Double-layer Steps Research Articles

Analysis, identification, and application of distribution of relaxation times in CMP slurry

This study delves into the mechanistic roles of oxidants, complexing agents, and inhibitors in slurries through of Distribution of Relaxation Times (DRT) analysis. The influence of varying concentrations of these chemicals on electrochemical processes is discussed. This article divides the spectrum obtained through DRT analysis into four different frequency regions and differentiate naming through electrode reaction process: the double-layer charging step, the charge transfer step, the mass transfer step, and the surface transformation step by adsorption or chemical alterations at the electrode surface. The results reveal that the peak observed in the low-frequency range at 10−2–10° Hz is associated with mass transfer step, while the relaxation peak at 10°–102 Hz corresponds to the charge transfer process, while the relaxation peak occurring at 102–104 Hz is linked to the surface transformation step, and the first peak emerging in the 104–106 Hz range is attributed to double-layer charging step.

Theoretical and experimental study on epitaxial growth of antiphase boundary free GaAs on hydrogenated on-axis Si(001) surfaces

The formation of double-layer atomic steps on Si(001) surfaces is an efficient way to eliminate the antiphase boundaries (APBs) on GaAs/Si(001) interfaces. The surface energy of on-axis Si(001) surfaces with different atomic step structures was calculated and analyzed from the first principles. An optimal hydrogen-annealing process condition, the hydrogen pressure of 800 mbar and the annealing temperature of 800 °C for 10 min, was obtained experimentally. Under this annealing condition, a 420 nm APB-free GaAs epitaxial layer grown on on-axis Si(001) substrates was achieved by metal-organic chemical vapor deposition. The effect of the annealing temperature on the APB density of the GaAs/Si(001) samples was explained from the aspects of thermodynamics and kinetics. It is of great significance to optimize the growth conditions of APB-free III–V epitaxial materials on on-axis Si(001) surfaces for the large-scale silicon monolithic integration of optoelectronic devices.

A new technique for uterine incision closure at the time of cesarean section: does it make a difference?

The purpose of this study was to compare the short-term operative outcomes of three different surgical techniques for uterine incision closure during caesarean section (CS). This trial enrolled 120 patients scheduled for primary caesarean delivery. Patients were randomised into either classical double-layer uterine closure, purse-string double-layer uterine closure (Turan), or our new approach of uterine closure (double layer step up-step down technique). For short-term comparison, transvaginal ultrasonography was planned for all patients 6 weeks after surgery. Compared to group II and Group III, residual myometrial thickness was significantly thinner in group I (p < .001). The number of patients with uterine niche was 10 (50% of all scar defects) in group I whereas it was 4 (20%) in group II and 6 (30%) in group III. Operative time was significantly longer in group II (p < .001). This led to our conclusion that Turan technique and our new approach are associated with thicker myometrial thickness and less frequency of uterine scar defect than classical double-layer uterine incision closure; however, our approach takes less operative time. Clinical Trial Registration: ClinicalTrials.gov Identifier: NCT04681378 Impact statement What is already known on this subject? Many variations in CS technique have been studied. For example, single-layer uterine incision closure has been compared to double-layer uterine incision closure. Purse string double layer (Turan) has been also compared to the traditional double-layer technique. Double layer unlocked closure has been shown to result in a thicker residual myometrium and as a consequence can possibly lead to the decrease of niche development after a CS compared to single-layer closure with lower frequency of uterine scar defect with Turan technique. What do the results of this study add? Here we introduce a new uterine closure technique, which we have named double-layer step up–step down technique, as an alternative method. With this technique, the uterine incision contract more than with the traditional double layer technique and has similar results to the Turan technique; however, our approach takes less operative time. What are the implications of these findings for clinical practice and/or further research? These alternative techniques of uterine incision closure decrease the frequency of uterine niche that may be associated with many clinical problems such as ectopic pregnancy at the CS scar, placenta accreta, rupture of the uterus during a subsequent pregnancy. Future studies are needed to investigate the frequency of uterine rupture in a subsequent pregnancy following different uterine incision closure techniques.

