A group of researchers at MIT investigated MoTe2, a two-dimensional transition-metal dichalcogenide (TMD), with the aim to search for silicon-compatible photonic materials for on-chip integration of optical communications.
While inherent lattice mismatch impact III–V light sources grown on silicon through heteroepitaxy, 2D MoTe2 layers can be directly adhered to a silicon substrate via van der Waals interactions.
MIT researchers described a silicon waveguide-integrated light source and photodetector based on a p–n junction of bilayer MoTe2, emitting at 1160nm (infrared) in the paper title “A MoTe2-based light-emitting diode and photodetector for silicon photonic integrated circuits”.
The MoTe2 p–n junction comprise of an exfoliated bilayer of MoTe2 separated by a hexagonal boron nitride (h-BN) dielectric layer from a dual graphite gate. The p- and n-type doping is induced electrostatically by applying different voltages to the graphite gates. The on-demand electrostatic split-gate configuration allows for diverse functionalities to be programmed, including transistors, light-emitting diodes (LEDs) and photodetectors, coupled with an underlying silicon photonic-crystal (PhC) waveguide (a holey silicon membrane).
The carrier concentration in MoTe2 is controlled by the split graphite gates, the separation of the two gates is 400 nm, the dielectric layer is h-BN on top of the MoTe2, and the thickness is 80nm. The source (S) and drain (D) electrodes are thin graphite flakes connected to Cr/Au leads.
The researchers fabricated a grating coupler at the end of the waveguide. When the p–n junction is operated as a LED, the emitted light travels in-plane to the grating coupler where it can be output. In the photodetector mode, incident light coupled into the waveguide by the grating coupler can be detected by the p–n junction.
The researchers noted that the MoTe2 device is compatible with passive waveguide layers, such as silicon nitride, which are available as top layers in most CMOS processes. This dual functionality means such 2D TMD devices could be transferred onto otherwise passive photonic integrated circuits for optical point-to-point interconnects operated in an emitter–receiver configuration, creating high-speed interconnects on top of processors and memory chips.
In LED mode, the light emitted from the p–n junction propagates through the waveguide and is coupled out at the grating coupler. In photodetector mode (bottom), incident light is coupled to the waveguide through the grating coupler and is detected by the p–n junction.
The researchers anticipated that narrowband lasers may well be fabricated in a similar way, by integrating electrically pumped TMD gain materials with PhC nanocavities coupled to waveguides.