Step-free Surface Research Articles

Te Pua Keith Park – Nau mai, Haere mai Let’s Play Together

Te Pua/Keith Park playground employed an innovative early program of co-design with an All Abilities Project Group (AAPG), representing disability organisations and key stakeholders from the community. Through ongoing engagement with disabled people as experts, the outcome was an inclusive and welcoming play space for a diverse range of children, young people and their caregivers.Play equipment included a range of vestibular, visual, and auditory pieces as well as a customised 2m high wheelchair accessible play tower for inclusive play experiences. Caregivers were enabled to play with their children through smooth and step-free surfaces as well as specific play equipment such as an adult and child swing. Children and young people of different ages and abilities were encouraged to sit/lie/stand in the basket swing and see-saw together. Unique to this playground, communication boards were innovatively and collaboratively designed with visual images representing various features of the playground and QR codes linking to online videos with New Zealand Sign Language.In addition to play equipment, the AAPG identified that the toilet facilities were crucial to ensuring accessibility to many families, including those with bigger children or teens with access needs who were often faced with the reality of needing to be changed in unsanitary and unsafe ways without the appropriate facilities being available. Keith Park worked with a leading toilet manufacturer to co-design a bespoke double toilet block with enhanced accessibility features including an adult-sized change table.Every aspect of the park was carefully selected and designed including fencing, furniture, plants and colours. Colour enhanced accessibility by guiding children with low vision and created a play circuit to assist the neurodiverse community. The resultant playground is one that welcomes all to play, which is a core tenet of child development, socialisation and participation.
 
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Selectively buried growth of heavily B doped diamond layers with step-free surfaces in N doped diamond (1 1 1) by homoepitaxial lateral growth

In this work, we proposed a novel method to form a buried heavily boron (B) doped p+-structure with a step-free diamond surface in nitrogen doped diamond (111) based on homoepitaxial lateral growth technique. The buried structure with a size of 10 µm × 10 µm × 0.085 µm was fabricated by inductively coupled plasma (ICP) etching of the mesa structure and subsequent heavy B-doping, with a boron to carbon (B/C) ratio of 15,000 ppm in the microwave-plasma-assisted chemical vapor deposition (MPCVD). The surface morphology of the buried area and its surrounding were observed by laser microscopy (LM) and atomic force microscopy (AFM), which showed atomically flat surfaces before and even after B doping. Surface potential difference measurements on both areas were applied using Kelvin-probe force microscopy (KFM). The intensity imaging of B concentration in and around the buried areas were measured by dynamic secondary ion mass spectrometry (SIMS), which confirmed the heavy B doping of the buried structure and the depth profile shows that B doping is uniform, with a concentration of 2 × 1020 atoms/cm3.

Step-free GaN surfaces grown by confined-area metal-organic vapor phase epitaxy

A two-step homoepitaxial growth process producing step-free surfaces on low dislocation density, Ga-polar ammonothermal GaN single crystals is described. Growth is conducted under very low supersaturation conditions where adatom incorporation occurs predominantly at step edges, and lateral growth is strongly preferred. The achievable step-free area is limited by the substrate dislocation density. For ammonothermal crystals with an average dislocation density of ∼1 × 104 cm−2, step-free mesas up to 200 × 200 μm2 in size are achieved. These remarkable surfaces create a unique opportunity to study the effect of steps on the properties and performance of semiconductor heterostructures.

Surface and Step Conductivities on Si(111) Surfaces.

Four-point measurements using a multitip scanning tunneling microscope are carried out in order to determine surface and step conductivities on Si(111) surfaces. In a first step, distance-dependent four-point measurements in the linear configuration are used in combination with an analytical three-layer model for charge transport to disentangle the 2D surface conductivity from nonsurface contributions. A termination of the Si(111) surface with either Bi or H results in the two limiting cases of a pure 2D or 3D conductance, respectively. In order to further disentangle the surface conductivity of the step-free surface from the contribution due to atomic steps, a square four-probe configuration is applied as a function of the rotation angle. In total, this combined approach leads to an atomic step conductivity of σ(step)=(29±9) Ω(-1) m(-1) and to a step-free surface conductivity of σ(surf)=(9±2)×10(-6) Ω(-1)/□ for the Si(111)-(7×7) surface.

Nucleus and spiral growth mechanisms of nitride semiconductors in metalorganic vapor phase epitaxy

Nucleus and spiral growth mechanisms of GaN and InN are investigated using the selective-area metalorganic vapor phase epitaxy (SA-MOVPE) technique on GaN bulk substrates. Nucleus growth of GaN occurs within selective areas having no screw-type dislocations, while spiral growth occurs within selective areas having screw-type dislocations. These growth modes are simultaneously observed on a single substrate in a single growth run. The nucleus and spiral growths of GaN result in the formation of step-free surfaces and growth spirals, respectively, wherein the interstep distance of growth spirals enables us to estimate the degree of surface supersaturation (σ). The σ dependences of nucleus and spiral growth rates of GaN are experimentally investigated. We found that these dependences are well explained by the classical crystal growth theories advocated by Burton, Cabrera, and Frank. We also investigate nucleation of InN using step-free GaN surfaces as an ideal platform.

