Multilayer Silicon Research Articles

Optimization of multilayer porous silicon antireflection coatings for silicon solar cells

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. In this work, multilayered refractive index stacks optimized for antireflection, in bare air and within modules, are modeled. The relation between porous silicon (PS) etching parameters and PS structure is carefully investigated using spectroscopic ellipsometry, gravimetry, x-ray photoelectron spectroscopy, and scanning electron microscopy. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate the optimized ARCs. Limits imposed by efficient application in photovoltaics, such as thickness restrictions and the angular distribution of incident light, are examined and accounted for. Low reflectance multilayer ARCs are fabricated with integrated reflectances of ∼3% in air and 1.4% under glass in the wavelength range 400–1100 nm.

Analysis of thin-film silicon solar cells with plasma textured front surface and multi-layer porous silicon back reflector

The optical performance of thin crystalline silicon solar cells with plasma textured front surface and with porous silicon stack back reflector is analysed. A rigorous analytical ray tracing model, that uses an accurately estimated porous silicon refractive index, is elaborated to predict the optical absorption, carrier generation, external quantum efficiency, and total photogenerated current in the cell. Calculated results best fitted to those measured for an experimental demonstration cell show that surface recombination is a major loss factor in the demonstration cell. The study also concludes that the plasma textured front surface has poor light diffusion properties, and predicts that the maximum gain in the photocurrent from back reflection at the porous silicon stack equals 10% compared to 20% if the back reflector is ideal. However, the proposed cell is realistic and promises excellent performance when state of the art front surface texturing, passivation and front metallization techniques are implemented.

Direct Synthesis of Oligonucleotides on Nanostructured Silica Multilayers

In this work, we have quantified the functionalization of some porous silicon multilayers that optically act as Bragg reflectors: 1.3 × 10−3 mol/g, corresponding to 3.25 nmol/cm2, of a ten base oligonucleotide has been estimated by a chemical procedure, based on the dimethoxytrityl monitoring. The oligonucleotide has been synthesized directly on two different porous silicon structures, one oxidized by a thermal treatment, the other by a chemical process based on exposure to I2/pyridine. We have also monitored the oligonucleotide synthesis by a label-free technique: a 11 nm red-shift of the optical spectrum can be observed by spectroscopic reflectometry.

Nanoporous silicon multilayers for terahertz filtering

We describe the fabrication, simulation, and measurement of a terahertz (THz) filter composed of nanoporous silicon multilayers. Using electrochemical etching, we fabricated a structure composed of alternating high- and low-index layers that achieves 93% power reflectivity at the target wavelength of 1.17 THz, with a stopband of 0.26 THz. The measured reflection and transmission spectra of the multilayer filter show excellent agreement with calculations based on the refractive indices determined separately from single-layer measurements. This technique could provide a convenient, flexible, and economical way to produce THz filters, which are essential in a variety of future applications.

Preparation and analysis of porous silicon multilayers for spectral encoding applications

AbstractThis paper compares two methods used to encode information into the optical spectrum of a porous Si film using a modulated current‐time waveform. In the first approach, a waveform consisting of a sequence of current steps is used to produce a porous silicon multilayer structure. Each layer in the multilayer acts as a Fabry‐Perot interference film, and the superposition of their contributions results in a complicated reflectivity spectrum. The spectrum can be deconvoluted by means of a Fast Fourier Transformation (FFT), which yields the optical thickness of each of the individual layers and their combinations.The second type of waveform consists of several superimposed cosine waves of differing frequencies. The resulting reflectivity spectrum consists of a series of peaks, each of whose position in the spectrum (frequency at which a peak is at maximum amplitude) corresponds to the frequency of a cosine wave component in the current‐time waveform. The two methods of encoding information are compared in terms of repeatability, tunability, and information capacity. The superimposed cosinusoidal waves are found to provide greater information density and a simpler means of decoding. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Refractive index contrast in porous silicon multilayers

AbstractTwo of the most important properties of a porous silicon multilayer for photonic applications are flat interfaces and a relative large refractive index contrast between layers in the optical wavelength range. In this work, we studied the effect of the current density and HF electrolyte concentration on the refractive index of porous silicon. With the purpose of increasing the refractive index contrast in a multilayer, the refractive index of porous silicon produced at low current was studied in detail. The current density applied to produce the low porosity layers was limited in order to keep the electrolyte flow through the multilayer structure and to avoid deformation of layer interfaces. We found that an electrolyte composed of hydrofluoric acid, ethanol and glycerin in a ratio of 3:7:1 gives a refractive index contrast around 1.3/2.8 at 600 nm. Several multilayer structures with this refractive index contrast were fabricated, such as dielectric Bragg mirrors and microcavities. Reflectance spectra of the structures show the photonic quality of porous silicon multilayers produced under these electrochemical conditions. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and characterization of porous silicon layers for applications in optoelectronics

In the present paper, several samples of porous silicon monolayers and multilayers were prepared at different anodization conditions with fixed HF concentration. The room temperature photoluminescence wavelength observed to be increased with increased etching time and current density respectively. By Raman measurement it has been observed that as the size of silicon crystallites decreased with increased etching time, the silicon optical phonon line shifted somewhat to lower frequency from 520.5 cm−1 and became broader asymmetrically. The surface roughness and pyramid like hillocks surface was confirmed by AFM measurement. In SEM images, the porous silicon layers were clearly observed by white and black strips. It was also observed that the reflectivity increased as the number of porous silicon layers was increased.

