Self-heating of vertical-cavity laser diodes is strongly affected by t
he thermal conductivity of the distributed Bragg reflectors (DBR's). B
inary GaAs-AlAs DBR's are expected to be good heat conductors, but inv
estigations of GaAs-AlAs superlattices indicate strong interface scatt
ering of phonons. For the first time, we present direct thermal conduc
tivity measurements of separated GaAs-AlAs DBR's with a quarter-wave l
ayer thickness of more than 100 nm (tuned to 1.55-mu m wavelength). Us
ing an ac calorimetric method and finite element analysis, we measure
about 50% thermal conductivity reduction compared to the average bulk
value. GaAs-AlAs DBR's for shorter wavelengths are expected to show an
even lower thermal conductivity.