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

Onchip optical interconnects promise to drastically reduce energy consumption compared to electrical interconnects, which dominate power dissipation in modern integrated circuits (ICs)One key requirement is a lowpower, highefficiency, and highspeed nanoscale light source. However, existing IIIV semiconductor light sources face a high surface recombination velocity (SRV ~ 104106cm/s) that greatly reduces efficiency at nanoscale sizes. An alternative material system is the monolayer transition metal dichalcogenides (TMDCs), singlemoleculethick directbandgap semiconductors that are being investigated for a variety of applications in photonics and electronics. In particular, they are intrinsically nanoscale inone dimension and lack dangling bonds at the surface, leading to high optical efficiency. In addition, they can be electrically injected, transferred to arbitrary substrates, and processed in a topdown manner similar to traditional semiconductors.
However, the development of electricallyinjected light emitting devices based on TMDCs is still in an early stage, with relatively few reports of highefficiency light emission. In this dissertation, we identify current decay over time as the main limitation preventing stable operation of lateraljunction TMDC lightemitting diodes (LEDs), particularly in ambient (nonvacuum) conditions. To solve this, we propose operating WSe2LEDs under pulsed voltage, which shows much more stable light emission over time than DC operation (hours vs. secondsof continuous device operation). Electroluminescence (EL) efficiency matches that of photoluminescence, confirming materialqualitylimited efficiency. In addition, we demonstrate fast 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 preferred over lasers due to their lack of threshold requirement. The slow speed of LEDs can be greatly enhanced by coupling the emitter to an opticalantenna, extending itsoptical transition dipole length and resulting in possible ~100x-1000x enhancement of spontaneous emission rate. We experimentally demonstrate three electrically-injected antenna-coupled designs: the double-gate LED coupled to bowtie antennas, the light-emitting capacitor coupled to a slot antenna array, and the single-gate LED coupled to a nanosquare antenna array. The light-emitting capacitor shows very high polarization ratios >30x, and the nanosquare array shows >10x improved intensity over control devices, both indicators of strong antenna enhancement. We discuss the tradeoffs involved in each design. Finally, we theoretically investigate high-speed, highly scaled TMDC nanoLED devices and identify the limits to speed and efficiency. We conclude that edge recombination can be largely overcome with sufficient antenna enhancement, while exciton-exciton annihilation ultimately limits efficiency at high injection levels. Under our model, high speeds >70 GHz can be achieved at modest quantum efficiencies >10%. However, much further work remains in improving material properties and devices.
Publication date: 
May 31, 2019
Publication type: 
Ph.D. Dissertation
Han, K. (2019). Monolayer Transition Metal Dichalcogenide NanoLEDs: Towards High Speed and High Efficiency. United States: University of California, Berkeley.

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