Plain Substrates Research Articles

Wet Etching of Silicon in Planar Nanochannels.

Silicon (Si) alkaline etching constitutes a fundamental process in the semiconductor industry. Although its etching kinetics on plain substrates have been thoroughly investigated, the kinetics of Si wet etching in nanoconfinements have yet to be fully explored despite its practical importance in three-dimensional (3-D) semiconductor manufacturing. Herein, we report the systematic study of potassium hydroxide (KOH) wet etching kinetics of amorphous silicon (a-Si)-filled two-dimensional (2-D) planar nanochannels. Our findings reveal that the etching rate would increase with the increase in nanochannel height before reaching a plateau, indicating a strong nonlinear confinement effect. Through investigation using etching solutions with different ionic strengths and/or different temperatures, we further find that both electrostatic interactions and the hydration layer inside the nanoconfinement contribute to the confinement-dependent etching kinetics. Our results offer fresh perspectives into the kinetic study of reactions in nanoconfinements and will shed light on the optimization of etching processes in the semiconductor industry.

Influence of electron beam irradiation on nanoscale adhesion during colloidal probe experiments inside the scanning electron microscope

Scanning electron microscope is not only a versatile visualization and characterization tool for nanomaterials, but it is also widely used for visual feedback during assembly and manipulation of micro- and nanoparticles, novel devices based on 2D and 1D materials, and microrobots. Due to increase in the surface-to-volume ratio, adhesion forces play a significant role in micro- and nanorobotic applications. We investigate the quantitative effect of electron beam irradiation on nanoscale adhesive interactions between different end effector types and plain substrates. For this purpose, we use an interferometry-based force measurement setup integrated into the chamber of a scanning electron microscope capable of a subnano Newton force resolution. We provide quantitative results for the influence of the electron beam on the adhesion forces depending on the exposure dose, geometrical configuration in which it is applied, as well as on the material of the end effector. Simulation results, atomic force microscope adhesion measurements, and measurements with piezoresistive cantilevers serve as references. Our findings indicate that the electron beam influences the adhesive interaction not only in the case of insulating probes but also for well-grounded conducting and semiconducting probes. Its influence can range from several 10% to several 100% in comparison to the adhesion values measured with an atomic force microscope in an ambient atmosphere and no charged particle beam is applied. Moreover, it is shown that the influence of a charged particle beam is a multidimensional phenomenon that depends on several factors such as end effector and substrate material, geometry and aspect ratio of the end effector, geometrical configuration between particle beam and probe–sample arrangement, as well as exposure dose and field of view. We provide recommendations for handling and characterization of micro- and nano-objects using end effectors under the influence of charged particle beams: not to rely on adhesion values acquired in different environmental and exposure conditions, not to expect the application of conductive materials and grounding to completely avoid the effect of charged particle beams, and to be aware that charged particle beams can lower or increase the adhesion force in vacuum depending on material properties, among others.

Angle dependent specular reflectance study on antimony triselenide photovoltaic absorber grown on periodically textured substrates

Typical thickness of the absorber layer in conventional thin film photovoltaics is of the order of few microns to few tens of nanometers depending upon the light absorber material used. Optimum values are often associated with the diffusion length of the minority charge carriers, that determine the upper limit and hence the optical absorption coefficient. On the other hand, it is desirable to increase absorption coefficient to maximize the utilization of incident photons. To achieve the same, the optical path length of the incident photons needs to be increased. Surface modifications via random/periodic texturing is considered for light trapping, as it induces multiple reflections of the incident photons, thereby increasing optical path length. In this work, glass substrates were textured with SU-8 2005 photoresist consisting of periodical array of micro cuboids (height ∼6 μm; side length ∼100/70/50/20μm) over an area of 20 mm × 20 mm using i-line lithography. Sb2Se3 absorber layer was conformally grown onto textured as well as on plain substrates via e-beam evaporation. UV–Vis-NIR spectra were recorded and the optical absorbance was analyzed as a function of cuboidal size. Optical absorption increases with decrease in the cuboidal feature size for side lengths up to 40μm and decreased for 20μm. Confirmative angle dependent specular reflectance study was done to correlate the observed absorption behavior, which was in corroboration with absorption measurements. Complementing absorption and reflectance results were understood through COMSOL Multiphysics optical path tracing simulations.

