Enhanced photoluminescence of GeSn by strain relaxation and spontaneous carrier confinement through rapid thermal annealing

In this work, the structural evolution and photoluminescence (PL) of 200 nm pseudomorphic Ge0.9338Sn0.0662 on Ge (001) substrate grown by low-temperature molecular beam epitaxy (MBE) after rapid thermal annealing (RTA) is studied. Under RTA at 350 °C or lower, the GeSn film is coherently strained on Ge substrate. As RTA temperature further increases, gradual strain relaxation of GeSn is enabled by generation of misfit dislocations and threading dislocations. As RTA temperature reaches 550 °C or beyond, Sn segregation occurs along with strain relaxation. The PL intensity of annealed samples is enhanced compared to that of as-grown sample probably due to improved crystal quality and strain relaxation (for RTA at> 350 °C) of GeSn. The sample annealed at 500 °C exhibits highest PL intensity due to formation of a Sn-component-graded (SCG) heterojunction with highest Sn content in surface region resulted from interdiffusion of Ge and Sn. The formation of SCG heterojunction renders spontaneous confinement of optically pumped carriers in the surface region and enlarges occupation probability of carriers in Γ valley. Additionally, the carrier confinement in the surface region reduces self-absorption of GeSn and suppresses nonradiative recombination near the GeSn/Ge interface. The results manifest that RTA is an appropriate approach to improve the light emitting property of GeSn grown by low-temperature MBE.