J.F. Trigo

Study of the optical constants determination of thin films: Dependence on theoretical assumptions

We present a critical study of the known reflectance (R) and transmittance (T) method to extract the optical constants of thin film–substrate systems. In order to study the effect of the usual assumptions (homogeneous layers, semi‐infinite substrate, normal incidence angle for reflectance measurement, etc.) in the loss of solution during the numerical inversion process, we have developed a theoretical model that overcomes these approximations. Simulated film–substrate systems as well as RT measurements from a ZnSe film onto a CaF2 substrate have been used to show how accurate film thickness and optical constants can be obtained using a more complete optical model.

Growth of Cu-Rich/Poor CuInS 2 thin films by the sequential modulated flux deposition technique

CuInS 2 has emerged during recent years as a good candidate to substitute CuInSe 2 as polycrystalline absorber in thin film solar cells, mainly due to its direct band gap energy of 1.5 eV. In this study, absorber layers of both Cu-rich and Cu-poor types have been grown on soda-lime glass substrates by proper selection of the deposition parameters. The morphology and the optical properties of the resulting CuInS 2 films were studied in dependence of the deposition order of the elemental constituents: alternate evaporation of the precursors, simultaneous deposition of the three constituents and sequential modulation of the evaporation fluxes.

Optical characterization procedure for large thin films

Our goal was to characterize large thin films from reflectance and transmittance (R-T) measurements. We focus on the homogeneity of the films related to the geometrical information of their deposition system. This is not an easy task in multisource deposition systems where film thickness mapping and chemical lateral distribution should be checked. In fact, the optical constants lateral variation influences the determination of the size parameters (thickness, roughness). A precise analysis requires determination of every optical parameter on every sample point. This independent point analysis is highly time consuming and results in noisy maps when the represented parameters are strongly correlated. We present a new approach to a global fitting strategy. We have modified our general-purpose software for R-T thin film characterization, adding this global mapping capability. The advantages of our procedure match the challenge of the in situ monitoring in the industrial film processing. As in other mapping procedures, a previous knowledge of our measured system is needed. A detailed optical determination of an In2S3 10x10cm film is presented as an example. Principal Component Analysis has been done with the extracted optical constants in order to reduce the data. This information has been introduced in our automatic global analysis software. The fitting of the same data set resulted on similar but consistent and smoother maps and in around 1/5 of the computing time.