Amorphous Carbon Coatings for Total Knee Replacements-Part I: Deposition, Cytocompatibility, Chemical and Mechanical Properties.

Diamond-like carbon (DLC) coatings have the potential to reduce implant wear and thus to contribute to avoiding premature failure and increase service life of total knee replacements (TKAs). This two-part study addresses the development of such coatings for ultrahigh molecular weight polyethylene (UHMWPE) tibial inlays as well as cobalt–chromium–molybdenum (CoCr) and titanium (Ti64) alloy femoral components. While a detailed characterization of the tribological behavior is the subject of part II, part I focusses on the deposition of pure (a-C:H) and tungsten-doped hydrogen-containing amorphous carbon coatings (a-C:H:W) and the detailed characterization of their chemical, cytological, mechanical and adhesion behavior. The coatings are fabricated by physical vapor deposition (PVD) and display typical DLC morphology and composition, as verified by focused ion beam scanning electron microscopy and Raman spectroscopy. Their roughness is higher than that of the plain substrates. Initial screening with contact angle and surface tension as well as in vitro testing by indirect and direct application indicate favorable cytocompatibility. The DLC coatings feature excellent mechanical properties with a substantial enhancement of indentation hardness and elastic modulus ratios. The adhesion of the coatings as determined in modified scratch tests can be considered as sufficient for the use in TKAs.

Effects of Palladium Chloride Concentration on the Nanoscale Surface Morphology of Electroless Deposited Palladium Thin Film

Understanding the effects of thin film electroless deposition parameters at nanoscale is crucial for complete understanding and control of the thin film deposition process. In this study, we investigated and optimized the effect of precursor concentration on the nanoscale surface morphology of electroless deposited Pd thin film. The FESEM characterization of plain substrates showed that the dominant features of plain alumina substrates were terraces, steps and bumpy microstructures and the final surface morphologies of the deposited Pd thin film was strongly dependant on the surface morphologies of the substrate. FESEM characterization results of seeding technique displayed a thin film of Pd nanoparticles on the surface of the alumina substrate. The number of times that the seeding process was carried out was optimized at five seeding times using hydrazine as a reducing agent. FESEM characterization revealed that the nanoscale surface morphology of the Pd thin film was strongly dependent on precursor concentration. Three types of secondary nanoscale surface morphologies formed were nanorods, nanoflakes and flower-like Pd nanostructures at various concentrations. The nanoflake surface density was strongly dependent on precursor concentration. Results of this research provided a foundation and method to tailor the nanoscale surface morphology to the specific requirement of surface dependent processes or reactions.

Thick Heteroepitaxy of Binary and Ternary Semiconductor Materials for Nonlinear Frequency Conversion in the Mid and Longwave Infrared Region

Thick hydride vapor phase heteroepitaxy of some III–V, II–VI, and III–VI binary and ternary semiconductor materials such as GaAs, GaP, GaP, GaAsxP1‐x, ZnSe, and GaSe is performed on GaAs substrates and orientation‐patterned GaAs templates. Up to 500 μm thick GaAs, GaP, GaP, GaAsxP1‐x, and ZnSe layers are deposited in single growths or after a regrowth. The GaSe layers deposited through van der Waals heteroepitaxy are up to 4–5 μm thick. When possible, growths are also performed homoepitaxially as the comparison of growth rate and layer quality indicates similar or better results in some of the heteroepitaxial cases. Material analysis using optical microscopy, scanning electron microscopy, atomic force microscopy, high‐resolution X‐ray diffraction, and energy dispersive X‐ray spectroscopy indicate smooth surface morphology, uniform layer thicknesses, high crystalline quality, and gradual substrate‐to‐layer material transition in the cases of heteroepitaxy. Respectively, the growths on the templates prepared by an optimized polarity inversion technique (assisted by molecular‐beam epitaxy) reveal excellent domain fidelity and good infrared transparency. The samples grown on templates are intended for frequency conversion devices operating in mid and longwave infrared. The layers grown on plain substrates are meant to support applications in optoelectronics, renewable energy, sensing, and solar cells.

