Multilayer Silicon Research Articles

Self- and Dopant Diffusion in Extrinsic Boron Doped Isotopically Controlled Silicon Multilayer Structures

AbstractIsotopically controlled silicon multilayer structures were used to measure the enhancement of self- and dopant diffusion in extrinsic boron doped silicon. 30Si was used as a tracer through a multilayer structure of alternating natural Si and enriched 28Si layers. Low energy, high resolution secondary ion mass spectrometry (SIMS) allowed for simultaneous measurement of self- and dopant diffusion profiles of samples annealed at temperatures between 850°C and 1100°C. A specially designed ion-implanted amorphous Si surface layer was used as a dopant source to suppress excess defects in the multilayer structure, thereby eliminating transient enhanced diffusion (TED) behavior. Self- and dopant diffusion coefficients, diffusion mechanisms, and native defect charge states were determined from computer-aided modeling, based on differential equations describing the diffusion processes. We present a quantitative description of B diffusion enhanced self-diffusion in silicon and conclude that the diffusion of both B and Si is mainly mediated by neutral and singly positively charged self-interstitials under p-type doping. No significant contribution of vacancies to either B or Si diffusion is observed.

Preparation of a structural color forming system by polypeptide-based LB films

Abstract To prepare novel display systems like chameleon which change the color by stimuli, we prepared a structural color forming system by use of LB films consisting of polypeptide multilayers. We think this system change its color depending on the molecular structure like photochemical systems. Poly(γ-hexyl- l -glutamate) (PHeLG) was used as the multilayer component and silicon was used as the substrate for reflection of incident light. PHeLG monolayers were transferred onto substrates hydrophobized with octadecyl trimethoxy silane (OTS). LB films with various layer numbers up to 160 layers were prepared and they showed different colors depending on the thickness of polypeptide layer due to interference of light. The structures of LB films were estimated by using AFM and FT-IR spectroscopy. Interference colors were characterized by reflective visible spectra and compared with theoretical ones. Silicon wafer pre-colored by excessive oxidization was also used, and it was confirmed that only 10 or 20 layers of PHeLG could show the structural color. In this study, the base for preparation of chameleon-type display system was established. We will introduce photochemical moiety (azobenzene moiety, etc.) into this system and produce display systems which change their colors by optical stimuli.

Development of a viable 3D integrated circuit technology

Three-dimensional integrated circuit technology with transistors stacked on top of one another in multi-layer silicon film has always been a vision in the future technology direction. While the idea is simple, the technique to obtain high performance multi-layer transistors is extraordinarily difficult. Not until recently does such technology become feasible. In this paper, the background and various techniques to form three-dimensional circuits will be reviewed. Recent development of a simple and promising technology to achieve three-dimensional integration using Metal-Induced-Lateral-Crystallization will be described. Preliminary results of 3D inverters will also be provided to demonstrate the viability for 3D integration.

Photon-stimulated ion desorption from mono- and multilayered silicon alkoxide on silicon by core-level excitation

Photon-stimulated ion desorption from Si(OCH3)4 [tetramethoxysilane (TMOS)] adsorbed on Si(100) following silicon K-edge excitation has been investigated using monochromatized synchrotron radiation. In order to separate the photochemical process induced by the substrate excitation from that induced by adsorbate excitation, we have prepared well-controlled monolayer, submonolayer and multilayer coverages of TMOS on Si(100). In the x-ray absorption near-edge structure (XANES) spectrum for a multilayer of TMOS, the peak energy of the resonant excitation from the Si 1s to the unoccupied σ* orbitals was shifted to higher energy by 6 eV than that for the silicon substrate. For a thin layer such as the monolayer and submonolayer, the XANES spectra taken by total electron yields were almost the same as those of the silicon substrate. However, the photon-energy dependencies of the CH3+ and Si(OCH3)3+ yields for the monolayer sample did not follow the XANES spectra of silicon but they followed those for multilayered TMOS. This result indicates that the secondary electrons produced in the silicon substrate scarcely contribute to the fragment-ion desorption for thin layers. It was concluded that the core-to-valence resonant excitation in monolayered TMOS is the main trigger of the C–O bond scission and the fragment-ion desorption.