Temperature effects on the nano-friction across exposed atomic step edges

Despite the practical and fundamental importance of friction, there are many issues that are still needed to be explored and revealed; one of them is how friction changes across an atomic surface step edge at different temperatures. In this article, a friction force microscope under ultrahigh vacuum conditions has been used to study the temperature dependence of nanoscale friction between a silicon tip and a freshly cleaved HOPG surface with exposed single- and double-layer step edges. For the upward scanning from the lower terrace to the upper terrace, a large resistive force which is linearly dependent on the normal force is observed. A similar resistive force but with a smaller magnitude together with an assistive force is observed for the downward scanning; however, the resistive force is found to be independent of the normal force while the assistive force increases with the normal force. Besides, the resistive force for the double-layer step edge is found to be twice as high as that of the single-step edge, while the assistive force seems to be less influenced by the height of the step. Finally, the experimental results reveal that temperature has a negligible effect on the friction coefficients at the step edges, which is inconsistent with the thermal activated process where friction should decrease with temperature. Based on the theoretical studies, this observation can be explained by a process where the temperature effect is very small compared with the edge Schwoebel–Ehrlich barrier. These findings may help in understanding the temperature effects on macroscopic friction having a lot of step edges at the interface.

Increased Stability of Subsurface C Induced by Ca on the C-Incorporated Si(001)-4°-off Substrate

The effects of Ca atoms adsorbed on the C-incorporated Si(001)-4°-off substrate have been investigated by using scanning tunneling microscopy (STM) and synchrotron photoemission spectroscopy (PES). The C atoms incorporated into the subsurface by C2H2 exposure and postannealing at 630 °C induce the c(4 × 4) structure and debunch the double-layer DB steps. The Ca atoms additionally adsorbed on this C-incorporated substrate induce terraces with a width twice that of the clean surface. Until postannealing up to 680 °C, no SiC trace is detected. The SiC islands start to be detected after postannealing at 780 °C, which is about 100 °C higher than that for the identical surface without Ca atoms. After the adsorbed Ca atoms are mostly desorbed by 880 °C postannealing, the SiC islands only remain on the surface. Such a result implies that the adsorbed Ca atoms act as a stabilizer to the subsurface C atoms and increase the onset temperature of SiC formation by 100 °C.

Double-layer stepped Si(1 0 0) surfaces prepared in As-rich CVD ambience

Low-defect III-V integration on Si(1 0 0) requires atomically well-ordered heterointerfaces. To this end, we study the interaction of As with Si(1 0 0) surfaces and the formation of dimerized Si(1 0 0):As surfaces in situ with reflection anisotropy spectroscopy during metalorganic chemical vapor deposition (MOCVD). Control of surface processes allows us to intentionally ‘rotate’ the dimer orientation and choose between predominantly (1 × 2) and (2 × 1) reconstructed Si(1 0 0):As surfaces both for low and higher surface misorientation. We also verify that the (2 × 1) surface reconstruction becomes energetically more favorable for vicinal Si(1 0 0):As surfaces. We explain the preparation of Si(1 0 0):As surfaces displaying an almost single-domain, (1 × 2) reconstructed surface with an RMS roughness ≤1 Å and exhibiting broad, atomically flat terraces as well as regular double-layer steps. This surface is highly suitable for subsequent low-defect III-V integration.

Control Over Dimer Orientations on Vicinal Si(100) Surfaces in Hydrogen Ambient: Kinetics Versus Energetics

In chemical vapor ambience, Si(100) surfaces and hydrogen are strongly interacting with each other at relevant process temperatures. Energetic considerations cannot solely describe step and terrace formation sufficiently, as the formation of equilibrium surface reconstructions can be counteracted by surface kinetics. In recent years, the combination of in situ reflection anisotropy spectroscopy (RAS) and complementary UHV‐based surface sensitive techniques helped to understand and control the entire Si(100) surface preparation, in particular step and domain formation, which strongly depend on the applied process conditions and surface misorientation. Here, we discuss vicinal, 6° misoriented Si(100) surfaces in comparison to results on larger terraces in order to derive a general view on how to promote either a kinetically or an energetically driven step formation. Dictated by the dominant driving force, monohydride Si(100) surfaces with “A‐type” (1 × 2) or “B‐type” (2 × 1) majority domain can be prepared, as we confirm with RAS as well as low energy electron diffraction, scanning tunneling microscopy and Fourier‐transform infrared spectroscopy. Well‐ordered DB double layer steps at the vicinal B‐type Si(100) surface cause a step‐related, derivative‐like RAS signal, in contrast to all other surfaces. In general, we confirm that high H2 pressures promote kinetically driven processes, while the impact of energetics increases with offcut magnitude.

