  Low-defect-density Ge thin films are critical in Ge based optical devices
(optical detectors, LEDs and Lasers) integrated with Si electronic devices for
low-cost, highly integrated photonic circuits. In this work, Ge thin films
prepared by wet etching with four different solutions were studied in terms of
the surface morphology, defect density and achievable thickness. Both
nanostrip-based solution (1:1:10) and HCl-based solution (1:1:5) were able to
wet-etch 535 micron thick bulk-Ge substrates to Ge films thinner than 10 micron
within 53 hours. The corresponding RMS surface roughness was 32 nm for the
nanostrip-based solution and 10 nm for the HCl-based solution. The good quality
of bulk-Ge was preserved before and after the etching process according to the
HRXRD results. The low threading dislocation density of 6000-7000 cm-2 was
maintained in the process of wet etching without introducing extra defects.
This approach provides an inexpensive and convenient way to prepare
sub-10-micron thick Ge thin films, enabling future studies of
low-defect-density Ge-based devices such as photodetectors, LEDs, and lasers.
