High-quality Ge films on Si substrates using Sb surfactant-mediated graded SiGe buffers

High-quality Ge films were grown on Si substrates by solid-source molecular beam epitaxy using SiGe graded layer and Sb surfactant-mediation technique. Transmission electron microscopy measurements show that samples grown using this method have a lower threading dislocation density than those grown by other typical methods, such as grading at high temperature (700°C) only, grading at intermediate temperature (510°C) only, and the use of low temperature Si buffer. A relaxed Ge film on a 4-μm-thick graded buffer was grown and shown to have a threading dislocation density of 5.4×10⁵ cm^-2 and surface roughness of 35 Å. Ge p-i-n diodes were fabricated and tested. Under a reverse bias of 1 V, the p-i-n Ge mesa photodiodes exhibit a very low dark current density of 0.15 mA/cm².