**5. Conclusions**

In this paper, we have demonstrated an optical method used for electron concentration determination in the calibration of InGaAs layers designed for waveguides of InP-based quantum cascade lasers. This method requires neither contacts nor an external magnetic field as are typically required in direct-contact methods such as Hall and C-V measurements. Moreover, the method can be applied to various types of structures, made of di fferent kinds of materials and grown in di fferent techniques such as metal-organic chemical vapor deposition, as well as di fferent techniques of doping. It is important to note that 15 K is a significant di fference from the point of view of manufacturing technology. There are several possible causes that would explain reported discrepancies between Hall effect method and the Berreman e ffect method, such as the degree of ionization of the dopant atoms in the sample, or gauging the magnetic field of the magnet, while using Hall setup. Further establishing causes of shown di fferences is crucial to optimize the manufacturing process and therefore the quality of ready devices due to the feasibility of the optical method, which makes it a perfect candidate to supersede other methods. By determination of the so called "Berreman e ffect minima" in the reflectance spectra and the derived carrier concentration versus plasma frequency, we were able to verify the nominal concentration, and finally establish Si source temperature versus concentration dependence.

**Author Contributions:** Conceptualization, M.M.; methodology, M.K. and M.M.; software, M.K.; validation, M.K. and M.M; formal analysis, M.K. and M.M.; investigation, M.K. and M.R.; resources, K.P. and P.G.; data curation, M.K. and M.R.; writing—original draft preparation, M.K.; writing—review and editing, M.M. and G.S.; visualization, M.K., M.R., and M.M.; supervision, M.M.; project administration, M.M.; funding acquisition, M.M. and G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the project SENSE, founded by National Centre for Research and Development TECHMAT-STRATEG1/347510/15/NCBR/2018.

**Acknowledgments:** The authors would like to thank K. Ryczko for his comments and discussions regarding the performed calculations.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
