An analysis of the ultrafast response of Ge quantum dots, with average radii of 4 nm and 16 nm, is reported here. Pump and probe experiments allowed investigation of the wavelength region between 450 and 750 nrm, characterized by the absorption peaks due to the E, and E1+Δ1 interband transitions. Two different time regimes have been identified: a first one (τ ≤ 1 ps) associated with pump-induced conduction band filling in the τ-L direction in the Brillouin zone; a second one (τ up to 100 ps) associated with band gap renormalization caused by the carrier interaction in the conduction band. Quantum confinement effects show up in terms of sizedependent blueshift of the E1+Δ1 spectral feature.

Mid-infrared intraband absorption in Ge/Si self-assembled quantum dots is reported. The self-assembled quantum dots are grown by ultra-high-vacuum chemical vapor deposition. The intraband absorption is observed using a photoinduced absorption technique. The mid-infrared absorption, which is in-plane polarized, is maximum around 300 meV. The absorption is attributed to a quantum dot hole transition between bound and continuum states. The absorption cross section is deduced from the saturation of the photoinduced intraband absorption. An inplane absorption cross section as large as 2 × 10−13 cm2 is measured for one dot plane.

Inter-sub-level transitions in p-type modulation-doped Ge quantum dots are observed. The structure is grown by molecular beam epitaxy and consists of 30 periods of Ge quantum dots separated by 6 nm boron-doped Si layers. An absorption peak in the mid-infrared range is observed at room temperature by Fourier transform infrared spectroscopy, and is attributed to the transition between the first two heavy hole states of the Ge quantum dots. This study suggests the possible use of modulation-doped Ge quantum dots for improved infrared detector application.

The recent success of bulk synthesis of pure nano-scale silicon quantum wires (SiQW's) enables us to evaluate their photoluminescence (PL) characteristics under ultra-violet photoexcitation. Intense multiple light emissions ranging from dark red to blue regions were revealed for as-grown and partially oxidized SiQW samples. The physical origin of the multiemissions is discussed on the basis of quantum confinements, or defect centers.

We report a novel poly-Si-thin-film-transistor based memory with Si-nano-crystals (Si dots) floating gate fabricated on a quartz substrate at a temperature below 400°C. Novel techniques of Si-dot and tunnel-oxide formation using excimer laser annealing were performed. A preliminary device shows a threshold voltage shift larger than 1 V with 20 V, 10 ms write/erase (W/E) operation and a retention time of 103 s at room temperature. The device operates 104 W/E cycles without significant degradation.

We have investigated the preferential positioning of self-organized Ge quantum dots on Si (001) mesas fabricated in a selective epitaxial growth (SEG) mode of gas-source molecular beam epitaxy. The surface morphology was evaluated by atomic force microscopy, and for the Si SEG structures, showed mass accumulation at the periphery. Using micro-Raman spectroscopic imaging, the strain-field distribution of Si structures was studied before Ge deposition, and the results suggest a tensile strain at/near the periphery of the structure while a compressive strain at the center. These structures were subsequently used as a template for Ge dots, which tended to align along the mesa edge. The strain field distribution on Si mesas is speculated to be the driving force for preferential nucleation of Ge dots.

We present the first comparative study of nucleation and growth of Si quantum dots on SiO2, SiOxNy and Si3N4 substrates using silane Low Pressure Chemical Vapor Deposition at low temperature (570–610°C). The samples are investigated by Atomic Force Micoscopy, Scanning Electron Microscopy, High Resolution Transmission Electron Microscopy and Spectroscopic Ellipsometry. We confirm that the chemical nature of the surface and precisely the presence of SiO bonds decreases the Si quantum dot density. By optimising the deposition parameters, a Si dot density of 1012 cm−2 can be obtained below 600°C on a pure Si3N4 surface. The influence of hydrogen, provided by silane decomposition, on the Si nucleation mechanism will be discussed.