Semiempirical model of carrier mobility in silicon carbide for analyzing its dependence on temperature and doping level

Experimental data on electron and hole mobility in three silicon carbide polytypes, 4H-SiC, 6H-SiC, and 3C-SiC, are analyzed. A semiempirical model is proposed for describing the dependence of the majority carrier mobility on temperature and doping level. The model describes well the accumulated body of experimental data and can be applied to model characteristics of multilayer silicon carbide structures.

The Effect of Thermal Processing on Multilayer Porous Silicon Microcavity

The effect of thermal processing, that gave limited oxidation from annealing under N2 atmosphere, on multilayer porous silicon (P-Si) microcavities has been investigated. The reflectivity spectra were measured before and after thermal processing. The observation of multilayer samples by scanning electron microscopy has shown unexpected expansion and compression of P-Si layers. It is found that thermal processing not only changed the effective refractive indices of the individual porous layers but also roughened their respective interfaces.

Low degradation and fast annealing effects of amorphous silicon multilayer processed through alternate hydrogen dilution

Alternately hydrogen diluted a-Si:H multilayers are shown to be a promising concept for the fabrication of stable a-Si:H solar cells or other a-Si:H based devices. The alternately hydrogen diluted amorphous silicon multilayers were obtained by toggling both the H2/SiH4 dilution ratio and the total flow rate of the gases under continuous UV light irradiation into the reaction chamber of a photochemical vapor deposition system. The films were characterized by Fourier transformed infrared spectroscopy, spectroscopic ellipsometry, cross-sectional transmission electron microscopy, and atomic force microscopy. We applied these multilayers as the active layer of p–i–n type thin film solar cells. The multilayer solar cells are compared to solar cells incorporating a-Si:H made from pure SiH4 gas and to solar cells incorporating a-Si:H made at a constant hydrogen dilution ratio containing nearly the same hydrogen amount as the multilayer. We report on the light-soaking and annealing behavior of the solar cells. The multilayer solar cell has an exceptionally high recovery rate at low temperatures, which makes the solar cell degradation behavior highly sensitive to the cell temperature during degradation. Following the relation, DH∝1/τ, where DH and τ are the diffusion coefficient for hydrogen and time constant for annealing, respectively, the layered structure in the multilayer possibly elevates DH, which accounts for rapid stabilization and annealing.

Optical properties of multilayered porous silicon

We present a short review of some optical devices based on multilayered porous silicon, which can be easily obtained by varying the formation current during the etching process. These include Bragg reflectors and Fabry–Pérot microcavities, which can be adjusted from the visible to the near infrared. The interface roughness, tragic in the case of multilayers, is studied. It can be drastically reduced when changing the electrolyte viscosity. The high reflectivities obtained in this way are measured by Cavity Ring–Down Spectroscopy. Problems occurring when realising thin layers and an efficient way to adjust precisely the optical thicknesses of the thin layers constituting the multilayered structure are also presented. Finally we present a method of calculation of the emission which takes absorption into account and is able to explain the angular dependence of the luminescence.

High-efficiency silicon solar cells

Multilayer silicon solar cells are very promising structures for high efficiency photovoltaic conversion. The author discusses the basic principles of solar cell operation and how to maximise the output power and current. However, one problem is the series resistance resulting from the thinness of the layers and the lateral contacts. Sophisticated computations can be undertaken to simulate and optimize these different layers' thickness.

Fracture toughness of multilayer silicon nitride with crack deflection

The fracture resistance of a multilayer silicon nitride consisting of alternate dense and porous layers was investigated by a single-edge-V-notched beam (SEVNB) technique. Since silicon nitride whiskers were aligned parallel to the laminar direction in the porous layer, the crack deflected macroscopically along the whiskers, resulting in high apparent K I values, 15–25 MPa m 1/2. The crack then propagated in mode I, and was arrested when K I was reduced to the fracture resistance without the crack deflection effects. These fracture resistance behaviors were well-explained in terms of the notch-insensitivity and the shielding effects of pull-out of the aligned whiskers.