GaAsP/Si tandem solar cells: In situ study on GaP/Si:As virtual substrate preparation

Realization of high efficiency III-V-on-Si solar cells requires preparation of an oxygen free Si substrate surface, which allows for subsequent antiphase domain free III-V epitaxy. For GaAsP/Si tandem cells, the impact of As on established Si processing in metal organic vapor phase epitaxy (MOVPE) is essential. Here, we study the interaction of As with vicinal Si(100) surfaces, the formation of atomically well-ordered As-modified Si(100) surfaces, and its impact on subsequently grown GaP epilayers. We apply optical in situ spectroscopy during MOVPE and show that process routes strongly affect the formation of double-layer steps on Si(100). We demonstrate that exposure to As, together with HF pre-treatment, enables processing of suitable Si(100) 2° and 6° offcut surfaces at temperatures below 820°C. Subsequently grown GaP epilayers exhibit smooth surfaces free of antiphase disorder and distinct Laue oscillations in X-ray diffraction. Such low temperature processing, which is controllable in situ, is promising for further metamorphic GaAsP integration.

Toward the III–V/Si co-integration by controlling the biatomic steps on hydrogenated Si(001)

The integration of III-V on silicon is still a hot topic as it will open up a way to co-integrate Si CMOS logic with photonic devices. To reach this aim, several hurdles should be solved, and more particularly the generation of antiphase boundaries (APBs) at the III-V/Si(001) interface. Density functional theory (DFT) has been used to demonstrate the existence of a double-layer steps on nominal Si(001) which is formed during annealing under proper hydrogen chemical potential. This phenomenon could be explained by the formation of dimer vacancy lines which could be responsible for the preferential and selective etching of one type of step leading to the double step surface creation. To check this hypothesis, different experiments have been carried in an industrial 300 mm metalorganic chemical vapor deposition where the total pressure during the annealing step of Si(001) surface has been varied. Under optimized conditions, an APBs-free GaAs layer was grown on a nominal Si(001) surface paving the way for III–V integration on silicon industrial platform.

In situ preparation of Si p-n junctions and subsequent surface preparation for III-V heteroepitaxy in MOCVD ambient

III-V integration on active Si-bottom cells promises not only high-efficiency multi-junction solar cells but also lower production costs. In situ preparation of an adequate Si p-n junction in metalorganic chemical vapor deposition ambient is challenging, particularly since the final Si surface should be atomically well-ordered to enable low-defect III-V nucleation. Precisely, a single-domain Si(100) surface with double layer steps needs to be prepared in order to suppress antiphase disorder in subsequently grown III-V layer structures on top of the Si p-n junction. We first investigate the formation of a n+-type collector in Si(100) as a result of annealing in tertiarybutylphosphine (TBP) or tertiarybutylarsine (TBAs) ambient. We illustrate how the n-type doping concentrations and their depth profiles depend on the essential preparation parameters, such as precursor partial pressures, exposure and annealing time, as well as reactor pressure. Subsequently, by applying in situ reflectance anisotropy spectroscopy, we find that exposure of Si(100) to TBP or TBAs leads to atomic disorder on the surface. Further, we apply an additional annealing step without precursor supply leading to predominantly (1×2) reconstructed Si(100) surfaces, which are suitable for subsequent low-defect III-V growth.

Diameter-selective alignment of carbon nanotubes on Si(001) stepped surfaces.

We report total-energy electronic-structure calculations based on the density-functional theory that provide stable adsorption sites, structural characteristics, and energy bands of carbon nanotubes (CNTs) adsorbed on the Si(001) stepped surfaces. We choose (5,5), (9,9), and (13,13) armchair CNTs with the diameters of 6.8Å, 12.2Å, and 17.6Å, respectively, as representatives of CNTs and explore all the possible adsorption sites either on the terrace or at step edges. We find that the (9,9) CNT is most favorably adsorbed at the edge of the double-layer step DB along the ⟨110⟩ direction, whereas the (5,5) and (13,13) CNTs favor the terrace site where the CNTs are perpendicular to the Si dimer rows. This finding is indicative of the diameter-selective self-organized alignment of CNTs by exploiting the Si surface steps along the particular direction. We also find that the electronic structure of each CNT is modified upon adsorption depending on the adsorption site and the diameter of the CNTs. In particular, the (9,9) CNT at the most stable step edge site becomes semiconducting and the resultant valence and conduction bands exhibit nearly linear dispersion with the effective mass of 0.085 m0 (m0: bare electron mass), preserving the characteristics of the Dirac electrons. We also find that the flat bands appear near the Fermi level (EF) when the (13,13) CNT is adsorbed at the metastable DB step edge, inferring that spin polarization is possible for the CNT on the Si(001) stepped surface.

