Surface analysis of thermally annealed porous silicon

Quasi-monocrystalline porous silicon (QMPS) has high potential for photovoltaic application for its enhanced optical absorption compared to bulk silicon in the visible range of solar spectrum. In this study, QMPS was formed from low porosity (∼20-30%) porous silicon (PS) produced by electrochemical anodization, and thermal annealing in the temperature range 1050-1100 °C under pure hydrogen ambient for a duration of 30 min. We analyzed the material surface by grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (SIMS) study. The crystallinity was confirmed by GIXRD while FESEM studies revealed that the surface layer is pore free with voids embedded inside the body. AFM studies indicated relatively smooth and uniform surface and the dynamic SIMS study showed the depth profiles of impurities present in the material.     

3D photonic crystals based on macroporous silicon: Towards a large complete photonic bandgap

Recently, a new three-dimensional photonic crystal consisting of two interpenetrating hexagonal pore sets was introduced. Here, we investigate the influence of possible experimental errors including shift, tilt and rotation of one pore set on the size of the bandgap theoretically and determine fabrication tolerances. A structure is fabricated in silicon using photo-electrochemical etching and subsequent focused ion beam milling. The change of the cross-section of the milled pores with pore depth is investigated and reflection measurements along different directions are performed indicating the low frequency edge of the photonic bandgap.     

Terahertz wave narrow bandpass filter based on photonic crystal

We designed a narrow bandpass terahertz wave filter using photonic crystals with a line defect. An inserted linear defect in one-dimensional photonic crystal structures for a channeled filtering in the terahertz range are studied and designed theoretically. By using transfer matrix method, we examined the transmittance spectra for the proposed terahertz wave filter has a 3 dB transmission loss bandwidth of 20 MHz ranging from 0.29998 THz to 0.30001 THz. The simulated results show that a very narrow transmission band and high transmission (higher than 99.99%) centered at λ₀, and very sharp edges can be achieved.     

Improved performance of solar cell based on porous silicon surfaces

Porous silicon (PS) surfaces were fabricated by electrochemical etching for both sides of the Si wafer. The objective of the present study is to investigate the PS effect on performance of silicon solar cells. Moreover, enhancement of solar cell efficiency can be obtained by manipulating of the reflected mirrors, and the process is very promising for solar cells manufacturing due to its simplicity, lower cost and suitability for mass production. The surface of PS is observed to have been discrete pores with smooth walls, and with short branches pores for the polished wafer side. In contrast, the etched backside of the wafer was observed to have bigger pore size than the etched polished side, and pores on the surface are in random location. PS formed on the both sides has lower reflectivity value in comparison to the other researcher group. The increase in efficiency of solar cell fabricated with PS formed on both sides of the wafer were extremely observed in comparison to one side PS and bulk silicon solar cells respectively. Solar cell fabricated shows that the conversion efficiency increased to 14.5% in comparison to unetched sample. The porous surface texturing properties could enhance and increased the conversion efficiency of silicon solar cells, these results also showed that the efficiency with this procedure is more promising in comparison to other solar cells, which are fabricated under similar conditions.     

Photonic drop splitters based on self-collimation ring resonators in a silicon photonic crystal

We report here 1 × 3 and 1 × 2 photonic drop splitters (PDSs) with different splitting ratios based on self-collimation ring resonators (SCRRs) in an air-hole silicon photonic crystal. An 1 × 3 PDS consists of four beam splitters and an 1 × 2 PDS consists of three beam splitters and one mirror. Light propagates in the PDSs employing self-collimation effect. The theoretical transmission spectra at different drop ports in PDSs were analyzed with the multiple-beam interference theory. Then they were investigated with the finite-difference time-domain (FDTD) simulation technique. The simulation results agree well with the theoretical prediction. For the drop wavelength 1550 nm, the free spectral range of the PDSs is about 29 nm, which almost covers the whole optical communication C-band window. Because of small dimensions, air-hole structure and whole-silicon material, the proposed PDSs hold great potentials for applications in photonic integrated circuits.     

Patterning of porous silicon by metal-assisted chemical etching under open circuit potential conditions

Porous silicon is the most studied Si-based light-emitting material. The potential for the application of porous silicon in optoelectronics and also for chemical or biochemical sensing is high. Therefore, the successful patterning of porous silicon on Si wafers is of great interest. HF-based aqueous solutions containing H₂O₂ as oxidizing agent, in combination with appropriate metal deposition, can supply the necessary current in order to sustain the electrochemical etching of single crystalline Si under no external anodic bias. The H₂O₂ concentration can tune the etching rate of the Si wafers as well as the observed photoluminescence intensity and photon energy. We demonstrate that porous silicon growth can be preferentially initiated at sites where metal (Pt) has been deposited and effectively be confined there, in order to form a well-defined pattern of desired geometry. Conventional DC sputtering using stainless-steel masks was applied in order to test various patterning geometries and lengthscales. Photoluminescence spectroscopy, atomic force and optical microscopy were used in order to characterize the produced porous silicon patterns. This method could be a simple, cost-effective way for the production of porous silicon patterns on Si wafers, which could be used in various fields of application.     

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.     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Terahertz wave switch based on photonic crystal ring resonators

In this paper, we have presented a compact and integrated terahertz wave switch design based on photonic crystal ring resonators. The photonic crystal structure with square lattice is investigated and applied for design of ring resonators. The switching mechanism of this novel switch is based on the variation of the resonant frequency of the ring resonator inserted between two parallel waveguides. The refractive index of the holes of the structure filled with polyaniline electrorheological fluids are varied by applied external electric field, the result of which is the variation of the ring resonant frequency. The proposed device is analyzed by using finite difference time domain method. Numerical simulation results show that this switch has high extinction ratio, small size, low voltage and advantages of selectivity of coupling THz wave to different output ports.     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

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.     