Scratch Resistance of High Velocity Oxy-Fuel Sprayed WC-20% Co Coatings Reinforced with Carbon Nanotubes

Tungsten Carbide-20 weight % Cobalt (WC-20 wt% Co) coatings were extensively used material to achieve a combination of extreme hardness and excellent strength. Hence, an effort has been made to improve the toughening properties of WC-Co coatings through reinforcement of Carbon Nano-Tubes (CNTs) using High-Velocity Oxy-Fuel (HVOF) spraying process. In this work, 2 wt%, 4 wt%, and 6 wt% of CNTs were blended by the ball milling process with WC-Co powders. These composed powders were deposited by HVOF process on to the plain mild steel substrates. The scratch test analysis on as-sprayed coatings showed that due to the addition of CNTs on WC-Co coatings, the resistance to penetrate the coating surface increased. Also, by varying the percentage of CNTs in the coating system, the pores were reduced. This phenomenon is attributed to the toughening mechanism by forming a CNT bridge which avoids the formation of internal cracks.

Effect of cell imprinting on viability and drug susceptibility of breast cancer cells to doxorubicin

This study demonstrates the effect of substrate's geometrical cues on viability and the efficacy of an anti-cancer drug, doxorubicin (DOX), on breast cancer cells. It is hypothesized that the surface topographical properties can mediate the cellular drug intake. Pseudo-three dimensional (3D) platforms were fabricated using imprinting technique from polydimethylsiloxane (PDMS) and gelatin methacryloyl (GelMA) hydrogel to recapitulate topography of cells’ membranes. The cells exhibited higher viability on the cell-imprinted platforms for both PDMS and GelMA materials compared to the plain/flat counterparts. For instance, MCF7 cells showed a higher metabolic activity (11.9%) on MCF7-imprinted PDMS substrate than plain PDMS. The increased metabolic activity for the imprinted GelMA was about 44.2% compared to plain hydrogel. The DOX response of cells was monitored for 24 h. Although imprinted substrates demonstrated enhanced biocompatibility, the cultured cells were more susceptible to the drug compared to the plain substrates. In particular, MCF7 cells on imprinted PDMS and GelMA substrates showed 37% and 50% higher in cell death compared to the corresponding plain PDMS and GelMA, respectively. Interestingly, the drug susceptibility of the cells on the imprinted hydrogel was about 70% higher than the cells cultured on imprinted PDMS substrates. Having MCF7 cell-imprinted substrates, DOX responses of two other breast cancer cell lines, SKBR3 and ZR-75-1, were also evaluated. The results support that cell membrane curvature developed by multiscale topography is able to mediate intracellular signaling and drug intake. Statement of SignificanceResearch in biological sciences and drug discovery mostly rely on two dimensional (2D) cell culture techniques which cannot provide a reliable physiologically relevant environment. Lack of extracellular matrix and a large shift in physicochemical properties of conventional 2D substrates can induce aberrant cellular behaviors. While chemical composition, topographical, and mechanical properties of substrates have remarkable impacts on drug susceptibility, gene expression, and protein synthesis, the most cell culture plates are from rigid and plain substrates. A number of (bio)polymeric 3D-platforms have been introduced to resemble innate cell microenvironment. However, their intricate culture protocols restrain their applications in demanding high-throughput drug screening. To address the above concerns, in the present study, a hydrogel-based pseudo-3D substrate with imprinted cell features has been introduced.

Highly Rectifying Heterojunctions Formed by Annealed ZnO Nanorods on GaN Substrates.

We study the effect of thermal annealing on the electrical properties of the nanoscale p-n heterojunctions based on single n-type ZnO nanorods on p-type GaN substrates. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and on the substrates locally patterned by focused ion beam lithography. Electrical properties of single nanorod heterojunctions are measured with a nanoprobe in the vacuum chamber of a scanning electron microscope. The focused ion beam lithography provides a uniform nucleation of ZnO, which results in a uniform growth of ZnO nanorods. The specific configuration of the interface between the ZnO nanorods and GaN substrate created by the focused ion beam suppresses the surface leakage current and improves the current-voltage characteristics. Further improvement of the electrical characteristics is achieved by annealing of the structures in nitrogen, which limits the defect-mediated leakage current and increases the carrier injection efficiency.

