Multilayer Silicon Research Articles

Structural tailoring of multilayer porous silicon for photonic crystal application

The 1D photonic crystals based on multilayer porous silicon are reported in this paper. The photonic structures are composed of alternating high and low dielectric function porous silicon layers by changing the porosity of porous silicon with a periodic electric pulse alternating between 10.0 and 70.0 mA/cm 2 during anodization on p +-type (1 0 0) silicon wafer with resistance range 0.01–0.02 Ω cm. Omni -directional mirrors as well as narrow band filters working in the range around 1.55 μm are designed and fabricated. Photonic crystal with such a structure exhibits high quality of filter behavior.

Interlayer composition in Mo–Si multilayers using X-ray photoelectron spectroscopy

Molybdenum–silicon multilayers have been synthesized using electron beam evaporation method. The chemical nature of surfaces and interfaces are probed using a depth profile X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy results indicate the formation of MoSi 2 phase on both the interfaces. The experimental results agree well with predicted results based on solid-state amorphizing reaction, as a result of large heat of mixing. The effective heat of formation model reveals the formation of MoSi 2 as the first phase.

Space instrumentation environmental verification: Dynamic tests on GLAST LAT tracker trays

The GLAST satellite is a mission for the detection of gamma rays of both galactic and extra-galactic origin in the energy range 20 keV–300 GeV. The satellite is planned to be launched in 2007 by NASA. Gamma rays detection is based on the pair conversion mechanism achieved by a multilayer silicon strip detector interleaved with a tungsten target for photon conversion and e + –e − pairs tracking. The GLAST program did foresee a wide use of vibration tests both in development and in qualification phases. The acceptance phase in aerospace environment is usually demanding in terms of vibration tests. Furthermore in this program the trays’ design and manufacturing procedure might have been highly vibration sensitive, and experimental validation was considered highly recommendable. This paper aims to give evidence of the performed tests. A critical analysis of the tests is proposed and some results will be also presented.

Resonant cavity enhanced optical microsensor for molecular interactions based on porous silicon

AbstractThe molecular binding between the glutamine binding‐protein (GlnBP) from Escherichia coli and L‐glutamine (Gln) is detected by means of an optical biosensor based on porous silicon technology. The binding event is optically transduced in the wavelength shift of the porous silicon optical microcavity (PSMC) reflectivity spectrum. The hydrophobic interaction links the GlnBP, which acts as a molecular probe for Gln, to the hydrogenated porous silicon surface area. We can thus avoid any preliminary surface functionalization process. The protein infiltrated PSMC results stable to oxidation at least for few cycles of wet measurements. The penetration of the proteins into the pores of the porous silicon matrix has been optimized: a strong base post‐etch process increases the pore size and removes any nanostructure on top and inside the porous silicon multilayer while does not degrade the optical response and the quality of the microcavity. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Inclusion of nanosized silicon grains in hydrogenated protocrystalline silicon multilayers and its relation to stability

Photoluminescence and Fourier transform infrared spectroscopy measured at room temperature produce strong evidence that nanosized silicon (nc-Si) grains embedded in hydrogenated protocrystalline silicon (i-pc-Si:H) multilayers. Thus, we propose the structure of the i-pc-Si:H multilayer possessing isolated nc-Si grains and their wrapping layers with a high hydrogen concentration embedded in highly hydrogen-diluted sublayers. The isolated nc-Si grains may act as radiative recombination centers of photoexcited carriers, and hence suppress the photocreation of dangling bonds caused by the nonradiative recombination in amorphous silicon matrix. Because of the repeatedly layered structure, the i-pc-Si:H multilayers have a fast light-induced metastability with a low degradation.

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.

Photoluminescence of erbium–oxygen-codoped silicon multilayers prepared by molecular beam epitaxy

Erbium–oxygen-codoped silicon multilayer film, which consists of alternate erbium–oxygen-codoped silicon layers and oxygen-doped silicon layers, was synthesized by molecular beam epitaxy. Er 3+-related luminescence from erbium–oxygen-codoped multilayer film is stronger than that of erbium–oxygen-codoped monolayer film and shows a weak temperature quenching behaviour. These improvements may be explained by the effect of O-doped Si layers in the multilayer film. The O-doped Si layers have a wide gap which prohibits the energy backtransfer effectively. At the same time, photogenerated carriers from silicon nanocrystals in O-doped Si layers could transfer the energy to neighboring Er 3+ ions in Er–O-codoped Si layers by a tunneling process, thus enhancing the Er 3+-related luminescence.

