  We present a study on the influence of strain-relieving InAlAs buffer layers
on metamorphic InAs/InGaAs quantum wells grown by molecular beam epitaxy on
GaAs. Residual strain in the buffer layer, the InGaAs barrier and the InAs
wells were assessed by X-ray diffraction and high-resolution transmission
electron microscopy. By carefully choosing the composition profile and
thicknesses of the buffer layer, virtually unstrained InGaAs barriers embedding
an InAs quantum well with thickness up to 7nm can be grown. This allows
reaching low-temperature electron mobilities much higher than previously
reported for samples obtained by metamorphic growth on GaAs, and comparable to
the values achieved for samples grown on InP substrates.
