Low temperature growth of hydrogenated nanocrystalline silicon film (nc-Si:H) by microwave electron cyclotron resonance chemical vapor deposition has been performed employing a double dilution of silane, using a He carrier for SiH4 and its subsequent dilution by H2. A series of Raman spectra and AFM pictures has shown that a very thin (<100Å) nc-Si:H layer initially grown with high H2 dilution on a glass substrate can serve as a seed layer for the subsequent growth of the film with lower H2 dilution, which results in a higher crystallinity of the whole film. The role of this thin layer in low temperature junction formation has been examined by the insertion of the layer between the interface of both nc-Si:H (deposited with lower H2 dilution)/c-Si and a-Si:H/c-Si heterojunction type photovoltaic cells. This is to address the knowledge that the device's performance is strongly influenced by the quality of the thin film silicon/crystalline silicon interface. Various thicknesses and H2 dilution ratios have been used to find the optimized condition providing the best performance of the cells. The maximum efficiency of 10.5% (Jsc=35.1mA/cm2, Voc=0.51V and FF=0.59) has been obtained, without an AR coating, by the successive deposition of nc-Si:H film with four different H2 dilution ratios on a crystalline silicon substrate. This is potentially a low-temperature, low-cost solar cell fabrication process.

Infrared photoluminescence (PL) and electroluminescence (EL) from erbium-doped porous silicon (PSi) structures are studied. The PL and EL from the Er-doped PSi structures and the absence of silicon band edge recombination, point defect, and dislocation luminescence bands suggest that the Er-complex centers are the most efficient recombination sites. PSi multilayers with very high reflectivity (R ≥ 90%) in the 1.5 gim range have been incorporated in the structures resulting in a PL enhancement of over 100%. Stable and intense EL is obtained from the Er-doped structures. The EL spectrum is similar to that of the PL, but shifted towards higher energy. The unexpected shift in emission opens up the possibility for erbium related luminescence to encompass a larger part of the optimal wavelength window for fiber optic communications.

The morphology of nanocrystalline (nc)-Si/amorphous (a)-SiO2 superlattices (SLs) is studied using Raman spectroscopy in the acoustic and optical phonon ranges, transmission electron microscopy (TEM), and atomic force microscopy (AFM). It is demonstrated that high temperature annealing (up to 1100°C) and oxidation in O2/H2O ambient do not destroy the SL structure, which retains its original periodicity and nc-Si/a-SiO2 interface abruptness. It is found that oxidation at high temperatures reduces the defect density in nc-Si/a-SiO2 SLs and induces the lateral coalescence of Si nanocrystals (NCs). The size, shape, packing density, and crystallographic orientation of the Si nanocrystals are studied as a function of the oxidation time.

The compatibility of both bulk and porous silicon at the subcutaneous site has been assessed for the first time, following ISO standard procedures. The in-vivo responses to implantation were monitored in the guinea pig and histopathological reactions evaluated at 1, 4, 12 and 26 weeks. Attention is focused here on the histological assessment protocols used, and the results demonstrating in-vivo evidence for good tissue compatibility, and porous Si bioactivity with regards calcification.

As-prepared porous silicon comes out covered with covalently bonded hydrogen. This hydrogen coating provides a good electronic passivation of the surface, but it exhibits limited stability, being removed by thermal desorption or converted into an oxide upon prolonged storage in air. Starting from the hydrogenated surface, an oxide layer with good electronic properties is also obtained by anodic oxidation or rapid thermal oxidation.

The hydrogenated surface may be nitridized using thermal treatments in nitrogen or ammonia. Fast halogenation of the surface may be obtained at room temperature, but the resulting coating is rapidly converted to an oxide in the presence of moisture. Many metals have been incorporated into the pores, using chemical or vacuum techniques, or even direct incorporation during porous silicon formation.

