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

On-chip optical interconnects promise to drastically reduce energy consumption compared to/nelectrical interconnects, which dominate power dissipation in modern integrated circuits (ICs)./nOne key requirement is a low-power, high-efficiency, and high-speed nanoscale light source./nHowever, existing III-V semiconductor light sources face a high surface recombination velocity/n(SRV ~ 10^4 – 10^6 cm/s) that greatly reduces efficiency at nanoscale sizes. An alternative material/nsystem is the monolayer transition metal dichalcogenides (TMDCs), single-molecule-thick/ndirect-bandgap semiconductors that are being investigated for a variety of applications in/nphotonics and electronics. In particular, they are intrinsically nanoscale in one dimension and/nlack dangling bonds at the surface, leading to high optical efficiency. In addition, they can be/nelectrically injected, transferred to arbitrary substrates, and processed in a top-down manner/nsimilar to traditional semiconductors./nHowever, the development of electrically-injected light emitting devices based on TMDCs is still/nin an early stage, with relatively few reports of high-efficiency light emission. In this/ndissertation, we identify current decay over time as the main limitation preventing stable/noperation of lateral-junction TMDC light-emitting diodes (LEDs), particularly in ambient (nonvacuum) conditions. To solve this, we propose operating WSe2 LEDs under pulsed voltage,/nwhich shows much more stable light emission over time than DC operation (hours vs. seconds of/ncontinuous device operation). Electroluminescence (EL) efficiency matches that of/nphotoluminescence, confirming material-quality-limited efficiency. In addition, we demonstrate/nfast rise and fall times of ~15 ns, a record for TMDC LEDs./nFor nanoscale light emitters that require high efficiency at low input power, LEDs are preferred/nover lasers due to their lack of threshold requirement. The slow speed of LEDs can be greatly/nenhanced by coupling the emitter to an optical antenna, extending its optical transition dipole/nlength and resulting in possible ~100x-1000x enhancement of spontaneous emission rate. We/nexperimentally demonstrate three electrically-injected antenna-coupled designs: the double-gate/nLED coupled to bowtie antennas, the light-emitting capacitor coupled to a slot antenna array, and/nthe single-gate LED coupled to a nanosquare antenna array. The light-emitting capacitor shows/nvery high polarization ratios >30x, and the nanosquare array shows >10x improved intensity/nover control devices, both indicators of strong antenna enhancement. We discuss the tradeoffs/ninvolved in each design. Finally, we theoretically investigate high-speed, highly scaled TMDC/nnanoLED devices and identify the limits to speed and efficiency. We conclude that edge/nrecombination can be largely overcome with sufficient antenna enhancement, while exciton-exciton annihilation ultimately limits efficiency at high injection levels. Under our model, high/nspeeds >70 GHz can be achieved at modest quantum efficiencies >10%. However, much further/nwork remains in improving material properties and devices.

Advisor: Wu, Ming C