Substrate Surface Preparation Research Articles

GaN nanowires on Si (111) substrates via molecular beam epitaxy: growth, electronic and optical properties

In this report, we study influence of the Si (111) substrate surface preparation on the growth, electronic and optical properties of the GaN nanowires (NWs) obtained via plasma-assisted molecular beam epitaxy. The substrate preparation varied from bare Si (111) surface and its deliberately nitridated counterpart to growth on AlN and Ga2O3 buffer layers and Ga droplets seeding layers. Statistical data on the morphology of the synthesized arrays was obtained and analyzed. The most homogeneous NW array in terms of length distribution was obtained on AlN buffer layer. It was demonstrated that the NWs surface density drastically depends on the surface preparation method. Electrical properties of the arrays were studied via analysis of volt-ampere characteristics and optical properties were investigated with photoluminescence. The highest conductivity and optical response were obtained with AlN buffer layer.

III–V Solar Cells Grown on Unpolished and Reusable Spalled Ge Substrates

Substrate reuse offers a promising route toward enabling high-efficiency III–V solar cells to become cost-competitive for one-sun terrestrial applications. In this study, Ge films were spalled using an electroplated Ni stressor layer, and the fracture surface was characterized. Initial heteroepitaxial GaAs films grown on spalled Ge substrates by hydride vapor phase epitaxy were found to be single crystal with nearly equivalent growth rates between growth on epi-ready and unpolished spalled Ge substrates. We show the first demonstration of III–V solar cells (GaInAsP, Eg ∼ 1.7 eV) grown on unpolished spalled Ge substrates to assess the viability of substrate reuse. We demonstrate equivalent performance for a solar cell grown on spalled Ge and on a coloaded epi-ready Ge substrate, despite the residual roughness on the spalled Ge substrate and the lack of any substrate surface preparation. This initial demonstration of combining substrate reuse with the ability to grow III–V solar cells using a high growth rate and high throughput deposition process is a promising step toward low-cost III–V photovoltaics.

CdZnTe Substrate Surface Preparation Technology at ASELSAN, Inc. for Molecular Beam Epitaxy Growth of High Quality HgCdTe Epilayers

High quality CdZnTe substrates with 4% ZnTe mole fraction are used for epitaxial growth of HgCdTe infrared detector layers. Molecular Beam Epitaxy (MBE) growth of HgCdTe epilayers requires high quality surface layer with sub-nanometer surface roughness values as well. We report on the development of a CdZnTe substrate surface preparation process for MBE growth of high quality HgCdTe epilayers. Surface preparation processes were performed on both commercially available CdZnTe substrates and substrates obtained from in-house grown CdZnTe boules. We developed a multi-step substrate surface processing flow to achieve sub-nanometer surface roughness, low total thickness variation (TTV), and wax or slurry residue free CdZnTe substrate surfaces. Each process step was optimized with the aim of removing the subsurface damage caused by the previous process step; so that we reduce the amount of damaged layer needed to be etched prior to epitaxy. Our developed surface preparation process can be applicable to commercially available CdZnTe substrates with high surface roughness and high TTV. This process was also optimized for as-cut CdZnTe slices. We are capable of processing typically 25 mm × 25 mm CdZnTe substrates with achievable surface roughness values (Rrms) down to below 0.5 nm.

Mechanical performance of statically loaded flat face epoxy bonded concrete joints

One of the main challenges in the offshore renewable energy industry is the reduction in the levelised cost of energy of wind, wave and tidal devices. The use of concrete as the primary construction material in such devices presents a low unit cost, high marine durability alternative to steel, however, to maximise material efficiency factors such as mix constituent design, structural detailing and manufacturing processes have to take into account the specific conditions of the marine environment. Pre-cast segmental construction can be considered as one of the fastest and cheapest construction options. However the challenges regarding performance of epoxy bonded concrete in marine environment should be taken into account. This paper presents the results of an experimental programme on the performance of shear and tensile capacity of flat face concrete joints, focussing on the effect of substrate surface preparation, joint thickness, properties of epoxy resins, exposure to seawater and presence of joint defects on the ultimate failure load. The ultrasonic pulse velocity (UPV) method for detection of defects in the adhesive layer was examined and digital image correlation is used to observe the surface strain flow through the joint. The results indicate that the epoxy joints behave monolithically and remain undamaged under different types of static loading. The joints do not significantly interrupt the flow of strain but can locally affect the distribution of strain (and thus stiffness and stresses) in a structure. An increase in the density of the epoxy (and the filler content) leads to the increase in the joint strength and thicker joints are less affected by small defects in the bonding layer. The majority of tested specimens failed by cracking of concrete rather than by debonding of the joint, whilst compressive stresses acting on the joint can help to augment its shear strength. Sandblasting of bonded surfaces can improve performance of joints, whereas UPV testing may be used for quality control of epoxy-bonded joints.

