Si Stripes Research Articles

Luminescence behavior of semipolar (101¯1) InGaN/GaN “bow-tie” structures on patterned Si substrates

In this work, we report on the innovative growth of semipolar “bow-tie”-shaped GaN structures containing InGaN/GaN multiple quantum wells (MQWs) and their structural and luminescence characterization. We investigate the impact of growth on patterned (113) Si substrates, which results in the bow-tie cross section with upper surfaces having the (101¯1) orientation. Room temperature cathodoluminescence (CL) hyperspectral imaging reveals two types of extended defects: black spots appearing in intensity images of the GaN near band edge emission and dark lines running parallel in the direction of the Si stripes in MQW intensity images. Electron channeling contrast imaging (ECCI) identifies the black spots as threading dislocations propagating to the inclined (101¯1) surfaces. Line defects in ECCI, propagating in the [12¯10] direction parallel to the Si stripes, are attributed to misfit dislocations (MDs) introduced by glide in the basal (0001) planes at the interfaces of the MQW structure. Identification of these line defects as MDs within the MQWs is only possible because they are revealed as dark lines in the MQW CL intensity images, but not in the GaN intensity images. Low temperature CL spectra exhibit additional emission lines at energies below the GaN bound exciton emission line. These emission lines only appear at the edge or the center of the structures where two (0001) growth fronts meet and coalesce (join of the bow-tie). They are most likely related to basal-plane or prismatic stacking faults or partial dislocations at the GaN/Si interface and the coalescence region.

Guided cell adhesion, orientation, morphology and differentiation on silicon substrates photolithographically micropatterned with a cell-repellent cross-linked poly(vinyl alcohol) film

In this work, silicon substrates with poly(vinyl alcohol) (PVA) patterns created by a simple, low-cost and high-fidelity photolithographic procedure were evaluated with respect to cell adhesion and alignment, viability, metabolic activity, proliferation and cell cycle progression using the human glioblastoma cell-line U87MG and human skin fibroblasts. In addition, rat adrenal pheochromocytoma cells (PC-12) were employed to evaluate a modified photolithographic protocol appropriate for adhesion of cells requiring extracellular matrix components to adhere on the surface and to demonstrate that the proposed patterned substrates could provide unhindered cell differentiation. Regarding U87MG cells and skin fibroblasts, it was found that as the stripes width increased from 10 to 50 μm, the percentage of cells attached to Si versus the total area (Si + PVA) increased from 78% and 72% to 98.5% and 94.5% (p < 0.05), for U87MG cells and skin fibroblasts, respectively, with optimum cell alignment (≥95% of adherent cells with fidelity between 0.90 and 1.0; p < 0.05) for stripes width ranging between 20 and 22.5 μm. Concerning the viability, metabolic activity and proliferation of adherent cells, no statistically significant differences were observed compared to cells cultured onto non-patterned surfaces. Regarding PC-12 cells, a modification of the patterning procedure was followed involving coating of the substrate with type IV collagen prior to the photolithographic procedure, since they could not adhere on plain Si substrates. It was found that PC-12 cells adhere selectively (>95%) to collagen-coated Si stripes when the pattern width was equal to or wider than 10 μm. Following treatment with nerve growth factor, approximately 80% (p < 0.05) of the adherent cells differentiated to neuron-like cells extending neurites exclusively within the pattern. Given that the proposed patterning procedure allows highly selective cell adhesion without affecting cell proliferation, metabolic activity, and differentiation it could serve as a useful tool in various fields including tissue engineering, cell-based sensors and analytical microsystems.

Experimental investigation on the mechanical buckling of one-dimensional Si nanoribbons with a thickness contrast

The mechanical buckling of one-dimensional Si nanoribbons with a thickness contrast was investigated. For Si stripes with a small thickness difference, each section was found to undergo its own buckling with characteristic wavelengths and wave amplitudes, depending on their thicknesses. On the other hand, unusual buckling occurred for Si ribbons with large thickness differences. For this case, the thinned sections were uniformly buckled, and absorbed a large portion of externally applied strain. On the contrary, the thicker parts of the ribbon were buckled nonuniformly, i.e. buckling occurred in central regions only, with an almost flat configuration at both ends of the thicker parts. Buckling mechanics can qualitatively explain the observed buckling of the Si ribbons with a small thickness contrast, while the finite length effect should be considered for the case of the Si ribbons with a large thickness contrast. Considering the fact that real functional devices are not uniform in thickness, the present work helps to understand the complex buckling behavior of mechanical systems with a wide range of thickness contrasts.

