The Stranski-Krastanov growth mode of Ge thin films on Si and the clustering behavior of Ge on calcium fluoride have been exploited to grow self-assembled nanocrystals by molecular beam epitaxy. The growth of the samples was monitored in situ with RHEED, and they were analyzed ex situ with AFM and RBS. For each system (Ge/Si and Ge/CaF2/Si), the dependence of Ge islanding on substrate temperature and on substrate misorientation was studied. When grown on Si(111) at temperatures between 500°C and 700°C, Ge clusters nucleated at step edges on vicinal wafers and nucleated homogeneously on on-axis wafers. Above 600°C, no transition to a spotty RHEED pattern, which would be expected for island growth, was observed for the vicinal samples. Ge grown at 500°C on on-axis Si(111) formed islands with a relatively narrow size distribution, typically 160nm in diameter and 10nm to 20nm in height. When grown on a CaF2 buffer layer, Ge islands nucleated homogeneously at a substrate temperature of 750°C, resulting in randomly distributed, oblate crystallites approximately 100nm to 200nm in diameter. At 650°C and 700°C, although we still observed many randomly distributed, small crystallites, most islands nucleated at step bunches and had a length scale of over 500nm.

We have investigated the formation of micron-sized single crystal dots of germanium by pulsed laser ablation. The laser ablation of a Ge target (KrF excimer laser λ=248 nm, pulse rate 10Hz, pulse duration 25x10−9 seconds, and energy 10J /cm2) results in the formation of micron and submicron liquid droplets which are ejected from the target. These droplets can be crystallized into single crystal dots on lattice-matched substrates by rapid liquid-phase recrystallization. We report the details of microstructure as a function of dot size. It is found that under these conditions, below a critical size (about 2μm), the dots are crystalline;above which dots become polycrystalline. We discuss the implications of the results in producing doped and undoped single-crystal quantum dots for device applications.

Possible mechanisms which may explain the strong blue/violet (410 nm) photoluminescence (PL) in spark-processed Si (sp-Si) are critically reviewed and discussed. They include involvement of silicon-oxides, silicon-hydrides, silanol groups, siloxene, and silicon carbide. It is concluded from the experimental evidence that none of the above mechanisms can satisfactorily explain the major contribution(s) to PL in sp-Si.

Ion implanted crystalline Si samples show luminescence (PL) in the visible at cryogenic temperatures after annealing treatments. This phenomenon is studied by varying the ion fluences and the sample doping density, and it is discussed considering the damage caused by the H implantation and its evolution with thermal treatments. A working hypothesis about the PL has been formulated which considers the exciton localisation in nanoconstrictions formed in the regions among the different microvoids produced by the implantation.

Implantation of Si+ ions into thermal oxides grown on silicon has been used to synthesise a two phase structure consisting of Si nanocrystals in a SiO2 matrix. Various processing conditions have been used in order to modify the size and population distributions of the Si inclusions. Photoluminescence spectra have been recorded from samples annealed in nitrogen, forming gas and oxygen. Both red and blue shifts of the luminescence peaks have been observed. It is concluded that the photoluminescence is a consequence of the effects of quantum confinement but is also dependent on the presence of irradiation-induced defects or Si/SiO2 interface states.

Ion implantation and thermal processing have been used to synthesize compound semiconductor nanocrystals (SiGe, GaAs, and CdSe) in both SiO2 and (0001) Al2O3. Equal doses of each constituent are implanted sequentially at energies chosen to give an overlap of the profiles. Subsequent annealing results in precipitation and the formation of compound nanocrystals. In SiO2 substrates, nanocrystals are nearly spherical and randomly oriented. In Al2O3, nanocrystals exhibit strong orientation both in-plane and along the surface normal.

Nanocrystals of group IV (Si, Ge and SiGe), III-V (GaAs), and II-VI (CdSe) semiconductor materials have been fabricated inside SiO2 by ion implantation and subsequent thermal annealing. The microstructure of these nanocrystalline semiconductor materials has been studied by transmission electron microscopy (TEM). The nanocrystals form in near-spherical shape with random crystal orientations in amorphous SiO2 Extensive studies on the nanocrystal size distributions have been carried out for the Ge nanocrystals by changing the implantation doses and the annealing temperatures. Remarkable roughening of the nanocrystals occurs when the annealing temperature is raised over the melting temperature of the implanted semiconductor material. Strong red photoluminescence peaked around 1.67 eV has been achieved in samples with Si nanocrystals in SiO2.

