The many and diverse approaches to materials science problems have greatly enhanced our ability in recent times to engineer the physical properties of semiconductors. Silicon, of all semiconductors, underpins nearly all microelectronics today and will continue to do so for some time to come. However, in optoelectronics, the severe disadvantage of an indirect band gap has limited the application of elemental silicon. Here we review a number of diverse approaches to engineering efficient light emission in silicon nanostructures. These different approaches are placed in context and their prospects for application in silicon-based optoelectronics are assessed.
Optical gain has been recently observed in ion implanted Si nanocrystals (nc). Critical issues to the observation of optical gain are the formation of a waveguide structure to improve the mode confinement and a large nanocrystal area den-sity in the samples. Here we confirm these results by measuring optical gain by the variable stripe length (VSL) method on a set of silicon nanocrystals (nc) formed by plasma enhanced chemical vapor deposition (PECVD) and annealing treatments. Time resolved VSL measurements with ns pulses at high pumping fluencies have revealed fast component in the recombination dynamics under gain conditions. Lifetime shortening and superlinear emission have been unambi-guously observed. The spectral shape of the fast luminescence is consistent with the amplified spontaneous emission lineshape (ASE) observed under CW pumping conditions and overlaps the gain spectral band. The observation of light amplification is critically dependent on a very delicate balance among the nc gain cross sections, the optical mode losses of the waveguide structure, and the fast non radiative Auger processes. Within a four levels model we quantify the strong competition among all these processes and we obtain a satisfactory agreement with the experiments.
Phase separation and thermal crystallization of SiO/SiO2 superlattices result in ordered arranged silicon nanocrystals. The preparation method enables independent control of particle size as well as of particle density and spatial position by using a constant stoichiometry of the layers. Infrared absorption and photoluminescence spectra are measured as a function of annealing temperature to study the phase separation process. Three photoluminescence emission bands are observed. A band centered at 560 nm is found in as-prepared samples and vanishes for annealing above 700oC. A second band around 760 nm to 890 nm is detected for annealing temperatures above 500oC. The superlattices show a strong luminescence and a size dependent blue shift in the visible and near-infrared region after crystallization for temperatures above 900oC. The origin of the different photoluminescence bands at different phase separation stages of ultra thin SiOx layers are discussed based on transmission electron microscopy investigations and on correlations seen in photoluminescence spectra and infrared absorption. In addition, we report the PECVD preparation of amorphous SiO/SiO2 superlattices which show a similar size dependent luminescence after crystallization.
Single nanometer-thick layers of crystalline silicon (c-Si) confined by amporphouse SiO2 have been prepared from silicon-on-insulator wafers. The photoluminescence from these ultra-thin quantum wells shows an increase in peak energy with decreasing c-Si layer thickness. Comparison with experimental results for the c-Si band gap and also with theory shows that the increase in photoluminescence peak energy is not as rapid as the measured or predicted energy gap. This difference is attributed to recombination of confined electron-hole pairs at the c-Si/SiO2 interface rather than within the quantum well.
We report the fabrication of dielectric mirrors microcavity with silicon nanocrystals layer as the emitting layer. The fabrication process allowed the non degradation of the optical properties of the emettors. By angular-resolved photoluminescence, we investigated changes in angular emission pattern caused by a half cavity and a full cavity. We show that the whole energy emitted in the half cavity vertical cone emission is concentrated in the sharp full cavity vertical cone emission.
We report on a strong intrinsic optical anisotropy of silicon induced by dielectric nanopatterning. As a result, in-plane birefringence of anisotropically nanostructured (110) oriented Si is found to be 105 times larger than that observed in bulk silicon. The difference of the main values of the anisotropic refractive index (Δn) exceeds that of any natural birefringent crystal. Δn depends strongly on the typical size of the silicon nanowires assembling the layers and the dielectric constant of the medium surrounding these silicon nanoparticles. We show that dielectric stacks of anisotropically nanostructured Si can act as a dichroic distributed Bragg reflectors or optical microcavities. The reflection/transmission behavior of these structures is sensitive to the polarization of the incident linearly polarized light. These findings open the possibility of an application of optical devices based on birefringent silicon layers in a wide field.
