Why Silicon is used in VLSI instead of germanium?
The oxide of silicon is a stable insulating insoluble solid, making it possible to integrate the metal interconnects in planar layers over the semiconductor.
The oxide of germanium is unstable and soluble in water, making it necessary to connect the integrated components with individual wires which is far too labor intensive and is also impractical for integration levels beyond MSI. Texas Instruments made some prototype SSI germanium ICs this way in 1958 and 1959 but abandoned the process when they licensed Fairchild's planar silicon IC process.
No ICs are made of germanium now, only discrete transistors and diodes.
Some work has been done on ICs made of silicon-germanium alloy, but I am not sure of the current status.