Temperature-dependent resistance measurements of Bi-related nanowire arrays with different wire diameters and Sb concentrations are performed. The variation in the measured R(T) curves of these nanowires is closely related to the unique semimetal-semiconductor transition in Bi, and the results are explained by theoretical simulations. It is found that the special feature of the maximum in the resistance ratio R(10 K)/R(100 K) can be employed to experimentally identify the conditions for the semimetal-semiconductor transition.

Silicon (Si) nanocrystals formed by controlled thermal crystallization of amorphous silicon dioxide (a-SiO2)/amorphous silicon (a-Si)/amorphous silicon dioxide (a-SiO2) layers hold considerable promise for application in non-volatile memory products and optoelectronic devices. The size of the nanocrystals is fixed by the thickness of the Si layer and strong quantum confinement is provided in the vertical (growth) direction by the insulating a-SiO2 layers. However, the extent of quantum confinement in the lateral dimensions remains to be established. Electron energy loss spectroscopy (EELS) measurements performed within a scanning transmission electron microscope (STEM) indicate that the nanocrystals are laterally isolated by approximately 2nm of a-SiO2. The confinement potential provided by this barrier is insufficient to localize carriers within a nanocrystal for prolonged durations and can permit quantum mechanical tunneling via wave function overlap between adjacent nanocrystals. Charge leakage kinetics within a sheet of Si nanocrystals was studied using electric force microscopy. Approximately 750 electrons were injected within a 100nm radius circular patch with an atomic force microscope cantilever. The entire charge dissipated from this area in 70min via lateral conduction routes. With a goal of localizing the injected charge and enhancing its retention time, the samples were subjected to relatively low temperature dry oxidation at 750°C. After 20 min of oxidation, retention times above 400 minutes were observed.

We have observed visible photoluminescence (PL) spectra (peak energy: 3.1 eV) of size-controlled silicon (Si) nanoparticles annealed in oxygen gas. The PL peak energy did not depend on the temperature, and the PL lifetime was relatively fast (on the order of nanoseconds). It was inferred that the visible PL was attributed to localized states in the oxidized surfaces of size-controlled Si nanoparticles. We also observed the PL excitation spectra and studied the excitation process. In order to elucidate mechanisms of the visible PL, excitation and recombination processes are discussed.

Tunable porous silicon mirrors were fabricated as building blocks for optical interconnects and as a first step towards efficient routing of information on a small size scale. The basic structures for the tunable mirrors are porous silicon microcavities infiltrated with liquid crystals. The optical properties of the mirrors are influenced by thermal or electric field modulation. When the device is heated or a voltage is applied, the orientation of the liquid crystals changes, causing a change in the effective refractive index of the liquid crystals within the mirror. The position of the reflectance resonance of the porous silicon microcavity is particularly sensitive to such changes in the refractive index. A reversible 10nm shift of the reflectance resonance of the mirror, leading to a 30% change in the amplitude of reflectance, has been observed due to thermal effects. Not only do these results show potential for future devices, but they also confirm that the liquid crystals are able to rotate in the constricted geometry of the porous silicon microcavities. For voltage driven devices, careful attention needs to be given to the configuration of the electrical contacts. The effectiveness of various device geometries has been investigated. Using standard lithographic techniques, aluminum contacts with minimum feature sizes of 10 microns were patterned directly on the porous silicon surface. A study on the use of free-standing porous silicon films was also performed.

TiO2/CdS composite nanowire (∼ 200 nm diameter and ∼ 50 μm length) arrays were successfully prepared by the sol-gel template method. An anticipated good nanowire arrays was obtained by using liquid or supercritical CO2 to develop the target materials in purification process. The morphology, crystallinity, and composition of the product was characterized by SEM, XRD, and EDX.

A kind of hydrogenated diphasic silicon films has been prepared by a new regime of plasma enhanced chemical vapor deposition (PECVD) near the phase transition regime from amorphous to nanocrystalline. The microstructural properties of the films have been investigated by the micro-Raman and Fourier transformed Infrared (FT-IR) spectra and atom force microscopy (AFM). The obtained Raman spectra show not only the existence of nano-scaled crystallites, but also a notable improvement in the medium-range order of the diphasic films. For the FT-IR spectra of this kind of films, it notes that there is a blueshift in the Si-H stretching mode and a redshift in the Si-H wagging mode in respect to that of typical amorphous silicon film. We discussed the reasons responsible for these phenomena by means of the phase transition, which lead to the formation of a diatomic hydrogen complex, H2* and their congeries.

