Optical study of the strain effect in pseudomorphic In1-xGaxAs-InP heterostructures

hotoreflectance (PR) and optical absorption (OA) have been used to study the energy-gap dependence on temperature in In1-xGaxAs thin layers, metalorganic vapor-phase epitaxy grown on InP substrates under nearly lattice-matched conditions (‖Δa/a0‖≤10-3). Both PR and OA spectra show a split structure in the band-gap energy region. Moreover, the energy-gap values at all temperatures, obtained by Franz-Keldysh-oscillations analysis of PR line shapes, are blueshifted with respect to the values of the perfectly lattice-matched alloy (x=0.472). The results are related to the valence-band splitting at k=0 generated by the biaxial strain in In1-xGaxAs, due to the small lattice mismatch. We calculated the splitting and shift values using the orbital-strain Hamiltonian and accounting for compositional effects. The values are in agreement with the experimental results giving optical evidence of coherent growth of the alloy film.