Asymmetrically contacted germanium photodiode using a metal-interlayer-semiconductor-metal structure for extremely large dark current suppression.

In this study, we proposed germanium (Ge) metal-interlayer-semiconductor-metal (MISM) photodiodes (PD), with an anode of a metal-interlayer-semiconductor (MIS) contact and a cathode of a metal-semiconductor (MS) contact, to efficiently suppress the dark current of Ge PD. We selected titanium dioxide (TiO2) as an interlayer material for the MIS contact, due to its large valence band offset and negative conduction band offset to Ge. We significantly suppress the dark current of Ge PD by introducing the MISM structure with a TiO2 interlayer, as this enhances the hole Schottky barrier height, and thus acts as a large barrier for holes. In addition, it collects photo-generated carriers without degradation, due to its negative conduction band offset to Ge. This reduces the dark current of Ge MISM PDs by ×8000 for 7-nm-thick TiO2 interlayer, while its photo current is still comparable to that of Ge metal-semiconductor-metal (MSM) PDs. Furthermore, the proposed Ge PD shows ×6,600 improvement of the normalized photo-to-dark-current ratio (NPDR) at a wavelength of 1.55 µm. The proposed Ge MISM PD shows considerable promise for low power and high sensitivity Ge-based optoelectronic applications.

Synthesis of a Five-Membered Molecular Necklace: A 2+2 Approach.

Like with a string of pearls, four molecular "beads" are threaded on a molecular rectangle to form a molecular necklace. This rectangular species is synthesized from two L-shaped, preorganized pseudorotaxanes with two molecular beads each (cucurbituril, schematically symbolized by the barrels), held together by Cu2+ ions [Eq. (1)].