High indium content In0.78Ga0.22As photodetector structures have been grown on InP substrates with InxGa1-xAs or InxAl1-xAs graded buffer layers wherein x changed continuously by gas source molecular beam epitaxy. Their characteristics were investigated by atomic force microscope, x-ray diffraction, transmission electron microscopy and photoluminescence. The differences between samples with the two kinds of buffer layers were studied. Results show that moderate surface morphology can be obtained with either InxGa1-xAs or InxAl1-xAs buffer layers. Larger residual strain is present in the photodetector structure with InxGa1-xAs buffer layer. On the other hand, superior optical characteristics have been observed for the photodetector structure with InxAl1-xAs buffer layer.