The characteristics of ballistic electron transport in porous silicon (PS) are investigated in terms of the electron emission from PS diodes and the energy distribution of emitted electrons. The energy distributions show a behavior of ballistic electron emission that is quite different with the Maxwellian distribution. This is clearly observed at low temperatures below 150 K where the electrical conduction in PS is dominated by the tunneling mode. At 100 K, the peak position of distribution curve becomes more close to the energy corresponding to the energy gain expected from ballistic transport without any scattering losses. The simulated energy distribution suggests that the electrons having higher energies in a non-equilibrium state travel ballistically in PS via field-induced tunneling process. These results support that electrons can travel ballistically in nanocrystalline layer under a high electric field. The observed ballistic transport indicates the further technological potential of silicon nanocrystallites.

Short-period superlattices consisting of nanocrystalline Si wells and amorphous SiO2 barriers were analyzed using various structural (transmission electron microscopy, atomic force microscopy, and x-ray diffraction) and optical (Raman scattering and spectroscopic ellipsometry) characterization techniques. We observe parallel layers containing polycrystalline Si wells, primarily with <111> orientation, and an interesting surface morphology due to sputtering damage. Raman spectra show a redshift and broadening due to finite-size effects. The ellipsometry data can be described using the effective medium approximation (since the superlattice period is much shorter than the wavelength of the optical excitation) or a superlattice approach based on the Fresnel equations with a polycrystalline Si dielectric function.

Silicon nanocrystals have been fabricated by picosecond pulsed laser ablation. Size control over the nanocrystals can be achieved by careful selection of experimental geometry, laser fluence, laser wavelength, backing gas pressure and distance from plume center. Measurements of optical absorption and photoluminescence confirm that particle size variation does significantly affect bandgap and emission efficiency. Differences between results published previously for nanosecond pulsed laser ablation will also be discussed.

The influence of operating parameters in producing light-emitting porous silicon materials was investigated in ethanolic solutions of hydrofluoric acid. Photoluminescence spectra depended on applied potential, the intensity and wavelength of illumination, and electrolyte concentration. When the applied potential and the illumination wavelength increased, the photoluminescence shifted to longer wavelength. Change in HF concentration resulted in different intensity in photoluminescence.

A quantum structure based on Si/SiO2 and fabricated using standard Si technology has strong potential for applications in non-volatile and scaled dynamic memories. Among standard requirements, such as long retention time and endurance, a structure utilizing resonant tunneling offers lower bias operation and faster write/read cycle. In addition, degradation effects associated with Fowlher-Nordheim (FN) hot electron tunneling can be avoided. Superlattices of nanometer size layers of silicon and silicon dioxide were obtained by sputtering. The size of the silicon nanocrystallites (nc-Si) is fixed by the thickness of the silicon layer which limits the size dispersion. A detailed analysis of the storage of charges in the dots, as a function of the nanocrystals size, is investigated using capacitance methods. Constant voltage and constant capacitance techniques are used to monitor the discharge of the structure. Room temperature non-volatile memory with retention times as long as months is evidenced.

Photoluminescence (PL) of SiO2 films co-doped with Si nanocrystals (nc-Si) and Er was studied. The average size of nc-Si was changed in a wide range in order to tune the exciton energy of nc-Si to the energy separations between the discrete electronic states of Er3+. PL from exciton recombination in nc-Si and the intra-4f shell transition of Er3+ were observed simultaneously. At low temperatures, periodic features were observed in the PL spectrum of nc-Si. The period agreed well with the optical phonon energy of Si. The appearance of the phonon structures implies that nc-Si which satisfy the energy conservation rule during the energy transfer process can resonantly excite Er3+. The effects of the quantum confinement of excitons in nc-Si on the energy transfer process are discussed.

We applied fluctuation microscopy technique to study medium-range order in tetrahedral semiconductor materials, such as amorphous silicon, amorphous diamond-like carbon films. It is shown that this technique is very sensitive to local structure changes in the medium range order and promises solutions to open questions that cannot be answered by current techniques. For asdeposited amorphous germanium and silicon, we previously identified a fine-grain para-crystallite structure [1, 2], which will be relaxed into a lower-energy continuous random network structure after thermal annealing. With the same fluctuation microscopy technique, we however found that thermal annealing introduces medium-range order in amorphous diamond-like carbon films. Future studies will be focused on modeling and systematic exploration of annealing effects.