Communication: fully alignment-specified O2 chemisorption on vicinal Si(100).

A fully alignment-resolved O2 sticking experiment on a single domain Si(100)-(2×1) surface is presented. This provides the first experimental evidence that the reactivity of O2 depends on both the polar and azimuthal angles of the molecular axis relative to a surface. It has been found that, in case of side-on collision, an O2 molecule perpendicular to the dimer on Si(100) is about 40% more reactive than that parallel to the dimer. Comparison of the O2 sticking on flat and vicinal Si(100) surfaces indicates that barrierless dissociation channels exist at the double layer step.

Domain-sensitive in situ observation of layer-by-layer removal at Si(100) in H2 ambient

Double-layer step formation on Si(100) substrates is a crucial prerequisite for antiphase-domain free III–V compound semiconductor heteroepitaxy. Due to its unequaled relevance in microelectronics, the (100) oriented surface of silicon is by far the most studied semiconductor surface. However, Si(100) preparation in hydrogen process gas ambient, which is commonly employed for Si and III–V device preparation, is completely different from preparation in ultra-high vacuum due to strong interaction between H2 and the Si surface, leading to a kinetically driven different step formation. Here, we observe chemical layer-by-layer removal of surface atoms from the terraces at the Si(100) surface during annealing in hydrogen ambient. Mutually perpendicularly oriented dimers on subsequently removed monolayers induce oscillations in the in situ reflection anisotropy spectroscopy (RAS) signal. Scanning tunneling microscopy measurements support a model, where surface atom removal proceeds by formation and anisotropic expansion of vacancy islands on the terraces. We determined an activation energy Ed of 2.75 ± 0.20 eV for Si etching in H2 ambient by transient in situ RAS measurements. In situ control of the highly reactive Si(100) surface preparation is essential for subsequent defect-free III–V heteroepitaxy.

Electrochemical and physical characterisation of Pt-nanocluster activated molybdenum carbide derived carbon electrodes

Oxygen electroreduction (ORR) on Pt-nanoclusters activated electrodes, based on different microporous–mesoporous carbon supports, were studied in 0.5M H2SO4 solution using cyclic voltammetry, rotating disc electrode (RDE), and impedance methods. The C(Mo2C) carbon supports with variable specific surface area, microporosity–mesoporosity, good electrical conductivity and corrosion stability at positive potentials were prepared from Mo2C at 600°C and 800°C using chlorination synthesis method. Pt-nanoclusters were deposited onto/into carbons using sodium borohydride reduction method. XRD, XRF, XPS, SEM-EDX, and HRTEM data confirm that catalysts under study are free from chlorine, chloride ions, and other residuals of raw materials, and have highly porous hierarchical structure with homogenously dispersed Pt-nanoclusters. RDE data show that the kinetic current densities are higher and half-wave potentials are more positive for Pt–C(Mo2C)800°C than for Pt–C(Mo2C)600°C in 0.5M H2SO4 solution. The limiting diffusion currents are independent of the catalyst composition used due to the proceeding of ORR process in the outer-diffusion mode. Analysis of impedance data demonstrated nearly capacitive behaviour in low ac frequency region, explained by quick cathodic electroreduction of oxygen followed by slow electrical double-layer formation step limited by adsorption of ions and intermediates at/inside microporous–mesoporous C(Mo2C) and Pt-nanocluster modified electrodes. High series and parallel capacitance values (100–250Fg−1) have been established, attractive also for hybrid supercapacitor applications. High frequency series resistance values are higher for more amorphous C(Mo2C)600°C than for C(Mo2C)800°C supported electrode.

Anomalous double-layer step formation on Si(100) in hydrogen process ambient

We prepared Si(100) surfaces with anomalous atomic double-layer steps grown via chemical vapor deposition. Scanning tunneling microscopy resolved ${D}_{A}$-type steps, supported by low-energy electron diffraction, Fourier-transform infrared spectroscopy, and in situ reflection anisotropy spectroscopy, which enabled direct control of majority domain formation. We attribute the energetically unfavorable step structure to interaction of the surface with the H${}_{2}$ ambient, driving a dynamic step formation process governed by surface vacancy generation, diffusion, and annihilation at step edges.