Evaluación de la activación superficial de aceros inoxidables usando sanblasting y mecanizado para la aplicación de recubrimientos mediante aspersión de plasma atmosférico (APS)

En el presente trabajo se realizó una evalución de la preparación superficial de sustratos metálicos con chorro de alumina (sandblasting SB) y mediante el uso de herramientas de corte (HC) sobre aceros inoxidable AISI 304 para después aplicar un recubrimiento por APS. Se utilizo la estacion de rugosidad de la Universidad Nacional de Colombia Sede Medellin con el apoyo del grupo GTS de superficies, para analizar la textura y asi comparar los perfiles de rugosidad y sus respetivos parametros de rugocidad tomando de referencia los parametros más usados en contexto metalmecanico nacional. Como resultado de ejecutar ambas tecnicas sobre inoxidable 304, se evidencio que existe un efecto significativo en la activacion superficial cuando se combinan el sandblasting y los procesos de mecanizado para activar la superficie ya que la componente polar de energia libre disminuye de manera significativa al aumentar las variables de rugosidad.

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.

Experimental and Numerical Study of the Influence of Substrate Surface Preparation on Adhesion Mechanisms of Aluminum Cold Spray Coatings on 300M Steel Substrates

The effect of substrate surface topography on the creation of metallurgical bonds and mechanical anchoring points has been studied for the cold spray deposition of pure aluminum on 300M steel substrate material. The coatings adhesion strength showed a significant decrease from 31.0 ± 5.7 MPa on polished substrates to 6.9 ± 2.0 MPa for substrates with roughness of 2.2 ± 0.5 μm. Strengths in the vicinity of 45 MPa were reached for coatings deposited onto forced pulsed waterjet treated surfaces with roughnesses larger than 33.8 μm. Finite element analysis has confirmed the sole presence of mechanical anchoring in coating adhesion strength for all surface treatment except polished surfaces. Grit embedment has been shown to be non-detrimental to coating adhesion for the current deposited material combination. The particle deformation process during impacts has been studied through finite element analysis using the Preston–Tonks–Wallace (PTW) constitutive model. The obtained equivalent plastic strain (PEEQ), temperature, contact pressure and velocity vector were correlated to the particle ability to form metallurgical bonds. Favorable conditions for metallurgical bonding were found to be highest for particles deposited on polished substrates, as confirmed by fracture surface analysis.

Hydrogen Etching Influence on 4H-SiC Homo-Epitaxial Layer for High Power Device

The surface preparation of 4H-SiC substrate plays a crucial role for the epitaxial growth. In the present work, the Hydrogen etching influence on 4H-SiC surface of substrate before the growth process was studied. The epi-layer was grown .with a commercial low-pressure, hot-wall Chemical Vapor Deposition (CVD) reactor by Tokyo Electron Limited. The etching time of the surface was increased until three time (x3) respect with the normal value usually adopted for the growth. Photoluminescence and optical inspection analyses show a clear relationship between the etching time and the defectivity. Atomic Force Microscope (AFM) measurements also show an increase of step bunching with the etching time.

Optimization of the Sandblasting Process for a Better Electrodeposition of Copper Thin Films on Aluminum Substrate by Feedforward Neural Network

The influence of a proper surface preparation is essential for a better adhesion of copper thin films on aluminum substrate. In this work, the surface properties of the aluminum substrate have been modified through sandblasting process, in order to influence the quality of electroplating. To evaluate the correct adhesion of the thin film to the substrate non-destructive measurements of diffusivity by infrared thermography have been made. A combining of a feedforward artificial neural network (FFANN) and an external optimized algorithm (EOA) is proposed to optimize the substrate surface preparation process. A FFANN model is developed to map the complex non-linear relationship between the surface process conditions of the substrate and the thermal diffusivity of the electroplated sample. A good performance of the FFANN model is achieved. An EOA is used for the optimization of the sandblasting process conditions, in order to maximize the adhesion of the thin film to the substrate.

