We have carried out an in-depth structural study of Ge thick layers grown using the so-called "LT/HT" approach on Si(0 0 1). Si(0 11) and Si(1 11) wafers. The low temperature (400 degrees C) adopted for the first Ge layer plastically relaxes the strain in the Ge film without 3D islanding. The high temperature (750 degrees C) used for the growth of the second, topmost, Ge layer lowers the dislocation density and reduces the overall deposition time. High temperature thermal cycling (in-between 750 and 890 degrees C) was called upon to further reduce the amount of defects in the layers. Ge growth rates on (0 0 1) were consistently higher than the ones on (111), which were themselves higher than on (0 11) (be it at 400 degrees C, 100 Torr or at 750 degrees C, 20 Torr). The surfaces of Ge (0 0 1) layers were rather flat, as attested by their small root mean square (rms) roughness and Z ranges (< 1 and 10 nm, respectively). By contrast, Ge (0 11) (Ge (111)) thick layers were really rough, with rms roughness and Z ranges typically 30 (60) times higher than on (0 0 1). Almost all Ge layers were in a tensile-strain configuration, with macroscopic degrees of strain relaxation in-between 101% and 106% (slightly higher on (1 11) than on (0 0 1) and on (0 11), however). Misfit dislocations were found (by cross-sectional transmission electron microscopy) to be confined in the first few hundreds of nanometres of 2.5 mu m thick Ge layers on Si(0 0 1). As far as 2.5 mu m thick Ge layers on (0 11) and (111) are concerned, numerous (111) stacking faults were present at the Ge/Si interface and also threading all the way to the surface. The threading "defect" density, which is close to 10(7) cm(-2) on (001), is around 8 x 10(8) cm(-2) on (0 11) and roughly 2 x 10(9) cm(-2) on (111). The higher defect density on (0 11) and (1 1 1) is likely due to 60 degrees misfit dislocations dissociation (into 30 degrees and 90 degrees partial dislocations). For such surface orientations, the 90 degrees partial dislocation is expected to lead with in its trail the 30 degrees partial dislocation (hence the stacking faults multiplication). (C) 2008 Elsevier B.V. All rights reserved.