Inserting Hydrogen into Germanium Quantum Dots

Germanium quantum dots are very interesting for applications such as solar cells, photodetectors, and light emitters because their small bandgap can be tuned over a wide energy range by changing the particle size. One obstacle to applications is the presence of defects, both in the interior and at the surface of the nanoparticles. The defects function as nonradiative recombination centers or trap charge carriers, which will hinder further optical performance. Introducing hydrogen, as employed in a-Si:H solar cells, has proven to be a good method to counter such detrimental defect effects. In this work, germanium quantum dots were fabricated by an ultraclean, vacuum-based nanoparticle reactor in which hydrogen was supplied during growth. Optical spectroscopy of the a-Ge:H quantum dots, together with Raman and X-ray photoelectron spectroscopy, revealed a direct bandgap and that the presence of hydrogen resulted in amorphous Ge:H quantum dots. These a-Ge:H quantum dots are a step forward toward reducing charge carrier recombination in quantum dot solar cells.