Terrace Width Research Articles

Resistivity as a Witness of Local Crystal Growth Conditions

A detailed knowledge of the growth front geometry during physical vapor transport (PVT) growth of SiC single crystals is beneficial to achieve high quality n+ SiC substrates for power device applications. In this report we show how mapping of resistivity in SiC wafers can shed light on local growth conditions, which are very difficult-to-study in situ. We consider both thermodynamic quantities (absolute temperature T and partial pressure pN₂) and geometric characteristics of the growth surface relevant to growth kinetic parameters, namely atomic terrace width and atomic step velocity. Specifically, we show how an elevation map of the growth surface can be reconstructed from a spatially-resolved measurement of resistivity in a SiC wafer by integrating the spatial derivative of elevation with respect to the basal plane, which is assumed to be related to local resistivity through dependence of non-equilibrium nitrogen incorporation on the atomic step velocity.

Physical confinement versus adsorption driven reconstruction: The case of sulfate anion interaction with vicinal Cu(111) surfaces

Nano-electrochemistry, i.e., the research of the properties of nano-(structured) electrodes and their influence on electrochemical processes when immersed inside an electrolyte, represents a hot topic in view of applications in nano-electronics, electro-catalysis and energy storage devices. The role of physical confinement in the electrochemical fabrication and performances of the respective systems have been recently addressed in the context of metal-organic networks on surfaces, but rarely of nano-structured bare metal surfaces, for instance, regularly stepped (vicinal) surfaces. In this work we investigate the interplay between physical confinement and adsorbate induced restructuring by the electrochemical adsorption of sulfate anions on the flat and two distinctly different vicinal Cu(111) surfaces. Sulfate adsorption on the flat Cu(111) surface is known to create a long-range ordered Moiré-superstructure with lattice parameters in the 2–4 nm range due to an expansion of the topmost layer of copper atoms with respect to the underlying crystal planes. This restructuring is also observed on a vicinal Cu(111) surface whose original terrace width is considerably smaller than the lattice vectors of the sulfate induced Moiré-structure. The results clearly indicate not only that the Moiré formation lifts the physical confinement imposed by the initial terrace width, but also shine more light on the Moiré formation process itself. Such adsorbate induced restructuring, of course, depends on the respective adsorbate – electrode combination, but must, in principle, always be taken into account in order to understand electrochemical processes at nano-structured (and nano-sized) electrode surfaces.

Passive Design Strategy in Vernacular House of Samin, Indonesia

Vernacular architecture embodies passive design principles and strategies to create comfortable dwellings. This study investigates the impact of passive design strategies on temperature comfort in Samin vernacular houses. Visual observation is employed to assess the suitability of house elements based on passive design criteria. At the same time, field measurements are conducted to evaluate comfortable temperature conditions using data loggers over one month. The research focuses on the Original Samin house and the New Samin house situated in Klopoduwur Village, Blora Regency, Central Java, Indonesia. The visual observation reveals that the roof volume, slope, and thin walls with low conductivity in both Samin houses align with passive design principles. The New Samin house also incorporates other passive design strategies, including an east-west building orientation with the long side perpendicular to the wind direction, an open-plan room layout, and varying floor heights. The temperature comfort performance in both houses falls within the neutral temperature range during 16 hours. The New Samin house exhibits the largest daytime air temperature decrease at 5.5°C, while the Original Samin house experiences the largest nighttime decrease at 0.7°C. The evolution of passive designs in Samin vernacular houses encompasses considerations such as building proportion and width, terrace width, positioning and size of window openings, vegetation, and shading elements.

Step-edge-guided nucleation and growth mode transition of α-Ga2O3 heteroepitaxy on vicinal sapphire

Abstract Controlling the epitaxial growth mode of semiconductor layers is crucial for optimizing material properties and device performance. In this work, the growth mode of α-Ga2O3 heteroepitaxial layers was modulated by tuning miscut angles (θ) from 0° to 7° off the (10-10) direction of sapphire (0002) substrate. On flat sapphire surfaces, the growth undergoes a typical three-dimensional (3D) growth mode due to the random nucleation on wide substrate terraces, as evidenced by the hillock morphology and high dislocation densities. As the miscut angle increases to θ=5°, the terrace width of sapphire substrate is comparable to the distance between neighboring nuclei, and consequently, the nucleation is guided by terrace edges, which energetically facilitates the growth mode transition into the desirable two-dimensional coherent growth. Consequently, the mean surface roughness decreases to only 0.62 nm, accompanied by a significant reduction in screw and edge dislocations to 0.16×107 cm-2 and 3.58×109 cm-2, respectively. However, the further increment of miscut angles to θ=7° shrink the terrace width less than nucleation distance, and the step-bunching growth mode is dominant. In this circumstance, the misfit strain is released in the initial growth stage, resulting in surface morphology degradation and increased dislocation densities.

