Mono-crystalline Germanium functions as an absorber in near infra-red optoelectronics and as a waveguide for mid-IR photonics. Ge is lattice matched to GaAs and hence can be used as a substrate for III-V-based HEMTs, LEDs, and photovoltaic devices. Ge also has a higher electron and hole mobility than silicon, making it useful for next-generation CMOS. Ge wafers are very expensive and incompatible with CMOS. To reduce cost and increase CMOS-compatibility, it is preferable to grow epitaxial Ge directly on Si wafers. However, growth of device-grade Ge on Si is quite challenging due to a 4% lattice mismatch, which leads to dislocations and poor device performance. Techniques such as MBE, RPCVD, two-step RPCVD and three-step MOCVD have been used for growing Ge on Si(100), Si (111) and Si (110). However, these methods tend to be complicated with limited growth rates. Solid-phase crystallization of amorphous Ge using Ge-metal eutectics has also been reported.9,10 Unfortunately, eutectics lead to metal contamination. Furthermore, methods described before are always orientation specific. They do not work equally well for Ge (100), Ge (111) and Ge (110).
In this project we use a liquid-phase crystallization (LPC) method to obtain epitaxial Ge on silicon. The LPC method is significantly simpler than alternatives like MBE and CVD, requiring fewer steps and less-expensive equipment. The process works equally well for Ge (100), Ge (110), and Ge (111) films. The resulting epitaxial Ge does not suffer from metal contamination, making it suitable for optoelectronic applications. The best epitaxial Ge films are [100] oriented 1 μm thick, have a grain size of 2-5 μm, and a dislocation density of ~ 10$^9$ cm$^{-2}$.