We have obtained a porous silicon optical planar waveguide, with a submicronic periodic modulation of the optical index along one direction of plane, using a holographic process. Near-infrared continuous transmission spectra of guided light across not less than 3000 periods show several strong stopbands, with a decrease of intensity by two orders of magnitude. By means of the coupled-mode theory, we were able to deduce from the spectra a realistic map of the optical index at a microscopic scale, demonstrating a strong photo-induced index contrast (Δn = 0.5) at a submicronic scale.

We have developed silicon (Si) nanocrystallite light-emitting devices synthesized by a novel integrated process in which a size-controlling unit of differential mobility analyzer (DMA) is combined to a nanocrystallite formation unit of pulsed laser ablation (PLA). The size-controlled Si nanocrystallites as active layers have been deposited on Si substrates, and have been covered with stoichiometric indium oxide (In2O3) thin films synthesized also by the PLA process. The electroluminescence (EL) spectra had a narrow bandwidth of 0.15 eV peaked at slightly higher energy region (1.17 eV) than the bulk Si energy gap (1.10 eV), at room temperature.

Interest in the synthesis of semiconductor nanoparticles has been generated by their unusual optical and electronic properties arising from quantum confinement effects. We have synthesized silicon and germanium nanoclusters by reacting Zintl phase precursors with either silicon or germanium tetrachloride in various solvents. Strategies have been investigated to stabilize the surface, including reactions with RLi and MgBrR (R = alkyl). This synthetic method produces group IV nanocrystals with passivated surfaces. These nanoparticle emit over a very large range in the visible region. These particles have been characterized using HRTEM, FTIR, UV-Vis, solid state NMR, and fluorescence. The synthesis and characterization of these nanoclusters will be presented.

TiO2and W:TiO2 thin films have been prepared by a chemically modified sol-gel technique, that implies hydrolysis and condensation of tetraethylortotitanate in the presence of a polymer dissolved in ethanol. Oxide-polymer films were deposited by dip-coating. Annealing at 500°C results in nanosized structurally stable oxides films. For doping tungsten(V)ethoxide was used in concentration that led to a final W/Ti atomic ratio of 1/33, 5/33, 10/33. The morphological and structural characteristics of thin films were studied through microraman, GIXRD, SEM and TEM. A microstructural comparison between pure and doped TiO2 layers is reported. Ethanol and methanol sensing properties are tested.

We have studied the microstructure, the transport and the phototransport properties of the Si crystallites network in Si-SiO2 composites. We have found that in our co-sputtered samples the average crystallite diameter, d, decreases from 40 to 5 nm as the content of the silicon, x, decreases from 80 to 40 volume%, and that the percolation of the network sets is at x ≈ 40 vol%. A simultaneous study of the photoluminescence (PL) shows the, quantum confinement, expected red shift of its peak with increasing d. On the other hand the very strong observed decrease of the PL intensity with x is interpreted here as due to a deconfinement effect that is dominated by the increase in the cluster size of connected Si crystallites. The results suggest that a closed random packing of the Si crystallites will be the preferred network for high intensity electroluminescence.

The effect of varying the Si layer thickness on the Er3+ photoluminescence properties of Er-doped Si/SiO2 superlattice is investigated. We find that as the Si layer thickness is reduced from 3.6 nm down to a monolayer of Si, the Er3+ luminescence intensity increases by over an order of magnitude. Temperature dependence of the Er3+ luminescence intensity and time-resolved measurement of Er3+ luminescence intensity identify the increase in the excitation rate as the likely cause for such an increase, and underscore the importance of the Si/SiO2 interface in determining the Er3+ luminescence properties.

Using very-high-frequency (VHF) plasma decomposition of SiH4 and pulsed gas technique, we have successfully prepared nanocrystalline silicon (nc-Si) quantum dots having average diameter of 8 nm and dispersion of 1 nm. The role of natural oxide is very important. It controls the size of nc-Si dots. Of particular interest is that the oxidation of these dots can be self limited, due to the stress induced near Si/oxide interface, which would allow further reduction of size and improvement in dispersion. This paper deals with the systematic study of oxidation process of nc-Si dots. Nc-Si dots formed in an Ar plasma with SiH4 gas pulses are deposited onto a Pt mesh The dots are then oxidized at 750, 800 and 850°C from 20 minutes to 15 hours. The dimensions of the residual nc-Si and the grown oxide are measured directly from the TEM micrographs and analyzed. For comparison, field oxide is investigated using ellipsometry. Retardation in the oxidation rate of nc-Si is observed. The mechanism of the reduction of oxidation rate in nc-Si is discussed taking into account the effect of stress.