Origin of Distinct Insulating Domains in the Layered Charge Density Wave Material 1T-TaS2.

Vertical charge order shapes the electronic properties in layered charge density wave (CDW) materials. Various stacking orders inevitably create nanoscale domains with distinct electronic structures inaccessible to bulk probes. Here, the stacking characteristics of bulk 1T-TaS2 are analyzed using scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. It is observed that Mott-insulating domains undergo a transition to band-insulating domains restoring vertical dimerization of the CDWs. Furthermore, STS measurements covering a wide terrace reveal two distinct band insulating domains differentiated by band edge broadening. These DFT calculations reveal that the Mott insulating layers preferably reside on the subsurface, forming broader band edges in the neighboring band insulating layers. Ultimately, buried Mott insulating layers believed to harbor the quantum spin liquid phase are identified. These results resolve persistent issues regarding vertical charge order in 1T-TaS2, providing a new perspective for investigating emergent quantum phenomena in layered CDWmaterials.

Variation of (001) surface structures of strontium titanate single crystals caused by flash event under direct/alternative current electric fields

Flash events, which are electric power spikes occurring under an electric field and temperature above the threshold, can significantly accelerate mass diffusion over short periods of time. This study investigated the surface structure of the (001) surface of strontium titanate single-crystal treated by flash events caused by direct current (DC) and alternating current (AC) electric fields from a viewpoint of surface-related mass diffusion. The surface structuring caused by flash events was significantly different among DC and AC electric fields. The DC-flash event destabilized surface steps, causing them to wander and form surface mounds with a height of several tens of nanometers. In addition, numerous two-dimensional (2D) nuclei with a size of several nanometers are formed on the terraces. In contrast, the AC-flash event stabilized surface steps without the formation of surface mounds, causing them nearly straight with uniform terrace width at approximately 500 nm. 2D nuclei with a size of a few 10 nm were formed at the step edges with round shapes. Both methods were confirmed as evidenced by the formation of two-dimensional nuclei on the surface after the flash events, to enhance surface-related mass diffusion.

Effects of Miscut on Step Instabilities in Homo-Epitaxially Grown GaN.

The rough morphology at the growth surface results in the non-uniform distribution of indium composition, intentionally or unintentionally doped impurity, and thus impacts the performance of GaN-based optoelectronic and vertical power electronic devices. We observed the morphologies of unintentionally doped GaN homo-epitaxially grown via MOCVD and identified the relations between rough surfaces and the miscut angle and direction of the substrate. The growth kinetics under the effect of the Ehrlich-Schwoebel barrier were studied, and it was found that asymmetric step motions in samples with a large miscut angle or those grown at high temperature were the causes of step-bunching. Meandering steps were believed to be caused by surface free energy minimization for steps with wide terraces or deviating from the [11¯00] m-direction.

Suppression of Mixing of Metastable Zincblende Phase in GaN Crystal Grown on ScAlMgO4 Substrates by Radio‐Frequency Plasma‐Assisted Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB‐GaN is generated as stacking faults during the coalescence of the initially grown wurtzite‐ (WZ‐) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ‐GaN. Therefore, high‐temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB‐GaN also decreases with an increase in the growth time. Based on these results, a 940 nm‐thick GaN layer is grown at 750 °C on the SAM substrate with a terrace width of 820 nm. A pure WZ‐GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 × 109 cm−2 is obtained.