Failure Modes in Concrete Repair Systems due to Ongoing Corrosion

Corrosion of steel reinforcement is the main cause of deterioration in reinforced concrete structures. It can result in cracking and spalling of the concrete cover. After the damaged cover is repaired, reinforcement corrosion might continue and even accelerate. While the development of the corrosion cell is difficult to control, the damage can be possibly delayed and controlled by use of a suitable repair material. The lattice fracture model is used in this paper to investigate the performance of strain hardening cementitious composite (SHCC) in concrete repair systems exposed to ongoing corrosion. Numerical results were verified by experimental tests when SHCC, nonreinforced material (repair mortar), and commercial repair mortar are used as repair materials. In experiments, reinforcement bars (surrounded by a repair material) were exposed to accelerated corrosion tests. The influence of the substrate surface preparation, the type of repair material, the interface, and the substrate strength on the resulting damage and failure mode of repair systems are discussed. In general, SHCC repair enables distributed cracking with small crack widths, up to several times smaller compared to repair mortar. Furthermore, more warning signs prior to the final failure are present in the SHCC repair system.

Effect of substrate surface condition on fatigue behavior of cold sprayed Ti6Al4V coatings

Ti6Al4V coatings were deposited on Ti6Al4V substrates by using a high pressure cold spray system. Before deposition, the substrate surfaces were prepared with four different surface preparation methods, namely, grinding, milling, sand blasting and water-jet cutting. The fatigue behavior of the coated specimens was studied with three point-bending-fatigue test. The fracture characteristics of the specimens were observed by using scanning electron microscopy. The surface roughness and its effect on the crack initiation and propagation in the coatings were also investigated. The results indicated that the fracture mode of the coated samples was interface cracking. At two different stress levels such as 420 and 360MPa, the samples with the coatings deposited on the ground surfaces survived the longest fatigue life before failure, while the samples with the coatings deposited on the water-jet cut surfaces survived the shortest fatigue life. It was clearly shown that the fatigue strength of the cold sprayed Ti6Al4V coatings was strongly affected by the both spraying and substrate surface preparation conditions.

A 3D Lattice Modelling Study of Drying Shrinkage Damage in Concrete Repair Systems.

Differential shrinkage between repair material and concrete substrate is considered to be the main cause of premature failure of repair systems. The magnitude of induced stresses depends on many factors, for example the degree of restraint, moisture gradients caused by curing and drying conditions, type of repair material, etc. Numerical simulations combined with experimental observations can be of great use when determining the influence of these parameters on the performance of repair systems. In this work, a lattice type model was used to simulate first the moisture transport inside a repair system and then the resulting damage as a function of time. 3D simulations were performed, and damage patterns were qualitatively verified with experimental results and cracking tendencies in different brittle and ductile materials. The influence of substrate surface preparation, bond strength between the two materials, and thickness of the repair material were investigated. Benefits of using a specially tailored fibre reinforced material, namely strain hardening cementitious composite (SHCC), for controlling the damage development due to drying shrinkage in concrete repairs was also examined.

Detailed investigation on the mechanism of co-deposition of different carbon nanostructures by microwave plasma CVD

Co-deposition of carbon nanotubes (CNTs) along with nano-crystalline diamond and graphene nanowalls (GNWs) have been made possible by microwave plasma enhanced chemical vapor deposition. CNTs have been grown directly on Inconel and other nanostructures could be grown on the Si microwave attenuator placed above Inconel. The detailed mechanism involving co-deposition of sp3 and different sp2 hybridized nanostructures has been investigated through a series of experiments and windows for co-deposition have been found out. Deposited samples are characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and Raman imaging. Characterization results revealed that combination of substrate surface preparation, carbon precursor feeding rate and deposition time are the deciding factors for growth of a particular type of nanostructure. Raman spectroscopic results show that GNWs behave like nano form of turbostratic graphite and it is seen that they can be deposited even when plasma is highly perturbed. Optical emission spectroscopy results indicated that an increase in the intensity of C2 swan band increases deposition of graphitic nanostructures but decreases the required control for uniform deposition. The role of plasma sheath in co-deposition is explained with the help of a schematic model.