The enhancement of the third-harmonic generation (THG) in photonic crystal microcavities fabricated from alternating layers of mesoporous silicon is experimentally studied. Two types of THG resonances are observed in the third-harmonic intensity spectra measured in both angular and frequency domains. The THG enhancement is obtained as the fundamental radiation is in the resonance with the cavity mode and is attributed to the spatial localization of the fundamental field inside the cavity spacer and the fulfillment of the phase-matching conditions for THG. The intensive THG response is also observed as the fundamental radiation is tuned across the photonic band gap edge and is supposed to be attributed to the THG phase-matching. Additional factor for the THG enhancement is the three-photon resonance of the porous silicon cubic susceptibility.
We present ab initio results for the structural, electronic and optical properties of silicon nanostructures confined by silicon dioxide. We investigate the role of the dimension, symmetry and bonding situations at the interfaces. In particular we consider Si/SiO2 superlattices and Si nanocrystals embedded in SiO2 matrix. In the case of Si/SiO2 superlattices the presence of oxygen defects at the interface and the dimensionality are the key points in order to explain the experimental outcomes concerning photoluminescence. For Si nanocrystals embedded in SiO2 we show, in agreement with experimental results, the close interplay between chemical and structural effects on the electronic and optical properties.
Quantum confinement in anisotropic nanostructures has been studied within the effective-mass approximation. The calculation of the optical properties of elongated quantum dots is presented, showing the possibility of an anisotropic optical response depending on the dot geometry. The role of the radiation polarisation in the emission and absorption processes is pointed out. This can become quite relevant because the controlled fabrication of non-spherical quantum dots with variable aspect ratio is now available.
We developed a supercritical solution phase synthesis of silicon nanocrystals. High temperature and pressure (500°C, >140 bar) conditions allow a wet chemical approach to this challenging synthesis. Diphenylsilane was used as a silicon precursor and long chain thiols and alcohols were used to sterically stabilize the luminescent nanocrystals. Moderate size separation was achieved via size exclusion chromatography using crosslinked styrene divinylbenzene beads. Size separated fractions of silicon nanocrystals exhibit quantum efficiencies of 12% while polydisperse samples have quantum efficiencies of 5%. Nanocrystal size distributions have been determined with transmission electron microscopy and further characterized with atomic force microscopy (AFM). These silicon nanocrystals have size tunable photoluminescence as indicated by their ensemble spectroscopy and further verified through AFM and single nanocrystal photoluminescence spectroscopy. Fluorescence intermittency (characteristic of single CdSe nanocrystals) is present in our isolated silicon nanocrystals and is one of the criteria used to distinguish single crystals from clusters of particles.
The optical properties of both II-VI (direct gap) and type IV (indirect gap) nanosize semiconductors are significantly affected not only by their size, but by the nature of the chemical interface of the cluster with the embedding medium. This affects the light conversion efficiency and can alter the shape and position (i.e. the color) of the photoluminescence (PL). As the goal of our work is to embed nanoclusters into either organic or inorganic matrices for use as near UV, LED-excited phosphor thin films, understanding and controlling this interface is very important for preserving the high Q.E. of nanoclusters known for dilute solution conditions.
We describe a room temperature synthesis of semiconductor nanoclusters which employs inexpensive, less toxic ionic precursors (metal salts), and simple coordinating solvents (e.g. tetrahydrofuran). This allows us to add passivating agents, ions, metal or semiconductor coatings to identical, highly dispersed bare clusters, post-synthesis. We can also increase the cluster size by heterogeneous growth on the seed nanoclusters.
One of the most interesting observations for our II-VI nanomaterials is that both the absorbance excitonic features and the photoluminescence (PL) energy and intensity depend on the nature of the surface as well as the average size. In CdS, for example, the presence of electron traps (i.e Cd(II) sites) decreases the exciton absorbance peak amplitude but increases the PL nearly two-fold. Hole traps (i.e. S(II)) have the opposite effect. In the coordinating solvents used for the synthesis, the PL yield for d~2 nm, blue emitting CdSe clusters increases dramatically with sample age as the multiple absorbance features sharpen.
