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

Kevin Han

EECS Department
University of California, Berkeley
Technical Report No. UCB/EECS-2019-100
May 29, 2019

http://www2.eecs.berkeley.edu/Pubs/TechRpts/2019/EECS-2019-100.pdf

On-chip 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 low-power, high-efficiency, and high-speed nanoscale light source. However, existing III-V semiconductor light sources face a high surface recombination velocity (SRV ~ 104 – 106 cm/s) that greatly reduces efficiency at nanoscale sizes. An alternative material system is the monolayer transition metal dichalcogenides (TMDCs), single-molecule-thick direct-bandgap semiconductors that are being investigated for a variety of applications in photonics and electronics. In particular, they are intrinsically nanoscale in one 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 top-down manner similar to traditional semiconductors. However, the development of electrically-injected light emitting devices based on TMDCs is still in an early stage, with relatively few reports of high-efficiency light emission. In this dissertation, we identify current decay over time as the main limitation preventing stable operation of lateral-junction TMDC light-emitting diodes (LEDs), particularly in ambient (non- vacuum) 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 of continuous device operation). Electroluminescence (EL) efficiency matches that of photoluminescence, confirming material-quality-limited 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 optical antenna, extending its optical 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.

Advisor: Ming C. Wu


BibTeX citation:

@phdthesis{Han:EECS-2019-100,
    Author = {Han, Kevin},
    Title = {Monolayer Transition Metal Dichalcogenide NanoLEDs: Towards High Speed and High Efficiency},
    School = {EECS Department, University of California, Berkeley},
    Year = {2019},
    Month = {May},
    URL = {http://www2.eecs.berkeley.edu/Pubs/TechRpts/2019/EECS-2019-100.html},
    Number = {UCB/EECS-2019-100},
    Abstract = {On-chip 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 low-power, high-efficiency, and high-speed nanoscale light source. However, existing III-V semiconductor light sources face a high surface recombination velocity (SRV ~ 104 – 106 cm/s) that greatly reduces efficiency at nanoscale sizes. An alternative material system is the monolayer transition metal dichalcogenides (TMDCs), single-molecule-thick direct-bandgap semiconductors that are being investigated for a variety of applications in photonics and electronics. In particular, they are intrinsically nanoscale in one 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 top-down manner similar to traditional semiconductors.
However, the development of electrically-injected light emitting devices based on TMDCs is still in an early stage, with relatively few reports of high-efficiency light emission. In this dissertation, we identify current decay over time as the main limitation preventing stable operation of lateral-junction TMDC light-emitting diodes (LEDs), particularly in ambient (non- vacuum) 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 of continuous device operation). Electroluminescence (EL) efficiency matches that of photoluminescence, confirming material-quality-limited 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 optical antenna, extending its optical 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.}
}

EndNote citation:

%0 Thesis
%A Han, Kevin
%T Monolayer Transition Metal Dichalcogenide NanoLEDs: Towards High Speed and High Efficiency
%I EECS Department, University of California, Berkeley
%D 2019
%8 May 29
%@ UCB/EECS-2019-100
%U http://www2.eecs.berkeley.edu/Pubs/TechRpts/2019/EECS-2019-100.html
%F Han:EECS-2019-100