Silicon Surface Reconstruction Research Articles

Monolithic perovskite/black-silicon tandems based on tunnel oxide passivated contacts

<h2>Summary</h2> Perovskite/silicon tandems offer a promising pathway to increase the efficiency beyond the Auger limit of silicon at minimal additional cost. However, it is challenging to accommodate the solution-processed perovskite without flattening or reducing pyramidal texture of silicon subcells. Herein, we report the first monolithic perovskite/silicon tandem featuring an industrially applicable front-side-nanostructured black silicon with a tunnel oxide passivated contact (TOPCon). The TOPCon together with the surface reconstruction of black silicon contributes to the high-level surface passivation without sacrificing the broadband light trapping. Additionally, the reconstructed nanotexture significantly facilitates the wetting of perovskite and acts as a nanoconfining scaffold to guide the vertical growth of perovskite. The resulting tandem yields a certified efficiency of 28.2%, representing one of the highest efficiencies reported on either perovskite/TOPCon or double-side-textured perovskite/silicon tandems.

Improvement Silicon Nanostructured Surface by Laser Induced Etching Process

Surface reconstruction of silicon using lasers could be utilized to produce silicon nanostructures of various features. Electrochemical and photoelectrochemical etching processes of silicon were employed to synthesize nanostructured surface. Effects of current densities 5, 10 and 20 mA/cm2 on the surface features were examined. It is found that the surface porosity and layer thickness increase with the current density. Moreover, large surface area of 410 m2/cm3 can be achieved when laser power density 0f 0.6 W/cm2 was used during the etching process. Optimum operating conditions were found to achieve better silicon nanostructured surface features. The surface roughness can be reduced to 8.3 nm using laser beam of 650 nm irradiated the silicon surface during the photoelectrochemical etching process. The surface morphology of the nanostructured silicon surface using SEM and AFM could give rich details about the surface. Silver nanoparticles of 10 – 20 nm was embedded at the nanostructured silicon surface by LIFT process to reduce the surface resistance and maintain the large surface area. This technique enables silicon nanostructures to be efficiently used in many optoelectronic applications.

Excitation spectra of Bi/Si(001) interfaces

Present article deals with the theoretical and experimental investigation of optical properties of Bi/Si(001) interfaces in a wide spectral range (1–4 eV). Theoretical excitation spectra of vicinal Si(001) surface, Bi/Si(001)–0.5 monolayer and Bi/Si(001)–1 monolayer interfaces are calculated by time‐dependent density functional theory with the hybrid Becke and Lee, Yang and Parr pseudopotential. Calculations and experimental studies of the absorption spectrum of the samples revealed the blue shift of the optical absorption edge with increasing of the bismuth covering degree. Features of the experimentally obtained reflectance anisotropy spectra (RAS) and surface differential reflectance spectra (SDRS) concerned with changing of the silicon surface reconstruction from 2 × 1 to 1 × 1 with the increasing of the bismuth covering degree from 0.5 to 1 monolayer. It is shown that the correlation between the theoretically calculated and experimental data makes it possible to identify the electronic transitions in the surface region.

シリコン表面超構造における超伝導輸送現象

Silicon surface reconstructions with metal adatoms are two-dimensional systems with atomic-scale thickness. We have successfully measured the superconducting transition of a representative silicon surface reconstruction Si(111)-(√7×√3)-In by electron transport measurement. In this article, we report our results together with the experimental techniques and future prospect for surface superconductors.

Petascale molecular dynamics simulation of crystalline silicon on Tianhe-1A

An efficient and highly scalable bond-order potential code has been developed for the molecular dynamics simulation of bulk silicon, reaching 1.87 Pflops (floating point operations per second) in single precision on 7168 graphic processing units (GPUs) of the Tianhe-1A system. Furthermore, by coupling GPUs and central processing units, we also simulated surface reconstruction of crystalline silicon at the sub-millimeter scale with more than 110 billion atoms, reaching 1.17 Pflops in single precision plus 92.1 Tflops in double precision on the entire Tianhe-1A system. Such simulations can provide unprecedented insight into a variety of microscopic behaviors or structures, such as doping, defects, grain boundaries, and surface reactions.

