Surfactant behavior and limited incorporation of indium during in situ doping of GeSn grown by molecular beam epitaxy

GeSn is a promising group-IV semiconductor material for on-chip Si photonics devices and high-mobility transistors. These devices require the use of doped GeSn regions, achieved preferably in situ during epitaxy. From the electronic valence point of view, p-type dopants of group-IV materials include B, Al, Ga, and In. The latter element has never been investigated as p-type dopant in GeSn. In this work, we explore in situ In p-type doping of GeSn grown by molecular beam epitaxy. We demonstrate that In acts as a surfactant during epitaxial growth of GeSn:In, accumulating on surface and inducing Sn segregation in the form of mobile Sn-In liquid droplets, strongly affecting the local composition of the material. In nondefective GeSn, we measure a maximal In incorporation of 2.8 x 1018 cm-3, which is two orders of magnitude lower than the values reported in the literature for in situ p-type doping of GeSn. We further show that In induces the nucleation of defects at low growth temperatures, hindering out-of-equilibrium growth processes for maximization of dopant incorporation. This work provides insights on the limitations associated with in situ In doping of GeSn and discourages its utilization in GeSn-based optoelectronic devices.