Substrate-induced peak in the photoluminescence of heavily doped epitaxial GaAs

A peak at 1.38 eV is observed in the 300-K photoluminescence of n+ and p+ epitaxial GaAs. This peak appears as a shoulder of the principal band-to-band emission. The shoulder is an artifact produced by the substrate and should not be interpreted as an additional optical transition. It is demonstrated that the subband-gap luminescence, which arises from the band-gap reduction caused by the heavy doping, travels through the transparent semi-insulating substrate, reflects off the back surface, and is emitted from the epilayer. The shoulder intensity is enhanced by the scattering of light off the back surface. Thick substrates with polished back surfaces are optimum for reducing the shoulder peak.