Halide Vapor Phase Epitaxy α‐ and ε‐Ga2O3 Epitaxial Films Grown on Patterned Sapphire Substrates

The growth of Ga2O3 films by halide vapor phase epitaxy on plain and cone‐shaped patterned sapphire substrates (PSS) is reported. The obtained specimens are characterized by X‐ray diffraction, transmission electron microscopy, cathodoluminescence, optical transmission spectroscopy, and current–voltage measurements. Both types of Ga2O3 layers are of reasonably high crystal qualities; their physical properties, however, are very different. Under the same conditions, the growth on plain substrates results in a continuous α‐Ga2O3 layer, whereas the growth on PSS produces a regular array of α‐Ga2O3 columns on top of the sapphire cones with the space between them filled with ε‐Ga2O3. Ga2O3 films grown on plain sapphire are insulating; in contrast, Ga2O3 films grown on PSS are conducting. It is found that the conductivity of Ga2O3 on PSS follows the Arrhenius law with the activation energy of 0.33 eV. New luminescent bands for α‐ and ε‐phases are found. Spectral components of the defect‐related luminescence for α‐ and ε‐ phases are identified.

Electrical properties of nanoscale p-n heterojunctions formed between a single ZnO nanorod and GaN substrate

We study electronic transport in single vertically oriented n-type ZnO nanorods on p-type GaN substrates using a nanoprobe in a scanning electron microscope. The ZnO nanorods are prepared by chemical bath deposition on both plain GaN substrates and the substrates locally modified by focused ion beam. The heterojunctions on focused ion beam-modified substrates show rectifying current-voltage characteristics, while the characteristics of the plain structures are symmetrical. Adsorption/desorption processes on the surface of ZnO nanorods strongly affect their electrical properties. We demonstrate that the electronic transport in the nanorods grown on focused ion beam-modified substrates is less sensitive to adsorption/desorption processes, which is related to their uniform nucleation and higher crystalline quality.

Heteroepitaxial growth of nonlinear optical materials for frequency conversion in the mid and longwave IR

We report recent results on heteroepitaxy of ZnSe on GaAs, GaSe on GaP, GaAs and GaN substrates, and GaAsxP1−x ternaries on GaAs, as well growths of OPGaP, OPGaAsP and OPZnSe on orientation-patterned (OP) GaAs templates. ZnSe and GaSe were chosen for their wide IR transparency and good NLO properties, taking advantages from the close lattice match of ZnSe to GaAs, α–Ga4Se6 (cubic) to GaP and GaAs, and β–GaSe (hexagonal) to GaN. For the same reason growth of OPGaP was attempted on OPGaAs templates. As for the GaAsxP1−x ternaries, the idea was to determine whether these ternaries can combine in one material the lower 2-photon absorption of GaP with the higher nonlinear susceptibility of GaAs since a GaAsxP1−x ternary always has a closer lattice match to either GaAs or GaP than the two parenting materials. Up to 500 µm thick layers grown by Hydride Vapor Phase Epitaxy (HVPE) on plain substrates and up to 300 µm thick OP structures grown on OP templates revealed smooth surface morphology (RMS < 2 nm from 1 μm2 AFM scan) and superior crystalline quality (FWHM < 100 arcsec of the XRD Omega scan). The growth of α–GaSe/GaAs can be considered as a low-dimensional Van-der-Waals heteroepitaxy. Further characterization of the heteroepitaxial layers by Nomarski optical imaging, linear transmission and nonlinear absorption measurements, TEM, SEM, AFM, and EDX revealed that in some of the cases a smooth transition between the substrate and the layer material can be achieved by the formation at the interface of an intermediate ternary layer with a gradually changing composition. It also indicated that during fast growth processes, such as HVPE, the strain built due to the lattice and the thermal mismatch between substrate and growing layer shall be relaxed in faster and more energy absorbing processes such as formation of voids or roughening surfaces (GaP/GaAs) or an immediate formation of stacking faults that originate still from the substrate (ZnSe/GaAs). All this may postpone or even replace the formation of the expected misfit dislocations frequently observed in the slower MOCVD and MBE growths.