Muscovite mica: Flatter than a pancake

Muscovite mica is a widely used material because of its transparency, heat resistance and especially its flatness. This study investigates how flat muscovite mica really is. The surface of cleaved muscovite mica was studied with the help of several optical techniques, surface X-ray diffraction and with atomic force microscopy. The results show that for high-quality muscovite mica, large (>1cm2) step-free surface areas exist, which makes it one of the flattest materials around. Several reasons are given to explain why this crystal is so incredibly flat. The flatness of muscovite mica can be exploited in applications such as the surface force apparatus and creating a flat interface for organic solar cells.

Formation of Step-Free Surfaces on Diamond (111) Mesas by Homoepitaxial Lateral Growth

Formation of Step-Free Surfaces on Diamond (111) Mesas by Homoepitaxial Lateral Growth

Isotope Effect of Deuterium Microwave Plasmas on the Formation of Atomically Flat (111) Diamond Surfaces

This article describes the positive effect of replacing hydrogen with deuterium on the formation of an atomically flat surface in microwave plasma-assisted chemical vapor deposition (MPCVD) of homoepitaxial (111) diamond. Using atomic force microscopy (AFM) measurements, suppression of the density of surface islands and enlargement of the step-free surface area were identified. Isotope effects promoting preferential etching, which enhances the growth at steps (or kinks) and formation of the atomically flat surface, are discussed with respect to lateral growth.

Isotope Effect of Deuterium Microwave Plasmas on the Formation of Atomically Flat (111) Diamond Surfaces

This article describes the positive effect of replacing hydrogen with deuterium on the formation of an atomically flat surface in microwave plasma-assisted chemical vapor deposition (MPCVD) of homoepitaxial (111) diamond. Using atomic force microscopy (AFM) measurements, suppression of the density of surface islands and enlargement of the step-free surface area were identified. Isotope effects promoting preferential etching, which enhances the growth at steps (or kinks) and formation of the atomically flat surface, are discussed with respect to lateral growth.

Formation of Step-Free Surfaces on Diamond (111) Mesas by Homoepitaxial Lateral Growth

We present a technique for the array formation of atomically step-free diamond surfaces on diamond (111) substrates by microwave plasma-enhanced chemical vapor deposition. With an appropriate choice of plasma conditions, the atomic steps initially present on each mesa surface move by lateral growth, and then, atomically step-free surfaces are successfully formed on diamond (111) mesas by microwave plasma-enhanced chemical vapor deposition. The lateral growth of diamond (111) films results in the formation of step-free surfaces with device dimensions up to 100 µm square on diamond (111) mesas. A limiting factor in scaling up the size and yield of the step-free mesas is the density of screw dislocations in the diamond substrate.

Extremely Narrow Violet Photoluminescence Line from Ultrathin InN Single Quantum Well on Step‐Free GaN Surface

An ultrathin (one monolayer thick) InN single quantum well (SQW) formed on a step-free GaN surface shows very sharp violet PL emission. The size (16 μm in diameter) is large enough for state-of-the-art nanotechnology to handle. Longer wavelength emissions, such as green and red, are expected by increasing the thickness of the SQW through the utilization of the quantum size effect.

Morphological Instability of 4H-SiC (0001) Basal Plane Surface during Si-Vapor Thermal Etching

The morphological instability appeared at step-free 4H-SiC (0001) surfaces was investigated. The step-free surfaces were fabricated at the bottom of inverted-mesa structure by the method combining a laser digging and Si-vapor etching. By repeated Si-vapor etching treatments, randomly created crater and maze structures were cyclically appeared at the step-free surfaces. These structures were distinctly classifiable by their depths from the step-free surfaces. Crater structures have 0.2 - 0.3 nm depth and maze structures have 0.5 nm depth. The morphological evolutions indicate the process of destruction of the step-free (0001) basal plane and generation of steps from step-free surfaces in the Si-vapor etching process.

Fabrication of GaN Step-free Surfaces by Controlling the Surface Supersaturation(<Special Issue>Breakthrough in Nitride Crystal Growth for Next Generation Devices)

Fabrication of GaN Step-free Surfaces by Controlling the Surface Supersaturation(<Special Issue>Breakthrough in Nitride Crystal Growth for Next Generation Devices)

4H-SiC(0001) Basal Plane Stability during the Growth of Epitaxial Graphene on Inverted-Mesa Structures

The epitaxial graphene growth at the 4H-SiC(0001) surface with intentionally inserted step-free basal plane regions was performed by high temperature annealing in the range of 1600–1900 °C under ultrahigh vacuum. For fabricating inverted-mesa structures with the step-free regions at SiC surfaces, a combined process consisting of a direct laser digging and a Si-vapor etching at 1900 °C was utilized. The graphitized surfaces were characterized by atomic force microscopy, low acceleration voltage (0.1–1.0 kV) scanning electron microscopy and Raman spectroscopy. It was found that the graphene thickness at the SiC step-free surface tends to be suppressed compared with the thickness at background SiC step-terrace surfaces where the steps are intrinsically introduced from intentional/unintentional substrate miscut angles. From the characterization by Raman mapping, 1 ML graphene was obtained at the SiC step-free surface at 1600 °C graphitization in contrast to the case that multilayer graphene was grown at SiC step-terrace surfaces.

