The strained Si0.8Ge0.2 film has been prepared onto Si substrate by using an ultrahigh-vacuum chemical vapor deposition system. A low cost wavelength filter of photodetector has been demonstrated for the first time. This filter was simply carried out by just inserting a 60 nm thick a-Si:H capped layer onto Si0.8Ge0.2 thin film. The room-temperature photoluminescence shows that the sample with Si0.8Ge0.2 layer has a tendency to shift wavelength into longer regime than that of Si substrate. The full width at half maximum (FWHM) was 185 nm for Si0.8Ge0.2 photodetector without a-Si:H capped. By inserting a 60 nm thick a-Si:H capped layer, the FWHM was narrowed into 97 nm. This demonstrates that the a-Si:H capped layer has an ability acted as wavelength filter in our study. (c) 2005 Elsevier B.V. All rights reserved.