Variable downcanyon morphology controlling the recent activity of shelf-incised submarine canyons (Alboran Sea, western Mediterranean)

This research aims to distinguish genetic sedimentary processes building canyon geomorphological patterns and the factors driving different sedimentary activities in two nearby Mediterranean shelf-incised submarine canyons (Carchuna and Motril) that exhibit different degrees of incision on the narrow margin of the northern Alboran Sea.The straight Carchuna Canyon incises the shelf up to 200 m off the coastline and exhibit steep canyon walls featuring narrow terraces, muddy sands with high contents of organic matter along the thalweg, and transported shelf benthic foraminifera in distal settings. The Motril Canyon head is wider and incises the shelf edge, ca. 2 km off the coastline. It exhibits a sinuous morphology and less steep walls, wider terraces, and higher sedimentation rates with muddy sediments along the thalweg. In both canyons, cross-section relief, width, incision, and area decrease downslope, although these parameters increase locally.The downslope variations of geomorphological parameters are attributed to enhanced erosional/depositional processes promoted by tectonically controlled abrupt changes of the axial channel orientation. The degree of shelf incision, the location of the canyon heads in relation with the local sediment sources, and the seasonally variable hydrodynamic regimes determine the different degrees of recent canyon activity. The Motril Canyon is interpreted as a mature system that reflects episodic activity, collecting fine-grained sediments from the nearby Guadalfeo River. The Carchuna Canyon exhibits a youthful developmental stage whose activity is more continuous and involves sediment trapping of littoral cells and continuous downslope sand transport.

Site-specific reactivity of stepped Pt surfaces driven by stress release.

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research1-4, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps1,5-10. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites7,8,11,12, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization thatoverlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Controllable step-flow growth of GaN on patterned freestanding substrate

A new kind of step-flow growth mode is proposed, which adopts sidewall as step source on patterned GaN substrate. The terrace width of steps originated from the sidewall was found to change with the growth temperature and ammonia flux. The growth mechanism is explained and simulated based on step motion model. This work helps better understand the behaviors of step advancement and puts forward a method of precisely modulating atomic steps.

Kinetic Monte Carlo study on the effect of growth conditions on the epitaxial growth of 3C–SiC (0001) vicinal surface

Due to the lack of research on the microscopic evolution process and the formation of step growth patterns for SiC crystals, it is of great importance to deepen the understanding of the epitaxial growth of a SiC vicinal surface from a microscopic point of view. In this study, a three-dimensional lattice kinetic Monte Carlo algorithm was used to study the step flow growth characteristics of SiC crystals. The microscopic evolution of the step flow growth patterns for SiC vicinal surfaces was shown. C and Si were treated as the basic particles, and the net deposition and diffusion of atoms were considered in this model. The periodic boundary conditions were applied along the step edge and the helical boundary conditions were applied in the direction perpendicular to the step. The surface morphology evolution of SiC crystals grown on step substrates was simulated at different growth temperatures, partial deposition fluxes, and terrace widths. The results indicated that the growth patterns of the SiC vicinal surface could be transformed from a step flow growth to a nucleation growth pattern by decreasing the growth temperature, increasing the deposition flux, and increasing the terrace width.

Enhancing microstructure and device performance of InGaN quantum dot micro-LEDs through substrate off-cut angle modulation

InGaN quantum dots (QDs) are regarded as a compelling candidate material for the fabrication of high-quality GaN-based micro-LEDs. In this work, to study the impact of a substrate structure on InGaN QDs and QD-based micro-LEDs, GaN-on-sapphire substrates with off-cut angles toward the a-axis of 0.2°, 0.4°, and 0.7° were utilized as templates for the fabrication of InGaN QDs and InGaN QDs-based micro-LEDs. Experimental results show that GaN template with 0.4° off-cut angle exhibits the narrowest terrace width and enables InGaN QDs to be higher and more uniform. The InGaN QD sample grown on 0.4° substrate has a very small wavelength shift of 2.5 nm with temperature increasing and owns the longest photoluminescence peak wavelength implying the highest In content. Furthermore, electroluminescence (EL) spectra demonstrate that QD-based micro-LED array has excellent wavelength stability under various injection currents, and the stability can be improved further on a GaN template with narrower terraces. The results indicate that altering the terrace width of GaN template is a feasible scheme for improving the properties of GaN-based micro-LEDs.

