Narrow Results

In this paper we report the detailed fabrication and experimental characterization of porous silicon one-dimensional photonic bandgap structures for specific photonic applications. These structures have been prepared on silicon substrate as well as free standing and can be proven useful for a large variety of applications such as high quality dielectric mirrors and filters.

This presentation is a short review of some scientific insights on the possibilities of photonic applications of nanostructured silicon (NS–Si), porous Si(p-Si) and Si nanocrystals (NC–Si), one of the most interesting problems in nano-crystallite physics. The emission mechanism of a very bright photo-luminescence (PL) band and relatively weak electro-luminescence (EL) are presently the main issue. The basic question lies in whether the emission is an extrinsic or intrinsic property of nanocrystals. It is important from a fundamental physics viewpoint because of the potential application of Si wires and quantum dots in optoelectronic devices and information technology. Nanostructuring silicon is an effective way to turn silicon into a photonic material. It is observed that low-dimensional (one and two dimensions) silicon shows light amplification, photon confinement, photon trapping as well as non-linear optical effects. There is strong evidence of light localization and gas sensing properties of such nanostructures. Future nano-technology would replace electrical with optical interconnects, which has appealing potential for higher-speed performance and immunity to signal cross talk.

The photoluminescence spectrum of the freshly photosynthesized porous silicon (PS) has been investigated. This measurement was repeated after three and six months for the same sample after storage under ambient condition (open air at room temperature). Photoluminescence (PL) measurements of the stored PS show different peak positions and intensity width as compared with the results of the fresh PS. A blue shift in PL peak positions with aging time was observed. PL relative intensity is strongly diminished after 6 months of aging. Dark I-V characteristics of Al/PS/n-Si/Al structure shows a behavior of PS/n-Si isotype heterojunction for fresh device and a MIS (metal-insulator-semiconductor) device due to contribution of Al/PS Schottky barrier after aging, essentially after 6 months.

In this work, we show an algorithm to calculate the complex refractive index of porous silicon (PS) on its crystalline silicon (c-Si) substrate in UV-NIR range by means of the reflectance spectra only. The algorithm is based on the analytical relations established by Heavens to obtain both complex refractive index and thickness of an absorbing thin film on an absorbing substrate. Based on this model, some simplification is introduced at different wavelengths. We start with the NIR range (1000–2500 nm), where the c-Si substrate has a low extinction coefficient. Then, we continue with the near infrared to the optical range (300–1000 nm), where PS has a strong extinction coefficient and dispersion. The calculated n and k values are in agreement with those reported in the literature obtained from separated measurements of a free standing PS film. We consider that the algorithm can be applied to any thin film on a substrate with similar optical properties.

In this work, a colloid of nanocrystalline ZnO particles was prepared by chemical method, and sprayed on porous silicon (PS) substrate which was prepared by electrochemical etching under a current density of 15 mA/cm² for 10 min. The initial radius of the ZnO particles was found to be (2.2 nm). FTIR spectra exhibit the presence of Zn–O bond which indicates the formation of ZnO particles. Also spectra reveals the formation of SiH_x(x = 1–2) and Si–O bond which indicates the presence of porous layer. High-performance rectification was obtained, with high photoresponsivity of 0.54 A/W at 400 nm. The corresponding quantum efficiency was 166.7%. The results show that ZnO on PS structures act as good candidates for making highly efficient photodiodes.

Effects of current density on nanostructure and light emitting properties of porous silicon (PS) samples were investigated by field emission scanning electron microscope (FE-SEM), gravimetric method, Raman and photoluminescence (PL) spectroscopy. FE-SEM images have shown that below 60 mA/cm², macropore and mesopore arrays, exhibiting rough morphology, are formed together, whose pore diameter, pore depth and porosity are about 265–760 nm, 58–63 μ m and 44–61%, respectively. However, PS samples prepared above 60 mA/cm² display smooth and straight macropore arrays, with pore diameter ranging from 900–1250 nm, porosity of 61–80% and pore depth between 63–69 μm. Raman analyses have shown that when the current density is increased from 10 mA/cm² to 100 mA/cm², Raman peaks of PS samples shift to lower wavenumbers by comparison to crystalline silicon (c-Si). The highest Raman peak shift is found to be 3.2 cm^-1 for PS sample, prepared at 90 mA/cm², which has the smallest nanocrystallite size, about 5.2 nm. This sample also shows a pronounced PL, with the highest blue shifting, of about 12 nm. Nanocrystalline silicon, with the smallest nanocrystallite size, confirmed by our Raman analyses using microcrystal model (MCM), should be responsible for both the highest Raman peak shift and PL blue shift due to quantum confinement effect (QCE).