Macroscopic Superconducting Current through a Silicon Surface Reconstruction with Indium Adatoms:Si(111)−(7×3)−In

Macroscopic and robust supercurrents are observed by direct electron transport measurements on a silicon surface reconstruction with In adatoms [Si(111)-(√7 × √3)-In]. The superconducting transition manifests itself as an emergence of the zero resistance state below 2.8 K. I-V characteristics exhibit sharp and hysteretic switching between superconducting and normal states with well-defined critical and retrapping currents. The two-dimensional (2D) critical current density J(2D,c) is estimated to be as high as 1.8 A/m at 1.8 K. The temperature dependence of J(2D,c) indicates that the surface atomic steps play the role of strongly coupled Josephson junctions.

New Structural Model for theSi(331)−(12×1)Surface Reconstruction

A new structural model for the $\mathrm{Si}(331)\mathrm{\text{\ensuremath{-}}}(12\ifmmode\times\else\texttimes\fi{}1)$ surface reconstruction is proposed. Based on scanning tunneling microscopy images of unprecedented resolution, low-energy electron diffraction data, and first-principles total-energy calculations, we demonstrate that the reconstructed Si(331) surface shares the same elementary building blocks as the $\mathrm{Si}(110)\mathrm{\text{\ensuremath{-}}}(16\ifmmode\times\else\texttimes\fi{}2)$ surface, establishing the pentamer as a universal building block for complex silicon surface reconstructions.

Surface reconstruction and core distortion of silicon and germanium nanowires

We report the results of molecular dynamics simulations for structures of pristine siliconnanowires and germanium nanowires with bulk cores oriented along the [110]direction and bounded by the (100) and (110) surfaces in the lateral direction. Wefound that the (100) surfaces for both silicon and germanium nanowires undergo2 × 1 dimerization while their (110) surfaces do not show reconstruction. The direction of the dimerrows is either parallel or perpendicular to the wire axis depending on the orientation of thesurface dangling bonds. The dimer length for Si is in good agreement with the result obtainedby first-principles calculations. However, the geometry of Si dimers belongs to the symmetrical2 × 1 reconstruction rather than the asymmetrical buckled dimers. We also show that surfacereconstruction of a small nanowire induces significant change in the lattice spacing for theatoms not on the (100) surface, resulting in severe structural distortion of the core of thenanowire.

Photoelectron spectroscopy of atomic core levels on the silicon surface: A review

Recent studies of the atomic structure of the single-crystal silicon surface (both clean and covered by adsorbates) that are performed by high-resolution core-level photoelectron spectroscopy using synchrotron radiation are reviewed. The physical principles of the method, experimental techniques, the spectrum processing procedure, and the procedure of determining the energy shifts of the core levels in the subsurface layer are outlined. Emphasis is placed on the surface modes of silicon 2p spectra, which are observed for the main types of silicon surface reconstruction (Si(111)-7×7 and Si(100)-2×1), and on a correlation between these modes and the atomic structure of the (111) and (100) surfaces. Also, particular attention is given to the studies of the Ge/Si system, which is viewed as a promising material of nanoelectronics, as well as to those concerned with metal and gas adsorption on basic (low-index) silicon faces. These studies clearly demonstrate that core-level photoelectron spectroscopy provides extremely detailed information on the structure of adsorbed layers and on the adsorption-stimulated reconstruction of the substrate surface.

Modeling the adsorption of norbornadiene on the Si(001) surface: The predominance of non-[2+2]-cycloaddition products

