Geometrical Parameters Of Nanostructure Research Articles

Optical Model and Optimization for Coherent-Incoherent Hybrid Organic Solar Cells with Nanostructures

Embedding nanostructures in organic solar cells (OSCs) is a well-known method to improve the absorption efficiency of the device by introducing the plasma resonance and scattering effects without increasing the active layer thickness. The introduction of nanostructures imposes greater demands on the optical analysis method for OSCs. In this paper, the generalized rigorous coupled-wave analysis (GRCWA) is presented to analyze and optimize the performance of coherent-incoherent hybrid organic solar cells (OSCs) with nanostructures. Considering the multiple reflections of light scattered within the glass substrate by the device, the correction vector g is derived, then the modified expressions for the field and absorption distribution in OSCs are provided. The proposed method is validated by comparing the simulated results of various structures with results obtained by the generalized transfer matrix method (GTMM) and the “equispaced thickness method” (ETM). The results demonstrate that the proposed method can reduce the number of simulations by at least half compared to the ETM while maintaining accuracy. With the proposed method, we discussed the device performance depending on the geometrical parameters of nanostructures, and the optimization and analysis are accomplished for single and tandem OSCs. After optimization based on the proposed method, the performance of OSCs are significantly improved, which further demonstrates the practicality of the method.

Glass Flow Evolution and the Mechanism of Antireflective Nanoprotrusion Arrays in Nanoholes by Direct Thermal Imprinting.

Moth-eye-mimicking nanoprotrusion arrays are typical bioinspired broadband antireflection patterns that improve the transmittance and visibility of optical devices by adjusting different geometrical parameters of nanostructures, such as diameter, height, shape, and periodic arrangement, and widely used in solar cells, electronic displays, and so on. Rapid, net-shape, less complicated, and low-cost fabrication of the glass-based moth-eye nanostructure array is a huge challenge. This work adopted the nanohole array template to transform the moth-eye nanostructures on the optical glass by hot embossing combined with ultrasonic-assisted demolding. To investigate the mode transition and filling behavior of the glass nanostructures when compressed into the nanoholes, we conducted a series of hot embossing tests with various processing parameters and characterized the geometrical morphology of the glass-based nanostructure array, such as height and shape. In these tests, surface defects such as nanocracks will occur when inappropriate processing parameters were applied and we evaluated the transmittance performance of defective and fine glass nanostructures and surface with no nanostructures to reveal the effect of nanostructures with different levels of quality on antireflection. This work provides an effective and environmental-friendly method for the fabrication of moth-eye nanostructure arrays with an improved antireflection performance.

Study on AlxGa1-xAs nanocones with variable Al composition structures for highly efficient light trapping

In this study, various AlxGa1-xAs nanostructure arrays with variable Al composition are proposed and designed for highly efficient light capture. The optical properties of the proposed nanocone array with variable Al composition have been numerically investigated by COMSOL Multiphysics package based on the finite element method (FEM), and compared with the counterparts of cylinder and conical frustum. The results show that the variable composition AlxGa1-xAs nanocones with uniform sublayer distribution thickness can obtain an ultimate efficiency of 37.3%, which is higher than that of the cylindrical and conical frustum structures under the same conditions. In addition, the effects of geometric parameters of nanostructures on the light absorption of nanoarrays with different shape changing components are studied. Taking the variable component AlxGa1-xAs nanocone structure as an example, increasing the Al component range in the axial direction of the nanocone can significantly improve the absorption of short-wavelength light, which increases the overall absorption efficiency. In addition, for a specific Al composition distribution range, a uniform thickness distribution design of unit sub-layer in nanocone along the z-axis direction can provide optical absorption enhancement of more than 1.8% and 0.9% over than the decreasing and increasing distribution design. The design principles proposed in this work will provide a reference for selecting appropriate parameters in solar cell applications.

Application of Finite Element Method for Analysis of Nanostructures

Abstract The paper deals with application of the finite element method in modelling and simulation of nanostructures. The finite element model is based on beam elements with stiffness properties gained from the quantum mechanics and nonlinear spring elements with force-displacement relation are gained from Morse potential. Several basic mechanical properties of structures are computed by homogenization of nanostructure, e.g. Young's modulus, Poisson's ratio. The problems connecting with geometrical parameters of nanostructures are considered and their influences to resulting homogenized quantities are mentioned.

Optically excited THz generation from ordered arrays of GaAs nanowires

THz generation under excitation by ultrashort optical pulses from ordered arrays of GaAs nanowires is reported. It was found that the efficiency of THz generation is determined by the geometrical parameters of nanostructures and has a resonant character. Furthermore, it is shown that the terahertz generation efficiency at optimum geometrical parameters of an array of semiconductor nanowires is greater than the corresponding value for bulk semiconductor p-InAs which is the most effective THz emitter.

