Thin Si Layer Research Articles

Effectiveness of Si Seed for Selective SiGe Epitaxial Deposition in Recessed Source and Drain for Locally Strained pMOS Application

A thin layer of Si seed was employed to help nucleate low- temperature selective SiGe epitaxial film in recessed source and drain areas. In combination with pre-epi wet clean and low- temperature chemical bake, use of Si seed resulted in improved SiGe film morphology and micro-loading effect, and further improved dislocation on the lateral recessed interface.

Nucleation and Movement of Dislocations during Relaxation of He Implanted SixGe1-x/Six Heterostructures

The fabrication of strain relaxed SixGe1-xbuffer layers is technologically of great importance, because biaxial, tensile strain can be achieved within a thin Si layer grown onto these SixGe1-x virtual substrates. Strain relaxation of the SixGe1-x layer can be accomplished by He implantation and subsequent annealing. In this paper we address the nucleation and movement of dislocations in He implanted SixGe1-x/Si heterostructures. By in-situ transmission electron microscopy we find clear evidence, that He precipitates formed during annealing act as internal dislocation sources and therefore promote the relaxation. This can be understood in view of the results of quantitative evaluation of the shear stress around He filled precipitates. Clearly, for some of the precipitates, the critical resolved shear stress for dislocation formation is exceeded at the edge of the platelets.

Fabrication of dislocation-free tensile strained Si thin films using controllably oxidized porous Si substrates

A method to fabricate strained Si films is reported via the oxidation of a thin Si film on a porous Si substrate. The Si film can be put under tensile stress in a controllable fashion through the expansion of the porous Si upon low temperature oxidation. The thin Si layer on porous Si substrate can be fabricated using a self-limiting anodization of epitaxially grown intrinsic Si on a heavily doped p-type Si substrate. Tensile strain of up to ∼1% is observed in 100nm thick Si films, making it suitable for the various device applications based on strained Si.

Design of all-optical switches based on carrier injection in Si/SiO/sub 2/ split-ridge waveguides (SRWs)

All-optical switches based on optical carrier injection in high-index-contrast Si/SiO/sub 2/ split-ridge-waveguide (SRW) couplers are analyzed. The waveguide devices are suitable for the construction of low-loss optical switch matrices as well as fast optical switching. These devices exhibit robustness against fabrication tolerances, improved heat sinking, good carrier confinement, and high uniformity in transmission over the entire C-band of optical communications, in contrast to comparable devices based on buried or ridge waveguides. Reasonably low electrical switching power of 1-10 mW is predicted for switching frequencies of 1 MHz to 1 GHz. Carrier recombination measurements in thin Si layers passivated with different oxide layers confirm the feasibility of the designed switches and modulators.

Structure and properties ofTiN(111)∕SixNy∕TiN(111)interfaces in superhard nanocomposites: First-principles investigations

We perform first-principles calculations based on density-functional theory to investigate the role of interfaces in superhard nanocomposites. A prerequisite is clearly knowledge of the detailed atomic structure, which is addressed in the present paper. In particular, we study the relative stability of $\mathrm{TiN}(111)∕{\mathrm{Si}}_{x}{\mathrm{N}}_{\mathrm{y}}∕\mathrm{TiN}(111)$ interfaces, which form in the highly thermally stable $nc\text{\ensuremath{-}}\mathrm{TiN}∕a\text{\ensuremath{-}}{\mathrm{Si}}_{x}{\mathrm{N}}_{y}$ nanocomposites. For nitrogen-rich conditions, the most favorable configurations involve very thin layers of Si, which are purely nitrogen coordinated and tetrahedrally bonded. For increasingly nitrogen-poor conditions, interfaces involving $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{Si}\text{\ensuremath{-}}\mathrm{N}$, and predominantly octahedral $\mathrm{Ti}\text{\ensuremath{-}}\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ti}$, bonding are preferred. The atomic geometry and associated electronic structure are discussed for these interfaces, as well as properties of the bulk $\ensuremath{\alpha}$, $\ensuremath{\beta}$, and $\ensuremath{\gamma}$ phases of ${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, $\mathrm{TiN}$, and ${\mathrm{TiSi}}_{2}$.

