Monolayer Transition Metal Dichalcogenide NanoLEDs: Towards High Speed and High Efficiency

On-chip optical interconnects promise to drastically reduce energy consumption compared toelectrical interconnects, which dominate power dissipation in modern integrated circuits (ICs).One key requirement is a low-power, high-efficiency, and high-speed nanoscale light source.However, existing III-V semiconductor light sources face a high surface recombination velocity(SRV ~ 10^4 – 10^6 cm/s) that greatly reduces efficiency at nanoscale sizes. An alternative materialsystem is the monolayer transition metal dichalcogenides (TMDCs), single-molecule-thickdirect-bandgap semiconductors that are being investigated for a variety of applications inphotonics and electronics. In particular, they are intrinsically nanoscale in one dimension andlack dangling bonds at the surface, leading to high optical efficiency. In addition, they can beelectrically injected, transferred to arbitrary substrates, and processed in a top-down mannersimilar to traditional semiconductors.However, the development of electrically-injected light emitting devices based on TMDCs is stillin an early stage, with relatively few reports of high-efficiency light emission. In thisdissertation, we identify current decay over time as the main limitation preventing stableoperation of lateral-junction TMDC light-emitting diodes (LEDs), particularly in ambient (nonvacuum) conditions. To solve this, we propose operating WSe2 LEDs under pulsed voltage,which shows much more stable light emission over time than DC operation (hours vs. seconds ofcontinuous device operation). Electroluminescence (EL) efficiency matches that ofphotoluminescence, confirming material-quality-limited efficiency. In addition, we demonstratefast rise and fall times of ~15 ns, a record for TMDC LEDs.For nanoscale light emitters that require high efficiency at low input power, LEDs are preferredover lasers due to their lack of threshold requirement. The slow speed of LEDs can be greatlyenhanced by coupling the emitter to an optical antenna, extending its optical transition dipolelength and resulting in possible ~100x-1000x enhancement of spontaneous emission rate. Weexperimentally demonstrate three electrically-injected antenna-coupled designs: the double-gateLED coupled to bowtie antennas, the light-emitting capacitor coupled to a slot antenna array, andthe single-gate LED coupled to a nanosquare antenna array. The light-emitting capacitor showsvery high polarization ratios >30x, and the nanosquare array shows >10x improved intensityover control devices, both indicators of strong antenna enhancement. We discuss the tradeoffsinvolved in each design. Finally, we theoretically investigate high-speed, highly scaled TMDCnanoLED devices and identify the limits to speed and efficiency. We conclude that edgerecombination can be largely overcome with sufficient antenna enhancement, while exciton-exciton annihilation ultimately limits efficiency at high injection levels. Under our model, highspeeds >70 GHz can be achieved at modest quantum efficiencies >10%. However, much furtherwork remains in improving material properties and devices