A new method for the fabrication of nanocrystalline silicon (nc-Si) in SiH4 plasma with very-highfrequency (VHF; 144MHz) excitation is proposed to increase the deposition rate, to control the size, and to minimize size dispersion of nc-Si. Nanocrystalline silicon is formed in the gas phase of the SiH4 plasma cell by coalescence of radicals. Supplying Ar enhances the nucleation of nc-Si because of high efficiency of SiH4 excitation into SiH2 radicals resulting in the nucleation. The deposition rate is thus increased by a factor of 100 to 1012/cm2.h. At the low flow rate of SiH4, smaller nc-Si with small dispersion is obtained. Moreover, when pulsed-SiH4 is supplied into Ar plasma, the growth of nuclei is limited by the time when SiH4 flows. The size of nc-Si and its dispersion are adjusted by the duration of SiH4 gas pulse.

We describe a quantitative method for measuring the strain in small islands, and show results for coherent Ge islands on Si. The method uses dark field images from backside thinned samples in the transmission electron microscope. We show that no independent strain models are needed in the measurement, which employs an excellent “abrupt displacement” approximation. Results show that the strain in Ge domes is higher than in pyramids as expected.

We study theoretically and experimentally the effects of the Coulomb and the spin-orbit coupling on the electronic structure of small quantum dots. A tight-binding calculation with restricted configuration interaction is developed in a typical case: very small cubic quantum dots for which the electron-hole exchange interaction is of the order of magnitude of the spinorbit interaction. Experimentally, resonant photoluminescence and photoluminescence excitation are used to obtain information on a single size of CdS quantum dot obtained by a chemical growth method.

In this paper we present results of the absorption and photoluminescence (PL) of CdSdoped Si02 films fabricated by RF co-sputtering (semiconductor volume fraction f=1–15%, nano-crystallite's mean size 5–7nm) and matrix-free films of close-packed CdS nanocrystallites (f∼30%, size 2–5nm) produced by an original chemical method. The absorption spectra have been modelled using the modified Maxwell-Garnett model. This gives the e-h pair state energies and evidence of a strong absorption in the glass matrix containing CdS. The temperature dependence of the spectral position and broadening of the PL peak is analysed. It is concluded that a photo-generated hole is captured on an acceptor-type trap before the radiative recombination with a confined electron. The excitation of this ‘band-edge’ PL occurs through some states in the matrix and directly in the CdS crystallites for the two kinds of samples, respectively. The temperature coefficients of the optical transition energies for the nearly matrix-free films are similar to those of bulk CdS, while for the CdS/glass films they are smaller. This may be because of the different boundary conditions for the thermal expansion of CdS crystallites.

High-temperature colloidal synthesis of InAs/InP and InAs/CdSe core/shell nanocrystal quantum dots is reported. InP and CdSe shells with several thicknesses were grown on InAs cores ranging in diameter between 20 to 50 Å. Optical spectra, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were used to analyze the core/shell quantum dots and determine their chemical composition, average size, size distributions, and structures. The experimental results indicate that shell growth is uniform, expitaxial, and controllable. For both InP and CdSe shells, growth is accompanied by a red shift of the band gap energy as a result of the extension of the electron wavefunction into the shell region. An increase of the room temperature photoluminescence quantum yield by a factor of∼4 is observed with CdSe shell growth on InAs Cores. The growth of InP shells, however, quenches the photoluminescence quantum yield. The difference is assigned to outer surface effects in core/shell nanocrystals.

Monodispersed CdSe nanocrystals with average diameter of ∼5 nm were synthesized from SeSO32− and rich Cd2+ ionic reagents in aqueous solution. Two-dimensional (2D) close-packed colloidal CdSe films were formed on the electrode surface by electric-field induced deposition method. Transmission electron microscopy (TEM) images show that colloidal CdSe films exhibit different states, named “gas”, “liquid” and “solid” states, which can be transformed by adjusting applied field (E) or deposition time (T). The experimental results demonstrate that the average nearest-neighbor distance of clusters on the electrode surface is inversely proportional to the ET product.