Solid phase crystallization of amorphous Fe–Si layers synthesized by ion implantation

Microstructural changes of ion-beam-synthesized amorphous Fe–Si layers on thermal annealing were investigated using transmission electron microscopy. Single crystal Si(111) substrates were irradiated with 120keV Fe+ ions at cryogenic temperature to a fluence of 4.0×1017cm−2, followed by thermal annealing at 200–700°C. The amorphous Fe–Si layer in the as-implanted sample crystallized to polycrystalline ε-FeSi and β-FeSi2 layers after annealing at 500°C for 2h.ε-FeSi transformed into β-FeSi2 and the β-FeSi2 region extended with increasing annealing temperature. Excess Fe atoms from ε-to-β phase transformation migrate toward the Si substrate via β-FeSi2 grain boundaries. We discuss the recrystallization process of amorphous Fe–Si thin layers and the growth mechanism of β-FeSi2 thin layers formed in high-dose Fe ion-implanted Si.

Structural characterization of amorphous Fe–Si and its recrystallized layers

We have synthesized amorphous Fe–Si thin layers and investigated their microstructure using transmission electron microscopy (TEM). Si single crystals with (111) orientation were irradiated with 120keV Fe+ ions to a fluence of 4.0×1017cm−2 at cryogenic temperature (120K), followed by thermal annealing at 1073K for 2h. A continuous amorphous layer with a bilayered structure was formed on the topmost layer of the Si substrate in the as-implanted specimen: the upper layer was an amorphous Fe–Si, while the lower one was an amorphous Si. After annealing, the amorphous bilayer crystallized into a continuous β-FeSi2 thin layer.

High Sensitive Imaging of Atomic Arrangement of Ge Clusters Buried in a Si Crystal by X-ray Fluorescence Holography

We have performed high sensitive imaging of the atomic arrangement of Ge buried in a Si crystal as Ge clusters using the internal detector scheme of X-ray fluorescence holography. The atomic arrangement of Ge buried in a Si crystal is expected to have the strained diamond structure. We have measured holograms of a thin Si/Ge multilayer film composed of thin Si layers and Stranski–Krastanov Ge islands using a crystal analyzer in combination with synchrotron radiation. The atomic arrangement reconstructed from the holograms shows that the Ge islands are indeed of the diamond structure. Comparison of atomic arrangements reconstructed from experimental and simulated holograms allows us to determine the average lattice constant of the Ge islands. The lattice constant obtained shows that the Ge islands are partially relaxed in Si matrix lattice.

Determination of band gap in polycrystalline Si/Ge thin film multilayers

The valence band (VB) photoemission supported by ultraviolet–visible–near infrared spectroscopy techniques were used to determine the band gap values of polycrystalline Si and Ge single layers as well as of Si/Ge multilayer structures. The band gap values obtained from VB photoemission measurements for these structures were found to be much larger than their corresponding bulks and to match well with those determined from standard optical absorption measurements. In each case, the VB offset values were obtained by considering the corresponding VB maximum as a reference. The increase in band gap in case of thin single layers of Si and Ge with respect to bulks were interpreted in terms of quantum confinement effect, while in case of multilayer sample, the effect of various factors such as (i) intermixing leading to the formation of SiGe alloy, (ii) roughness at the interface, (iii) particle size, and (iv) strain seem to play an important role in the observed change in band gap.

Effects of silicon concentration in SOFC alloy interconnects on the formation of oxide scales in hydrocarbon fuels

Oxide scale formations on Fe Cr alloy interconnects were investigated in anode gas (mixtures of CH 4 and H 2O) atmospheres for solid oxide fuel cells. The silicon concentration in Fe Cr alloy changed the microstructures of oxide scales, elemental distribution and oxide scale growth rates. Oxide scale is composed of the following phases from surface to inner oxides: Fe Mn spinel, Cr 2O 3, oxide scale/alloy interface and internal oxides of Si and Al. With decreasing the Si concentration from 0.4 to 0.01 mass%, formation of thin Si and Mn layer was observed inside the Fe Cr alloy. Oxide scale growth rate constants decreased by lowering the Si concentration in Fe Cr alloy from 4.2 × 10 −18 to 2.1 × 10 −18 m 2 s −1 at 1073 K. Diffusivity of Fe and Cr was changed by the concentration of Si in Fe Cr alloy, which affects the growth rates of oxide scale. The electrical conductivity of oxidized Fe Cr alloy shows almost same level regardless the Si concentration (in the orders of 10 S cm −2 at 1073 K).

