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On-chip photonics devices relying on the weak, volatile thermo-optic or electro-optic effects of silicon usually suffer from high insertion loss (IL) and a low refractive index coefficient. In this paper, we designed two novel 1 × 1 and 1 × 2 phase-change optical switches based on a signal-mode Si waveguide integrated with a Ge2Sb2Te5 (GST) top clad layer, respectively. The three-state switch including amorphous GST (a-GST), face centered cubic crystalline phase (FCC-GST) and hexagonal crystalline phase (HCP-GST) operated by utilizing the dramatic difference in the optical constants between the amorphous and two crystalline phases of GST. In the case of the 1 × 1 optical switch, an extinction ratio (ER) of 8.9 dB and an extremely low IL of 0.8 dB were achieved using an optimum GST length of only 2 μm. While for the 1 × 2 optical switch, low ILs in the range of 0.15 ∼ 0.35 dB for both ‘cross’ (a-GST) and ‘bar’ (FCC-GST and HCP-GST) states were also obtained. Additionally, we found that both ILs and mode losses of the switch with HCP-GST were about half lower than those with FCC-GST, which means FCC-GST could be instituted by HCP-GST in the traditional ovonic switch with the consideration of low loss. This research provides the fundamental understanding for the realization of low loss and non-volatile Si-GST hybrid optical switches, with potential for future communication networks.

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The scandium (Sc) and aluminum (Al) co-doped antimony-selenium (Sb45Se25Sc7Al23) thin film was fabricated and systematically investigated in this study. The results reveal that our examined Sb45Se25Sc7Al23 thin film has good thermal stability (the failure temperature for 10 years data retention ~103 °C) and the fast phase change speed (5 ns) at same time. Besides, the Sb45Se25Sc7Al23 thin film shows no multiple phase change, which derives from the fact that no formation of new Sc or Al related phases that are able to trigger multiple states in the thin film. However, it is found that more Sb precipitation is formed by the increase of dopants, which may increase the tendency of multiple phase change. All results suggest Sb45Se25Sc7Al23 thin film would be a good candidate used for phase change random access memory (PCRAM) applications.

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