High electron mobility in modulation-doped Ga_xIn _1-xAs/Al_yIn_1-yAs heterostructures with highly strained AlInAs grown by molecular beam epitaxy

N-type modulation-doped Ga_xIn_1-xAs/Al _yIn_1-yAs heterostructures grown by molecular beam epitaxy on InP substrate have been characterised by double-crystal X-ray diffraction and X-ray topography, temperature-dependent Hall measurements, and photoluminescence and absorption spectroscopy. Both the constituent Ga _xIn_1-xAs (x=0.453) and the Al_yIn _1-yAs (y=0.421) layers are strained. X-ray Pendellosung fringes from the Ga_xIn_1-xAs as well as the Al_yIn _1-yAs epilayers are observed, indicating an almost perfect interface smoothness and homogeneity of the layer composition. Despite the highly strained Al_yIn_1-yAs the electron mobility reaches 9.0*10³, 6.5*10⁴, and 8.9*10⁴ cm² V^-1 s^-1 at 300, 77 and 4 K, respectively, thus indicating only a minor influence of the strain of the Al _yIn_1-yAs upon the 2D electron mobility. In photoluminescence the excellent material quality manifests itself in the narrow bound exciton peak from the Ga_xIn_1-xAs layer and in the absence of any transition related to the 2D electrons and holes trapped near the heterointerface.