Silicon Multilayer Structures Research Articles

Enhanced photoluminescence of active ions in rugate type multilayer structures

Multilayer rugate-like structures based on porous silicon with incorporated active ions are theoretically investigated. Multilayer structures consist on different sinusoidal refractive index profiles designed to show the greatest amplification of electric field at a wavelength of 1 µm. We show that it is possible to increase the localization of the electric field by optimizing the design parameters, and thereby directly enhances the photoluminescence of the active ions. The luminescent properties of active ions incorporated in the porous silicon structures are calculated from a model based on the Förster Resonance Energy Transfer (FRET) theory. This model considers the effect of the electric field due to the structures and the energy transfer processes between donors and acceptors and allows studying the luminescent dynamics in this type of systems. It is shown that a greater intensity in the emission of ions is obtained from the structures with the highest amplification of the electric field. Furthermore, it is observed that the emission of active ions depends on the porous silicon multilayer structure and on the concentration of ions. Likewise, greater luminescence is observed as the acceptor density increases. In the same way, donor and acceptor lifetimes also depend on the acceptor density, although it does not depend on the porous silicon structure. The structures proposed here are prospective for Si-based optoelectronic applications for the near-infrared spectral region.

The Effect of the Dielectric Layer with Surface Plasma Treatment on Characteristics of Amorphous Silicon Antifuse

With the advantages of lower capacitance, faster operation and smaller size, the amorphous silicon antifuse is very promising for high-speed and high-density FPGA. However, the leakage current of the conventional amorphous silicon antifuse in the off-state is high, which decreases its performance and reliability. Although the leakage current of the SiNX or SiOX antifuse is small, the proper thickness of the dielectric layer is not easily controlled by PECVD processes. In addition, the highly undesirable switch-off behavior is common to almost all metal-to-metal antifuse structures. Focusing on the study of amorphous silicon multilayer dielectric structures, a novel antifuse with the structure of Al/α-Si:H,N/α-Si:H/Al was proposed, which was manufactured by nitrogen plasma treatment for an α-Si:H film surface. Through surface plasma treatment, the hydrogen content of the dielectric layer is stable, the film surface is smoother, the leakage current is reduced, the switch-off behavior is eliminated, the programming voltage is more concentrated and the on-state resistance distribution is more compact. The results demonstrated that surface plasma treatment with proper time for the dielectric layer could significantly improve the performance and reliability of the Al/α-Si:H,N/α-Si:H/Al antifuse. Furthermore, the fabrication process of the α-Si:H,N/α-Si:H structure has excellent compatibility, controllability and simplicity.

Temperature distribution inside a porous silicon photonic mirror

Photonic devices require precise and high-cost procedures to evaluate their performance which is related to their temperature increase. The fundamental understanding of thermal phenomena, ergo measurement of temperature, inside radiation controlling devices is of great relevance to study their performance. In this paper, we carry out a comprehensive campaign of experiments to study the temperature profile inside a porous silicon multilayer 1D photonic structure by using a thermographic camera. In particular, we have analyzed broad-range reflective devices and found that the electromagnetic radiation does not travel beyond the photonic structure showing a clear maximum inside of it. We have compared this result with a pure silicon wafer under the interaction with the same radiation. To compare these samples, we used a normalization procedure that can be implemented on many microstructured devices to simplify their performance evaluation.

Optimal phase shift mask and multilayer stack with the evaluation of imaging performance and process latitude in extreme ultraviolet high numerical aperture

In high numerical aperture (NA) extreme ultraviolet lithography, which is used to implement a finer linewidth of 10 nm or lower, serious problems arise in patterning as the NA increases. To alleviate such problems, a thin absorber and a multilayer with good reflective efficiency and improved pattern quality are required. To develop an effective EUV photomask for the commercialization of high-NA systems, we determined the optimal ruthenium (Ru)/silicon (Si) multilayer structure using a phase-shift mask (PSM) absorber. A Ru/Si multilayer using PSM as an absorber has a smaller best-focus range and placement error compared to the molybdenum (Mo)/silicon (Si) multilayer. At the same time, it provides improved image contrast, enabling more stable patterning. Even when the number of layers of the Ru/Si multilayer was reduced, it was confirmed that the reflectance efficiency and image quality were maintained.

