Optical determination of strains in heterostructures: GaAs/Si as an example

Raman and photoluminescence spectroscopies are used to characterize crystalline quality and interfacial strain in heterostructures. The effect of a biaxial stress on electronic and vibronic energies is reviewed and then applied to the case of a GaAs layer. Measurements on GaAs grown on Si(100) by molecular-beam epitaxy are made over a wide temperature range (4→700 K). The evolution of the strain is deduced from the shift of both the energy-band gaps and the long-wavelength transverse and longitudinal-optical-phonon frequencies. The sensitivity of the Raman probe is dramatically enhanced by excitation under resonant conditions at the E1 edge of GaAs. The measurements confirm the anisotropy of the strain and demonstrate that both its sign and value at room temperature result from a balance between two reverse phenomena: the thermal expansion and the lack of complete relaxation of the lattice mismatch during growth.