The photoreflectance spectroscopy method has been used to study stressed GaAs/GaAs$_{0.6}$P$_{0.4}$ superlattices grown along the [100] and [111] directions. An analysis of the Franz-Keldysh oscillations observed in photoreflectance spectra determines the built-in fields in these superlattices. The fields amount to 80 kV cm$^{-1}$ for the (100) orientation and 430 kV cm$^{-1}$ for the (111) orientation. The difference has been explained by the presence of a strain-induced piezoelectric field in stressed superlattices grown along the [111] direction.
$^1$Department of General Physics, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
$^2$ Sapphire" Research Institute
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