Depth-resolved microspectroscopy of porous silicon multilayers

We have measured depth-resolved microphotoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. The PL emission band gets stronger, blueshifts, and narrows at the high porosity layers. On the contrary, the Raman band weakens and broadens. This band is fitted to the phonon confinement model. With the bulk silicon phonon frequency and its linewidth as free parameters, we obtain crystallite size, temperature, and stress as a function of depth. Sizes are larger than those estimated from PL. Laser power was reduced to eliminate heating effects. Compressive stresses in excess of 10 kbar are found in the deepest layer due to the lattice mismatch with the substrate.

A multilayer edge-on silicon microstrip single photon counting detector for momography mammography

A 3-layer edge-on silicon microstrip detector for the SYRMEP/FRONTRAD project has been designed and realised. The image matrix is made by 764 pixels with dimensions 300 (thickness of the single detectors) × 200 (strip pitch) μm 2. The system has a sensitive area of 50 × 1 mm 2, an inter-layer distance of ≈ 100 μm and an efficiency of ≈ 80% for 20 keV photons. The image is acquired by scanning the object across the beam cross-section and the overall statistics on the single pixel is obtained by summing up the information of corresponding pixels in the three layers, thereby reducing the effect of possible noisy or not functioning pixels. Experimental results obtained at the SYRMEP/FRONTRAD beam line of the ELETTRA synchrotron radiation machine with this innovative detector are presented.

Spatiotemporal dynamics of current-density filaments in a periodically driven multilayered semiconductor device

Results of numerical calculations of a set of reaction-diffusion equations for silicon ${p}^{+}\ensuremath{-}{n}^{+}\ensuremath{-}p\ensuremath{-}{n}^{\ensuremath{-}}$ devices under dc and ac bias are presented. For pure dc bias, we observe self-sustained oscillations of the device voltage accompanied with pendulumlike oscillations of a current-density filament. By imposing an ac bias the device voltage shows frequency locking, quasiperiodicity, and chaos depending on the amplitude and the frequency of the ac bias. It is shown that the global oscillations are connected with corresponding frequency-locked, quasiperiodic, and chaotic filament motions. The numerical results reproduce the bifurcation sequences observed recently in experiments on multilayered silicon structures.

A Low‐Thermal‐Budget In Situ Doped Multilayer Silicon Epitaxy Process for MOSFET Channel Engineering

This paper describes an in situ boron‐doped, multilayer epitaxial silicon process that can be used to obtain doping profiles for channels in the deep‐submicron regime. We have extensively studied lightly doped channel structures in which an intrinsic silicon layer is grown on an in situ doped epitaxial silicon film. Low‐thermal‐budget processing is achieved by the ultrahigh‐vacuum rapid thermal chemical vapor deposition technique which combines low‐temperature surface preparation and deposition (≤800°C) while providing high growth rates using disilane . Boron doping is achieved using diborane diluted in hydrogen (500 ppm) as the precursor. Temperature and gas switching are compared in terms of doping transition, interface contamination (carbon and oxygen incorporation), and impurity diffusion upon annealing. Our results reveal that for a contamination‐free epitaxial silicon interface, interfacial carbon contamination must be eliminated or reduced to a minimum level. Using this process, short‐channel n‐channel metal‐oxide semiconductor devices μm) have been fabricated for the first time demonstrating the potential use of the technique. It was found that lightly doped channel metal‐oxide semiconductor field effect transistors are more easily scalable into the 0.1 μm regime with superior short‐channel characteristics. © 1999 The Electrochemical Society. All rights reserved.

Depth-Resolved Microspectroscopy of Porous Silicon Multilayers

AbstractWe have measured micro-photoluminescence (PL) and micro-Raman spectra on the cross section of porous silicon multilayers to sample different layer depths. We find noticeable differences in the spectra of layers with different porosity, as expected from the quantum confinement of electrons and phonons in silicon nanocrystals with different average sizes. The PL emission band gets stronger, blue shifts, and narrows at the high porosity layers. The average size can be estimated from the shift. The Raman phonon band at 520 cm−1 weakens and broadens asymmetrically towards the low energy side. The line shape can be related quantitatively with the average size by the phonon confinement model. To get a good agreement with the model we add a band at around 480 cm−1, which has been attributed to amorphous silicon. We also have to leave as free parameters the bulk silicon phonon frequency and its line width, which depend on temperature and stress. We reduced laser power to eliminate heating effects. Then we use the change of frequency with depth to monitor the stress. At the interface with the substrate we find a compressive stress in excess of 10 kbar, which agrees with the reported lattice mismatch. Finally, average sizes are larger than those estimated from PL.

