Determination of the composition of strained InGaAsP layers on InP substrates using photoreflectance and double-crystal x-ray diffractometry

Determining the composition of strained InGaAsP films through measurements of the lattice spacings and band-gap energies (Eg) requires converting the measured values to ones which would correspond to unstrained material. In strained layers the lattice constants perpendicular and parallel to the growth plane can vary significantly from the relaxed value, and the optically measured Eg is affected by a strain-induced splitting of the valence band and a shifting of the direct gap energy. By combining double-crystal x-ray data with room-temperature photoreflectance results, we determine the InGaAsP composition accurately using an iterative procedure. Film compositions calculated using strain adjusted values of Eg agree with those determined by energy dispersive spectroscopy to within 1–2 at. %, whereas if energy shifts are not considered, the error approaches 10 at. % for strain on the order of 0.4%.