Norbornadiene (NBE) chemisorbs to a Si(001) surface in a flagpolelike structure that has potential as an anchor point for nanoscale molecular devices to the surface. Its bindings to the reconstructed Si(001)-(2×1) surface and a partially depassivated Si(001)-(2×1)-H surfaces are modeled by slab-based density functional theory using the PW91 density functional. This method is shown to quantitatively and qualitatively reproduce many known properties of bulk silicon, the silicon surface reconstruction, and the gas-phase NBE molecule. Four strongly bound adsorbate configurations are found, with the C–C bonds located either above a Si–Si dimer row or trough, oriented either parallel or perpendicular to each other. The calculated binding energies are 96, 85, 81, and 72 kcal mol−1 for the perpendicular row, perpendicular trough, parallel row, and parallel trough configurations, respectively, evaluated at quarter-monolayer coverage on the bare surface, with hydrogen passivation of the surrounding sites having little influence. These results indicate that the observed structural disorder for NBE adsorption on the bare surface at very high coverage results from kinetic rather than thermodynamic control of the reaction products. Such kinetic control is shown to be associated with large barriers in excess of 40 kcal mol−1 for possible adsorbate annealing processes, with desorption into a (partially or fully) physisorbed precursor state being required. Enhanced disorder is also predicted arising from the strong partial binding of NBE through one alkene linkage only, with the analogous four structural motifs being calculated to be very similar in energy. The lowest-energy single-alkene-bonded structure is predicted to be of the parallel–above-row type, consistent with the observed structures for most monoalkene adducts. Preference for the uncommon perpendicular binding of NBE is predicted to arise from unfavorable interactions within the silicon lattice when parallel binding occurs on adjacent rows, a binding motif that is observed for only the simplest monoalkene, ethylene, and only at high coverage. The primary reaction products of NBE are not those predicted by a [2+2] cycloaddition reaction between C=C and Si=Si double bonds, suggesting that, in general, this is not the mechanism for chemisorption of alkenes on Si(001). Rather, the reaction products are those expected on the basis that the silicon dimer bond is biradical in nature. Careful structural, polarization, and band-structure analyses of the reconstructed surface are also shown to provide no evidence for the existence for a doubly bonded silicon dimer.

Silicon surface reconstruction lost upon cobalt adsorption

We have studied the room-temperature adsorption of cobalt in the range of submonolayer coverages on a reconstructed Si(100)2×1 surface. The measurements were performed by methods of high-resolution (∼140 meV) photoelectron spectroscopy using synchrotron radiation (hν=130 eV). An analysis of changes in the Si 2p line shape in the course of cobalt deposition showed that the metal adsorption leads to the loss of the initial substrate surface reconstruction. The results are interpreted using a model whereby adatoms arriving at the silicon surface are incorporated into the uppermost atomic monolayer, occupying positions between four Si atoms and forming rows parallel to the 〈110〉 directions in the substrate.

アモルファスシリコンの表面エネルギ・表面応力

Surface energy and surface stress is a key parameter for predicting of the intrinsic stress of thin film. We have evaluated the statistical properties of the surface energy and the surface stress of amorphous silicon by using molecular dynamics, and obtained values of 1.44±0.03J/m2 and -0.48±0.06N/m respectively for 67nm2 surface area. Since it has been determined that dispersion shows a normal distribution, that of an area of any size can be predicted by the central limit theory. The coefficients of variation in an area of 1nm2 are 20.1% and 108.3%, respectively. It is found that the dispersion of surface stress is significantly large. For applications in the continuum mechanics, the surface stress can be treated as the initial stress generated in a surface region with a thickness of about 0.4nm. Though the elastic constants are decreased due to the surface effect, the difference of the strain is only about 7% for a film with 3.8nm thickness. The surface stress is strongly dependent on the reconstruction of the surface atoms. The contribution of surface reconstruction of amorphous silicon is larger than that of Si (100) 1×1 surface.

Structural and photoluminescence properties of Ge–Si ultra-thin films andheterostructures

This paper reviews recent advances in our current level of understanding of thephysics underlying the growth process of Ge thin films, (GenSin)p superlatticesand GeSi heterostructures on the Si(001)2 × 1 reconstructed surface. The role ofthermodynamics and silicon surface reconstruction is discussed, together withthe effects of Sb as a surfactant. Careful investigation of structural andphotoluminescence properties is reported.

A new reciprocal space based treatment of long range interactions on surfaces

A new formalism designed to treat long range interactions on surfaces, systems which are infinitely replicated in two spatial directions but have finite extent in the third, is developed. The new formalism is based in reciprocal space and, thus, permits the facile extension of standard plane-wave based density functional theory, Ewald summation, and smooth particle-mesh Ewald methods to handle surfaces efficiently. The method is tested on both model (body centered cubic lattices) and realistic problems (an ice surface with a defect and the 2×1 surface reconstruction of silicon) and found to be accurate, efficient, and a marked improvement on existing formulations in speed, accuracy, and utility.

