Development Points to Potential for 100-Fold Improvement in Nanopillar LED Efficiency

Researchers from Eindhoven University of Technology have created a very efficient nanopillar LED. The team fabricated the nanopillar LED with a metal cavity in a III-V layer stack attached to a silicon substrate.

The device achieved “relatively high” on-chip external quantum efficiency (10−4 to 10−2 for room-temperature and 9.5 K, respectively) with output power ranging from nW to tens of nW. They also pointed to additional developments which could increase the efficiency 100 fold compared to previous nanopillar LEDs. The team said that the results encourage the development of future, ultra-low power, high-density optical interconnect systems with Gbps data rates.

They published their findings in Nature Communications.

Scanning Electron Microscope images of fabricated devices. (a) False-colored image showing the device structure before metallization with the nanopillar on top of a waveguide connected to a grating coupler. (b) Metal-coated nanopillar after silver evaporation and rapid thermal annealing, before the sputtering of gold (100 nm) was used to prevent silver oxidation. (c) False-colored image of the device after metallization, displaying the electrical contacts.

Scanning Electron Microscope images of fabricated devices. (a) False-colored image showing the device structure before metallization with the nanopillar on top of a waveguide connected to a grating coupler. (b) Metal-coated nanopillar after silver evaporation and rapid thermal annealing, before the sputtering of gold (100 nm) was used to prevent silver oxidation. (c) False-colored image of the device after metallization, displaying the electrical contacts.

They found the LED far exceeded previous nanoscale LEDs which exhibited up to hundreds of pW of output. The researchers noted that while recently some promising photonic crystal lasers have also been integrated on silicon substrates, metal nanocavities still offer advantages such improving heat dissipation and reducing footprint.

The group found that the nanopillar LED achieved a power level at low temperature (>50 nW). According to the scientists, this power level translates to over 400 photons per bit at 1 Gb s−1, well above the shot-noise-limited sensitivity of an ideal receiver.

The team asserts that with the continuous progress in integrated receivers and the expected low loss of short-distance interconnects, this power level may enable intrachip data communications with an ultracompact source.

Furthermore, they said claimed that percentage-level efficiency at low temperature that they achieved suggests that of metal nanocavity light sources could potentially perform very efficiently even at room temperature if better passivation techniques suppress the non-radiative recombination processes (found to be dominant in their devices).

The group pointed to the very recent development of a surface passivation method using a wet chemical ammonium sulfide treatment and SiO2 encapsulation. This treatment lead to a two-orders-of-magnitude (∼500 cm s−1) decrease in the surface velocity for nanopillars with similar dimensions and an identical epitaxial layer stack to their nano-LEDs.

The team claims that such surface velocity reduction could provide up to a 100-fold increase in the efficiency of their nano-LEDs. They also said that they might further reduce energy consumption with improved ohmic contacts, using heavily doped InGaAsP layers and a thicker p-doped InGaAsP layer.

Another possible efficiency improvement could come from improved spatial matching of the active region with the optical mode that may increase coupling of spontaneous emission into the cavity mode, resulting in an increase in efficiency of greater than 1%, they said.

The findings point to the potential use of nanopillar LEDs in future high-density optical interconnect systems needing Gbps data rates at ultra-low power consumption. The researchers claim that such Gbps data rates might eventually be achievable with arrays of directly modulated integrated nanoscale sources.

Reference

Dolores-Calzadilla, V. et al. Waveguide-coupled nanopillar metal-cavity light-emitting diodes on silicon. Nat. Commun. 8, 14323 doi: 10.1038/ncomms14323 (2017).

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