Fs-laser significantly enhances both above- and below-bandgap absorption in germanium

Fs-laser irradiation is a promising fabrication method for future broadband optoelectronic applications as it creates antireflective micro- and nanoscale structures on semiconductor surfaces and introduces below-bandgap absorption; however, its application has mainly been limited to silicon. This paper demonstrates that fs-laser technology enables high optical absorption both above and below the bandgap in germanium (Ge). With optimized laser parameters, we achieve a maximum above-bandgap absorptance of 95% and over 70% below-bandgap absorptance, due to the creation of surface microstructures and structural defects, respectively. Raman spectroscopy reveals that under intense laser irradiation, Ge may undergo a phase transition to structures with a narrower bandgap extending the absorption to the mid-infrared region. Furthermore, we develop a hyperdoping process using Ti coating pre-laser processing followed by rapid thermal annealing, which results in 90% above-bandgap absorption and a 12% relative increase in below-bandgap absorption along with a high degree of crystallinity. The increased below-bandgap absorption is attributed to Ti doping and is twice as high as reported earlier. Our findings should have significant implications for the future Ge-based infrared applications.