Thermal-Responsive Anisotropic Wetting Microstructures for Manipulation of Fluids in Microfluidics.

We show morphology-patterned stripes modified by thermal-responsive polymer for smartly guiding flow motion of fluid in chips. With a two-step modification process, we fabricated PNIPAAm-modified Si stripes on silicon slides, which were employed as substrates for fluid manipulation in microchannels. When the system temperature switches between above and below the lower critical solution temperature (LCST) of PNIPAAm, the wettability of the substrates also switches between strong anisotropy and weak anisotropy, which resulted in anisotropic (even unidirectional) flow and isotropic flow behavior of liquid in microchannels. The thermal-responsive flow motion of fluid in the chip is influenced by the applied pressure, the thickness of PNIPAAm, and dimension of the microchannels. Moreover, we measured the feasible applied pressure scopes under different structure factors. Because of the excellent reversibility and quick switching speed, the chip could be used as a thermal-responsive microvalve. Through tuning the system temperature and adding the assistant gas, we realized successive "valve" function. We believe that the practical and simple chip could be widely utilized in medical detection, immunodetection, protein analysis, and cell cultures.

Efficient and broadband Terahertz plasmonic absorbers using highly doped Si as the plasmonic material

The property of highly doped Si as the plasmonic material in the THz regime is analyzed, based on which the design of efficient and broadband Terahertz plasmonic absorbers is proposed and the performance of these absorbers is numerically investigated. Numerical results from the reflection spectra demonstrate that these structures exhibit high absorption in the terahertz frequencies with large bandwidth and high tunability. It is also shown that the same level of absorptivity and bandwidth can be achieved when the top layer of highly dopes Si stripes are replaced with regular metal materials e.g. copper, highly facilitating the fabrication and practical use of the proposed structure in real Terahertz applications.

Spatial Distribution of Photocurrent in Si Stripes under Tilted Illumination Measured by Multimode Scanning Probe Microscopy

Spatial distribution of photocurrent in Si stripes of 50–1000 nm in width were investigated as a function of optical excitation wavelength by multimode scanning probe microscopy (MSPM). Inhomogeneous distribution of the MSPM photocurrent in the Si stripe interior was attributed to the light intensity profile. A model that included light absorption depth profile and the probe-induced band-bending region reproduced the photocurrent profile for tilted illumination of the stripes. An effective spatial resolution of ∼10 nm was deduced from the photocurrent measurements with multimode SPM in the constant-force mode.

Spatial Distribution of Photocurrent in Si Stripes under Tilted Illumination Measured by Multimode Scanning Probe Microscopy

Spatial Distribution of Photocurrent in Si Stripes under Tilted Illumination Measured by Multimode Scanning Probe Microscopy

Fabrication of silica mesh patterns via self-assembly of block copolymers

In this study, we developed a simple way to fabricate well-ordered nanoporous structures in square arrays, or mesh patterns. Inorganic silica mesh patterns in nanometer scale were generated by using self-assembly of block copolymers (BCPs). Polystyrene-block-poly(2-vinylpyridine) copolymers (S2VP) were deposited on a silicon wafer by the spin-coating method. Well-ordered cylindrical microdomains with parallel orientation to the surface were obtained after solvent-annealing in tetrahydrofuran (THF) vapor. Immersion of thin films into ethanol induced the reconstruction of their surface and caused trenches, which were porous structures. Polydimethylsiloxane (PDMS) was coated on the film and then exposed to UV-ozone to generate silica stripes from PDMS. As the previous steps were repeated, it was found by scanning probe microscopy (SPM) measurements that cylindrical microdomains were preferentially directed orthogonal to the ridge of Si stripes due to the wetting behavior of (BCPs). Finally, silica mesh patterns were fabricated from this distinct orientation of cylindrical microdomains on underlying Si stripes.