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III–V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(100) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(100) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(100) with 0.1°, 2° and 6° off-cut in [011] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(100) surfaces indicated two-domain (2×1)/(1×2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III–V layers. However, double-layer steps developed on 6° misoriented Si(100) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.

MOLECULAR DYNAMICS SIMULATIONS OF SINGLESiADATOM DIFFUSION ON THESi(001)SURFACE AND ACROSS SINGLE-LAYERSi(001)STEPS

By using the Stillinger–Weber atomic interactional potential, we have carried out molecular dynamics simulations of single Si adatom diffusing on the Si(001) surface and single-layer Si(001) steps at temperatures ranging from 1000 K to 1300 K. We have presented one new diffusion pathway of a single Si adatom diffusing on the Si(001) along the direction perpendicular to dimer rows, that can weaken the diffusion anisotropy. We have investigated the process of the single Si adatom diffusing across single-layer Si(001) steps as well and given adatom diffusion pathways of step-flow and transformation of single-layer into double-layer steps. Our results show that the exchange between an adatom and a surface atom plays an important role in the adatom diffusion process above 1000 K.

Metalorganic chemical-vapour-deposition (MOCVD) of InGaAs, BGaAs, and BInGaAs: Quantum chemical calculations on the mechanisms

Using the density-functional method, reaction energies and barrier heights are calculated for reactions of precursor molecules at steps of the GaAs(0 0 1) surface modelled by molecular clusters. The reaction behaviour of XZ 3 molecules (X=B, Ga, In, As; Z=H, CH 3) is investigated at double-layer steps of Ga- and As-rich surfaces. The energetic data provide assertions to the rate-limiting step, the efficiency of the precursor molecules, and the formed structures. The precursor molecules or the formed precursor fragment dimers preferentially bind in dimers at steps. In general, the deposition with hydrides XH 3 is faster than with methyl molecules X(CH 3) 3. B(CH 3) 3 is less suitable for the boron deposition in alloys. The boron deposition by borane is very probable at boron dimers, is reduced at arsenic dimers (isovalent deposition), and is not possible at gallium and indium dimers (antisite deposition). The first case leads to the formation of a boron phase, what can be suppressed by working under excess of arsenic and reduced exposure of boron.

Elastic analysis of periodic dimer forces on a Si(001) surface

A method to reduce the dependence of a continuum elastic model of surface energetics on a number of parameters from total energy calculation is presented. It uses sinusoidal surface forces representing dimer rows as boundary conditions for computing two-dimensional elastic fields. A vicinal cutting performed to a stepped surface and the sinusoidal forces can yield step formation energies and force dipoles from surface stress data. Anisotropic periodic dimers on a Si(001) surface produce constant elastic energy for the terrace with the dimerization direction perpendicular to its steps but yield the logarithmic dependence on terrace width for the terrace with the dimerization direction along the steps. Double-layer step bunching can occur, but a large coalescence of steps is not thermodynamically preferred. The double-layer step bunching ensures that only terraces with the parallel dimerization direction survive, resulting in double-layer steps of type DB. Formation energy for step SA is found negative, while for SB it is positive. The elastic-field-induced diffusion barrier provides a confinement for diffusing species on the lower terrace of a step.

Interacting dimer rows on terraces: Reconstructed Si(001) surfaces

A finite sum of dimer-row elastic interactions on a two-dimensional surface yields a logarithmic stress-domain interaction energy. Using dimer rows as the building blocks of a reconstructed surface generalizes the Alerhand et al. and Marchenko models of the stress-domain interaction. Our model is applied to step-height transition on vicinal Si(001) surfaces. The double-layer step phase is determined to be more stable than the single-layer step phase for typical temperatures and miscut angles. A sizable mixed phase region on the temperature-versus-miscut-angle phase diagram is found. The onset of the mixed phase region decreases to a 0\ifmmode^\circ\else\textdegree\fi{} miscut angle if the density of forced kinks is zero. Formation energies of two step types in the Si(001) are positive and consistent with total energy calculations. Our results suggest that the single-layer step phase is stable only for flat Si(001) surfaces.