To observe the Quantum Anomalous Hall Effect, unlock the exciting potential of magneto-electric effects, robust ferromagnetic doping of topological insulators has to be achieved. Ferromagnetic doping, which leads to a gap opening in the topological surface state band structure while also having a low background carrier density, is required. A promising materials system which allows for both requirements to be met at the same time is Cr-doped (Bi,Sb)2Te3. We propose to study MBE grown thin films of varying stoichiometry and substrate type to gain insight into carrier concentration and strain dependencies of the ferromagnetic ordering. Our goal is to understand the underlying mechanisms that provide the fine balance of ferromagnetic doping and low carrier density in great detail. Our goal is to study the interfaces and thin films of precisely engineered thin films using POLREF.