We have fabricated Ge/Si heterojunction photodetectors with high responsivi
ties of 550 mA/W at 1.32 mu m and 250 mA/W at 1.55 mu m and time responses
shorter than 850 ps. High quality Ge was epitaxially grown on Si using ultr
ahigh vacuum/chemical vapor deposition followed by cyclic thermal annealing
. The beneficial effect of the post-growth thermal annealing on the electri
cal properties of Ge epilayers, due to the reduction of threading-dislocati
on densities, is confirmed by the dramatic enhancement of the performance o
f the photodetectors. (C) 2000 American Institute of Physics. [S0003-6951(0
0)03410-0].