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Register a new userInfrared Transmissometer to Measure the Thickness of NbN Thin Films
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We present an optical setup that can be used to characterize the thicknesses of thin NbN films to screen samples for fabrication and to better model the performance of the resulting superconducting nanowire single photon detectors. The infrared transmissometer reported here is easy to use, gives results within minutes and is non-destructive. Thus, the thickness measurement can be easily integrated into the workflow of deposition and characterization. Comparison to a similar visible-wavelength transmissometer is provided.
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We have implemented three different optical methods to quantitatively assess the thickness of thin GaSe flakes transferred on both transparent substrates, like Gel-Film, or SiO2/Si substrates. We show how their apparent color can be an efficient way to make a quick rough estimation of the thickness of the flakes. This method is more effective for SiO2/Si substrates as the thickness dependent color change is more pronounced on these substrates than on transparent substrates. On the other hand, for transparent substrates, the transmittance of the flakes in the blue region of the visible spectrum can be used to estimate the thickness. We find that the transmittance of flakes in the blue part of the spectrum decreases at a rate of 1.2%/nm. On SiO2/Si, the thickness of the flakes can be accurately determined by fitting optical contrast spectra to a Fresnel law-based model. Finally, we also show how the quantitative analysis of transmission mode optical microscopy images can be a powerful method to quickly probe the environmental degradation of GaSe flakes exposed to aging conditions.
In spectroscopic ellipsometry, the optical properties of materials are obtained indirectly by generally assuming dielectric function and optical models. This ellipsometry analysis, which typically requires numerous model parameters, has essentially been performed by a try-and-error approach, making this method as a rather time-consuming characterization technique. Here, we propose a fully automated spectroscopic ellipsometry analysis method, which can be applied to obtain dielectric functions of light absorbing materials in a full measured energy range without any prior knowledge of model parameters. The developed method consists of a multiple-step grid search and the following non-linear regression analysis. Specifically, in our approach, the analyzed spectral region is gradually expanded toward higher energy while incorporating an additional optical transition peak whenever the root-mean-square error of the fitting analysis exceeds a critical value. In particular, we have established a unique algorithm that could be employed for the ellipsometry analyses of different types of optical materials. The proposed scheme has been applied successfully for the analyses of MoOx transparent oxides and the complex dielectric function of a MoOx layer that exhibits dual optical transitions due to band-edge and deep-level absorptions has been determined. The developed method can drastically reduce a time necessary for an ellipsometry analysis, eliminating a serious drawback of a traditional spectroscopic ellipsometry analysis method.
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We have studied the effect of deposition rate and layer thickness on the properties of epitaxial MgB2 thin films grown by hybrid physical-chemical vapor deposition on 4H-SiC substrates. The MgB2 film deposition rate depends linearly on the concentration of B2H6 in the inlet gas mixture. We found that the superconducting and normal-state properties of the MgB2 films are determined by the film thickness, not by the deposition rate. When the film thickness was increased, the transition temperature, Tc, increased and the residual resistivity, rho0, decreased. Above about 300 nm, a Tc of 41.8 K, a rho0 of 0.28 mikroOhm.cm, and a residual resistance ratio RRR of over 30 were obtained. These values represent the best MgB2 properties reported thus far.
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