The morphological evolution and the effect of growth temperature on size, orientation and composition of molecular beam epitaxy grown Ge-Si islands on Si(5512) surfaces have been investigated in the temperature range from room temperature to 800°C. Two modes of substrate heating, i.e. radiative heating (RH) and direct current heating (DH) have been used. The post-growth characterization was carried out ex situ by scanning electron microscopy, cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry. In the RH case, we found spherical island structures at 600°C with a bimodal distribution and upon increasing temperature, the structures got faceted at 700°C. At 800°C thick (122nm) dome-like structures are formed bounded by facets. While in the case of dc heating, after the optimum critical temperature 600°C, well aligned trapezoidal Si 1xGe x structures with a graded composition starts forming along the step edges. Interestingly, these aligned structures have been found only around 600°C, neither at low temperature nor at higher temperatures. © 2012 IOP Publishing Ltd.