Synthesis and size-dependent photoluminescence has been performed for Ge nanocrystals in SiO2 matrices with average diameters between 2 nm and 9 nm, formed by room temperature ion implantation into SiO2 followed by precipitation during vacuum thermal anneals. Nanocrystal size distributions obtained from electron microscopy data were used in conjunction with a quantum-confined exciton recombination model[l] to generate calculated photoluminescence spectra, which were compared with experimental spectra.

The PL (photoluminescence) and EL (electroluminescence) demonstrated for porous silicon and silicon rich SiO2 films have provided exciting opportunities for the integration of optoelectronics and Si digital electronics. Recent work has suggested that the porous silicon luminescence results from either quantum size or surface state effects. In this report, we review PL demonstrated in the visible spectral range from quantum nanocrystals (QNC) formed from Si embedded in an SiO2 matrix. We used Raman scattering to estimate the QNC particle size and correlated the shift in the luminescence spectra to the observed change in the Raman spectra. The PL spectrum peak shifted from 7000 to 8000 Å as the average particle size increased from ∼50 to 70 Å, measured from Raman scattering. Further, High Resolution Transmission Electron Microscopy, HRTEM, and X-ray Diffraction, XRD, measurements confirmed the particle size range. PL lifetime measurements and excitation intensity studies are also presented. The stable nature of the QNC embedded in an insulating or semiconducting matrix offers further advances towards the integration of optoelectronics with Si devices.

A systematic study of the PL spectra of Si quantum nanocrystals in the SiO2 matrix has been performed. The results have been fitted to a quantum-confinement model that includes the nanocrystal size dispersion rather than a specific size of the nanocrystal. This serves as a strong confirmation of the confinement-induced nature of the PL. It has been shown that if the dispersion is taken into account, the position of the emission peak as well as the PL width can always be correlated with the average size of the nanocrystal.

Raman scattering results on porous silicon, and silicon and gallium arsenide nanocrystals show that almost all vibrational modes become Raman active and remarkably soft in these nanocrystal systems. The experimental results further demonstrate that the carrier-induced strain effects play an important role on the optical properties of such nanocrystal systems.

Degenerate four-wave mixing with picosecond pulses at 532 nm has been used to study the third-order nonlinear optical susceptibility (x(3)) for a series of passivated thin films of silicon nanocrystallites. A pulsed laser ablation supersonic expansion source of isolated silicon nanocrystallites was used to deposit thin films onto inert substrates. These films were subsequently passivated using chemical etches or oxidation steps. We observe a strong dependence of the measured x(3) as a function of the degree of passivation, indicating the fundamental importance of the surface of the nanocrystallite in enabling the nonlinear optical behavior. Systems providing more complete passivation were found to have greatly enhanced X(3) behavior when compared to poorly passivated systems. Surface passivation is also shown to be critical to the visible photoluminescence behavior of the thin films, as poorly passivated nanocrystallites exhibit very weak light emission, while well passivated systems show efficient emission. In both cases, the passivation controls the recombination pathways of excited carriers and determines the material's optical properties.

CdS nanoclusters ranging in diameter between 1 and 4 nm were prepared in aqueous solution using aliphatic mercaptoalcohols as ligands. The photon energies of the Is Is absorption and the respective oscillator strengths are in accordance with size quantization theory. Some clusters crystallize as macroscopic 3-D superlattices which were investigated by single crystal x-ray analysis. The neutral Cd17S4(RS)26 clusters are covalently linked in the superlattice the structure of which exhibits self similarity to the interior structure of the clusters.

Onion-shell-like composite particles from CdS and HgS were prepared by successive substitution and re-precipitation processes. Particles with a core radius of 2 nm, a shell of up to 1 nm HgS followed by a final shell of up to 1.5 nm CdS were obtained. Electrons and holes were localized in the HgS shell giving rise to excitonic fluorescence.

