We demonstrate a GeSn growth method that uses new source gases to increase the Sn concentration of GeSn thin films, which are attracting attention as a next-generation semiconductor material for replacing for Si. Synchrotron radiation techniques are very useful for physical analysis of ultrathin films, particularly hard X-ray photoelectron spectroscopy, which can simultaneously analyze the outermost surface of the thin film and the bulk GeSn just underneath. As a result, we obtained results suggesting the growth of a high-quality GeSn thin film with suppression of surface Sn segregation and surface oxidation. Analysis of Sn concentration using a combination of Rutherford backscattering spectroscopy and secondary ion mass spectrometry revealed that the concentration of Sn was about 6.6 atom % and that the Sn content was uniform in the depth direction. These synchrotron techniques are expected to facilitate the preparation of high-quality, high-Sn GeSn thin films.