Liquid chromatographic (LC) separation of the nanoclusters from other chemicals and different sized clusters is used to investigate the intrinsic optical properties of the purified clusters and identify which clusters are contributing most strongly to the PL. Both LC and dynamic light scattering, show that as the nanocluster concentration approaches 1 x 10-4M and above, a large loss in light emission occurs due to association or "clumping" of clusters. Overcoming this natural tendency toward aggregation may be the most significant technical obstacle to the use of nanoclusters in thin film phosphors.
Hybrid organic-inorganic thin films doped with lead sulfide nanocrystallites were synthesized by a combination of colloidal chemistry and sol-gel processing. In order to study the influence on the spectroscopic properties of the crystallite surface, and hence the related defect states, PbS-doped films with different sulfur to lead ratios and different surface capping agent concentrations were fabricated. X-ray diffraction measurements showed the presence of nanoparticles with a mean diameter raging from 3 to 5 nm. The absorption spectra showed a large blue shift of the absorption edge to shorter wavelength, indicating strong quantum confinement. Strong photoluminescence emission in the near infrared was found by pumping at 514 or 532 nm. The emission intensity and its position were found to be dependent on the elaboration parameters. The film fabrication process influences only slightly the good particle size distribution of the colloidal PbS solutions. Best results were obtained for films with low sulfur to lead ratio or with high capping agent concentration.
The silica sol-gel derived glasses co-doped with CuxO and CuxSe nanoparticles and Eu3+ ions have been fabricated. The analysis of luminescence spectra of a series of glasses with different composition allows us to suppose the direct energy transfer between copper oxide nanoparticle and Eu3+ ion. A luminescence signal of europium ions occurs as the result of excitation of the complex active centres (SiO2:Cu2O:Eu3+) in the absorption range of copper oxide.
The mechanisms of porous gallium phosphide formation by anodic etching are studied. Gallium phosphide porous samples <100> oriented were prepared in sulfuric acid solution with different concentrations of NaF. The current-voltage characteristic curve depends on the NaF concentrations and shows the typical behavior for porous semiconductors formation. Four regions can be distinguished in the I-V curve: a pore formation zone, a dielectric layer growth, a transition region in which the two processes compete for the control of the surface morphology and finally the GaP surface electropolishing. The oxide formation shifts to higher potential and the pore formation zone is widened by adding fluorides in the etching solution. Raman spectroscopy is applied to investigate the surface morphology of samples prepared in different anodizing current density conditions and in different acid solutions. As regards the dielectric growth, the direct observation of the sample surface and the analysis of the vibrational spectra indicate that in different potential regimes two chemically different oxides can be formed.
A concept of quantum dots in photonic dots has been realized by incorporating highly luminescent CdSe nanocrystals (quantum dots) into various spherical microcavities, or 3-D photonic dots (micron-size glass, polymeric microspheres). Coupling of discrete electron states of quantum dots and photon states inside photonic dots strongly affects onto both stationary and dynamic photoluminescence properties of nanocrystals. A number of interesting optical effects is demonstrated on such structures: increase in radiative recombination rate in the vicinity of ultranarrow photon modes (Purcell effect); room temperature nearly thresholdless lasing; blinking of photon modes; single photon mode switching by single quantum dot emission.
