Tunable photoluminescence of porous silicon by liquid crystal infiltration

The photoluminescence (PL) of porous silicon films has been investigated as a function of the amount of liquid crystal molecules that are infiltrated into the constricted geometry of the porous silicon films. A typical nematic liquid crystal 4-pentyl-4′-cyanobiphenyl was employed in our experiment as the filler to modify the PL of porous silicon. It is found that the originally red PL of porous silicon films can be tuned to blue by simply adjusting the amount of liquid crystal molecules in the microchannels of the porous films. The chromaticity coordinates are calculated for the recorded PL spectra. The mechanism of the tunable PL is discussed. Our results have demonstrated that the luminescent properties of porous silicon films can be efficiently tuned by liquid crystal infiltration.     

Peculiarities of photoluminescence of porous silicon with luminescent liquid crystal fillers

Photoluminescence spectra of porous silicon filled by luminescent liquid crystals 5CB and H109 were investigated. It was observed that there were photoluminescence bands corresponding to both porous silicon and liquid crystal in experimental spectra. In addition, the band corresponding to porous silicon increases in comparison with photoluminescence of porous silicon without the filler. Experimental results are explained by the radiating and nonradiating energy transfer from liquid crystal to porous silicon.     

Enhancement of saturation magnetization in Cr-ion implanted silicon by high temperature annealing

Magnetic properties and microstructure of Cr-implanted Si have been investigated by alternating gradient magnetometer (AGM), superconducting quantum interference device (SQUID) magnetometer, and transmission electron microscopy (TEM). p-Type (1 0 0) Si wafers were implanted at 200 keV at room temperature with a dosage of 1 × 10¹⁶ cm⁻² Cr ions and then annealed at 600-900 °C for 5 min. The effect of annealing on the structure and magnetic properties of Cr-implanted Si is studied. The as-implanted sample shows a square M-H loop at low temperature. Magnetic signal becomes weaker after short time annealing of the as-implanted sample at 600 °C, 700 °C, and 800 °C. However, the 900 °C annealed sample exhibits large saturation magnetization at room temperature. TEM images reveal that the implanting process caused amorphization of Si, while annealing at 900 °C led to partial recovery of the crystal. The enhancement of saturation magnetization can be explained by the redistribution and accumulation of Cr atoms in the vacancy-rich region of Si during annealing.     

Enhancement of Raman scattering intensity in porous silicon

An enhancement in inelastic light scattering intensity from porous-silicon quantum wires has been discovered. It is shown that this effect is caused by a decrease in the absorption coefficient of the optical medium formed by quasi-one-dimensional structures, with the crystal structure of the wires themselves remaining unchanged. Fiz. Tverd. Tela (St. Petersburg) 41, 1320–1322 (July 1999)     

Simulation and fabrication study of porous silicon photonic crystal

The present work reports design and fabrication of porous silicon based one-dimensional (1D) photonic crystal. Distributed Bragg reflector (DBR) is a 1D photonic crystal composed of multilayer stack of high and low refractive index layers. Design of porous silicon DBR is a complex one and requires appropriate control in optical parameters of its constituent layers. In order to design DBR, two porous silicon single layer samples were fabricated using current density of 10 and 50 mA/cm². Optical characterization of single layer samples showed series of interference fringes. Reflective interferometric Fourier transform spectroscopy (RIFTS) method was employed to determine optical constants of porous silicon single layers. DBR simulation was carried out based on transfer matrix method. DBR was then fabricated using optical parameters obtained from RIFTS method. Reflection bandwidth of prepared DBR was found to be 216 nm, which is comparable to the simulated value of 203 nm.     

Enhancement of parametric efficiency by saturation suppression

The energy conservation equations due to Westervelt are used to show that saturation effects in a finite amplitude wave can in principle be suppressed, if the attenuation coefficient at the second harmonic frequency of the primary wave can be selectively increased. Introduction of microscopic air bubbles which are resonant at the second harmonic frequency is proposed as a scheme for applying this idea to a parametric array operating in water. The parameter of non-linearity also increases when the bubbles are introduced; hence parametric efficiency would be enhanced by both of these effects.     

Study on the thermal isolation properties of porous silicon photonic crystal

The thermal isolation properties of porous silicon photonic crystal structures have been designed and discussed theoretically. Excellent thermal isolation properties can be obtained on such a porous silicon photonic crystal, even better than that of porous silicon with high porosity. Due to the excellent thermal isolation properties of the porous silicon photonic crystal structures, they can be used as the thermal isolation substrates in infrared detectors     

Enhancement of room temperature sub-bandgap light emission from silicon photonic crystal nanocavity by Purcell effect

We report the enhancement of sub-bandgap photoluminescence from silicon via the Purcell effect. We couple the defect emission from silicon, which is believed to be due to hydrogen incorporation into the lattice, to a photonic crystal (PhC) nanocavity. We observe an up to 300-fold enhancement of the emission at room temperature at 1550 nm, as compared to an unpatterned sample, which is then comparable to the silicon band-edge emission. We discuss the possibility of enhancing this emission even further by introducing additional defects by ion implantation, or by treating the silicon PhC nanocavity with hydrogen plasma.     

基于相位硅基液晶的光衍射演示实验

计算虚拟物体的衍射光场相位分布,然后加载到相位硅基液晶器件,利用平行光照射硅基液晶器件,可在光屏上看到虚拟物体的实像.根据相位硅基液晶的采样间隔,计算出虚拟物体的采样间隔,可预计虚拟物体大小.演示结果表明:处于菲涅尔衍射区域,衍射图像清晰,亮度高;而夫琅禾费衍射区的衍射图像虽然清晰,但比较暗淡.     

