We investigated the initial growth stages of Si x Ge 12x ͞Si͑001͒ by real time stress measurements and in situ scanning tunneling microscopy at deposition temperatures, where intermixing effects are still minute (#900 K). Whereas Ge͞Si(001) is a well known Stranski-Krastanow system, the growth of SiGe alloy films switches to a 3D island mode at Si content above 20%. The obtained islands are small (a few nanometers), are uniform in shape, and exhibit a narrow size distribution, making them promising candidates for future quantum dot devices.