Two-dimensional photonic crystals are promising structures for photonic applications. Here, we show that the optical properties of two-dimensional photonic crystal membranes fabricated from silicon-on-insulator substrates can be probed at room temperature by the internal continuous wave photoluminescence of Ge/Si self-assembled islands. Quality factors up to 15 000, limited by the spectrometer resolution, are measured. We also show that two-dimensional photonic crystals can be fabricated as well with germanium-on-insulator substrates by using the same processing techniques. The optical properties of these two-dimensional photonic crystals can be probed by the room-temperature photoluminescence associated with the direct band gap of the bulk Ge layer. Enhanced resonant extraction efficiency can be obtained for wavelengths longer than 1.55 mu m. The quality factors of HI hexagonal cavities are found in good agreement with those obtained by finite-difference time domain calculations. (C) 2008 Elsevier B.V. All rights reserved.