Guess why LEDs are made of gallium arsenide instead of silicon? Because the optical properties of GaAs are much better. Doing optics with Si might be possible, but it will always remain inferior to GaAs.

Wikipedia has a comparison between GaAs and Si which is worth reading:

http://en.wikipedia.org/wiki/Gallium_arsenide#Comparison_with_silicon

However, please keep in mind that the third advantage of Si mentioned there does not apply anymore, since Dr. Taylor has resolved the problem of GaAs's slow hole mobily somehow.