RAMAN PHONONS OF SILICON WIRES

Using the Keating model and the Raman polarizability of Alben et al., the phonon Raman spectra of silicon wires are calculated. With the calculation results, the Raman spectra of porous silicon of some published papers are analyzed. Until now different authors have had different views on the luminescence mechanism of porous silicon, which may mainly result from the fact that they had different samples.

Strong visible photoluminescence from amorphous silicon grains in a-SiOx:H films

Two strong luminescence bands were observed from a-SiOx:H in the spectral range of 550-900 nm at room temperature. One is a main broad peak which blueshifts with oxygen content and the other is a shoulder fixed at about 835 nm. In conjunction with TR and micro-Raman spectra, we have proposed that the main band may originate from the amorphous silicon grains embedded in SiOx network, while the shoulder might be due to some defects induced by excess-silicon in these films. (C) 1997 Elsevier Science Ltd.

Scanned probe oxidation on an octadecyl-terminated silicon (111) surface with an atomic force microscope: kinetic investigations in line patterning

Scanned probe oxidation (SPO) nanolithography has been performed with an atomic force microscope (AFM) on an octadecyl-terminated silicon (111) surface to create protuberant oxide line patterns under ambient conditions in contact mode. The kinetic investigations of this SPO process indicate that the oxide line height increases linearly with applied voltage and decreases logarithmically with writing, speed. The oxide line width also tends to vary with the same law. The ambient humidity and the AFM tip state can remarkably influence this process, too. As compared with traditional octadecylsilated SiO2/Si substrate, such a substrate can guarantee the SPO with an obviously lowered voltage and a greatly increased writing speed. This study demonstrates that such alkylated silicon is a promising silicon-based substrate material for SPO nanolithography.

Investigation of precision grinding process for production of silicon diaphragms

The application of precision grinding for the formation of a silicon diaphragm is investigated. The test structures involved 2-6 mm diam diaphragms with thicknesses in the range of 25-150 //m. When grinding is performed without supporting the diaphragm, bending occurs due to nonuniform removal of the silicon material over the diaphragm region. The magnitude of bending depends on the µNal thickness of the diaphragm. The results demonstrate that the use of a porous silicon support can significantly reduce the amount of bending, by a factor of up to 300 in the case of 50 m thick diaphragms. The use of silicon on insulator (SOI) technology can also suppress or eliminate bending although this may be a less economical process. Stress measurements in the diaphragms were performed using x-ray and Raman spectroscopies. The results show stress of the order of 1 X107-! X108 Pa in unsupported and supported by porous silicon diaphragms while SOI technology provides stress-free diaphragms. Results obtained from finite element method analysis to determine deterioration in the performance of a 6 mm diaphragm due to bending are presented. These results show a 10% reduction in performance for a 75 µm thick diaphragm with bending amplitude of 30 fim, but negligible reduction if the bending is reduced to

Role of structural saturation and geometry in the luminescence of silicon-based nanostructured materials

The structural saturation and stability, the energy gap, and the density of states of a series of small, silicon-based clusters have been studied by means of the PM3 and some ab initio (HF/6-31G* and 6-311++G**, CIS/6-31G* and MP2/6-31G*) calculations. It is shown that in order to maintain a stable nanometric and tetrahedral silicon crystallite and remove the gap states, the saturation atom or species such as H, F, Cl, OH, O, or N is necessary, and that both the cluster size and the surface species affect the energetic distribution of the density of states. This research suggests that the visible luminescence in the silicon-based nanostructured material essentially arises from the nanometric and crystalline silicon domains but is affected and protected by the surface species, and we have thus linked most of the proposed mechanisms of luminescence for the porous silicon, e.g., the quantum confinement effect due to the cluster size and the effect of Si-based surface complexes.

Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapor deposition

Previous results concerning radiative emission under laser irradiation of silicon nanopowder are reinterpreted in terms of thermal emission. A model is developed that considers the particles in the powder as independent, so under vacuum the only dissipation mechanism is thermal radiation. The supralinear dependence observed between the intensity of the emitted radiation and laser power is predicted by the model, as is the exponential quenching when the gas pressure around the sample increases. The analysis allows us to determine the sample temperature. The local heating of the sample has been assessed independently by the position of the transverse optical Raman mode. Finally, it is suggested that the photoluminescence observed in porous silicon and similar materials could, in some cases, be blackbody radiation

Photoluminescence in silicon/silicon oxide films produced by the Pulsed Electron Beam Ablation technique

In this work we report studies of the photoluminescence emission in samples based on Si/SiOx films deposited by the Pulsed Electron Beam Ablation (PEBA) technique. The samples were prepared at room temperature using targets with different Si/SiO2 concentrations. The samples were characterized using X-ray Absorption Edge Spectroscopy (XANES) at the Si-K edge, Raman spectroscopy, Photoluminescence (PL) and X-ray Photoelectron Spectroscopy (XPS). The concentration of a-Si and nc-Si in the film was dependent on the silicon concentration in the target. It was also observed that the PL is strongly dependent on the structural amorphous/crystalline arrangement. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.

Theoretical estimation of optical absorption and photoluminescence in nanostructured silicon; an approach to improve efficiency in nanostructured solar cells

Renewable energy is growing in demand, and thus the the manufacture of solar cells and photovoltaic arrays has advanced dramatically in recent years. This is proved by the fact that the photovoltaic production has doubled every 2 years, increasing by an average of 48% each year since 2002. Covering the general overview of solar cell working, and its model, this thesis will start with the three generations of photovoltaic solar cell technology, and move to the motivation of dedicating research to nanostructured solar cell. For the current generation solar cells, among several factors, like photon capture, photon reflection, carrier generation by photons, carrier transport and collection, the efficiency also depends on the absorption of photons. The absorption coefficient,α, and its dependence on the wavelength, λ, is of major concern to improve the efficiency. Nano-silicon structures (quantum wells and quantum dots) have a unique advantage compared to bulk and thin film crystalline silicon that multiple direct and indirect band gaps can be realized by appropriate size control of the quantum wells. This enables multiple wavelength photons of the solar spectrum to be absorbed efficiently. There is limited research on the calculation of absorption coefficient in nano structures of silicon. We present a theoretical approach to calculate the absorption coefficient using quantum mechanical calculations on the interaction of photons with the electrons of the valence band. One model is that the oscillator strength of the direct optical transitions is enhanced by the quantumconfinement effect in Si nanocrystallites. These kinds of quantum wells can be realized in practice in porous silicon. The absorption coefficient shows a peak of 64638.2 cm-1 at = 343 nm at photon energy of ξ = 3.49 eV ( = 355.532 nm). I have shown that a large value of absorption coefficient α comparable to that of bulk silicon is possible in silicon QDs because of carrier confinement. Our results have shown that we can enhance the absorption coefficient by an order of 10, and at the same time a nearly constant absorption coefficient curve over the visible spectrum. The validity of plots is verified by the correlation with experimental photoluminescence plots. A very generic comparison for the efficiency of p-i-n junction solar cell is given for a cell incorporating QDs and sans QDs. The design and fabrication technique is discussed in brief. I have shown that by using QDs in the intrinsic region of a cell, we can improve the efficiency by a factor of 1.865 times. Thus for a solar cell of efficiency of 26% for first generation solar cell, we can improve the efficiency to nearly 48.5% on using QDs.

Wideband antireflection film fabricated by glancing angle deposition

Graded-index ZrO2 films has been fabricated on K9 glass by glancing angle deposition. Because the index mismatch at the interface has been reduced, the film results in wideband high-transmission antireflection. From 400nm to 1200nm, the film reflection is lower than 0.8% and the lowest value is 0.2% at 432nm.