Enhancement of optoelectronic properties in multicrystalline silicon via combination of grooving grain boundaries and porous silicon gettering

In multi-crystalline silicon (mc-Si), the detrimental effect of impurities and grain boundaries (GBs) on charge carrier transport has driven the research focus since many years. In view of curing these limitations, we present an innovative method to enhance the optoelectronic performance of mc-Si wafers via a combination between GBs grooving and porous silicon (PS) gettering. A preferential grooving of GBs was achieved using the HF/HNO₃ based solution, the PS layers were formed on both sides of the samples using stain-etching method and the gettering experiment was performed at temperatures ranging from 750 to 900 °C. As a result, it has been shown that the rapid thermal annealing process with chemical grooving gives a positive trend of improvement of the electronic quality and found to be more efficient when used in combination with PS. After removing the PS layer, the minority carrier lifetime increases by a factor of more than 27. In addition, a significant enhancement of majority carrier mobility was obtained, which led to an important decrease of the resistivity.     

Electro-tunable one-dimensional photonic crystal structures based on grooved silicon infiltrated with liquid crystal

The paper reports on composite periodic structures fabricated by means of wet anisotropic etching of (1 1 0)-oriented Si on a SOI platform and infiltrated with liquid crystal E7. The electro-optical effect under low voltages was registered for inter-digital structures by both optical microscopy and micro-Raman spectroscopy.     

The molecular dynamics of nematic liquid crystal confined in porous membranes treated by different surfactants

The molecular dynamics of the well-known nematic liquid crystal 4-n-pentyl-4′-cyanobiphenyl geometrically restricted in Anopore and Synpor porous membranes with various pore structure and treated by different surfactants (namely decanoic acid and lecithin) is compared. In the Anopore membrane the chosen surfactants induce the homeotropic orientation of the molecules on the walls of the cylindrical pores and observed corresponding relaxation processes (librational modes) are practically the same. The dielectric measurements of lecithin treated Synpor membranes reveals the reorientation of the molecules from planar to homeotropic on the complex multilayer structure present in their volume. The dielectric strengths of the observed two molecular processes (δ-process and librational mode) are inversed in the ratio compared to the untreated membranes. The observed differences in molecular dynamics results from the orientation of the liquid crystal molecules in untreated and treated membranes and the structure of the membranes themselves.     

Formation of silicon carbide nanocrystals by high-temperature carbonization of porous silicon

Based on X-ray diffraction analysis, Auger spectroscopy, and Raman scattering, it is shown that carbonization of porous silicon at temperatures of 1200–1300°C results in formation of silicon carbide nanocrystals 5–7 nm in size. The growth of 3C-SiC nanocrystals of fixed size d proceeds as follows. Silicon nanocrystals with d = 3–7 nm pass into the liquid phase, thereby effectively participating in the growth of silicon carbide. After the size of a crystallite has achieved a critical value determined by the equality of its melting point and environmental temperature, the crystallite solidifies and virtually ceases to grow. As a result, a nanocrystalline Si-SiC-amorphous SiC heterostructure is obtained.     

Synthesis and characterization of porous silicon layers for 1D photonic crystal application

In this paper, we studied the optical and physical properties of electrochemically prepared porous silicon layers. The atomic force microscopy analysis showed that the etching depth, pore diameter and surface roughness increase as the etching time increased from 30 to 50 mA/cm². By tuning two current densities J₁ = 50 mA/cm² and J₂ = 30 mA/cm², two samples of 1D porous silicon photonic crystals were fabricated. The layered structure of 1D photonic crystals has been confirmed by scanning electron microscopy measurement which showed white and black strips of two distinct refractive index layers. Finally, the measured reflectance spectra of 1D porous silicon photonic crystals were compared with simulated results.     

Effect of photonic crystal structure on the nonlinear optical anisotropy of birefringent porous silicon

Anisotropic photonic crystal structures consisting of birefringent porous silicon layers with alternating porosity were fabricated. The in-plane birefringence formed as a result of anisotropic etching in Si(110) results in unique multilayered structures with two distinct photonic bandgaps for orthogonal light polarizations. Nonlinear optical studies based on the third-harmonic generation from these structures demonstrate variation in the symmetry of the nonlinear optical response.     

Elastic characterization of porous silicon by acoustic microscopy

Acoustic microsocopy methods and particularly microechography have made it possible to determine porosity and mechanical properties of porous silicon. Nevertheless, these techniques are limited when porosity becomes important or when the layer thickness is too thin. This problem can be solved by detecting and analysing guided waves in the layers (Lamb's waves) which are contained in the acoustic signature V(z).     

Characterization of porous silicon prepared by powder technology

In this study, we have proposed the powder technology as new method for preparation of bulk porous silicon. Formation of porous silicon by high-energy ball milling followed by pressing and sintering was studied by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy (XPS). A crystalline wafer with (1 1 1) orientation was extensively ball milled up to 72 h leading to a decrease in average crystallite size up to 15 nm. The most significant reduction of crystallite size was observed after milling process for about 24 h. The nanopowders were then pressed into pellets at a pressure up to 400 MPa and sintered at 1173 K for 60 min in a high purity argon atmosphere. Results showed that after sintering the material became porous with uniform porosity in whole volume, independently of the sinter size. It is not possible to prepare such porous materials using the conventional electrochemical etching, where the porous structure depth usually does not exceed tens of micrometers. Core-level XPS studies showed very good agreement between peak positions of the sintered porous silicon and in-situ prepared polycrystalline 20 nm-Si thin film or single-crystalline Si (1 1 1) wafer. Furthermore, the valence band spectra measured for sintered samples are broader compared to those measured for the Si (1 1 1) wafer or polycrystalline Si thin film. On the other hand, the shape and broadening of the valence bands measured for the sintered samples are in very good agreement with those reported for electrochemically prepared porous silicon.     

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.     

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.