Development of a novel micromirror based on surface micromaching technology

A novel 3 × 3 micromirror array is designed and successfully fabricated with multi-layer silicon surface micromaching technology. It is composed of bottom electrode, support part and mirror plate, in which a T type beam structure is used to support the mirror plate. It can provide mirror with the vertical movement and the rotation about two horizontal axes, thus enabling phase modulation and amplitude modulation for the incident light. The test results show that the maximum deflection length along the vertical direction of the mirror plate is 2 μm, while the rotation angle about X- and Y-axis are ±2.3° and ±1.45°, respectively.

In situ observation of the evolution of porous silicon interference filter characteristics

AbstractPorous silicon multilayer formation was observed by in situ monitoring of the reflectivity spectra in the visible range. In order to reproduce the formation process optical model simulation was carried out. For demonstration of this method a 24‐layer microcavity structure was selected. Although in this low wavelengths region some absorption and scattering effects complicate the overall picture, the combined analysis throws new light upon the evolution of the porous silicon multilayer. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Laterally stacked porous silicon multilayers for subquart micron period UV gratings

AbstractA novel method was presented to prepare porous silicon Bragg grating structures. With this technique the grating period can be easily tuned in a wide range. In the porous silicon multilayer formation there is no theoretical limit for the period to be lowered to even below 100 nm. Both phase and mixed type (amplitude and phase) optical grating are demonstrated. Another unique feature of these structures is, that contrary to the case of commonly used porous silicon layers, here the pores are lying parallel to the wafer surface. This might be a valuable asset e.g. in microfluidic‐type sensor development. The paper discusses design and processing constraints. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Force modulation microscopy of multilayered porous silicon samples

AbstractIn this paper we report on Force Modulation Microscopy (FMM) study and force‐distance curve analysis of porous silicon layers grown on silicon. The characterization has been carried out on the cross section of porous silicon. The FMM images allowed us to investigate the morphological thickness of the layers through local elasticity differences resolving both between porous silicon layers of different porosities and between porous silicon and silicon itself. Force‐distance curves showed different adhesion behaviour: porous silicon is more hydrophobic than bulk silicon in cross sectional view. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photocurrent simulation in an n-p-n-p silicon multilayer solar cell

In this work, we simulate and optimize the photocurrent densities in a model of an n - p - n - p type thin film multilayer silicon solar cell for space applications. The incident light penetrates the cell perpendicularly to the junctions. The electrodes tailored inside the structure connect the n -layers together and the p -layers together. The equations giving the photocurrent density produced in each abscissa of the structure was developed. We used Matlab software to simulate and optimize the different parameters of the model. The results of simulation show that the optimized n - p - n - p silicon multilayer solar cell could deliver a photocurrent density of more than 46 mA/cm 2 under Air Mass 0 (AM0) solar spectrum (solar constant of 1.36 KW/cm 2 ) and that the photocurrent density produced by the n - p - n - p multilayer silicon solar cell is at least 10% higher than the photocurrent density produced by the simple n - p junction solar cell. We also show that the most important components of the total photocurrent densities (94%) is due to the minority carrier collection which happens on both side of the three space charge regions tailored inside the cell.

Versatile micromirror with a multimovement mode

A 3 x 3 micromirror array has been designed and successfully fabricated by multilayer silicon surface micromaching technology. It is composed of a bottom electrode, a supporting part, and a mirror plate and can modulate both phase and amplitude of incident light. The maximum deflection length along the vertical direction of the mirror plate is 2 microm, and the rotation angles about the Y and X axes are +/-2.3 degrees and +/-1.45 degrees, respectively; one can obtain an even larger deflection by simply increasing the thickness of the sacrificial layers.

Highly and Rapidly Stabilized Protocrystalline Silicon Multilayer Solar Cells

AbstractWe have developed highly stabilized (p-i-n)-type protocrystalline silicon (pc-Si:H) multilayer solar cells. However, the source of the superior light-induced stability of the pc-Si:H multilayer absorbers compared to conventional amorphous silicon (a-Si:H) absorbers remains unclear. Photoluminescence (PL) and Fourier transform infrared (FTIR) spectroscopy measured at room temperature produce strong evidence that nano-sized silicon grains embedded in regularly arranged highly H2-diluted sublayers suppress the photocreation of dangling bonds. To achieve a high conversion efficiency, we applied a double-layer p-type amorphous siliconcarbon alloy (p-a-Si1-xCx:H) structure to the pc-Si:H multilayer solar cells. The less pronounced initial short wavelength quantum efficiency variation as a function of bias voltage, and the wide overlap of dark current - voltage (JD-V) and short-circuit current - open-circuit voltage (Jsc-Voc) characteristics prove that the double p-a-Si1-xCx:H layer structure successfully reduces recombination at the p/i interface. Thus, we achieved a highly stabilized efficiency of 9.0 % without any back reflector.