The measurement of patients’ dosages of radiation caused by medical diagnostics continues to be challenging. A Cantor sequence photonic crystal structure using porous silicon doped with a polymer of polyvinyl alcohol, carbol fuchsin and crystal violet (DPV) is proposed. The influence rules of geometrical and optical parameters such as the radiation doses, number of periods, porosity of porous layers, incident angle and thickness of layers are investigated using MATLAB based on the transfer matrix method. The transmittance of the Cantor sequence of a defective photonic crystal sensor under different conditions is investigated to select the optimum conditions. The proposed system recorded the accepted sensitivity of 0.265nm/Gy, FoM of 6.5Gy${}^{-1}$, Q of 12,701, RS of $6\times 10^{-3}$ and LoD of $8\times 10^{-3}$ for gamma radiation. The suggested detector has simple design, accurate monitoring efficiency and immense potential for gamma radiation sensing.

We presented fluorescence spectra and cathode-luminescence spectra of as-prepared porous silicon under different preparation conditions. The luminescence of porous silicon may be related to the luminescent centers on porous silicon surface. The luminescent efficiency depended on the porosity of porous silicon, which further depended on the oxidative level of porous silicon with metal assistance. Under back illumination, the higher the oxidative degree of the metal is, the higher is the porosity and the luminescent efficiency of porous silicon. But under front illumination, the results exhibited an opposite tendency, which can be explained by the formation theory of PS.

In this paper, the microstructure of porous silicon was discussed by scanning electron microscopy and Raman spectra. The compositions of the samples were studied by X-ray diffraction and electron diffraction. The luminescent properties of porous silicon were explored by the PL measurements. The results showed that the structure of the top porous silicon prepared by epitaxial silicon consists of meso-pores and that of the underlying porous silicon consists of macro-pores. Furthermore, the structure of porous silicon prepared by heavily doped silicon is made of nano-pores. The composition of porous silicon consists of the single-crystal phase, poly-crystal phase and amorphous-crystal phase, respectively. In addition, the luminescence of porous silicon may come from the defects on the silicon-rod surface and in Si complexes including siloxene, Si oxide and Si hydrides, and may not be related to amorphous phase and nano-phase.

Aiming at the problem of poor reliability of beam membrane fabrication in silicon microstructure processing technology, a technique for preparing microbeams using porous silicon sacrificial layer is proposed. The comparison of different porosity, different depths of porous silicon to beam membrane release and the comparison of different beam structures to beam membrane release were studied. The experiment was carried out by selecting different process parameters and testing the prepared beam membrane. According to the test results, the appropriate electrochemical corrosion process parameters were selected to prepare the porous silicon layer with appropriate porosity, large depth, good support effect and fast release speed, which can realize the release of the beam membrane and improve the reliability of the beam membrane. The field of application of microsensors and microactuators has been greatly increased.

Aiming at the problem of heterogeneity of sealed cavity in silicon microstructure processing technology, the technology of preparing micro-cavity by using porous silicon sacrificial layer is proposed. The effect of current density on the preparation of porous silicon and the effect of porous silicon with different porosity on the formation of micro-cavity in the preparation process of porous silicon were studied. Different process parameters were selected for experiments and the prepared micro-cavities were tested and analyzed. According to the test results, the suitable electrochemical corrosion process parameters were selected to prepare porous silicon, and the micro-cavity was realized by changing the process parameters, which greatly increased the application fields of micro-sensors and micro-actuators.

In this work, palladium (Pd) nanoparticles (NPs) were synthesized using laser ablation in liquids (LAL) at laser energies of 360, 660, and 800mJ and a laser wavelength of 1064nm with 200 pulses per shot. The Pd nanoparticle suspension deposited on porous silicon (PS) was investigated. X-ray diffraction (XRD) was used to determine the crystal structure. Field emission-scanning electron microscopy (FE-SEM) revealed the sponge-like structure of the PS and spherical clusters of Pd NPs within it. The optical properties of the prepared solution samples were investigated by testing the absorbance between 250–850nm, and energy gap values of ∼2.35, 2.56, and 2.75eV were calculated for the laser energies 360, 660, and 800mJ, respectively. The electrical properties, including $I\text{–}V$ and $I_{ph}\text{–}V,$ of the Al/PS/Si/Al and Al/Pd NPs/PS/Si/Al heterojunctions were calculated from $I\text{–}V$ measurements. The observed enhancement of the fabricated Pd NPs/PS is shown by the broad spectral response in the visible to near-infrared (vis-NIR) range. The increased spectral responsivity due to coating the PS surface with Pd NPs promotes using these nanostructures in photodetectors.

Porous silicon structures were observed in laser-assisted etching in a TMAH solution. The porous structures significantly increase the active surface areas and therefore are of great interest to the pharmaceutical and fine chemicals industry, in the areas of high throughput drug discovery, reaction optimization and process development. Laser-assisted etching of silicon using a Nd:YAG laser with a TMAH solution is rarely studied but desirable as the solution is nontoxic. To the best of our knowledge, the porous structures directly produced this way have not been reported previously. In this paper, we discuss our experimental results and the possible mechanism of formation of the porous structures. A dummy micro-reactor was fabricated with selective porous surface structures to demonstrate its potential industrial applications.

