Direct-to-indirect energy-gap transition in strained ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP quantum wells

We present direct experimental evidence that in ${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/InP quantum wells the band structure undergoes a direct-to-indirect gap transition in k space above a critical value of x. We observe a drastic increase of the measured radiative exciton lifetimes for samples where x>${\mathit{x}}_{\mathit{c}}$, with ${\mathit{x}}_{\mathit{c}}$ depending on well width. using a six-band k⋅p calculation of the valence subbands, we show that for x>${\mathit{x}}_{\mathit{c}}$ the valance-band maximum is at k≠0, i.e., the band structure becomes indirect.