Ge Quantum Dots Coated with Metal Shells (Al, Ta, and Ti) Embedded in Alumina Thin Films for Solar Energy Conversion

A method to enhance the optoelectronic properties of thin films containing three-dimensional ordered germanium quantum dots (QDs) coated with metal shell (Al, Ta, and Ti) in alumina matrix is presented. The conditions for the achievement of self-assembled growth of the metal-coated Ge QDs by magnetron sputtering deposition are explored. The influence of the metal shell on the structural, optical, and optoelectronic properties is found to be radical and desirable. First, it reduces Ge oxidation. Second, it enhances overall absorption by an order of magnitude. Third, it enables manipulation of the absorption curve shape in two ways. The Ge absorption peak, useful for photoelectric conversion, can be tuned significantly via control of the Ge energy gap because of the quantum confinement effect. On the other hand, infrared absorption can be controlled by the amount of metal because the geometry of the system can enhance absorption in the core by excitation of plasmon resonances in the shell. The mentioned effects result in a high increase of the photocurrent and quantum efficiency of core–shell nanoparticles compared to simple Ge particles. The experiments are supported by theoretical predictions, showing that the intensity of the incident radiation is strongly amplified in the region around the metal–semiconductor interface upon the excitation of plasmon resonances, hence increasing the probability of photon absorption and, thus, of charge carrier generation in the photo-active material. Therefore, the presented materials are very promising for photoelectric devices.