Nanotextured Samples Research Articles

Study on Microgratings Using Imaging, Spectroscopic, and Fourier Lens Scatterometry

With new fabrication methods for mass production of nanotextured samples, there is an increasing demand for new characterization methods. Conventional microscopes are either too slow and/or too sensitive to vibrations. Scatterometry is a good candidate for in-line measuring in an industrial environment as it is insensitive to vibrations and very fast. However, as common scatterometry techniques are nonimaging, it can be challenging for the operator to find the area of interest on a sample and to detect defects. We have therefore developed the technique imaging scatterometry, in which the user first has to select the area of interest after the data have been acquired. In addition, one is no longer limited to analyze areas equal to the spot size, and areas down to 3 μm × 3 μm can be analyzed. The special method Fourier lens scatterometry is capable of performing measurements on misaligned samples and is therefore suitable in a production line. We demonstrate characterization of one-dimensional and two-dimensional gratings from a single measurement using a Fourier lens scatterometer. In this paper, we present a comparison between spectroscopic scatterometry, the newly developed imaging scatterometry, and some state-of-the-art conventional characterization techniques, atomic force microscopy and confocal microscopy.

Imaging scatterometry for flexible measurements of patterned areas.

Characterization of micro/nano-textured surfaces is time consuming using scanning probe and electron microscopy techniques. Scatterometry, where the intensity of scattered light is used as a 'fingerprint' to reconstruct a surface, is a fast and robust method for characterization of gratings. However, most scatterometry techniques are measuring the averaged signal over an area equal to the spot size of the light source. In this paper we present the imaging scatterometry technique, which is capable of locally measuring topographic parameters of gratings spanning an area down to a few µm(2) with nm accuracy. The imaging scatterometer can easily find areas of interest on the cm scale and measure multiple segments simultaneously. We demonstrate two imaging scatterometers, one built into an optical microscope and one in a split configuration. The two scatterometers are targeted characterization of mm(2) and cm(2) areas, respectively, and both setups are validated using nano-textured samples.

Rigorous modelling of light scattering in solar cells based on finite element method and Huygens' expansion.

Finite element method is coupled with Huygens' expansion to determine light intensity distribution of scattered light in solar cells and other optoelectronic devices. The rigorous foundation of the modelling enables calculation of the light intensity distribution at a chosen distance and surface of observation in chosen material in reflection or in transmission for given wavelength of the incident light. The calculation of scattering or anti-reflection properties is not limited to a single textured interface, but can be done above more complex structures with several scattering interfaces or even with particles involved. Both scattering at periodic and at random textures can be efficiently handled with the modelling approach. A procedure for minimisation of the effect of small-area sample, which is considered in the finite element method calculation, is proposed and implemented into the modelling. Angular distribution function, total transmission and total reflection of the scattering interface or structure can be determined using the model. Furthermore, a method for calculation of the haze parameter of reflected or transmitted light is proposed. The modelling approach is applied to periodic and random nano-textured samples for photovoltaic applications, showing good agreement with measured data.

Nanopyramid Formation by Ag Metal-Assisted Chemical Etching for Nanotextured Si Solar Cells

We investigated the formation of a nanopyramidal structure and fabricated nanotextured Si solar cells using an Ag metal-assisted chemical etching process. The nanopyramidal structure was formed on a Si flat surface and the nanotexturing process was performed on the p-type microtextured Si surface. The nanostructural formation shows a transition from nanopits and nanopores to nanowires with etching time. The nanotextured surfaces also showed the photoluminescence spectra with an enhanced intensity in the wavelength range of 1,100~1,250 nm. The photoreflectance of the nanotextured Si solar cells was strongly reduced in the wavelength range of 337~596 nm. However, the quantum efficiency is decreased in the nanotextured samples due to the increased nanosurface recombination. The nanotexturing process provides a better p-n junction impedance of the nanotextured cells, resulting in an enhanced shunt resistance and fill factor which in turn renders the possibility of the increased conversion efficiency.

Elimination of strength degrading effects caused by surface microdefect: A prevention achieved by silicon nanotexturing to avoid catastrophic brittle fracture.

The unavoidable occurrence of microdefects in silicon wafers increase the probability of catastrophic fracture of silicon-based devices, thus highlighting the need for a strengthening mechanism to minimize fractures resulting from defects. In this study, a novel mechanism for manufacturing silicon wafers was engineered based on nanoscale reinforcement through surface nanotexturing. Because of nanotexturing, different defect depths synthetically emulated as V-notches, demonstrated a bending strength enhancement by factors of 2.5, 3.2, and 6 for 2-, 7-, and 14-μm-deep V-notches, respectively. A very large increase in the number of fragments observed during silicon fracturing was also indicative of the strengthening effect. Nanotextures surrounding the V-notch reduced the stress concentration factor at the notch tip and saturated as the nanotexture depth approached 1.5 times the V-notch depth. The stress reduction at the V-notch tip measured by micro-Raman spectroscopy revealed that nanotextures reduced the effective depth of the defect. Therefore, the nanotextured samples were able to sustain a larger fracture force. The enhancement in Weibull modulus, along with an increase in bending strength in the nanotextured samples compared to polished single-crystal silicon samples, demonstrated the reliability of the strengthening method. These results suggest that this method may be suitable for industrial implementation.

Formation of nanotextured surfaces on microtextured Si solar cells by metal-assisted chemical etching process.