Enhancing the critical current of YBa2Cu3O7 thin films by substrate nanoengineering

The critical current density, Jc, can be increased by introducing defects throughout YBa2Cu3O7 superconducting thin films. We propose a new approach of substrate nanoengineering to produce well-controlled defects. LaAlO3 substrates have been ion-etched with different 34 nm deep patterns prior to the deposition of the films. An annealing step at 1000 °C after substrate etching has been undertaken to negate the Ar-ion damage to the surface. The Jc of the so-prepared samples measured at different temperatures has been compared to thin films deposited on plain substrates at the same time. In general, an increase in Jc is observed across all temperatures and fields as a result of both patterning and annealing. In particular, at zero field and 85 K, the largest enhancement of &amp;gt;40% has been recorded for circle and triangle patterns. This new substrate nanoengineering technique is very promising for flux trapping of superconducting devices, particularly because an increase to critical current (Ic) also occurs. Further optimization of depth, size, and shape of the patterns is expected to produce further improvements to Jc.

Auxetic structure for increased power output of strain vibration energy harvester

This paper develops an auxetic (negative Poisson’s ratio) piezoelectric energy harvester (APEH) to increase the power output when harnessing strain energy. The APEH consists of a piezoelectric element bonded to an auxetic substrate. The auxetic substrate concentrates the applied stress and strain into the region of the piezoelectric element and introduces auxetic behaviour in the piezoelectric element, both of which increase the electric power output. A finite element model was developed to optimise the design and verify the mechanism of the power increase. Three APEHs were manufactured and characterised. Their performance was compared with two equivalent strain energy harvesters with plain substrates. Experimental results show that the APEHs, excited by sinusoidal strains of 250 με peak-to-peak at 10 Hz, are able to produce electric power of up to 191.1 μW, which is 14.4 times that of the peak power produced by the plain harvesters (13.4 μW). The power gain factor is constant between samples as the amplitude and frequency of their applied strains are varied. The model and experimental results are in good agreement, once accounting for the imperfect bonding of the epoxy using the spring constant of the Thin Elastic Layers on the modelled epoxy surfaces.

Heteroepitaxy of orientation-patterned nonlinear optical materials

We report some recent results on thick heteroepitaxial growth of GaP on GaAs substrates and on orientation-patterned (OP) GaAs templates conducted in a hot-wall horizontal quartz reactor for Hydride Vapor Phase Epitaxy. The growths on the plain substrates resulted in up to 500 µm thick GaP with smooth surface morphology (RMS < 1–2 nm) and high crystalline quality (FWHM = 100–150 arcsec), comparable to the quality of the related homoepitaxial growths of GaP on GaP. Up to 300 µm thick OPGaP quasi-phase matching structures with excellent domain fidelity were also heteroepitaxially grown with high reproducibility on OPGaAs templates in support of frequency conversion laser source development for the mid and longwave infrared. We studied the GaAsxP1−x ternary transition layer that forms between the growing film and the substrate. We also undertook steps to determine some important characteristics of heteroepitaxy such as thickness of the pseudomorphous growth and periodicity of the expected misfit dislocations. The formation of these and some other defects and their distribution within the layer thickness was also investigated. Samples were characterized by Nomarski optical microscopy, transmission optical measurements, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. The focus was predominantly on the interface and, more precisely, on what influence the pre-growth surface treatment of the substrate has on the initial and the following stages of growth, as well on the mechanisms of the strain relaxation from the lattice and thermal mismatch between layer and substrate. The efforts to accommodate the growing film to the foreign substrate by engineering an intermediate buffer layer were extended to thick growths of GaAsxP1−x ternary with the idea to combine in one material the best of the nonlinear properties of GaP and GaAs that are strictly relevant to the pursued applications.

Photocatalytic soot oxidation on TiO2 microstructured substrate

Photocatalytic soot oxidation was investigated using TiO2 microstructured substrates. Plain and micropillar substrates of sintered TiO2 nanoparticles were prepared using a soft lithography process. Photon absorption analysis revealed substantial photon transmission for plain substrate within the current substrate thickness range (<2.5μm). The porous structure of substrate was confirmed by electron microscopy and by dimensional changes due to sintering, allowing gaseous species and radicals to diffuse across micrometer-thick substrates. It was found that the soot oxidation rate increased with the thickness of the plain TiO2 substrate; this was related to the increase in electron-hole pair generation associated with increased photon absorption. The soot oxidation rate increased further with the use of TiO2 micropillars, enhancing photon absorption through multiple scattering. These multiscale phenomena in the photocatalytic soot oxidation on TiO2 microstructured substrates were analyzed.

