Thin highly p-doped Si1-xGex/Si layers were grown by molecular beam epitaxy (MBE) on [100] silicon substrates for mid-IR absorption and photoresponse measurements. The SiGe layers were boron doped in the range 1 to 5 x 10(20) cm(-3) and had a thickness of 15 nm for x = 0.4 and x = 0.5. We measured the IR absorption for the samples between 2 and 5 mu m at 77 K. Mesa diodes were fabricated as lest devices for IR detectors, and the quantum efficiency and dark current were measured. A maximum quantum efficiency of eta = 1.6% was achieved at 4 mu m and 77 K; the spectral dependence of the photocurrent showed a broad responsivity in the technological important range between 3 and 5 mu m.