Manipulation of BiFeO3 nanostructure by substrate terrace morphology

Topological domain structures, such as vortex and center domain have been found in BiFeO3 nanoislands fabricated by self-assemble approaches, which exhibit great application potential in nonvolatile storage. However, how to control the self-assembled nanostructure during preparation process is still an open question. Here, we successfully grew BiFeO3 films with nano-stripe, nano-maze and dense nanoisland structures on LaAlO3 (001) substrate by pulsed laser deposition. We found the formation of different nanostructures is related to the terrace morphology of the substrate. Nano-stripe structure with strip domain and high conduction tail-to-tail domain wall preferred to form on wide terrace, nano-maze structure with three-fold or four-fold center-type domain usually form on bunched terrace, while dense nanoisland structure with two/three-fold domain preferred to from on the narrow or rough terrace LaAlO3 substrate. These findings provide guidance for manipulating the ferroelectric topological domain structure during film preparation process.

Soil microbial properties are improved by the adoption of soil management and conservation practices in no-tillage system

ABSTRACT No-tillage system (NTS) plays a prominent role in conservation agriculture, however, its benefits can be further improved by adopting complementary soil management and conservation practices, such as using autumnal cover crops, contour seeding, and terraces. This study aimed to evaluate how soil biological activity responds to soil management and conservation systems. The treatments consisted of three macroplots with an area of 11.000 m 2 each, as follows: a) Non-Terraced catchment (NTC), cultivated in NTS similar to most farmers of the region, in which the agricultural operations are carried out in the direction of the slope and without terraces used; b) Best Management Practices (BMPs) were adopted in NTS with additional autumnal cultivation of cover crops, and also the direction of machine traffic was transverse to the slope direction; and c) Terraced catchment (TC), cultivated in NTS was associated to mechanical practices to erosion control, using wide base terrace on level. Soil microbial properties sampled in the 0.00-0.10 m layer were evaluated during 2019, 2020, and 2021, all shortly after the summer crop harvest. Natural inoculum potential of arbuscular mycorrhizal fungi (AMF), respirometry, metabolic coefficient, acid phosphatase activity, and organic carbon and nitrogen in the microbial biomass were assessed. Averages of each microbiological properties were compared through the confidence intervals (p<0.05). The results showed a greater potential for AMF inoculum in BMPs and TC systems. The NTC showed the highest values of respirometry and metabolic quotient, releasing 31.7 and 27.3 % more CO 2 compared to BMPs and TC, respectively. The BMPs and TC were able to retain 13.8 and 16.5 % more carbon in the microbial biomass and 8.0 and 8.8 % more nitrogen in the biomass than NTC, respectively. Adopting soil management and conservation practices such as autumn cover crops, level seeding, and wide base terrace on level improved the soil microbial properties, with an increase in AMF inoculum potential, higher levels of acid phosphatase activity, and increment of carbon and nitrogen in microbial biomass.

Sn-mediated transformations on Si(111) surface: Reconstructions, Electromigration, Homoepitaxy

We have studied Si mass transport and morphological transformations on the Si(111) surface during (√3 × √3)-Sn reconstruction formation and following Si homoepitaxy in 600–825°C interval by in situ ultrahigh vacuum reflection electron microscopy (UHV REM) and ex situ atomic force microscopy (AFM). We have shown that the formation of a metallic α-(√3 × √3)-Sn phase during Sn deposition at T < 650 °С leads to Si 2D island nucleation on > 1 μm terraces, while the formation of a mosaic γ-(√3 × √3)-Sn phase at T > 650 °С is followed by monatomic step shift in the step-up direction and provides Sn/Si(111) interface with wide atomically flat terraces. The step shift value has been measured as functions of substrate temperature and Sn deposition rate to determine emitted by the steps Si coverage participating in γ-phase formation. The electromigration-induced drift of disordered “1 × 1”-Sn domains has been shown to entail enhanced noncompensated Si mass transport followed by the Si(111)-(√3 × √3)-Sn surface roughening. The kinetics of 2D island nucleation and growth near monatomic steps and on wide (up to 10 μm) terraces on the Si(111)-γ-(√3 × √3)-Sn surface during Si deposition at 700°C is limited by surface diffusion only, and critical nucleus i consists of ∼ 20–100 Si atoms.