More interestingly, organic derivatization may increase surface stability or provide chemical functionalities. The poor reactivity of the hydrogenated surface can be remedied by using various methods: thermal desorption of hydrogen, hydroxylation or halogenation of the surface, thermal or UV assisted reaction. However, most promising results have been obtained through either Lewis-acid catalyzed grafting or electrochemical activation of the surface. The latter method has been used for grafting formate, alkoxy, and recently methyl groups. In most of these methods, oxidation is present as a parallel path, and care must be taken if it is not desired. Also, 100% substitution of the hydrogens by organic groups has never been attained, due to steric hindrance problems. The electrochemical method appears especially fast, and has led to 80% substitution of the hydrogens by methyl groups, with no photoluminescence loss and a chemical stability increased by one order of magnitude.

This paper describes the covalent attachment of various organic molecules to the hydrogenterminated surface of porous silicon using alcohols and Grignard reagents. With alcohols, the chemical reaction forms Si-O-C attachments to the silicon substrate and requires modest heating (40–70 °C). With Grignard reagents, the reaction proceeds at room temperature and forms a covalent film that is attached by Si-C bonds to the silicon support. Evidence for these reactions is provided by infrared and x-ray photoelectron spectroscopies.

Hydrosilylation of alkynes and alkenes on silicon surfaces utilizing the native Si-H termination can be smoothly and rapidly carried out (30 s to 24 h) at room temperature through hydrosilylation mediated by Lewis acid catalysts or photoinduction with white light. Insertion of alkynes and alkenes into surface silicon hydride bonds yields covalently bound alkenyl and alkyl groups, respectively. Different chemical functionalities can be incorporated through these hydrosilylation reactions, including ester, hydroxy, chloro, nitrile and chiral groups. Hydrophobic porous silicon surfaces demonstrate remarkable stability with respect to boiling aqueous aerated pH 1 to 10 solutions, and protect the bulk silicon from attack. Modification and tailoring of surface properties through this series of reactions induce wide variations in photoluminescent behavior of porous silicon, leading to almost complete quenching in the case of substituted and unsubstituted styrenyl termination, and minor decreases for alkyl and alkenyl functionalization. Because of the broad range of stable, modified surfaces produced using this chemistry, the work described here represents an important step towards technological applications of silicon surfaces.

The establishment of tuning techniques of visible emission from porous silicon (PS) is very important from both physical and technological viewpoints. As previously reported that the photoluminescence (PL) spectra of PS can continuously be controlled from red to blue simply by postanodization illumination method without any growth of the surface oxide. Details of this oxide-free blue emission have been studied in terms of the PL decay dynamics and surface chemistry. We report here that post-preparation exposure of PS to hydrogen gas is very useful for improvement in efficiency and stability of blue PL. Based on this technique, it has become possible to get blue electroluminescence even at low bias voltages.

The effect of surface passivation and crystallite size on the photoluminescence of porous silicon is reported. Oxygen-free porous silicon samples with medium to ultra high porosities have been prepared by using electrochemical etching followed by photoassisted stain etching. As long as the samples were hydrogen-passivated the PL could be tuned from the red (750nm) to the blue (400nm) by increasing the porosity. We show that when surface oxidation occurred, the photoluminescence was red-shifted. For sizes smaller than 2.8nm, the red shift can be as large as 1eV but for larger sizes no shift has been observed. Comparing the experimental results with theoretical calculations, we suggest that the decrease in PL energy upon exposure to oxygen is related to recombination involving an electron or an exciton trapped in Si=O double bonds. This result clarifies the recombination mechanisms in porous silicon.

We use high resolution transmission electron microscopy (XTEM) to image the nanostructure of (100) p-type porous Si. A network of pore tracks subdivide the material into nanoislands and nanocrystallites are resolved through out the material. With distance from the substrate, electron diffraction develops, in addition to coherent diffraction, amorphous-like patterns that dominates the coherent scattering in the topmost luminescent layer. Also, with distance from the substrate, crystalline island size diminshes to as small as 1 nm in the topmost luminescence material. Although their uppermost layer has the most resolved nano crystallites, it has the strongest diffuse scattering of all regions. This suggests that the diffuse scattering is due to a size reduction effects rather than to an amorphous state. We discuss the relevance of a new dimer restructuring model in ultra small nanocrystallites to the loss of crystalline effects.

