  Distributed Bragg reflectors (DBRs) composed of an AlN/AlGaN superlattice
were grown of Si (111) substrates. The first high-reflectance III-nitride DBR
on Si was achieved by growing the DBR directly on the Si substrate to enhance
the overall reflectance due to the high index of refraction contrast at the
Si/AlN interface. For a 9x DBR, the measured peak reflectance of 96.8% actually
exceeded the theoretical value of 96.1%. The AlN/AlGaN superlattice served the
added purpose of compensating the large tensile strain developed during the
growth of a crack-free 500 nm GaN / 7x DBR / Si structure. This achievement
opens the possibility to manufacture high-quality III-nitride optoelectronic
devices without optical absorption in the opaque Si substrate.