Theoretical and experimental study of Fermi Bragg reflector as application for thin silicon solar cells

The conception of efficient Bragg reflectors (BRs) based on dielectric layers is needed to improve the light trapping in thin film epitaxial silicon solar cells. Porous silicon multilayer structures (PSMS) used as BR show its potential to enhance optical confinement of low energy photons. In the present work, a novel design of PSMS will be studied in order to extend the reflectivity to long wavelength by adjusting BR thicknesses according to the Fermi profile law. Such new design named Fermi Bragg reflector (FBR) includes different configurations. Several FBR parameters are used in the simulation study, which is carried out by transfer matrix method. The optimal structures were realized using electrochemical etching dissolution of P+ type silicon substrate in hydrofluoric acid solution (HF) at two temperature conditions: room temperature (20 °C) and −20 °C. The measured optical response of FBR shows an enlargement in the width of high reflection range in comparing to the conventional BR. More than 80% of the photons were reflected in the spectral range 636–1155 nm for the FBR performed at ambient temperature and in the range 750–1353 nm to that carried out at −20 °C. This result proves that the FBR formed on crystalline silicon could enhance the light trapping in thin film silicon solar cells.

All-solution-processed Si films with broadband and omnidirectional light absorption

Silicon photonic structures have attracted a great deal of attention due to their potential benefits of efficient light management in optoelectronic applications. In this paper, we demonstrate broadband optical absorption enhancement in solution-processed amorphous silicon (a-Si) by leveraging the advantages of silicon photonic structures. Graded refractive index silicon multi-layer structures are employed by modulating optical constants with simple process optimization, resulting in significantly improved reflectance over a broad range of visible wavelengths. In addition, nanopatterning flexibility of solution-processed silicon provides benefits for tailoring silicon optical properties. With the incorporation of the two-dimensional submicron pattern into silicon films, the absorptivity of silicon films improves considerably below the wavelength of the bandgap (λ ∼ 800 nm), and the limited bandwidth of absorptivity in silicon films can be extended to near-infrared wavelengths by coating with a thin gold layer. The methodology is generally applicable to a platform for improving the broadband optical absorption of photonic and optoelectronic devices.

Reflectivity of 1D photonic crystals: A comparison of computational schemes with experimental results

We report the reflectivity of one-dimensional finite and semi-infinite photonic crystals, computed through the coupling to Bloch modes (BM) and through a transfer matrix method (TMM), and their comparison to the experimental spectral line shapes of porous silicon (PS) multilayer structures. Both methods reproduce a forbidden photonic bandgap (PBG), but slowly-converging oscillations are observed in the TMM as the number of layers increases to infinity, while a smooth converged behavior is presented with BM. The experimental reflectivity spectra is in good agreement with the TMM results for multilayer structures with a small number of periods. However, for structures with large amount of periods, the measured spectral line shapes exhibit better agreement with the smooth behavior predicted by BM.

Density of states measurements in a-Si:H and a-Si:H/nc-Si:H multilayer structures prepared by hot wire chemical vapor deposition

In this study, the density of states (DOS) were determined in single layer hydrogenated amorphous silicon (a-Si: H) and a-Si:H/nc-Si:H (hydrogenated nanocrystalline silicon) multilayered structures, prepared by hot wire chemical vapor deposition, by measuring space-charge limited currents in sandwich geometry using ITO(Indium Tin Oxide)/a-Si:H/Ag and ITO{a‐Si:H/nc‐Si:H}n/Ag structures. Comparisons showed that the DOS of a-Si:H layer in a-Si:H/nc-Si:H multilayer structures were higher than that of single layer a-Si:H film for the same shift in ΔEfn. The higher DOS in multilayer structures may be due to the existence of interface states between the a-Si:H and nc-Si:H layers, which could also change the local electric field at the junction under the no-bias condition. These interface states were also shown to be responsible for the observed persistent photoconductivity in these a-Si:H/nc-Si:H multilayer structures.

Vapor sensing using a bio-inspired porous silicon photonic crystal

Naturally occurring photonic crystals present on the wing scales of Papilio butterflies are known to exhibit vivid iridescent coloration as well as various optical properties, including vapor sensing properties. This work reports the fabrication of a periodic array of concave multilayers in porous silicon mimicking the Papilio blumei nanostructure and its application as an optical vapor sensor. We compared the variations in the reflectance spectrum due to ethanol vapor adsorption and condensation inside the bio-inspired concave porous silicon multilayer structure to the variations of a standard flat porous silicon Bragg mirror. This results in an enhanced response time in the case of the bio-inspired concave structure.