We investigated a nanotexturization process on the microtextured surface of monocrystalline Si solar cells which utilized a metal assisted chemical etching process. P-type Si solar cell wafers were used for nanotexturing followed by saw damage removal and a microtexturing process. As the nanotexturing time was increased, green and red-orange photoluminescence spectra at wavelengths of 506, 507, and 637 nm were observed from the nanotextured cells, indicating that the quantum size effect caused the confinement of charge carriers in nanocrystalline silicon. The nanotextured cells showed a low photoreflectance of 4.5% in the visible spectral region of 400-600 nm. However, the reduced quantum efficiency from the nanotextured samples suggests that a shallow nanopore depth and density are required to prevent surface-related phenomena of charge recombination and surface current leakage.

Dropwise condensation on superhydrophobic nanostructured surfaces: literature review and experimental analysis

It is well established that the dropwise condensation (DWC) mode can lead up to significant enhancement in heat transfer coefficients as compared to the filmwise mode (FWC). Typically, hydrophobic surfaces are expected to promote DWC, while hydrophilic ones induce FWC. To this end, superhydrophobic surfaces, where a combination of low surface energy and surface texturing is used to enhance the hydrophobicity, have recently been proposed as a promising approach to promote dropwise condensation. An attractive feature of using superhydrophobic surfaces is to facilitate easy roll-off of the droplets as they form during condensation, thus leading to a significant improvement in the heat transfer associated with the condensation process.High droplet mobility can be obtained acting on the surface chemistry, decreasing the surface energy, and on the surface structure, obtaining a micro- or nano- superficial roughness. The first part of this paper will present a literature review of the most relevant works about DWC on superhydrophobic nanotextured substrates, with particular attention on the fabrication processes. In the second part, experimental data about DWC on superhydrophobic nanotextured samples will be analyzed. Particular attention will be paid to the effect of vapour velocity on the heat transfer. Results clearly highlight the excellent potential of nanostructured surfaces for application in flow condensation applications. However, they highlight the need to perform flow condensation experiments at realistic high temperature and saturation conditions in order to evaluate the efficacy of superhydrophobic surfaces for practically relevant pure vapor condensation applications.

The effect of nanometric surface texture on bone contact to titanium implants in rabbit tibia

Designing biomaterial surfaces to control the reaction of the surrounding tissue is still considered to be a primary issue, which needs to be addressed systematically. Although numerous in vitro studies have described different nano-metrically textured substrates capable to influence bone cellular response, in vivo studies validating this phenomenon have not been reported. In this study, nano-grooved silicon stamps were produced by laser interference lithography (LIL) and reactive ion etching (RIE) and were subsequently transferred onto the surface of 5 mm diameter Titanium (Ti) discs by nanoimprint lithography (NIL). Patterns with pitches of 1000 nm (500 nm ridge and groove, 150 nm depth), 300 nm (150 nm ridge and groove, 120 nm depth; as well as a 1:3 ratio of 75 nm ridge and 225 nm groove, 120 nm depth) and 150 nm (75 nm ridge and groove, 30 nm depth) were created. These samples were implanted in a rabbit tibia cortical bone. Histological evaluation and histomorphometric measurements were performed, comparing each sample to conventional grit-blasted/acid-etched (GAE) titanium controls. Results showed a significantly higher bone-to-implant contact at 4 weeks for the 300 nm (1:3) specimens, compared to GAE (p = 0.006). At 8 weeks, there was overall more bone contact compared to 4 weeks. However, no significant differences between the nano-textured samples and the GAE occurred. Further studies will need to address biomechanical testing and the use of trabecular bone models.

Surface-nano-texturing by aluminum-induced crystallization of amorphous silicon

Aluminum-induced crystallization (AIC) of amorphous silicon (a-Si) has been studied extensively to produce large poly-Si grains for electronic and photovoltaic applications such as thin-film transistors, solar cells, and display panels. This paper reports a novel use of AIC of a-Si technique in nano-scale surface-texturing for tribological applications. Various nano-textured samples were fabricated by using AIC of a-Si technique employing different a-Si thickness and annealing conditions. The crystalline properties of these nano-textured surfaces (NTSs) were then characterized using scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), transmission electron microscope (TEM), and atomic force microscope (AFM). The adhesion and friction properties of the NTSs and a smooth Si surface were studied by a Triboindentor. These characterizations show that the nano-textures on the NTSs were poly-silicon crystallites and the NTSs significantly reduced the adhesion and friction forces compared to the smooth surface. Both texture height and density were found to affect the adhesion and friction performances of the NTSs.

A comparative study on nanotextured high density Mg-doped and undoped GaN

Nanotextured high density Mg-doped and undoped GaN were obtained using photoelectrochemical etching. Interesting features are observed in the temperature dependent photoluminescence (PL) studies of these nanotextured materials. First, the PL intensity of the excitonic emissions shows more than three orders of enhancement. At low temperature, the peak energy shows a blueshift with temperature. This phenomenon is attributed to the formation of excitonic band-tail states. Second, the excitonic emissions in the nanotextured samples are redshifted compared to the as-grown GaN suggesting strain relaxation. Third, the blue luminescence band (2.7–2.9eV in Mg-doped GaN) shows a large redshift, which is not consistent with strain relaxation calculated from excitonic band. Furthermore, temperature dependence of the blue luminescence band energy shows an asymmetric S-shaped behavior in nanotextured GaN. All these observations are explained by invoking an increase in carrier localization due to an increase in potential fluctuation created by the nanotexturization process.