High-quality Ge/Si0.4Ge0.6 multiple quantum wells for photonic applications: growth by reduced pressure chemical vapour deposition and structural characteristics

Strain-symmetrized Ge/SiGe multiple quantum wells have been grown on a thin (2.1 µm) relaxed Si 0.2 Ge 0.8 /Ge/Si(1 0 0) virtual substrate (VS) by reduced pressure chemical vapour deposition. Such structures are of interest in optoelectronic applications for which the structural integrity of the quantum well layers, after processing, is critical. The layer composition, thickness and interface quality have been studied for wafers both as-grown and after annealing between 550 and 700 °C. Transmission electron microscopy indicated precise thickness control of ±0.1 nm and sharp abruptness between the Ge QWs and SiGe barrier layers. A smooth surface was observed, with an average rms roughness of 1.5 ± 0.1 nm determined by atomic force microscopy. High-resolution x-ray diffraction (HR-XRD) indicated that both the QWs and barriers were fully strained compared with the relaxed VS. The thermal stability of the epilayers was investigated both by ultra low energy secondary ion mass spectroscopy of post-growth annealed layers and by in situ annealing in a high temperature HR-XRD stage. No obvious interdiffusion and strain relaxation was observed provided the annealing temperature was below 600 °C, but significant atomic rearrangement was evident for greater thermal budgets, thereby setting an upper processing temperature for this type of structure.