Mechanism of silicon epitaxy on porous silicon layers

The mechanism of silicon epitaxy on porous Si(111) layers is investigated by the Monte Carlo method. The Gilmer model of adatom diffusion extended to the case of arbitrary surface morphology is used. Vacancies and pendants of atoms are allowed in the generalized model, the activation energy of a diffusion hop depends on the state of the neighboring positions in the first and second coordination spheres, and neighbors located outside the growing elementary layer are also taken into account. It is shown that in this model epitaxy occurs by the formation of metastable nucleation centers at the edges of pores, followed by growth of the nucleation centers along the perimeter and the formation of a thin, continuous pendant layer. Three-dimensional images of surface layers at different stages of epitaxy were obtained. The dependence of the kinetics of the epitaxy process on the amount of deposited silicon is determined for different substrate porosities. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 512–517 (10 April 1998)     

Study of porous silicon structure by Raman scattering

In this paper, the effect of etching time on light emitting porous silicon has been studied by using Raman scattering. Enhancement of Raman intensity by increasing the porosity is observed. Also there is a red shift, about 4 cm⁻¹, from the Raman peak of crystalline silicon to that of porous silicon. The phonon confinement model suggests the existence of spherical nanocrystalline silicon with diameter around 7 nm. But SEM images show that the samples have a sheetlike structure that confines phonons in one dimension. This should not cause any shift in their Raman spectra. It is suggested that the observed Raman peak shift is due to the spherical nanocrystals on the surface of these sheets.     

Variation of double-peak structure in photoluminescence spectra from porous silicon with excitation wavelength

We have systematically observed at room temperature the variations of photoluminescence(PL) from porous silicon(PS) with excitation wavelength in a range from 260 to 460nm at 20nm intervals. In the range from 260 to 320nm, the PL spectra for one of the two studied samples show clear double-peak structure. Each spectrum can be fitted by two Gaussian functions with their peaks centered at about 610nm and 710nm, respectively. The peak position and the full width at half maximum of the two Gaussian functions change little with excitation wavelength. The above phenomena seem hard to interpret using the quantum confinement model, but can be understood in the quantum confinement/luminescence centers model (G.G.Qin and Y.Q.Jia, Solid State Commun. 86, 559(1993)), if we suppose that there are two kinds of luminescence centers adsorbed on the surfaces of nane-scale silicon (NS) units or situated in the SiO_x layers covering the NS units in PS.     

Propagation of light in a structure with a regular magnetic inhomogeneity

The reflection and transmission of a light wave normally incident on a layer with a regular magnetic inhomogeneity described by a harmonic function are investigated. Solutions to the Maxwell equations for such a medium and the Jones matrices determining the magneto-optical characteristics of the structure under consideration (the coefficients of transmission and reflection and the degrees of polarization of transmitted and reflected waves) are obtained.     

Kinetics of photoreactions in a regular porous nanostructure with cylindrical cells filled with activator-containing macromolecules

The kinetics of a two-stage laser-induced reaction between electronically excited molecules of two sorts (an organic luminophore and oxygen) in a regular porous nanostructure whose cells are filled with linear macromolecules is studied. A mathematical model of the process is proposed that takes into account an inhomogeneous distribution of polymer chain links in a nanopore, with this distribution determining the radial profile of reagents. One version of the theory considers a radial diffusion flux of activated oxygen molecules, while the other version considers a thermodiffusion flux of unexcited molecules. The numerical results of the model are compared to the data of molecular dynamic calculations and experiment.     

一种正三角形结构的双光束干涉装置

依据波动光学的原理,介绍了一种能产生双光束干涉现象的正三角形干涉装置,解析推导并分析了该干涉结构发生等倾干涉的条件及干涉斑纹的分布规律;同时讨论了这一光学干涉结构的可能应用,及其对光学干涉的教学研究的补充与辅助作用.     

Birefringence of porous silicon

The paper reports the observation and studies of the birefringence in porous silicon samples obtained onto different crystallographic planes of silicon single crystals.     

Luminescence centers in porous silicon

Photoluminescence studies on porous silicon show that there are luminescence centers present in the surface states. By taking photoluminescence spectra of porous silicon with respect to temperature, a distinct peak can be observed in the temperature range 100–150 K. Both linear and nonlinear relationships were observed between excitation laser power and the photoluminescence intensity within this temperature range. In addition, there was a tendency for the photoluminescence peak to red shift at low temperature as well as at low excitation power. This is interpreted as indicating that the lower energy transition becomes dominant at low temperature and excitation power. The presence of these luminescence centers can be explained in terms of porous silicon as a mixture of silicon clusters and wires in which quantum confinement along with surface passivation would cause a mixing of andX band structure between the surface states and the bulk. This mixing would allow the formation of luminescence centers.     

