier densities can be well controlled in this system. We report theoretical calculations for Z in this system, based on which Si/Si1-xGex quantum-well structures were grown by molecular-beam epitaxy. Thermoelectric and other transport measurements were made, indicating that an increase in Z over bulk values is possible through quantum confinement effects in the Si/Si1-xGex quantum-well structures.

Opal consists of a three dimensional array of silica balls with diameter in the range 150 -350 nm and acts as a 3-D grating. Within the microcrystals (a few hundred microns) of opal there is typically 5% variation in ball diameter arranged in a fee structure. Using a specially modified MOCVD process InP can be grown within the voids between the touching silica balls. These interconnecting voids are approximately 1/5 and 2/5 the silica sphere diameter, Raman spectroscopy confirms the crystallinity of the InP for two samples loaded with 2.5 volume % of InP. The LO phonon is shifted to the red by 10 cm−l from the bulk value of 345 cm−1, while the TO phonon is shifted 22 cm−l to the blue from the bulk value of 303.7 cm−l. This shift is attributed to strain in the nanocry stals.

The half width of the reflectance is dependent upon InP loading with the 2.5 vol.% samples giving 0.19 eV and the lower loaded 0.4 vol. % giving 0.17 eV for samples with the same ball diameter. Photoluminescence emission near 1.7 eV is attributed to confined InP compared with a bulk value of 1.4 eV. The energy of the emission is determined by the combined contribution of the laterally confined InP and its interaction with the opal matrix.

In this paper we report on the limits and properties of size quantization effects in InAs self assembled quantum dots (QDs). Size, density and character of the InAs islands are investigated by transmission electron microscopy. The electronic and optical properties of the islands in the coherent and dislocated growth regime are studied using capacitance, photoluminescence, photovoltage and photocurrent spectroscopy. In the data measured with the different techniques, the change in dot size and density as well as the transition from coherent to dislocated island growth is clearly observable. An increasing QD size causes a red shift in the energetic position of the QD features while the density of the islands is reflected in the intensity of the QD signal. The decrease in intensity at high InAs coverage is attributed to dislocated island formation.

We report optical properties of the micro-facetted InGaAs quantum wells and quantum wires on non-planar substrates employing magnetophotoluminescence (MPL). The InGaAs/GaAs structures were grown by chemical beam epitaxy on V-groove patterned GaAs substrates. In the presence of a magnetic field of 18 T, the diamagnetic shifts of exciton ground states of the (001)-and side-QWLs are ΔE=15.6 and 10.3 meV, respectively. In MPL of the facetted microstructure, we found that the different diamagnetic shifts strongly depend on the magnitude of the effective magnetic field as well as the quantum confinement. From comparing the intensities and full widths at half maximum, we easily found that side-QWLs are of higher quality than (OOl)-QWLs. We also fabricated InGaAs/GaAs quantum wires with a size of about 200 Å × (500–600) Å. By fitting the diamagnetic shifts (ΔΕ = 9.5 meV) of the exciton ground state with the calculated results of a variational method, we estimated that the reduced mass of the exciton is approximately 0.052 me.