The surface passivation of porous silicon is a determining factor in the emission efficiency of the material. The hydrogen surface coverage has been shown to provide very efficient passivation. In this work, we have monitored Si–H_n surface coverage, which is readily-obtained porous silicon formed at 1:3 concentrations of HF: ethanol with/without HCl content in the electrolytes, and relate it to the emission efficiency. SEM, FTIR and PL studies were carried out to understand the role of HCl content in the electrolytes. When samples are anodized with HCl content in the electrolytes, there is a net decrease in the concentration of the Si–H groups on the material surface. This treatment provokes desorption of the hydrogen atoms and results in a drastic decrease in the photoluminescence.

Structural and Photoluminescence (PL) studies are carried out for p-type porous silicon materials prepared with different Hydrogen Fluoride (HF) concentration. Surface morphology of porous silicon was analyzed using atomic force microscope (AFM). Fourier Transform Infra Red (FTIR) spectra indicate that surface of porous silicon contain chemical species like SiH_n. PL studies reveal that there is a prominent emission peak around 625 nm. No spectral shift is observed. It is further observed that the emission intensity increases with HF concentration. The PL of the porous silicon layer was found to come from its upper part, which was confirmed by a simulation of the PL spectrum employing an optical model. The refractive indices of the porous silicon layer were also determined through this simulation. The large hydrogen concentration in freshly prepared material has led to the suggestion that a silicon–hydrogen alloy effect is responsible for the luminescence.

The electroluminescence (EL) studies on the aluminum-doped nanoporous silicon (NPSi:Al) have been conducted. Nanoporous silicon (NPSi) layers have been prepared by anodically etching the unpolished p-type Si[100] wafer with surface resistivity of 4–8 Ω cm^-1 in hydrofluoric solution at 1:1 ratio of ethanol. Aluminum (Al) was doped on NPSi using cathodic electrodeposition composed of aluminum chloride (AlCl₃) and ethanol electrolyte. A diode structure has been fabricated comprising a semi-transparent Au/NPSi:Al/p-Si/Al ohmic contact electrode showing rectification on a forward bias I–V curve. EL from NPSi and NPSi:Al/p-Si has also been observed using the diode structure. The NPSi:Al/p-Si device shows increasing EL quantum efficiency at about 30%, and blue-shift EL spectra are observed. Possible reasons for the enhancement will be discussed.

The aim of this study is the formation of porous silicon (PS) and the heat treatment effect on the optical properties of the PS layer. An optimized HF:HNO₃ chemical solution is used at ambient temperature on the n + p silicon surface. Scanning electron microscopy (SEM) pictures show the form of the nanopores. We studied the effect of the heating on the morphology and the reflection of the PS layer. A chemical analysis of the surface is also carried out. The measurements show that the pore shape and the oxygen content on the surface are changing with temperature. Specular reflection spectra under variable incidence angles are measured on each treated surface. Curves of weighted reflection R_w are drawn to illustrate the evolution with temperature. The modification of oxygen content of the porous surface is correlated with the sheet resistance of the emitter. Results show the lower the oxygen percent the lower the resistivity. The layout of R_w according to the temperature of annealing indicates that the shape of the curve is the same for the angles of incidences 20°, 30°, 40°, and 50°. At a low temperature R_w is minimal indicating the presence of an oxide coating on the porous layer an indication of optical adaptation between the air and the substrate of silicon.

Porous silicon layers have been prepared from n-type silicon wafers of (100) orientation. SEM, XRD, FTIR, and PL have been used to characterize the morphological and optical properties of porous silicon. The influence of varying HF concentration in the anodizing solution, on structural and optical properties of porous silicon has been investigated. It is observed that pore size increases with HF:ethanol concentration ratio and attain maximum for 1:2 ratio and then decreases.

In this work, a porous layer on an n⁺ emitter by a chemical route was realized. The MEB observation shows a nanoporous shape of the surface. The subsequent contact depositions needs a heat treatment that has an influence on hydrogen and oxygen distributions in the porous layer. After heat treatments, the porous silicon layer is analyzed by secondary ion mass spectroscopy. The concentration profile of light elements like H, O, C, F and N are measured and compared with untreated porous layer. The results show that oxygen is present at high level at ambient temperature and then decreases from 25°C to 775°C. This means that oxygen desorption is observed on the surface. At 800°C the oxygen content increases again showing an oxidation of porous layer surface. For hydrogen, the concentration decreases from the ambient temperature until 750°C was noted. Then the hydrogen is restored at its first concentration. The FTIR spectra correlate this hydrogen distribution. The absorption spectra show the appearance of Si–H_x bonds (with x = 1, 2, 3) at 2089, 2115, and 2140 cm^-1, respectively. Both elements O and H are present deeply in the porous silicon layer as shown by the sputter time.

Transparent conducting indium tin oxide (ITO) films are deposited on Porous Silicon (PS) substrates by spray pyrolysis technique. In this process, the films are formed over the surface and also incorporated into the pores of PS and thereby making a protecting layer as well as a contacting terminal. Thus, the ITO/PS/Si heterojunction light-emitting devices are fabricated. The growth of ITO on PS is thoroughly investigated by SEM and X-ray diffraction techniques. The features of growth on other substrates like single-crystal p-type (100) silicon and glass are also taken into consideration. The influence on the PS interface is correlated with the electrical and luminescent behavior of the resulting heterojunction diode structure.