A precise optical determination of nanoscale diameters of semiconductor nanowires

Abstract

Electrical and optical properties of semiconducting nanowires (NWs) strongly depend on their diameters. Therefore, a precise knowledge of their diameters is essential for any kind of device integration. Here, we present an optical method based on dark field optical microscopy to easily determine the diameters of individual NWs with an accuracy of a few nanometers and thus a relative error of less than 10%. The underlying physical principle of this method is that strong Mie resonances dominate the optical scattering spectra of most semiconducting NWs and can thus be exploited. The feasibility of this method is demonstrated using GaAs NWs but it should be applicable to most types of semiconducting NWs as well. Dark field optical microscopy shows that even slight tapering of the NWs, i.e. diameter variations of a few nanometers, can be detected by a visible color change. Abrupt diameter changes of a few nanometers, as they occur for example when growth conditions vary, can be determined as well. In addition a profound analysis of the elastic scattering properties of individual GaAs NWs is presented theoretically using Mie calculations as well as experimentally by dark field microscopy. This method has the advantage that no vacuum technique is needed, a fast and reliable analysis is possible based on cheap standard hardware.

Far-Field Imaging for Direct Visualization of Light Interferences in GaAs Nanowires

“…Indeed, it has been shown that even a small tapering of semiconductor NWs affects the scattering resonances. 23,32 In the present case the tapering of the NW is about 1 magnitude higher (i.e., the change of the radius is about 10 times higher within approximately the same length) than that of the tapered GaAs NW studied in ref 23. A more fitting description of these resonances is that of volume resonances determined by the complete three-dimensional structure.…”

“…It has been shown earlier that this is a good approximation for the light scattering of individual semiconductor NWs. 21− 23 The radii of the investigated NWs are smaller than 80 nm. These NWs support only a single mode, the so-called fundamental mode.…”

Single-nanowire solar cells beyond the Shockley–Queisser limit

“…1a-c) 26 . We find that light absorption in single standing nanowires is more than one order of magnitude more efficient than is predicted from the Lambert-Beer law.…”

“…For low-absorbing materials (for example, indirect bandgap materials such as silicon), waveguiding effects plays a key role 23,24 , whereas highly absorbing semiconductors (such as direct-bandgap GaAs) exhibit resonances that increase the total absorption several times. Nanowires lying on a substrate also exhibit such resonances, often described by Mie theory 25,26 , although the total absorption rate is significantly lower 27,28 . Even though the optical absorption of nanowires arranged in an array has been shown to be far more complex than in thin films, nanowire vertical arrays currently seem to be the most reasonable device proposal.…”

Design of Nanowire Optical Cavities as Efficient Photon Absorbers

“…[12][13][14]17,[20][21][22][23][24][25][26][27] For example, absorption or scattering cross sections have been estimated qualitatively, 17,[20][21][22][23] while in other works wavelength-dependent photocurrent measurements performed on single NW PV devices were used to assign quantitatively the resonances of NW optical cavities. [13][14][15] Concurrent with experiments, numerical and analytical calculations of the resonant modes and key optical figures of merit (e.g.…”