Synthesis and structural properties of Ge nanocrystals in multilayer superlattice structure

Abstract

Ge nanocrystals (Ge NCs) were grown in a multilayered superlattice using magnetron co-sputtering and subsequent thermal annealing. The purpose is to produce a material in which the band gap can be controlled by controlling the Ge NC size and to investigate the potential of this material for use in tandem solar cells. The presence of size-controlled Ge NCs was revealed by Raman spectroscopy, glancing incidence X-ray diffraction (GIXRD) and Transmission Electron Microscope (TEM), and this was supplemented by the observation of blue shifts in the absorption and photoluminescence (PL) properties. Raman spectra showed Ge-Ge active phonon modes at around 300 cm-1 implying the formation of high quality Ge NCs. With increasing annealing temperature and duration, more Ge precipitate changed from a non-crystalline phase to a crystalline phase. However, calculation of degree of crystallinity indicated that a considerable amount of non-crystalline Ge remained at our chosen annealing conditions. GIXRD measurements exhibited three Bragg peaks associated with crystalline Ge. TEM images showed direct evidence of the crystal lattice of the Ge NCs. The size of nanocrystals increased with annealing duration indicating nanocrystal growth by diffusion. The growth of nanocrystals was found to be confined by the GeO2/SiO2 spacing layers, and the average crystallite size was determined by the thickness of the GeRO layers. However, enhanced interdiffusion at elevated annealing temperature weakened the size confinement effect of the multilayer structure. Hence an optimum annealing condition is needed to produce high quality and reproducible Ge NCs. Our preliminary work indicates that it may be promising to use Ge NCs as absorber materials in tandem solar cells..