Rapid thermal annealing of hot wire chemical-vapor-deposited a-Si:H films: The effect of the film hydrogen content on the crystallization kinetics, surface morphology, and grain growth

The ability to crystallize thin amorphous Si layers into large grain Si can lead to significant improvements in Si solar cells and thin-film transistors. Here we report on the effect of the hydrogen content in as-grown films on the crystallization kinetics, surface morphology, and grain growth for hot wire chemical-vapor-deposited a-Si:H films crystallized by rapid thermal annealing (RTA). At RTA temperatures >750°C for high-hydrogen-content films, we observe the explosive evolution of hydrogen, with a resultant destruction of the film. Little or no damage is observed for films containing low hydrogen content. At a lower RTA temperature (600°C), the films remain intact with similar morphologies. At this same lower RTA temperature, both the incubation time and crystallization time decrease, and the grain size as measured by x-ray diffraction increases with decreasing hydrogen film content. Measurements of the crystallization time versus H evolution time indicate that the vast majority of the hydrogen must evolve from both films before crystallization commences. To examine the relationship between hydrogen evolution and crystallization, a two-step annealing process was utilized. For the high hydrogen content films, the final grain size increases if a large portion of the hydrogen is driven out at temperatures well below the crystallization temperature.

Self-organization of semiconductor nanocrystals by selective surface faceting

The formation and ordering of Si nanocrystals in dewetting and agglomeration of the thin single crystalline Si layer in silicon-on-insulator has been investigated using low-energy electron microscopy. The evolution of the Si dewetting and agglomeration is captured in real time, revealing the detailed process for the formation and ordering of the nanocrystals. A surface faceting mechanism governing this self-organized process is proposed and supported with first-principles calculations.

Transmission electron microscopy study on ion-beam-synthesized amorphous Fe–Si thin layers

Ion-beam-synthesized amorphous Fe–Si thin layers have been characterized using transmission electron microscopy (TEM) in combination with imaging plate techniques. Si single crystals with a (111) orientation were irradiated with 120keV Fe+ ions to a fluence of 4.0×1017cm−2 at cryogenic temperature (120K). Cross-sectional TEM observations indicated the formation of an amorphous bilayer on the topmost layer of the Si substrate. It was found that the upper layer is an amorphous Fe–Si with the composition, in terms of atomic ratio, of Fe∕Si ∼1∕2, while the lower one is an amorphous Si. Atomic pair-distribution functions extracted from microbeam electron diffraction patterns revealed that the nature of short-range order in amorphous Fe–Si thin layer can be well described by the atomic arrangements of crystalline iron silicides.

Visible light emission from macroporous Si

Macroporous silicon structures have been fabricated by electrochemical etching. Such fabrication process is known to result in the presence of a thin microporous Si layer at the walls of the macropores and at the surface. Photoluminescence measurements conducted in plan-view and cross-section exhibit a wide emission peak around 650 nm which can be attributed to the microporous Si. The combination of a photonic crystal and a light emitter in one structure represents a potential for applications that has not been studied previously. We initiate this study by investigating the influence of the main fabrication parameters, namely the current density and the etchant solution, on the emission properties of the microporous Si layer.

Coupling atomistic and continuum length scales in heteroepitaxial systems: Multiscale molecular-dynamics/finite-element simulations of strain relaxation inSi∕Si3N4nanopixels

A hybrid atomistic-continuum simulation approach has been implemented to study strain relaxation in lattice-mismatched Si/ Si3N4 nanopixels on a Si111 substrate. We couple the molecular-dynamics MD and finite-element simulation approaches to provide an atomistic description near the interface and a continuum description deep into the substrate, increasing the accessible length scales and greatly reducing the computational cost. The results of the hybrid simulation are validated against full multimillion-atom MD simulations. We find that strain relaxation in Si/ Si3N4 nanopixels may occur through the formation of a network of interfacial domain boundaries reminiscent of interfacial misfit dislocations. They result from the nucleation of domains of different interfacial bonding at the free edges and corners of the nanopixel, and subsequent to their creation they propagate inwards. We follow the motion of the domain boundaries and estimate a propagation speed of about 2.5 10 3 m / s. The effects of temperature, nanopixel architecture, and film structure on strain relaxation are also investigated. We find: i elevated temperature increases the interfacial domain nucleation rates; ii a thin compliant Si layer between the film and the substrate plays a beneficial role in partially suppressing strain relaxation; and iii additional control over the interface morphology may be achieved by varying the film structure.

Plasma hydrogenation of strained Si∕SiGe∕Si heterostructure for layer transfer without ion implantation

We have developed an innovative approach without the use of ion implantation to transfer a high-quality thin Si layer for the fabrication of silicon-on-insulator wafers. The technique uses a buried strained SiGe layer, a few nanometers in thickness, to provide H trapping centers. In conjunction with H plasma hydrogenation, lift-off of the top Si layer can be realized with cleavage occurring at the depth of the strained SiGe layer. This technique avoids irradiation damage within the top Si layer that typically results from ion implantation used to create H trapping regions in the conventional ion-cut method. We explain the strain-facilitated layer transfer as being due to preferential vacancy aggregation within the strained layer and subsequent trapping of hydrogen, which lead to cracking in a well controlled manner.