Large-scale nanostructure metrology using Mueller matrix imaging ellipsometry

In order to achieve effective process control, the fast, inexpensive, nondestructive and accurate nanoscale feature measurements are extremely useful in high-volume nanomanufacturing. The optical scatterometry has currently become one of the important approaches for in-line metrology of geometrical parameters of nanostructures in high-volume nanomanufacturing due to its high throughput, low cost, and minimal sample damage. Conventional scatterometry techniques can only obtain the mean geometrical parameter values located in the illumination spot, but cannot acquire the microscopic variation of geometrical parameters less than the illumination region. In addition, conventional scatterometry techniques can only perform monospot test. Therefore, the sample stage must be scanned spot by spot in order to obtain the distribution of geometrical parameters in a large area. Consequently, the final test efficiency will be greatly reduced. Accordingly, in this paper, we combine conventional scatterometry with imaging techniques and adopt the Mueller matrix imaging ellipsometry (MMIE) for fast, large-scale and accurate nanostructure metrology. A spectroscopic Mueller matrix imaging ellipsometer is developed in our laboratory by substituting a complementary metal oxide semiconductor camera for the spectrometer in a previously developed dual rotating-compensator Mueller matrix ellipsometer and by placing a telecentric lens as an imaging lens in the polarization state analyzer arm of the ellipsometer. The light wavelengths in the developed imaging ellipsometer are scanned in a range of 400-700 nm by using a monochromator. The spectroscopic Mueller matrix imaging ellipsometer is then used for measuring a typical Si grating template used in nanoimprint lithography. The measurement results indicate that the developed instrument has a measurement accuracy of better than 0.05 for all the Mueller matrix elements in both the whole image and the whole spectral range. The three-dimensional microscopic maps of geometrical parameters of the Si grating template over a large area with pixel-sized lateral resolution are then reconstructed from the collected spectral imaging Mueller matrices by solving an inverse diffraction problem. The MMIE-measured results that are extracted from Mueller matrix spectra collected by a single pixel of the camera are in good agreement with those measured by a scanning electron microscope and the conventional Mueller matrix ellipsometer. The MMIE that combines the great power of conventional Mueller matrix ellipsometry with the high spatial resolution of optical microscopy is thus expected to be a powerful tool for large-scale nanostructure metrology in future high-volume nanomanufacturing.

Phonon focusing and features of phonon transport in silicon nanofilms and nanowires at low temperatures

The influence of the elastic anisotropy on the thermal conductivity in silicon nanofilms and nanowires for the diffuse scattering of phonons at boundaries at low temperatures has been investigated. It is shown that for the correct description of the phonon transport in silicon nanofilms, in contrast to isotropic medium, it is necessary to introduce two orientation parameters, which take into account the dependencies of the kinetic characteristics of phonon system on the direction of heat flow and the orientation of the film plane with respect to the crystal axes. The optimum orientations of the silicon film plane and directions of heat flow, which provide maximum or minimum heat transfer from elements of the silicon microcircuits have been determined. It is shown that for the nanowires with a square cross-section the values of thermal conductivity depend mainly on the direction of heat flow. However, in rather wide silicon nanofilms they are mainly determined by the orientation of the film plane with respect to the crystal axes. The dependencies of the thermal conductivity and the mean free paths of the phonons in the boundary scattering regime on the geometric parameters of nanostructures have been analyzed.

Low-temperature anisotropy of the thermal conductivity of single-crystal nanofilms and nanowires

The effect of phonon focusing on the phonon transport in single-crystal nanofilms and nanowires is studied in the boundary scattering regime. The dependences of the thermal conductivity and the free path of phonons on the geometric parameters of nanostructures with various elastic energy anisotropies are analyzed for diffuse phonon scattering by boundaries. It is shown that the anisotropies of thermal conductivity for nanostructures made of cubic crystals with positive (LiF, GaAs, Ge, Si, diamond, YAG) and negative (CaF2, NaCl, YIG) anisotropies of the second-order elastic moduli are qualitatively different for both nanofilms and nanowires. The single-crystal film plane orientations and the heat flow directions that ensure the maximum or minimum thermal conductivity in a film plane are determined for the crystals of both types. The thermal conductivity of nanowires with a square cross section mainly depends on a heat flow direction, and the thermal conductivity of sufficiently wide nanofilms is substantially determined by a film plane orientation.

A metrological large range atomic force microscope with improved performance

A metrological large range atomic force microscope (Met. LR-AFM) has been set up and improved over the past years at Physikalisch-Technische Bundesanstalt (PTB). Being designed as a scanning sample type instrument, the sample is moved in three dimensions by a mechanical ball bearing stage in combination with a compact z-piezostage. Its topography is detected by a position-stationary AFM head. The sample displacement is measured by three embedded miniature homodyne interferometers in the x, y, and z directions. The AFM head is aligned in such a way that its cantilever tip is positioned on the sample surface at the intersection point of the three interferometer measurement beams for satisfying the Abbe measurement principle. In this paper, further improvements of the Met. LR-AFM are reported. A new AFM head using the beam deflection principle has been developed to reduce the influence of parasitic optical interference phenomena. Furthermore, an off-line Heydemann correction method has been applied to reduce the inherent interferometer nonlinearities to less than 0.3 nm (p-v). Versatile scanning functions, for example, radial scanning or local AFM measurement functions, have been implemented to optimize the measurement process. The measurement software is also improved and allows comfortable operations of the instrument via graphical user interface or script-based command sets. The improved Met. LR-AFM is capable of measuring, for instance, the step height, lateral pitch, line width, nanoroughness, and other geometrical parameters of nanostructures. Calibration results of a one-dimensional grating and a set of film thickness standards are demonstrated, showing the excellent metrological performance of the instrument.

Scanning tunneling microscopy of films of amorphous carbon doped with copper

The results of experimental studies of the copper-doped hydrogenated amorphous carbon films by scanning tunneling microscopy and spectroscopy are reported. These results are indicative of the effect of spatial ordering of nanostructures in thin films based on carbon and copper. Geometric parameters of nanostructures were measured. In the context of the Simmons model, the work function was estimated to be equal to ϕ ∼0.05 eV. Oscillations in differential conductance with a period in a voltage of ΔV=200–400 mV were detected in tunneling contacts of Ir with the films. The origin of the observed oscillations is discussed.