Energy gaps in strained In1-xGaxAs/In1-yGayAszP1-z quantum wells grown on (001) InP.

${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As single quantum wells with strain between -2.5% and 1.7% confined by ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{Ga}}_{\mathit{y}}$${\mathrm{As}}_{\mathit{z}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathit{z}}$ barriers, lattice matched to InP substrates, have been studied by electroabsorption, photocurrent, and luminescence spectroscopy. The results agree with current models of the strain-induced shift of energy levels and confirm a transition to a type-II superlattice, which for a barrier gap of 1.10 eV takes place for a Ga content near 70% at a tensile strain of 1.7%. The transition is not sharp in the presence of an electric field where confined states are gradually replaced by resonant states. Deep wells of compressively strained samples are not sensitive to a small variation of the position of the conduction band in contrast to wells under tensile strain. The data indicate a negligible shift of the valence band with hydrostatic strain and suggest a valence-band offset for unstrained ternary wells and quaternary barriers between 55% and 60% of the band-gap difference, similar to that of ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/InP heterostructures. Samples with strain larger than 1% become increasingly inhomogeneous resulting in level splitting by thickness variation and strong broadening of the spectral features. Under large tensile strain, luminescence is observed that is attributed to defect luminescence rather than to exciton recombination.