The impact of buffer schemes on the strain relaxation and structural characteristics of In0.83Ga0.17As photodetector structures with relatively high lattice mismatch (5.9%) grown on GaAs substrate by gas source molecular beam epitaxy has been investigated. Reduction of surface roughness, full widths at half maximum of X-ray diffraction signals and threading dislocations, as well as an enhancement of photoluminescence intensity were observed for the In0.83Al0.17As photodetector structure with fixed-composition In0.83Al0.17As buffer compared to those with continuously graded InxAl1-xAs buffer. The role of fixed-composition In0.83Al0.17As buffer layer is investigated, and it is believed that a couple monolayers of In0.83Al0.17As at the initial growth stage can provide high density of nucleation site by the formation of quantum dots at the interface and thus to reduce the strain energy caused by the large lattice mismatch between In0.83Al0.17As and GaAs. (C) 2015 Elsevier B.V. All rights reserved.