Vibrational properties of self-assembled InAs quantum dots (QD’s) embedded in AlAs and aluminium oxide were studied by Raman spectroscopy. The InAs/AlAs QD structures were grown by molecular beam epitaxy on GaAs (001) substrates. The following main features of the phonon spectra of InAs/AlAs QD nanostructures were observed: 1) asymmetric lines of QD LO phonons affected by strain, confinement and size inhomogeneity of QD’s; 2) confined phonons of InAs wetting layer (WL); 3) two bands of interface phonons in the AlAs frequency region, attributed to modes associated with the planar interface WL/AlAs matrix and the three-dimensional QD/matrix interface; 4) doublets of folded acoustic phonons caused by periodicity in the multilayer QD structures. The influence of growth temperature, varied from 420 to 550°C, on the morphology of QD’s was investigated. QD’s grown at 420°C are found to have the smallest size. Increasing the temperature up to 480°C leads to the formation of larger InAs islands and improved size homogeneity. Further temperature elevation (above 500°C) causes partial re-evaporation of InAs leading to a decrease of QD size and density, and, finally, their complete disappearance. InAs QD’s embedded in aluminium oxide were fabricated by selective oxidation of the AlAs matrix in self-assembled InAs/AlAs QD’s. Micro-Raman spectroscopy data show that depending on oxidation conditions (humidity, temperature) InAs QD’s in an oxide matrix can be even more strained than before oxidation, or become fully relaxed. At the boundaries of oxidized/non-oxidized areas the presence of amorphous and crystalline As clusters is evident.

Three samples of self-assembled In0.44Ga0.56As quantum dots (QDs) grown on (001) GaAs by molecular beam epitaxy (MBE) were studied using atomic force microscopy (AFM) and high-resolution transmission electron microscopy (TEM) in order to characterize the height, faceting, and densities of the QDs. The cross-sectional TEM images show both pyramidal dots and dots with multiple side facets. Multiple faceting has been observed only in dots more than 8.5 nm in height and allows increased dot volume without a substantial increase in base area. Addition of a GaAs capping layer is found to increase the diameter of the QDs from roughly 40 nm to as much as 200 nm. The areal QD density is found to vary up to 50 % over the central 2 cm x 2 cm section of wafer and by as much as 23 % on a length scale of micrometers.

Semiconductor nanocrystals (i.e., Quantum Dots, QDs) exhibit size-dependent emission properties and have synthetically adjustable ligand shells, making them interesting materials for applications ranging from luminescent displays to biomolecular tags. In this paper, the electrochemical properties of two types of nanocrystal are studied with an emphasis on the effect of core/shell vs core structures. The band gap energy of CdSe particles, measured using optical spectroscopy, was shown to increase slightly with the application of a ZnSe shell, as expected based on the increased energy required to transfer an electron through the shell material. The electrochemically determined band gaps are overestimated in the case of CdSe/ZnSe core/shell nanoparticles, reflecting the band gap of the ZnSe shell. Finally, QDs were self-assembled onto gold surfaces by electrostatic and covalent attachment, and their presence confirmed by fluorescence spectroscopy. The high intensity of emitted light shows that the QDs can be self-assembly onto metallic surfaces, without energy transfer quenching of the luminescence.

To understand the mechanisms of nanoparticle formation and its potential applications, we have investigated silicon particles formed in various gasses in an inductive coupled plasma (ICP) system and have measured their structural properties by electron microscopy. Particle generation in pure SiH4 and SiH4/H2 are reported. ICP silane plasmas are shown to be an interesting and versatile source of nanoparticles. Three regimes are mapped out: a regime of no observable particle growth at the lowest pressures, a regime of polydisperse and agglomerated particles at the highest pressures, and a regime yielding highly monodisperse particles at intermediate pressures. Conditions that generate these nonagglomerated, extremely monodisperse silicon particles are emphasized. For H2 dilutions less than 92%, the growth rate is almost independent of H2 partial pressure. Particle growth decreases steadily when the H2 dilution is increased further. TEM images, however, indicate that the addition of hydrogen decreases the particle density. At higher dilution ratios, polycrystalline particles are obtained. Under all other conditions the particles are amorphous. Reasons for this behavior are explored.

