  Bulk Ge crystals, characterized by significantly lower threading dislocation
densities (TDD) than their epitaxial counterparts, emerge as optimal candidates
for studying and improving Ge laser performance. Our study focused on the Ge
thickness and TDD impacts on Ge's photoluminescence (PL). The PL peak intensity
of a bulk Ge sample (TDD = 6000 cm^(-2), n-doping = 10^16 cm^(-3)) experiences
a remarkable 32-fold increase as the thickness is reduced from 535 to 2 micron.
This surpasses the PL peak intensity of a 0.75 micron thick epi-Ge on Si
(biaxial tensile strain= 0.2 %, n-doping = 10^19 cm^(-3)) by a factor of 2.5.
Furthermore, the PL peak intensity of a 405 micron thick zero-TDD bulk Ge
sample (n-doping = 5 * 10^17 cm^(-3)) is ten times that of the 0.75 micron
thick epi-Ge, rising to twelve times when thinned to 1 micron. The TDD
reduction method is highly effective, which relaxes the requirements of high
n-doping and stress in enhancing Ge laser performance and thus reduces the side
effects of high optical absorption, high non-radiative recombination, bandgap
narrowing, and large footprints associated with these two techniques.