Ion-beam induced atomic mixing in isotopically controlled silicon multilayers

Implantation of germanium (Ge), gallium (Ga), and arsenic (As) into crystalline and preamorphized isotopically controlled silicon (Si) multilayer structures at temperatures between 153 K and 973 K was performed to study the mechanisms mediating ion-beam induced atomic mixing. Secondary-ion-mass-spectrometry was applied to determine concentration-depth profiles of the stable isotopes before and after ion implantation. The intermixing is analytically described by a depth-dependent displacement function. The maximum displacement is found to depend not only on temperature and microstructure but also on the doping type of the implanted ion. Molecular dynamics calculations evaluate the contribution of cascade mixing, i.e., thermal-spike mixing, to the overall observed atomic mixing. Calculated and experimental results on the temperature dependence of ion-beam mixing in the amorphous and crystalline structures provide strong evidence for ion-beam induced enhanced crystallization and enhanced self-diffusion, respectively. On the other hand, the former process is confirmed by channeling Rutherford backscattering analyses of the amorphous layer thickness remaining after implantation, the latter process is consistently attributed to the formation of highly mobile Si di-interstitials formed under irradiation and in the course of damage annealing. The observed ion-beam mixing in Si is compared to recent results on ion-beam mixing of Ge isotope multilayers that, in contrast to Si, are fully described by thermal-spike mixing only.

Optical characterization of one-dimensional porous silicon photonic crystals with effective refractive index gradient in depth

In this paper, we report the results of the investigation about the optical properties of one-dimensional photonic crystals based on porous silicon multilayer structures having unit cells with optical properties that vary as a function of the device thickness. These devices were obtained by anodizing crystalline silicon in electrolyte aqueous solution of HF at different concentrations. The effective refractive index and extinction coefficients, of porous silicon layers of unit cells had been obtained by fitting the experimental reflectance spectra using the transfer matrix method. The effective–refractive indexes, for devices obtained in electrolyte solution of high HF concentration, were shown to have a significant gradient in the depth of the layers, whereas it was low (negligible) for devices obtained in electrolyte solution with low HF concentration, but the mechanical stability of these latter devices is poor. It was found that this fragility depends on the layer number and upon their physical thicknesses ratio.

Tunneling times of acoustic phonon packets through a distributed Bragg reflector.

The longwave phenomenological model is used to make simple and precise calculations of various physical quantities such as the vibrational energy density, the vibrational energy, the relative mechanical displacement, and the one-dimensional stress tensor of a porous silicon distributed Bragg reflector. From general principles such as invariance under time reversal, invariance under space reflection, and conservation of energy density flux, the equivalence of the tunneling times for both transmission and reflection is demonstrated. Here, we study the tunneling times of acoustic phonon packets through a distributed Bragg reflector in porous silicon multilayer structures, and we report the possibility that a phenomenon called Hartman effect appears in these structures.

Tunable optical filters with wide wavelength range based on porous multilayers.

A novel concept for micromechanical tunable optical filter (TOF) with porous-silicon-based photonic crystals which provide wavelength tuning of ca. ±20% around a working wavelength at frequencies up to kilohertz is presented. The combination of fast mechanical tilting and pore-filling of the porous silicon multilayer structure increases the tunable range to more than 200 nm or provides fine adjustment of working wavelength of the TOF. Experimental and optical simulation data for the visible and near-infrared wavelength range supporting the approach are shown. TOF are used in spectroscopic applications, e.g., for process analysis.

Macromesoporous silicon Bragg and microcavity structures

In this paper, macromesoporous porous silicon Bragg and microcavity structures are fabricated from highly doped n-type silicon based on HF/ethanol electrolyte system without oxidising components. Well defined controllable pore sizes at the range of 50–100 nm have been obtained by systematically optimisation of the etching parameters. Alternately arrangement of straight and branched mesoporous layers are first achieved successfully under the condition of current density no more than 100 mA cm−2. Macroporous Bragg reflector is also produced successfully. Layers stack of the macroporous Bragg reflector relies on nanowires. As well as the Bragg reflectors with different pore morphologies, microcavity structure can be obtained based on the same silicon wafer using the same HF/alcohol electrolyte. In this study, work has been done on macromesoporous silicon multilayer structures successfully under the condition of simple and easy to implement experimental environment.