We report 1.6 μm emission from InAs QDs (QDs) grown on a GaAs substrate. The ensemble is grown on a graded digital alloy (DA), which increases the matrix lattice constant from 5.65 Å to 5.77 Å. The reduced lattice mismatch between the InAs and matrix material produces larger QDs and thereby allows longer wavelength emission compared to standard growth techniques. The resulting QD density ranges from 2×1010 to 8×1010/cm2 with QD dimensions of 5nm x 30nm measured using atomic force microscopy (AFM). According to x-ray diffraction (XRD) data and transmission electron microscopy (TEM), the metamorphic buffer is unstrained with low defect density.

Ultrathin single quantum wells of crystalline silicon (c-Si) confined by SiO2 have been prepared by chemical and thermal processing of silicon-on-insulator wafers. The photoluminescence (PL) produced by these nanometer-thick single wells contains two bands: one exhibits a peak energy of ∼1.8 eV, while the second increases rapidly in peak energy with decreasing c-Si layer thickness. Comparison with theories based on self-consistent first-principles calculations shows that the increase in PL peak energy of the second band is consistent with that predicted for the c-Si energy gap of such wells. It also agrees with the measured band gap variation. The ∼1.8 eV PL band is attributed to the recombination of electron-hole pairs confined at the c-Si/SiO2 interface.

We have extensively studied the effects of oxygen on the optoelectronics properties of various types of isolated silicon nanodots, through ab initio total energy calculations within the density functional theory. Varying the cluster size we have considered different Si/O bonding geometries and different levels of oxidation. We provide strong evidences that the role of the interface region surrounding the silicon nanostructures have to be carefully taken into account in order to understand the striking optical properties of these systems. Moreover the multiple presence of silanone Si=O bonds at the nanodots surface is shown to provide a consistent interpretation of the photoluminescence red-shift observed in oxidized porous silicon samples. Finally for the first time we have performed ab initio calculations on small silicon nanodots embedded in a SiO2 matrix stressing the strong interplay between the nanodot and the surrounding host environment and the active role of the interface region between them.

Currently, there is considerable interest in producing patterned metallic structures with reduced dimensions for use in technologies such as ultra large scale integration (ULSI) device fabrication, nanoelectromechanical systems (NEMS), and arrayed nanosensors, without sacrificing throughput or cost effectiveness. Research in our laboratory has focused on the preparation of precious metal thin films on semiconductor substrates via electroless deposition. This method provides for the facile interfacing of metal nanoparticles with a group (IV) and III-IV compound semiconductor surfaces. Morphologically complex films composed of gold, platinum, and palladium nanoparticles have been prepared as a result of the immersion of germanium and gallium arsenide substrates into dilute, aqueous solutions of tetrachloraurate (III), tetrachloroplatinate (II), and tetrachloropalladate (II), respectively. Continuous metallic films form spontaneously under ambient conditions, in the absence of a fluoride source or an externally applied current. This facile electroless deposition methodology provides an alternative to complex and expensive vacuum methods of metallization, yet allows for the preparation of both thin and thick nanostructured films with control over surface morphology and deposition rate. Furthermore, precious metal films prepared in this way exhibit excellent adhesion to the underlying semiconductor substrate. The resultant films were characterized utilizing scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning probe microscopy (SPM). In order to apply this novel metallization method toward the development of useful technologies, patterning utilizing photolithography, microcontact printing (μCP), and scanning probe nanolithography (SPN) has been demonstrated.