Aging of porous silicon in physiological conditions: Cell adhesion modes on scaled 1D micropatterns

The surface properties of porous silicon (PSi) evolve rapidly in phosphate-buffered saline. X-ray photoelectron spectra indicate the formation of a Si-OH and C-O enriched surface, which becomes increasingly hydrophilic with aging time. Multiscale stripe micropatterns of Si and PSi have been fabricated by means of a high-energy ion-beam irradiation process. These micropatterns have been aged in physiological conditions and used to analyze human mesenchymal stem cell (hMSC) adhesion. The actin cytoskeleton of hMSCs orients following the uniaxial micropatterns. In the wider Si stripes, hMSCs are dominantly located on Si areas. However, for reduced Si widths, adhesion is avoided on PSi by a split assembly of the actin cytoskeleton on two parallel Si areas. These results confirm that nanostructured Si-OH/C-O-rich surfaces with hydrophilic character are specially adapted for the creation of cell adhesion surface contrasts.

The molecular dynamics simulation of ion-induced ripple growth

The wavelength-dependence of ion-sputtering induced growth of repetitive nanostructures, such as ripples has been studied by molecular dynamics (MD) simulations in Si. The early stage of the ion erosion driven development of ripples has been simulated on prepatterned Si stripes with a wavy surface. The time evolution of the height function and amplitude of the sinusoidal surface profile has been followed by simulated ion-sputtering. According to Bradley-Harper (BH) theory, we expect correlation between the wavelength of ripples and the stability of them. However, we find that in the small ripple wavelength (lambda) regime BH theory fails to reproduce the results obtained by molecular dynamics. We find that at short wavelengths (lambda<35 nm) the adatom yield drops hence no surface diffusion takes place which is sufficient for ripple growth. The MD simulations predict that the growth of ripples with lambda>35 nm is stabilized in accordance with the available experimental results. According to the simulations, few hundreds of ion impacts in lambda long and few nanometers wide Si ripples are sufficient for reaching saturation in surface growth for for lambda>35 nm ripples. In another words, ripples in the long wavelength limit seems to be stable against ion-sputtering. A qualitative comparison of our simulation results with recent experimental data on nanopatterning under irradiation is attempted.

Enhancement of second-harmonic generation from silicon stripes under external cylindrical strain

The enhanced second-harmonic (SH) generation from Si (111) stripes induced by external cylindrical strain is investigated. The dependence of the intensity of the strain-induced SH on the sample azimuth shows that the Si under cylindrical strain has 3m symmetry, which is similar to that of the Si (111) surface. Further studies indicate that the intensity of the enhanced SH is a quadratic function of the cylindrical strain within the magnitude that the Si stripe can bear. For the p-polarized and s-polarized SH, the intensities are, respectively, enhanced by 47.9% and 13% at epsilon(0)=2.93x10(-4). The enhancement of SH is due to the contributions from the strain-induced second-order nonlinear susceptibility chi(strain)(2) to the bulk dipole.

Study of stress in a shallow-trench-isolated Si structure using polarized confocal near-UV Raman microscopy of its cross section

Stress in shallow-trench-isolated structures with 250-nm-wide Si stripes was examined by 364nm excitation confocal Raman microscopy. Using objective lenses with the experimentally measured ∼150nm spatial resolution, we studied polarized Raman spectra from the section perpendicular to the stripes. Detection of two optical phonons enabled us to analyze orientation and intensity of local stress quantitatively. Uniaxial [001] compressive stress was found in the Si substrate under both the trench and the stripe, while the stress in the stripe was uniaxial [110] perpendicular to the stripe.

Surface modification of an amorphous Si thin film crystallized by a linearly polarized Nd:YAG pulse laser beam