ZnO Nanorod-Based Ultraviolet Photodetector Prepared on Patterned Sapphire Substrates

A patterned sapphire substrate with a submicron hole array structure was used to fabricate ZnO nanorod (NR) array by low-temperature hydrothermal method. A ZnO seed layer was spin-coated using sol-gel process and then etched to form separated seed islands. Different concentrations of zinc acetate in the hydrothermal solution were performed for ZnO NR synthesis to investigate ultraviolet (UV) photo-response properties. The ZnO NRs synthesized on the patterned substrates were oriented at various inclined angles. The diameter of the NRs increased and their surface-to-volume ratio decreased as the concentrations of zinc acetate increased from 0.01 to 0.04 M. X-ray diffraction and photoluminescence analyses showed that the NRs synthesized at a higher zinc acetate concentration had better crystal quality and fewer defects. The current–voltage characteristics of the ZnO NR-based metal-semiconductor-metal photodetectors exhibited a linear relationship, suggesting an Ohmic contact between NRs and Al electrodes. The UV response, defined by the resistance ratio (Rdark/RUV), of the photodetectors increased with the decreasing zinc acetate concentration from 0.04 to 0.01 M. Compared to traditional vertically aligned NR array on plain substrates, the proposed ZnO NR-based photodetector on the patterned substrates achieved improved UV responses, and the average increment for samples of 0.01–0.04 M was 145%.

A model for patterned interfaces debonding – Application to adhesion tests

Patterned interfaces are widely used in the industries of microelectronics and micro-electromechanical systems and are of general interest for obtaining specific interface properties by playing on geometry as an additional parameter influencing physical processes (interface engineering). In this paper, a novel analytical model is proposed to characterize the debonding of patterned interfaces. This model is accounting for both stretching and rotation of the finite width bond lines composing a patterning between two plain substrates. The model highlights how the choice of pattern periodicity and line width allows for controlling the interface mechanical behavior during debonding: presence of stable and unstable crack growth regimes. It is shown that models considering a continuum interface are to be used with care when dealing with patterned interfaces. The model proposed here, used in conjunction with an experimental procedure providing the sample bending profile during a wedge test (or a DCB test) should provide valuable guidance for determining the interface fracture energy Gc (mode I) for a patterned interface.

Chemical bath deposition growth and characterization of zinc oxide nanostructures on plain and platinum-coated glass substrates for hydrogen peroxide gas sensor application

Growth of zinc oxide on plain and Pt-coated glass substrate via chemical bath deposition technique (CBD) were studied. Aqueous solutions of ammonium hydroxide (NH4OH) and zinc sulfate (ZnSO4) were used as the precursor substances in the synthesis. Ultraviolet-visible spectroscopy (UV-Vis) was performed to determine the energy band gap and X-ray diffraction (XRD) to examine crystallinity. Sensitivity measurements were carried out in order to examine its potential to be fabricated as hydrogen peroxide (H2O2) gas sensor. Experimental results in the sensitivity experiment show that in the presence of H2O2 gas, the resistance of ZnOincrease which can be used as the basis for H2O-2 detection. UV-Vis showed variation of energy band gap values but were all near the generally accepted value. XRD spectra further verify that ZnOwere indeed synthesized.

Large-scale topographical screen for investigation of physical neural-guidance cues.

A combinatorial approach was used to present primary neurons with a large library of topographical features in the form of micropatterned substrate for high-throughput screening of physical neural-guidance cues that can effectively promote different aspects of neuronal development, including axon and dendritic outgrowth. Notably, the neuronal-guidance capability of specific features was automatically identified using a customized image processing software, thus significantly increasing the screening throughput with minimal subjective bias. Our results indicate that the anisotropic topographies promote axonal and in some cases dendritic extension relative to the isotropic topographies, while dendritic branching showed preference to plain substrates over the microscale features. The results from this work can be readily applied towards engineering novel biomaterials with precise surface topography that can serve as guidance conduits for neuro-regenerative applications. This novel topographical screening strategy combined with the automated processing capability can also be used for high-throughput screening of chemical or genetic regulatory factors in primary neurons.