Luminescent porous silicon films were created by electrochemical anodization of n-type substrate under light illumination. Semi-transparent conducting polypyrrole films were deposited by electrochemical polymerization at a low current density. The SEM micrographs showed that the polymer film impregnates into the wide vertical pores of 1–5 µm and grows sideways suggesting strong current distribution on the walls. The AFM images of polymer surface reveal nanometer size polymer aggregates on the porous layer. The impregnation of the polymer film due to sideways growth provides a useful mean to fabricate stable contact for light emitting diodes from porous silicon.

Here we report a nanoparticulate route to Y2O3 nanofibers (~50 nm in diameter and a few micrometers in length) and for the radial growth of ZnS spheres (200-800 nm diameter). Well-defined higher order structures are developed upon thermostatically aging the dispersions of monomeric nanocrystals. The shapes of the “macromolecules„ are correlated to primary monomeric nanocrystallites, the growing time and temperature, and surfactant templating agents. It is anticipated that this approach should inspire fabrication of nanoparticulate structures by using primary nanoparticles as monomers.

Femtosecond (fs) broad-band transient absorption (TA) is used to study the intra-band relaxation and depopulation dynamics of electron and hole quantized states in CdSe nanocrystals (NC's) with a range of surface properties. Instead of the drastic reduction in the energy relaxation rate expected due to a 'phonon bottleneck', we observe a fast sub-picosecond 1P-to-1S relaxation, with the rate enhanced in NC's of smaller radius. We use fs IR TA to probe electron and hole intraband transitions, which allows us to distinguish between electron and hole pathways leading to the depopulation of NC quantized states. In contrast to electron relaxation, which is controlled by NC surface passivation, depopulation of hole quantized states is extremely fast (sub-ps-to-ps time scales) in all types samples, independent of NC surface treatment (including NC's overcoated with a ZnS layer). Our results indicate that ultrafast hole dynamics are not due to trapping at surface defects, but rather arise from relaxation into intrinsic NC states.

We report here the first measurement of strong near-infrared room temperature photoluminescence (PL) from colloidally-prepared HgTe nanocrystals. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) measurements indicate that the nanoparticles are in the cubic coloradoite phase, with a diameter of approximately 4 nm. The absorption spectrum shows a pronounced electronic transition in the near-infrared, and the broad PL appears to consist of several overlapping features between 800 and 1400 nm with a peak at 1080 nm, which represent a dramatic shift from bulk HgTe behaviour. The quantum efficiency (QE) of the freshly prepared sample is around 50%, which is among the highest ever reported for a nanocrystalline material. Over a period of several days, the luminescence shifts further into the infrared yielding more dominant longer wavelength features. The observation of this strong infrared luminescence makes this material a promising candidate for application in optical telecommunication systems.

We have fabricated zincblende GaAs nanocrystals by means of Ga+ and As+ coimplantation into SiO2 matrices. A broad photoluminescence band is observed in the visible spectral region. Under selective excitation at energies within the visible luminescence band, GaAs-related phonon structures are observed at low temperatures. The photoluminescence mechanism in GaA/SiO2 nanocomposites is discussed.

We investigated the plasmon characteristics on luminescent porous silicon using electron energy loss spectroscopy. The samples were prepared from p-type crystalline silicon, (100) face, using the conventional electrochemical etching technique with the usual solution of HF, ethanol and water, followed by a critical point drying process. The energy of the bulk plasmon was measured both before and after sputter cleaning the sample with argon-ion bombardment. We found that initially the plasmon energy was slightly higher, ∼18 eV, than the plasmon energy of crystalline silicon. After sputter cleaning the sample with 5 keV Ar+ ions, the plasmon energy increased to ∼20 eV. Exposure to the electron beam used for the measurements caused a slow upward shift of the plasmon energy as a function of time, toward a saturation energy of 22-23 eV, an energy close to the plasmon energy of SiC. Auger spectroscopy performed in parallel showed an increasing carbon coverage. We prepared also samples without ethanol in the etching solution and/or with no critical point drying. Samples that did not undergo the critical point drying process showed consistently a practically constant plasmon energy, with almost no change upon sputtering and/or exposure to the electron beam. On the other hand, samples that were prepared with or without ethanol but using the critical point drying process, showed an appreciable increase in the plasmon energy upon exposure to the electron beam.

