Photoreflectance and Photoluminescence Study of (GaAs)m/(AlAs)5 (m=3-11) Superlattices: Direct and Indirect Transition

Photoreflectance and photoluminescence measurements are carried out to investigate optical properties of (GaAs) m /(AlAs) 5 superlattices with m =3, 5, 7, 9 and 11 at room temperature, in order to clarify the cross over of the direct and indirect optical transition. Combining the transition energies determined from the photoreflectance measurements with the photoluminescence data, the superlattices are shown to be indirect for m 7. Energy band calculations based on the sp 3 s * tight-binding method are found to explain these results consistently. The lowest allowed direct transition energy obtained from the tight-binding theory agrees well with the energy gap determined from the photoreflectance measurements, whereas the lowest-lying indirect gap agrees with the photoluminescence peak energy for m <7.

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