why silicon is used in silicon controlled rectifier not germanium?

Answer 1

Because Si sustains a larger reverse voltage typically 70V at room temp; compared to about only 30V max for Ge. This is because the bangap of Si at room temp is 1.2 eV where as that of Ge is only 0.75 eV (both are in the the far infrared so both types of diode junctions must be in complete darkness to work). Bandgap is the photon energy required to make a pure, no doped hence no intermediary potential stepping stones, semiconductor electron to jump out of its potential barrier to be a free and electrically conducting one.

Even when the p-n junction forward drop of 0.7V of a Si diode is not desirable, a Ge diode (p-n forward drop of about 0.3V) is today more costly than a Schottky Si diode (a metal-on-Si, not a p-n junction, of 0.2 to 0.4 V forward junction voltage drop).

Answer 2

Mainly, because it's easier to provide higher peak inverse voltages (PIV) in silicon, over germanium.

I don't think it's possible to make a germanium "S"CR with a PIV of, say 1000 volts. (maybe some semiconductor physicist knows otherwise.)

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