Synthesis and light absorption mechanism in Si or Ge nanoclusters for photovoltaics applications

Photon absorption in the solar energy range has been investigated in semiconductor nanostructures. Different synthesis techniques (magnetron sputtering, plasma enhanced chemical vapor deposition, ion implantation) followed by thermal annealing, have been employed to fabricate Si or Ge nanoclusters (1-25 nm in size) embedded in SiO2 or Si3N4 matrices. The thermal evolution in the formation of Si nanoclusters (NCs) in SiO2 was shown to depend on the synthesis technique and to significantly affect the light absorption. Experimentally measured values of optical bandgap (Eg OPT) in Si NCs evidence the quantum confinement effect which significantly increases the value of Eg OPT in comparison to bulk Si. Eg OPT spans over a large range (1.6-2.6 eV) depending on the Si content, on the deposition technique and, in a most significant way, on the structural phase of NC. Amorphous Si NCs have a lower Eg OPT in comparison to crystalline ones. The matrix effect on the synthesis and light absorption in semiconductor NCs was investigated for Ge NCs. Large difference in the Ge NCs synthesis occurred when using SiO2 or Si3N4 matrices, essentially due to a much lower Ge diffusivity in the latter, which slows down the formation and growth of Ge NCs in comparison to silica matrix. Light absorption in NCs is also shown to be largely affected by the host matrix. Actually, Ge NCs embedded in Si3N4 material absorb photons in the solar energy range with a higher efficiency than in silica, due to the different confinement effect. In fact, Si3N4 host offers a lower potential barrier to photogenerated carriers in comparison to silica, thus a lower confinement effect is expected, leading to slightly smaller optical bandgap. These effects have been presented and discussed for potential application in light harvesting purposes.