Nano-porous Silicon Microcavity Sensors for Determination of Organic Fuel Mixtures

We present the preparation and characteristics of liquid-phase sensors based on nano-porous silicon multilayer structures for determination of organic content in gasoline. The principle of the sensor is a determination of the cavity-resonant wavelength shift caused by refractive index change of the nano-porous silicon multilayer cavity due to the interaction with liquids. We use the transfer matrix method (TMM) for the design and prediction of characteristics of microcavity sensors based on nano-porous silicon multilayer structures. The preparation process of the nano-porous silicon microcavity is based on electrochemical etching of single-crystal silicon substrates, which can exactly control the porosity and thickness of the porous silicon layers. The basic characteristics of sensors obtained by experimental measurements of the different liquids with known refractive indices are in good agreement with simulation calculations. The reversibility of liquid-phase sensors is confirmed by fast complete evaporation of organic solvents using a low vacuum pump. The nano-porous silicon microcavity sensors can be used to determine different kinds of organic fuel mixtures such as bio-fuel (E5), A92 added ethanol and methanol of different concentrations up to 15%.

Aminosilane functionalizations of mesoporous oxidized silicon for oligonucleotide synthesis and detection

Direct solid phase synthesis of peptides and oligonucleotides (ONs) requires high chemical stability of the support material. In this work, we have investigated the passivation ability of porous oxidized silicon multilayered structures by two aminosilane compounds, 3-aminopropyltriethoxysilane and 3-aminopropyldimethylethoxysilane (APDMES), for optical label-free ON biosensor fabrication. We have also studied by spectroscopic reflectometry the hybridization between a 13 bases ON, directly grown on the aminosilane modified porous oxidized silicon by in situ synthesis, and its complementary sequence. Even if the results show that both devices are stable to the chemicals (carbonate/methanol) used, the porous silica structure passivated by APDMES reveals higher functionalization degree due to less steric hindrance of pores.

Reduced thermal conductivity of isotopically modulated silicon multilayer structures

We report measurements of the thermal conductivity of isotopically modulated silicon that consists of alternating layers of highly enriched silicon-28 and silicon-29. A reduced thermal conductivity of the isotopically modulated silicon compared to natural silicon was measured by means of time-resolved x-ray scattering. Comparison of the experimental results to numerical solutions of the corresponding heat diffusion equations reveals a factor of three lower thermal conductivity of the isotope structure compared to natural Si. Our results demonstrate that the thermal conductivity of silicon can be effectively reduced with isotopically modulated structures. This offers a promising approach to optimize silicon for thermoelectric applications.

Rationally designed porous silicon as platform for optical biosensors

Optical porous silicon multilayer structures are able to work as sensitive chemical sensors or biosensors based in their optical response. An algorithm to simulate the optical response of these multilayers was developed, considering the optical properties of the individual layers. The algorithm allows designing and customizing the porous silicon structures according to a given application. The results obtained by the simulation were experimentally verified; for this purpose different photonic structures were prepared, such as Bragg reflectors and microcavities. Some of these structures have been derivatized by the introduction of aminosilane groups on the porous silicon surface. The algorithm also permits to simulate the effects produced by a non uniform derivatization of the multilayer.

Diamond-Like Carbon Thin Films with Extremely High Compressive Stress (>8~12GPa) for Advanced CMOS Strain Engineering

ABSTRACTDiamond-like carbon (DLC) films as a new strain-capping material with compressive stress up to 12GPa for strained silicon technology were fabricated by filtered cathodic vacuum arc (FCVA) deposition system. The films’ compositions and bonding structures were characterized using multi-wavelength Raman spectroscopy. The relationship between intrinsic stress and G peak dispersion of the films’ Raman spectra were discussed. The results showed that the bias voltage applied to substrate during deposition determines films’ sp3 bonding content and intrinsic stress. Process compatibility of the DLC films with standard CMOS technology was confirmed by using WDXRF measurement. Also diffusion behavior of carbon atoms in DLC films with copper and silicon was studied with a Cu(200nm)/DLC(40nm)/silicon multilayer structure annealed at 500℃ in N2 atmosphere for an hour. At last, stress induced on silicon surface by DLC strips was characterized using surface sensitive UV-Raman spectroscopy. The results showed that DLC films with extremely high compressive stress have potential application in future CMOS strain engineering.

Sensing characteristics of the organic vapors according to the reflectance spectrum in the porous silicon multilayer structure

We fabricated a porous silicon multilayer and investigated its reflectance spectra before, during, and after exposure to various organic vapors. During exposure of the porous silicon multilayer to isopropanol, ethanol, methanol, and acetone vapors, the reflectance peak shifted toward longer wavelengths by about 5, 12, 26, and 39 nm, respectively. The shift of the reflectance peak arises from refractive index changes induced by capillary condensation of the organic vapor in the pores of the porous silicon multilayer. In addition, we observed that the shift value of the reflectance peak increased with increasing organic solvent concentration in the organic solvent-water mixture. After removing the organic vapor, the reflectance spectrum returned completely to its original state.