Lateral coupling between separate quantum dots has been observed in a system of In0.55A10.45As self-assembled quantum dots. The experiment was performed by taking photoluminescence excitation (PLE) spectra in the optical near-field at 4.2 K. The high spatial resolution afforded by the near-field technique allows us to resolve individual dots in a density regime where interactions between neighboring dots become apparent. In the PLE spectra, narrow resonances are observed in the emission lines of individual dots. A large fraction of these resonances occur simultaneously in several emission lines, originating from different quantum dots. This is evidence of interdot scattering of carriers, which additional data show to be mediated by localized states at energies below the wetting layer exciton energy. A very rich phonon spectrum generated by the complicated interfaces between barrier and dot material is also evident in the data.

We report on the experimental observation of localized surface plasmons sustained by small metallic particles using a photon scanning tunneling microscope (PSTM). The surface plasmons are excited in gold nanostructures tailored by electron beam lithography. The constant height operation of the PSTM allowed a direct comparison with theoretical computations of the distribution of the optical near-field intensity. Plasmon coupling above a chain of Au particles and electromagnetic energy transfer from a resonantly excited nanoparticle to a nanowire are demonstrated. Our experimental results appear to be in good agreement with theoretical computations based on the Green's Dyadic Technique.

Electromigration is one of the main failure mechanisms limiting the miniaturization of microelectronic devices. As a consequence of the high current densities in the interconnections, hillocks and voids are formed and their evolution can modify the electrical performances of devices till failure. To characterize electromigration damages,. today's failure analysis techniques require removal of the protection passivation to allow scanning electron microscope or focused ion beam microscope imaging, but the removal process itself can damage the surface of the metal stripes. Due to the optical transparency of the passivation, near-field scanning optical microscopy can be used to overcome this problem. We succeeded in obtaining the first near-field images in resolution < 150 nm of electromigration-damaged metal structures without complete removal of the passivation. The latter was thinned to 100 – 200 nm to allow the evanescent waves to reach the metal structures and illuminate a subwavelength zone of the sample. Near-field images show the presence of hillocks and voids of dimensions down to 250 nm which can be due only to electrornigration, and, in this sense, they are more reliable than the usual scanning electron microscope images.

In the present work we study the effect of vertical alignment in the quantum dot array formed by successive deposition of several rows of InAlAs and InGaAs quantum dots separated by thin AIGaAs spacer layers. Transmission electron microscopy and photoluminescence studies revealed that the InAlAs QDs characterized by high areal density force InGaAs to be transformed into the denser array as compared to the case of spontaneous transformation. Using denser array of composite quantum dots in the active region of a diode laser leads to the increase in modal gain, decrease in internal loss, and decrease in the threshold current density for short cavity diodes. Room temperature continuous wave output power as high as 3.3 W at 0.87 µm is achieved.

Two methods are presented which have been successfully used to fabricate highly ordered 2D and 3D arrays of nano-scale particles. The first method uses a combination of microcontact printing (μCP) and surface-templated reactions to form 2D patterned arrays of nanoparticles on silicon substrates. The second method uses confined self-assembly to crystallize colloidal particles into 3D cubic-close-packed (ccp) arrays (or opaline structures).

The combination of self-organized growth on high-index substrates with lithographic patterning and atomic hydrogen assistance allows the fabrication of GaAs quantum wires and quantum dots as well as coupled wire-dot arrays with superior structural and electronic properties by molecular beam epitaxy. The selectivity of patterned growth on high-index substrates differs qualitatively from that on low-index ones thus producing quantum wire and quantum dot structures which fulfill the stringent criteria on the geometrical features to be met for useful devices.

We applied low temperature diffraction limited confocal optical microscopy to spatially resolve, and spectroscopically study photoluminescence from single self-assembled semiconductor quantum dots. Using selective wavelength imaging we unambiguously demonstrated that a single photoexcited quantum dot emits light in a few very narrow spectral lines. By solving numerically a many body Hamiltonian for a model quantum dot, we show that the multi-line emission spectrum is due to optical transitions between confined exciton multiplexes. We explain the measured spectrum and its dependence on the power of either cw or pulsed excitation by analytically solving the coupled rate equations for the excitation relaxation between these exciton multiplexes.