Optical Far-Field Method with Subwavelength Accuracy for the Determination of Nanostructure Dimensions in Large-Area Samples

Abstract

The physical, chemical, and biological properties of nanostructures depend strongly on their geometrical dimensions. Here we present a fast, noninvasive, simple-to-perform, purely optical method that is capable of characterizing nanostructure dimensions over large areas with an accuracy comparable to that of scanning electron microscopy. This far-field method is based on the analysis of unique fingerprints in experimentally measured reflectance spectra using full three-dimensional optical modeling. We demonstrate the strength of our method on large-area (millimeter-sized) arrays of vertical InP nanowires, for which we simultaneously determine the diameter and length as well as cross-sample morphological variations thereof. Explicitly, the diameter is determined with an accuracy better than 10 nm and the length with an accuracy better than 30 nm. The method is versatile and robust, and we believe that it will provide a powerful and standardized measurement technique for large-area nanostructure arrays suitable for both research and industrial applications.

Design for strong absorption in a nanowire array tandem solar cell

“…Different computational methods, such as the finite-element method (FEM)33233, the rigorous coupled wave analysis (RCWA)46 and the scattering matrix method18132425, have been used for studying the diffraction and absorption of light in nanostructures through the solution of the Maxwell equations, which give results in good agreement with experiments13. We chose to employ the scattering matrix method to solve the Maxwell equations for normally incident light in order to calculate the absorption spectrum A 1(2) (λ) of the nanowire top and bottom cells.…”

“…It is known that the nanowire diameter affects strongly the absorption of light15781335. Therefore, to study the effect of the nanowire diameter on the absorption in the tandem cell, we fix P  = 530 nm, L top  = 2000 nm, and L bot  = 2900 nm [Fig.…”

“…Optimization of the geometry of the single-nanowire geometry is necessary to obtain maximum photocurrent12 and open circuit voltage412. The resonances can lead to a 20 times stronger absorption per volume semiconductor material in a III-V nanowire as compared to a bulk sample13.…”

“…Furthermore, the resonant absorption by designing the nanowire geometry holds the prospect of lower material usage than in thin-films13. Therefore, to enable high-efficiency nanowire tandem solar cells, we need to understand the optimum choice of materials for the subcells as well as the optimum nanowire geometry to have the best absorption characteristics for photovoltaics.…”

“…1) with the scattering matrix method18132425. This modeling allows us to perform a Shockley-Queisser detailed balance analysis to study and optimize the efficiency potential of the nanowire solar cell as a function of material choice and geometrical design of the nanowires.…”