Optical and morphological properties of SiN x /Si amorphous multilayer structures grown by Plasma Enhanced Chemical Vapor Deposition

Very thin layers of Si were grown in between silicon nitride layers using Plasma Enhanced Chemical Vapor Deposition (PECVD) technique and SiH2Cl2/H2/NH3 mixtures. Deposition conditions were selected to favor Si cluster formation. Room Temperature Photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical and structural properties of the films. Scanning Electron Microscopy (SEM) of the cross section of cleaved samples allowed to observe a clear pattern of Si clusters embedded in the SiN matrix. The UV-VIS absorption spectra present two band edges. We assume that the higher band gap is due to the amorphous Si clusters. RT-PL spectra are characterized by two broad bands: one centered at 1.5 eV and the other at 2.1 eV. The broad luminescence centered at 2.1 eV could be associated with the higher band gap observed in absorption spectrum. After vacuum annealing of the samples at 400 ºC, the band at 2.1 eV disappears. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SiC epitaxial growth on Si(0 0 1) substrates using a BP buffer layer

Decreased stacking fault densities of 3C-SiC films have been demonstrated by the CVD method using a BP buffer layer on Si(001) substrates. The BP layer effects a lattice relaxation between the SiC and Si. Although the thermal decomposition temperatures of BP are lower than the SiC growth temperatures, the SiC epitaxial growth on a BP buffer layer was achieved at a temperature of 1150°C through a carbonization process of a thin Si layer grown on a BP film. The BP surface morphologies were affected by the film thickness, and the SiC morphologies were also seriously affected by the BP surface morphologies. Extremely smooth SiC surfaces were obtained by using an optimized thickness of a BP layer. However, several domains still remained within the films, and they appeared to be anti-phase domains.

Enhanced optical absorption in a thin silicon layer with nanovoids

A detailed theory for enhanced optical absorption in thin silicon with a distribution ofnanovoids has been worked out in this paper. It is demonstrated that significantenhancement of the effective optical absorption coefficient (by a factor of about two tomore than four) in a thin Si layer can be achieved by optimizing the dimensions anddistribution of nanovoids. In this work, the absorption in a thin Si layer has been modelledtaking into account the diffraction of light by the nanocrystallites between the voids as wellas the scattering of light by the voids. This modelling is supposed to be applicable to anysemiconductor film having a distribution of nanovoids since the modelling incorporatesscattering phenomena due to Rayleigh scattering for small voids and the gradual transitionfrom Rayleigh scattering to diffraction phenomena in the case of large voids includingmultiple-and back-scattering effects. The consideration of the diffraction of lightinstead of Mie scattering greatly simplifies the calculation and still predicts thecorrect behaviour of absorption phenomena in such films. The simulated resultsobtained using this modelling agree excellently with Brendel’s recently reportedexperimental results. This enhancement of the optical absorptance in a thin Si film withnanovoids has potential application in different devices, e.g. thin Si solar cells.The realization of nanovoids can be achieved by high temperature annealing ofdouble-layer porous silicon, i.e. a quasi-monocrystalline porous silicon (QMPS)layer.

Properties of hydrogen induced voids in silicon

After heat treatment, silicon samples implanted with high doses of hydrogenexhibit blistering and defoliation of thin silicon layers. The process is usedcommercially in the fabrication of thin silicon-on-insulator layers (SmartCut®). In the present study we investigate the behaviour of hydrogen after different processingsteps, which lead to thin Si layers bonded to glass substrates. A set of hydrogenimplanted samples is studied by means of low temperature photoluminescence, Ramanspectroscopy, x-ray diffraction and optical microscopy (visible and infrared). Theformation of Si–H bonds is detected after implantation together with a build-up ofinternal strain. After annealing, the relaxation of the implanted layers is found to beconnected with the formation of hydrogen saturated vacancies and the formation ofH2 molecules filling up larger voids. A comparison is made with hydrogen plasma treatedsamples, where well defined platelets on {111} planes are found to trap hydrogen molecules.No direct evidence of the role of {111} and {100} platelets in the blistering process is found inthe implanted layers from our study. We determine considerable compressive stressesin the bonded Si layers on glass substrates. The photoluminescence is stronglyenhanced in these bonded layers but red-shifted due to a strain reduced band gap.