Evolution of structural defects in SiOx films fabricated by electron cyclotron resonance plasma chemical vapour deposition upon annealing treatment

We study the structural defects in the SiOx film prepared by electron cyclotron resonance plasma chemical vapour deposition and annealing recovery evolution. The photoluminescence property is observed in the as-deposited and annealed samples. [-SiO3](2-) defects are the luminescence centres of the ultraviolet photoluminescence (PL) from the Fourier transform infrared spectroscopy and PL measurements. [-SiO3](2-) is observed by positron annihilation spectroscopy, and this defect can make the S parameters increase. After 1000 degrees C annealing, [-SiO3](2-) defects still exist in the films.

Anomalous temperature dependence of photoluminescence from stoichiometric GD(2)O(3-x) film

A stoichiometric Gd2O3-x thin film has been grown on a silicon (10 0) substrate with a low-energy dual ion-beam epitaxial technique. Gd2O3-x shares Gd2O3 structures although there are many oxygen deficiencies in the film. The photoluminescence (PL) measurements have been performed in a temperature range 5-300 K. The detailed characters of the peak position, the full-width at half-maximum (FWHM) and the peak intensity at different temperature were reported. An anomalous intensity behavior of the PL spectra has been observed, which is similar to that of some other materials such as porous silicon and silicon nanocrystals in silicon dioxide. Therefore, we suggest that the nanoclusters with the oxygen deficiencies contribute to the PL emission and employ the model of singlet-triplet exchange splitting of exciton to discuss the four peaks observed in the experiment. (C) 2003 Elsevier B.V. All rights reserved.

Chemical trends of defect formation in Si quantum dots: The case of group-III and group-V dopants

Using first-principles methods, we have systematically calculated the defect formation energies and transition energy levels of group-III and group-V impurities doped in H passivated Si quantum dots (QDs) as functions of the QD size. The general chemical trends found in the QDs are similar to that found in bulk Si. We show that defect formation energy and transition energy level increase when the size of the QD decreases; thus, doping in small Si QDs becomes more difficult. B-Si has the lowest acceptor transition energy level, and it is more stable near the surface than at the center of the H passivated Si QD. On the other hand, P-Si has the smallest donor ionization energy, and it prefers to stay at the interior of the H passivated Si QD. We explained the general chemical trends and the dependence on the QD size in terms of the atomic chemical potentials and quantum confinement effects.

Strain-induced anodization of SiGe/Si multiple layers to form high density SiGe/Si heterogeneous nanorods

Nation Natural Science Foundation of China 50672079 60676027 60837001 60776007; National Basic Research Program of China (973 Program) 2007CB613404; China-MOST International Sci & Tech Cooperation and Exchange 2008DFA51230

Photoluminescence from C+ ion-implanted and electrochemical etched Si layers

The microstructural and optical analysis of Si layers emitting blue luminescence at about 431 nm is reported. These structures have been synthesized by C+ ion implantation and high-temperature annealing in hydrogen atmosphere and electrochemical etching sequentially. With the increasing etching time, the intensity of the blue peak increases at first, decreases then and is substituted by a new red peak at 716 nm at last, which shows characteristics of the emission of porous silicon. C=O compounds are induced during C+ implantation and nanometer silicon with embedded structure is formed during annealing, which contributes to the blue emission. The possible mechanism of photoluminescence is presented. (c) 2005 Elsevier B.V. All rights reserved.

Photoluminescence behaviors from stoichiometric gadolinium oxide films

Stoichiometric gadolinium oxide thin films have been grown on silicon (100) substrates with a low-energy dual ion-beam epitaxial technique. Gadolinium oxide shares Gd2O3 structures although the ratio of gadolinium and oxygen in the film is about 2:1 and a lot of oxygen deficiencies exist. Photoluminescence (PL) measurements have been carried out within a temperature range of 5-300 K. The detailed characters of the PL emission integrated intensity, peak position, and peak width at different temperature were reported and an anomalous photoluminescence behavior was observed. The character of PL emission integrated intensity is similar to that of some other materials such as porous silicon and silicon nanocrystals in silicon dioxide. Four peaks relative to alpha band and beta band were observed also. Therefore we suggest that the nanoclusters with the oxygen deficiencies contribute to the PL emission and the model of singlet-triplet exchange splitting of exciton was employed for discussion. (C) 2003 American Institute of Physics.