Significant photoluminescence improvements from bulk germanium-based thin films with ultra-low threading dislocation densities

Abstract

Listen

Translate article

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 = 1016 cm-3) experiences a remarkable 32-fold increase as the thickness is reduced from 535 µm to 2 µm. This surpasses the PL peak intensity of a best-performing epitaxial-Ge on Si (epi-Ge) (0.75 µm thick, biaxial tensile strain= 0.2%, n-doping = 7 ×1018 cm-3) by a factor of 2.5. Furthermore, the PL peak intensity of a 405 µm thick zero-TDD bulk Ge sample (n-doping = 2.5 × 1018 cm-3) is 9.7 times that of the 0.75 µm thick epi-Ge, rising to 12.1 times when thinned to 1 µm. Although the bulk Ge-based TDD reduction approach doesn’t boost the direct band transitions, it can work alongside n-type doping and strain engineering to enhance Ge laser performance and relax the requirement on the latter two approaches, which reduce the associated side effects of high optical absorption, high non-radiative recombination, and large footprint.