We describe a method to synthesize optical quality thin and thick films of close packed CdSe nanocrystallites (quantum dots). The average dot size is tunable from ∼12-150 Å in diameter with ∼3.5% rms size distribution. The identity of each particle and the high monodispersity of the sample are maintained in the films as revealed by small-angle x-ray scattering, transmission electron microscopy, and optical spectroscopy. We use small-angle x-ray diffraction patterns to obtain form factors for the individual dots and to generate radial distribution functions for the densely packed films. We observe the random close packing of dots in the solid state using transmission electron microscopy. Comparing optical spectra of particles close packed in a film with those of nanocrystallites dispersed in alkanes reveals similar optical resonances in absorption while the emission lineshape is modified and its position is red shifted. Differences in film emission are consistent with electronic energy transfer between close packed dots within the inhomogeneous distribution of the sample.

We have measured and analyzed optical absorption, Raman scattering and x-ray absorption spectra of CdS nanocrystals grown by solid-state reaction in a borosilicate glass. We observe the appearance and growth of an optical absorption peak which is attributed to CdS nanoparticles. Using x-ray absorption we quantify the progressive replacement of O nearest neighbors of Cd by S with heat treatment and the appearance and growth of a well-defined second nearest neighbor peak.

A CdSxSe1−x semiconductor doped sodium zinc borosilicate glass has been made and investigated by Raman, linear absorption, and photoluminescence measurements. The glass color changes from colorless through yellow and orange to red after annealing at 625 C with increasing annealing time. The average size of Cd(S,Se) quantum dots in glass has been determined by low-frequency Raman scattering in the order of several nanometers and increases with annealing time. The resonant Raman spectra from colored glasses show the longitudinal optical phonons of CdSe and CdS-like modes in alloy microcrystals, at 204 cm−1 and 284 cm−1, respectively. The stoichiometric composition of CdSxSe1−x nano-particles, x, determined by Raman scattering, is about 0.3. The photoluminescence spectra of colored glass samples show two emission bands, which are attributed to the electron-hole recombination and emission from trap centers of surface states, respectively.

We report pressure and laser tuned Raman scattering studies on CdSxSei1-x nanocrystals. The electron-phonon coupling strength was determined as a function of pressure beyond the bulk phase transition pressure point. The coupling strength at atmospheric pressure determined from the Stokes shifted photoluminescence and resonant Raman scattering is not drastically smaller than the bulk value as might be expected theoretically. As a function of pressure it also shows no abrupt changes at the bulk phase transition pressure point. These results indicate that deep traps play a critical part in the mechanism of the electron-phonon coupling.

Intensity, temperature and frequency-dependent photoabsorption spectroscopy are used to probe trap states in CdSo.44Seo.56 nanoparticles in glass. The photoabsorption signal from 6.2 nm radius particles is found to be a combination of two effects: bleaching, due to phase space filling, and an electric-field effect. Frequency-dependent photoabsorption data show that the electric-field effect is caused by a long-lived (r=30 /µs at T=293 K) trap state and that the bleaching occurs on a faster time scale. The trap activation energy obtained from the temperature dependence of the lifetime is 0.23 ±0.06 eV. Only bleaching is observed in the smaller nanocrystallites.

Sol-gel matrices doped with lead sulfide nanoparticles showing a structured absorption spectrum were synthesized by using the procedure previously reported for CdS doped matrices. The morphology of the PbS aggregates was investigated by high resolution electron microscopy, and the observations are correlated with the absorption spectra. Finally, preliminary results concerning some linear optical properties of the materials are presented.

Colloid semiconductor particles of PbS have been prepared in reversed micelles of AOT-heptane. The effects of the AOT, water and reactants concentrations have been studied. The initial crystallite size and its growth with the time aging are strongly dependent on the water/AOT ratio and the ionic pool concentration. The absorption spectroscopic characteristics of the PbS particles have been examined. As the particle size decreases the band gap shifts in color from orange-brown to yellow and approaches the transition energy of the first allowed excited state of a PbS molecule. Moreover, the optical nonlinear property of PbS particles was investigated by Degenerate Four-Wave Mixing (DFWM) experiment using a frequency-doubled Nd:YAG laser.