To determine if your results are good, you should look at several different criteria. You should check the χ2 value first to determine how well the model fits the data. You should then compare the results visually, check if the fit is reasonable and physical, check the error bars on the resulting values, and examine the correlation matrix. If the error bars are large, or the correlation matrix shows strong correlations, for example, there may be an issue with your model. One other indicator of a good fit is quick convergence of the fitting algorithm, which suggests the fit is unique.
Typically, for a simple single layered thin film, a χ2 value of around 1 is very good. As the samples get more complex, the χ2 values will start to rise. For a complex sample, an χ2 of up to 10 or more is acceptable, as long as all other results are physical and reasonable. Keep in mind that even if the χ2 value is small, the results may not always be physical or reasonable, so be sure to check how well the other fit criteria are, as well.
If you see strong correlations, you should add any additional data that you might have, whether it is data taken at different angles of incidence, or reflectance/transmittance data. You can also identify the parameters which are correlated, and check if they both need to be fit at the same time. If not, it might be possible to fix one, if this makes sense to the model.
Each dispersion function is used for a certain type of material. For example, a Drude dispersion function is commonly used for metallic films and a Lorentz dispersion function is commonly used for transparent or weakly absorbing films. For a list of all of the dispersion functions used in the DeltaPsi 2 software, as well as what types of materials they are used for, please email us at info-sci.fr(at)horiba.com.
The most commonly used dispersion functions include Cauchy and Lorentz (Classical) for transparent or weakly absorbing films, amorphous, new amorphous, and Tauc-Lorentz for semi-transparent materials (dielectrics, polymers, semiconductors absorbing in the VIS/FUV), and Drude for metals.