Anisotropic photonic crystals: Generalized plane wave method and dispersion symmetry properties

This article presents a generalized vector plane wave expansion method, applicable to isotropic and anisotropic periodic dielectric media of arbitrary geometry and dimension. The influence of anisotropic material orientation on the symmetry properties of photonic crystal dispersion surface is discussed. It is shown that the overall Brillouin zone symmetry is formed by the intersection of the photonic crystal lattice symmetry and the symmetry determined by the anisotropic material orientation. This work explains how to define the irreducible Brillouin zone of a two-dimensional anisotropic photonic crystal and demonstrates that doing it correctly allows one to avoid erroneous results, when calculating band gap diagrams of anisotropic photonic crystals. With the help of the methods presented, the possibility of controlling the band gaps of anisotropic photonic crystals by means of external electric field is shown.     

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.     

Numerical analysis of a temperature sensor based on the photonic band gap effect in a photonic crystal fiber

In this paper we investigate, by the plane wave expansion method and an analytical model, the temperature effect on the photonic band gap fiber, and we report on a numerical demonstration of a temperature sensor based on the photonic band gap (PBG) shift in a solid core photonic crystal fiber (PCF) infiltrated with a high refractive index oil. The bandwidth and the position of the central wavelength of the band gap are the parameters of interests for our temperature sensing purpose. Simulation results were found to be in excellent agreement with the refractive index scaling law and the highest sensitivity of 3.21?nm/°C was achieved, and it will be even higher than the grating based sensors written in PCFs with similar structure.     

Efficient luminescence from porous silicon

Photoluminescence measurements are carried out on porous silicon layers. We show the enhancement and stabilization of the luminescence when depositing a silicon nitride layer on top of porous layers.We also demonstrate that direct- and remote-plasma nitridation are good ways to reduce the ageing effect of porous silicon layers due to a passivation of dangling bonds.     

Light propagation in polytype Thue–Morse structures made of porous silicon

For the first time, two different sets of polytype Thue–Morse multilayered porous silicon structures are studied to investigate the reflection of light in aperiodic dielectrics. The optical response of the samples was studied before and after oxidation. The results were compared with the classical periodic structure, and an enhancement in the number of photonic bandgaps with a significant blue shift in reflectance peaks, in some of the structures, were observed. Numerical simulation along the lines of the transfer matrix approach is also presented.     

Effect of anisotropy on the photonic band gap and surface polaritons of a one-dimensional single-negative photonic crystal

The effect of anisotropy on the photonic band structure and surface polaritons of a one-dimensional photonic crystal made of uniaxially anisotropic epsilon-negative (ε<0,μ>0) and mu-negative (ε>0,μ<0) metamaterials is theoretically investigated. Two different cases of uniaxially anisotropic epsilon-negative and mu-negative metamaterials are considered. It is found out that for one case of anisotropy, one-dimensional photonic crystal does not have any single-negative band gap. As a result, it can not support the surface polaritons. While, for another case, the structure shows single-negative band gaps. So, the surface polaritons can be excited at the interface of such a photonic crystal. However, these surface polaritons, unlike the isotropic case, are not omnidirectional and they are restricted to a limited rang of the propagation constant.     

Surface treatment of nanoporous silicon with noble metal ions and characterizations

A very large surface to volume ratio of nanoporous silicon (PS) produces a high density of surface states, which are responsible for uncontrolled oxidation of the PS surface. Hence it disturbs the stability of the material and also creates difficulties in the formation of a reliable electrical contact. To passivate the surface states of the nanoporous silicon, noble metals (Pd, Ru, and Pt) were dispersed on the PS surface by an electroless chemical method. GIXRD (glancing incidence X-ray diffraction) proved the crystallinity of PS and the presence of noble metals on its surface. While FESEM (field emission scanning electron microscopy) showed the morphology, the EDX (energy dispersive X-ray) line scans and digital X-ray image mapping indicated the formation of the noble metal islands on the PS surface. Dynamic SIMS (secondary ion mass spectroscopy) further confirmed the presence of noble metals and other impurities near the surface of the modified PS. The variation of the surface roughness after the noble metal modification was exhibited by AFM (atomic force microscopy). The formation of a thin oxide layer on the modified PS surface was verified by XPS (X-ray photoelectron spectroscopy).     

Photoluminescence and electroluminescence from porous silicon

The properties and origins of the red, blue and infrared photoluminescence bands of porous silicon are reviewed and discussed in the light of the models that have been proposed to explain the experimental and theoretical results. The red band is due to quantum confinement possibly supplemented by surface states; the blue band is linked to the presence of silicon dioxide; the infrared band is correlated with dangling bonds and bandgap luminescence in large crystallites. The fabrication and characterization of light-emitting devices made of porous silicon are reported and discussed with respect to critical issues such as the device stability, efficiency, modulation speed, emission wavelength, and compatibility with microelectronic processing.     

Comparison of photonic bandgaps of two-dimensional periodic and quasi-periodic photonic crystals with different relative permittivity dielectric

Photonic bandgaps (PBGs) of two-dimensional (2D) triangular-lattice and square-lattice and decagonal quasi-periodic photonic crystals (PCs) have been analyzed, with a given scatterer radius and dielectric relative permittivity changing from 1 to 30 within air-cylinders-in-dielectric and dielectric-cylinders-in-air constructions. The results have shown that 2D quasi-periodic PC is more likely to generate PBG and complete PBG than 2D periodic PC. For the given scatterer radius and two constructions, PBG widths of the two types of 2D PCs vary little, whereas the corresponding center frequencies decrease in smooth “hyperbola-like” curves with dielectric relative permittivity increasing monotonically. The present results will guide the design of PBG-type microstructure photonic devices.