Empirical correlation of minority carrier lifetime to defect density profile in germanium on silicon heteroepitaxy

High-quality Ge-on-Si (GoS) heterostructures are pursued for many applications, including near infrared (NIR) photodetectors and integration with III-V films for multi-junction photovoltaics. However, challenges such as thermal expansion coefficient mismatch and lattice mismatch between Ge and Si often leads to a high density of defects which can propagate through any III-V material grown subsequently. Introducing annealing steps prior to and after full Ge island coalescence are found to reduce the defect density. It has been observed that the defect density in Ge also decreases with increasing dopant density in Si substrates, likely by the defect pinning near the Ge/Si interface by dopants. Using the effective minority carrier lifetime measured as a function of Ge film thickness, an empirical correlation is established between the minority carrier lifetime and the defect density in the Ge film as a function of distance from the Ge/Si interface and relating the surface recombination velocity on Ge film surface average lifetime within Ge film, and recombination velocity at the Ge/Si interface. Two photoconductance decay techniques were used in this work: quasi-steady-stay photoconductance decay and microwave photoconductance decay. The interface recombination velocity for Ge films grown on low-doped, high resistivity (HDLR) and high-doped, low resistivity (LDHR) Si substrates were found to be 370 cm/sec and 0.22 sec/cm2, respectively. Also established is an empirical correlation between minority carrier lifetime of the Ge film (tauGe) and the defect density (rhoD) within the Ge film as a function of distance from the Ge/Si interface: tauGe = C/rhoD, where C is a proportionality constant and a fitting parameter which is determined to be 0.17 sec/cm2 and 0.22 sec/cm2 for Ge films grown on LDHR Si substrates and HDLR Si substrates, respectively.