To realize three-dimensional (3D) optical interconnection on large-scale integration (LSI) circuits, layer-to-layer couplers based on Si-photonics platform were reviewed. In terms of optical cross talk, more than 1 µm layer distance is required for 3D interconnection. To meet this requirement for the layer-to-layer optical coupler, we proposed two types of couplers: a pair of grating couplers with metal mirrors for multi-layer distance coupling and taper-type directional couplers for neighboring layer distance coupling. Both structures produced a high coupling efficiency with relatively compact (∼100 µm) device sizes with a complementary metal oxide semiconductor (CMOS) compatible fabrication process.

We experimentally demonstrate transistor operation in a vertical p+-MoS2/n-HfS2 van der Waals (vdW) heterostructure configuration for the first time. The HfS2/MoS2 heterojunction transistor exhibits an ON/OFF ratio of 104 and a maximum drain current of 20 nA. These values are comparable with the corresponding reported values for vdW heterojunction TFETs. Moreover, we study the effect of atmospheric exposure on the subthreshold slope (SS) of the HfS2/MoS2 transistor. Unpassivated and passivated devices are compared in terms of their SS values and IDS-VGS hysteresis. While the unpassivated HfS2/MoS2 heterojunction transistor exhibits a minimum SS value of 2000 mV/dec, the same device passivated with a 20-nm-thick HfO2 film exhibits a significantly lower SS value of 700 mV/dec. HfO2 passivation protects the device from contamination caused by atmospheric moisture and oxygen and also reduces the effect of surface traps. We believe that our findings will contribute to the practical realization of HfS2-based vdW heterojunction TFETs.