Amorphous Si films of 60 and 10nm thick on glass substrates were irradiated by a linearly polarized Nd:YAG pulse laser with the wavelength λ=532nm at the incident angle θi=0. The surface of the irradiated 60-nm-thick film had both periodic ridges perpendicular to the electric field vector E and aperiodic ridges roughly parallel to E, where the spatial period of the periodic ridges was almost λ. From the continuous 10-nm-thick film, the separate rectangular Si islands were formed with a periodic distance of λ, with the edges parallel or perpendicular to E. When θi was increased from normal incidence of the s-polarized beam for a 60-nm-thick film, the aperiodic ridges were reduced while the periodic ridges were still formed. For a 10-nm-thick film, the Si stripes were formed perpendicular to E, using the s-polarized beam at θi=12°. In order to investigate the mechanisms of the surface modifications of, in particular, aperiodic ridges, islands, and stripes, we improved the previous theoretical model of the periodic distribution of the beam energy density (periodic E-D) generated by irradiation of the linearly polarized laser beam, taking account of the multireflection effect in the Si film which is semitransparent for λ. Further, the calculated E-D was corrected with respect to the thermal diffusion in the irradiated Si film. The calculation results show that the two-dimensional E-D consists of a constant or a dc term and a sinusoidal or an ac term which contains various spatial periods. The multireflection effect strongly influences the amplitude and phase of every ac term, which means that the amplitude and phase depend on the film thickness. The thermal diffusion during the heating of the irradiated film greatly reduces the amplitudes of the ac terms with periods below the thermal diffusion length. The theoretical calculation showed that, by increasing θi, the temperature distribution in the irradiated Si film was changed from two-dimensional toward one-dimensional, which can explain the above experimental results reasonably.

High-spatial-resolution Raman microscopy of stress in shallow-trench-isolated Si structures

Stress in single and periodic shallow-trench-isolated Si structures was examined by 364nm excitation confocal resonance Raman microscopy, laser penetration being restricted to the near-surface region. Using a 1.3 numerical aperture microobjective lens with a theoretical ∼140nm spatial resolution, the authors show that the configuration with both incident and scattered lights polarized parallel to each other and perpendicular to Si stripes is favorable for stress detection in the middle of the stripes, suppressing contributions from their edges. The stresses located in different areas of the structures were identified and analyzed.

Electronic steps and band offset ofSi−Getwo-dimensional superlattices onBi∕Si(111)

The electronic and structural properties of the Bi-covered $\mathrm{Si}\ensuremath{-}\mathrm{Ge}$ two-dimensional superlattices (SLs) have been investigated employing ab initio total energy calculations. The $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SLs are formed by Ge stripes with a height of one atomic layer, $\ensuremath{\sim}0.3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, separated by Si layers on the Si(111) surface covered with a monolayer of Bi trimers. At the equilibrium geometry we find that the Bi trimers on $\mathrm{Ge}∕\mathrm{Si}(111)$ are higher by $0.10\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ compared with the Bi trimers on the $\mathrm{Si}∕\mathrm{Si}(111)$ region. In both regions the Bi trimers lie on the $T4$ sites. Our simulated scanning tunneling microscopy images exhibit bias-dependent ``electronic steps,'' which are in accordance with recent experimental findings [Phys. Rev. Lett. 91, 096102 (2003)]. These bias-dependent electronic steps, which can reach up to $0.8\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, are due to the different electronic density of states of the Bi trimers adsorbed on the Ge stripes compared with the Bi trimers on the Si stripes. Since the Si and Ge regions of the SL are electronically distinct, we compared the electronic structures of the isolated systems, $\mathrm{Bi}∕\mathrm{Si}∕\mathrm{Si}(111)$ and $\mathrm{Bi}∕\mathrm{Ge}∕\mathrm{Si}(111)$, with the one of the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SL. We find a valence band offset of $0.21\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for the $\mathrm{Si}\text{\ensuremath{-}}\mathrm{Ge}$ SL, with the highest occupied states lying on the Ge layers. In contrast, for the lowest unoccupied states we find an almost negligible conduction band offset.

Highly sensitive strain detection in silicon by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) measurements were performed on strained silicon (Si) stripes cut from crystalline silicon wafers. Strains were externally applied using a device developed especially for the study of layers and layered structures. The dependence of the RAS signal intensity on strain was measured for (100), (110), and (111) silicon wafers strained along [001] and [011]. In these configurations, the RAS spectra show a derivativelike structure at 3.4 eV, which increases in amplitude linearly with strain. While RAS line shapes depend on the orientation of the Si wafer and the crystallographic directions along which strains are applied, RAS intensities depend on strain magnitude. Strains as low as ${10}^{\ensuremath{-}5}$ can be measured, which is two orders of magnitude smaller than those detected with standard techniques such as Raman, piezoelectroreflectance spectroscopy (PERS), or x-ray diffraction (XRD). The experimental RAS spectra are found to be in good agreement with spectra calculated on the basis of the spectral response of the published piezo-optical tensor components. It is concluded that RAS provides a highly sensitive tool for the detection of strain induced bulk anisotropies. Strain calibrated RAS spectra can be used for strain-stress characterization of semiconductor layers and microstructures with a higher efficiency than that achieved by Raman, PERS, and XRD. Combined with growth techniques, RAS spectroscopy can be also used for in situ control of strain during semiconductor growth.

