We report a single-crystal Si-Ge structure which works as an efficient photodetector in the wavelength region of up to 1.5 µm. The multilevel structure is grown by molecular-beam epitaxy on an n-type 3-in silicon substrate and consists of the following layers: n+silicon (1000 Å), n+GexSi1 - xalloy (1800 Å, graded in ten steps fromx = 0tox = 1), n+germanium (1.25 µm), undoped germanium (2.0 µm), and p+germanium (2500 Å). Top three layers form a germanium p-i-n diode, which is removed from the Ge-Si interface by a buffer layer of high conductivity. An advantage of this structure is that its performance is insensitive to material defects in the buffer layers. Moreover, transmission electron microscopy shows that the density of dislocations introduced by lattice mismatch at the Ge-Si interface falls off with the separation from the interface. Our first experimental structures do exhibit the characteristics of a germanium p-i-n diode. The spectral response curves agree with those given in the literature for germanium, both at room and liquid nitrogen temperatures. For the incident light wavelength of 1.45 µm we have measured a quantum efficiency of 41 percent at T = 300 K. we believe that our approach opens an attractive possibility of fabricating complete infrared optoelectronic systems on a silicon chip.