Vacancy Mediated Diffusion at Surface-Confined Atomic Intermixing

The present study deals with diffusion behavior of adsorbed atoms on stepped crystal surfaces. In volume-immiscible systems, two-dimensional (2D) atomic intermixing at epitaxial interface could be completely blocked on step-free surface domains. This is a result of high diffusion barrier for direct atomic exchange between adsorbed layer and substrate. In that case, diffusion takes place exclusively across the steps of atomic terraces. In such systems, dynamic competition between energy gain by mixing and substrate strain energy results in diffusion scenario where adsorbed atoms form alloyed stripes in the vicinity of terrace edges. At high temperatures, the stripe width increases and finally completely destroys the terraces. This process leads to formation of alloyed 2D atomic islands on top of pure substrate layer. The atomistic Monte Carlo simulations reveal vacancy-mediated mechanism of diffusion inside atomic terraces as a result of spontaneous generation of vacancies at high temperature. Being in agreement with recent experimental findings, the observed surface-confined alloying opens up a way various surface pattern to be configured at different atomic levels on the crystal surface.

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH 4/H 2 ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm 2 mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.

HCl Etching Behavior on Low-Tilt-Angle and Step-Free 4H-SiC Surfaces

We report on new observations made, when 4H-SiC, Si-face substrate mesas, having either low tilt-angle (< 1°) with steps or step-free top surfaces, were exposed to three separate HCl etching conditions for five minutes at temperatures of 1130°C, 1240°C and 1390°C. We observed that HCl was ineffective at 1130°C, as etching was incomplete with abundant surface contamination. At 1240°C, screw dislocations were aggressively etched by HCl, while multiple shallow flat-bottomed etch pits were formed on step-free mesa surfaces. At 1390°C, step-flow etching dominated as large etch pits were formed at screw dislocations and previously step-free surfaces etched inward from mesa edges to form parallel rows of organized steps.

Determination of the step dipole moment and the step line tension on Ag(0 0 1) electrodes

Using impedance spectroscopy, we determined the step dipole moment and the potential dependence of the step line tension of silver electrodes in contact with an electrolyte: (0 0 1) and vicinal surfaces (1 1 n) with n = 5, 7, 11 in 10 mM ClO 4 −-solutions were investigated. The step dipole moment is determined from the shift of the potential of zero charge ( pzc) as a function of the surface step density. The dipole moment per step atom was found to be 3.5 ± 0.5 × 10 −3 e Å. From the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tensions of stepped (1 1 n) and the nominally step-free (0 0 1) surfaces. The results are compared to a previous study on Au(1 1 n) surfaces. For gold, the step line tension decreases roughly linear with potential, whereas a parabolic shape is observed for silver.

A novel approach to measure the step line tension and the step dipole moment on vicinal Au(0 0 1) electrodes

In this paper we introduce a new experimental approach to determine the potential dependence of the step line tension on metal electrodes in contact with an electrolyte: (0 0 1) and (1 1 n) surfaces of single crystal gold electrodes were investigated by impedance spectroscopy in solutions containing weakly adsorbing anions, such as ClO 4 - , F − and SO 4 2 - . Within the limits of error the shift in the potential of zero charge is proportional to the step density of the vicinal surfaces indicative of a well-defined dipole moment per step length. The dipole moments per step atom are 6.8 ± 0.8, 5.2 ± 0.4, 5.8 ± 0.5 × 10 −3 eÅ for SO 4 2 - , ClO 4 - , and F − containing electrolytes, respectively. Using the values for the pzc and the potential dependence of the capacitance curves, the potential dependence of the surface tension of the vicinal surfaces is determined. The line tension of the steps is then calculated from the difference between the surface tension of the stepped and the step free surface. Our results represent the first experimental confirmation of a recent theoretical model proposing that in absence of specifically adsorbed ions the step line tension should decrease (roughly linear) with potential.

Approaches to Reduced-Defect Active Regions for III-N Devices

Two approaches to achieving reduced-defect active regions in III-N devices are discussed - confined epitaxy and heteroepitaxy on step-free SiC surfaces. In confined epitaxy, sapphire substrates (either GaN coated or not) are patterned with a dielectric mask and then III-N device structures are selectively and vertically grown in the openings using metalorganic chemical vapor deposition (MOCVD). In heteroepitaxy on step-free SiC surfaces, SiC mesas are created that have surfaces completely free of atomic steps and then used as substrates for conventional MOCVD III-N growth. In both approaches significant reductions in extended defect densities (10-100x) are observed and manifest in improved electroluminescence efficiency of UV emitters and leakage currents in UV detectors. Extensions of these efforts and other structural characterization results will be presented. Modeling results suggesting directions for future efforts will also be discussed.