Effects of substrate on the domains and electrical properties of epitaxial graphene formed on on-axis C-face 4H-SiC

The effect of substrate surface preparation on the quality of epitaxial graphene grown on on-axis C-face 4H-SiC has been investigated. Structural epitaxial graphene has been achieved using a graphite enclosure. The homogeneity and domain size of epitaxial graphene on different pretreated substrates were studied using Raman spectroscopy, field emission scanning electron microscopy and hall measurements. The etching of C-face SiC substrate using C3H8/H2 is shown to have a pronounced effect on the domain size and hall mobility of multilayer epitaxial graphene. From Raman measurements, it is found that the pretreatment of substrate has a great influence on the domain size of epitaxial graphene. From hall results, it is found that the effect of substrate pretreatment on carrier mobility follows the same tendency reflected in the results of Raman spectra. It is concluded that the pretreatment of substrate surface, mainly the etching for generating high density, regular and periodic step structure with flatter terrace, would allow the growth of graphene with significantly large area and high mobility, even on large on-axis C-face 4H-SiC wafer.

Effect of Growth Parameters and Substrate Surface Preparation for High-Density Vertical GaAs/GaAsSb Core–Shell Nanowires on Silicon with Photoluminescence Emission at 1.3 μm

GaAs/GaAsSb nanowire (NW) arrays are ideally suited to meet the demands of the next generation infrared (IR) photodetectors with potential for improving detection. NWs in a core–shell geometry have the advantage of providing axial direction for a long optical path for enhanced optical absorption and a short radial path for charge diffusion and collection. For the Ga-assisted molecular beam epitaxial growth of vertical, dense and uniform GaAs core NWs on Si (111), the effects of substrate surface preparation in combination with growth parameter variation were examined. On the epiready substrate without any surface preparation, both initial Ga shutter opening duration and V/III beam equivalent pressure ratio play a vital role in achieving almost all vertical NWs with moderate density ~107 cm−2. Also the spatial uniformity of the NWs was poor. Substrate surface preparation by chemical cleaning followed by oxidation in air led to highly vertical and uniform NWs with high density (8 × 108 cm−2). The GaAsSb shell was then successfully grown around the highly dense and vertical core GaAs NWs at growth temperatures ranging from 550°C to 590°C. It was found that growth temperature has a strong influence on Sb incorporation in the NWs and, hence, the NW morphology and 4K photoluminescence (PL) spectra. The presence of x-ray diffraction peaks corresponding to (111) reflection only and its higher-order reflections attest to the vertical alignment of NWs. Strain in the NWs as estimated using the Williamson–Hall isotropic strain model increases with Sb incorporation, which results in bending of the NWs with increasing Sb. Structural properties of these NWs using scanning transmission electron microscopy (STEM) are also presented. The temperature dependence PL of the NWs exhibited “S-curve” behavior, which is a well-known signature of localized excitons and a room temperature band edge PL emission occurring at ~1.3 μm.

Mechanization of the Grit Blasting Process for Thermal Spray Coating Applications: A Parameter Study

The bond strength between a thermal spray coating and substrate is critical for many applications and is dependent on good substrate surface preparation and optimized spray parameters. While spray parameters are usually carefully monitored and controlled, most surface preparation is carried out by manual grit blasting, with little or no calibration of blast parameters. Blasting is currently highly dependent on operator skill and often surface finish is only assessed visually, meaning a consistent, reproducible surface profile cannot be guaranteed. This paper presents investigations on the effect of blast parameters (including blast pressure, standoff distance, media feed rate, blast angle, traverse speed, and media size) on surface profile for seven different metallic substrates using a mechanized, robotic blasting system and employing a brown fused alumina blast medium. Substrates were characterized using non-contact focus variation microscopy. Average surface roughness was found to be most affected by blast pressure, media size, and traverse speed, while changes to media feed rate and standoff distance had a limited effect on surface profile. Changes to blast angle resulted in limited change to average roughness, but microscopy examinations suggested a change in the mechanism of material removal.