Optical Properties of Non-Periodic Dielectric Systems Made of Nanostructured Porous Silicon

To investigate the optical properties in non-periodic dielectric systems, we study here the reflection of light from nanostructured porous-silicon-based period doubling heterostructures. The multilayered systems are fabricated in such a way that the optical thickness of each layer is one quarter of 650nm. The results for the optical reflectance are presented and compared with that of Fibonacci, Thue-Morse, and random structures fabricated under the same conditions. Numerical simulation for the reflectance along the lines of the transfer matrix approach is performed. In addition, optical reflection from Gaussian porous silicon multilayers is also briefly discussed. We find that porous silicon Period Doubling dielectric multilayers could demonstrate the optical properties similar to the classical periodic Febry-Perot interference filters with one or multiple resonant peaks, but with an advantage of having total optical thickness much lesser than that of the periodic structures.

Correlating Raman-spectroscopy and high-resolution transmission-electron-microscopy studies of amorphous/nanocrystalline multilayered silicon thin films

The nanostructure of multilayered silicon thin films was studied using Raman spectroscopy (RS) and high-resolution transmission electron microscopy (HRTEM). Since the properties of nanocrystalline silicon layer depend on the size of the nanocrystals, an accurate determination of the crystallite sizes and the crystalline fraction is of primary importance. The average sizes of the nanocrystals estimated by RS, assuming bi-modal distribution of crystal sizes, were close to 2 nm and above 5–20 nm. HRTEM confirmed the existence of nanocrystals with a mean square value of around 2 nm and certain number of larger nanocrystals, embedded in an amorphous matrix. The correlation between the results obtained by these two techniques is discussed. The optical properties of measured samples corresponded to an amorphous-crystalline mixture with indication of confinement effects compatible with 2 nm nanocrystals.

Omnidirectional photonic bandgaps in porous silicon based mirrors with a Gaussian profile refractive index

We have designed and fabricated one dimensional photonic bandgap (PBG) structures from dielectric multilayers of porous silicon, with a periodic repetition of a unit cell consisting of 21 layers (95%) with the refractive index varying according to the envelope of a Gaussian function and another layer (5%) with a fixed refractive index. The structures can be designed to demonstrate the wavelength scalability within the visible as well as near infrared region. Three different structures have been stacked together to enhance the width of the PBG. The omnidirectional nature of the PBG was verified experimentally and theoretically up to 68° and 89° angles of incidence, respectively.

Porous Multilayers as a Dielectric Host for Photons Manipulation

Porous silicon multilayers are a suitable tool to modify the photon mode density in one dimension. In this way, new phenomena such as photon Bloch oscillations, Zener tunneling of light, slow light, photon Anderson localization, necklace light states and optical bistability may occur. A review of these phenomena and their actual demonstration is here reported.

Enhancement of magneto-optical response in nanocomposite-hydrogenated amorphous silicon multilayers

The dependence of the magnetic and magneto-optical properties on the semiconductor layer thickness has been studied for a [(Co45Fe45Zr10)35(Al2O3)65(X)/α-Si:H(Y)]30 multilayer. It is found that an increase in the Si layer thickness to 1.3–1.7 nm leads to an increase in the transverse Kerr effect, magnetization, and coercive force. The changes in the properties of the nanomultilayer system are related to the percolation transition between CoFeZr granules through Si streaks. This percolation leads to effective exchange interaction between isolated ferromagnetic granules of Co45Fe45Zr10 alloy and increase in magneto-optical response.

X-ray diffraction study of the morphology and structure of pulse-anodized porous Si multilayers

Porous Si layers, obtained by pulsed electrochemical etching of n-Si(001) substrates (resistivity 0.01 Ω cm) in a 1: 1 mixture of hydrofluoric acid and ethanol, have been investigated by high-resolution X-ray diffraction and electron microscopy. The average structural parameters of the layers grown (thickness, strain, porosity, pore size) are determined. It is found that pulsed anodic oxidation leads to a decrease in the average strain of layers. It is established that, at frequencies up to 1 Hz, anodic oxidation makes obtaining porous silicon multilayers with layer thicknesses of 20–300 nm containing Si nanocrystallites possible. It is shown that X-ray diffuse scattering from pores yields information about their ordering and can be used to monitor the processes of electrochemical etching used to form porous layers.