We conclude that traces of CO2, used in the critical point drying process, are stored in the pores of the porous silicon surface and serve as a source of carbon. Apparently, upon activation by argon bombardment or by the electron beam, the carbon interacts with the porous Si surface forming a carbon-silicon compound, most probably SiC.

The use of nanoparticle precursors for electronic materials including sulfides, selenides, oxides and the elements has potentially wide ranging implications for improving device properties and substantially reducing the deposition costs. To realize this goal the complex interfacial chemistry of these small particles must be controlled. In this paper we present a number of cases demonstrating the complexity of this chemistry. These include CuInSe2 where the kinetics of phase formation dominate the sintering process; CdTe where sintering proceeds with and without the sintering enhancement of CdCl2, but produces materials different electronically than bulk materials; and the use of compound and elemental nanoparticles ( Ag, Al, Hg-Cu-Te and Sb-Te) for contacts to elemental and compound semiconductors (Si and CdTe).

GaN films have been deposited over a range of temperatures from 50 C to 650 C by ECR plasma MOCVD on silicon (111) and (100), sapphire and quartz using triethylgallium and molecular nitrogen or ammonia as reagents. Growth rates of 2 um/hr are achieved on temperature-controlled substrates (total reactor pressure 0.5 mTorr, 250 watts at 2.45 GHz).

Films deposited at 200, 600 and 650 C on sapphire show the GaN(0002) diffraction peak and sharp photoluminescence lines (at 10 K) between 370 and 400 nm and broad emission at 530-550 nm. Broad photoluminescence at 390 nm is observed from GaN/Si( 11). Films deposited at 50 and 100 C show no evidence of a crystalline phase or GaN(0002) diffraction peak. The films are smooth and optically transparent. A broad photoluminescence peak at 520 nm, with a fwhm of about 150 nm is also observed (at 10K). The optical bandgap is measured to be about 2.6-2.7 eV. All of these films show a GaN LO phonon mode at 736 cm-l. IR spectra indicate some hydrocarbon impurities in the low temperature films.

Prototype devices (Schottky barrier diodes) have been made from MOCVD GaN and amorphous GaN.

Amorphous, polycrystalline, and single crystal nanometer dimension particles can be formed in a variety of substrates by ion implantation and subsequent annealing. Such composite colloidal materials exhibit unique optical properties that could be useful in optical devices, switches, and waveguides. However colloids formed by blanket implantation are not uniform in size due to the nonuniform density of the implant, resulting in diminution of the size dependent optical properties. The object of the present work is to form more uniform size particles arranged in a 2-dimensional lattice by using a finely focused ion beam to implant identical ion doses only into nanometer size regions located at each point of a rectangular lattice. Initial work is being done with a 30 keV Ga beam implanted into Si. Results of particle formation as a function of implant conditions as analyzed by Rutherford backscattering, x-ray analysis, atomic force microscopy, and both scanning and transmission electron microscopy will be presented and discussed.

CdS/HgS/CdS nanoparticles consist of a CdS core, epitaxially covered by one or two monolayers of HgS and additional cladding layers of CdS. The present paper describes our efforts to identify the influence of CdS/HgS/CdS interfaces on the localization of the photogenerated carriers deduced from the magneto-optical properties of the materials. These were investigated by the utilization of optically detected magnetic resonance (ODMR) and double-beam photoluminescence spectroscopy. A photoluminescence (PL) spectrum of the studied material, consists of a dominant exciton located at the HgS layer, and additional non-excitonic band, presumably corresponding to the recombination of trapped carriers at the interface. The latter band can be attenuated using an additional red excitation. The ODMR measurements show the existence of two kinds of electron-hole recombination. These electron-hole pairs maybe trapped either at a twin packing of a CdS/HgS interface, or at an edge dislocation of an epitaxial HgS or a CdS cladding layer.