Optical constants of porous silicon films and multilayers determined by genetic algorithms

Porous silicon (PS) can be optically described as a homogeneous mixture of air, silicon, and, eventually, silicon dioxide, which explains the great tunability of its refractive index. This behavior makes porous silicon a very interesting material for the development of interference filters for use in optoelectronic applications. For the accurate design of such filters, the precise determination of the optical constants of PS is mandatory. However, since PS is a dispersive and absorbing material, determination of its complex refractive index is not trivial. In the present work, a genetic algorithm has been used to precisely determine the complex refractive index and thickness of thin films from their reflectance spectra in the visible wavelength range. This algorithm was applied to porous silicon. The precise spectral values of the refractive index obtained for PS layers of different characteristics have been used to design and fabricate interference filters with a predetermined optical behavior. In addition, the performance of the algorithm in two different working modes is discussed.

Interfaces and roughness in a multilayer silicon structure

The results of spectral ellipsometric studies of a complex multilayer system consisting of a Si substrate, a SiO2 layer, and a layer of polycrystalline Si are reported. A method for analysis of the multilayer structure is suggested; the method is based on a heavy dependence of experimental data of spectral ellipsometry on the depth of penetration of probing radiation at various wavelengths in the visible region of the spectrum. The actual structure of the system and the parameters of the layers are determined. Processing the ellipsometry data made it possible to identify a natural rough surface oxide SiO2 layer and also a transition layer at the interface between the layer of polycrystalline Si and the layer of natural oxide. The composition and thickness of these layers are determined. It is shown that the deposited Si layer consists of a mixture of the crystalline and the amorphous phases; the percentage of these phases is determined.

Development of a novel 3 × 3 micromirror array for light modulation

A novel 3 × 3 micromirror array is designed and successfully fabricated with multi-layer silicon surface micromachining technology using phosphosilicate glass as sacrificial layer and polysilicon as structural layer. It is composed of a bottom electrode, a support part and a mirror plate, in which a T-shaped beam structure is used to support the mirror plate. It can provide the mirror with vertical movement and rotation about two horizontal axes, thus enabling phase and amplitude modulation of the incident light. The results show that the maximum deflection length of the mirror plate along the vertical direction is 2 µm, while the rotation displacements about X- and Y-axes are ±1.45° and ±2.3°, respectively, and an even larger deflection magnitude can be obtained through simply increasing the thickness of the sacrificial layers.

Microelectromechanical Devices for Satellite Thermal Control

Future space missions will include constellations of spacecraft, including nano- and picosatellites, where adaptive thermal control systems will be needed that fit the constraints of space applications with limited power and mass budgets. A microelectromechanical systems (MEMS) solution has been developed that will vary the emissivity on the surface of the small satellite radiator. The system is based on louver thermal controllers, where panels are mechanically positioned to modulate the effective radiator surface area. This system consists of MEMS arrays of gold-coated sliding shutters, fabricated with the Sandia ultraplanar, multilevel MEMS technology fabrication process, which utilizes multilayer polycrystalline silicon surface micromachining. The shutters can be operated independently to allow digital control of the effective emissivity. This first demonstrator technology is limited in the possible emittance range to a 40% change. Early prototypes of MEMS louvers that open away from the structure have shown the capability of a much wider dynamic range. The first generation of this active thermal management system will be demonstrated on NASA's New Millennium Program ST-5 spacecraft. With the opportunity to validate the MEMS thermal control technology in space on ST-5, lightweight, low-power MEMS radiators offer a possibility for flexible thermal control on future nanosatellites.

Investigation of silicon oxynitride and amorphous silicon multilayers

Optical (refractive index) and structural properties of silicon oxynitride (SiON) and amorphous silicon (a-Si) multilayers grown by RF sputtering with thickness in the 10–30 nm range have been analysed by ellipsometry and TEM. Satisfactory agreement between the two techniques is obtained as regards the thickness determination of the SiON films. Disagreement with values obtained by the stylus method by extrapolation for the two types of layers is discussed. The interfaces of the SiON films are very good when n-type P doped Si is used as a target. They are wavy with average periodicity and amplitude on the order of 50 and 2 nm, respectively, when a semi-insulating Si target is used, despite the presence of a buffer layer. Hypothesis is made that P incorporation may improve the reconstruction of the SiON surface.

Elemental composition of nanoclusters formed by pulsed irradiation with low-energy ions during Ge/Si epitaxy

Multilayer silicon structures with built-in layers of Ge nanoclusters were studied experimentally by Raman light scattering. The built-in layers were formed by the pulsed action of a low-energy beam of intrinsic ions during molecular-beam epitaxy. It is found that the ion-stimulated nucleation and the subsequent growth make it possible to obtain Ge nanoclusters almost free of Si.