OPTICAL COMMUNICATIONS: Plasmonic LED approaches 10 GHz modulation speed

Commercial light-emitting diodes (LEDs) are only capable of 1 GHz maximum modulation speeds because of slow carrier recombination, limiting them to applications in short-haul optical communication links. Because the carrier radiative recombination rate, R_r, in a semiconductor is defined by R_r = Bnp (where B is the recombination rate of an isolated electron-hole pair, and n and p are the active-layer concentrations of electrons and holes, respectively), two methods to increase this rate are to increase the electron and hole concentrations, or to increase B using the Purcell effect. Unfortunately, increasing n and p dopant concentrations causes nonradiative recombinations that decrease internal device efficiency. So researchers at Hewlett-Packard Laboratories (Palo Alto, CA) instead exploited the Purcell effect by creating surface-plasmon polaritons (SPPs) at a metal-dielectric interface within an LED that interact with quantum-well excitons and create a large radiative enhancement.¹ Tenfold carrier lifetime reductions from 462 to 44 ps were observed for their plasmonic LED.

Generating surface-plasmon polaritons

To optimize the LED design, a rigorous coupled-wave analysis used a series of calculations to determine that a silver layer at some distance from the tensile strained gallium arsenide phosphide (GaAsP) quantum-well structure would maximize SPP generation. The LEDs were fabricated with different gaps between the quantum-well layer and the 2000 Å silver layer. To test the LEDs, 2 ps pulses from a 780 nm diode laser with 100 MHz repetition rate were used to excite the LED structures. Spectrally resolved photoluminescence plots showed emission peaks for the LEDs at 805 nm, with light intensity decreasing as the gap between the quantum well and silver decreases. This intensity drop is attributed to two interactions: first, as the quantum-well layer gets closer to the silver layer, more optical energy is emitted in the SPP mode rather than the free radiative mode; and second, silver diffusion in the semiconductor gap region causes nonradiative decay of carriers and a corresponding drop in emitted light intensity.

Time-resolved photoluminescence was also studied for these LEDs using a single-photon detector. The decay time for an LED with a 40 nm gap between the quantum well and silver layer was approximately 44 ps, corresponding to a 3 dB modulation speed of 3.6 GHz. Decay time for a sample with an 80 nm gap increased to approximately 462 ps, nearly the same as the 500 ps delay observed for an LED without the silver layer altogether. Essentially, decay time decreases as the gap separation decreases (see figure).

Even though the device with a 40 nm gap between the quantum well and silver layer achieves a modulation speed of only 3.6 GHz, the researchers expect that a device fabricated with higher free-carrier densities on the order of 10¹⁹ cm^-3 could easily achieve a modulation speed of 10 GHz.

The next step for the research team is to study electrically driven devices with good external efficiencies. “LEDs capable of 10 Gbit/s modulation speed can be the next-generation low-cost optical source for very short distance links within computer racks,” says Michael Tan, a senior scientist at Hewlett Packard Laboratories. “The cost of the optical components is one of the largest mitigating factors for introducing photonics inside the box. These LEDs are much easier to manufacture and would cost 50 to 100 times less than today’s data communication VCSELs.”

REFERENCE

1. D. Fattal et al., Appl. Phys. Lett. 93, 243501 (Dec.15, 2008).