Optimized light emission from layered porous silicon structures

We discuss in detail the physics of light emission from porous silicon microcavities formed by periodically modulating the porosity to produce multilayered structures. Changing the porosity alters not only the refractive index and absorption but also the luminescence, resulting in a complex interplay of effects that has not yet been addressed in the literature as far as we know. A transfer matrix model is developed that accounts for the dispersion of the refractive index, absorption, and photoluminescence. A multilayer porous silicon mirror is found to emit light almost as well as a conventional distributed feedback microcavity system with a mid-stop-band resonant state.

Photoluminescence properties of alternating nanocrystalline silicon/amorphous silicon multilayers

Photoluminescence (PL) properties of alternating hydrogenated nanocrystalline silicon/amorphous silicon multilayers were investigated at room temperature and low temperature (77 K). The room-temperature PL spectra clearly show two peaks at about 1.54 and 1.7 eV. The ∼1.7 eV peak has no crystallite size dependence of the peak energy. The low-temperature PL spectra can be decomposed into three bands peaked at 1.53, 1.58 and 1.65 eV. The 1.53 eV band is related to the growth process of our samples. The 1.58 eV band is believed to be related to a-Si : H tissue. For the 1.65 eV band, spectral analysis indicates that the radiative recombination of photogenerated carriers in the near-surface region of nanocrystallites is responsible for the visible PL band.

Thin layers and multilayers of porous silicon: X-ray diffraction investigation

Porous silicon is a unique example of a porous material exhibiting the properties of a nearly perfect single crystal. High resolution x-ray diffraction has been used to investigate thin p− and p+ type porous silicon layers in the 100–1000-nm-thickness range. Since several thickness fringes are observed, the comparison between experimental results and simulations enables one to deduce information about the main structural parameters such as porosity, lattice parameter, thickness, and heterotransition width. Porous silicon multilayers have also been investigated: some satellites are clearly observed. The obtained results are then compared and discussed with the literature.

Observation of anomalously polarized photoluminescence in alternating nanocrystalline Si-amorphous Si multilayers

Polarization properties of visible photoluminescence (PL) from alternating hydrogenated nanocrystalline silicon-amorphous silicon multilayers excited with linearly polarized light are examined. The PL is almost linearly polarized in the direction perpendicular to the polarization of excitation light, independent of the sample orientation. The degree of linear polarization \ensuremath{\delta} negatively increases with decreasing emission energy, reaching a maximum of $\ensuremath{\sim}90%$ on the low-energy side, and a different temperature dependence is observed. In addition, it was found that the sample with large structural inhomogeneity displays low PL efficiency and large \ensuremath{\delta}. Since the obtained PL spectra show no crystallite size dependence of the peak energy and fine structures, we attribute these anomalous polarization properties to the radiative recombination of carriers confined in the interfacial region of optically anisotropic and randomly distributed Si:H nanocrystals.

Structural Analysis of Porous Silicon Multilayer using X-Ray Diffraction

A porous silicon monolayer (PSL) and four types of porous silicon multilayers (PSMLs) with 2-, 3-, 7-, and 50-layers were studied using the X-ray double crystal method. PSMLs were formed by controlling the anodization current density. Only three distinct peaks were observed in each rocking curve for these PSMLs, which showed two kinds of porous silicon layers with good crystallinity. It was also shown that the PSL and PSMLs were slightly different in the state of lattice strain. To observe the structure and depth-distribution of lattice strain of PSMLs, a new X-ray cross-sectional topographic technique (XCST) has been proposed. Applying XCST to a three-layer porous silicon consisting of 5 µm, 10 µm, and 5 µm layer thicknesses resulted in a clear X-ray cross-sectional topograph. From the topograph and the corresponding rocking curves, the precise distribution of the lattice strain was revealed. The XCST is useful for evaluating not only the PSML, but also general crystal multilayer structures such as superlattice hetero structures.