We describe the synthesis of colloidal mercury chalcogenide quantum dots (QDs) using a combination of strong Hg(II) coordinating ligands and precursor phase separation. This synthetic strategy provides a means of controlling the growth kinetics of mercury based II-VI QDs and addresses some of the problems which have heretofore made the synthesis of such compounds difficult. In particular, the simultaneous use of mercury coordinating ligands and precursor phase separation overcomes both the rapid precipitation of bulk mercury chalcogenides that occurs when only weak ligands are used and the reduction of Hg(II) when a strong ligand/high temperature combination is pursued. In the case of both HgS and HgSe this scheme has yielded one of the first examples of mercury chalcogenide QDs to date. The linear absorption/emission of HgS is size-dependent and ranges from 500 nm to 800 nm with corresponding sizes between 1 to 5 nm in diameter. For HgSe the band edge absorption/emission are also size dependent, ranging from 600 to 900 nm. The zincblende phase of both HgS and HgSe QDs is determined from wide angle x-ray diffraction experiments and reveals potentially large (band edge) spectral tunabilities for either material given their zero or slightly negative (bulk) band gaps.
The significance of surface states in nano-structures is studied using CdS nanoparticles. Spectral features like peak red-shift due to organic capping and influence of surface states have been observed. The pronounced enhancement of emission from surface states can be dominant with certain modiciation of CdS nanoparticles. Spectral behaviors of electroluminescence in different temperature are also studied.
The third-order nonlinear optical response of Au nanocrystals embedded in BaTiO3 and ZrO2 matrices were investigated by off-resonance femtosecond optical Kerr effect. The nonlinearity of composite films show an enhancement comparing with the pristine matrices films, which is Au particle size and matrices refractive index dependence. A calculation based on Lorenz-Mie scattering theory was performed to explain the experimental results. Additionally, optimization of nonlinearity was obtained by considering the particle size and matrices refractive index.
In this work we investigate the transport properties of Si/SiO2 superlattices with a multiband one-particle Monte Carlo simulator. The band structure of the system is obtained analytically by solving the Kronig-Penney potential in a tight binding approximation along the growth direction z while we have assumed parabolic dispersion in the in-plane directions. We have introduced in the simulator confined optical phonons, both polar and non polar, as scattering mechanisms. Owing to the very flat shape of the bands along the growth direction, very low drift velocities are obtained for vertical transport. However it turns out that for oblique fields, the in-plane component of the electric field strongly influences the transport properties along the vertical direction as effect of carrier heating. In particular higher vertical drift velocities can be obtained.
The spatial distribution of the local optical field at the cleavage of photonic crystal smicrocavity has been obtained by the scanning near-field optical microscope (SNOM). The localization of optical radiation at microcavity resonant wavelength in the vicinity of the λ/2 spacer layer is demonstrated. Samples of photonic crystal microcavity are prepared from silicon wafer by electrochemical etching technique. The wavelength of the microcavity mode is optimized for resonance with wavelengths of lasers. The image of the spatial distribution of optical field at the cleaved edge of the facing vertically microcavity is observed. Sample is pumped through external single-mode fiber perpendicularly to the microcavity. SNOM
operates in the collection mode with the apertureless tip. We observe the localization of the resonant optical field in microcavity but we do not reveal such localization of the radiation at the non-resonant wavelength.
The enhancement of the second-harmonic generation (SHG) in all-silicon coupled microcavities (CMC) based on one-dimensional photonic crystals is experimentally studied. CMC are fabricated from alternating layers of electrochemically grown mesoporous silicon and consist of two identical half-wavelength-thick cavity spacers separated by additional porous silicon photonic crystal. The enhancement of the second-harmonic response of CMC in the vicinity of splitted cavity modes is experimentally observed in both angular (wave vector) and frequency domain spectra. The variation of transmission of the intermediate photonic crystal, which controls the interaction between coupled cavity spacers, leads to monotonic dependence of the SHG resonances splitting on the number of pairs of the intermediate Bragg reflector.
Our efforts to nucleate and grow AgI nanoparticles in Sb-doped Ag thin films by a closed chamber ambient iodization process has been delineated. The Ag-Sb thin films are characterized by XRD to be nanocrystalline; Sb prolongs the iodization process by delaying the nucleation of AgI nanoparticles, so that optimization of concentration, thickness and iodization duration yields films with self-assembled nanoparticle structure. The retarded growth of AgI particles is reflected in the slow but controlled evolution of the exciton band and variation in the Sb concentration dependent cluster density directly seen in the SEM micrographs.