Effect of Pulsed Waterjet Surface Preparation on the Adhesion Strength of Cold Gas Dynamic Sprayed Aluminum Coatings

It has been observed that the method of substrate surface preparation can have a profound effect on the adhesion strength of cold-sprayed metallic coatings. In this investigation, pure aluminum powder was sprayed onto aluminum alloy substrates using cold spray. The substrates used in this work had undergone a variety of surface preparations to impart varying degrees of surface roughness. The pulsed waterjet technique was used to increase the substrates’ surface roughness beyond what can be achieved using traditional grit blasting procedures. Surfaces prepared using pulsed waterjet resulted in substantial increases in the pure aluminum coating adhesion strength. This increase may be the result of increased mechanical anchoring sites available as well as their favorable geometries. It is hypothesized that compressive residual stress may also contribute to increased adhesion strength.

Pulse Electrodeposition of Sn-Ni-Fe Alloys and Deposit Characterisation for Li-ion Battery Electrode Applications

Characterisation of Sn-Ni-Fe ternary alloy deposits obtained by pulse electrodeposition show that novel alloys of varying composition can be successfully deposited exhibiting amorphous structures using specific on/off pulse plating parameters for electrodeposition in combination with an electrolyte based on gluconate as a complexing agent. The deposits obtained exhibited high quality bright metallic surface morphologies combined with a distinct pronounced spherical/nodular grain structure across a range of average current densities. The influence of on/off period pulse current and the effect of the average current density on the surface morphology of the electrodeposited films were studied using Scanning Electron Microscopy (SEM). The cathode efficiency results obtained show that the pulse electrodeposition process chosen is significantly more efficient than the constant current electrodeposition. Results suggest that the substrate surface deformation condition and preparation are critically important in achieving a surface morphology suitable for an effective Sn-Ni-Fe alloy battery electrode. The alloy deposits with a morphology, which exhibits a high surface area per unit volume, are shown to be promising as an alternative anode material for lithium ion batteries.

The key process parameters influencing formation of columnar microstructure in suspension plasma sprayed zirconia coatings

The paper discusses the screening of experimental variables leading to formation of a columnar microstructure in suspension plasma sprayed zirconia coatings. These variables tested in 12 experimental runs included: (i) 2 types of zirconia powder; (ii) 4 concentration of solids in suspensions; (iii) 4 substrate preparation methods; and (iv) 2 plasma spray setups. Two different, commercially available, powders were used to formulate the suspensions. Yttria and ceria stabilized zirconia of composition ZrO2+24wt.% CeO2+2.5wt.% Y2O3 (YCSZ) was milled the decrease the particles sizes. The yttria stabilized zirconia of composition ZrO2+14wt.% Y2O3 (14YSZ) was used as received. The coatings were deposited on 304L stainless steel substrates which had the surface prepared by: (i) grid blasting; (ii) grinding; (iii) turning; and (iv) laser treatment. The 3D topographies of substrates' surfaces were characterized and their roughnesses were measured. The suspensions were plasma sprayed using the following plasma torches: SG-100 of Praixair and Triplex of Sulzer-Metco. The microstructure of powders and coatings was analyzed by optical microscopy, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) as well as by X-ray diffraction. The columnar microstructure was formed in coatings sprayed with both plasma setups sprayed using finer 14YSZ powder suspensions. The substrate surface preparation as well as low concentration of solids in suspension promoted their formation. Rietveld method was applied to determine the quantity of different phases in the structure of coatings and to calculate the lattice parameters. The YCSZ coatings crystallized in mainly tetragonal phase with a small content of monoclinic phase. The 14YSZ crystallized in cubic phase. Finally, the thermal diffusivity of coatings was characterized up to 523K with the use of laser flash method and thermal conductivities of coatings were determined. The conductivities were in the range from 0.6 to 1.1W/(mK) depending on temperature for YCSZ and 14YSZ coatings.

A review comparing cathodic arcs and high power impulse magnetron sputtering (HiPIMS)

High power impulse magnetron sputtering (HiPIMS) has been at the center of attention over the last years as it is an emerging physical vapor deposition (PVD) technology that combines advantages of magnetron sputtering with various forms of energetic deposition of films such as ion plating and cathodic arc plasma deposition. It should not come as a surprise that many extension and variations of HiPIMS make use, intentionally or unintentionally, of previously discovered approaches to film processing such as substrate surface preparation by metal ion sputtering and phased biasing for film texture and stress control. Therefore, in this review, an overview is given on some historical developments and features of cathodic arc and HiPIMS plasmas, showing commonalities and differences. To limit the scope, emphasis is put on plasma properties, as opposed to surveying the vast literature on specific film materials and their properties.