Improvement in epitaxial quality of selectively grown Si 1−xGe x layers with low pattern sensitivity for CMOS applications

The selective growth of Si 1− x Ge x structures ( 0.09 < x < 0.29 ) on patterned substrates aimed for channel layer application in metal oxide semiconductor field effect transistor (MOSFET) structures by using chemical vapor deposition (CVD) has been investigated. By optimizing the growth parameters, the surface roughness of these structures was improved and layers with better epitaxial quality was obtained. Furthermore, two methods are proposed to decrease the pattern-dependency of the epitaxy process. The key point in these methods is based on the reduction of surface diffusion of the adsorbed species on the oxide. This can be obtained by introducing square polycrystalline Si stripes around the oxide openings acting as diffusion barriers. Subsequently, chemical mechanical polishing was performed to remove the polycrystalline stripe on the oxide. Evaluation of the proposed solutions for issues such as defect generation and the possibility for integration in device structures have also been performed.

Methods to reduce the loading effect in selective and non-selective epitaxial growth of sigec layers

Various methods to reduce both global and local loading effect during non-selective and selective epitaxial growth of Si1−x−yGexCy (0.09≤x≤0.28 and 0≤y≤0.01) layers have been proposed. Evaluation of the proposed solutions for issues such as defect generation and the possibility for integration in device structures have been performed. The key point in these methods is based on reduction of surface diffusion of the adsorbed species on the oxide. In non-selective epitaxy, this was achieved by introducing a thin silicon polycrystalline seed layer on the oxide prior to Si1−x−yGexCy deposition. The thickness of this seed layer had a crucial role on both the global and local loading effect, and also on the epitaxial quality. Higher carbon content (y≥0.006) in Si1−x−yGexCy layers had no noticeable influence on the loading effect, however, the defect density was clearly increased in these layers. In selective epitaxy case, introducing square polycrystalline Si stripes around the oxide openings acting as diffusion barriers have reduced the loading effect effectively. Meanwhile, using Si nitride stripes showed no visible effect on Si1−x−yGexCy layer profile. Further decrease in loading effect can be performed by increasing the HCl partial pressure during epitaxy. Chemical–mechanical polishing (CMP) was performed to remove the polycrystalline stripe on the oxide.

Positioning of self-assembled Ge islands on stripe-patterned Si(001) substrates

Self-assembled Ge islands were grown on stripe-patterned Si(001) substrates by solid source molecular beam epitaxy. The surface morphology obtained by atomic force microscopy and cross-sectional transmission electron microscopy images shows that the Ge islands are preferentially grown at the sidewalls of pure Si stripes along the [−110] direction at 650 °C or along the trenches, whereas most of the Ge islands are formed on the top terrace when the patterned stripes are covered by a strained GeSi buffer layer. Reducing the growth temperature to 600 °C results in a nucleation of Ge islands both on the top terrace and at the sidewall of pure Si stripes. A qualitative analysis, based on the growth kinetics, demonstrates that the step structure of the stripes, the external strain field, and the local critical wetting layer thickness for the islands formation contribute to the preferential positioning of Ge islands on the stripes.

Ge island formation on stripe-patterned Si(001) substrates

Self-assembled Ge islands were grown by solid-source molecular-beam epitaxy on the submicron stripe-patterned Si(001) substrates at 650 °C. Atomic-force microscopy shows that the Ge islands grow preferentially at the sidewall of the Si stripes, oriented along the [−110] direction. The migration of the Ge adatoms from the top terrace down to the sidewall accounts for the island formation at the sidewall of the stripes. However, most of the Ge islands are formed on the top terraces when the patterned stripes are covered by a strained GeSi multilayer buffer prior to Ge island growth. Apparently, the strained buffer layer acts as a stressor and contributes to the preferential growth of islands on the top terrace.