Towards the new generation of courtyard buildings as a healthy living concept for post-pandemic era

COVID-19 has laid a context for holistic research and practical approaches towards health issues in buildings. This study focuses on one particular residential building type, which is a combination of a modern apartment building with private double-oriented terraces, and a traditional courtyard building. This principle improves several aspects of healthy buildings and contributes to address indoor-outdoor interactions, daylighting, and the use of natural ventilation. The purpose of this study is to determine the factors underlying a particular type of semi-outdoor space within building forms and to explain their microclimatic behavior in buildings. One solid model and twelve porous apartment buildings with different numbers of porous sides, and terrace widths are evaluated using computational fluid dynamics. The k–ε turbulence model is adapted to simulate airflow in and around a four-story building. CFD simulations were validated against the wind-tunnel measurements. Investigations indicated that increasing the number of porous sides reduces the internal mean and maximum ages of air by -15.75 and -36.84%, which means improved ventilation performance. However, it leaves a negative trace on ventilation of the semi-outdoor spaces. Meanwhile, increasing the width of the terraces enhances the ventilation performance by reducing the mean age of air in units, courtyards, and terraces by -20%, -20%, and -9%, respectively.

From dissolution to controlled macrostepping of 4H-SiC during liquid Si-induced structuring in a sandwich configuration

The structuring of Si face 4°off 4H-SiC surfaces was investigated using Si melting in a SiC/Si/SiC sandwich configuration. The stacks were treated at 1550–1600 °C under H2 in a RF-heated cold-wall reactor. By fixing the liquid Si thickness to 30 µm, the vertical thermal gradient inside the stack generates carbon transport from the bottom to the top SiC wafer. The constant dissolution of the bottom SiC wafer (1.7 µm/h at 1550 °C) leads to surface structuring in macrosteps. The regularity of these macrosteps can be reproducibly controlled when performing the process on an epitaxial layer thanks to the pre-structuration in parallel microsteps of such kind of surfaces. The best regularity of the steps was obtained after a structuring process of 2 h, with an average terrace width of ∼3–5 µm.

Modulation of the LaFeO3 film growth by the terrace width of SrTiO3 substrates

We have investigated the growth of polar LaFeO3 thin films on SrTiO3 (001) substrates with various terrace widths by pulsed laser deposition. It is found that the growth of LaFeO3 films undergoes a transition from a two-dimensional (2D) mode to a three-dimensional (3D) mode due to polarity compensation. Notably, however, the critical thickness of the growth mode transition from 2D to 3D can be modulated by the terrace width of the substrate, i.e., when the film is 30 monolayers thick, it becomes 3D growth on the substrates with narrow terrace width (∼100 nm) while it maintains 2D growth on those with wide terrace width (∼400 nm). Combined with dynamic and kinetic models, we have found that the modulation of the critical thickness from 2D to 3D is rationalized as a result of competition between the dynamic process and the kinetic process. These findings highlight the key influence of substrate surface morphology on the epitaxial growth of complex oxide films, shedding light on the construction of multifunctional artificial low-dimensional structures with polar stackings.

Terrace Engineering of the Buffer Layer: Laying the Foundation of Thick GaN Drift Layer on Si Substrates

AbstractVertical GaN‐on‐Si devices are promising for the next‐generation high‐voltage power electronics with low cost and high efficiency. However, their applications are impeded by the limited thickness of crack‐free GaN layers and high threading dislocation density (TDD) in the layer. Buffer layers are crucial for stress control while they usually behave with poor surface morphology, which causes early stress relaxation in GaN and limited thickness. Hereby, a terrace engineering approach for the buffer layers is proposed. Through tuning the supersaturation ratio, the terrace width can be manipulated and an atomically smooth AlGaN buffer layer can be realized, which effectively reduces the compressive stress relaxation and provides a firm foundation for thick GaN growth. As a result, a 7.5 µm thick GaN drift layer with TDD as low as 8.6 × 107 cm−2 is achieved on Si substrates. The room temperature electron mobility of the GaN drift layer can be raised up to 1210 cm2 V−1 s−1. The fabricated PiN diode shows a high breakdown voltage of 1058 V as well as a high on/off ratio of 1012. This work thus truly demonstrates the potential of high‐performance and low‐cost GaN‐based electronic as well as optoelectronic devices on Si platforms.