Diffusion in porous silicon carbide

By the example of vanadium and erbium diffusion in porous silicon carbide, the semiconductor porous structure modification during thermal annealing has been simulated and the effect of this modification on impurity diffusion has been considered. A comparison of calculated and experimental profiles of the erbium and vanadium distributions in porous silicon carbide shows that the consideration of porous structure modification due to vacancy redistribution makes it possible to adequately describe diffusion in the porous semiconductor.     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

Sensor properties of structures with porous silicon layer

We have measured at room temperature current-voltage and noise characteristics of structures with a porous silicon (porosity 80%) layer at adsorption of gases ammonia, propane and butane mixture, and ethyl alcohol vapor. It was obtained that the largest change in CVC and low-frequency noise is observed under action of ammonia gas on the structure. Physical reasons of sensor properties of studied samples are discussed.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Method of porous diamond deposition on porous silicon

In this paper, we discuss the experimental results of the fabrication of porous diamond/porous silicon and porous diamond structures by chemical vapor deposition of diamond over a skeleton of porous silicon, replicating the porous surface geometry around the Si pores and also creating new porous diamond structures. Scanning electron microscopy (SEM) revealed that the diamond nuclei are deposited on the top of the porous silicon skeleton, forming isolated grains in the first nucleation stages, and then growing like the usual structure of most ceramic materials, making a self-sustained porous diamond structure. Raman spectroscopy revealed that the diamond films are of good quality, close to that of diamond films grown on crystalline silicon.     

Triangular pattern formation on silicon through self-organization of GaN nanoparticles

Nanoparticles of gallium nitride, synthesized by a low-temperature reaction between triethyl gallium and ammonia, were introduced onto silicon wafers containing a thin layer of chemically prepared silicon dioxide. At room temperature, the nanoparticles form unstructured agglomerates on the surface. However, upon annealing the samples beyond the decomposition temperature of the silicon dioxide layer, the gallium nitride particles self-organize to form triangular structures. The pattern formation is attributed to the domain separation associated with the (1 × 1)-(7 × 7) surface phase transformation followed by selective incorporation of the nanoparticles.     

Confinement in a planar waveguide with porous silicon omnidirectional mirrors as confining walls

We present the design and study of waveguide structures based on porous silicon where the light confinement is not due to the usual total reflection effect but to the use of photonic crystals (PCs) as confining walls. These PC are omnidirectional mirrors (OMs), consisting of the periodic repetition of two porous silicon layers with different refractive indices and thicknesses. They reflect the radiation for all angles of incidence within a frequency range called the omnidirectional band gap (OBG). We have followed the PC formalism to investigate the properties of the OM as a multimode waveguide: the number of modes within the band gap, their field spatial distribution and their confinement as a function of the frequency and the core thickness.     

Properties of erbium silicate doped with chromium in porous silicon

The possible formation of chromium-doped erbium silicate Er₂SiO₅: Cr in thin layers of porous silicon is demonstrated. This paper reports on studies of the photoluminescence, electron paramagnetic resonance, and transverse current transport in porous silicon layers (with different chromium and erbium contents) grown on n-and p-silicon single crystals heavily doped with shallow impurities. The Er₂SiO₅: Cr phase with the photoluminescence maxima at approximately 1.3 and 1.5 μm manifests itself after high-temperature annealing at 1000°C. The introduction of erbium and annealing at 700°C increase the intensity of the red photoluminescence of porous silicon by several factors. The decrease in the electrical conductivity of porous silicon suggests the onset of the formation of erbium silicate. The current-voltage characteristics exhibit a nonlinear behavior with an exponential dependence of the current on the voltage due to the discrete electron tunneling. An electron paramagnetic resonance spectrum of P _b centers in p-type heavily doped silicon is observed for the first time.     

Properties of magnesium silicate doped with chromium in porous silicon

Forsterite doped with Cr⁴⁺ ions is prepared in silicon-based structures according to a simple technique. These structures are of interest due to the characteristic luminescence in the near-IR range. Forsterite is synthesized by impregnation of porous silicon layers on n⁺-Si and p⁺-Si substrates with subsequent annealing in air. A photoluminescence response at a wavelength of 1.15 μm is observed at room temperature in porous silicon layers doped with magnesium and chromium for which the optimum annealing temperature is close to 700°C. The photoluminescence spectrum of porous silicon on the p⁺-Si substrate contains a broad band at a wavelength of approximately 1.2 μm. This band does not depend on the annealing temperature and the magnesium and chromium content and is most likely associated with the presence of dislocations in silicon. The experimental EPR data and eletrical properties of the structures are discussed. It is found that layers of pure porous silicon and chromium-doped porous silicon on n⁺-Si subtructures exhibit indications of discrete electron tunneling.     

Resonance effects in the non-radiative de-excitation of silicon in porous silicon

Resonant silicon Auger KLL and 2s and 2p photoemission spectra of a porous silicon sample have been studied when excited by photons in the energy domain of the 1s edge in pure silicon and silicon oxide. Characteristic features of a resonant process could be detected. In particular, the constant initial state spectrum of the 2p state of silica behaves similarly to that encountered in systems which present a well-defined atomic level. This is due to the existence of a well-localized molecular orbital built in the SiO₄ unit. The use of high-energy photons, which generate high-energy electrons, allows these photoemission experiments to be quite bulk sensitive.     

Ellipsometric studies of porous silicon

Multiple-angle-of-incidence (MAI) ellipsometry at 632.8 nm is used to characterize P and P⁺ porous silicon of high porosity. Complex dielectric constants are obtained, from which the porosity can be estimated qualitatively. The properties of the imaginary part of the dielectric constants are studied and the possible causes are discussed. Two dielectric constants, perpendicular and parallel to the interface, respectively, are measured based on a semi-infinite anisotropic model. Ellipsometric studies demonstrate a larger difference between the two dielectric constants in P⁺ porous silicon, but both P and P⁺ samples only show weak anisotropy, i.e., a network-like structure, which tends towards isotropy, is more suitable for porous silicon than a column-like one, which shows strong anisotropy.