Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures

What is it about?

Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles (NPs) were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing - RTA (550–900 °C, 1 min) in N2. The structures were investigated using XRD, HRTEM together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe NPs (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such NPs acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell NCs. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.

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Why is it important?

SiGe NCs sandwiched between SiO2 layers were fabricated by co-sputtering using HiPIMS and dcMS followed by RTA (1 min) at different temperatures. It is shown that HiPIMS deposition facilitates the formation of small nanoparticles/clusters in the as-grown structures. A suitable selection of annealing temperature and time results in the columnar self-assembly of SiGe core–shell NCs, as comprehensively studied by GiXRD and TEM analysis. The self-assembly is attributed to a dominant strain relaxation process, further assisted by already present small NPs in the as-grown structures, acting as seed crystals for heterogeneous nucleation. The photocurrent study reveals that strain and its influence on the NCs/matrix interface morphology play a vital role in determining spectral features and sensitivity.

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