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

Abstract

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.

Selection rules of Raman scattering by optical phonons in strained cubic crystals

“…2,3 Often, ''forbidden modes'' appear in the spectra, in measurable intensities, owing to the deviation of q from exact backscattering. 4,5 ͑Actually the intensity of forbidden modes is fully predictable, provided the correct selection rules are applied.͒ 6,7 The noncoincidence of q with any of the eigenvectors may occur not only when q deviates from exact backscattering, but also when the eigenvectors are not in simple geometrical relation with the crystal system of axes 5 and, therefore, q and eigenvectors are not collinear, not even for exact backscattering.…”

Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si

“…From GaAs grown on a planar Si substrate, the emission peak is at 1.416 eV, as shown in figure 3(b). According to previous literature [12], the red shift of 5 meV is caused by the tensile strain, which is caused by the mismatch of thermal expansion coefficients of GaAs and Si. For GaAs grown in trenches, as shown in figures 3(c)-(g), the peak position is initially blue-shifted as the trenches become narrower, and is then red-shifted as the width of the trench is further reduced to less than 80 nm.…”

Photoluminescence measurements of tensile-strained GaAs/In0.07Al0.93As quantum wells

“…Because of the tensile strain, the strained LO phonon frequency decreases as expected from the elastic model. 33 The strain-induced shift of the GaAs LO phonon frequency is 34,35 …”