  The morphological evolution and the effect of growth temperature on size,
orientation and composition of molecular beam epitaxy grown Ge-Si islands on
Si(5 5 12) surfaces have been investigated in the temperature range from room
temperature (RT) to 800$^\circ$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 (SEM),
cross-sectional transmission electron microscopy (X-TEM) and Rutherford
backscattering spectrometry (RBS). In the RH case, we found spherical island
structures at 600$^\circ$C with a bimodal distribution and upon increasing
temperature, the structures got faceted at 700$^\circ$C. At 800$^\circ$C thick
($\sim$ 122nm) dome like structures are formed bounded by facets. While in the
case of DC heating, after the optimum critical temperature 600$^\circ$C, well
aligned trapezoidal Si$_{1-x}$Ge$_x$ structures with a graded composition
starts forming along the step edges. Interestingly, these aligned structures
have been found only around 600$^\circ$C, neither at low temperature nor at
higher temperatures.
