Thermal Stability of Annealed Germanium-Tin Alloys Grown by Molecular Beam Epitaxy

The thermal stability of undoped and boron-doped germanium tin (Ge1-x Sn (x) ) alloys grown by molecular beam epitaxy with varying composition and layer thickness was investigated. The alloys were annealed in forming gas at various temperatures up to 800A degrees C for 1 min using rapid thermal processing, and were characterized using high-resolution x-ray diffraction and Rutherford backscattering spectrometry. It was found that the Ge1-x Sn (x) alloys were stable to well above the growth temperature, but the stability decreased with increasing thickness, Sn content, and doping. Ge1-x Sn (x) alloys with low Sn composition (x similar to 0.025) were stable up to 700A degrees C, and for a given Sn composition, the undoped alloys were more thermally stable than the doped alloys. As the thickness of the Ge0.975Sn0.025 alloys increased to about 950 nm, the temperature of thermal stability dropped to 500A degrees C. As the Sn composition of the 90 nm-Ge1-x Sn (x) alloys increased up to x = 0.08, the temperature of thermal stability dropped to 300A degrees C. At higher annealing temperatures, the Ge1-x Sn (x) alloy degraded with lower crystal quality, and a gradient in the Sn composition appeared, which may be due to Sn diffusion or segregation.