We report a direct experiment evidence for holes and electrons charging in a new nanocrystalline Si (nc-Si) floating gate structure (SiO2/nc-Si/SiO2/c-Si) fabricated in-situ by plasma oxidation and layer by layer deposition technique in a plasma enhanced chemical vapor deposition (PECVD) system. In this nc-Si floating gate structures, the thickness of tunneling SiO2 layer is about 2 nm and the mean grain size of nc-Si is 6 nm obtained from Raman scattering and AFM measurements. The discrete quantum level and Coulomb charging energy for a single electron have been observed in large ensemble of nc-Si dots by frequency dependent capacitance spectroscopy, which demonstrates that the Coulomb blockade for electron in nc-Si dots is larger than size fluctuation effects on the quantum confinement for our nc-Si floating gate structure. Quantitatively, the experiment results of capacitance spectroscopy are in good agreement with the theoretical calculations. By contrasted with silicon single electron transistor memory made by using ultra fancy nanotechnology, nc-Si based memory can be fabricated with a minimum perturbation of conventional silicon technology and may be closest to industrial application.

Dispersive cobalt nanoparticles are fabricated directly on Si (001) substrates by DC magnetron sputtering at room temperature. During deposition, the parameters chosen for the investigation are substrate bias (from +525 To –100 Volts), target-to-substrate distance (from 6 to 12 cm) and deposition time (from 10 to 30m sec), while the other parameters are kept the same, including the power of 50 watts. Atomic force microscope (AFM) is employed to determine the density and morphology of cobalt nanoparticles whereas high-resolution electron microscope (HRTEM) is used to visualize the resulting microstructure in the nanoparticles. It is found that Co nanoparticle array can be formed by combining the optimum substrate bias and target-to-substrate distance. The size uniformity of the nanoparticle array can be enhanced by positive bias due to charging effects. The nanoparticle of as small as a few nanometers can be successfully fabricated by DC-sputtering and can be applied to nanotube growth as catalysts.

In polar semiconductor samples of the self-organized quantum dots, grown by the Stranski-Krastanow growth method, the lowest-energy extended states of the motion of the electronic excitations, are assumed to be the wetting-layer states. The coupling between these extended states and the electronic states localized in the individual quantum dots, may influence the optical spectra of such samples in the sub-wetting layer region of energy. This effect is studied assuming the Fröhlich's coupling between the electrons and the polar optical phonons. The contribution of this interaction to the appearance of the sub-wetting layer continuum in the optical spectra, and to the level broadening of the localized states, pointed out in some experiments, is estimated.

We investigated the effect of the chemical etching on light-emitting porous silicon. It is well known that the treatment with the mixture of HF acid and ethanol noticeably enhances photoluminescent efficiency but the mechanism is not clarified yet. We found that ethanol plays a key role to oxidize silicon hydride species that dissolves with the aid of HF acid. In addition, the chemical treatment with methanol or propanol instead of ethanol gave the same result.

The optical absorption from an indirect L-T point valence band transition in bismuth nanowires is simulated based on a surface sensitive term, which is very weak in bulk bismuth, but effectively couples the L and T point bands in bismuth nanowires. The experimental Fourier transform infrared absorption spectra in the energy range 0.08 to 0.5 eV are compared to theoretical simulations for absorption resulting from indirect interband transitions between subbands at the L point valence band to subbands at the T point valence band as the polarization, tellurium doping, and wire diameter are varied. The simulated absorption from this indirect transition reproduces many of the experimentally observed trends and allows us to conclude that the dominant feature in the experimental absorption spectra of Bi nanowires results from an indirect L-T point transition which is enhanced in Bi nanowires.

Self-organized InAs quantum islands (QIs) were grown in the Stranski-Krastanov regime, by solid source molecular beam epitaxy (SSMBE), on In0.52Al0.48As layer lattice matched to InP(001) substrate. The growth parameters are chosen to produce dot shaped InAs islands as indicated by the photoluminescence (PL) linear polarization which is about 9%. The PL spectrum reveals several resolvable components. PL versus power excitation and photoluminescence excitation (PLE) measurements show clearly that this multi-component spectrum is related to emission from transitions associated to fundamental and related excited states of quantum dots (QDs) having monolayer-height fluctuations. The integrated PL intensities have been measured as a function of temperature in the 8–300 K range. The PL intensity measured at 300K is only 8 times lower than at 8 K, indicating good carrier confinement in these InAs/InAlAs QDs. An enhancement of the PL intensity in the 8–90 K temperature range has been tentatively attributed to the exciton dissociation from the InAlAs barriers which then recombine radiatively in the InAs QDs.