Reflectance analysis of a multilayer one-dimensional porous silicon structure: Theory and experiment

We present a method for treating birefringent effects in layered media and apply the formalism to analyze reflectance from a multilayer one-dimensional (1D) porous silicon (PS) structure at off-normal incidence. The approach is to characterize the fields in terms of s- and p-polarized amplitudes in each layer, and the calculations then naturally employ Fresnel reflection and transmission coefficients for the uniaxially anisotropic media. We observe an excellent agreement between the theoretical and experimental curves by including optical absorption and macroscale waviness of the PS layers, and the resolution of the spectrophotometer. In particular, we point out the importance of birefringent effects that cause the splitting of the resonance wavelengths between two different polarizations. The investigated 1D PS structure can be used, for example, as a polarization sensitive optical switch.

Multilayer SiC for thermal protection system of space vehicles: Manufacturing and testing under simulated re-entry conditions

Two types of laminated multilayer silicon carbide plates were processed by tape casting, de-binding and pressureless sintering. The specimens were subject to thermal re-entry testing under conditions as derived from the HERMES study: up to 100 combined thermal and air pressure cycles were performed. After the first cycle, all samples lost about 1.5% of their initial mass. This was caused by burn-off of the carbon added as sintering aid or left after thermal decomposition of binder and plasticizer used in the tape casting process. During subsequent thermal cycles, all samples slightly gained mass due to formation of silica on the surface and partial oxidation at the core. After completing 100 cycles, microstructure and mechanical behaviour of specimens were compared to that of the as-processed material. A flexural strength higher than 100 MPa and a Young modulus of about 130 GPa were retained by all the specimens.

Advanced Multilayer Amorphous Silicon Thin-Film Transistor Structure: Film Thickness Effect on Its Electrical Performance and Contact Resistance

We report the intrinsic and extrinsic electrical characteristics of advanced multilayer amorphous silicon (a-Si:H) thin-film transistor (TFT) with dual amorphous silicon nitride (a-SiNX:H) and a-Si:H layers. The thickness effect of the high electronic quality a-Si:H film on the transistor's electrical property was investigated; with increasing film thickness, both field-effect mobility and subthreshold swing show improvement and the threshold voltage remain unchanged. However, the contact resistance increases with the a-Si:H film thickness. Using the two-step plasma enhanced chemical vapor deposition process, we fabricated TFT's with acceptable field-effect mobility (∼1 cm2 V-1 s-1) and threshold voltage (<1.5 V) with enhanced throughput.

Prediction of effective elastic modulus of plain weave multiphase and multilayer silicon carbide ceramic matrix composite

The bottom up multiscale finite element modelling that is based on the sequential consideration of the fibre/interface/matrix scale and the tow/matrix/coat scale is employed to predict the effective elastic modulus of the plain wave multiphase and multilayer silicon carbide ceramic matrix composite. Instead of using the homogenisation method, the strain energy based method that is advantageous in computing efficiency and numerical implementation is adopted for the multiscale analysis and evaluation of effective properties. First, the effective properties of tows are computed on the fibre/interface/matrix scale. They are then incorporated into the tow/matrix/coat scale to evaluate the effective properties of the composite. Numerical results obtained by the proposed method and model show a good agreement with the results measured experimentally.

Optimisation of a smooth multilayer nickel silicide surface for ALN growth

For use in thin film electroacoustic (TEA) technology a few hundred nanometre thick nickel silicide (NiSi) electrode would need to be fabricated. A complete fabrication process for the formation of over 200 nm thick silicide films has been optimised for use as an electroacoustic electrode. Optimisation of silicidation temperature and identification of the mono phase of silicide is demonstrated. Thick electrodes are formed by depositing multilayers of silicon and nickel pairs onto silicon (Si) substrates before rapid thermal annealing. The numbers of multilayers and relative material thicknesses are optimized for both surface roughness and electrical resistivity. The growth of textured aluminium nitride (AlN) has been investigated on the optimised surfaces.

Multilayer silicon rich oxy-nitride films characterization by SIMS, VASE and AFM

In this work secondary ion mass spectrometry (SIMS), variable angle spectroscopy ellipsometry (VASE) and atomic force microscopy (AFM) are used to investigate the structure, composition and morphology of multilayer SRON films. Three/four SRON sequential layers were deposited on silicon wafers by PECVD and silicon, nitrogen and oxygen content was varied by changing the N2O/SiH4 ratio. The total thickness of the resulting SRON stack is about 50nm. SIMS analyses of NCs+, OCs+, SiCs+, in MCs+ methodology are performed by a Cameca SC-ultra instrument. Depth profiles are obtained at 500eV of primary beam impact energy with sample rotation. An approximate method to obtain silicon concentration is used. Total layer thickness are obtained from both SIMS and VASE measurements. In addition, we compare the thickness of the single layers obtained from VASE with the SIMS depth profiles. A detailed analysis of films morphology is obtained by AFM. The SRON stack is sputtered by SIMS until a certain layer is exposed, which is then analyzed by AFM. The sputtered layers are then etched in HF solution to better resolve the exposed nano-crystals.

Single and multilayer porous silicon structures for photonic applications

Porous silicon (PS) apart from its diverse properties from bulk Si is also identified to be a photonic material. PS single and multilayer samples are prepared on a polished surface of (100) Si substrate. A peak fitting technique has been discussed for XRD analysis in the case of a dichromatic beam. Theoretical simulations of light transmission for some special structures are also presented.