Surface reconstruction of silicon and polysilicon by Nd:YAG laser etching: SEM, Raman and PL studies

A comparative study of porous silicon (PS) fabricated by laser-induced etching on n-type crystalline-silicon and n-type polysilicon substrates is presented here under near resonance conditions by employing a Nd:YAG laser. A scanning electron microscopic study of the two PS samples, prepared under identical processing conditions, elucidates differences in surface morphology. An analysis of the Raman and PL spectra from the two PS materials, employing quantum confinement models, is presented and it further elucidates differences in the nanocrystallites size distributions. A qualitative explanation for the differences is presented.

Adsorption stage in the formation of Eu-Si(111) thin-film structures

The adsorption stage in the formation of the Eu-Si(111) interface has been studied within a broad temperature range by thermal and isothermal desorption spectroscopy, low-energy-electron diffraction, Auger electron spectroscopy, and the contact potential difference method. It is shown that the ordering of an adsorbed europium film is accompanied by silicon surface reconstruction throughout the coverage range studied, 0<θ≤1.8. This self-organized process is also shown to be thermally activated. Ordered adsorbed europium layers have been found to be made up of 2D islands, whose structure depends on the amount of the metal deposited on the surface. The energy required to remove atoms from an island to vacuum has been determined. This energy decreases with decreasing 2D lattice constant of the islands. This pattern of its variation is accounted for, in the final count, by the decrease of the number of the Si surface atoms not bound directly to Eu atoms.

High-quality AlN grown on Si(111) by gas-source molecular-beam epitaxy with ammonia

Hexagonal AlN layers were grown on Si(111) by gas-source molecular-beam epitaxy with ammonia. The transition between the (7×7) and (1×1) silicon surface reconstructions, at 1100 K, was used for in situ calibration of the substrate temperature. The initial deposition of Al, at 1130–1190 K, produced an effective nucleation layer for the growth of AlN. The Al layer also reduced islands of SiNx that might be formed due to background NH3 on the silicon surface prior to the onset of epitaxial growth. The transition to two-dimensional growth mode, under optimum conditions, was obtained after the initial AlN thickness of ∼7 nm.

Surface reconstruction of hydrogen annealed (100) silicon

The hydrogen termination and surface reconstruction of (100) silicon annealed at high temperature in a H2 atmosphere at 1 bar is investigated with multiple internal reflection infrared spectroscopy and atomic force microscopy. The surface flattens and becomes 2×1 reconstructed and terminated by strained monohydrides. This surface is shown to be very stable against contamination and oxidation.

Use of multilayer techniques for XPS identification of various nitrogen environments in the Si/NH3 system

The experimental ability to reach rapidly, under UHV and on the same substrate holder, the high temperatures (1100 K) required for silicon surface reconstructions and the low temperatures (80 K) needed to condense ammonia molecularly or to adsorb this gas dissociatively on specific silicon surface states, allowed us to elaborate thin multilayers made up of various combinations of the nitrogen chemical environments involved in the Si/NH3 system. Included in a surface laye (< 50 Å) corresponding to the analysing electron escape depth of a X-ray photoelectron spectrometer, they lead one to obtain the binding energy signatures for molecular ammonia (NH3), amide (NH2) and imide (NH) fragments and nitrides (Si3N) by simultaneously and easily comparable N 1s core level recordings. The results confirm that beside the Ts dependence of the degree of dissociation of NH3, the silicon surface state configuration (Si(100)-2 × 1. Si(111)-7 × 7 a-Si) is another important parameter explaining the presence of differently adsorbed N, NH or NH2 groups.

Lattice gas model for semiconductor (111) surface reconstruction

A simple lattice gas model is proposed for semiconductor (111) surface reconstruction. By signing energy parameters to the gas particles and the interaction among them we found that the local structure for the silicon (111) surface (7 × 7) reconstruction and the germanium (111) surface c(2 × 8) reconstruction should be different, and that the formation of the dimers on the surfaces plays a very important rule in the surface reconstructions. We suggest that a dimer instead of a simple adatom should be the basic building unit for the germanium (111) surface c(2 × 8) reconstruction.