Under proper and critical growth conditions, very uniform Ge islands could be formed on Si(100) by using molecular beam epitaxy. The atomic force microscopic and cross-sectional transmission electron microscopic observations illustrate that the size and height uniformities of the islands are about +/-3%. The Auger electron spectroscopic measurements show that the wetting layer is converted into a SiGe alloy layer due to its intermixing with the substrate during the growth. A very narrow peak with the full width at half maximum (FWHM) of 1.6 meV at the energy of 0.767 eV is observed at the temperature of 16 K. We attribute this peak to the free exciton longitudinal acoustic (LA) phonon replica originated from the Ge islands. As the temperature increases, this narrow peak splits into a doublet. This 'camel-like' peak is temporarily explained as the exciton-polariton luminescence.