The growth mechanism of chains of silicon nanocrystallites was investigated. Energy-filtered transmission electron microscopy observations provided strong evidence that the chains were formed as a result of surface oxidation of silicon nanowires. In addition, the periodic instability in the wetting property of molten catalysts was simulated numerically.

To verify the availability of porous silicon (PS) device for application to optical switching, the photonic response of PS for carrier injection has been studied for a microcavity configuration. The observed reflectance spectral shift in the PS microcavity with increasing diode current is analyzedunder the assumption that the refractive index of nanocrystalline silicon is increased by carrier injectionand the subsequent accumulation possibly in localized states. This is consistent with the experimental results obtained from dynamic response measurements: there are fast and slow components in the refractive indexchange. It is also demonstrated that under the pulsed operation, the PS microcavity operates as a current-induced nonlinear optical switching device.

Amorphous silicon dioxide/silicon/amorphous silicon dioxide single quantum well structures were fabricated by oxygen implantation followed by thermal oxidation. No photoluminescence (PL) was observed from the as grown samples. We found that annealing in hydrogen allows the single quantum well (SQW) structures to emit two-peak (blue and yellow) PL at room temperature (RT). The blue PL (2.9 eV) does not change with the thickness of Si layers or the temperature. The yellow peak varied from 2.0 eV to 2.4 eV with thinning of the Si layer from 5 nm to 0.5 nm. Lowering the temperature also changed the yellow peak position of the 1.5 nm Si-SQW structure from 2.3 eV (RT) to 2.6 eV (8.4 K). We conclude that the blue PL is from SiO2 and the yellow PL is caused by a recombination process in the Si-SQW.

Amorphous silicon quantum dots (a-Si QDs), which show a quantum confinement effect, were grown in a silion nitride film by plasma enhanced chemical vapor deposition. Red, green, blue, and white photolumiscence were observed from the a-Si QD strictures by controlling the fot size. An organe light-emitting device (LED) was fabricated using a-Si QDs with a mean size of 2.0 nm. The turn-on vottage was less than 5 V. An external quantum effiency of 2×10−3 % was also demonstrated. These results show that an LED using a-Si QDs embedded in the silicon nitride film is superior in terms of electrical and optical properties to other Si-based LEDs.

An atomic force microscope (AFM) has been used to optimize the radio frequency (rf)/ radio frequency magnetron sputtering deposition conditions for obtaining atomically smooth, hydrogen free amorphous silicon (a-Si) and silicon dioxide (a-SiO2) thin films. Superlattices composed of periodically repeating units of nanoscale (a-Si/a-SiO2) units were fabricated at these optimized conditions and subsequently crystallized. The amorphous and crystallized superlattices were characterized by AFM and Raman spectroscopy. Raman spectroscopy of the superlattices was performed by enhancing the weak scattered signal and eliminating the silicon substrate signal by using either waveguiding, cross polarization or interferometric enhancement techniques. The enhanced Raman spectrum clearly indicates formation of nanocrystals after crystallization.

A new technique of bulk micromachining using anodic etching of (100)-oriented n-type silicon is presented. For particular conditions the transition regime between porous silicon formation and electropolishing enables the formation of high aspect ratio microtips which correspond to inverted macropore structures. This unusual property can be explained by the distortion of current lines near the basis of formed structures. The distortion, which prevents the tip dissolution, is due to the electrical field in the space charge region at the silicon-electrolyte interface. The same property can be used to form three-dimensional microstructures. The position and shape of the structures can be defined by forming steps of a few microns depth, prior tothe electrochemical etching. Then the etching parameters (HF concentration, light intensity, etching current density) are adjusted in order to electropolish the sample except where vertical walls are needed. This enables to form microstructures without a periodic pattern. The feasibility of this technique is demonstrated by forming 100μm wide pores, free-standing beams as well as high aspect ratio micro-needles and micro-tubes.