We have used transmission electron microscope to examine the microscopic structure of the germanium nanowires grown at low temperatures via a single-step and two-step growth techniques. For the single-step growth technique, the temperatures were ramped gradually to 300 and 260 degrees C. Gold diffusion from the tip to the sidewalls and longitudinal defects occurred at 300 degrees C. By growing the germanium nanowires at 260 degrees C, gold diffusion and longitudinal defects can be circumvented. Germanium nanowire which has both tapered and straight structure was grown via the two-step growth technique by starting at 300 degrees C and lowering the temperature gradually to 260 degrees C. We observed the coexistence of crystalline germanium and amorphous germanium at the tapered structure near the nanowire base, whereas the straight structure near the tip contains only crystalline germanium. The nanowire base diameter is larger and becomes narrower towards the nanowire tip as the growth temperature decreases. A model that explains the mechanism of nanowire diameter modulation via the two-step growth technique was developed. (C) 2013 Elsevier B.V. All rights reserved.